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Potassium Research - Review and Trends
Congress on Occasion of the 25th Anniversary of the
Scientific Board
of the International Potash Institute
Potassium Research Review and Trends
UN
Proceedings of the 11th Congress of the International
Potash Institute, 1978
International Potash Institute
CH-3048 Worblaufen, Bern/Switzerland
Phone 031/585373 Telex 33430 ipibe ch
Printed by <Der Bund
AG, Bern/Switzerland 1979
Contents
Page
The members of the Scientific Board of the
International Potash Institute
7
1st Session
25 years IPI
N. Cello
Welcome address
15
R. Bach
Developments in potassium research
17
T. Walsh
The International Potash Institute; twentyfive years on - the achievements and the
challenge
21
2nd Session
Potassium in the soil/plant root system
D.Schroeder
Structure and weathering of potassium containing minerals
43
H. Laudelout
The physical chemistry of equilibria involving the potassium ion in soils
65
A. van Diest
Factors affecting the availability of potassium
in soils
75
H.Grinme and K.N~meth
The evaluation of soil K status by means of
soil testing
99
3rd Session
The role of potassium in yield formation
A. Lduchli and R. Pfliger
Potassium transport through plant cell membranes and metabolic role of potassium in
III
plants
0. Steineck and H. E. Haeder
The effect of potassium on growth and yield
165
components of plants
H.lBeringer and G. Trolldenier
Influence of K nutrition on the response to
environmental stress
189
5
4th Session
Potassium requirements of crops
K.Mengel
A consideration of factors which affect the
potassium requirements of various crops
225
P. Quintanilla Rejado
Potassium requirements of cereals
239
G.Fauconnier
Potassium requirements of grain legumes
259
S. L. Jansson
Potassium requirements of root crops
267
A. Malquori and F. Parri
The potassium requirements of fruit crops
283
H. R. von Uexkiill and A. Cohen
Potassium requirements of some tropical tree
crops (oil palm, coconut palm, rubber,
coffee, cocoa), and cotton
291
G.de Beaucorps
The potassium requirements of crops harvested green, with special reference to grassland
325
5th Session
Potassium fertilization in agricultural practice
G. Drouineau
Potassium effects in long-term experiments
345
G. W. Cooke and P.A. Gething
Changing concepts on the use of potash
361
A.Louj
The interaction of potassium with other
growth factors, particularly with other
nutrients
407
A. Dam Kofoed
The potassium cycle in cropping systems
6th Session
Translation of research into practice
l.Arnon
Fertiliser use as a lead practice in modernising agriculture
451
A.von Peter
The economics of fertiliser use and fertiliser
resources
479
6
435
The Members
of the Scientific Board
of the
International Potash Institute
(1978)
The Members of the Scientific Board of the International Potash
Institute (1978)
Dr. T. Walsh
Prof. Dr. I. Arnion
Dublin,[ ire
Reh ovot' srael
Member since 1953
Member since 1959
I)r, (i \. Cooke
L ondn Gircat Britain
Member since 9 62
Prof. Dr. R. Bach
Zirich SwitzerIand
Member ince 1963
8
Director A. D.Kolbed
Askov pr. Vejen Denmark
Member since 1963
Prof. Dr. H. Laudelout
Louvain-!a-Neuv< Belgium
Member since 1963
t
G, Drou neaU
Prof. Dr. S. L Jansson
t
Director o Reearh
llon rr
Paris rnce. Member since 1967
L .ppsa Su eden
Member since 1967
The Xlembers of the Scientific Board of the International Potash
I nstitute (1978)
Prot" )t l)j. B J-elcim
BeIgraidvc "YotgIaai
\lcmlbcr since 1967
Prl Dr. A M lqu]I
I>lurerce Itakv
Menber i nce 1967
Protl* P Qm)t1iuilk Repclo)
Madirid Spihl
Mem1nlbCr
IM6cr1er
0i1CtC
Prol. Dr. ).Schroeder
Kiel iederal Rep, of (erman\
'i1,e 9(6)
1(
a
Prof. Dir
) Stcincck
Vienna Austria
NAember sminc 1969
Pr,).D r. . an ) iCSt
\VLaeningen Fhe NetherlandR
Meher since 1977
Z..
::...
U
Pril" I)r. [l Fieringcr
l)ire.ro,-
I Iatnnto e I-e. Rep ,I (iviarn
Sdeartifie Sceret] tthe Boatrd
PI'r
de lcttleor>ip-
Iljfce
Neutlie Scmar
ll
f the I'tovrd
25th Anniversary of the Scientific Board
11th IPI-Congress held in Bern/Switzerland
from September 4 to 8, 1978
1st Session
25 Years IPI
13
Welcome Address
Dr. N. Celio, Former President of the Swiss Confederation: President of the International Potash Institute, Bern/Switzerland
First, L address to you all a hearty welcome to Switzerland. The honour of opening
this Jubilee Congress of the Scientific Board of the International Potash Institute falls
to me and I must tell you how happy I am to have this opportunity to greet in Berne
so many specialists in agricultural research who have come from forty-eight different
countries of Africa, America, Asia and Europe and from FAO and five other major
international organisations. You all have at heart the wish to improve agriculture and
to better the lot of mankind.
The title of this our eleventh Congress is: 'Potassiun Research - Review and Trends'.
You have come to Berne in order to survey the progress achieved over the past twenty
five years. This does not just involve writing a history of research; in our work we are
at least as much concerned with the future as we are with the past. The aim is to arrive
at a critical evaluation of past research results and to assess the present state of
knowledge in our particular field of science. The future will not be a repetition of the
past. Science and research move forward constantly; your objective will be to identify
gaps in present knowledge, then to decide on priorities for future work and further, if
possible, to suggest how the improvements made possible by research may best be
translated into agricultural practice.
Such, Ladies and Gentlemen, is the challenge facing you during your stay in Berne.
Allow me to hope that the work which will occupy you during this coming week will
be carried out in an atmosphere of objective yet creative criticism, a spirit which has
always been characteristic of our Scientific Board. It is my opinion that the very great
achievements of this Board are due entirely to the fact that its members - who have
over the years become true friends - have never feared to express their ideas emphatically, and to compare their opinions; to take a strong stand whenever they have
thought it useful to do so but also to be open to accept sometimes the views of their
colleagues in order to progress.
I must take the opportunity which this occasion gives me to express to all the members
of the Scientific Board, in the name of all those who are responsible for the destinies
of this Institute, the most sincere thanks for the way in which they have supported
research on the element potassium with their great scientific knowledge and expertise.
That our Institute enjoys such a high reputation in international circles is largely due
to you, Gentlemen.
In our Congresses, we have a tradition of hard work; this congress continues that
tradition. Nevertheless, the short breaks in the working programme will give you the
15
opportunity to make friendly personal contacts, contacts which do much to enrich
and enliven the formal proceedings. I am convinced that your discussions will bear
fruit.
The technical excursions planned for the last day will, we hope, give you an impression
of Swiss agriculture. Much of the farming in this country is done on small but very
intensive holdings. These are often worked under conditions which would lead some
countries to abandon such land. While agriculture might be thought to be of somewhat
minor significance in a country so heavily industrialised as Switzerland this is not the
case. The great majority of the Swiss people considers it to be of the utmost public and
cultural value; the reasons for this will be explained to you. The excursions will also
offer you some opportunity to see the scenic beauties of our country and I sincerely
hope that the sun will shine.
I cannot conclude without offering our heartfelt thanks to our Directors and all their
staff for their contributions to the organisation of this Congress.
In wishing you a lively, interesting and fruitful meeting, I now declare that the Eleventh
Congress of the International Potash Institute is open!
16
Developments in Potassium Research
R. Bach, Chairman of the Congress, Swiss Federal Institute ofTechnology, Zfilrich/Switzerland*
1. Introduction
This Congress of the International Potash Institute marks a Jubilee. The Scientific
Board of the Institute was founded almost exactly twenty five years ago.
It is a main aim of IPI to collect and make known scientific knowledge about potassium. In this way research workers may be directed towards new goals and practical
agriculturists may be shown the way to higher yields and better quality. The Scientific
Board has throughout its existence been concerned to maintain the activity and
reputation of the International Potash Institute on the highest level.
The first Congress of the Institute was held in Ztirich in 1954 to review the then
status of knowledge about potassium. The four sessions of that Congress dealt with:
potassium in the soil, potassium in the organisms and in agriculture and with problems
of analysis. From the point of view of agriculture potassium was at that time recognised as being important in making possible higher yields and much current
research was directed to improving fertiliser recommendations. There have been
many developments since 1954 which have broadened and altered the aims of potassium research.
2. World political, social and economic changes
I would first like to mention some of the changes which have taken place in the political,
social and economic environment.
a) World population and the need for food have increased in spectacular fashion.
Population increase has been particularly great in the countries with the least
well developed agriculture.
b) The developed countries mostly have food surpluses and in these countries there
is a demand for higher and higher quality.
c) Food supplies and needs have become, geographically and temporally, more and
more out of balance. Storage, transport and distribution have imposed new
demands on the quality of agricultural produce.
*
Prof. Dr. R. Bach, Dept. of Food Science and Technology, Swiss Federal Institute of
Technology, ETH-Zentrum LFO, 8029 Zirich/Switzerland
17
d) The environment has been damaged and is increasingly threatened. Conservation
measures are an urgent necessity.
e) There are indications that some of our natural resources may become exhausted
in the foreseeable future.
f) Agriculture is increasingly concerned not only with the production of food of
good quality but also with the quality of life offered to those who work in it.
3. Changes in agriculture
There have been significant changes in agriculture.
a) The plant breeder has developed higher yielding cultivars of the majority of crops.
b) In a number of cases quality of produce has assumed more importance than
mere yield.
c) Certain modern practices may have unwanted and damaging consequences in
the long term.
d) Agriculture is more and more looked upon as a whole whose inter-related parts
must be in sympathy: nature, technology, economics and social aspects.
e) In the long term, the conservation of soil fertility is just as important as theintensification of production.
4. Changes in scientific methods and concepts
Agricultural research has been improved and much influenced by progress in the
basic scientific disciplines.
a) Experimental methods have been improved and new methods have been introduced,
for example: atomic absorption- and infra-red-spectroscopy, scanning electron
microscopy, the use of radioactive tracers.
b) Many analytical processes have been automated enabling the handling of many
more samples.
c) Computers are available for the fuller evaluation of the results of complex experiments.
d) There is increased use of mathematical modelling in the study of complex systems.
e) Specialist research has been improved through the multidisciplinary approach.
5. Activities of the International Potash Institute
The International Potash Institute has kept a close watch on worldwide changes in
the political, economic and social fields as it has on scientific developments. This is
shown in the topics chosen for the congresses and colloquia which have been held
from 1955 to 1977: Potassium and the quality of agricultural products, manuring
of grassland, forests and glasshouse crops, the use of fertilisers in the mediterranean
and tropical areas, the role of fertilisers in the intensification of agriculture, fertilisers
and plant health, fertilisers and protein production, fertilisers and the production
of carbohydrates and lipids. The colloquia have offered the opportunity to bring
18
together workers from different countries engaged in related disciplines. The plan
has been to complete each cycle of three colloquia with a congress in which the
findings of the individual colloquia are brought together, generalised and brought
to the attention of a wider audience.
6. The present congress
The Congress with which we are celebrating the twenty-fifth anniversary of the
Scientific Board is of a different kind. The plan is that members of the Scientific
Board, with some other scientists, will review 25 years progress in research in various
fields, evaluate the results and attempt a forecast of what should be the aim for the
next twenty-five years.
The sessions are organised under the following headings:
a) Potassium in the soil/plant/root system.
b)
c)
d)
e)
The role of potassium in yield formation.
The potassium requirements of crop plants.
Potassium fertilisation in agricultural practice.
Translation of research into practice.
The lecturers have naturally reviewed their fields of activity comprehensively, but,
owing to the limited time available here they will only be able to deal with the salient
features of their reviews. The full papers will be published in the Congress Proceedings which should serve as an important 'Reference to Potassium Research'.
Finally, I should like to say that this Congress must not be content with looking
at the past; it must look into the future. The lectures and discussion should identify
the gaps in current scientific knowledge and point out the roads to be followed in
future research. They should also show how scientific knowledge can be better used
to improve practical agriculture for the benefit alike of producers and consumers
and, indeed, to the benefit of all mankind. I hope we shall achieve this goal and that
this Jubilee Congress will stand as a milestone in potassium research.
I hope you will all benefit from this Congress and wish you all a pleasant stay in
our country.
19
These were ambitious objectives, especially for an organisation which derived its
resources primarily from the commercial background of two major potash producers*
viz. Verkaufsgemeinschaft Deutscher Kaliwerke and Soci&t Commerciale des Potasses
d'Alsace, where the commercial and financial objectives would inevitably be seen
as having a major influence. Yet 25 years have passed, and the Institute is still in
existence and flourishing. It is however, an appropriate time to assess what it has
accomplished, the extent to which its objectives have been achieved, what contribution it has made, how it stands to-day and where it goes in the future. Such an
evaluation is at all times difficult but especially so in the case of an institute such as
this where the end product - knowledge - can have an effect in so many intangible
ways and on so many institutions and people.
It is, I believe, necessary to attempt this assessment against the background of overall political, economic, scientific and societal developments during the period in
which the Institute has operated. It might be useful, for instance, to see the extent
to which the Institute, in its work, has reflected modern scientific and technological
developments in agriculture, developments in fertiliser technology and in agricultural research. On a global scale it might be queried to what extent the Institute
considered or took into account the macro, international issues of energy, resource
conservation, environmental protection and human welfare, especially food needs.
Having considered the position against this background we can then proceed to
examine the various means by which the Institute has attempted to achieve its objectives and to see how far it has succeeded.
2. The challenge of changing times
The period under review has been a most dynamic one, with rapid and continual
change the order of the day. Political changes in the aftermath of the Second World
War have been particularly dramatic. Many new States have emerged; new major
economic groups such as the European Economic Community have been created;
new levels of affluence have been reached in the developed countries and new horizons
set for those underdeveloped. For the greater part of the period progressive economic
development has been an outstanding feature, greater of course in some countries
and regions than others but positive in most.
The twin problems of inadequate food supply and population increase have become
matters of special concern and have involved action by many international agencies
and foundations. World population has increased by about 60%, from 2650 million
in 1953 to an estimated 4250 million in 1978 (Figure 1). The oil crisis highlighted
our overdependence on fossil fuels and provided a timely warning of the urgent
need to develop alternative energy sources. Food and petro power today exert major
political influence almost everywhere.
There has also been international unrest, conflict and war, which have set their own
problems. Towards the end of the period, new attitudes in relation to development
have inevitably emerged, with major consideration now being given to the quality
of living, to human welfare and the conservation of natural resources. A feature of
* To be joined later by five others viz. Potasas Espafolas 1954, DeadSea Works, 1959, Polasas
de Navarra, 1964 and Seifa, 1965, Cleveland Potash, 1970.
22
4500
4000
3500
3000
2500
20001500.
1000.
5001950
55
60
65
70
75
78
Fig.). World population (millions)
these developments, especially in the developed countries, is the greater concern of
people with the ways in which food nutrients affect health, virility, age and general
welfare, a concern being increasingly reflected in food quality needs and indeed
becoming an increasingly important pressure in consumerism. In other words,
societal objectives in development have supplemented, if not to some degree supplanted
the more limited economic objectives envisaged at the beginning of the period.
2.1 Scientific developments:
The period has been one of unparalleled scientific growth, with major increases in
scientific and technological institutions and manpower in most countries. For instance,
in France and Ireland there was a four to five fold increase in the manpower for
agricultural research. The same held for other countries. In EEC countries, for
instance, there are now some 8000 scientists in agricultural research and some 12 000
projects under way, with effective international co-operation and coordination.
In other fields of scientific endeavour directly related to agriculture there has also
been substantial growth. Many new universities and research institutes have been
founded, with increased emphasis on post-graduate studies. There has been substantial development in international foundations and other research centres.
While the main emphasis has been on large scale scientific projects and indeed prestige
science, especially in the space research field, developments towards the end of the
period have seen a healthy change towards concentration on matters which affect
the quality of life and on factors which affect human satisfaction. In effect, this
scientific explosion means that today there are more research workers, more scientists
and more technologists requiring information servicing than ever before, and more
highly specialist information available for dissemination and use. People in general
23
are now also much more interested in and sensitive to scientific and technological
developments. Indeed public demand for scientific information and informal learning about science and technology is now leading to the proliferation of science centres
for the public in countries such as the United States.
2.2 Developments in agriculture
The need to expand food production to counter starvation and undernourishment,
and the demonstrated capacity, through scientific methods of land development,
to meet this problem, has brought agricultural resources, land, water and human,
under increasing pressure. FAO responded by mounting its Freedom from Hunger
Campaign, while international agencies made major investment in research and
development projects. Government food aid programmes helped to meet specific
needs. Food production has continued to expand. Under the impact of modern
agricultural technology many soils are now yielding more produce than ever before,
and much more than our forebears thought possible. Indeed many such soils now
under intensive use were traditionally considered very infertile and incapable of
yielding crops or supporting livestock.
In the developing countries striking advances have taken place in areas once fooddeficient, such as Mexico, the Philippines, India and Pakistan, where cereal production
has now more than matched needs in many areas. In practically every country throughout the world a new awareness has been created of the food production capacity of
the land. This has been prompted by the new technology which has emerged or has
been developed on the spot, which has stimulated adaptive research and created a
new demand for a more quantitative approach to soil productivity. Practically all
these developments have been research based and the creation and use of new technology from such research is escalating. Another important and relevant development
is that of the technology of synthetic food production - challenging conventional
sources either directly as food products or as ingredients or extenders.
It has been accepted that the agricultural sector, including the food industry, in both
developed and developing countries is in a primary position to initiate and sustain
economic growth, both as the source of food and as the generator of agri-business
and agri-industry. We have merely to consider what the fertiliser industries are contributing by way of employment and commercial activity in many countries to understand the significance of agricultural development in priming economic growth, and
they constitute only one factor in the complex of industries and activities involved.
I dealt with this subject and described an Irish case study in detail elsewhere (Walsh
[1973]).
Agricultural research has made a major contribution to these new developments
in agricultural production through the introduction of new plant varieties, through
a greater understanding of disease resistance in both animals and plants, through
development of new knowledge in animal fertility and production and through the
impact of new techniques on the properties of soils and on the nutrient needs of
plants as a basis for sustained production. As expected farm mechanisation, increased
productivity and output per man resulted in displacing people from the land in many
countries. The gainful redeployment of redundant rural workers has constituted a
real challenge for economic planners, just as has the displacement of factory workers
through automation in industrialised economies.
24
2.2.1 Changes in farm structure:
Statistical data usually indicate that farm structure changes slowly in most countries,
but this trend is not universal. In Great Britain, for instance (Anon [1977]), some
30 000 smaller farms have disappeared during the past 10 years, while the number of
large farms over 350 acres has increased from 35 000 to 40 000. Although these
larger farms only represent 15% of all farms in Britain they produce more than 50%
of total farm output. In Ireland the total number of farms declined by 44 000, or
13%, in the 20-year period between 1955 and 1975 (Fennel [1968]; Embleton [1977]).
There also medium to larger farms now account for a greater proportion of total
output. In 1966/67 farms of 100 acres and over contributed 33% of total output, by
1974 this had risen to 39%. Over the same period the number of Irish creamery milk
producing farms dropped from 96 000 to 73 000 approximately, while the total
number of dairy cows on the smaller number of farms increased from 655 000 to
1 052 000. This represented an increase of 112% in cows per farm (Kearney [1977]).
These are some of the pointers to the results, both economic and social, of intensification.
Farm structure is also affected by developing new land. Scientific developments have
enabled new areas to be reclaimed, and in a more precise and permanent fashion.
The introduction of powerful bulldozers and excavators has expedited land reclamation and made possible the complete reorganisation of farm layout - field shape,
size, farm roads, fences, water supplies and other services. "
2.2.2 Changes in farming organisations:
Strong national and international farming organisations emerged during this period
and farmers became increasingly involved in agri-business. Farmers in many countries
became not only more market oriented but also more business conscious. Agribusiness itself also changed dramatically over the period. For example, in my
own country the Irish dairy industry underwent major rationalisation. The result
was the amalgamation of 160 co-operative agricultural societies and some 600 branch
creameries into 12 major dairy processing and trading co-operatives. Twenty-five
smaller creameries still process independently but market their products under a
national brand. In effect there have been major developments in the co-operative
approach both nationally and internationally. National and multinational amalgamations and takeovers have meant the annexation of many relatively large undustrial
and business organisations and the complete disappearance of others during this
dynamic period in the international business and financial world.
2.3 Developments in the fertiliser industry
2.3.1 Fertiliser use:
The core of action by the PI is fertiliser use. This involves considerations not only
in overall usage but in methods of manufacture, sources of supply, use efficiency
and other related subjects. Consequently I am focussing special attention on developments in this sphere in this review.
There have been many developments in the fertiliser industry. World consumption of
fertiliser plant nutrients jumped from 17.4 million tonnes in 1953 to an estimated
100 million tonnes in 1977/78 (Figure 2). The steady growth in world fertiliser use
25
10080
70"
6050"
40
30
20
10
1950 3 55
60
65
70
75 78
Fig.2. World fertiliser consumption (million tonnes nutrients N+P 2 0 5 +K 20)
160
1.0
World Summary- Forecast Fertiliser Demand
u 120
Future Demand
Production
.,100 z
60
ActuaL
Consumption
-40
0
0
N P2 qK 2 0
1975
K20
1980
N P
N PAOK 2O
1990
N P2qK 20
2000
Fig.3. World summary forecast of fertiliser demand (UNIDO [1977])
26
over the period was temporarily upset by the abnormal price increases in fertiliser
raw materials following the oil crisis in 1973-74. However, the scare about shortages
of fertiliser materials quickly evaporated as consumption dropped and new production
capacity for nitrogen and phosphate came on-stream.
The United Nations Industrial Development Organisation (UNIDO) (Anon [1976])
has projected a massive growth in fertiliser demand over the last quarter of this
century. This expansion is more or less in line with the growth rate achieved during
the 1953-73 period, but in real terms the quantities of fertilisers involved are predicted
to represent a fourfold increase in nitrogen and a threefold increase in the use of
phosphorus and potassium compared with 1975 fertiliser consumption (Figure 3).
These projections envisage that by the year 2000 nearly 40% of the demand for nitrogen and phosphorus and 30% of the demand for potassium will be in the developing
countries (Stangel [1975]). This compares with less than 9% of total nutrients in
1953, 12% in 1963 and over 24% in 1975. If achieved, this increase in fertiliser use,
coupled with the adoption of improved crop and livestock husbandry practices
and the findings of agricultural research and development, should greatly facilitate
the production of adequate food supplies for the growing world population up to
the year 2000.
2.3.2 Fertilisersupplies:
Have we the capacity to meet nutrient needs in the long term? The position seems
reasonably secure. Discoveries of new deposits and sources of fertiliser raw materials
over the past 25 years have added considerably to known world reserves.
Stangel [1976] reported that even with only a slight improvement in technology,
there should be sufficient phosphate ore for the next 300-400 years. Neither is sulphur
likely to be a limiting factor in the production of fertiliser since pollution control
measures now planned will lead to the recovery of more sulphur than the industry
requires.
Stangel also notes that 80% of presently known recoverable deposits of potash are
located in the USSR and Canada, but recently discovered deposits in China, Thailand, Brazil and Ethiopia have not yet been quantified. However, he estimates that
the known reserves are more than adequate to meet the one thousand million tonnes
K2O required in the period 1980-2000. Another estimate based on projected 1980
consumption levels indicates that known potash reserves would be sufficient to meet
requirements for 1800 years (Stangel [1975]).
Future nitrogen fertiliser supplies are a different matter and are dependent on the
availability of energy sources. Reserves of atmospheric nitrogen are unlimited but
unfortunately, the conversion of elemental nitrogen into usable fertiliser requires a
suitable source of energy. Virtually all world nitrogen fertiliser production is now
based on the Haber process. While natural gas is the major feedstock, naphtha oil
and coal are used to a lesser extent. Stangel discounts concern that current reserves
of natural gas are inadequate to sustain world demand by the turn of the century
in the developed countries. Reserves of natural gas also appear to be ample to meet
regional nitrogen needs in the developing countries in Asia, Latin America and
Africa for the next 25 years. Assumptions, therefore, that a major shift in ammonia
production from natural gas to coal is necessary appear to be unfounded (Stangel
[1976]).
27
Also, greater understanding of the legume nitrogen fixation process and its possible
transfer to non-legumes, and indeed a better understanding of the dynamics of soil
nitrogen, may substantially change the whole future with regard to nitrogenous
fertiliser usage.
2.3.3 Fertiliserefficiency:
There have been major advances in the formulation and conditioning of fertiliser
e.g. increased concentration in the nutrient content of compound fertilisers, new
ways of application in the form of liquids and slurries, and totally new fertilisers
such as those produced by the Tennessee Valley Authority. Side by side with these
developments in fertiliser technology, it has been possible to reach new levels of
precision in fertiliser application, through a better understanding of the nutrient
requirements of plants and of the individual needs of soils under the impact of different
cropping regimes. But the switch from lower grade 'straights' and simple mixed
fertilisers to virtually chemically pure concentrated compounds may be a mixed
blessing. The consequent economy in materials handled, and in packaging and transport must be counterbalanced by the elimination of essential secondary and trace
elements which were supplied by the less concentrated fertilisers.
2.4 The information explosion
The increase in scientific manpower has caused a veritable explosion in the supply
of literature, with many new highly specialised journals appearing. It has also brought
a greater diversity in fields of knowledge. Fortunately, side by side with these developments there has been major progress in the information technology field with the
development of new systems of classification and new techniques of retrieval with
computers now playing a very important part. The extent of expansion is reflected
in the fact that there are now more than one million information specialists working
throughout the world. Compared with the early days of IPI there has then been a
significant increase in activity in the information field which is one of the primary
functional areas of the Institute.
3. How the Institute responded
These are some of the factors responsible for the changing environment in which
the Institute has had to develop and work. How has it responded to the challenge
posed by these changing times?
In promoting its objectives and in meeting new situations and requirements a number
of techniques have been used and actions taken mainly relating to the users to be
serviced. These are now discussed under the headings of congresses and colloquia,
publications, encouraging young research workers, field missions, response to major
issues, working with other agencies and finally the role of the Administrative and
Scientific Boards in promoting and developing these objectives.
3.1 Congresses and colloquia
In the belief that the most effective form of communication is direct contact between
scientists, technologists and advisers, the Institute has, from the beginning, concentrat28
ed on holding congresses and colloqui.. These have been planned in a systematic
fashion. Firstly the subject requiring treatment at a particular time was identified;
this was then broken down into meaningful areas for detailed attention; finally the
best available contributor for each particular subject was chosen. The mechanism of
congresses developed in this way, and, in conjunction with the discussions and personal
contacts, has led to the production of a series of reports, which, from the beginning,
made a significant impact. As the material accumulated, it has proved a valuable
source of reference for people in many walks of scientific and technological life. In
effect a very substantial information bank has been created which like most banks,
grows stronger with age.
In the beginning the congresses were of a relatively general nature and were attended
by people from a wide spectrum of activity in the scientific, technological, agribusiness and agri-industrial world. In time it became evident to the Scientific Board
that a change was necessary. It was felt that a series of colloquia would be desirable,
aimed either at a regional problem or the probing of some emerging aspect of work,
or some new scientific or technological development, in condiserable depth, and
that they should primarily involve scientists, research workers and others actively
engaged in the subject under examination. After thorough examination a decision
was made by the Scientific Board early in 1963 to recommend the undertaking of
these colloquia, to be planned in the context of an overall, broadly based theme
which would be the subject for an ensuing congress, one of which would be held every
four years. Thus, another chapter was written in the systematic evolution of the
Institute, primarily for the purpose of meeting more specialised needs arising from
overall scientific and technological developments. In the intervening time a number
of such colloquia have been held, as can be seen from the attached list and again the
published material represents a key source of reference to-day. The colloquia have
touched in considerable depth on practically every aspect of fertiliser use, in the
process identifying for research workers new areas requiring attention. A significant
feature of these meetings has been the range of subject matter covered and the fact
that while there was in-depth probing, at the same time it was under the umbrella of
the wider spectrum of agricultural science. Perhaps one of the best examples of this
was the colloquium on Potassium in Soil at Landshut, 1972, where the subject,
although highly specialised, was treated in the context of the much broader field of soi
chemistry, soil fertility and land use as a whole. One might say that this kind of
treatment has enabled the people attending these colloquia to see the wood for the
trees. A special feature of the colloquia is that there is a sessional co-ordinating paper,
generally by a member of the Scientific Board, which assesses and appraises the
significance of the contributions and discussions at each session and highlights the
practical significance of the information.
At the first meeting of the Advisory Board, later the Scientific Board, in Zurich, it
was decided, as a matter of principle, that potassium could not be dealt with in a
vacuum as a single nutrient but that its role should be seen against the background
of other nutrients and its effects on soil, plants, animals and indeed human nutrition,
and of its place in production systems as a whole. Even a cursory examination of
the proceedings of the congresses and colloquia, with their very wide range of subjects,
will show how well this objective has been achieved.
As an example, we can take the 9th Congress held at Antibes in 1970. The theme of
this congress was 'The role of fertilisation in the intensification of agricultural pro.
29
duction'. While a number of aspects of this question had been dealt with at previous
meetings, it was considered that the subject should be explored in depth, firstly
because of the paramount role of fertilisers in improving the economy of developing
nations, and secondly because in countries where production was already high it
was necessary to foresee developments clearly. Consequently. as a basis for this
congress an in-depth exploration of particular aspects of the subject was carried
out at the Colloquia held in Finland in 1967 on Forest Fertilisation, in Italy in 1968
on the Fertilisation of Protected Crops, and in Israel in 1969 on the Transition from
Extensive to Intensive Agriculture with Fertilisers. An examination of the papers
presented at these colloquia and the subsequent congress shows clearly the wide
range of subjects dealt with, from production aspects on the one hand to technology
transference and rural social problems on the other. It emerged clearly that in fertiliser
use we must consider not only the physical, chemical and biological factors of production, but economic and social factors as well. If farmers are not motivated through
the actions of such agents as advisers, then the research worker may find his work
unused and his efforts less rewarding. For those concerned with the production,
formulation and sale of fertilisers, the results clearly substantiate the philosophy
of the Institute concerning the processes for the transfer of research results from the
laboratory to the field.
In the process of information recording, retrieval and dissemination through congresses and colloquia people from many disciplines meet for discussions and exchange
of views. Consequently, the results are made available in many countries of the world.
Already some of the more practical results have been integrated into daily usage
by agronomists in their work. While no detailed study has been carried out on the
subject, enquiries have shown that the proceedings of these congresses and colloquia
are being quoted more and more as reference sources in scientific literature and are in
constant use by agronomists. This approach of the Institute is being closely followed
by its offshoot, Potassium Institute Ltd., in the United Kingdom and Ireland, which,
though still in its early years, has already held a number of valuable symposia which
catered for intensive probing of important subjects by research and extension workers.
3.2 Publications
Another important vehicle for dissemination is the Potash Review, published monthly
and widely distributed throughout the world. In the Review, papers and relevant
materials are abstracted from appropriate sources in many countries and made
available through the dissemination channels of the Institute to some 8000 agents
or agencies. Literature on fertiliser use and related activities is dispatched frequently
to research workers, agronomists and others concerned. To date some 600 separate
items have been included in this Review, which, together with some 150 bibliographies, may in all represent 1500 reports. In pinpointing advances and in calling
attention to deficiencies in knowlegde, this method of publication achieves significant
results. In effect Potash Review was a pioneer in the field of current awareness for
scientists and advisers and in procedures of classification and production. Side by
side with this regular system of publication, monographs covering a variety of specialised subjects are produced from time to time. These also constitute a valued reference source. Another important activity is the preparation and issue of the IFC/CIA
30
Review, essentially a press bulletin, which deals with information on fertiliser use
and experimentation statistics, as well as short items of information concerning
the activities of IPI and other relevant organisations.
3.3 Encouraging young research workers
In line with the Institute's objective of encouraging research, special attention has
been focussed on young research workers. In 1961 a competition for research workers
under 40 years of age was initiated, to be implemented through the submission of
scientific papers, theses or other appropriate publications, for assessment by a panel
of experts from the Scientific Board. The competition has attracted wide attention
with 31,33, 22, 16 and 44 candidates for prizes in the years 1963, 1966, 1969, 1972 and
1975 respectively. Two prize were awarded in 1963, three in 1966 and one each in
the other two years, the prizes winners coming from Mexico, United Kingdom,
Belgium, Germany, Poland and Italy. The submissions were generally of high scientific
merit and have contributed substantially to the literature of the chosen subjects.
3.4 Field missions
While congresses and publications have rightly received much attention, the role
of the IPI in sponsoring field missions to a variety of countries has received rather
less emphasis. In practical terms the results of such work are of critical value, providing an opportunity primarily for 'on the spot' study of problems of special importance to under-developed countries, with a consequent injection of technical know-how.
At the meeting of the Scientific Board at Baden in January 1972, an account of
these missions was ably presented by Dr. von Peter. The IPI has sponsored or cosponsored missions to a number of countries and has had staff active for considerable
periods in Brazil, East Africa, Singapore (covering a large part of S.E. Asia and the
Far East), Japan, Korea and Turkey. These missions have stimulated agricultural
research and they work in close cooperation with government institutions.
One could cite a litany of projects and, perhaps more important, a record of acknowledgements and tributes paid to the achievements of these missions, especially those
which have been actively working with agronomists and farmers in the field. In India,
for instance, the IPI programme did much to extend NPK experiments from the
experimental stations to the farmer's field. Indeed it is a tribute to the Institute
that its methods were later applied by FAO in other countries. From these acknowledgements there can be little doubt that the Institute's work has had an important
impact on fertiliser use in a number of densely populated, under-developed areas,
by underpinning new fertiliser application programmes, including of course potassium. In some of these areas where traditionally phosphorus alone was used, the
important role of potassium has been clearly demonstrated, even though there has
rightly been considerable stress on nitrogen as a major growth producer. It may be
emphasised that this corresponds with the experience arising from the colloquia,
especially that in Israel, which stressed the fact that while the 'pull' in soil potassium
by slow-growing low-yield crops could generally be met by normal release from the
31
soil, intensification produced entirely new requirements entailing the addition of
fertiliser potassium not only for growth but also for quality.
It is patently clear that these missions are of very great value, synchronised as they
have been with those of other agencies concerned with similar work.
3.5 Technology transfer
In essence action along the foregoing lines has aimed at achieving the transfer of
knowledge from research institutes of one type or another into practice on the farms.
The ways in which this could be effectively achieved were discussed at the Colloquium
in Israel, while at the meeting in Florence attention was drawn to the role of the
'blueprints' or 'systems' approach in the adoption of research results under intensive
horticultural conditions. More recently at the York meeting, this matter received
special attention, being brought forward by contributions from our United Kingdom
and Nigerian colleagues. It is obvious that in the future this aspect of the work of
the Institute will become increasingly important. Indeed in the past the lack of
transfer of technology or knowledge in agriculture has been often criticised, but in
this sphere it can be justly claimed that the Institute has played a valuable part.
3.6 Response to important global issues
What has been said so far is indicative of success in what may be classed as the more
technical aspects of the work. This is not the only criterion, however. Another is
that of how responsive or reactive the Institute has been to relevant major issues
of the time. This role can perhaps best be assessed by reference to the timeliness
and subject matter of its colloquia in identifying important global issues and responding to them by providing a forum for international discussion on these subjects
and in the publication and dissemination of the results of such expert deliberations.
Let us now look at the action which has been taken on a number of these issues.
3.7 The energy question
During the period under discussion the question of energy supplies became increasingly
important. A primary consideration in this respect is the extent to which, through
the photosynthetic process, solar energy is fixed by plants, and equally the loss which
takes place through photo-respiration and other avenues before final use. In several
of the colloquia and in the Institute's other publications the vital role of potassium,
for instance, in stomatal regulation and in the energy cycle has been discussed, the
existing knowledge brought together, and the need for future research highlighted.
For instance, today, biomass production from trees as an alternative renewable
source of energy is receiving increasing emphasis and here the information on the
use of nutrients in forest tree growth brought together at the Colloquium in Finland
in 1967 has been especially relevant.
32
3.8 Use and conservation of resources
Central to the work of the Institute, and indeed a major reason for its existence,
has been the development and improvement of soils. Much valuable information
has been brought together on the way this renewable resource can be conserved
and further developed. At another level also there has been special emphasis on the
conservation and use of such major food sources as proteins, carbohydrates and
lipids in publications and colloquia, e.g. Denmark in 1975, which dealt with protein
production by different crops, and more recently at York in 1977 in relation to the
production of carbohydrates and lipids.
Another important contribution in this field is that concerned with nutrient conservation through the process of recycling and more efficient use of farm residues.
This is reflected, for instance, in developments involved in transition from extensive
to intensive agriculture, with the latter raising questions concerning the effective
use of slurries and other nutrients in farm residues.
3.9 Environmental protection
The question of environmental protection in fertiliser use has become a major global
issue. Methods of fertiliser use in different terrains, the inherent capacity of soil to
act as a nutrient sink, and the rational use of nutrients in farm systems, have been
the subject of discussion, debate and publications by the Institute. In 1975, the Institute
published jointly with C.E.A. and I.S.M.A. a 'Handbook on Environmental Aspects
of Fertiliser Use'. Earlier at the Colloquium in Florence (1968), the use of nutrients
n relation to the production of decorative and landscaping plants was highlighted.
3.10 Food supply and quality
At a number of points in this paper the concern of the Institute with world food
supply and the actions taken to promote food production in developing countries
has been emphasised.
The more affluent societies are increasingly concerned with the quality of food and
what it means in terms of human well being. Again this has been the subject of
attention at a number of colloquia. For instance at the Colloquium in Sweden in
1971 the question of potassium in the biochemistry and physiology of animals and
man received emphasis. In considering fertiliser use in the production of carbohydrates and lipids at York in 1977 much attention was devoted to human nutrition,
the effect of processing on food nutrients and the role of energy, lipids and phospholipids in this respect. This obviously will be an area for increasing concern and action.
These, then, are some of the major issues to which the Institute has contributed
over the past quarter-century. Perhaps it is not for us to judge the merits of that
contribution. Nor can the work of the Institute be easily measured in conventional
terms. But I believe that our efforts have been worthwhile, both in terms of scientific
achievements and the more immediate and material task of human betterment by
increasing agricultural, horticultural and forest productivity.
33
4. Planning and co-ordination of activities
The continuous progress of the Institute over the past 25 years can be attributed
to a number of factors. These may be categorised as external and internal.
4.1 External
Through the colloquia, congresses, and publications contact has been created throughout the world with many agencies and a level of acceptance and confidence has
been created. In other words a valuable fund of goodwill has been established on
the basis of established performance. By its very nature the Institute has been able
to make valuable contacts with international agencies such as UNESCO, FAO,
OECD and other foundations. Its acceptability as an agency in this respect is due,
I believe, to the fact that its resources are given without strings attached, for the
purpose of activating, stimulating, and generally advancing agriculture. In this
advancement the staff of the Institute have played a major role through attendance
at international meetings and congresses, as observers at many official meetings,
and through active participation in the many activities required in the international
field. The work of the IPI has been facilitated in this respect through the members
of the Scientific Board, many of whom have played such a significant part on the
international scene, both in developed and underdeveloped countries. It would be
invidious to single out personalities in this respect but even a cursory glance at
the membership of the Board since its inception will indicate the level of its contacts
and experience.
4.2 Internal
A major element in the development of any organisation is its structure and the
manner in which the different components work together. In the Institute the three
main components are the Administrative Boards, the Scientific Board and the Directors
and Staff.
4.2.1 Administrative Board:
As mentioned earlier the Institute depends for its finances on a group of companies
engaged in potash production, originally composed of French and German interests
but later joined by companies from other countries. The general government and
administration of the Institute is in the hands of the Administrative Board, on which
these companies are represented. Much has depended on the attitude and approach
of the Board. As seen from my place on the Scientific Board, down the years, this
has, by any standards, been impressive. The finances made available have been
'untied', the basic stipulation being that they should be used in the best possible
way to meet the objectives of the Institute in advancing agriculture. While no other
similar group in the world has spent as much money on research, development and
education, and has worked so closely in affiliation with official research and other
institutions, as this group, on no occasion have I seen any efforts being made to
use the Institute as a sales promotion agency. The Administrative Board has, to my
34
knowledge, on all occasions insisted on a high scientific level, despite, I am sure,
substantial difficulties from time to time in mobilising the necessary financial resources. Its liberal approach has, without doubt, contributed significantly to the
success of the Institute. It has, I believe, been primarily responsible for the fact that
the Scientific Board has been able to attract to its membership scientists and agronomists of high calibre. Indeed it might be said that the activities of the two Boards
have meshed harmoniously and effectively in advancing the work of the Institute.
4.2.2 Scientific Board:
I have already referred at some length to the activities of the Scientific Board in
organising congresses, and colloquia and in the field of international contacts. In a
recent report, the Board is recorded as consisting of 'research workers who are in
the first rank in the domain of agronomy in their home countries and enjoying an
established reputation' and also as making 'authoritative recommendations for
important scientific projects and technical decisions'. Leaving aside the fact that
I have been a member of the Scientific Board from the beginning, I feel it only right
to record that it has met the requirements placed upon it effectively.
Through the congresses and other mechanisms, scientists and research workers from
many different spheres of activity, from universities and research institutes, official
and semi-official bodies and organisations concerned with manufacturing, commercial
and business aspects, have been brought together in a number of countries. While in
this way the activities of the Institute have been brought to the attention of a very
wide user audience, each decision in relation to location, subject matter and attendance has been subjected to penetrating discussion. Indeed an examination of the
deliberations of the Scientific Board would show that they are themselves of considerable scientific merit, reflecting the contemporary scientific milieu at any particular
time. To one who has participated in these meetings this has been an especially
rewarding aspect of working on the Board. Through its action it has been able to
bring together, and place at the disposal of the Institute, people operating in disciplines covering a wide scientific field, from clay mineralogy on the one hand to medical,
social, and behavioural sciences on the other. One is justified, then in regarding
the IPI as a scientific foundation in its role of research stimulation, information
retrieval, dissemination and contact making.
While this is a board of scientists it is one of scientists sensitive to development,
who understand the needs of agriculture and accept the societal responsibility of
scientists. In the complex world of today the capacity to create confidence in science
and what it can accomplish is a very important attribute. The Scientific Board is in
essence a scientific club, where from time to time one is able to meet colleagues
engaged in the same kind of activities in other countries, in this way valuable contacts and indeed many new friendships are established.
4.2.3 Directors and Staff:
From the beginning the Institute has been fortunate in having had at its disposal
Directors and staff dedicated to the purposes for which it was founded. This has
contributed in a major way to its continued progress. They merit our very best
thanks as do also the Secretaries of the Scientific Board who have done so much
over the years to advance the purposes of the Institute.
35
The Institute has achieved a high standing in the world of agricultural research and
development and can take pride in what has been accomplished, especially considering the background against which developments have had to take place. It is not
possible to put a financial value on its achievements - research is by and large impossible to cost-benefit -, but the overall effect has been both creative and beneficial.
5. The future
Specialisation in the various disciplines and fields of research has led to an in-depth
examination of scientific matters previously merely skimmed over. Many new research tools have been developed, while basic research has opened up new avenues
for exploration to applied research workers. These developments have created new
needs and new orientation in the agricultural research field, with the economic,
social and behavioural sciences partnering those concerned with production in
providing for these needs. Experience from programmes of development and extension of research results has provided a new approach to meeting user requirements.
This new orientation received particular focus in a summary of a high-level OECD
meeting which stated that 'agricultural research needs to be seen in a wide and changing context, in relation to the structure of the agricultural industry and to society as
a whole, in relation to human needs for food, employment, and recreation in nature,
and in the whole relation to the environment, as well as in its relationship to science
and technology' (Anon [1972]). In this context soil resources occupy a kernel role.
Resting on past achievements, is not the path to future progress. There are many
new spheres for action and much work to be undertaken. Just as major developments
in the scientific, agricultural, and fertiliser use fields have taken place in the past
25 years so also will there be progress in the future. As yet, only the surface of many
problems has been explored but the future has generally been signposted by developments now in the research pipeline.
Essentially new developments will be conditioned by the capacity of research workers
to explore and then control the biological process that now limit further advances
in crop and livestock production.
With intensified production especially, new problems will, no doubt, arise, relating
to such matters as the cycling and use of nutrients, the nutrition of plants and animals,
the balance and interaction between major and trace elements and the capacity of
soils under intensification to supply the latter. Agronomic problems grow more
complex, leading to even more specialisation, while on the other hand the economic
pressures on farmers to achieve a high degree of efficiency will compel them to use
fertilisers so as to maximise returns. In such a business-oriented agriculture, farmers
will more and more require an integrated system, or 'blueprint' approach to production problems, while on the other hand societal requirements relating to the
conservation of the environment will increasingly emerge.
Projected world population growth from an estimated 4250 million in 1978, of whom
less than half are adequately fed, to 6000 million by the year 2000 demands maximum
efforts in the application of technology to the production, storage, processing and
distribution of food. Not only will there then be more to feed but many more to
feed better as development proceeds. However, we must also take adequate steps
to ensure that the ever increasing use of agro-chemicals and other growth promoters
in crop and animal production does not impair the quality of food for human con36
sumption. The potential of work now under way on gene engineering, in the breeding of crop varieties capable of extracting hitherto unavailable plant nutrients from
soils, and in the transfer of nitrogen fixation properties from legumes to non-legumes
via plasmids and cell fusion must be further explored. The scientific and commercial
development of single cell biological protein production is well advanced.
Developments in biomass production from trees as an alternative energy source
will require the reclamation, planting and cropping of very large areas of hitherto
unproductive marginal land. This will open up a new outlet for fertilisers. The role
of potassium and other nutrients in regulating the energy-fixing potential of the short
term forest species on different soils and in different climatic regions must befullyexplored. Although the UNIDO study Anon [1976] indicates that there are large world reserves of fertiliser raw materials it is only prudent that these resources be utilised
with maximum efficiency and conserved as far as possible through the recycling of
nutrients so that requirements of future generations can be ensured. Protection of
the environment also demands that pollution and eutrophication of water supplies
be prevented through the judicious use of fertilisers to prevent runoff and by the
recycling of nutrients in animal and plant residues. In essence, then, as far as crops
are concerned, we must look to greater photosynthetic efficiency, improved biological
nitrogen fixation, new techniques for genetic improvement, better methods of crop
protection and better methods of reducing losses and new methods of conserving
and utilising waste and nutrients to ensure future progress.
New developments in animal genetics and physiology, both reproductive and growthwise, in neuro-biology and in hormone use will ensure greater efficiency in animal
production. It has been shown, for instance, that the metabolically active hormone
prolactin acts as a growth stimulant, giving up to 15% increase in growth rate for
a 3% increase in feed intake. Controlled environmental work in progress is aimed
at stimulating pituitary gland activity and prolactin production by increasing exposure to light and raising temperature, where necessary, in winter above the critical
minimum level of 4C (Peters et al. [1978]). Work on the use of both hormone
and synthetic growth promoters with cattle in Ireland and elsewhere has shown that
they give similar effects to natural prolactin stimulation by increasing growth rate
and improving feed conversion efficiency (Roche et al. [1978]). Further research
is required to explore the mechanism of action of the various growth promoters.
If it can be shown that their mode of action is through the stimulation of natural
growth hormones such as prolactin this should allay fears of possible deleterious
effects on human health. In this work the role of creative research is paramount.
In future research on potassium Drouineau [1977] has specified the need for both
fundamental and applied work. This is essential. For example, we must further
explore the role of potassium in photosynthesis and the biosynthesis of proteins;
its influence on the efficiency with which plants utilise solar energy, and its effect
on crop and food quality. In the applied field work is required on nitrogen - potassium
interactions in the nutrition of new high yielding crop varieties, and nutrient balance
for optimum production, particularly under intensive cropping systems.
Information retrieval systems must continually be improved to cope with the everincreasing literary output of the worlds' scientists, now estimated at two million
scientific papers per annum or some 6000 articles and reports per day (Anderla
[1973]). This output increased about five fold over the past 25 years and has an
estimated growth rate of about 10% per annum. Consequently the problems of
37
information handling, storage, retrieval, interpretation and use will constitute a
major challenge to an organisation such as the IPI.
Finally in the developed countries as yields begin to plateau, new yield horizons
must emerge. Here basic speculative research in such fields as bio-energetics, bioengineering, neuro-biology, physiology and nutrition will contribute as these new
vistas are revealed. There is also little doubt now that progress in the developing
countries can be short circuited and telescoped timewise to meet their particular
needs. In these new departures the role of research is basic, hence the manifest importance of creating an environment in which such research can be nurtured and
brought to fruition. It is fair to state that the environment created through the
activities of this Institute has been a potent factor in furthering agricultural production and increasing the world's food supply. The responsibility rests on us all to
ensure that such activities never diminish.
Acknowledgements
I wish to acknowledge the assistance of Mr. B.M. Lewis, Dr.E. Culleton and Mr. H.
de Tarragon in preparing this paper.
6. References
Anderla, G.: Information in 1985. OECD, Paris, 1973
Anonymous: Summary report of the second working conference of directors of agricultural
research, OECD, Paris, 1972
Anonymous: World wide study of the fertiliser industry, 1975-2000, United Nations Industrial
Development Organisation, Vienna, 1976
Anonymous: The changing structure of agriculture 1968-1975. HMSO, London, 1977
Drouineau, G.: The tasks of IPI in the future. International Potash Institute, Berne, 1977
Embleton, F.A.: The structure of Irish agriculture, 1960-1975. Statistical and Social Enquiry
Society of Ireland. Unpublished, 1977
Fennell, R.: Structural change in Irish agriculture. Jr. J. agric. Econ. rur. sociol. 1, 171-193
(1968)
Kearney, B.: The structure of dairy farming and its future development. An Foras Taluntais,
Dublin, 1977
Peters, R.B., Chapin, L.T., Tucker, N.A. and Leining, K.B.: Supplemental light stimulates
growth and lactation in cattle. Science 199, 911-912 (1978)
Roche, J. ., Davis, W. and Sherington, J.: Effect of trenbolone acetate and resorcyclic acid
lactone alone or combined on daily liveweight and carcass weight in steers. Ir. J. agric. Res.
17, 7-14 (1978)
Stangel, P.J.: World supplies of fertiliser in relation to need, 1974-1980. American Society of
Agronomy, Madison, 1975
Stangel, P..: World fertiliser reserves in relation to future demand. Workshop, National
Agricultural Library, Beltsville, 1976
Walsh, T.: The effective use of fertilisers under temperate conditions. An Irish case study.
Pontifical Acad. Scientarum Scripta Varia, No. 38, Rome (1973)
38
Appendix
Congresses and Colloquia organized by I.P.I.
Potassium Congresses/Symnposia
1. Potassium in the soil and in living organisms
2. Potassium in the soil
3. Potassium in the plant
4. Potassium and grassland
5. Potassium manuring in relation to problems of water
6. Potassium in the animal organism
7. Potassium manuring under Mediterranean conditions
8. Potassium and the quality of agricultural products
9. Role of fertilisation in the intensification of
agricultural production
10. Potassium research and agricultural production
11. Potassium research - Review and trends
Potassium Colloquia
I. Potassium in relation to grassland production
2. Potassium and the quality of agricultural products
3. Potassium and the quality of agricultural products
4. Potassium and the quality of agricultural products
5. The fertilisation of forest trees
6. The fertilisation of protected crops
7. Transition from extensive to intensive agriculture
with fertilisers
8. Potassium in biochemistry and physiology
9. Potassium in soil
10. Potassium in tropical crops and soils
II. Fertiliser use and protein production
12. Fertiliser use and plant health
13. Fertiliser use and production of carbohydrates and lipids
14. Potassium research - review and trends
Zurich
Rome
London
Vienna
Madrid
Amsterdam
Athens
Brussels
1954
1955
1956
1957
1958
1960
1962
1966
Antibes
Budapest
Bern
1970
1974
1978
Wexford
(Ireland)
Murten
(Switzerland)
Lisbon
Belgrade
Jyvdiskyla
Florence
1963
Tel-Aviv
Uppsala
Landshut
Abidjan
Bornholm
Izmir
York
Bern
1969
1971
1972
1973
1975
1976
1977
1978
1964
1965
1965
1967
1968
39
25th Anniversary of the Scientific Board
11th IPI-Congress held in Bern/Switzerland
from September 4 to 8, 1978
2nd Session
Potassium in the Soil/
Plant Root System
41
Structure and Weathering of Potassium
Containing Minerals
D. Schroeder, Institute for Plant Nutrition and Soil Science, Christian-Albrechts-University,
Kiel/Federal Republic of Germany*
1. Introduction
The soil as an ingredient of the pedosphere is a part of the ecosphere which envelops
the Earth and in which the life of plants, animals and man is based. This ecosphere
is an open and dynamic system in which gains and losses of energy and materials
are in dynamic equilibrium. Potassium in the plant root/soil system can be looked
at in a similar way; it is also part of an open and dynamic system in which gains and
losses are involved in transformation and translocation processes (Schroeder [1974,
1978]).
I shall discuss here the structure and weathering of the K containing minerals,
especially the 'primary' pyrogenic minerals which constitute the source of all other
potassium, whether it be found in high concentration in the potash deposits, in the
water of the sea, rivers, soil or rainfall, in living plants or decaying vegetable matter
or adsorbed on the mineral and organic colloids of the soil.
Potassium, comprising 2.6% of the lithosphere - the Earth's crust - is the seventh
most abundant element and, after iron, calcium and sodium is the fourth most
abundant mineral plant nutrient (Figure I). Through weathering, potassium is set
free to the soil which has a K content varying between <0.1 and >3% K, most
frequently about 1%. These orders of K content result in total K contents in the
soil ranging between <3000 and >I 00 000 kg K/ha in the upper 20 cm of soils
(Schroeder [1976]). Out of this total K content, more than 98% is bound up in
the minerals while less than two per cent is found in organically bound or adsorbed
form or in the soil solution.
Though plants can take up potassium only in the 'available' form, i.e. solution and
adsorbed potassium, the mineral potassium constitutes a very large reserve and,
even though this can be mobilised only to a variable extent, it is of the greatest importance.
Thanks to the great progress which has been made in the field of mineral analysis
(X-ray analysis, electron microscopy, microprobe, infra-red spectroscopy and thermal
* Prof. Dr. D. Schroeder, Institute for Plant Nutrition and Soil Science of the ChristianAlbrechts-University Kiel, Olshausenstr. 40-60, D-23 Kid/Federal Republic of Germany
43
analysis) we are today well informed about the structure and chemistry of the Kcontaining-minerals.
VI.
50
0
44.7
40
30
S,
27
20
to
V~
37
a 2
26
21
1
Plant nutrients
Fig. 1. Average chemical composition of the lithosphere (weight %).
2. Survey of K-containing minerals, mineral structure and bonding strength
2.1 K-containing minerals
Potassium combined in minerals is found predominantly in 'primary' and 'secondary'
crystalline silicates and only to a small extent in non-crystalline (amorphous) or paracrystalline compounds (v. Section 5).
The K-feldspars, at about 16%, compared with the K-micas at about 5.2%, predominate (Ahrens [1965]). Among the micas, biotite (at 3.8%) is more abundant
than muscovite (1.4%). However, in sediments formed by weathering, transport
and deposition, which cover 75% of the world's dry land and of which the majority
of soils consist, this situation is usually reversed because biotite is less stable to
weathering (v. Section 4.4). Secondary K-containing minerals, typified by illite
and the transitional clay minerals, are found in such variable proportions that no
quantitative generalisation can be made. This holds also for the K-containing minerals
in the soil. Their contents vary greatly depending upon parent rock and the conditions
under which the soils have been formed, for example, podsols formed on glacial
sands contain only a small percentage of feldspar, mica and illite; chernozems formed
on young pleistocene loess contain 15% feldspars, 10% micas and 6% illites and
transitional minerals (Bronger, Kalk and Schroeder [1976]).
44
2.2 Mineral structure
The structural units of the crystalline silicates are predonnantly SiO4 ' tetrahedra
octahedra (Figure 2 right). In these
(Figme 2 left) and. in lesser quanittv AI(OH),
(diameter 078 A )* and Al' ions
Ion
tetrahedra and octahedrahethemaill central Si
(diameter 1.14 ) are surroLunded b' large 0- ions** (2(4 A) and O{ ions (2.77 A)
so the centres of the 0 and Oil ions form the poins of the tetrahedron or octahedron
as the case ma' he ( Figurc 2). Thus, In the silicon teirahedra, ,fe ha'e 4-coordination
and. in the Al octahedra, 6-coordination. Si in the tetrahedra can he su bs ituted by
2
in the octahedra b, Nig2 and FC , resulting in stronger negatike
AI , and Al
4.1 and 4.3)
3.1,
(Sections
charges on the tetrahedra and octahedra
O---ions
O-tn
t
t
AA
i,,,2.1O
-otahedra I righ I
Accordin to the kind of boitding betkcen letrahedra among lhenisehes and fitlh
ocuahedra. .arious crl11alline stLIctures Cal be ormed and, anong these, the
only ones of interest to ls are the framework ruictttre of the feldspars (tectosilicates)
and the laser sliructure of micas, illites and Iransitional clai minerals (phslIosilicates)
Figure 3 illustrates the arrangement ot Si-O-tctrahedra in the ideal frarne, ork sirrctelrahedra through 0
ture of quait. Osli ng to the mutuial cross linkage of Si(),
unit is associaled %kith4 half 0 units and the
bridges in rt diensions, each Si
resulting crvsttlline lattice is clectric-ail neutral; the general lormuta is (Si()C).
. 3.11l
I vorn this basic structure, the trucLture of feldpar can be der ed
riI0
In mm.
'm
The designatin'ion is noi quitcl tc
ao1t and Lon ; -. 3)
I \ 2 I0
*
sice
the oxgen ;l ma statetcnmdiite hetsee
45
A
10
9
8
7
6
5
4
3
-0
00
I igurv 4 shosss diagraimaticallv the faver strlCture ol
rica, here still
ithou K ions,
I ach la5r consslts or two sheets of teltahedra eneloping an octahedral shect.
Ihr helts are bonded
atit
0 bridges. The tetralhedra ot cach shCet Ire joined alt
their apices and form a hexagonal or ditigonal arrangemelnt Of 0 ions On the (itCr
sttltCco,. albove ald clow. ilto which K ions can be installed, and adjoining laer,
are linked together throutgh lhesc 1. 4 1 and Ftigtires 8 and I2, The octahedra are
not linked throtghithe apices ht at he edge and are i close packing.
I At
/c.4. \irodcl
46
rutlIgenlcs
4
of r lehetllrA ani Il
ot1 held
inl 111-l
mcl lasller
/
97f
As well as the crystalline silicates there are non- or paracrystalline silicates in which
the units are arranged with little or no order (e.g. allophane, v. 5.1).
2.3 Bonding strength
The bonding within the tetrahedra and octahedra and between them is of a type
intermediate between heteropolar and covalent. For instance, in the Si-O-tetrahedra
the bonds are 50% ionic and 50% covalent (Bach [1972]). When Si in the tetrahedra is replaced by Al the bonds are 63% ionic (Correns [1968]). As the size of
the central ion incieases and charge decreases, bonding strength is reduced, whether
in tetrahedra by replacement of Si4+ by Al 3+ or in octahedra by replacement of
A13+ by Mg 2+ or Fe 2+ (Si-O>AI-O>Mg-O). It is also influenced by other substituents remaining in the framework or layer structure.
The monovalent K ion has a large diameter (2.66 A), the largest of the mineral
nutrients. It is therefore less strongly bound than smaller ions and those of higher
charge. Because of its size, the K ion can be enveloped by 7-12 01- ions in the various
minerals (v. 3.1 and 4.1), so that the strength of each K-O bond is correspondingly
weak (Rich [1968]).
3. Potassium in feldspars
3.1 Chemistry and structure
The feldspars are potassium aluminosilicates, their general formula being KAISi 3O8,
according to which the theoretical K content is 14%. However, in naturally occurring feldspars the Si : A ratio is never exactly 3 :1 and part of the potassium can be
replaced by sodium (and to a lesser degree by calcium) so that the actual K content
3
is always below 14%. The central Si+ ion is replaced by A1 + in one of every four
o
tetrahedra in the lattice: AlSi 3 Og- compared with Si4 O8 in quartz. One K+ ion is
incorporated into the crystal lattice to compensate for the resulting excess negative
charge. Thus the tetrahedra form 4-jointed rings forming zig-zag chains and these
in turn are joined through O-bridges and as a result a framework structure is preserved. This configuration results in large interstices, which are occupied by K ions
in 8-coordination with O-ions. For a survey of the literature of the chemistry and
structure of feldspars see Huang [1977], Radoslovich [1975]. Ribhe [1975], Rasnussen [1972], among others.
The following polymorphs can be distinguished among feldspars of varying detailed
structure and origin:
Orthoclase:
Sanidine:
Microcline
monoclinic-prismatic, in plutonic rocks, normal arrangement of
tetrahedra
monoclinic, less ordered high temperature modification, mainly in
volcanic rocks
triclinic, well ordered low temperature modification, consisting of
magmatite-pegmatite
47
Adularia:
similar to orthoclase, mainly found in lodes and fissures.
All the above four polymorphs have the general formula KAISi3 0 8 . The following
Na-containing form is also found:
Anorthoclase: (K, Na)AISi 3 O8 and furthermore the so-called 'substituted feldspars'
Leucite:
KAISiO 6 cubic, with lower Si and higher K content (up to 18%)
Nepheline:
(Na, K)AISiO4 hexagonal-pyramidal, containing more Na than K.
Plagioclases: (Ca, Na-feldspars) may also contain small amounts of potassium.
Feldspars of the formula KAISi 3O8 have a hardness of 6 on the MOHS scale and the
substituted feldspars a hardness of 5/ to 6.
3.2 Weathering
Chemical weathering of feldspars by which K ions are released is of dominant importance but physical weathering is an important prerequisite as it is through this
that the feldspars are freed from the parent rock and, with decreasing grain size,
their specific surface increases so that chemical weathering can intensify.
Temperature and frost are especially important in physical weathering. Cryoclastic
breakdown, due to ice formation, can produce feldspar particles of fine silt (6.3-2
ztm) or coarse clay (2-0.6 tin) size.
The small particle size, with high specific surface, favours hydrolysis - chemical
reaction with H+ and OH- ions of water-and protolysis (or acidolysis) - attack
by H + ions (or protons) of various acids - (Pedro [1973], Scheffer and Schachtschabel [1976]). The acids concerned are mainly H2 CO3 and several organic acids
formed in the decomposition of organic matter, and also strong inorganic acids like
H2SO 4 and HNO 3 (as a result of the oxidation of reduced S- and N-compounds).
Weathering of feldspars is incongruent (v. Engelhardt [1976]) that is, not all parts
of the mineral are dissolved simultaneously (cf. NaCI or CaCO3 ) but weathering
operates first on the weakly-bound K ions at surface sites and only later attacks the
more stable Al- and Si-tetrahedra.
Figures 5 and 6 clearly illustrate this phenomenon. An aqueous solution of pH 3 was
percolated through finely-ground feldspar and Figure 5 shows the total amounts
S)
40-
'Z360
__r
2200C
o
C0>u
120
40
200
,0
6000
emff
V=O
Filtrate (ml)
Fig.5. Dissolution of K feldspar in aqueous solution, pH 3 (von Engelhardt [1976]).
48
dissolved. Dissolution is rapid initially and then approaches equilibrium. Figure 6
shows the composition of the dissolved material; in the early stages predominantly
potassium is dissolved and, as percolation proceeds, aluminium and silicon follow.
The substances are liberated in the ionic form for they can pass a collodion membrane.
The unweathered portion of the particles becomes enveloped in a shell rich in silicon
and low in K and Al which hinders futher weathering.
4o
0X,
0
EP
/do1000
0
000
8M
6000
11000
Filtrate (ml)
Fig.6. Behaviour of potassium, aluminium and silicon in the dissolution of K feldspar in
aqueous solution at pH 3 (von Engelhardt [ibid.]).
The hydrolysis (without excess H+ ions) can be represented in simplified form as
follows:
KAISi3 O8 + HOH = HAISiJO 8 + K + + OH- (rapid first step)
HAISi 30 + 4HOH = AI(OH) 3 + 3H2 SiO 3 (second very slow reaction)
The addition of free H+ ions leads to increased liberation of K ions and to the rupture
of AI-O bonds so that the liberated Al is then found as AIOH2 groups in 4-coordination:
=Si-O-Al= + HO + H+-KI
=Si-O +
Al-OH, + K +
I
H
The breaking of the strong Si-O-Si bond probably results from the attachment of
OH- ions to Si as Si-OH groups. In this way the covalent double bond is broken
(Bach [1972], Scheffer and Schachtschabel [1976]).
Another possible way in which Si-O and AI-O linkages might be broken is the
chelation (formation of soluble complexes) of Al and Si by phenols, ketones and
aliphatic and aromatic acids formed in the humification of plant material or excreted
by plant roots (Bach [1972], Huang [1977]).
49
Marshall [1977] proposed a joint reaction of H 2 0 and H + ions in the breakdown
of orthoclase according to:
3 KAISi3 O 8 + 12 H 2 0 + 2 H + = KAISi 3 0 6 .A 2O 4 (OH)2 + 2 K + + 6 H 4 SiO 4
Pedro [1973] proposes the following scheme for the weathering of orthoclase to
kaolinite:
2 KAISi3 O8
H1O
AISi 2O(OH) + 2 K + + 2 OH- + 4 H 4SiO 4
The final weathering products of feldspar (aluminium hydroxide, silicic acid, allophane, kaolinite, beidellite and mica-type minerals) are not of immediate concern
within the present terms of reference. The important point is that first of all the K at
exterior sites is rapidly liberated, further potassium will only be released from internal
sites after breaking of the stable AI-O and Si-O bonds which involves destruction
of the Si rich protective shell (v.supra).This is only about 3.10 - mmthick but is most
effective in protecting the feldspar crystal from further weathering (Correns [1968]).
3.3 Factors concerned in weathering and its consequences
The many factors that influence the weathering of feldspars can be divided into two
groups, internal and external. The following are among the internal factors:
a) Regularity of the crystal lattice. Microcline is more stable to weathering than
sanidine and orthoclase,
b) Na content of crystals: Anorthoclase weathers more easily than orthoclase.
c) Si content: Substituted feldspars are less stable than feldspars,
d) Particle size: The smaller the particles the greater is the surface exposed to hydrolysis and acidolysis.
The following are some of the external factors:
a) Temperature: Weathering processes proceed more rapidly at higher temperature,
b) Solution volume: Wet conditions favour weathering,
c) Migration of weathering products, so that disequilibrium is preserved; weathering
is hindered if these products accumulate,
d) The formation of difficulty soluble products of hydrolysis which are precipitated
and shift the equilibrium to the right,
e) pH value: The higher the H+ ion concentration the more intensive protolysis
(and neutralisation of the freed OH ions),
f) The presence of chelating agents and the formation of soluble complexes which
migrate,
g) Intensive K uptake by higher plants, reducing the K concentration of the soil
solution so that K release is favoured.
One more important factor should be mentioned: time. The longer conditions remain
favourable to weathering the greater will be the breakdown and dissolution of feldspar.
The various factors will be met with in all kinds of combinations so that it is easy to
understand the variation in findings quoted in the literature, any given results are
only strictly applicable to the prevailing natural or experimental conditions.
50
A' a generalt-ation it can be said that appreciable amounts or K will [tc liberated from
feldspar on Iv when enat herg is 5Cry in tenI e. notably iIIthe hu id tropic, Pc>/ro
/973
But under such conditions the feldspars are also substantially destrocd
b
Rble
to serve as a source of remsere K. igu e 7
It. in litosols) and are then no more
illustrates trong ksathering of i microcline particle.
ijrticline particle about 100 'm)
7,,
IatosoL Photo Ka1Th.
hoasit
evidcnc of strong dissolution, Nigerian
Litle w eathcrng of Ifldspar ocur, in temperte~a clitates. [ or eampl. in Voting
feldskpars, the feldspar
soils formed fro
ting pleiniocte oesscomtaining 15 20
content of the top soil is a he most only 5", Ibelo% tril of The parent mtlerial.
In a brown earth fron miiddle pleistocenc oess the loss of K Feldspar was ip to N",
arid K iA
/976 - li coarse-gianined glacial sands
Brwier at al. /974 ,$1-ile
part iculail nucirocaIni be gmcacr; howes er. e'cit here, K fcldspar,
the Incafkdo
1977
and 01, (on/,mA
ihlasd relutivsel intense \weathering Ihc-ni
can
ile
identilied feldspar, in the clay fractin of a prdsol ocr I 0X0 yeis old.
1972 wc can conclude that in the tcnlperatC zone the feldspars
\With Rasntwse
lIage but rather intiaccessilble poitassitins reser v.
coIstitUe ia ''
4. PIotassium in micas. illites and transitional cla
4.1 (hemistr
minerals
and structure of K micas
The potasiLimn ciintatinirsg micas arc, like fldspar> pi"aim>mslitlt summo-ilicates.
but their conmpt),aiion is nwire compi\. the arc phNlhivicancs ixd colmai along,
qI
'6th the S
ICrahedra Al ticihedt]
with karing
bbti tiuton ol the cental ins
22 and Figure 4). l the ideal case, in the tetrahedra each 4th Si is replaced [hyAl
and as in the feldspars, the resulting ecess negatixe charge i neutra ised by the env
ot K ion,( K to 4 tetiahedr'lz , These K ions are enclosed mall intterlaye position
H igues N and 12) so that each K ion is surrounded by 12
ions (1. 23) il the
6-ings of the adjoining letrahed-a (6 on each sidet (se also Figure 14)
In the octahedral shen two of the three octahedra are tilled xkitih \* ions (dioctahedral ttica. with one 'eripl, octahedron) or the three oclahedra are filed with
three Nlg- or Ie- ions (tioctahedctl mica, all octahedral posinon, filled). Accorditg to chemical composition, colour and origin, the following mica- Call e distirtgtNshed:
Muscovite:
K 'IA(ASt, tO., (0)i, dioctahed raI, plates, bright coloured, derived
rtont ia gin ait es aid pegnia tes
Ltrhie :
Like niusco tie but iner grain, manly ot metan orphic origin
lliolite :
K(Mg, F-c' t(Si
rjOOHv. triocrahedral, plates. dark in colour:
derived from matgmaites, peginatites and metaiorphi es
Phlog..pir: KMgjAISiI)O>(O,)I , trioctahedral. transparent plates of pne.i
niatolxtic origiht furthermore
LcpitlAil
and
/imrn-,ldit': with additional lithium in the octahedra.
Ihe For111LII ae represent in Ihe 1tA position the interlaxer K. in the 2nd oct ahedial
central ions, in the 3rd tetrahedral central ions and in the 4th and 5th the 0- and
O1I-ons of the elraihedra and ocahedra. The K content of mtlscovite should theoretically be AN". and that of biotite 8.7', II ok.e\er. such conitenls are practically
never fottrd int natural ivicas because the ce,, res of the retrahedra and octahedra are
variously occupied and the average negalie charge per Lnii never reaches one unit
(- I K ) and, also. K in lhe interilaver p)sitions can e sibstit utred by Na or Ca.
( oner sely, the Na Imlica p ' guiw alld Ilhe Ca iica nt tg-i; - In a' a Iso co III ain so ilec
K.
The Wicas are easily issihle and ha -e a relatix cl lox hardness of 2* 3 on tile \Ol-lS
scale.
4.2 Weathering of micas
Physical wealhlming is an iniportant first step as it is witl the weathering of"feldspar.
I he succeeding chemical wearhering is cxeni iore i ncolgmtren t than it is in feldspar,.
Chemical weathcring starts "xiih ie echange of K in inkLrla,.er positions which is
relatively weakly held b If and 11 P (omonium) ions / 23 and 4
[he process
stars fron the edge of the crystal as shown schenaiica ll ii I gure .
The dissoltior, of K kmlis does not proceed on the whole surface. as in feldspar.
but zon1all, atl xchange-fronts which tiertetrate lurther into the paltice as weathering proceeds ton Rt-iacth 1972, 1976 . " his is well seen in f igures 9 and 10.
Figure 9 illustrates a hiotite particle frmu which iinerler potasiurn has been
iemoved elperi iet tall by ion escehange xvih Ba. Severat inrlaer posit ots are
52
alread' reed of potassium and expanded: the wa'es to the right of'the bioite platelet
show ho". far the exchange of potas,urn has procceded in the upper iavers Iigure 10
shows the hreakdow nof a iotite particle b% natural ixeatlhering sho ing separation
into several
Ihooks" of taers.
Plainar surface
Broken edge
0
Si-O tetrahedra
-. a
Al-OH octahedra
Si-O tetrahedra
+
K*- ions
-4
lit,.S Rcleae of K I ol'1
HO-
I,()
iltt
fonl tIItusco- 0 crsa I itike
Iti' !' Scanntling elc2t[ron titero photograph of atbiotile particle after expe:rilentl extratiton
ofiltcrlarer pola,sium i97 R I hl'tib6t/t P76
1"'
S cSarninmg CeC'troiI o.i o-photograph of tle edge ( naturtlly wCathemd biotite
tt
Re
ii +II
]hII
I
1970
I hc Cxpalisrl auted h, remloval of* potasium (ssa\. area in i gure 9) prootoes
ph'-.i>al 'seathcrmg hx rraeturingi leading to the produitittiI
of smaller particle,.
In this manner, t. physical and chemical xiathering. illite cit minerals de%,Jop
from the mi, *ia tihe inlrmediaC stage of hixdroi mica
ifthe mean particle
Si, les, hin 2 cm and further rcduction in patitck site tabhutt 0I in. fbiudl
1976 and withdra,,al ot K leads to the o- called transitional l a\ miner tl and
further to the K free minmcra, mt'ntmorillonit imeclici and %Crmi uliteI igoe II
represent, the prtcess of tranforma), tionl of il'iCt
It cla, millerls %' til
he sCtting
fre oQ potassium in a .chematic fathion
Decrease of particle size and lowering K content
K-content -10%
6-8%
-3%
4-6%
ansitionamilite
Increase of H20, specific surface and exchange capacity
I-*,
/ ile
hi
'sorttiuoi
m9poidtnt
release oh K,
Of Jof
tIa to Ja' i1ninleraits sNnh corr
In addition to [tic lost imiportant w<eathering procsses so Lr described in which
K is releacsd from the inteiter postnions. destnution of tile
ociahedra and tetrahedra bk proklysis is aIso possible; as a result lg-. Ie and Alion s ai fheed from
the crystal lattice and this can bring about intensified K relCease J cVtlhaumn and
e
Shumt
1975 ),
4.3 (henistr
and structure ol" illite and transitional clay minerals
The name illiteill
he Lsed here in Gvil"' Original sense
Io
found in the cla
l
ifraction. The meaning of 'transitional cla
include all clay minerals interniediale bctweecn th
rtinnicace ous minerals
mincrals is taken to
ilitcs and the moitmorillotes
and xCermitliles. for discussion of the )OOitnlattle, origin and lroI)t* ties of this
miscellaneous class of minerals. see ti
tt,
and Kcrmida /977 and o.,Rcichaa
hack and Rich
1975 )
Examples of the chemical coliposition of iite are gi\en in the Ifllowing foImulae
UIt fhiuit and Acranidax 1977 Sc ht.'/ir and , hachtvcha/e/ 1976
Dioahcdral ifet (dti\cd from musco'-i h)
C
- 0.75
0.25
1
0501
,\LISi
, 0 t0ut
f
t
1C)
iK
(
denotes calions w hich. tollo%h g tnh
e iremos alof potassium tron intci Itet 1,,itions.
are2 Olnd at edge sites i1n
excha gCa ble lot t to Pf co. tece lec
t rica I ncu t ra Iit,
The general
trucitre of lile is simila to that ot muscovite (f igurC 12 f I LcUcs 4
adt S butl, st1tbtitution in the tetrahcdri f\ tot Sitt less and, instead (1containing
onlv Al, the octalhedra also conhatn Ic' ati Mg' -I[here thus aircS a ncgatlvc
excess charge (C) of 0.75 per lot lula tntt
/ the formula ftr mls1Oki1C in4.AI
so it order to retloie ilCtlt-alitv. cattiOns eqttltitlf to the charge --075 ij, potasstlhm and exchangeable tons) are hound and adsorbed in the interlaer sitcs. [In
the
example gt'en thCsC compi'e 05M8 pals phtassutn iot vet reToved by wkeathi]hng
pit O5
0I I7 exchangeable oins
7ris l/ctd/al illitt tdcrmed Itot biotitc
C
K
54
- o6
A'
fMg
1.05
019
Ic1
\l,
)t\1,.Si
h
jO (01l)
- n H1,O
-704
N
Si (A)
AI(Mg)
Si(AI)
1
Iit ite
Fig. 12. Schematic illustration of the structure of illite with two layers and interlayer-K
(Scheffer and Schachtschabel [1976]).
Tetrahedral substitution is somewhat higher than in biotite (cf. biotite formula in
3+
3+
4.1); thus the octahedra here contain 0.39 parts Fe and A] in place of divalent
ions, resulting in excess positive charge which leads to a total negative charge on the
tetrahedra and octahedra of -0.66 which is neutralised by 0.45 parts interlayer
potassium (coming from the biotite) plus 0.21 parts exchangeable cations.
Dioctahedral glauconite, of very variable composition, is also a mica-like clay mineral
but should not be regarded as a mica derivative as it originates from marine sediments
3+
and is distinguished from dioctahedral illite in that it has a high Fe content in the
2+
octahedra and besides K+ also, Na+ and Ca ions in the interlayer positions.
As can be seen from Figure 11, transitional clay minerals can be derived from illite
by further weathering and these are variously structured (Figure 13). The transitional
minerals comprise: edge-expanded illite, expanded illite and interstratified minerals
made up in regular or irregular manner by mixed layers of illite, montmorillonite or
vermiculite. The lower their K content and the higher the substitution of the central
ions of the tetrahedra the less useful are they as potassium sources, but they are,
on the other hand, capable of fixing potassium from the soil solution in their interlayer spaces (v. van Diest's paper in this volume).
4.4 Factors influencing weathering and their consequences
As with the feldspars, internal and external influences should be distinguished. The
internal factors are peculiarities of structure (dioctahedral and trioctahedral structure
and particle size) while the most important external factors are the same as in the case
of feldspar (ef van Diest, in this volume) and in addition the kind and concentrations
of ions in the soil solution and the redox potential (see Section 4.4.3).
55
.
0
0
so..
*ld
.
0
o
* 1 lpy.nd..d
0
o
0
nits
and
0+
0
o
epanded ilt
varmlcuIlt. or
f
p
o
O
lt
Oifit
eaisedsayr,
0
o
.. oandd
0
0
n
Iontmolllonite
or
v0rmlullt.
* K+ Ions
0 exchanOilbla ions
Fig. 13. Schematic illustration of the transitional minerals between illite and montmorillonite
or vermiculite.
4.4.1 Dioctahedral and trioctahedral structure
Given equal particle size, the di- or trioctahedral structure of micas and illites determine the ease with which they weather.
The ideal hexagonal arrangement of tetrahedra is not possible for steric reasons
(Figure 14 left, cf. Figure 4) because the tetrahedra cannot exactly match the octahedra (v. ion sizes in 2.2 and Figure 2). In order to form a layer structure with the
octahedra, the tetrahedra must rotate to some extent and thus form a ditrigonal
arrangement (Figure 14 right). Due to this, the 12 coordination of the 02- ions around
the K+ ions (v. 4.1) changes to 2 x 6 coordination through which positions 2, 4
and 6 in Figure 14 (right) cause stronger bonding (because of the shorter K-O distance)
than positions 1,3 and 5. As, in trioctahedral biotite and illite, all the centres of the
octahedra are occupied by divalent ions, and in dioctahedral muscovite and illite
only two out of three centres with trivalent ions (with one empty octahedron), the
configuration of 0 ions in the tetrahedra of dioctahedral minerals is shifted increasingly towards a ditrigonal arrangement on account of the increased asymmetry of
the charges in.the octahedra (more rotation of the tetrahedra) and this results in K
56
Fig. 14. Ideal hexagonal (left) and actual ditrigonal arrangement (right) of tetrahedra in
micas and illites (Rich [1972]).
being more strongly held in the interlayer positions. Thus, muscovite and the dioctahedral clay minerals are more stable to weathering than biotite and its derivatives.
The different structure of di- or trioctahedral minerals also affects the influence of
the octahedral OH ions on the intensity with which K is held between the layers.
Because they are strongly dipolar, the OH ions in trioctahedral minerals (with equal
charge in the surrounding centres) are perpendicular to the basal plane (Figure 15
top left); in the dioctahedral minerals they are, on the other hand, markedly inclined
AR)
--empty
--
(-- --- H -
trioctahedral
dioctahedral
0--
reduced Fe
oxidised Fe
Fig. 15. Effect of composition of octahedral sheet of micas on OH dipole moment; dioctahedral vs. trioctahedral (above) and oxidation of Fe2 + to Fe 3+ (below) (Rich [1972]).
57
to the basal plane as a result of charge asymmetry (Figure 15 top right). Thus the
repulsion of the K + ions in the interlayer by the positive poles of the OH dipoles
is weakened so that the K + ions in dioctahedral micas are more strongly held than
in trioctahedral minerals. Muscovite and its derivatives are for these reasons also
more stable to weathering than biotite and its derivatives.
The difference in stability is well illustrated in Figure 16 which gives the results of
extraction by tetraphenylborate of muscovite and biotite of equal particle size.
Orientation of the OH dipole can also be influenced in trioctahedral minerals by
the oxidation of Fe2 + ions (Figure 15 bottom). When Fe2+ is oxidised to Fe 3 + the OH
dipole is inclined to the basal plane as a result of unequal charge distribution thus
the repelling effect on the interlayer K is less (v. supra).
%K
extracted
100
80
itt
60
4040
-
Muscovite
10- 20 pmn
20
0.01
0.1
Extraction
,,
,
1
10
i
time
100 h
Fig. 16. K extraction by tetraphenylborate from muscovite and biotite of equal particle size
(Scott [1973]).
An increase in positive charge due to oxidation (with consequent reduction in negative
charge and weaker K bonding) could operate counter to this effect. However, it has
several times been confirmed that there is no lowering of the negative charge through
Fe2 + oxidation.
A possible explanation is found in electron transfer from Fe 2+ to OH- with loss of
hydrogen through which there is no alteration in charge though K bonding is strengthened due to elimination of K repulsion. A more likely explanation is the loss of
protons (H + ions) from the OH- ions of the octahedra which would also result in
K being more strongly held without any change in charge (Ross and Rich [1974],
Veith and Jackson [1974]).
The influence of oxidation of octahedral FeZ+ ions has not yet been fully clarified.
It has been established that spontaneous oxidation of Fe2+ follows experimental
58
3
K liberation and that the ratio of adsorbed Fe 2+ and Fe + ions influences the oxidation of lattice Fe2 + (Beyme and von Reichenbach [1977].
The influence of the site of negative charges is also unclear. It would be expected
that, in all micas and their derivatives, increased substitution of central ions in the
tetrahedra would result in K being more strongly held between the layers. However,
this has not been widely confirmed. On the other hand, charge site has an influence
on the fixation of added potassium (van Diest, in this volume). There is no doubt
of the connection between total negative charge and the intensity with which interlayer K is held.
The first two structural aspects (ditrigonal arrangement of tetrahedra and orientation
of OH dipoles) are the main causes of differences in stability to weathering of muscovite and biotite.
4.4.2 Particle size
Dependance of K liberation on particle size is not as clear in the phyllosilicates as
with feldspars. True enough K liberation by weathering is generally greater the smallkr
the particle size but several workers (Fanning and Keramidas [1977], von Reichenbach and Rich [1975]) have contradicted this general view. It has been established
that interlayer K is initially released rapidly from large mica crystals due to the
preferential liberation of K from the interlayers between adjoining bundles of layers
(cf. Figure 10). On the other hand, smaller illite particles initially give up interlayer
K quite readily, though as liberation proceeds the remaining K is held more strongly
and the K in specific interlayer sites is relatively strongly bound. Several explanations
are offered for this behaviour. Very probably, expansion and curvature of the layers
at the edges - by the entry of larger exchangeable ions - (Figure 9 and expanded
illite in Figure 13) with corresponding entrapment of K ions further in is responsible.
It is also possible that protons of the OH dipoles in the expanded zones are directed
towards free sites so that the K ions in the adjoining interlayer are held more strongly
(Norrish [1973] see also 4.4.1).
4.4.3 External factors
In general external factors have effects on weathering of phyllosilicates similar to
those on feldspars but, because the mica-type minerals function as ion exchangers and
some contain Fe2 +, the following factors are also important:
a) Ionic composition of the soil solution:
3+
A low K content and high content of other easily adsorbed ions (mainly A]
under acid conditions and Ca 2 + under near neutral to slightly acid conditions)
favour K release (Laudelout, in this volume). If a critical threshold K concentration
is exceeded and the ratio of K + to other cations is high, not only will K release be
constrained but K may become fixed (van Diest, in this volume). The critical K
concentration is higher for the trioctahedral illites and transitional minerals
than for the dioctahedral clay minerals (Fanning and Keramidas [1977]).
b) Redox potential:
At high redox potentials more Fe2+ in the trioctahedral minerals can be oxidised;
this results, among other things, in the biotites and their derivatives being more
stable at high than at low redox potential (see 4.4.1).
59
4.4.4 Consequences
Resulting from the differing stability to weathering of di- and trioctahedral phyllosilicates, biotite and its derivatives weather more rapidly than muscovite and its
derivatives. Though there is great variation according to location, the content of
dioctahedral phyllosilicates is usually higher in soils than that of the trioctahedral.
Above all, because of the instability of trioctahedral phyllosilicates, the total phyllosilicate content of soils is usually below that of feldspars, though higher mica content
is found in some parent material. In loess soils of the temperate region many investigations (Bronger et al. [1976]) have shown that the phyllosilicates (>2l±m) decay
1.2-1.8 times as fast as the feldspars (comparing the original loess with surface
soil). In strongly weathered tropical soils only the more stable muscovite - if phyllosilicates are present at all - can, after feldspars, play any part as K sources (Pedro
[1973]).
Figure 17 gives an example of the decline in phyllosilicate (>2[im) and feldspar
content in the formation of a recent chernozem from the parent loess. Although the
course of decline in absolute content is parallel, the relative decline in phyllosilicate
content is greater. This tendency was expressed more strongly in some other soils but
interpretation was more difficult because the loess was not vertically homogenous.
Von Reichenbach and Rich [1975] give a comprehensive review of phylbosilicate
content and distribution in various sediments and soils under different climatic
conditions.
0
S
=
A
horion;.'
Et
10'.
80j
S
I0
S
10
25
IS
60'I.
Fig. 17. Mineral distribution in a recent chernozem (Stillfried) developed from young pleistocene loess (Bronger el al. [1974]).
60
5. Allophanes
5.1 Chemistry and structure
Non-crystalline (amorphous) or para-crystalline mineral substances, which arise
predominantly from the weathering of volcanic ash, tuffs, pumice and glasses but
also by the weathering of primary and secondary silicates, are, here, designated as
allophanes. They can also be encountered as stages in the formation of new clay
minerals from the end products of silicate weathering. Because, in the sense of the
present discussion, they are not strictly K-containing minerals (like feldspars, micas,
illites and transitional minerals) but are mainly of interest for the part they play in
the adsorption of potassium and other ions, they will only be treated briefly. The K
content of allophane is very low, only around 0.1% in various representative allophanes (Sticher [1972]).
Summaries of the chemistry and structure are given by Fieldes and Claridge [1975]
and Wada [1977]. Allophanes are hydrous aluminosilicates of varying composition.
They contain discrete Si tetrahedra and Al octahedra with AI-O-Si bonds, but without
any regular arrangement. Their molar ratio SiO 2/AIO 3 lies between I and 2. Allophanes are found as weathering products of porous volcanic rocks (v. supra) mainly
in the fine clay fraction of andosols.
The hnogolites are often classified with allophane, though they have more defined
chain and tube-like paracrystalline structure and a closer SiO/AIO3 ratio (but no
significant K content). The zeolites have a more framework-type structure (but very
irregular and with large cavities containing water). Among the zeolites found in
soils derived from volcanic material only phillipsite contains some potassium as
well as sodium.
Amorphous substances can also occur as end products of silicate weathering and the
destruction of clay ( Tributh [1976]) and these can give rise to crystalline clay minerals,
(e.g. kaolinite and halloysite) by rearrangement of the structural units. Gebhardt
[1976] found in temperate climates these amorphous substances covering surfaces
of clay minerals.
5.2 Potassium relationships
Because the non- or paracrystalline substances mentioned above contain almost no
potassium, they are not of prime importance to the present discussion. However,
they are important in K adsorption since the allophanes have a high cation exchange
capacity (40-100 meq/100 g) and they show high selectivity for potassium ions (as the
succeeding paper will show).
6. Summary
This survey of the K-containing minerals has dealt with their chemistry and structure
and their transformation by weathering.
The widely occurring K feldspars (KAISi 3 O8 ) are framework-structured tectosilicates;
they are generally relatively stable to weathering. In the early stages they easily and
61
rapidly release edge situated K but are then very resistant to further weathering.
Under intensive weathering (in the humid tropics) they are much more rapidly
broken down and constitute a good K source. In the temperate zone they make up a
large but difficultly available K reserve.
The micas are phyllosilicates with layer structure; they are less stable to weathering.
The trioctahedral biotite - K(Mg, Fe2+)3 AlSi3 O0 (OH) - weathers much more rapidly
than the dioctahedral muscovite - KAI 2 AISi 3 O,0 (OH)2 -. The same holds for the diand trioctahedral illites and transitional clay minerals formed in the weathering of
mica, which lead to the K-free clay minerals montmorillonite (smectite) and vermiculite.
The trioctahedral minerals are very rapidly broken down under intensive weathering (in the humid tropics) and then only the dioctahedral muscovite approaches
feldspar as a K source. In the temperate zone the biotites and their derivatives are
the more readily accessible K sources. Because of the difference in stability, the ratio
muscovite to biotite in the soil is the reverse of that in the parent material.
The non-crystalline or paracrystalline allophanes (including imogolite and zeolite)
which mainly arise from porous volcanic material contain little or no potassium. They
are significant not as K sources but because of their significance in K adsorption.
Significant advances in the identification and description of the K-containing minerals
have been made possible by improvements in research methods (X-ray analysis,
electron microscopy, electron microprobe, infra-red spectroscopy and thermal
analysis).
7. References
I. Ahrens, L.H.: Distribution of elements in our planet. McGraw-Hill, 1965
2. Bach, R.: Report on mineralogy of soil potassium. Proc. 9th Coll. Intern. Potash Institute,
67-71 (1972)
3. Beyne, B., und von Reichenbach, H.: Oxidation of structural Fe in an altered biotite
with H2O2 under controlled redox conditions. Proc. 3rd Europ. Clay Conf. in Oslo,
10-21 (1977)
4. Bronger, A. und Kalk, E.: Zur Feldspatverwitterung und ihre Bedeutung fcir die Tonmineralbildung. Z. Pflanzenern. Bodenk. 139, 37-55 (1976)
5. Bronger, A., Kalk, E. and Schroeder, D.: Zur Silikatverwitterung sowie Entstehung und
Umwandlung von Tonmineralen in Lbssbbden. Mitt. Dtsch. Bodenk. Ges. 18, 394-401
(1974)
6. Bronger, A., Kalk, E. and Schroeder, D.: Ober Glimmer- und Feldspatverwitterung
sowie Entstehung und Umwandlung von Tonmineralen in rezenten und fossilen Lbssb6den. Geoderma 16, 21-54 (1976)
7. Correns, C. W.: Einfflhrung in die Mineralogie, Springer-Verlag Berlin-HeidelbergNew York, 56-71, 1968
8. von Engelhardt, W.: Bodenkunde und Mineralogie. In: 100 Jahre Geowissenschaften
in Hohenheim. Ulmer-Verlag, Stuttgart, 29-40, 1976
9. Fanning, D.S. and Keramidas, V.Z.: Micas, in: Dixon, Weed, Kittrick, Milford, White:
Minerals in soil environments. Soil Sci. Soc. Amer., Madison, 195-258, 1977
10. Feigenbaum, S. and Shainberg, I.: Dissolution of illite - a possible mechanism of potassium release. Proc. Soil Sci. Soc. Amer. 39, 985-990 (1975)
II. Fieldes, M. and Claridge, G.G.C.: Allophane: in; Gieseking: Soil components, Vol. 2,
inorganic-components. Springer-Verlag Berlin-Heidelberg-New York, 353-393, 1975
12. Gebhardt, H.: Bildung und Eigenschaften amorpher Tonbestandteile in RBden des
gemassigt-humiden Klimabereiches. Z. Pflanzenern. Bodenk. 139, 73-89 (1976)
13. Huang, P.M.: Feldspars. Olivines, Pyroxenes and Amphiboles; in: Dixon, Weed,
Kittrick, Milford, White: Minerals in soil environments. Soil Sci. Soc. Amer., Madison,
553-602, 1977
62
14. Jasmund, K.: Tonmineralogie. Bericht Ober ein DFG-Schwerpunkt-Programm. Deutsche
Forschungsgemeinschaft, Bad Godesberg, 30, 1976
15. Jensen, W.H. and de Coninck, F.: Transformation of clay minerals during pedogenesis
in glacial- and fluvioglacial deposits, Okstindan, Norway. Proc. 3rd Europ. Clay Conf.
in Oslo, 80-81 (1977)
16. Mackenzie, R.C.: The classification of soil silicates and oxides; in: Gieseking: Soil
components, Vol. 2, inorganic components. Springer-Verlag Berlin - Heidelberg - New
York, 1-25 (1975)
17. Marshall, C.E.: The physical chemistry and mineralogy of soils. John Wiley, New York,
1977
18. Norrish, K.: Factors in the weathering of mica to vermiculite. Proc. Int. Clay Conf. in
Madrid, 417-432 (1973)
19. Pedro, G.: La pIdog6n se sous les tropiques humides et la dynamique du potassium.
Proc. 10th Coll. Intern. Potash Institute, 23-49 (1973)
20. Radoslovich, E. W.: Feldspar minerals; in: Gieseking: Soil components, Vol. 2, inorganic
components. Springer-Verlag Berlin - Heidelberg - New York, 433-448, 1975
21. Rasmussen, K.: Potash in feldspars. Proc. 9th Coll. Intern. Potash Institute, 57-60 (1972)
22. von Reichenbach, H.: Factors of mica transformation. Proc. 9th Coll. Intern. Potash
Institute, 33-42 (1972)
23. von Reichenbach, H.: Neubildung von Tonmineralen in Bdden; in: Jasinund: Tonmineralogie. Bericht 0ber ein DFG-Schwerpunktprogramm, Bad Godesberg, 62-91,
1976
24. von Reichenbach. H. and Rich, C.I.: Fine-grained micas in soil; in: Gieseking: Soil
components. Springer-Verlag Berlin - Heidelberg - New York, 59-95, 1975
25. Ribbe, P.H.: Feldspar mineralogy. Min. Soc. Amer., Short course notes, Vol. 2, Blacksburg, 1975
26. Rich, CI.: Mineralogy of soil potassium; in: Kilmer, Younts, Brady: The role of
potassium in agriculture, Soil Sci. Soc. Amer., Madison, 79-108, 1968
27. Rich, CI.: Potassium in soil minerals. Proc. 9th Coll. Intern. Potash Institute, 15-31
(1972)
28. Ross, G.J. and Rich, C.!.: Effect of oxidation and reduction on potassium exchange of
biotite. Clays and Clay Min. 22, 355-360 (1974)
29. Scheffer, F. and Schachtschabel, 0.: Lehrbuch der Bodenkunde. 9. Auflage, Enke-Verlag
Stuttgart, 12-17, 33-44, 1976
30. Schroeder, D.: Relationships between soil potassium and the potassium nutrition of
plants. Proc. 10th Congr. Intern. Potash Institute, 53-63 (1974)
31. Schroeder, D.: Kalium im Boden und Kalium-Ern~hrung der Pflanze. Kali-Briefe,
Fachgebiet I, Nr. 3, 1-13 (1976)
32. Schroeder, D.: Bodenkunde, 3. Aufi., Hirt-Verlag Kiel, 1978
33. Scott, A. D.: Effect of particle size on interlayer potassium exchange in micas, Transact.
9th Int. Congr. Soil Sci., Adelaide II, 649-660 (1968)
34. Sticher, H.: Potassium in allophane and zeolites. Proc. 9th Coll. Intern. Potash Inst.,
43-51 (1972)
35. Tributh, H.: Die Umwandlung der glimmerartigen Schichtsilikate zu aufweitbaren
Dreischicht-Tonmineralen. Z. Pflanzenern. Bodenk. 139, 7-25 (1976)
36. Veith, J.A. and Jackson, M.L.: Iron oxidation and reduction effects on structural hydroxyl and layer charge in aqueous suspensions of micaceous vermiculites. Clays and Clay
Min. 22, 345-353 (1974)
37. Wada, K.: Allophane and imogolite; in: Dixon, Weed, Kittrick, Milford, White: Minerals
in soil environments. Soil Sci. Soc. Amer., Madison, 603-638, 1977
63
The Physical Chemistry of Equilibria
Involving the Potassium Ion in Soils
Hf.Laudelout, Soil Science Department, University of Louvain-la-Neuve/Belgium*
Various theories have been proposed during the past fifty or sixty years to give a
quantitative description of the ionic exchange reactions in which potassium takes
part. There is a simple reason for the attention given to these reactions: the exchange
of potassium between the surface of the soil colloids and the soil solution controls
concentration in the latter, while the reverse exchange is a preliminary to the incorporation of potassium in the crystalline lattice (retrogradation or fixation) and
contributes to the prevention of loss by leaching. The theories have had both empirical
and fundamental bases. The more fundamental are those founded directly upon
chemical thermodynamics.
Consider the following example of ionic exchange (a very common one in soils)
written in the form of a chemical reaction:
Soil K + Ca 2+ :- Soil Ca + 2K
+
(i)
The symbols K+ and Ca 2+ have the same meaning as in any ordinary electrochemical
reaction, soil K and soil Ca representing amounts of soil providing two equivalents
of exchange capacity whose negative charges are balanced by the ions K + or Ca 2 +.
The expression describes the conservation of electrical charge in the system and the
conservation of matter present before and after the exchange reaction. As the anions
of the salts whose cations are exchanged take no part in the exchange reaction there
is no need to include them in the model even though the systems represented to left
or right of the expression cannot actually exist. Finally the reversibility symbol = is
important, signifying as it does that the distribution of K + in the presence of Cal +
between the soil solution and the soil colloid surface is the same whether the exchange
reaction proceeds from left to right or vice versa. If this is really so then the methods
of chemical thermodynamics may be rigourously applied. In the case of potassium,
the effect of reactions commonly known as 'fixation' is that true reversibility will
not be observed experimentally. Such behaviour is not confined to the exchange of
potassium in soils: the variation in the accessibility of exchange sites according to
the direction of the reaction can cause irreversibility but this is easily eliminated,
the reaction of H+ ions with the crystalline lattice of clays does not rule out the thermodynamic treatment of exchange reactions in which hydronium ions take part. It is
* Prof. H. Laudelout, Dpartement Science du Sol, Universitd de Louvain, Place Croix du
Sud 2, B-1348 Louvain-la-Neuve/Belgium
65
the same for potassium. Since the empirical descriptions of ionic exchange reactions
can be related to the thermodynamic treatment, it is with the latter that we shall
begin.
The thermodynamics of ionic exchange
The thermodynamic study of chemical reactions in general and of ionic exchange is
in point of fact very straightforward. The difficulties and abstractions involved are
only apparent. If it is accepted that, according to the second law of thermodynamics,
it is impossible starting with a system in equilibrium to obtain net work, it is useful
to define a function of the condition of the system (defined by pressure, temperature
and its chemical composition) such that its change is equal to the net work done in
any transformation. Clearly, if the system is in equilibrium such change must be
zero. Perhaps the application of this concept to an extremely simple system will
make this clearer. Imagine a mass m resting on a plane surface. Assuming that the
state of this system can be completely defined by its height h above the plane surface,
we can define a function f= mgh the change of which 8f is equivalent to the net work
done. If for a given displacement 8h the corresponding 8f=O, that is if there is no
change in height above the plane, the mass is in equilibrium. If f is negative, the mass
has fallen and the reverse is the case if f is positive. These excessively simple considerations may allow us to understand that when it is the composition which defines the
state of the system one can use a function which has the same characteristics as f in
exactly the same way: it can only be defined in relation to a given reference level and
its sign and magnitude show whether the system is at equilibrium or approaching
or receding from equilibrium. Such a function is the chemical potential, JL,and this
is defined more simply and less explicity that the function f above. The meaning of
the chemical potential of the Ca2± ion in the earlier example (i) is the increase of the
capacity of the system to furnish work when one Ca2+ ion is added. This extremely
simple definition, and inexact if taken literally, is sufficient to meet the needs of this
discussion. Returning to equation (i) describing ionic exchange between Ca+ and
K + , if a potassium ion passes from the soil solution to the colloid surface, the net
energy change of the system involved in this process is 2lX'K - 2v-"K+ where p.'refers
to the chemical potential in the solution and 1Z to the surface phase, similarly in the
case of calcium the change will be ."Ca2+ - [L'ca2+. From this viewpoint, therefore,
the balance of the exchange (AG) is:
AG = 211'K+-/L'ca2+ +
1"ca2+
- 21z"K+
(ii)
The chemical potential is only defined in comparison with a given reference level.
Put in another way, it is only differences in chemical potential that matter. In just
the same way, in topography, heights are only measurable as differences and their
numerical values can be compared when related to a known reference level. Such a
reference level will be constant within a country, but neighbouring countries may not
use the same reference. Similarly for the two phases (soil solution and colloid surface)
in which potassium may exist it is possible to choose different reference levels for the
expression of its chemical potential.
66
As the choice of reference level is arbitrary it can be dictated by convenience only,
one may use for instance the same reference for the two phases: colloid surface and
solution, whatever it is.
It is useful at this stage to bring in the definition of a new function, that of 'activity'
which is derived from the chemical potential. If the index (o)
is assigned to the chemical
potential corresponding to the state of the system chosen as reference the activity
is defined thus:
(a)
tLK+ = [LK+ + RTIn aK+
where R is the gas constant and T the absolute temperature. The reason for choosing
this relation to define this new function is that it replaces a difference in values of a
function by a ratio as a result of its logarithmic nature. By introducing the concept
of activity into the expression of AG above (ii) it becomes:
2
(0)
(a)
AG
= 2[L'K+ - 2p"K+ +
(0)
(o)
V"Ca2+ - J'Ca2+
+ RTIn
a' K+
a"Ca2+
a',2K+
a'ca+(iii)
The first four terms can be grouped as one expression AG(0). Then, at equilibrium
we shall have:
AG = 0
and consequently
2
AG(0) = -RTIn
(iv)
2
a" K+. a Ca2+
If identical reference levels have been chosen to describe the chemical potentials in
the soil solution and at the colloid surface we would have:
(a)
(0)
t'K+ = ["K+
(0)
to)
and V'ca2+ = 11Ca2+
and consequently:
AGo = 0
2
and a'K+
2
" a Ca + =
2 +
a' K
a'ca2+
Here we must introduce a third definition, that of the activity coefficient which is
such that:
fK+ "CK+ = aK+
where CK is the concentration of potassium whose numerical value will depend upon
the units in which the concentration is expressed. The choice of unit is quite arbitrary.
67
Now, taking the symbol y for activity coefficient in solution and f for activity coefficient in the adsorbed state:
2
2
2
2
YK C' K+ - fCa C'ca2+ = fK C' K+ YC'Ca2+
(v)
The choice of the standard state which defines the level to which change in chemical
potential is related or with reference to which the activities are measured can be made
so that in dilute solution we have:
YK+ = YCaZ+ = 1
Then, we have:
C'K+
a"K+
Va-C2± VCca2+
which is the so-called 'ratio law'.
+
2
If the soil solution is assumed to contain only salts of the cations Ca + and K and
concentrations are expressed in milli-equivalents per litre, if C. is the value for this
concentration, we shall then have:
- (1-
C'ca2+ =
2
C'K+ = Co a
+
where a is the proportion of the K ion in the soil solution. Similarly, if P is the
proportion of the K+ ion adsorbed on the surface of the colloids which have an
adsorption capacity of B mE per g.
C'ca2+
=
BB
- (I-p) and C'K±
_2
p B
Then the equilibrium condition becomes:
C.
C0
at2
__
B
I- a
.
I
-3 3
2
f2
fK
(vi)
fca
Conclusions can be drawn from this relationship about variation of the proportion
of K+ in the soil solution with its total salinity or with the cation exchange capacity.
The relationship has important practical consequences for plant nutrition. Unfortunately they are not quantitative for reasons which are all too evident: they only
indicate that the activity coefficient ratios of the adsorbed cations are effectively
constant. The treatment of results on the basis which has just been described is
usually presented as a consequence of the Donnan equilibrium but we have seen
that it is just a consequence of the thermodynamic treatment of ionic exchange
choosing identical references for the chemical potentials of adsorbed and solution
ions, the consequence of this choice being that the value of AGo is always zero whatever may be the ionic exchange under consideration.
However, it is of use in accounting for some well known experimental facts such as,
68
for example, that the Mg± ion displaces reaction (i) less towards the right than does
Ca 2+ or, in other words, that Ca 2+ has a higher affinity for the soil colloids.
The quantitative expression of this selectivity is quite possible in the Donnan equilibrium
2
because the ratios fCa/f 2 K and fM.f K are not identical. It is, however, more convenient
to dispose of a scale in the measurement of the substitution capacities of the cations,
one pair in relation to the others, just as in the study of oxidation-reduction reactions
the redox couples are classed in such a way that it is possible to predict in which
direction the reaction will go when two redox systems interact. It suffices to choose
different reference levels for the surface and the soil solution such that the balance,
AG(o) is not always zero. We shall then have for the exchange under consideration
a characteristic value as, for example, for the following exchange reaction:
Clay - K + Ca 2+ ± Clay Ca + 2K +
AG(o) = I kcal per equivalent
This arrangement permits the use of an inherent property of thermodynamic functions of the type AG(o) which has just been defined: the change involved in the
passage from state A to state B is dependent of the path by which it proceeds.
(o
Suppose we are faced with the problem of the experimental determination of AG )
2
for the exchange between Mg + and K+ and that for one reason or another this
measurement is difficult or impossible. The alternative is to proceed in two stages:
2
first the replacement of K in the soil by the Ca 2+ ion, then the replacement of Ca
+
2+
Mg ion. The net result of both operations being the exchange of K by
by the
Mg 2+ as shown in the following equations:
Soil K + Ca 2+ =- Soil Ca + 2K +
Soil Ca + Mg 2+ =- Soil Mg + Ca 2+
Soil K + Mg2+ - Soil Mg + 2K +
(I)
(2)
(3)
If AG(o) is known for reactions (1) and (2) it can be calculated for reaction (3).
It would be the same if we had measurements of the heats of exchange for reactions
(I) and (2); this would suffice to obtain that for reaction (3). This demonstrates the
predictive value of the thermodynamic method and of a simple method for checking
experimental findings. Clearly, if the heats of exchange are measured for all three
reactions, the values observed should be such that the sum of the first two is equal
to the third.
We have not yet considered how to obtain the value of AG(o) when it has been decided
not to choose the same reference levels for adsorbed and solution ions. It is a condition of the thermodynamic equilibrium that the chemical potentials of the cations
must be the same in solution and in the adsorbed states, but, if they are measured in
relation to different reference levels, the activities are no longer identical. The condition of the equilibrium
AG()
=
2
+
a xK
. a Ca ±
aK+-aCaz+
In
-RT
a K
+
•a'ca2+
can be written:
69
AG(o) = RTIn K, - RTIn
Y2K+
fca2+
2
YCa + f 2 K+
where K. is a selectivity coefficient defined by
2
C' K++ C'ca2+
C'K
C'ca2+
and y and f are the activity coefficients in the solution and adsorbed phases respectively.
There is nothing in the above treatment to dictate the units in which the concentrations
should be expressed; the only guide is experimental convenience. The selectivity
coefficient is an experimental value and if, at a given value of the ratio C'2K+ / C'ca2+
in the solution with which the soil is in equilibrium, the value of the ratio
C'2K+/C'caz+ for the adsorbed cations is measured, it is possible to calculate the
selectivity coefficient.
When we have a range of values for the selectivity coefficient covering the possible
composition of the exchange complex, we can calculate AG(O). An argument that
we shall not reproduce here shows that AG) is substantially equal to the mean value
of the selectivity coefficient for all possible values for composition of the exchange
complex.
Some might raise the objection that, if the result of applying the principles discussed
is to summarize a number of experimental values in a single parameter, too much
condensation of information is involved.
In fact, it is interesting to have a method of calculating how potassium will be distributed between the solution and the colloid surface. This distribution depends upon
the nature of the cations present, their relative concentrations, the concentrations
and types of salts and the electrical properties of the surface of the soil colloids.
Without any doubt the thermodynamic treatment of experimental results is of practical
value but it has not been widely used by the majority of authors who have been
concerned with the study of ionic exchange of potassium during recent years. Such
research has resulted in the formulation of a selectivity coefficient which is sufficiently
constant to make possible the calculation of the proportion of potassium adsorbed.
One of these is very well known and is much used though it dates from nearly half a
century ago. In place of the selectivity coefficient described above, the following
expression can be used:
2
K G =
Ke
X C'ca2+
If the concentrations are expressed in milliequivalents per litre this takes the name
'Gapon constant' whose value is independent of the units chosen to express the
quantities of adsorbed ions. It is directly related to the thermodynamic constant
since it can be shown that its logarithm over the whole range of possible values for
70
the proportion of potassium adsorbed is equal to half the value of the logarithm of
the thermodynamic exchange constant.
Actually, it is impossible that KG should be constant, its usefulness lies in that it
scarcely varies over a relatively large range of composition of adsorbed cations.
Attempts have been made to use deviations of the Gapon constant to characterise
exchange sites in the soil which fix potassium with varying strength. Other researches
on 'constants' have used the selectivity coefficient as expressed in the equation above.
Whatever units are chosen to express the concentrations of the two cations involved
in the exchange reaction, the equilibrium condition remains the same though the
numerical values of concentrations, of activity coefficients and expressions derived
therefrom will alter. Such 'constants' are those of Vanselow or Kerr and they are
now of historical interest only.
The search for more or less empirical constants for calculating the degree of adsorption of potassium has gone on for a long time because of the difficulty of taking
into consideration at the same time all the factors which determine this degree.
In parallel with this type of research has been the elaboration of a model of ionic
exchange and its adaptation to the study of problems in soil science; this is the diffuse
double layer theory. The bases of this theory have been known since the beginning
of the century, the theory has been developed and much used in the last forty years
and still has its supporters
2
In fact, it presents a very much simplified picture of ionic exchange between Ca ±
and K+: the colloid surface carries a uniformly distributed electrostatic charge and
is surrounded by an array of cations arranged in accordance with the spatial charge
distribution. The monovalent and divalent cations are distributed in this array in a
manner depending upon their proportions in the solution and the electrical properties
of the colloid surface. In the main these are the factors listed above but with considerable simplification: the only property of the cations which is taken into consideration
is their charge, and the distribution of mono- and divalent cations follows the basic
electrostatic laws and the distribution law of Maxwell-Boltzmann. Obviously, this
description is oversimplified and one would not expect that the double layer theory
would give meaningful results as far as concerns equilibria between K+ and the
divalent cations. In fact the theory is not confirmed directly by experimental results
on ionic exchange.
The logic of these conclusions has been confirmed experimentally. If we calculate
the value of the thermodynamic equilibrium constant predicted by this theory for
mono- and divalent cations, the value obtained will be applicable to point charges
and not to cations of finite dimensions which are affected by other than electrostatic forces. One can attain this value experimentally by extrapolating the values
of exchange constants for real cations to a zero value for their respective volumes.
Agreement between the limiting experimental value and theory is as good as might be
expected. It can be said that the diffuse double layer theory has very limited application in the case of our problem. This observation is only an apparent contradiction with the successful application of this theory in other fields of soil science as,
for example, the negative adsorption of anions: evidently in the latter case, the
restriction imposed on the theory by assuming point charges loses much of its weight.
This review of the different approaches used in the study of ionic exchange equilibria
would not be complete without mention of the technique of modelling equilibrium
71
reactions and the kinetic processes in which the potassium ion in soils may be involved. It must be pointed out that it is difficult to take into account all the reactions
implicating the cations which take part in an exchange reaction with potassium
even in the simplest system, let alone the clay-water-electrolyte system. This is
especially the case when ion pairs of the type CaCI+ or CaSO 4 + are formed and of
which account should be taken in calculating exchange equilibria and correcting
activities. The position in the soil is obviously even more complex. Allowance must
be made for precipitation of calcium in the form of carbonate and sulphate as, once
it is precipitated it takes no more part in the exchange with potassium. Again, ionic
equilibria between cations of different valency are much influenced by the concentration and composition of the soil solution. Finally, equilibria are modified by movements of the soil solution and they are not established strictly instantaneously.
Certainly, these are not all unknowns but they can only be taken into account individually.
It is now possible to calculate ionic concentrations in the soil solution which satisfy
many of the equilibrium conditions, and their parameters can be measured. The next
ten years will be marked by further progress in research in this field.
References
The following paper deals with the general background of problems of ionic exchange of
potassium
Thomas, G. W.: Historical Developments in Soil Chemistry: Ion Exchange. Soil Sci. Soc. Am.
J.41, 230-238 (1977)
The following group of papers deals with the application of various empirical or theoretical
approaches to potassium exchange equilibria
Schalscha, E.B., Pratt, P.F. and Deandrade, L.: Potassium-calcium exchange equilibria in
volcanic-ash soils. Soil Sci. Soc. Am. Proc. 39, 1069-1072 (1975)
Bolt, G.H., Sumner, M.E. and Kamphorst, A.: A study of the equilibria between three categories of potassium in an illitic soil. Soil Sci. Soc. Am. Proc. 27, 294-299 (1963)
Carson, C.D. and Dixon, J.B.: Potassium selectivity in certain montmorillonite soil clays.
Soil Sci. Soc. Am. Proc. 36, 838-843 (1972)
Rich, C.I. and Black, W.R.: Potassium exchange as effected by cation size, pH, and mineral
structure. Soil Sci. 97, 384-390 (1967)
Beckett, P.: Potassium-calcium exchange equilibrium in soils. Specific adsorption sites for
potassium. Soil Sci. 97, 376-383 (1964)
Pasricha, N.S. and Ponnamberuma, F.N.: Ionic equilibria in flooded saline, alkali soils. The
K+- (Ca+ + and Mg--) exchange equilibria. Soil Sci. 122, 315-320 (1976)
Woodruff, S.M.: Energy of replacement of calcium by potassium in soils. Soil Sci. Soc. Am.
Proc. 19, 167-171 (1955)
Munns, D. N.: Heterovalent cation exchange equilibria in soils with variable and heterogeneous
charge. Soil Sci. Soc. Am. J. 40, 841-845 (1976)
For thermodynamic theory developed for clays see:
Gaines, G.L. and Thomas, H.C.: Adsorption studies on clay minerals. 11. J. Chem. Phys. 21,
714-718 (1963)
On the irreversibility of some ion exchange reactions, see:
Van Bladel and Laudelout, H.: Apparent irreversibility of ion-exchange reactions in clay
suspension. Soil Sci. 104, 134-137 (1967)
72
For the thermodynamics of ionic exchange reactions, see:
Laudelout, H., Van Bladel and Robeyns, J.: Hydration of cations adsorbed on a clay surface
from the effect of water activity on ion exchange selectivity. Soil Sci. Soc. Am. Proc. 36,
30-34 (1972)
For comparison of predictions of the diffuse double layer theory with thermodynamic methods
for calculation of the exchange constant see:
Laudeloul, H., van Bladel, R., Bolt, G. H. and Page, A. L.: Thermodynamics of heterovalent
cation exchange reactions in a montmorillonite clay. Trans. Faraday Soc. 64, 1977-1988
(1968)
Van Bladel, R., Gaviria, G. and Laudeloul, H.: A comparison of the thermodynamic, double
layer theory and empirical studies of the Na-Ca exchange equilibria in clay water systems.
Proc. 1972 Int. Clay Conference, 385-398 (1972)
73
Factors Affecting the Availability of
Potassium in Soils
A. van Diest, Department of Soil Science and Plant Nutrition, Agricultural University,
Wageningen/The Netherlands
*
1. Introduction
1.1 Early history
There have been important changes in the first twenty-five years of the existence of
the Scientific Board of the InternationalPotash Institute in ideas on the mechanisms
of nutrient uptake from the soil.
Interest in these mechanisms developed about a century ago. The earliest workers
thought that the total quantity of a nutrient in the soil was all important but, during
the last two decades of the nineteenth century, the thought gained ground that a
part of the nutrient which was easily extractable would better indicate true availability to the plant. The extracting agent used was intended to simulate the extracting
power of an absorbing root.
It was known that roots excrete carbon dioxide, thus the idea developed that carbonic
acid would solubilise that part of the soil minerals most exposed to its action. This
concept, published by Czapek in 1896, must now be looked upon as an important
breakthrough in agronomic thinking. It led to the use of CO2 -enriched water as an
extractant of plant-available nutrients. At the same time, Dyer [1894] advanced the
thought that the acidity of root sap was responsible for dissolving certain fractions
of nutrients in the soil and he introduced the 1% citric acid reagent that has been
much used in soil testing, even up to the present day.
The usefulness of these weak-acid extracting agents was largely responsible for the
firm establishment of the idea that plant roots acidify their soil environment. In
retrospect, it is remarkable that soil scientists remained unaware of the fact that, in
nutrient solution culture, many plants cause the pH of the medium to rise, but it
must be admitted that the pH-raising effect is most easily discernible when dilute
nutrient solutions are used and that early workers with nutrient cultures often used
rather concentrated solutions. Also, during the first half of the 20th century, soil
* Prof. Dr. Ir. A. van Diest, Dept. Soil Science and Plant Nutrition, Agricultural University,
de Dreyen 3, Wageningen/The Netherlands
75
scientists did not seem to pay much attention to the work of plant physiologists,
while the latter showed little inclination to delve deeply into processes taking place
outside the root.
In 1939, Jenny and Overstreet published a paper that gave additional support to the
thought that plant roots acidify the soil. They advanced the concept of contact
exchange which had been suggested earlier by Devaux [1916] and by Comber [1922].
These authors stated that H+ ions on the root surface could be directly exchanged
for nutrient cations on the negatively charged clay surfaces. They saw no need for
the involvement of the soil solution as an intermediary between plant roots and
soil colloidal material.
Although some contemporaries disagreed with the views advanced by Jenny and
Overstreet, there was still a general consensus of opinion that root action was responsible for solubilising nutrients in the immediate root environment, thus making
them available for uptake. As yet, little thought was given to the influence that the
mobility of a nutrient in the soil might have on its availability to plants.
1.2 Mobility
In 1954, Bray published a thought-provoking article, in which he stated that 'the
mobility of nutrients in soils is one of the most important single factors in soil fertility
relationships'. Bray recognised that 'regardless of whether the root and clay surfaces are so close that contact exchange, as postulated by Jenny and Overstreet, can
take place, the amount obtained from the immediate contact would be small and
insufficient to make the root functional. The significant source of nutrients to the root
surface comes from movement of diffusion into the film of water between the root
surface and the soil surface'.
Bray realised that the primary functioning of an active root is not to be sought in
any solubilising action but in the fact that, by absorbing nutrients, the root lowers
the nutrient concentration in its immediate environment to such an extent that a
concentration gradient is created in the soil solution surrounding the root, causing
nutrients to diffuse towards the root. Such diffusion, in turn, will lead to the solubilisation of soil minerals. Bray felt that when, as in the case of phosphate, such soil
minerals were sparsely soluble, the main factor limiting availability would be their
rate of dissolution rather than the rate of movement of phosphate ions through the
soil solution. However, Fried and Shapiro [1956] published data on phosphate
patterns in various soils from which it could be calculated that the quantities of
phosphate that can be removed from soils by frequent periodic leaching with water
are many times larger than the quantities of phosphate actually removed by plants.
Thus it became evident that, in general, physical and chemical factors inhibiting
rapid movement of a sparsely soluble nutrient to an absorbing root must be held
responsible for limiting supply rather.than the potential of a soil to supply the nutrient.
1.3 Interaction with other ions
Most of the work on nutrient availability done during the first half of the twentieth
century concerned single nutrients only or, in a few cases, just nutrient cations or
76
nutrient anions. Little attention was paid to possible interaction between cations and
anions which could affect the availability of nutrients to plants. A review article by
Walker [1960j drew attention to the fact that many plants do not absorb equivalent
quantities of nutrient cations and anions. He pointed out that, generally, when plants
are grown on a medium containing available N mainly as NO,-, they absorb more
nutrient anions than cations. To prevent the development of a difference of potential
between the root and the soil, the plant is likely to make adjustments, possibly by
excreting HCO,- or OH- ions, or by taking up H+ ions. Walker emphasised that,
as long as cation uptake does not exceed anion uptake, there is little need to postulate
a theory of contact exchange for cation uptake.
1.4 Mass flow and diffusion
In 1962, Barber proposed an important amendment of Bray's concept by advancing
the thought that two processes are concerned in the movement of ions through the
soil, namely mass flow of the soil solution to the root induced by transpiration, and
diffusion of ions along a concentration gradient due to lowering of their concentrations
as a result of uptake at the root surface.
Mass flow is caused by water moving through the soil to the root in response to the
plant's demand for water. This soil water is a more or less dilute solution containing
a number of chemical elements, some essential to the growth of plants and others
not, present either in ionic or non-ionic form, as, for instance, soluble organic compounds. The quantity of such a solute arriving at the root surface depends on its
concentration in the soil solution and on the volume of water withdrawn by the root.
Both entities, viz. concentration of a nutrient in the soil solution and total volume
of water absorbed by a plant from its root medium can be measured.
It is also possible to determine the total quantity of a nutrient which is taken up by
a plant during its growth period, so it is possible to calculate whether or not the
quantity of nutrient arriving at the root by mass flow matches nutrient uptake. When
the solution moving to the root contains a relatively high concentration of an ion,
mass flow may bring more of that ion to the root surface than the root absorbs.
Consequently, the ion will accumulate in the soil near the root surface. The increased
concentration of the ion sets up a concentration gradient along which the ion will
diffuse in a direction counter to that of mass flow. If the concentration of the ion in
the soil solution is low, the quantity arriving at the root by mass flow may well be
insufficient to meet the plant's needs. Uptake by the root will then lower its concentration in the soil solution so that a concentration gradient is established along which
the ion will diffuse towards the root so that the latter obtains nutrient both through
mass flow and diffusion.
Barber listed the factors which together determine the extent of the concentration
gradient of an ion in the soil solution. These factors are:
a)
b)
c)
d)
e)
Initial concentration of the ion in the soil solution.
Rate of uptake of the ion per unit root surface.
Rate of diffusion of the ion to the root surface.
Rate of movement of the ion to the root by mass flow.
Rate of diffusion of the ion along the surface of soil particles.
77
f) Rate of replenishment of the ion from solid constituents of the soil containing
the nutrient under consideration.
g) The capacity of the soil to replenish.
1.5 Capacity and intensity
The concepts of 'capacity' and 'intensity' are implicit in Barber's thinking. Capacity
measures the ability of a soil to maintain a steady supply of a particular nutrient
from the solid to the liquid phase of the soil. It is also called 'buffering capacity'.
The total quantity of a nutrient that can be drawn from the solid phase into the soil
solution when this is continually depleted is called the 'labile pool' of that nutrient.
The capacity factor is more or less identical to Barber's factor (g), which is one of
the factors determining the extent of the concentration gradient of an ion in the soil
solution. The size of this 'capacity factor' or 'buffering capacity' is an important
determinant of nutrient availability. A fuller description of nutrient availability
demands knowledge of the concentration of the nutrient in the soil solution which is
in equilibrium with the solid phase of the soil. The concentration of the nutrient in the
equilibrium soil solution or the quantity that can be drawn into solution by extraction
with water or with a dilute solution of a neutral salt represents the so-called 'intensity
factor', which can be identified with factor (a) in Barber's list.
Knowledge of both 'capacity' and 'intensity' provides a foundation for defining
nutrient availability in soil. Both can be determined easily in the laboratory, even
on a routine basis. Nevertheless, it must be kept in mind that the factors (b) to (f)
listed by Barber will supply additional information on rates of movement of nutrients
in soils towards the root surface and, thus, on availability. These factors are not as
easily determined in the laboratory as are capacity and intensity, but knowledge of
them is needed for a complete description of availability.
In the following, we shall be concerned with the influence of some soil constituents
and some soil characteristics which determine the availability to plants of soil potassium and the fate of potassium applied as fertiliser will be included in the discussion
as appropriate.
2. Soil minerals
The behaviour of potassium in soil can be simply described in summary fashion by
the following scheme:
K non-exchangeable - K exchangeable - K in soil solution - K in plant. (I)
The scheme indicates that the reactions between the solution and solid phases are
reversible, suggesting that the soil minerals can function as both sources of and sinks
for K. This simple model is not concerned with the nature of the minerals which can
retain K in exchangeable or non-exchangeable positions. The discussion which follows
will make it clear that the conditions obtaining in a soil at any time will determine
whether either primary or secondary minerals will release or entrap K, thus regulating
the availability of soil- or fertiliser-K to plants.
The capacity of a soil to supply potassium to crops over an extended period of time
is fundamentally dependent upon:
78
a)
b)
c)
d)
The
The
The
The
K content of the primary minerals.
rate of release of K by the primary minerals.
quantity of clay (secondary) minerals present.
type of clay minerals.
The preceding paper (Schroeder [1979]) discusses fully the structure of primary and
clay minerals and the chemistry of weathering.
2.1 Primary minerals
Most of the potassium contained in primary minerals is found in feldspars and micas.
Orthoclase and microcline usually dominate among the feldspars and the most
important micas are biotite and muscovite. Temperate soils usually contain both
types but the micas are the main source of K supply to plants as shown in Table I
giving the results of an early experiment by Plumner [1918]. He ground each mineral
to a fine powder screened through the finest grade of bolting cloth. The K in freshly
ground micaceous material was comparatively readily available to plants. CO, enriched water was much more effective than plain water in releasing K from primary
minerals and in predicting its availability.
Table 1. The availability of K in freshly ground K-bearing minerals
Source of K
K2SO ...................
Biotite ..................
Muscovite ..............
Orthoclase ..............
M icrocline ..............
K removed by oats,
mg per pot
253
202
177
62
13
mg K removed by 5 extractions*
with
4.37
4.02
3.39
3.00
H20
with CO-enriched H2O
43.4
28.1
15.6
10.2
* 30 g of material in 200 ml total extractant, shaken for 96 hours (J.K.Plumner [1918])
It should not be inferred from Plunner's results that K is only released when the
minerals are finely ground. In fact release rates are often slower from finely ground
than from coarsely ground materials, as von Reichenbach and Rich [1969] found.
The amount of K removed from muscovite by Ba 2 + exchange was larger for the
20-5 [ fraction than for the finer particles. Mortland and Lawton [1961] found the
same in NaCI extraction of biotite. The bending of mica layers is thought to promote
the release of K and bending appears to be more pronounced in coarser particles.
Besides feldspars and micas, there are many other primary minerals that contain
sizeable quantities of K which becomes available on weathering. In many parts
of the world the so-called greensand soils, containing glauconite, are well known for
their ability to supply K to plants, but there are also some minerals which are known
for their ability to fix K added to the soil, making it unavailable to plants. Allophane
and zeolite are examples of such minerals.
79
Allophane contains very little K but has a definite affinity for K, which can cause
fertiliser K to become fixed and unavailable. From their work with synthetic aluminosilica gels, comparable to natural allophanes, van Reeuwijk and de Villiers
[1968] concluded that these materials could fix considerable quantities of K in a way
similar to illitic clay minerals and that no specific chemical reaction was involved.
However, Sticher [1972] pointed out that the properties of the aluminosilicates
used may have been characteristic of freshly prepared gels and not representative
of natural allophanes.
The formation of zeolites is favoured by high pH of the lake water in which the
volcanic ash from which they are formed has settled. They are also formed in dry
conditions by the presence of alkalis. The Na/K ratio of zeolite in sedimentary rock
is considerably lower than that in the surrounding lake- and soil-water, indicating
that they have a strong affinity for K, and this has led to the suggestion that soils
containing zeolites have the capacity to fix K. Schuffelen and van der Marel [1955]
included an artificial zeolite in the series of soils and minerals which they tested for
K fixation capacity and found that it fixed more K than most of the other materials
examined. They found that allophanes fixed large amounts of K and that K could
also be fixed by feldspars.
These results show that, in addition to the secondary minerals, some primary minerals
and minerals like allophane and zeolite, which form a transitional group between
the primary and secondary minerals, have the capacity fo fix K. Thus the K-bearing
soil minerals should not always be looked on as constituting a source of K; they can
also function as a sink for K originating from other soil components of from fertiliser.
2.2 Clay minerals
When a primary mineral like biotite acts as a source of K, releasing it to the soil
solution, it undergoes structural change and gradually loses its identity. Mortland
et. al. [1956] grew four successive crops of wheat on a mixture of quartz sand and
finely ground biotite and found (Table 2) that as K was depleted, the cation exchange
capacity of the biotite increased. The removal of K by the growing plants allowed
cations other than K to enter the mineral along with water, causing the layers of the
mineral to expand. X-ray analysis showed that during cropping the original diffraction
pattern of biotite largely disappeared, to be replaced by that of vermiculite.
Table 2. Effect of four crops of wheat on some properties of biotite added in various quantities
as a K source to the growth medium
Properties of biotite residue
Biotite added
per culture, g
25 ..........................................
50 ..........................................
100 ..........................................
at the completion of the experiment*
K content, %
CEC, me/100 g
2,4
3,6
4,5
54
38
30
* original biotite: K content, 5,8%; CEC 14 me/100 g (M.M.Morland et al. [1956])
80
Thus, removal of K causes a primary mineral to lose its original characteristics
and to take on the properties (cation exchange capacity and layer expansion) of a
clay mineral. If, however, the cations that replace K in the formation of vermiculite
from mica are later replaced again by K, the mineral layers may be drawn together
so tightly that the regained K is entrapped in non-exchangeable form.
To determine whether a soil contains K-fixing minerals, the soil is suspended for a
certain period in water containing a known quantity of K. After the appropriate
time, an electrolytic extractant is percolated through the soil to determine how far
the K taken up by the soil can be recovered by exchange with another cation. That
part of the K not recovered is considered to have become fixed in the crystal lattice.
Drying of the soil between contact with the K-containing solution and extraction
often increases K fixation, the likely cause being the removal of water molecules
from the inter-layer position and subsequent closure of the lattice layers entrapping
K+ ions.
The quantity of K recovered during the percolation is also influenced by the cation
used. Because NH 4 has a similar charge and similar ionic size, it can be fixed in the
sites which fix K. Thus, if ammonium is used in the percolation treatment, it may
give misleading results because NH 4 is likely to replace inter-layer K at the edges of
the particles, but it also causes the layers to collapse at the edges, entrapping K at
sites in the inner inter-layer positions and this will be erroneously included in the nonexchangeable fraction.
It is generally difficult to extrapolate from laboratory measurements of fixation to
predict field behaviour. The amount of native soil-K or added fertiliser-K which can
be released to a crop is governed largely by the intensity of cropping, as exemplified
in results reported by Hemingway [1963]. He grew a grass-legume mixture for three
years in field plots under various fertiliser treatments. All herbage was removed
and both non-exchangeable and exchangeable K were measured before and after
the 3-year period (Table 3).
Under severe K stress (N fertiliser without K), non-exchangeable K made a sizeable
contribution to the K supply of the crop. This suggests that the K concentration in
the soil solution was kept so low that equilibrium in equation I (above) was shifted
to the right. Conversely, when K was given without N, growth was impeded by N
starvation and there was an increase in the non-exchangeable fraction during the
course of the experiment. Thus the laboratory technique used would indicate that K
Table 3. Loss of exchangeable and non-exchangeable potassium from soil during a three-year
period of growth of a legume-grass mixture
Form of fertiliser added
Loss of K from soil, kg per ha*
exchangeable
N one ........................................
N itrogen .....................................
Potassium ....................................
Potassium + nitrogen ...........................
179
202
II
191
nonexchangeable
36
64
-31
1
* K added as fertiliser was counted to become a part of the exchangeable fraction
(R.G. Hemingway [1963])
81
had occupied positions in which it was fixed, though a more vigorously growing
crop would have been able to extract K from these positions. Such findings indicate
that the laboratory measurements have only limited application to field conditions.
Cropping intensity largely determines whether or not the designations 'exchangeable'
and 'non-exchangeable' are synonymous with 'available' and 'non-available' and
whether or not K designated as 'fixed' by laboratory standards is really useless in
terms of plant nutrition.
The name illite is given to a variety of secondary minerals belonging to the mica
group. Although they show some similarity to primary micaceous minerals, it is
doubtful whether illites are formed as degradation products of the weathering of
micas. They have a high K content (usually around 6%) and are thus thought to make
a large contribution to the K nutrition of plants. However, much of the K in illites
is an intimate part of the clay-mineral structure and is not readily released. Because
they contain native K, the cation exchange capacity of illites is lower than that of
smectites, even though the former have a higher charge density. The information
given in Figure 1 shows that illite can supply comparatively large quantities of K to
a rapidly growing crop.
2000
E
bimb*
1690
k
ilfile
1-200
800
o
40
0
5
70
15
Cropping period. dc)ys
Fig. 1. Release of K from different minerals during intensive cropping of mixtures of the
minerals with quartz sand. Particles of biotite, muscovite, and orthoclase <50 [L,those of
illite <20 [t. All minerals were added in quantities to supply the same total amount of K per
culture. (G. P. Verma: Ph.D. thesis, Iowa State Univ. [1963])
Due to the pivotal function performed by K in illites, they have a relatively large
percentage of exchange sites showing marked preference for K. Thus, when K saturation is low, K is held with a high binding energy in the interlayer positions, mainly
at the so-called K-specific sorption sites. K will only be found on non-specific sites
when K saturation is increased. Consequently, a given K concentration in the soil
solution will be at equilibrium with a larger amount of exchangeable K in a soil
containing illitic minerals than in a soil of lower clay content. Thus, less K fertiliser
82
is needed to achieve a certain K intensity in a light-textured soil than in a heavy
soil. However, the K capacity of the light soil is generally too low to sustain an adequate K intensity once plants start to draw K from the soil solution, while the higher
K capacity of a heavier soil can maintain the K intensity for much longer. Grinmme
et al. [1971] have contrasted the relationships between K intensity and K capacity in
two soils containing widely different quantities of illitic minerals (Figure 2).
. 3%7 clay
.25.
clay
5
4
2
oo
Ili
exchangeabie K, mg KI O0 g soil
Fig.2. The relationship between exchangeable K and K concentration in the soil solution for
two soils differing in clay content. (f.Grinane et al.: Landw. Forsch. 26, 1,Sonderheft, 165176 [1971])
Clay minerals of the smectite group and those of the kaolinitic and halloysite types
have few K-specific sorption sites. The K-fixation capacities of soils containing
these minerals are low and the clay minerals easily release their K to the soil solution
as it is depleted of K by plant uptake.
kn primar,
dissolulionof
I
ofsecod r
as o
mlnerals
N firatiOn
a'rele
S.le
ortdary
adsorption or
K in secondary
mninera...s,
e. kolinie
nlecs. of
adsorbed or
fired X
Fig.3. Locations and pathways of potassium in soil.
83
It was said above (2.1) that primary minerals can serve as both sinks for and sources
of K. When such minerals lose K from the lattice structure, they lose their identity
and are converted to secondary clay minerals. Hutchings [1890] long ago suggested
that the reverse process could occur, i.e. the conversion of clay minerals like kaolinite
into micaceous minerals. Volk [1934] showed that K added to soil as fertiliser increased the mica content. This process is regarded as a reversal of the weathering
process in which K is entrapped in expanding 2 : 1 layer silicates. Thus the reaction
sequence presented at the beginning of this section can be extended as in Figure 3.
3. Soil organic matter
The ways in which soil organic matter may influence the availability of K depend
upon its characteristics. Because K is not a constituent of any quantitatively important
organic plant component, the amount of K contained in undecomposed or partially
decomposed plant material in soil is bound to be small.
Humification of plant residues and soil organisms can produce a type of organic
matter with high cation exchange capacity. It is possible that organic matter is important in holding soil K in exchangeable form. However, humus retains divalent
cations (Mg, Ca) more strongly than the monovalent (K, Na). Whether this promotes
or hampers the availability of K to plants depends on the conditions. Weaker retention of K relative to Ca and Mg may increase K availability but, at the same time,
it renders the K more liable to leaching.
In tropical soils, in which kaolinitic clay minerals having low CEC predominate,
organic matter can make a sizeable contribution to the retention capacity, as is
shown in Table 4 (van Raij [1969]).
Table 4. Some characteristics of topsoils in Sao Paulo, Brazil
Soil No.
% clay
% O.M.
CEC, me/100 g soil
in clay
S .......................
2 .........................
3 . .......................
4 ......................... ..
5 ........................
6 . .......................
7 ........................
I
2
3
4
5
...........................
...........................
...........................
...........................
...........................
(B. van Raij [1969])
84
in O.M.
Soils with an argillic B horizon
2.2
5
0.8
1.0
2.1
0.6
1.2
6
1.8
8.2
2.5
12
6.0
2.4
1.4
19
2.0
4.3
2.3
38
1.2
7.9
3.2
18
9.4
15.0
4.5
64
Soils with a latosolic B horizon
4.5
12.8
16.1
59
3.1
6.4
2.7
52
2.9
1.0
1.2
24
4.0
2.2
25
2.0
14.9
6.5
1.4
56
% of CEC
in O.M.
69
64
82
81
68
87
62
56
67
74
65
91
These findings emphasise the importance of organic matter in tropical soils for the
maintenance of soil fertility, not just for nutrients like N, P and S, which may be
structural constituents of the organic material from which it is formed, but also for
nutrient cations, including K, whose availability may largely depend on the contribution of soil organic matter to cation exchange capacity.
Organic matter has well known indirect effects on the availability of soil K, in that
it promotes aggregate formation and stability and thus water-holding capacity and
aeration which favour root extension. These latter two subjects will be dealt with
in later sections.
4. pH
pH has a complex influence on the availability of soil nutrients. In the case of nutrients,
such as nitrogen, whose availability depknds largely on the activity of soil microbes,
any change in pH may affect their avaiiability in that such a change affects the type,
size and activity of the micropopulation. For nutrients whose availability is a function
of solubility of chemical compounds, e.g. phosphate, a change in pH may affect
their solubility and thus their availability.
Both microbiological and chemical factors can influence the availability of soil K
as well, but more remotely so. In many soils of temperate regions, soil microbes are
an important factor in creating and maintaining soil aggregate stability which, in
turn, governs soil moisture-and soil air characteristics. The effects of these two
soil-physical factors on K availability will be discussed in sections 5 and 6. In the
present section, attention will be given to the influence of pH on some soil-chemical
factors influencing K availability.
4.1 Influence on K fixation
It is well known that pH affects K fixation, but again here, the influence is indirect
in that pH largely determines which cation predominates in the inter-layer positions
of clay minerals. It is now generally agreed that in acid soils aluminium can occupy
many of the exchange sites. Acidification may lead to accumulation of polymeric
hydroxy-aluminium ions in inter-layer positions so that such sites nolonger contribute to the total cation-exchange capacity of the clay minerals. However, in vermiculite, aluminium ions also prevent K ions from occupying these sites and may thus
be instrumental in lowering the K-fixation capacity of the soil. Hence, the effect of
increasing acidity on K fixation will depend on whether or not the soil contains
vermiculite-type clay minerals and whether or not increasing acidity releases aluminium
ions which block inter-layer K-fixation sites. Conversely, the effect of liming an acid
soil on K fixation depends on whether or not the increasing Ca concentration leads
to a displacement of aluminium and thus to an exposure of sites with a potential
for fixing K.
It is therefore not surprising that the literature contains conflicting information on
the influence of pH on K fixation. York et al. [1953] give an example where liming
an acid soil increases K fixation (Figure 4). This Figure also shows that alternate
wetting and drying increased K fixation. However, there was little interaction between
85
.
moist constanfly
ted and dri.d
..
120
a
100
-
S
-
.
200 pounds K
-0
-K
0
20.
t
0!
no
--
K
. ..
..0
-20CCO03 appled In.. Cool00 gi
Fig.4. Effect of drying Mardin silt loam on lime-induced potassium fixation. (E. T. York et al.:
Soil Sci. 76, 379-387 [1953])
the two effects, as the effect of drying and wetting was about the same at all liming
levels.
The effect of liming on K-fixation capacity will also determine whether the soil
minerals will release or fix K. In this respect, Rich [1964] investigated the behaviour
of a soil containing vermiculite. The soil was washed repeatedly with IN Ca(OAc)
solution containing a small quantity of K (K/Ca= 1.35 x lO - 4) and adjusted to
three different pH levels. His results (Figure 5) show that below pH 4.35 the soil
acted as a K source, and above it as a sink for K.
It is to be expected that laboratory findings like that shown in Figure 5 can be extrapolated to field conditions. This would imply that liming an acid soil to improve
0.2
LeocQhot.no
Fig.5. Release of K to, or removal from, N Ca(OAc)2 by Nason soil as affected by pH.
(C.J. Rich: Soil Sci. 98, 100-106 [1964])
86
general growth conditions might increase the need for K fertiliser to ensure adequate
crop nutrition. An example will be given in the following.
In the Netherlands, estimates of plant-available soil K are obtained by extracting
soil with 0.1N HCI. It was found, however, that the quantity of K extractable with
0.IN HCI needed to be valued differently for soils differing in clay content or in pH.
The higher the clay content and the higher the pH, the more the numerical value
of the K-HCI reading had to be reduced in order to give a true index of the K status
of a soil.
The negative effect of clay content on K availability was discussed in Section 2
(see Figure 2). It was shown that a given quantity of exchangeable K per unit weight
of a light-textured soil gives a higher K concentration in the soil solution than does
the same quantity of exchangeable K in a heavy-textured soil. The negative effect
of pH was demonstrated by York et al. [1953] and Rich [1964] (Section 3).
The use of empirical factors to allow for varying pH and clay content is realised in
the calculation of K values, as follows:
K value
=
K-HCI value x b
0.15 pH(KCI)-0.05
'b' varies with the percentage of soil particles< 16 t (the higher the percentage, the
lower 'b'). At pH below 7, the K-HCI value will be multiplied by more than 1, indicating that each unit of K extractable with 0.1 N HCI represents a larger quantity
of available soil K, the lower the pH value of the soil is (Henkens [1977]).
Extension workers saw the need for such corrections long before the scientific background was understood. However, scientific progress in the last 25 years has provided
the physico-chemical basis for the findings of practical workers in the field.
The correction factors used were intended only for Dutch conditions. Nevertheless,
they have also proved to be useful under conditions very different from those obtaining in the Netherlands. In greenhouse trials, Muchena [1975] tested K availability
in II soil types from various regions of Kenya. The soils were cropped with sorghum
and the K content of the above-ground parts of the plants was correlated with the
quantities of K extracted from the soils by different extractants. The correction
factors used in the Netherlands were employed to transform the values obtained
with the Mehlich method (extraction with 0.IN HCI and 0.025N H2 SO 4 ) and the
Dutch method (O.IN HCI). Correlation was notably improved by use of the correction
factors.
Linear correlationcoefficients
Mehlich
corrected Mehlich
K-HCI
K-HCI corrected (i.e. K value)
0.905
0.956
0.912
0.961
4.2 Influence on organic matter
pH also influences the quantity of available K in organic matter. However, in contrast
to its effect on the availability of K in clay minerals, the effect of increasing pH is
likely to improve the K-supplying power of organic matter. The CEC of soil organic
87
matter is much more pH-dependent than that of clay minerals. The difference is
shown clearly in Figure 6 after Helling et al. [1964]. Soils whose capacity to retain
cations in exchangeable positions is located mainly in the organic-matter fraction
are in a much better position to function as sources of plant-available K when the
pH is kept high. When, as in tropical soils, the clay mineral is largely kaolinite, with
very low CEC and K content, organic matter is most important for safeguarding
the supply of K to plants. Whether organic matter can serve as a K source for plants
in such soils depends much on the soil pH. However, while liming increases the
CEC of organic matter, it also increases its rate of mineralisation. Little research
appears to have been done in the tropics on the influence of liming on the capacity
of organic matter to serve as a source of plant-available K.
400
Organi
C
300
a
'
200
-100
0
1
2
3
4
5
6
7
a
pH
Fig.6. Effect of pH on the CEC of organic matter and clay in 60 North American soils.
(C.S. Helling et al.: Soil Sci. Soc. Amer. Proc. 28, 517-520 [1964])
4.3 Influence on other cations
Measures taken to modify soil pH may be the addition of liming materials to a soil
considered too acid, or addition of sulphur or other acidifying agents to a soil thought
to be too alkaline. The latter is seldom practised so it is usually taken for granted
that adjustment of soil pH is more or less synonymous with liming. Liming materials
normally contain Ca, sometimes also Mg. It is to be expected that the mere addition
of these alkaline-earth cations, apart from their influence on pH, would also directly
affect the availability of K to plants, as will be discussed in 7.1. By influencing the
activities of Ca, Mg and Al in the soil solution, pH also affects the activity of K and
thus its availability.
88
5. Aeration
Soil organic matter content and pH can both affect soil aeration. Soil organic matter
directly or indirectly promotes aggregate stability. It plays a direct part in forming a
matrix in which mineral particles may become entangled to form soil aggregates.
Between soil aggregates there may be relatively wide pores which are important
for the diffusion of gases.
More indirectly, soil organic matter forms a substrate on which soil organisms live.
Such organisms may produce substances which act as cementing agents in the formation of aggregates from mineral particles (Costerton et al. [1978]).
The role of pH in promoting soil aeration is likewise associated with the action of
soil organisms. Earthworms and bacteria are particulary active in producing cementing
agents. Both types of organisms prefer alkaline conditions and are practically absent
from acid soils. Liming may therefore stimulate the growth of earthworms and bacteria,
and the cementing agents produced enhance the formation of aggregates which are
important for the establishment of wide pores through which both oxygen and carbon
dioxide can diffuse.
Good aeration favours both root extension and the functioning of uptake mechanisms
in the root responsible for the selective withdrawal of K from the soil solution. At
this point it is appropriate to refer back to Barber's factor 'the rate of uptake of an
ion per unit root surface' (1.4). It is obvious that total uptake will be determined
not only by the rate of uptake per unit root length but also by the total root length
(root extension).
The rate at which K is taken up appears to be particulary dependent on oxygen supply.
Table 5 shows that the absorption of K is much more affected by soil compaction
and soil aeration than is the absorption of Ca or Mg and that the growth of maize
is closely related to the K content of the tops.
K translocation in plants was also found to be influenced by soil aeration. Shapiro
et al. [1956] observed that K content of maize tops decreased with decreasing aeration,
although the K content of the roots increased.
Table 5. Effects of soil aeration and soil compaction on the growth and cation composition
of maize plants
Treatment
Maize yield, g/pot
tops
At 15% soil moisture
NP .........................
NP, soil compacted ...........
NPK .......................
NPK, soil compacted ..........
At 40% soil moisture
N P .........................
NP, soil aerated ..............
NPK .......................
NPK, soil aerated .............
Composition of maize tops,
me/100 g
roots
K
Ca
Mg
20.8
8.7
24.5
10.7
12.9
4.3
14.2
5.2
54
21
80
73
16
20
18
28
29
38
23
39
13.3
17.3
18.1
27.7
5.6
8.7
9.0
17.8
24
33
53
84
18
16
24
16
33
29
30
26
(K. Lawton [1945])
89
6. Soil moisture
Drew and Nye [1969] showed that only about 6% of the total K required by ryegrass
would be found in exchangeable form in the soil within the root hair cylinder. So,
much K must diffuse to the root from soil outside this zone. K diffuses to the absorbing root over a longer distance than does P but not as far as NO3 . Kauffnann
and Bouldin [1967] found that the diffusion path in moist soil was about 1 cm long.
Later, Grimne et al. [1976] showed that the length of the diffusion path depended
on soil moisture content (Figure 7). Near the absorbing surface (in this case an ion
exchange resin) the exchangeable K content of the soil is lower in the wet than in the
dry soil. Also the cylinder around the resin which is depleted of K has a larger diameter in the wet than in the dry soil.
N 20
01'
pF3.2(2?X H20)
to
---
--
10
04Y H0)
pFl.8
F
2030
diffusot,
Ptoh (m )
Fig. 7. Decrease in exchangeable soil K caused by diffusion into an ion exchange resin at two
soil moisture levels. (H. Grhinne: Bull. Indian Soc. Soil Sci. 10, 3-22 [1976])
Bearing in mind the importance of aeration for K uptake by the root, it will be clear
that there is a limit to the extent to which increasing moisture content will improve
the K nutrition of plants and for each species there will be a specific moisture content
above which any advantage of accelerated diffusion will be offset by a reduced capacity of the root's K uptake mechanism to translocate K from the ion free space into
the symplast. This mechanism has a high oxygen demand.
K diffuses through the soil by way of both water-filled soil pores and water films
surrounding soil particles. When the soil is moist and part of the larger pores is
filled with water, movement of K+ ions will be relatively little hampered by the
attractive forces exerted by the solid soil particles. Under certain conditions, such
forces can considerably retard the movement of K+ ions, and it will be clear that
this retaidation will be particularly evident when much of the soil particles consists
of negatively charged clay particles which tend to retain K on their absorption sites.
In a drier soil only the narrow pores are completely filled with water, water in the
larger pores having retracted to form a film around the particles. In these conditions
diffusion can take place only in the immediate vicin'ity of solid particles whose attractive forces will strongly retard the movement of K.
When the larger pores are empty of water, the distance over which ions have to travel
is increased by the tortuosity of the path. Equations describing the diffusive flux of
ions in a soil include a so-called impedance factor representing the degree of tortuosity of the diffusion pathway.
90
Lowering the water availability may reduce K uptake by plants through a combination of two factors: 1. reduced root activity caused by water shortage, and 2. reduced
diffusive flux due to the longer diffusion pathway and the resistance offered by the
solid particles. Experiments in which the roots of test plants were divided between
two media were designed to test the influence of the latter factor alone (Grimme
[1976]). In one medium, half the root system was supplied with all the nutrients
except K and with a constant water source. The other medium was a soil in which
the moisture and available K contents were varied. The results are shown in Figure 8
and indicate that uptake of K by maize was greatly reduced by lowering the soil
moisture content but that the reduction could be partially offset by raising the quantity
of available K in the soil.
!
500 4
K ade,
mgl/O0 g sot'
29
-
j9
40
30
20
moistu/re content at soil (per cet)
Fig.8. The effect of variations in soil moisture content at 4 soil K levels on the K uptake of
maize plants grown in a split-root experiment. (H. Grimme, Ind. Soc. Soil Sci. Bull. 10, 3-22
[1976])
7. Other ions in the soil
7.1 Cations
It is well established that metabolic absorption by roots is regulated by enzymecontrolled uptake mechanisms, the so-called carriers (Osterhout [1935]). It was shown
that there are a number of mechanisms in the root, each of which appears to be
specific to one nutrient and, as well as these 'specific carriers, the root also has 'common carriers', possibly one for cations and one for anions. Cations compete for
sites on the common cation carrier and, similarly, anions compete with respect to
the common anion carrier. Much of our knowledge about carrier-controlled ion
uptake stems from work done during the past 25 years (Epstein [1966 and 1972]).
The ion-uptake characteristics of many plants suggest that K is absorbed in preference to other cations. When, in short-term experiments, normally fed plants are
91
temporarily placed on a series of nutrient solutions each containing only K and one
other cation, it is usually found that, on an equivalence basis, the plants take up
more K than the complementary cation. This characteristic might have been acquired
during evolutionary processes in which sea-borne plants with relatively high K
demands were forced to adapt to situations in which the Na supply was many times
larger than the K supply. Later, when the plants had invaded the continents, the
situation changed, but only insofar as in soil solutions Ca++, Mg+ + , H + or AI + + +
dominate K+. The plant can only obtain sufficient K from a soil solution containing
much more Ca for example, by selective ion uptake.
Selective ion uptake implies that, while roots absorb K preferentially, they can exclude a large part of the Ca which arrives at the root surface in the flow of moisture.
This Ca may accumulate at the root wall or in the ion free space but such an accumulation is limited as it must set up a process by which Ca diffuses back into the soil
solution and away from the root. Such a process is, however, retarded by the effect
of soil moisture flowing in the opposite direction. We would therefore expect that
the Ca: K ratio in the immediate vicinity of the root or in the ion free space would
be even higher than it is in the soil solution outside the zone influenced by the root.
The value of this ratio in the neighbourhood of the root is a function of both the Ca
and K concentrations of the original soil solution and of the absorption characteristics
of the plant root and the rate of transpiration.
It sometimes appears as though soil chemists underestimate the influence of the
plant itself and the influence of climatic conditions on the Ca: K ratio in the rhizosphere when they emphasise the role played by Ca in affecting the K-supplying
capacity of the soil.
The intensity of K supply can be measured either by simply determining the K
concentration in the solution in contact with the root or, indirectly, by measuring
the K potential or activity ratio aK/(aca±Mg) / . Those who advocate the use of
the activity ratio contend that the availability of soil K to plants depends on both
the potential of K + and the potentials of Cal + and Mg2+ (Beckett [1964]). If, for
example, two soils differ in their combined Ca+Mg activities, the value in soil I
being four times higher than in soil 2, the K activity in the solution of soil 2 would
have to be twice as high as that in soil I for K availability to be equal in the two
soils. Mengel [1963], working with solution cultures and Wild et al. [1969] with
sand cultures, showed, however, that K uptake is a function simply of the K concentration in the soil solution rather than of the activity ratio.
Any benefit to be expected from linking the availability of K to the activities of
Ca and Mg in solution- and sand-culture experiments may stem from the assumption
that the divalent cations influence the ease with which K is absorbed by plant roots.
It is not possible to measure exactly the Ca concentration in the immediate vicinity
of an active root, but whether Ca will obstruct or promote the uptake of K depends
upon the concentration of Ca in the solution (Epstein [1962]). It was further shown
(Rains and Epstein [1967]) that the K concentration determines whether Ca will
stimulate or inhibit the uptake of K. Mg has sometimes been found to suppress
(Carolus [1938]) and sometimes to enhance ( Viets [1944]) the uptake of K.
The ways in which Mengel and Wild et al. tested the validity of the activity ratio
concept are open to criticism. They worked with systems in which the growing media
contained no material able to retain cations in exchangeable form. Thus their experiments were not fit to test how activity ratios may reflect the ratios of thecapac92
ities of K and (Ca + Mg) in natural soils. However, it can be said that activity ratios
may be useful in supplying information on relative capacity values but not on absolute capacity values.
The latter disadvantage can be overcome when activity ratios are used to determine
the so-called potential buffering capacity (PBC) of a soil. The PBC is expressed as
the ratio dQ/dI or the rate of change in K capacity per unit change in K intensity.
If experimentally obtained values for K capacity are plotted against the correspondthe slope of the relationship expresses the ability
ing values for aK/(aca+Mg),
of the soil to maintain K supply (or K availability) as the labile pool of K is depleted.
But again we may ask the question whether there is any advantage in using the
activity ratio rather than the simple K concentration as a measure of K intensity.
In many acid soils, Al is the dominant cation on the exchange sites. Liming often
lowers the availability of soil- and fertiliser K (4.2). Ca applied in liming material
may raise soil pH to a level at which hydrous Al-oxides are precipitated. The resulting disturbance in the equilibrium between exchangeable and solution Al forces Al
out of the lattice to be replaced by Ca. Calcium is not as effective as Al in blocking
K-specific sites and hence, as Ca replaces Al, exchangeable K, or the labile pool,
will increase. Whether such an increase in the labile pool raises or lowers K availability depends upon the circumstances. If the removal of exchangeable Al means
that K has access to inter-layer positions with a high K specificity, the K gained by
the clay mineral will become more or less fixed. However, if the Al was blocking the
passage of K held on non-specific sites, its removal will increase K availability. On
the other hand, should Al block the passage of fertiliser K to K-specific sites, adding
K may lead to a considerable rise in K availability. This is evident from Figure 9,
2.6
pH 4.)
2.0
~pH
5.1
pHSI
t
/IP70
0.2
10
X0
jo
40
K application n rnp/O0 soil
Fig.9. Relation between amount of K applied and the concentration of K in the solutions of
four gray-brown podzolic soils, having similar CEC values and differing in pH. (K.Nemdth:
Plant and Soil 42, 97-107 [1975])
93
where addition of fertiliser K was most effective in raising K concentration in the
solution of the most acid soil.
Obviously, in acid soils, the activity ratio aK/(aca+Mg) '/ cannot be an indicator
of K intensity. Tinker [1964] has proposed using Al 3+ as reference ion in such soils,
but, again, it remains to be shown that activity ratios really reflect the capacity of
the soil to supply K to plants. In an early paper, Beckett [1964] stated that 'the
activity ratio should provide an adequate comparative measure of the potential of
labile K and of the availability to plants of K in the soil so long as its uptake is not
limited by metabolic factors or antagonisms at the root surface'. It is clear from the
discussion above that, even though they may not limit K uptake, metabolic factors
certainly affect it. It may be for this reason that simple measurements of K concentration in the soil solution or of exchangeable K are often more useful estimates of K
availability than are activity ratios.
If it is wished to have information on both the K intensity and the K capacity of a
soil, the electro-ultrafiltration method of Nenu&h [1972] is worth considering. This
method is quicker and less laborious than the measurement of PBC and it can be used
irrespective of the main cation complementary to K.
7.2 Anions
Anions influence K availability mainly indirectly. There is no strong evidence to
show that variations in the quantities of NO,, H 2PO4 , Cl or SO4 in the soil will affect
the labile pool of soil K. However, the tendency of soil K to pass from the solid soil
particles into the solution depends on the difference between the actual and equilibrium K concentration in the soil solution. Anions may be indirectly involved in creating
this difference.
It stands to reason that a rapidly growing plant needs more K than a slow growing
one. Thus the former is more effective in lowering K concentration in the soil solution
and setting up the concentration gradient that is needed to induce K to leave the
solid phase. So anything that promotes growth will indirectly promote the release
of K. An example was given in Table 3, which showed that applying N made more
non-exchangeable K available.
It has been postulated (Higinbotham et at. [1967]) that plants take up anions actively,
i.e. against an electro-chemical potential gradient and that, under a number of circumstances, cations enter the plant more or less passively as counter-ions of the actively
absorbed anions. Such evidence is, however, stronger for Ca and Mg than it is for K.
There is much evidence that ions are absorbed by a dual transport mechanism (Epstein et at. [1963]). Generally, uptake of cations by the low concentration mechanism
is indifferent to the nature of the anion, whereas uptake by the high concentration
mechanism is much affected by the anion, but the concentration of K at which K
uptake starts to be affected by the nature of the anion is higher than the concentrations
ordinarily found in soil solutions.
Nitrate uptake followed by nitrate reduction in plants gives rise to the formation
of organic anions. Monocotyledonous plants fed with NO, usually take up more
anions than cations, and malate is the predominant organic anion formed. It is
likely that some of the malate is translocated back to the root where it is broken down
to bicarbonate and this is believed to be excreted by the roots in quantities equivalent
94
to the difference between nutrient anions and cations absorbed. K is the main cation
accompanying malate during transport to the root. Having delivered the malate
in the root, the K ions are thought to serve again in accompanying NO3 taken up in
excess of cations, in an upward direction. Thus, nitrate uptake is thought to stimulate
K uptake (Dijkshoorn [1962]; Kirkby [1974]).
8. Summary
The availability of soil potassium to plants depends on a number of chemical and
physical soil characteristics as well as on the effects these characteristics have upon
the activity of plant root systems. Many of these characteristics are interrelated.
In the past 25 years, our knowledge of the influence of soil minerals, soil organic
matter, pH, aeration, soil moisture and other ions in soil on soil K availability has
increased greatly. It is to be expected that, in the years ahead, further attention will
be given to the interactions between these factors in influencing soil potassium
availability.
9. References
Barber, S.A.: A diffusion and mass-flow concept of soil nutrient availability. Soil Sci. 93,
39-49 (1962)
Beckett, P.H. T.: Studies on soil potassium. I. Confirmation of the ratio law:measurement of
potassium potential. J. Soil Sci. 15, 1-8 (1964)
Bray, R.H.: A nutrient mobility concept of soil-plant relationships. Soil Sci. 78, 9-22 (1954)
Carols, R. T.: Effect of certain ions, used singly and in combination, on the growth and
potassium, calcium and magnesium absorption of the bean plant. Plant Physiol. 13,
349-363 (1938)
Costerton, J. W., Geesey, G.G. and Cheng, K.-J.: How bacteria stick. Scientific American
238, 86-95 (1978)
Comber, N. H.: The availability of mineral plant food. J. agric. Sci. Camb. 12, 363-369 (1922)
Czapek, F.: Zur Lehre von den Wurzelausscheidungen, Jahrb. 6. wiss. Bot. 29, 321-390 (1896)
Devaux, H.: Action rapide des solutions salines sur les plantes vivantes: ddplacement r6versible d'une partie des substances basiques contenues dans la plante. C.R. Acad. Sci. Paris
1962, 561-563 (1916)
Dijkshoorn, W.: Metabolic regulation of the alkaline effect of nitrate utilization in plants.
Nature 194, 165-167 (1962)
Drew, M. C. and Nye, P. H.: The supply of nutrient ions by diffusion to plant roots in soil.
11. The effect of root hairs on the uptake of potassium by roots of ryegrass (Loliun multiflorum). Plant and Soil 31, 407-424 (1969)
Dyer, B.: On the analytical determination of probably available 'mineral' plant food in soil.
J. Chem. Soc. 65, 115-167 (1894)
Epstein, E.: Mutual effects of ions in their absorption by plants. Agrochimica 6, 293-322
(1962)
Epstein, E., Rains, D. W. and Elzam, E.: Resolution of dual mechanisms of potassium absorp-
tion by barley roots. Proc. Nat. Acad. Sci. 49, 684-692 (1963)
Epstein, E.: Dual pattern of ion absorption by plant cells and by plants. Nature 212, 1324-1327
(1966)
Epstein, E: Mineral Nutrition of Plants: Principles and Perspectives. John Wiley and Sons,
1972
Fried, M. and Shapiro, R.E.: Phosphate supply patterns of various soils. Proc. Soil Sci. Soc.
Amer. 20, 471-475 (1956)
95
Grimme, H., Nemeth, K. and von Braunschweig, L.C.: Beziehungen zwischen dem Verhalten
des Kaliums im Boden und der Kaliumernahrung der Pflanze. Landw. Forsch. 26/1,
Sonderheft 165-176 (1971)
Grimme, H.: Soil factors of potassium availability. Ind. Soc. Soil Sci., Bull. 10, 3-22 (1976)
Helling, C.S., Chesters, G. and Corey, RB.: Contribution of organic matter and clay to soil
cation-exchange capacity as affected by the pH of the saturating solution. Proc. Soil Sci.
Soc. Amer. 28, 517-520 (1964)
Hemingway, R.G.: Soil and herbage potassium levels in relation to yield. J. Sci. Food Agric.
14, 188-195 (1963)
Henkens, C.H.: Bodem en Bemesting. Adviesbasis voor Landbouwgronden. Rijkslandbouwconsulentschap voor Bodem en Bemesting, Wageningen, Netherlands (1977)
Higinbotham, N., Etherton, B. and Foster, R.J.: Mineral ion contents and cell trans-membrane
electropotentials of pea and oat seedling tissue. Plant Physiology 42, 37-46 (1967)
Hutchings, W. M.: Notes on the composition origin of some slates. Geol. Mag. 7, 264-273,
316-322 (1890)
Jenny, H. and Overstreet, R.: Cation interchange between plant roots and soil colloids. Soil
Sci. 47, 257-272 (1939)
Kauffman, M.D. and Bouldin, D.R.: Relationships of exchangeable and non-exchangeable
potassium in soils adjacent to cation-exchange resins and plant roots. Soil Sci. 108,
145-150 (1967)
Kirkby, E.A.: Recycling of potassium in plants considered in relation to ion uptake and organic
acid accumulation. In: Plant Analysis and Fertilizer Problems, Vol. 2, 557-568. Proc.
7th Intern. Colloq. Hannover (1974)
Lawton, K.: The influence of soil aeration on the growth and absorption of nutrients by corn
plants. Proc. Soil Sci. Soc. Amer- 10, 263-268 (1945)
Mengel, K.: Untersuchungen fiber das 'Kalium-Kalzium-Potential'. Z. Pflanzenernihr.,
Ding. u. Bodenkunde 103, 99-111 (1963)
Mortland, M.M., Lawton, K. and Uehara, G.: Alteration of biotite to vermiculite by plant
growth. Soil Sci. 82, 477-481 (1956)
Mortland, M.M. and Lawton, K.: Relationship between particle size and potassium release
from biotites and its analogues. Soil Sci. Soc. Amer. Proc. 25, 473-476 (1961)
Muchena, F.N.: The availability of potassium in some Kenya soils. M.Sc. thesis, Agric. Univ.
Wageningen, Netherlands, 1975
Nemeth, K.: The determination of desorption and solubility rates of nutrients in the soil by
means of electro-ultrafiltration (EUF). Proc. 9th Coll. Int. Potash Inst. 171-180 (1975)
Osierhout, W.J. V.: How do electrolytes enter the cell? Proc. Nat. Acad. Sci. U.S. 21, 125-132
(1935)
Plumner, J.K.: Availability of K in some common soil-forming minerals. J. Agric. Res. 14,
297-315 (1918)
Raij, B. van: Cation exchange capacity of the organic and mineral fractions of soils. Bragantia
28, 85-112 (1969)
Rains, D. W. and Epstein, E.: Sodium absorption by barley plants: its mediation by mechanism
2 of alkali cation transport. Plant Physiol. 42, 319-323 (1967)
Reeuwijk, L.P. van, and de Villiers, J.M.: Potassium fixation by amorphous aluminosilica
gels. Proc. Soil Sci. Soc. Amer. 32, 238-240 (1968)
Reichenbach, H.G. von, and Rich, C.T.: Potassium release from muscovite as influenced by
particle size. Clays and Clay Minerals 17, 23-29 (1969)
Rich, C. T.: Effect of cation size and pH on potassium exchange in Nason soil. Soil Sci. 98,
100-112 (1964)
Schroeder, D.: Structure and weathering of potassium-containing minerals. Potassium
Research - Review and Trends. Proc. 1Ith Congress Int. Potash Inst., Berne (1978)
Schuffelen, A.C. and van der Marel, H. W.: Potassium fixation in soils. Proc. 2nd Symposium,
Int. Potash Inst. Berne, 157-201 (1955)
Shapiro, R. E., Taylor, S. and Volk, G. W.: Soil oxygen contents and ion uptake by corn. Proc.
Soil Sci. Soc. Amer. 20, 193-197 (1956)
Sticher, H.: Potassium in allophane and in zeolites. Proc. 9th Colloq. Int. Potash Inst. Berne,
43-51 (1972)
Tinker, P.B.: Studies on soil potassium. 3. Cation activity ratios in acid Nigerian soils. J. Soil
Sci. 15, 24-34 (1964)
96
Verma, G.P.: Release of non-exchangeable potassium from soils and micaceous minerals
during short periods of cropping in the greenhouse. Ph.D. thesis, Iowa State Univ., Ames,
Iowa, U.S.A., 1963. Quoted by C.A.Black, Soil-Plant Relationships, 2nd edition, John
Wiley and Sons, Inc. p. 679, 1968
Viers, F.G.: Calcium and other polyvalent cations as accelerators of ion accumulation by
excised barley roots. Plant Physiol. 19, 466-480 (1944)
Volk, N.J.: The fixation of potash in difficultly available forms in soils. Soil Sci. 37, 267-287
(1934)
Walker, T. W.: Uptake of ions by plants growing in soil. Soil Sci. 89, 328-332 (1960)
Wild, A., Rowell, D.L. and Ogunfowora, M. A.: The activity ratio as a measure of the intensity
factor in potassium supply to plants. Soil Sci. 180, 432-439 (1969)
York, E. T., Bradfield, R. and Peech, M.: Calcium-potassium interactions in soils and plants:
1. Lime-induced potassium fixation in Mardin silt loam. Soil Sci. 76, 379-387 (1953)
97
The Evaluation of Soil K Status by
Means of Soil Testing
H. Grimme and K. Nmeth, Agricultural Research Station
Blintehof, Hannover/Federal Republic of Germany
1. Introduction
Fertiliser is used when the nutrient demand of a crop expected to yield at a given
level exceeds the amount of nutrient which the soil can supply within a growing
season. Chemical soil tests are needed to determine the nutrient supplying ability
of the soil and hence the quantity of fertiliser which must be applied to overcome
any shortcoming.
There are many soil testing methods and a great deal of work has been expended
in the search for the 'best' method. So far, however, no method has been accepted
as universally applicable. There are many reasons for this. Some of the relevant
ones will be discussed in this paper.
When a method of analysis is assessed by measuring crop response to the nutrient
in question it is found that results are not consistent between different locations and
from one season to another. Usually soil analysis does not account for more than
50% of the variance in crop response, frequently for much less than 50% (Cooke
[1972]). Against the background of all the effort that has gone into devising and
calibrating soil tests it seems disappointing that the correlation between soil analysis
and crop response is so poor, but it also seems that the low level of correlation cannot
be surpassed.
2. A brief historical review
The idea that the soil solution is the medium from which plants draw their nutrients,
is not new. ( Woodward [1699], de Saussure [1804], Schloesing [1866], Whitney and
Cameron [1907]), Daubeny [1846]) introduced the idea of the buffering of the soil
solution at an early date. This led to the discovery of the 'base' exchange properties of
soils by Way [1850] and it was soon generally realized that the quantities of nutrients
Dr. H. Grimme and Dr. K. Nmeth, Agricultural Research Station, Bflnteweg 8, D-3000
Hannover/ Federal Republic of Germany
99
dissolved in the soil solution were not sufficient to meet the plants' requirements and
that nutrients combined with the solid components of the soil were also involved in the
nutrient supply to plant roots. Dyer [1894], [1891] tried to simulate the dissolving
action of plant roots by using 1% citric acid as an extractant for the available nutrient
fraction and was probably the first to use field experiments for calibrating a soil testing
method. In the wake of Dyer's work many extractants were tried among which concentrated mineral acids were found to be the least suitable (Opitz [1907]). However, in
the years to follow several lanes of approach were still being pursued. V. Wrangel and
co-workers ([1926], [1930]) investigated soil solutions and their usefulness as
indicators of nutrient availability. Mattson [1926] and Wilson [1928] introduced
electro-dialysis and Kittgen and Diehl [1929] electro-ultrafiltration. Electro-ultrafiltration (EUF) was taken up again and modified by Ncmeth [1971a, b].
But the main trend was for rapid methods, and the search went on for extractants
which would dissolve the plant available fraction with the least possible expenditure
of time and labour. (Lunt et al. [1950]; Egnjr el al. [1960], Ahmad et al. [1973]).
Devaux [1916] introduced the theory of contact exchange and this was later elaborated
by Jenny and Overstreet [1939] lending support to those who considered that fraction
of nutrients which was bound to the surfaces of the soil particles to be the most
important. Later this theory of contact exchange was refuted by Lagerwerff [1960]
and Olsen and Peech [1960] who were able to demonstrate that nutrient uptake
took place via the soil solution.
A completely new development was initiated by Bray [1954] with a paper on the
importance of nutrient mobility as a factor of nutrient supply to plant roots. These
ideas were taken up by Barber [1960], Barber and associates [1962] and by Nye and
his group (see Nye and Tinker [1977]). This approach proved to be more fruitful
than the 'nutrient potential' approach of Woodruff [1955] and Schofield [1955], on
whose work Beckett [1964] based his Q/I concept.
However, there has been little response to the new ideas that have evolved over the
past two decades in practical soil testing and most methods in current use are empirically based.
3. Methods of soil analysis
It is not intended to give a complete account of soil testing methods in present use
nor will technical details of individual methods be explained. This review will be
restricted to a discussion of the potentialities of the various methods and the information to be gained from the results obtained with them.
The various soil testing methods can be classified according to the fractions of soil
K which are covered. The different forms of soil K are discussed in this volume by
Schrdder [1979] and van Diest [1979] and their characteristic properties need not
be described here.
3.1 K concentration in the soil solution
K concentration in the soil solution is an important index of availability (Rowell
et al. [1967], Nye [1972]) because K diffusive flux towards the roots takes place in
100
the soil solution (see van Diest [1978]) and the rate of diffusive flux depends on the
concentration gradient that develops in the soil adjacent to an actively absorbing
root. Under conditions of a given K demand by the plant and the ability of the root
to reduce the K concentration at its surface to a certain minimum concentration the
concentration gradient is, among other things, a function of the initial concentration
in the soil solution.
The K concentration in the soil solution appears to provide a very good common
measure of K availability in soils of very different properties (Nineth and Harrach
[1974], Jankovic and Nbneth [1974], Ne,,eth and Forster [1976]) (Figure I).
Beckett's [1964] activity ratio (ARo) is a relative measure of K concentration in the
soil solution in that K concentration is expressed relative to the square root of standardized Ca+Mg concentrations. A number of criticisms have been levelled against
the use of 'K potentials' and AR, in soil fertility studies (Mengel [1963], Wildet al.
[1969]). In fact, Beckett himself has discussed the limitations of the method (Beckett
[1967]).
3000
r2=0.70
200
x
•
Ck.700
0
02
0.4
0.6
0.8
K- conc. soil solution (meK /I)
Fig. 1. Relationship between K concentration in the soil solution and K uptake of broad
beans (6 soils with 4 K levels on each soil; clay content 14-38 per cent).
The amount of K present in the soil solution represents only a very small proportion
of total soil K and is much less than a crop requires in a growing season. A measurement of the K concentration does not reveal whether this concentration is well buffered
or not and how much fertilizer would have to be added should the concentration be
considered inadequate.
101
K dissolved in the soil solution is a very labile fraction and may change without K
being added because it is sensitive to fluctuations in soil water content and total
electrolyte concentration. The plants, however, do respond to changes in K concentrations caused by the above mentioned factors (Nmeth and Grimmne [1974], Grinne
and Nimeth [1975]).
3.2 Easily extractable K
Schachtschabel and Heinemann [1974] proposed 0.025 N CaCI2 which was already
in use for Mg as an extractant for 'available' potassium, and this was shown by
Grimme and NbMeth [19 76 a, b] to be a good indicator of the K status of soils. The
quantities of K determined by this method are well correlated with K concentration
in the soil solution and with crop response. The proportion of total exchangeable
K extracted varies from 40 to 80% and depends on clay content and clay mineralogy
thus taking into account the selective bonding effect of soil clays for K.
There is a variety of methods which one would expect to yield results similar to those
obtained with the CaCl2-method (Black [1968], Ahmedet al. [1975]) because only a
fraction of total exchangeable K is determined.
3.3 Exchangeable K
The majority of soil testing methods for K employ the extraction of exchangeable
K or of quantities that come close to exchangeable K (Black [1968], SchefferSchachtschabel [1976]). A variety of extracting solutions are in use: neutral unbuffered electrolyte solutions, weak or strong acids, buffer solutions. There is probably
300
r2
35X
-"
200a
-z
0®
4
100
04
* *e
a
0
0
10
20
Exchangeable K (mgKIlOOg)
14 % cloy
* 17-20 % clay
* 36-38 % clay
30
40
Fig. 2. Relationship between exchangeable K and K uptake of broad beans (same experiment
as in Fig. I).
102
no feasible variation which has not been tried, but none has been proved to give
unambiguous and universally applicable results. Figure 2 demonstrates the poor
relationship between exchangeable K and K uptake, if soils of different clay contents
are included (14-38%). However, this is to be expected since, while exchangeable K
represents that fraction adsorbed on external and accessible internal surfaces, it is
not directly related to the K flux towards the roots (Grimne et al. [1971], Nye [1972]).
It constitutes only a quantity measurement. It is mainly the clay content and the
clay mineralogy which modifies the availability of exchangeable K (Grimme et al.
[1 9 71a, h], Grinme [1976], McLean [1978]) so that the relation between exchangeable K and K uptake is very much improved if exchangeable K is expressed as a
fraction of C.E.C. In that case a relationship like that in Figure 1 is obtained. Some
authors use the total exchange capacity as reference (Black [1965], McLean [1978]
others merely clay content (von Braunschweig [1965], Qubmener [1976]). In the
Netherlands the so called K-value is employed, which is the 0.1 N HCI-extractable K
multiplied by a correction factor taking account of the effects of pH, organic matter
and the fraction < 16 t (van Diest [1978]).
3.4 Non-exchangeable K
There is no question that plants are able to take up more K from a soil than the
exchangeable complement (Schachischabel [1937]). The 'available' non-exchangeable reserves are usually extracted with I N strong acids (De Turk et al. [1943],
Schachtschabel [1961], Haylock [1956]) and give a measure of the long-term K
supplying power of soils. Electrodialysis, exchange resins and Na-tetraphenylborate have also been used (Reiteneier et al. [1946], Quinener [1976]). One has to
bear in mind, however, that it is only when growth rates are low - i.e. low yields - that
non-exchangeable K can be considered a useful K source. In many cases yields are
reduced, if a large proportion of the K requirement has to be covered by non-exchangeable K, because the release rate is too low to meet the K demand of a vigorously
growing crop (Grimme, 1974).
3.5 Electro-ultrafiltration (EUF)
To the knowledge of the authors the EUF method of soil analysisis the only method
in practical use which takes dynamic aspects of nutrient supply into account. It
allows the determination of intensity, quantity and buffering parameters (N~meth
[1976]). It also allows an assessment of the change of mobile soil K with time
(Figure 3).
The EUF procedure makes use of nutrient desorption in an electric field. A soil
suspension is subjected to an electric field which causes ions to move out of the
suspension. This initiates a desorption process which continues as long as the electric
field acts on the soil. The quantities removed are plotted as a function of time. Either
constant or variable field strengths are used. Both procedures have their advantages.
Variable field strength (Figure 3) is usually employed in routine work and from the
results of a large number of field experiments a system of recommendations for
extension work has been developed (Table 1) (N mneth [1978]).
103
(pH 6,3
(pH 5.0
(pH 6.8
Oxisol
.----
Alfisol
Alfisol
10
26,2 mg exchangeable K )
24.1 mg exchangeable K)
23.8 mg exchangeable K)
( b)
(a)
1'
70
.8'
""/
_ 30
.
01
IA
30I
b
S1
0
05
75
25
0
35
Ist
2nd
3rd
4th cut
Desorption time in minutes (t
50V
200V
400V
Fig. 3. Comparison of EUF desorption characteristics of 3 different soils (2) and the decrease
of rye-grass yield with time (b). The soils contain similar amounts of exchangeable K but
differ in K selectivity and buffering power. The yield curves are parallel to the desorption
curves.
Table 1.Quantity of K required to raise the K-EUF values to 15 mg/l100 g of soil as a function
of initial K-EUF value and clay content. In field experiments 15 mg100 g was found to give
optimum K supply for high yielding sugar beet (- 54 to/ha).
K application (kg K/ha)
KEUF
(mg/100 g/35 min) 10
10-20
clay content (%)
20-30
30-40
1
2
3
4
5
6
7
8
9
10
600
560
480
420
370
330
270
240
210
190
1200
1050
900
800
700
600
450
300
250
200
1600
1300
1100
900
800
700
550
400
300
250
3000
1800
1400
1100
900
800
650
500
350
300
11
12
13
14
15
150
120
90
60
0
150
120
90
60
0
150
120
90
60
0
200
150
90
60
0
104
3.6 Biological methods
One would expect plants to be efficient in measuring available soil K. In fact, field
trials are always needed to calibrate chemical methods, but for the purpose of extension
services they are far too cumbersome. Tests with small soil samples such as the
Neubauer and the Stanford-de Ment methods (Schachtschabel [1937], de Mert et al.
[1959], QukMener [1976]) are still very tedious and too time consuming for practical
purposes. But the most important drawback is the fact that, because of the high
plant density employed, they impose a much higher stress on the soil K reserves
than would a normal crop and thus overestimate the supply rate under field conditions.
4. Conclusions
The determination of the K status of soils is usually carried out by extracting the
soils with salt solutions, weak or strong acids, or buffers of acids and their salts.
Depending on their composition these solutions extract the loosely bound K and a
variable proportion of more tightly bound K from external and internal surfaces
of the soil matrix and the amount extracted more or less approximates with exchangeable K. Exchangeable K is a measure of the quantity which can be relatively easily
mobilised as compared to the non-exchangeable K. But no information is obtained
as to the rate at which it is mobilized, thus no true evaluation of availability can be
made. There are instances, where exchangeable K may even be misleading (Grinule
[1976]).
Since the factors affecting availability are known it is possible to select methods to
provide the information wanted, and group them according to the parameters they
measure. One has to distinguish between intensity, quantity and capacity, which give
information on the immediate availability, the mobile reserves and the storage
capacity, respectively.
The availability is mainly governed by the rate of transport towards the roots. But
diffusion measurements are difficult and time consuming. It is, therefore, expedient
to measure a parameter that is closely related to diffusion. This would be the K
concentration in the soil solution, or the K saturation of the inorganic CEC, that is,
the quantity/capacity ratio. For routine purposes, extraction with 0.025 N CaCd2
appears to be a promising method (Schachtschabel and Heineinann [1974], Grinne
and N6,neth [1976]). This method takes into account the clay content of soils and
extracts a decreasing proportion of the exchangeable K with increasing clay content.
The results are closely correlated with K concentration and K diffusion. This method
is essentially an intensity method.
From a knowledge of K concentration alone, it is not possible to know whether the
reserves of exchangeable K are large enough to sustain an adequate concentration
for the duration of the growing period. This requires a knowledge of the quantity of
exchangeable K and if possible the buffering capacity. The buffering capacity is
given by the slope of the adsorption isotherm, and is a useful parameter when adjusting
the fertilizer dose. K concentration in the soil solution, or for that matter the degree of
K saturation, does not provide the information needed to estimate the fertiliser
requirement, since the change of K concentration with increasing K content of the
soil depends on the inorganic CEC.
The correct and complete evaluation of the K status of soils and, hence, the correct
105
forecasting of K fertiliser need may seem to be somewhat complicated. However,
once the desired level of K in a soil has been achieved, one of the conventional methods
is quite adequate for monitoring soil K and thus checking the effects of fertiliser
policy since it will certainly indicate whether soil K is declining, increasing or being
maintained at a constant level.
Because there is a correlation between K concentration and the degree of K saturation,
it is quite acceptable to use exchangeable K values, modified for clay content, when
assessing the K status for routine advisory purposes. With these two parameters it is
possible to estimate the degree of availability and to estimate the K fertiliser requirement. Yet, we should always remember that the nutrient supply to plants depends upon
dynamic processes in the soil and that the routine analytical methods now in use
reflect only a static situation and therefore give only a partial and approximate
measure of true K availability. Soil testing is essential for efficient fertiliser use. The
usefulness of soil testing rests on the correct choice of method.
The term 'availability' describes an obviously complex situation and must not be
confused with the term 'available quantity' as is often done. Quite often, the quantity
extracted by some extractant is termed available K. In the last analysis, all the potassium in a soil is available, not just the exchangeable K but also the nonexchangeable K.
Only the degree of availability differs. For a plant it is not so much the quantity
present which counts but the rate at which the required quantity is supplied. It is,
therefore, not surprising that often there is no correlation between soil test results and
response because the method chosen determined a 'quantity' parameter, without
supplying information on the degree of availability. Actually, both parameters are
needed, because it is necessary to know both whether the rate at which a nutrient is
supplied to the roots is adequate to keep up with the growth rate of the plant and
whether the quantity of nutrient present is sufficient to meet the plant's demand. As
far as potassium is concerned it is also necessary to know the shape of the buffer curve
in order to be able to calculate correctly the quantity of fertiliser needed. This amounts
to taking into account clay content and mineralogical composition of the clay fraction.
Knowing that a number of factors influence K availability and that plant demand
depends on the crop grown and the growing conditions it is evident that there is not
one optimum K level. The optimum K level varies with crop, yield level and weather
conditions. A high yielding crop having a root system with a low surface area requires
a higher supply rate than a low yielding crop with an extensive root system. In a dry
soil, a higher K concentration is required to maintain an adequate supply than is
needed in a wet soil. There is therefore a choice either to recommend a K dressing
sufficiently large to ensure that crop growth will not be limited by K supply even under
adverse growing conditions or a more modest rate that will be near optimum for
average conditions but below optimum if conditions are adverse. The latter recommendation carries a risk that yield will be sacrified. The soil nutrient level that can
be considered adequate will depend on the agricultural system, the overall productivity
and the prevailing economic conditions.
5. Summary
Following a brief account of the history of soil testing, various methods are discussed.
There is no detailed description of methodology, the discussion centring on the
evaluation of information obtained by measuring the different fractions of soil K
106
(K in the soil solution, easily extractable K, exchangeable K, non exchangeable K
E UF-K). The significance of these fractions with respect to plant growth is discussed
and a distinction is made between available K and the availability of soil K.
6. References
Ahnad, N., Cornforth, L S. and Walmsley, D.: Methods for measuring available nutrients
in West Indian soils. Il. Potassium. Plant and Soil 39, 635-647 (1973)
Barber, S.A.: A diffusion and mass flow concept of soil nutrient availability. Soil Sci. 39-42
(1962)
Barber, S.A., Walker, J.M. and Vasey, E. H.: Mechanisms for the movement of plant nutrients
from the soil and fertilizer to the plant root. J. Agric. Food Chem. 11, 217-229 (1963)
Beckett, P. H. T.: Studies on soil potassium. 1. Confirmation of the ratio law: measurement
of potassium potential. 11. The immediate Q/1 relations of labile potassium in the soil.
J. Soil Sci. 15, 1-23 (1964)
Beckett. P.H.T.: Potassium potentials. Ministry of Agric. Fish. Food. Tech. Bull. No. 14, 32-47
(1967)
Black, C.A.: Soil-plant relationships. Willy and Sons, 2nd ed., 1968
Bray, R.H.: A nutrient mobility concept of soil nutrient availability. Soil Sci. 93, 39-42 (1954)
Cooke, G. W.: Fertilizing for maximum yield. Crosby Lockwood, London, 1972
Daubeny, C.G.B.: On the distinction between the dormant and active ingredients of the soil.
J.R. agric. Soc. Eng. 7, 237-245 (1846)
DeMent, J.D., Stanford, G. and Bradford, B.N.: A method for measuring short term nutrient
absorption by plants: 11. Potassium. Soil Sci. Soc. Amer., Proc. 23, 47-50 (1959)
DeTurk, E.E., Wood, L.K. and Bray, R.H.: Potash fixation in corn belt soils. Soil Sci. 55,
1-12 (1943)
Devaux, H.: Action rapide des solutions salines sur les plantes vivantes: ddplacement r6versible d'une partie des substances basiques contenues dans ]a plante. C.R. Academie Sci.,
Paris 162, 561-563 (1916)
Diest, A. van: Factors affecting75the availability of potassium in soils. Proc. l1th Congr.
Intern. Potash Inst., Bern, p. (1978)
Dyer, B.: On the analytical determination of probably available mineral plant food in soil.
J. Chem. Soc. 65, 115-167 (1894)
Dyer, G.: A chemical study of the phosphoric acid and potash contents of wheat soils of
Broadbalk, Rothamsted. Phil. Trans. R. Soc. B. 194, 235-290 (1901)
Egner, H., Riehm, H. and Domingo, W. R.: Untersuchungen fiber die chemische Bodenanalyse als Grundlage fOr die Beurteilung des Nihrstoffzustandes. Kungl. Lantbruksh6gsk Ann. 26, 199-215 (1960)
Grimme, H,: Potassium release in relation to crop production. Proc. 10th Congr. Int. Potash
Inst., Bern, 131-136 (1974)
Grimme, H.: Factors of potassium availability. Ind. Soil Sci. Soc., Bull. 10, 144-163 (1976)
Grinme, H., Nmeth, K. and von Braunschweig, L.C.: Beziehungen zwischen dem Verhalten
des Kaliums im Boden und der Kaliumernahrung der Pflanze. Landw. Forsch. Sonderh.
26/1, 165-176 (1971a)
Grimme, H. and Ntneth, K.: Einfluss einer Diingung auf den Diffusionsfiuss nicht gedaingter
Kationen. Z. Pflanzenernihr., Bodenk. 138, 253-261 (1975)
Grimme, H. and Nt'meth, K.: Bietet die CaCI,-Methode Vorteile bei der Bestimmung der
K-Versorgung von B6den? Landw. Forsch. 29, 1-12 (1976)
Grimme, H. and Ndmeth, K.: Beziehungen zwischen K (CaCIl)-Gehalten im Boden und dem
Ertrag im GefAss-Kleinparzellen- und Feldversuch. Landw. Forsch. 29, 13-20 (1976)
Haylock, O.F.: A method for estimating the availability of non-exchangeable potassium.
VIth Intern. Congr. Soil Sci. II, 402-408 (1956)
Jankovic, M. and Nemeth, K.: The effect of K dynamics on yield. Proc. 10th Congr. Int.
Potash Inst., Bern (1974)
Jenny, H. and Overstreet, R.: Cation interchange between plant roots and soil colloids.
Soil Sci. 47, 257-272 (1939)
Kigen, P. and Diehl, R.: Uber die Anwendung der Dialyse und Elektro-Ultrafiltration zur
Bestimmung des Nafhrstoffbedfirfnisses des Bodens. Z. Pflanzenernihr., Dring. u. Bodenk.
A 12, 65-105 (1929)
107
Lagerwerff, J. V.: The contact exchange theory amended. Plant and Soil 13, 253-264 (1960)
Lunt et al.: The Morgan soil testing system. Com. Agri. Exp. Sta. Bull., 541 (1950)
McLean, E.O.: Influence of clay content and clay composition on potassium availability.
In: Potassium in Soils and Crops, Potash Res. Inst. India, p. 1-19 (1978)
Mattson, S.: Electrodialysis of the colloidal soil material and the exchangeable bases. J.
agric. Res. 35, 553-566 (1926)
Mengel, K.: Untersuchungen fiber das Kalium-Calciumpotential. Z. Pflanzenernaihr. Dflng.
Bodenk. 103, 99-111 (1963)
Nemeth, K.: Die Charakterisierung des K-Haushaltes von Bden mittels K-Desorptionskurven. Geoderma 5, 99-101 (1971a)
Ntmeth, K.: Miglichkeiten zur Bestimmung massgeblicher Faktoren der Bodenfruchtbarkeit
mittels Elektro-Ultrafiltration (EUF). Landw. Forsch. Sonderh. 26/1, 192-198 (1971a)
Nneth, K.: The determination of effective and potential availability of nutrients in the soil
by electro-ultrafiltration. Appl. Sci. Develop. 8, 89-111 (1976)
Nimeth, K.: Limitations of present soil test interpretation for K and suggestions for modifications. A European experience. In: Potassium in Soils and Crops. Symp. Potash Res.
Inst. Ind. Counc. Agric. Res., p. 96-111 (1978)
Nemeth, K. and Grinune, H.: Einfluss einer Diingung auf die Aufnahme nicht gedcingter
NAhrstoffe im Geftissversuch. Z. Pflanzenernihr. Dung. Bodenk. 137, 203-213 (1974)
Nemeth, K. and Harrach, T.: Interpretation der chemischen Bodenuntersuchung bei Ltssbflden verschiedenen Erosionsgrades. Landw. Forsch., Sonderh. 30/1, 131-137 (1974)
NSmneth, K. and Forster, H.: Beziehung zwischen Ertrag und K-Entzug von Ackerbohnen
sowie verschiedenen K-Fraktionen von Bden. Die Bodenkultur 27, 111-119 (1976)
Nye, P.H.: Localized movement of potassium ions in soils. Proc. 9th Colloq. Intern. Potash
Inst., 147-155 (1972)
Olsen, R.A. and Peech, M.: The significance of the suspension effect in the uptake of cations
by plants from soil water systems. Soil Sci. Soc. Amer. Proc. 24, 257-261 (1960)
Opitz, K.: Vergleichende Untersuchungen Ober die Ergebnisse von chemischen Bodenanalysen und Vegetationsversuchen. Landw. Jahrb. 36, Berlin (1907)
Qtnener,J.: Analyse du potassium dans les sols. Dossier K 20, No. 1, SCPA Mulhouse 1976
and IPI-Research Topics No. 4 (in English), 1978
Reitemeier, R.F.: Release of non-exchangeable potassium by greenhouse, Neubauer and
laboratory methods. Soil Sci. Soc. Amer. Proc. 12, 158-162 (1947)
Saussure, Th. de: Recherches chimiques sur ]a vdgdtation. Paris, Masson, 1804
Schloesing, M. Th.: Sur I'analyse des principes solubles de ]a terre vdgdtale. Compt. Rend.
63, 1007-1012, 1866
Schachtschabel, P.: Aufnahme von nicht austauschbarem Kalium durch die Pflanzen. Bodenk.
u.Pflanzenern. 3,107-133 (1937)
Schachtschabel, P.: Fixierung und Nachlieferung von Kalium- und Ammoniumionen.
Beurteilung und Bestimmung des Kaliumversorgungsgrades von B6den. Landw. Forsch.
Sonderh. 15, 29-47 (1961)
Schachischabel, P. and Heineinann, C.G.: Beziehungen zwischen den Kaliumgehalten in
Boden und in jungen Haferpflanzen. Z. Pflanzenernahr. Bodenk. 137, 123-134 (1974)
Scheffer-Schachtschabel: Lehrbuch der Bodenkunde, Stuttgart, 1976
Schroeder, D.: Structure and weathering of potassium containing minerals. Proc. 1 th Congr.
Intern. Potash Inst. Bern (1979)
Way, L T.: On the power of soils to absorb manure. J. Roy. agric. Soc. Eng. II, 317-379
(1850) and ibid. 13, 123-143 (1852)
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Dept. Agr. Bur. Soils Bull. 22 (1903)
Wild, A., Rowell, D. L. and Ogunfowora, M.A.: The activity ratio as a measure of the intensity
factor in potassium supply to plants. Soil Sci. 108, 432-439 (1969)
Wilson, B.D.: Exchangeable cations in soils as determined by means of normal ammonium
chloride and electrodialysis. Soil Sci. 26, 407-421 (1928)
Woodward, J.M.D.: Some thoughts and experiments concerning vegetation. Phil. Trans.
Roy. Soc. 21, 382-398 (1699)
Wrangel, N. von: Die Bestimmung der pflanzenzugnglichen Nhrstoffe des Bodens. Landw.
Jahrbflcher 67, 149-169 (1930)
108
25th Anniversary of the Scientific Board
11th IPI-Congress held in Bern/Switzerland
from September 4 to 8, 1978
3rd Session
The Role of Potassium
in Yield Formation
109
Potassium Transport through Plant
Cell Membranes and Metabolic Role
of Potassium in Plants
A. Lauchli*, Department of Land, Air and Water Resources, University of California, Davis,
CA/USA, and,
R. Pfliger**, Botanisches Institut, Tierarztliche Hochschule, Hannover/Federal Republic
of Germany
1. Introduction
Potassium is essential for all plants. The roots of higher plants absorb potassium
from the soil as K+ and this is translocated to all the tissues and organs of the plant.
Thus K + becomes available at the sites of physiological and biochemical activity
in the plant. Less is known of the extremely complex metabolic role of potassium
in plants than of the roles of other essential elements such as nitrogen, phosphorus
and iron. This review considers four important aspects:
(i) The efficient, easy and selective transport of K+ through cell membranes.
(ii) The function of K + in osmoregulatory processes, at least in the salt-sensitive
glycophytes, to which most crop species belong.
(iii) The function of K + in photosynthesis and respiration.
(iv) Its involvement as activator in the function of many enzymes.
2. Uptake of potassium by cells of higher plants
To enter the cell K+ has to pass two main barriers. The plasmalemma, the outer
plasma membrane, separates the free space of the cell wall from the cytoplasmic phase
which contains all the important cell organelles (nucleus, mitochondria, chloroplasts and others). The tonoplast is the membrane between the cytoplasm and the
vacuole which forms the inner compartment of most living plant cells. It has long
been known that these plant cell membranes have low permeability to K + (Epstein
[1972]). Nevertheless, most membranes probably transport K+ efficiently. Although
the exact mechanism of K+ transport through cell membranes of higher plants is
not known, several features of this transport can be described and possible models
of K + transport can be suggested.
It became evident in the 1930's and 1940's that K + transport is selective against
Na+ and energy-dependent, leading to significant K+ accumulation in the plant with
K+ concentrations within cells far higher than in the external medium. Basic evidence
*
Professor Dr. A. Liuchli, Department of Land, Air and Water Resources, University of
California, Davis, CA 95616/U.S.A.
Dr. R. Pflfiger, Botanisches Institut der Tierratlichen Hochschule, BOnteweg 17d,
D-3000 Hannover 71/Federal Republic of Germany
Ill
on the energy requirement of ion transport through plant cell membranes and on
ion accumulation in plant cells has been obtained independently in several laboratories using such diverse plant materials as fresh water algae and roots and storage
tissues of higher plants (Lundegdrdh and Burstrdm [1933]; Steward [1933]; Hoagland and Broyer [1936]; Hoagland [1944]; Robertson and Turner [1945]).
2+
2.1 Effect of Ca
In 1944 Viets discovered that the rate of K+ transport into root cells is enhanced
by polyvalent cations, in particular by Ca 2 +. This so-called 'Viets effect' has since
been confirmed many times. Undoubtedly, Cal + is essential for the integrity and
function of plant cell membranes (Epstein [1972]). However, its mode of action
on membrane transport is debated.
Around 1960 the importance of CaZ+ for unimpaired ion transport was established
in several laboratories (Hanson [1960]; Jacobson et al. [1960]; Epstein [1961]).
The rate of absorption of Rb+ (used as a tracer analog of K+) by barley roots declined rapidly in the absence of Ca 2+ (Epstein [1961]). Maize roots showed a loss
of K + to the solution (Maas and Leggett [1968]), but no Ca2+ was included in the
experimental solution. As is shown in Figure 1, this net loss of K+ does not occur
6
86Rb
K
W
._j
66R
BaRb
C'Ca
CS
+C
m
0
I
-n
1.0
005
2 0
.5
HOURS
2+
Fig. 1. Effect of Ca
[1970])
112
+
on absorption of K and
Rb by excised corn roots (LMuchli and Epstein
6
in the presence of Ca 2 + (Liiuchli and Epstein [1970]). Although Ca2 + generally
appears to stimulate K + uptake by root cells, inhibition of K + uptake by Ca 2+ was
observed in some instances (e.g. Elzamn and Hodges [1967]; Sze and Hodges [1977]).
These differences in Ca2 + effects may reflect differential behaviour of various plant
species.
Epstein [1972] thought the role of Ca 2± in membrane function was to minimise ion
diffusion and to maintain selective ion transport mechanisms (e.g. K+/Na+ selectivity). Other investigators supposed the role of Ca 2 + to be mainly to decrease membrane permeability. H+ increased membrane permeability to K+; this H+ effect was
alleviated by Cal+ (Marschner et al. [1966]; Marschner and Mengel [1966]).
Following the findings that removal of Ca 2 + from the membranes induced leakage
of ions (Van Steveninck [1965]), Mengel and Helal [1967] suggested that Ca 2 +
decreased ion efflux and thus increased net uptake of ions. A possible direct involvement of Ca 2 + in the K+ transport mechanism requires further study.
The maintenance of structural integrity of the membranes by Ca 2 + appears to be
due to Ca 2+ being bound to some membrane components. There may be Ca 2 +
bridges between proteins and phospholipids (Mengel and Kirkby [1978]), or CaZ +
may compete with H+ for a lipophilic anionic carrier in the membrane (Robertson
and Boardman [1975]). The significance for ion transport of the phospholipid polar
groups in membranes was pointed out recently by Robertson and Thompson [1977].
In fact, Ca 2 + interacts with phosphatidylserine in vitro to form a tightly-bound
complex (Hauser et al. [1977]).
2.2 Kinetics of transport and carrier concept
What possible mechanisms are there for K+ to be transported through plant cell
membranes exhibiting low permeability to K+? Van den Honert [1937] first introduced the carrier concept. In experiments on phosphate absorption by sugar cane,
the rate of absorption as a function of external concentration closely fitted a saturation curve. The author drew an analogy between the operation of the phosphate
transport mechanism and a rotating mechanical conveyor. Epstein and Hagen
[1952], studying the kinetics of absorption of K + and other alkali cations by barley
roots, linked the transport process to an enzymic reaction. Formally, MichaelisMenten kinetics could be applied to ion transport through membranes and a selective
ion carrier was postulated in analogy to a substrate-binding enzyme. This carrier
would form a carrier-ion complex and mediate the transport of the bound ion from
one side of the membrane to the other.
Detailed investigations by Epstein et al. [1963] on the kinetics of K + uptake by
barley roots over a wide range of external K + concentrations showed that K + uptake
followed Michaelis-Menten kinetics in the low concentration range(< I mM), indicating a carrier-type mechanism I (Figure 2). This carrier is undoubtedly located at
the plasmalemma of plant cells. As the K+ concentration is raised above I mM, the
rate of K+ uptake increases again, which suggests a second mechanism coming into
play (Figure 2). For agricultural situations only the carrier-type mechanism I is
relevant, as it appears that the concentration of K+ in soil solutions lies mostly
within the low concentration range (Figure 3). The carrier concept was criticised,
because these experiments were not done under sterile conditions and micro-organisms
113
25
0
K
20
00
310
5
0.50
0'20
0"10
10
50
25
K (mmolar)
+
Fig.2. Rate of K + absorption by excised barley roots as a function of K concentration in the
medium (Epstein [1966])
120
d o)
!510
LU
. .4.
Oh.
1
2
.. -.-.-. . .. -....
3
........
: . ..
4
. ... ..
5
K in so solution (nrM)
Fig.3. Frequency distribution of K + concentration in soil solutions. Data compiled from 155
soils (redrawn from Asher [1978]; based on data from Reisenauer [1966])
could have contributed substantially to the apparent ion absorption by the roots
(Barber and Frankenburg [1971]). But in a study by Epstein [1968], the roots were
+
in fact grown under sterile conditions and the rates of K absorption as a function
+
to that in Figure 2. The
similar
pattern
a
dual
of external K concentration showed
concept of carrier-mediated K+ transport through plant cell membranes is also
supported by the finding that marine algae in comparison with sea water do not
fractionate the K-isotopes 39 K and 4 K (Wagener el al. [1967]).
Strong support for the carrier concept of ion transport through plant cell membranes
comes from evidence that membrane transport is under genetic control, and specific,
inducible transport systems are supposed which would involve new synthesis or
114
activation of proteins or enzymes (Epstein [1972]: Lduchli [1976b]). Assuming
that the postulated ion carriers consist of some kind of polypeptides (Epstein [1972];
Mengel [1977]), their synthesis must also be controlled genetically. As regards K +
binding carriers, little genetic information is yet available to indicate their existence.
Mutants of Phaseolus vulgaris, which differ in the efficiency of utilising K+, did not
show consistent differences in K + uptake and, hence, cannot be considered transport
mutants (Shea et al. [1967, 1968]). Equally unresolved is the reason for inefficiency
in K + utilisation in a tomato mutant which was produced by Epstein [1978] by
treating seed with a chemical mutagen. Excised roots of Capsicum annutm scarbrous
diminutive, a wilty mutant of pepper, absorbed less Rb + (K +) than those of the normal
genotype, particularly in the low concentration range of mechanism I (Benzioni
and Tal [1978]). This indicates that the K+-selective carrier system is impaired in
the mutant roots, although the root cell membranes of this pepper mutant showed a
higher permeability to K+, too. Nonetheless, such mineral nutrition mutants offer
new research possibilities in the search for postulated ion carriers in membranes.
In the last few years research on carrier-mediated K+ transport through plant cell
membranes was directed toward regulation of transport (Cram [1976]). It has
often been observed that during K + accumulation in root cells K+ uptake declines
due to increasing internal K+ content (Cseh et al. [1970]; Johansen el al. [1970];
Pitman et al. [1971a]; Leigh and Wyn Jones [1973]; Glass [1977]). Cram [1976]
postulated that this effect is due to negative feedback regulation of K + influx. In
experiments on K + influx by barley roots during transfer of high-salt roots to lowsalt conditions, the disappearance of the influx system was measured (Glass [1975]).
It was proposed that the observed negative feedback inhibition by the absorbed K +
was either mediated by repression of carrier synthesis or by allosteric inhibition of
influx with the internal K + [Ki + ] serving as the allosteric effector (Glass [1975]).
In a subsequent study Glass [1976] observed that the Michaelis constant Km increases
with increasing [K + ] and K + influx is sigmoidally related to [Ki+]. This was taken
as support for an allosteric model of regulation (Figure 4). According to this model,
the carrier might possess a single binding site for external K+ and four allosteric
binding sites for [Ki+]. Saturation of these allosteric binding sites at high [Ki + ] may
inhibit K + influx by causing conformational changes in the carrier which reduce
the K + affinity of the external binding site. This model, which was developed from
experiments with excised roots, may also be operative in roots of intact barley plants
(Glass [1978]). Jensen and Petersson [1978] also suggested an allosteric model
for regulation of K+ uptake by plant roots. On the other hand, Bange [1978] envisioned a tetrameric transport protein, similar to that proposed by Stein et al.
[1973], with no special feedback regulation operating.
2.3 K+/Na + selectivity
The phenomenon of K+/Na + selectivity in alkali cation uptake by plant cells has
been much studied. Two transport components probably contribute to this selectivity.
One is selective K+ influx at the plasmalemma in the low K+ concentration range,
even in the presence of excess Na+ in the external medium (Figure 5). According to
Epstein et al. [1963] this is brought about by selective K + transport through the
membrane via the carrier of mechanism 1. However, there is little selectivity for
115
LOW SALT ROOTS
K"
K'
K'
K"
inside
membrane
outside
K
K'
K
K"
K'
•
K
K'
K
K"
HIGH SALT ROOTS
K"
K"
K'
K'
°
K'
K'
K
K'
K"
K'
K
*
K'
K'
K"
K'
+
Fig.4. Model of allosteric control system for carrier-mediated K influx in barley root cell
(Glass [1976])
0
E 10.0
E
-E 5.0
I
0.05
I
0.10
K, mM
I
0.15
0.20
of K concentration in the
Fig.5. Rate, v, of absorption of K+ by barley roots as a function
+
solution, and the effect of Nat. o control; & +0.5 mM Na (Epstein [1972])
116
transport of K+ over Na+ in the higher range of concentrations (Epstein et al.
[1963]; Rains and Epstein [1967 a, b]).
The second transport component involved in K+/Na+ selectivity is Na+ efflux from
the cells back into the medium. Pitman and Saddler [1967] estimated fluxes of K +
and Na+ for barley roots and suggested the operation of an active Na+ efflux pump
at the plasmalemma. Mengel and Pflger [1972] considered the efflux of Na + from
maize roots to be passive, because it was not influenced by different metabolic conditions. In barley roots Na+ efflux is stimulated by K+ in the external medium
(Jeschke [1970, 1973]; Jeschke and Steher [1973]); this is demonstrated in Figure 6.
The K + stimulated Na + efflux is inhibited by metabolic inhibitors (Jeschke [1974]).
The high K+/Na+ ratio in the cytoplasm of barley root cells may be the consequence
of a K + stimulated Na+ efflux pump at the plasmalemma and also may be mediated
by a Na+/K+ exchange across the tonoplast leading to Na - sequestration in the
vacuole (Jeschke and Stelter [1976]; Jeschke [1977]). Accumulation of Na + in the
No*
ImMl
A
1,500
IS
"fNo*I
1000
*•.
ImMiK
InI
Na'
S0*
.I
Zn
NN
15
th]7
1'
2
3
Fig.6. K- stimulated Na- efflux from excised barley roots.
A. Nal concentration in the medium after frequent changes ()and without change ()
B. Net efflux of Na (O) and accumulation of K, in the roots (a) (Jeschke and Sterter [1973])
117
vacuoles is probably most important in halophytes (Jennings [1968]; Flowers et a.
[1977]) and may also be significant in some of the relatively salt-tolerant crops
such as sugar beet.
2.4 Membrane ATP-ase and K+ transport
Most of the early hypotheses on the energy requirement of ion transport through
plant cell membranes linked transport to respiration, either directly to respiratory
electron flow (reviewed e.g. by Lfittge [1973] or to charge separation yielding H +
on one side of the membrane and OH- on the other (Robertson [1968]). Alternatively, ion transport through membranes may be powered by ATP. The highenergy intermediate metabolite ATP is synthesised in mitochondria by oxidative
phosphorylation and in green cells also in the chloroplasts by means of photosynthetic
phosphorylation. Since ATP is exported from these organelles, ion transport at the
plasmalemma (and other cell membranes) could readily be driven by an ATP-dependent mechanism (Epstein [1972]). Experiments by Robertson et al. [1951], in
which ion uptake by carrot discs was shown to be inhibited by the uncoupler DNP,
can be interpreted as evidence that ATP from oxidative phosphorylation may power
ion transport. Nowadays there is overwhelming evidence for ATP being the primary
energy source for ion transport. As regards ATP-dependent K+ transport, a pertinent
early reference is by Higinbothan [1959]. In non-photosynthetic cells the mitochondria are undoubtedly the source of ATP, while in green cells transport may be
driven by ATP synthesised by photosynthesis or alternatively by aerobic respiration
(Johanson and LUttge [1974j; Littge and Pitnan [1976]).
Recent experimental evidence suggests, though it does not prove, that an ATP-ase
bound to the plasmalemma may represent the energy-transducing agent between
ATP and K + transport (review: Hodges [1976]). The first experimental support
(Fisher et al. [1970]) came from a correlation between the absorption of K+ or
Rb+ by roots of barley, oats, wheat and maize and the K+ - or Rb+-stimulated ATPase activity of membrane preparations from these roots (Figure 7). One membranebound ATP-ase from oat roots appears to be located at the plasmalemma, as indicated from studies on isolated plasmalemma vesicles (Hodges et al. [1972]). This
enzyme shows a pH optimum of 6.5 to 7.0, has a requirement for Mg 2+ and its
activity is further stimulated by monovalent cations, particularly by K + or Rb +
(Hodges [1976]). From the work of Leonard and Hodges [1973] it is obvious that
the kinetics of K+-stimulation of the ATP-ase and K + absorption by the root are
similar (Figure 8). The complex nature of the kinetics may be explained in terms of
an enzyme that exhibits negative cooperativity, suggesting that the enzyme consists
of subunits which interact during cation binding and undergo conformational
changes (Hodges [1973]). Furthermore, the specificity of the plasmalemma ATPase toward the alkali cations is similar to the specificity of alkali cation transport
into the cells of oat roots (Sze and Hodges [1977]). Similar results were obtained
with maize roots (Leonard and Hotchkiss [1976]). The location of ATP-ase activity
at the plasmalemma of root cells has also been demonstrated by cytochemical techniques (Hall [1969, 1971, 1973]; Malone et al. [1978]; Winter-Sluiter et al. [1978]).
Such evidence supports the concept that the ATP-ase located at the plasmalemma
of the root cells is involved in K+ transport across this membrane.
118
'30.
30
INFLUX
BARLEY
0
OAT
S20
z.X
WHEAT
10
30
OATS
20.
EWHEAT
t=o.
30
S--CORN
bA
20
30
40
50O
OASo
'€,-20
Rb* or le CONCENTRATION (rM)
EAIfT
OT+ and K + or Rb + stimulated ATP-ase
K + it
or Rb
Fig. 7. Correlation between absorptionWHof
activity of excised roots of corn, wheat, oat, and barley (r =0.94) (Fisher et aa/[1970])
U-
30
20
0
0
302
0
50
05
IN (mM)
Rb'or iCOCNR
veorion
abs
oersus
veoritRbtrante
oreRbratinlate A-tPuated
Fig.7.
beieHftween
+
exiser et roo[s197
od]e
o
ATP-ae
ofplarootsm ofcrm wat, ot, and b abrley (r094
[197; opadapte
from
m
onadat
rodots an[197
orto3b]xcsd)a
oos(Hde
119
The plasmalemma-bound ATP-ase described by Hodges [1976] is clearly different
from that found in animal cell membranes. Theanimal membrane ATP-ase is closely
related to K+/Na + exchange, is synergistically stimulated by K + and Na + and inhibited by the glycoside ouabain. The inhibitor ouabain seems to have little effect
on plant membrane ATP-ases (Hodges [1976]). There have been some reports of
(Na + + K+) stimulation of plant membrane ATP-ases. In sugar beet such an enzyme
was related to salt tolerance and transport of Na+ and K+ (Hansson and Kylin
[1969]; Kylin and Hansson [1971]; Karlsson and Kylin [1974]). However, most
other investigators were unable to detect a synergistic effect of Na± and K + on
membrane ATP-ase activity. From inhibitor experiments, Jeschke [1974] suggested
that ATP-ase participated in K+/Na + exchange at the plasmalemma of barley root
cells. Although Ratner and Jacoby [1973] concluded that ATP-ases from the roots
of various grasses were not cation-specific and not related to cation absorption,
the weight of evidence favours the involvement of a membrane ATP-ase in cation
transport (ef. Bowling [1976]; Hodges [1976]; Nissen [1977]).
2.5 Active or passive K+ transport through cell membranes of higher plants
There has been considerable controversy as to whether K± transport across plant
cell membranes is thermodynamically active or passive. Measurements of the electrical potential across the plasmalemma and of the permeability coefficient for K+
permit calculation of the passive component of K + flux. This approach, as applied
to cell membranes of higher plants, has recently been outlined in detail, e.g. by
Higinbotham [1973], Liittge [1973] and Clarkson [1974]. The membrane potential
at the plasmalemma is of the order of -00 to -200 mV (inside negative); at the
tonoplast it is difficult to measure but seems likely to be small (LUittge and Zirke
[1974]; Pitman [1976]; Fischer et al. [1976]). The magnitude of the membrane
potential decreases with increasing external KCI concentration (Pitinan et al. [1971b];
Mertz and Higinbothan [1974]), indicating that membrane permeability to K + is
greater than that to CG-.
Some estimates of permeability coefficients are available for cells of roots (Table 1).
The values for K+ are about 10- 9 to 10- 0 m s - 1 and are slightly smaller for Na+ and
Cl-. Pitman [1976] combined these estimates of permeability coefficients with
measurements of membrane potentials and calculated the passive component of K+
fluxes in relation to Epstein's dual mechanisms of ion uptake (cf 2.2). He concluded,
and there is general agreement with this, that K+ transport through membranes of
higher plants is active at least in the concentration range of mechanism I (< 1 mM)
Table 1. Permeability coefficients estimated for cells of roots at 250 C (from Pitman [1976])
Plant
Permeability coefficients (m s - 1)x 1010
Plasmalemma
Tonoplast
K+
Na + ClK+
Nat CI-
Hordeum vulgare ............................
Triglochin maritima .........................
33
48
120
10
0.3
1.2
-
1.1
2.9
0.23
0.07
1.3
-
which is found in soil solutions. In the higher concentration range, however, the
passive component of K + flux appears to become more prominent.
Part of the K + influx may be related to H + efflux. It has long been known that during
excess cation influx there is a decrease in external pH due to H+ release from the
cell (see e.g. Raven and Smith [1974]). H + efflux from barley roots increases with
increasing external K+ concentration above I mM, i.e. in the range of mechanism 2
(Pitnan [1970]). Uptake of K + by barley roots appears to be coupled to H + efflux,
as demonstrated by the stimulation of both influx of K+ and efflux of H+ by the
phytotoxin fusicoccin (Pitnzan et al. [1975]). Fusicoccin, a growth-promoting
substance, induces large changes in ion transport such as stimulations of H + efflux
and K + influx at magnitudes much greater than those of comparable effects by
natural growth regulators (Marrd [1977]). The K+-activated ATP-ase (see 2.4)
may be implicated in the H + extrusion process, because its activity is also stimulated
by fusicoccin (Befagna et al. [1977]). H + efflux from the root cells is also balanced
in part by diffusive influx of H + (Pitnman el al. [1977a]), and is thought to reflect
the action of an active H+ extrusion pump at the plasmalemma (Anderson et al.
[1977]: Poole [1978]). However, it is still controversial whether K + is carried by
the H + pump itself or wether a separate carrier for K + is involved (Poole [1978]).
Moreover, complex models involving active and passive H+/K+ exchanges at the
plasmalemma of root cells have also been proposed (Hanson and Lin [1977]).
A hypothetical model is shown in Figure 9 and this may account for cation and
Outside
Plasmalemma
Cytoplasm
K> NaCATION
ATPase
NN H
H2 0
ATP
_ADP +
,
H+
OH-
CI-. anions
- -
Na+
Fig.9. Hypothetical model of cation and anion transport across the plasmalemma of root cells
(modified after Hodges [1973])
121
anion transport across the plasmalemma of root cells (cf. Hodges [1973]). In this
model a central role is played by an ATP-ase that is activated by monovalent cations.
This ATP-ase moves K+ selectively inwards in exchange for Na + or H + moving
outwards. Thus, the ATP-ase is directly responsible for active K± influx and active
Na + effiux. The ATP-ase can also give rise to a charge separation and pH gradient
(Mitchell [1961]) which is in line with the view that there is active H + extrusion.
The charge separation represents an intermediate conservation of energy.
The second important feature of the model in Figure 9 is an anion carrier that brings
about the exchange of CI- and of other anions for OH-, leading to a collapse of
the pH gradient. Hence, the ATP-ase is indirectly involved in active anion influx,
because anion influx is driven by the energy that was conserved temporarily in the
pH gradient. In addition, external anions can also be exchanged for HCO3 - derived
from aerobic respiration (Hodges [1973]); this is not shown in Figure 9. Finally,
the electrochemical gradient generated by the ATP-ase which powers H+ extrusion
also causes a chemical gradient through which passive H + influx can be mediated
in exchange for internal Na + .
In most other models recently published, the cation activated membrane ATP-ase
may also bring about directly the active uptake of K + into the cells, and the ATP-ase
would therefore act as a carrier, as depicted in Figure 9 (see Bowling [1976]; Mengel
and Kirkby [1978]; Littge and Higinbotham [1979]). In contrast, Ratner and
Jacoby [1976] believe that the membrane-bound ATP-ase is not directly responsible
for active K + transport. Such models are obviously hypothetical and need to be
tested further. Nevertheless, they mirror the almost unanimous agreement on the
connection between Epstein's carrier-type mechanism I of K + uptake (see 2.2) and
an ATP-ase at the plasmalemma of the root cells.
3. Uptake and distribution in the plant
3.1 Uptake by the root
Potassium uptake in higher plants is mediated by active uptake at the plasmalemma
of the root cortex cells, as outlined above (2.) The magnitude of K+ uptake, i.e. the
efficiency with which plant roots 'mine' the soil for K+ (cf Epstein [1973]), depends
on many external and endogenous factors; only a few outstanding features are
mentioned here. As described above (see 2.2) the rate of K+ uptake decreases with
increasing K+ content of the root. The supply of energy is also extremely important.
This is indicated by the fact that reduction of sugar transport from the leaves to the
root via the phloem results in reduced ion uptake by the root. This was demonstrated
in several ways, e.g. by cooling the stem of sunflower plants (Bowling [1968]) or by
illuminating the leaves after a period in the dark (Pitinan and Cram [1973]). Such
treatments also led to the elimination of the metabolic component of the membrane
potential in root cortical cells (Graham and Bowling [1977]). Figure 10 demonstrates
a direct effect of illumination on K + (-'Rb) uptake by barley seedlings growing in a
2-hr light/22-hr dark regime. About an hour after illumination K + uptake increased
and there was a concomitant increase in radioactivity of the root which was due to
"C-labeled sugars (Pitman and Cram [1973]).
122
50
Light
4-0-
To
Accumulation
-7500
3.0
5000
75 20
S
Eo
o
0
2000
W lo
AAAA
2
3
Hours
4
5
mM K1,in 2 h light/22 h dark
Fig. 10. Barley seedlings growing on nutrient solution with 10
around the shoots. Measureillumination to a 15 min pulse of '4CO,
regime, were exposed upon
+
4
ments were made of ' C in the root (.) and of the rates of K ('6Rb) accumulation over I h
(redrawn from Pinan and Crain [1973])
The process of ion uptake by the root is further complicated by the discovery that
the epidermis may be differentiated into transfer cells with wall in-growths and increased surface-area of the plasmalemma. This was demonstrated in Atriplex (Kramer
[1978a]) where the transfer cells may be involved in selective uptake of Kt over
Na + (Kramer [1978b]). The general significance of transfer cells in solute transport
was reviewed by Gunning [1977].
The rate of ion uptake by the root seems to be proportional to the relative growth
rate of the plant (Pitman [1976]). Phytohormones may also play an important role
in the ion transport characteristics of the root; this will be mentioned below. In the
intact plant (the situation in the 'real world') the shoot is certainly important in the
regulation of ion uptake by the root, possibly as a result of some information transmitted from it (Pitman and Cram [1977]). This exciting area of research requires
much more work before the process of ion uptake by the root can be fully integrated
in the general physiology of the whole plant (cf. Lfittge [1974]).
3.2 Transport through the root into the xylem
In the root of higher plants, two pathways are available for lateral K+ transport
through the root into the conducting xylem vessels (Figure 11). The apoplasmic
pathway is restricted to the cell walls and blocked at the endodermis, because the
radial walls of the latter cell layer are incrusted with the Casparian strip which has a
low permeability to ions and water (Lduchli [1976a]). Hence, K+ can enter the stele
123
X. Epidermis,
Cortex J.
Endlodermis
Xylem
Prnhm
Vessel
CC
Fig. II. Model of ion transport pathways in roots.
C=cytoplasm, V=vacuole, ER=endoplasmic reticulum, Cs=Casparian strip (Lauchli
[1976])
of roots only through the symplasmic pathway (IUuchli [1976e]). The concept of
symplasmic transport in plants was first discussed by Arisz [1956]. Experimental
evidence on this point came from a correlation between the rates of K+ transport
to the xylem along the length of the root and the development of the endodermal
cells (Clarkson and Robards [1975]). The symplasmic pathway is provided by the
existence of plasmodesmata which form a cytoplasmic continuum all the way from
the surface of the root through the endodermis to the stele (Robards and Clarkson
[1976]). The endoplasmic reticulum, which appears to be continuous through the
plasmodesmata, represents the intracellular compartment for intercellular symplasmic
transport (Figure 11).
Direct information on the localisation of ions in the root symplast came from a
study on CI- localisation in barley roots using the AgCl-precipitation technique
(Stelzer et al. [1975]). From this evidence the localisation of K + in the symplasmic
pathway of roots may also be inferred. Interestingly, the halophilic grass Puccinellia
was recently shown to transport K + preferentially through the root to the shoot
but to exclude Na+ from upward transport (Stelzer and Lduchli [1978]). This
may serve as an example for ready movement of K + through the root symplast;
furthermore, it raises the problem of how mobile Na + is in the symplast.
In the stele K+ is exported from the symplast to the xylem vessels, i.e. it moves back
into the apoplast. This transport process has received much attention; recent reviews
are by Anderson [1976]; Bowling [1976]; Lauchli [1976c]; Pitman [1977] and
Baker [1978]. The first question that needs to be answered is: are ions released into
living or dead vessels? Although it is commonly held that the young, developing
vessels in the root tip are not involved in ion transport from root to shoot, contrasting reports have been published by several authors during the last 25 years. Davis
and Higinbotham [1976] summarised the evidence in favour of the involvement of
living vessels. For barley roots, however, it has now been clearly demonstrated that
ion transport to the xylem can be inhibited selectively in parts of the root where
all the xylem vessels are mature (Lduchli et al. [1978]). This strengthens the hypothesis that ion transport to the xylem is into dead vessels and is due to release from
the xylem parenchyma cells (Figure 11).
124
Most workers suppose that the xylem parenchyma cells are the site of ion release
to the vessels. However, the possible mechanism of this release is still controversial;
one view is that ions move passively into the vessels, and that there is only one active
step in transport through the root, located at the plasmalemma of the cortex cells.
This view was favoured by Littge and Laties [1966], Laties [1969], Bowling [1976]
and Baker [1978], among others. However, there are several objections to this view.
Firstly, by means of X-ray microanalysis (Lduchli [1972a]), it was demonstrated
that the xylem parenchyma cells accumulate K + (Lduchli et af. [1971]; Lduchli et al.
[1977]; Eshel and Waisel [1978]): an example is shown in Figure 12. Other ions
may also be accumulated in living cells of the stele (Liuchli [1972b]). Stelar cells
may therefore possess active transport processes (Pitnan [1977]). On the other
hand, using K+-sensitive microelectrodes, the vacuolar K+ activities of the various
cells of maize roots were found not to differ significantly (Dunlop and Bowling [1971]).
The apparent discrepancy between the results from measurements with X-ray microanalysis and K+-sensitive microelectrodes cannot yet be explained.
Secondly, ion release to the xylem is sensitive to metabolic inhibitors such as CCCP,
an uncoupler of oxidative phosphorylation (Lduchli and Epstein [1971]; Pitman
[1972]). In this case, however, uptake by the root is also inhibited and it is not clear
K(Ka)
,
X),
XyP
Pa
P
En
CP
H Ep
R
Fig. 12. K distribution in the corn root; relative amounts measured by X-ray microanalysis.
R--medium, Ep=epidermis, H=hypodermis, CP=cortex, En=endodermis, P=pericycle,
Pa =stelar parenchyma, XyP =xylem parenchyma, Xy =xylem vessel (Lduchli et al. [1971])
125
whether the effect of the inhibitor is a consequence of reduced uptake by the root
or due to a specific inhibition of ion release to the xylem (Pitman [1977]). In spite
of this uncertainty, the electrochemical gradients for K+ between the xylem vessels
and the surrounding xylem parenchyma, determined by Davis and Higinbotham
[1976] in maize roots, indicate active K+ movements into the vessels. Furthermore,
the structural features of the xylem parenchyma in barley roots, i.e. the large proportion of cytoplasm with many mitochondria and extensive rough endoplasmic
reticulum, make them suitable to play an active role in the stele (Lduchli etal.[1974]).
In addition, these cells contain a K+-stimulated ATP-ase located at the plasmalemma,
indicating that active K + transport to the vessels is controlled by this membrane
(Winter-Sluiter et al. [1977]). Plasma membrane fractions from the stele of maize
roots also contain a K+-stimulated ATP-ase (Leonard and Hotchkiss [1978]).
Finally, certain substances were found to block ion release to the xylem, either without
reduction of uptake into the root, or else a much longer period of time is needed for
inhibition of uptake to become apparent, in comparison with inhibition of release
to the xylem. This response is shown by the phytohormone abscisic acid (ABA)
(Cram and Pitman [1972]) and by compounds that block synthesis of functional
protein (cycloheximide: Lduchli et al. [1973]; Liittge et al. [1974]; p-fluorophenylalanine: Schaefer et al. [1975]; azetidine 2-carboxylic acid: Pitman et al. [1977b]).
The mode of action of all these compounds on ion release to the xylem is a matter
of speculation. One suggestion has been that they block the formation of a carrier
protein that turns over rapidly (Schaefer et al. [1975]). An alternative possibility
would be that these substances lead to the production of compounds which could
signal a reduction in protein synthesis (Pitman and Cram [1977]).
In summary, there is mounting evidence to the effect that K+ release from the root
symplast to the xylem vessels is due to active K+ transport across the plasmalemma
of the xylem parenchyma cells. According to a stimulating recent hypothesis (Hanson
[1978]), in which Mitchell's chemiosmotic theory is applied to ion transport across
the root, ion release into the xylem vessels may be driven by a charge separation
and pH gradient at the plasmalemma of the xylem parenchyma cells and therefore
is energy-linked.
3.3 Circulation in the plant
Potassium is highly mobile in plants; this provides for its easy distribution in all
the organs. Not only is it translocated upwards in the xylem vessels to the leaves,
it also moves easily from the xylem to the phloem through which it is readily re+
distributed in the plant. Hence, K can circulate in the plant. This subject has been
extensively reviewed by Luchli [1972b] and Pate [1975, 1976].
In the xylem vessels K+ moves with the transpiration stream by mass flow, but there
may be some exchange of K + with the vessel wall (Lauchli [1967]). As the xylem sap
moves upwards through the shoot, some K+ is gradually removed from the xylem
by lateral transport to the phloem (Stout and Hoagland [1939]; Peel [1963]). This
lateral movement to the sieve tubes of the phloem is of course an outstanding feature
of the leaves; it is facilitated by a K+-Ioading process at the plasmalemrnma of the
sieve tubes (see 6).
126
Sodium accumulates selectively in the tissue around the vessels, mainly in the proximal
region of the root and in the lower portion of the stem, as it ascends through the
xylem of bean and maize plants (Jacoby [1964]: Rains [1969a, b]; Jacoby and
Ratter [1974]; Richter and Marschner [1974]). Na+ reabsorption from the vessels
is effected by the xylem parenchyma cells and probably occurs via a Na+/K+ exchange
process across the plasmalemma of these cells, leading to a depletion in Na+ and an
increased concentration or K+ in the xylem sap (Lduchli [19 76c]; Kramer et al.
[1977]; Yeo et al. [1977]; Lduchli and Wieneke [1978]). The possible significance
of this process in salt tolerance is discussed by Beringer and Trolldenier [1978]
elsewhere in this volume.
Analyses of the most important inorganic solutes in xylem saps and sieve tube fluids
show clearly that K + is the predominant cation in both fluids (Table 2). This mirrors
Table 2. Concentration of the most important inorganic solutes in xylem and phloem fluids
of two herbaceous plant species (from Pate [1976])
Constituents
-
Lupinus albus
Ricinus conmunis
(Fig ml )
Stem xylem
tracheal sap
Fruit tip
phloem exudate
Xylem root
bleeding sap
Stem incision
phloem exudate
Potassium ..........
Magnesium .........
Calcium ...........
Phosphorus .........
pH ........ ...... ..
90-179
27- 39
17- 95
100-200
6.3
1540-2260
85- 124
21- 63
300- 800
7.9
400-560
2300-4400
109- 122
20- 92
350- 550
8.0-8.2
-1
230-450
6.0
Analysis not available
the well known phenomenon of K+ circulation in the plant. Quantitatively, the
concentration of K+ in the two fluids depends on many factors and, therefore, varies
a great deal, but its concentration in the fluid of the phloem always exceeds that
of the xylem sap by about tenfold (Table 2). More data on K+ concentrations in
sieve tube sap have been compiled by Ziegler [1975]. The high K + concentration
in the phloem fluid allows the plant to redistribute K + readily to the young and
growing tissues and organs, particularly from the senescing leaves. For instance,
K+ moves preferentially to the young leaves of sunflower plants (Haeder and Mengel
[1969]. This appears to be brought about by the sum of upward movement of K+
in the xylem and redistribution from the older leaves through the phloem. Greenway
and Pitman [1965] demonstrated clearly that in barley seedlings the younger leaves
were supplied through the phloem with K + originating from older leaves, in addition
to import of K + via the xylem. This is presented in Table 3, from which it can be
Table 3. Turnover of K + in barley seedlings at the third leaf stage (from Greenuway and Pitnan
[1965])
Uptake of K+, vumoles day-'
First leaf
From the root (via the xylem) .............
1.9
From other plant organs (via the phloem).. . -1.6
Net uptake .............................
0.3
Second leaf
Third leaf
2.7
0.7
3.4
2.0
1.3
3.3
1 27
seen that the export from the oldest leaf almost balanced its import, and that the
exported K+ was redistributed to the second and youngest leaves; thus K+ meets the
requirements for a freely mobile solute in the circulatory pathways of the plant
(Pate [1976]).
4. Potassium and water relations
According to Mengel [1977], K + is the most important inorganic solute in plants
that is osmotically active. For this reason, K+ is significant in cell extension and
growth (this aspect will not be covered in this review). The osmotic role of K+ causes
this solute to be an outstanding factor in plant water relations, that is, in the absorption, translocation and loss of water. It is well established that plants sufficiently
supplied with K + have a lower water requirement (e.g. Linser and Herwig [1968])
and a lower water loss because of a reduced rate of transpiration (Brag [1972]).
In order to evaluate the osmotic role of K+, one needs to understand the compartmentation of K+ in plant cells. Higher plants can accumulate extracellular K+ in the
lumen of the xylem vessels of the roots leading to the phenomenon of root pressure;
this will be discussed in 4.1. The other point relevant to the osmotic role of K + is the
intracellular compartmentation, particularly the distribution of K+ between cytoplasm and vacuole of cells (4.2).
4.1 Root pressure
Active K+ transport from the xylem parenchyma cells into the xylem vessels of the
root (see 4.2) leads to net accumulation of K+ in the lumen of the vessels. An example
is presented in Table 4. Since the external medium, i.e. the soil solution under natural
Table 4. Concentration of K+ in the xylem exudate of excised corn roots (data from House
and Findlay [1966] and Davis and Higinbothant [1969])
KCI (mM)
in medium
K- (mM)
in xylem exudate
0 .1 .........................................................
I ...........................................................
5 ...........................................................
10 ..........................
.................................
50 ...........................................................
14 .7
22.1
27.3
28.5
53.4
conditions, is separated from the xylem sap by at least two membranes (plasmalemma
of cortical and xylem parenchyma cells), the K+ accumulated in the xylem cannot
diffuse back into the environment of the root. It lowers the water potential in the
vessels, thus creating an inward osmotic gradient along which water moves osmotically and sets up a positive hydrostatic pressure called root pressure (Baker and
Weatherley [1969]; Anderson et al. [1970]).
128
K+ is the most important cation in the xylem sap (cf. Table 2) and, hence, contributes
significantly to the development of root pressure. Mengel and Pfldger [1969], using
Rb+ as a physiological analog to K+, found good correlation between the rate of
fluid exudation and the Rb+ concentration in the xylem exudate of excised maize
roots (Figure 13). This result further emphasises the osmotic role of K + in moving
100
w
S
W£
60
X. 40
LL
0
w
LUU
F<
20
.
20
-40
60
100
Rb (86 Rb), REL.
Fig. 13. Correlation between rate of fluid exudation and Rb concentration in the xylem exudate of excised corn roots. r=0.72 (redrawn from Mengel and Pflfiger [1969])
H 20 from the external medium to the xylem. Root pressure exudation, that is, the
exudation of fluid from the cut end of an excised root, is the consequence of osmotic
water flow to the xylem. It was investigated quantitatively by House and Findlay
[1966] and Anderson et al. [1970] and reviewed by Anderson [1976]. Other possible
functions of root pressure in the water economy of plants were discussed by Epstein
[1972].
4.2 Compartmentation of K + in cells
To understand the osmotic role of K+ in the cell needs knowledge of the intracellular
distribution of this ion. In particular, the knowledge of its concentration in the
cytoplasmic phase and the vacuole is most relevant in this context, although K+ may
also accumulate in the chloroplasts (see 5.1) and the mitochondria (see 8).
Compartmental analysis may either be achieved by direct, in situ measurement of
ions in the cytoplasm and the vacuole or by estimation, using indirect methods such
as radiotracer efflux analysis. Two methods are available for in situ measurements
129
+
of K+, that is, the use of K+ sensitive microelectrodes to determine K activities
(see e.g. Hinke [1959]; Clarkson [1974]) or X-ray microanalysis (Lduchli [1972a,
+
1973]). The only direct measurement of K activity in the cytoplasm and vacuole
+
of higher plant cells was achieved by Etherton [1968] with a K sensitive microelectrode. He found K+ activity in the cytoplasm and the vacuole of pea root
+
cells to be 43 and 122 mM respectively, the external K concentration being 10 mM.
Assuming that most of the K+ in the cytoplasm occurs as free ions in solution (this
assumption certainly holds for K + in the vacuole), these K+ activities would approximately represent K+ concentrations (Clarkson [1974]). However, the cytoplasmic
layer of a plant cell may be as thin as 0.5 Itm and may occupy only about 3 per cent
of the cell volume (Macklon and Higinbothain [1970]). There is therefore some doubt
+
about the reliability of the cytoplasmic K value determined by Etherton [1968].
with K+ sensitive microelectrodes
determined
vacuoles
cell
the
root
for
The K+ values
are certainly more reliable. The values thus obtained for vacuoles of several plant
roots were in the vicinity of 100 mM or slightly higher (Etherton [1968]; Dunlop
and Bowling [1971]; Bowling [1972]; Dunlop [1973]).
X-ray microanalysis has thus far only yielded concentration values in relative units.
No great differences in K+ concentration were found between cytoplasm and vacuole
in root xylem parenchyma cells (Liuchli el al. [1971, 1977]). Pallaghy [1973] was
able to distinguish between cytoplasm, vacuole and other cell compartments in an
X-ray microanalysis study of maize leaves. In leaf cells of the halophyte Suaeda
there is good evidence derived from X-ray microanalysis that most of the Na+ and
Cl- is in the vacuole and most of the K+ in the chloroplasts, while the cytoplasm is a
region of low K + , Na+ and C- content (Harvey et al. [1978]). There are still serious
problems of specimen preparation and of absolute calibration in X-ray microanalysis.
Yet, recent advances both in specimen preparation (Harvey et al. [1976]; Lduchli
und Gullasch [1978]; Van Steveninck and Van Steveninck [1978]) and calibration for
absolute quantitative analysis (Spurr [1974, 1975]; Rick et al. [1978]; Steudle et al.
+
[1978]) will allow direct investigation of the compartmentation of K in plant cells.
Indirect compartmental analysis by tracer efflux studies is based upon the determination of the pattern of radioisotope effiux from a tissue previously loaded with labeled
ions. This approach was first used with the alga Nitellopsis by MacRobbie and Dainty
[1958]. Pertinent examples of higher plant tissues are by Pitinan [1963] on beetroot
tissue and by Macklon [1975] using onion root tissue. A review of the literature was
presented by MacRobbie [1970]. In general, the efflux analysis reveals three kinetic
phases, a fast phase from the free space of the cell wall, a slower phase assumed to be
due to loss from the cytoplasm and a still slower phase supposedly representing
loss from the vacuole. The estimation from efflux analysis of fluxes across the plasmalemma and the tonoplast and of ion concentrations in the cytoplasm and the vacuole
was discussed extensively by Walker and Pimnan [1976]. As regards the application
of this method to higher plant tissues, some serious criticisms were raised, as discussed
for instance by Epstein [1972] and Bowling [1976], and the estimated values particularly for K+ concentrations in the cytoplasm and the vacuole may be subject to
errors. Some K+ values for roots are compiled in Table 5; they are quite variable
and, hence, no general conclusions can, as yet, be drawn with regard to the reliability
of tracer efflux analysis in study of the compartmentation of K+.
An interesting new approach was chosen by Jeschke and Stelter [1976]. They measured K + profiles along the axis of single barley roots by analysing 0.5 mm sections
130
Table 5. Estimated K + concentrations in the cytoplasm and vacuole of root cells, obtained by
radiotracer efflux analysis (in comparison with other methods)
Plant
K- concentration (mM)
External CytoVacuole
medium plasm
References
Corn ....................
Barley ..................
Vicia faba ...............
Onion ...................
0.2
2.5
1.0
1.0
Liittge and Laties [1967]
Pitman and Saddler [1967]
Pallaghy and Scott [1969]
Macklon [1975]
21
102
20
100
22
74
34
83
Pea (measured with KT
sensitive microelectrode)
..
10
43
122
Barley (measurement of
longitudinal ion profiles)
..
0
110
20
Etherton [1968]
Jeschke and Stelter [1976]
with flameless atomic absorption spectroscopy. On the assumption that the meristematic tip cells are essentially devoid of vacuoles (see however Epstein [1972], Figure
5-1), these authors estimated the K+ concentration in the cytoplasm and the vacuole
of low salt barley roots to be about 110 and 20 mM, respectively (Table 5).
4.3 Role in stomatal opening and closure and interaction between K+ and abscisic acid
While K+ does not appear to be accumulated to dramatically high concentrations
in the vacuoles of root cells (see 4.2), it will now be demonstrated that K + is osmotically
active due to the possibility of its compartmentation in the vacuoles of the stomatal
guard cells. Already at the beginning of this century K + was detected light microscopically in guard cells after staining with cobaltinitrite (Macalhun [1905]). Subsequently, several studies in Japan established that K + is implicated in opening and
closure of stomata (Imamura [1943]; Fujino [1959, 1967]). Fujino concluded that
active transport of K+ into and out of guard cells is responsible for stomatal movement. In the late 1960's, extensive research started in the laboratory of Hsiao, using
strips of leaf epidermis, which led to the conclusion that K+ transport in guard cells
is a fundamental mechanism in stomatal movement (review: Hsiao [1976]). A more
general review on stomatal physiology was published by Raschke [1975].
Fischer [1968] and Fischer and Hsiao [1968] first demonstrated that K + is a major
osmotic agent in stomatal movement. With epidermal strips of leaves, K+ absorption
by the guard cells in the light appeared sufficient to account for the observed change
in osmotic potential of these cells, if C- was assumed to be a counterion of K+.
In Vicia faba leaves, the K + concentration of the guard cells increased by about
300 mM for an increase of 7 [ in stomatal aperture (Fischer [1968]; Fischer and
Hsiao [1968]). Moreover, for this species stomatal opening in the light was found
to be induced specifically by K+ concentrations that occur in the soil solution (and
also by low Rb + concentrations), while comparable opening required about 100 times
higher concentrations of Lit, Na+ or Cst (Humble and Hsiao [1969]; Figure 14).
131
12 - Light
10
8
W $
60
703
A A K +
6
4
0
O
0
Li*
No+
V
--2
'0
S 0
10 Dark
20
60
40
so
0
0
"6
8
Ai
V
4
Il.
0
•
'
I
20
I
60
40
Cation concentration t mM)
I
I
80
100
Fig. 14. Stomatal opening in epidermal strips of Vicia faba leaves in the light or dark as
dependent on monovalent cations in the medium (redrawn from Humble and Hsiao [1969])
In the dark, however, there was no specific K+ effect (Figure 14). Thus, there appears
to be a specific K + requirement at physiological concentrations for light-activated
+
stomatal opening in leaves of Vicia faba (Humble and Hsiaa [1969]). K is also
the osmotic cation involved in stomatal movements of many other plants (Willner
and Pallas [1973]) and in most natural situations, since Rb+ does not normally
occur in plants at concentrations sufficient to be a major osmotic factor in stomatal
movement. Moreover, stomatal activity is impaired in K+ deficient plants (Graham
and Ulrich [1972]; Terry and Ulrich [1973a]).
While stomatal opening is correlated with influx of K + ,there is effiux of K + when
stomata are closing. The implication of K+ in stomatal movement was detected by
many different methods, i.e. tracer methods using Rb+ and 42K+ (Humble and
Hsiao [1970]; Fischer [1972]), histochemical staining with cobaltinitrite (Fischer
[1971]), X-ray microanalysis (Sawhney and Zelitch [1969]; Humble and Raschke
[1971], Figure 15), K + determination in 'quasi-isolated' guard cells by flame photometry (Allaway and Hsiao [1973]), measurements with K+ sensitive microelectrodes
(Penny and Bowling [1974]) and also by an enzymatic test based on the requirement
of the enzyme pyruvate kinase for K + (Outlaw and Lowry [1977]).
The question has been raised in occasional reports as to whether Na + may replace
K + in stomatal movement (e.g. Wilhner and Mansfield [1969]). However, the latter
experiments were done in the absence of Ca 2+, in which no selectivity of K+ over Na+
can be observed (Pallaghy [1970]). The situation in Paphiopedilum whose epidermis
132
7
a-
2
Potassium
a
Chlorine
.............
Phosphorus
- --
-
6
i
5
4
Note different scoles for
Potassium and for
Chlorine ond Phosphorus
4C
\I
o
E
-
0
c
Fig. 15. Distribution of K, CI1and P in an open and closed stoma of Viciafjaba; relative
amounts measured by X-ray microanalysis (Humble and Raschke [1971j)
has a very low K + content is unclear, with no K++ localised in open guard cells (Nelson
and Mayo [1977]). In some halophytes, Na may take over the specific osmotic
role of K + under saline conditions (Eshel et al. [1974]), but there is only limited
evidence in support of such a specific function of Na + .
As postulated above, K + in guard cells apparently needs to be balanced by a counterion
with negative charge. This can be either CI- (Raschke and Fellows [1971]; Schnabl
and Ziegler [1977]) or malate (Allaway [1973]). Malate is synthesised within the
guard cells during K + influx, and electroneutrality can be maintained by release of
H + from the guard cells (Raschke and Humble [1973]). The relative role of CI- and
malate as counterions for K + seems to depend on the availability of CI- to the guard
cells ( Van Kirk and Raschke [1978]).
Gradmann [1977] challenged the idea of K + being the primary osmotic solute in
stomatal movement on grounds that most cell membranes exhibit a relatively high
K + permeability, but the experimental evidence available indicates a low membrane
permeability to K + in guard cells even under conditions inducive to stomata] closing
(Hsiao [1976]). +Furthermore, the measurements by Penny and Bowling +[1974]
indicate active K influx during stomata opening and active effux of K when
stomata are closing. Vacuolar pH measurements in guard cells gave a higher pH
value in open stomata than in closed ones (Penny and Bowling [1975]), again sug133
+
gesting that part of the K + influx is balanced by K+/H exchange (see also Raschke
[1977]). In fact, very recently the membrane potential of isolated guard cell protoplasts (see Zeiger and Hepler [1976, 1977]) was ascertained to be light-sensitive and
+
it was postulated that this membrane potential maintains a H gradient across the
+
plasmalemma that would drive K transport (Zeiger et al. [1977]).
Mittelheuser and Van Steveninek [1969, 1971] discovered that the phytohormone
abscisic acid (ABA) inhibits stomatal opening. Endogenous ABA levels are now
known to increase dramatically in leaves subjected to water stress (Hsiao [1973])
and to play a key role in the response of stomata to water deficit (Raschke [1975]).
In terms of ion movements in guard cells it was established that ABA inhibits both
stomatal opening and K + influx into guard cells and that this inhibition is reversible
(Mansfield and Jones [1971]; Horton [1971]; Horton and Moran [1972]). Hsiao
[1976] considers the action of ABA on stomatal movement to be due, not to increased
membrane permeability to K + , but rather to an effect on K+ influx or on an efflux
process. Using " C-ABA and microautoradiography, it was shown that radioactivity
from labeled ABA was localised in the region of guard cells (Itai et al. [1978]). Also,
under the influence of ABA, there was an increase in the concentration of solutes
in the epidermal cells, possibly due to release of solutes from guard cells (ltai and
Meidner [1978]). This result favours the idea of ABA stimulating some efflux process
from the guard cells. On the other hand, Van Steveninek [1976] and Raschke [1977]
+
suggested that ABA may inhibit the release of H from the guard cells. Obviously
there is not yet enough experimental evidence to elucidate the mode of action of
ABA on fluxes of K + and other solutes in relation to stomatal movement.
4.4 Involvement of K+ in turgor-regulation of higher plant cells
The fact that K + is a major osmotic solute in stomatal movement (4.3) emphasises
the important role of K+ in the regulation of turgor in the guard cells. We shall now
discuss whether the involvement of K + in turgor regulation is restricted to the stomatal
guard cells or is a general feature of cells of higher plants.
First of all, stomatal movement is not the only turgor-regulated movement involving
K + . In several families there are species whose leaves exhibit nyctinastic movements,
that is, the leaves are open during the day and stay closed at night. This phenomenon
is known particularly in leguminous plants (see e.g. Satter and Galston [1973]).
The turgor-regulated leaf movement in leguminous plants has been studied extensively
and demonstrated to be due to K + mediated turgor changes in the pulvinule, a special
tissue in the leaf that causes its movement by shrinking and swelling of its cells.
Relevant examples are Mitnosa (Toriyania [1955]; Allen [1969]), Alhizzia (Satter
et al. [1970]; Satter and Galston [1971]) and Phaseolus (Kiyosawa and Tanaka
[1976]). The rapid movement of the column of Stylidium, a mechanism for crosspollination of the flowers, is caused by turgor changes in the motor tissue of the
column, KCI being the principal osmoticum (Findlay and Pallaghy [1978]). Thus,
the involvement of K+ in turgor-regulated movements of plant organs is widespread.
Extension growth is thought to require positive turgor in the expanding cells (Cleland [1971]). That K + (together with malate) may act as a principal osmotic solute
in turgor-driven extension growth was shown for the growth of cotton fibres, whose
+
growth in vitro is inhibited in the absence of K (Dhindsa et al. [1975]). In many
134
higher plants K + , most often associated with organic anions, appears to be the
principal component of osmotic pressure in the cells (see e.g. Cram [1976]: Table 11.2).
Hence, one of the basic functions of K+ may indeed be as a principal contributor
to the maintenance of cell turgor. The question arises as to how cells of higher plants
regulate their turgor and how K+ is involved in turgor regulation. As outlined above
(2.2), K+ influx decreases with increasing internal K+ content, possibly due to negative
feedback regulation. This phenomenon could contribute to turgor regulation in the
cells. Indeed, it was found that K + influx into the marine alga Valonia can be increased
by reducing the turgor and can be reduced again by restoring the normal turgor
(Gutknecht [1968]). Although similar effects were observed with several other
algae, turgor has been found distinctly to influence the transport of K+ in cells of
higher plants in only a few cases (Crain [1976]; Hellebust [1976]). For instance,
+
Sutcliffe [1954] observed in sugar beet a stimulation of K influx by reducing the
turgor, similar to the situation in Valonia.
There is realistic hope that the use of new methods mainly developed in Zimmermann's
laboratory will aid in producing fresh information on the extent of the involvement
of K+ in turgor regulation in higher plant cells. Zimmermann et al. [1969] and
Zimmermann and Steudle [1974] developed a pressure probe for measuring directly
the pressure inside giant algal cells. With this device Steudle et al. [1977] and Zimnmermann and Steudle [1977] demonstrated that in cells of the alga Valonia K+
influx to the vacuole decreases with increasing turgor pressure up to 2 bar, whereas
the efflux of K+ from the vacuole increases over the entire turgor pressure range
(Figure 16).
100
90
/
6o
O
70
60
E
U
0
E
X 30
X:
20
0
0
0
1
2
3
4
5
6
7
PI bar
Fig.16. Effect of turgor pressure (P) on Kt fluxes in cells of the marine alga Valonia utricularis. K + influx from medium to vacuole (x) and K- efliux from vacuole to medium (0).
of K+effluxareforacell volume of 80 pl. Not shown in the figure is the dependence
The data
of K + efflux on cell volume, i.e. K t efflux increases with increasing cell volume. K+ influx does
not depend on cell volume (redrawn from Steudle et al. [1977] and Zimmermann and Steudle
[1977])
135
With the construction of a new pressure probe it is now possible to extend these
studies to cells with a minimum diameter of 20 [Lm in higher plant tissues (Hiisken
et al. [1978]). This new approach will add to our understanding of the complex
integration of the fluxes of K+, other ions and water and the regulation of cell turgor
(Zimmermann [1977, 1978]; Zimmermann and Steudle [1978]).
5. Potassium and photosynthesis
Photosynthesis is the fundamental process by which about 10l tons of carbon dioxide
per year are fixed on the earth. Only chlorophyll-containing plants and some photosynthetically active bacteria are capable of transforming light energy into chemical
energy, thus producing from the inorganic substances CO2 and H 2O organic compounds from which man, animals and all the other heterotrophic organisms can
synthesise their own compounds. Our main energy sources - coal, petroleum and
gases - are products of photosynthesis in ancient times.
Research in the last two decades has clearly shown that K+ is important in plant
metabolism particularly through its action on processess of photosynthesis and
respiration (Pirson et al. [1952]; Bierhuizen [1954]; Latzko and Mechsner [1958];
Mechsner [1959]; Schmidt [1959]). The last comprehensive review was made by
Jackson and Volk [1968] and our review concerns mainly results from the last
10 years, though some of the older results will be discussed in the light of new concepts.
5.1 Relation between K+ and photosynthesis
Recognition of the fact that metabolic processes can now be studied usefully with
subcellular particles like chloroplasts and mitochondria was an important step.
The isolation of such functionally active organelles and tests of their metabolic
capacity brought to light some interesting aspects of the function of K + in plant
metabolism.
The efficiency of photosynthetic CO2 fixation depends on the amounts of ATP and
NADPH available. Using isolated sugar beet chloroplasts Toibesi et al. [1969]
found that photoreduction of NADP was decreased by K + deficiency but there was
little or no effect on ATP formation. Later Pfliiger and Mengel [1972] showed that
K+ deficiency affected the photosynthetic reactions of chloroplasts (Table 6). They
Table 6. Effect of plant K + status on ATP synthesis and photosynthetic electron transport in
chloroplasts (Pflager and Mengel [1972])
Plant species
K, % of
dry matter
Electron transport,
jzeq. e- mg
chlorophyll-' h-I
ATP synthesis,
vamoles mg
chlorophyll -' h - I
P/2e-
Vicia faba .............
3.70
1.00
5.53
1.14
4.70
1.60
512
384
496
424
340
326
216
143
295
185
102
68
0.84
0.74
1.19
0.87
0.60
0.42
Spinacia oleracea .......
Helianthus annuus ......
136
found that the rates of electron transport (NADP reduction) and of ATP synthesis
were reduced in three species. It is difficult to explain these findings. Some possibilities
will be discussed in the following section.
The site of the photosynthetic reactions in a plant cell is the chloroplast. The topographical properties of a chloroplast are closely related to its function (Figure 17).
CHLOROPLAST
(photosynthesis)
Cytoplasm
MITOCHONDRION
(respiration)
r-tonspot ci.,ATPsynthisj
h,
e--
intr - thylCkoid
space
Fig. 17. Possible involvement of K + as counterion to H + fluxes across the thylakoid membrane
in chloroplasts and across mitochondrial membranes in relation to photosynthesis and
respiration
An intact chloroplast is surrounded by two membranes, called the chloroplast envelope.
The outer membrane is readily permeable even to larger molecules; the inner membrane is the functional barrier between the cytoplasm of the cell and the stroma of
the chloroplast (Heldt and Sauer [1971]). The site of the electron transport chain
and of ATP formation is in the thylakoid membrane which separates the intrathylakoid space from the stroma of the chloroplast.
When isolated chloroplasts are illuminated there is electron transport dependent
H + uptake into the intrathylakoid space, resulting in a pH gradient across the thylakoid membrane (Jagendorf and Hind [1963]). Such H + fluxes through the thylakoid
membrane are interesting in relation to the mechanism of ATP formation. According
to the chemiosmotic theory of phosphorylation this H + gradient drives ATP formation (Mitchell [1961, 1966]). The magnitude of the pH gradient itself is of the
order of 3 pH units (Schuldiner et al. [1972]; Heldt and Werdan [1973]). It is assumed
that a reversible ATP-ase is anisotropically located in the membrane. At the enzyme
site the passive outflow of H + couples the reaction ADP+P--*ATP. This assumption
was proved experimentally by Jagendorf and Uribe [1966] who showed in their
classical work that an acid-base transition of chloroplasts led to ATP synthesis
even in the dark. Similarly it was demonstrated that ATP formation occurred when
an artificial K + gradient was induced (Schuldiner ei al. [1972]; Uribe [1973]).
McCarty [1976]) has reviewed these ion fluxes. H + translocation into the intra137
thylakoid space is an electron transport dependent, active process and sets up an
electrical potential difference across the thylakoid membrane. Upon illumination
the electrical potential difference increases very quickly due to the active influx of
H + into the intrathylakoid space. The increasing potential difference across the
membrane causes secondary passive fluxes of ions other than H + to maintain electrochemical neutrality on either side of the membrane. Thus, as H± is taken up, cations
are released or anions are taken up, or even both fluxes occur (Figure 17). These
co- or counterion fluxes continue until equilibrium of the ion gradients is reached,
the extent of which is determined by the potential difference.
Various techniques have been used to find out which cations and/or anions are
involved in these fluxes. Dilley [1964] first discussed light-induced K + flux out of
the thylakoids into the stroma of the chloroplast. Dilley and Vernon [1965] found an
approximately stoichiometric release of Mg2 + and K+ using a Tris-acetate medium.
Crofts et al. [1967] and Deamer and Packer [1969] demonstrated compensatory
uptake of Cl- in a reaction medium containing NaCI. Using ion specific electrodes
Hind et al. [1974] measured simultaneously the changes in H + , CI1, Na + , K + , Mg2 +
and CaZ+ activities in suspensions of broken chloroplasts. They concluded that
H+ influx is accompanied mainly by Mg 2+ effiux and CI- cotransport and less by
2
K+ efflux. Nonetheless, their data also demonstrated the importance of the K+/Mg +
+
activity ratio in the test medium. As this ratio increases, more K takes part in the
total compensating fluxes. This should be kept in mind when discussing cation
contents of chloroplasts. It can be concluded that the composition of the incubation
or test medium is of crucial importance for the kind of ion that balances H+ uptake.
Furthermore, it is difficult to extend these results to the situation existing in the
chloroplast in situ.
Pfluiger [1974] attempted to overcome these complications by measuring the potential mobility of individual ions through the thylakoid membrane of chloroplasts
using single salt solutions. His findings indicate a greater membrane mobility of
monovalent cations, particularly of K+ (Table 7) which agrees with earlier experiments of Molotkovsky and Dzyubenko [1968], who found that alkali cations were
needed for the formation of the pH gradient.
In all the above experiments the ions were determined directly by flame photometry,
atomic absorption spectroscopy, radioactive labeling or by ion specific electrodes.
Now, results are available from the use of spectrophotometric techniques to study
indirectly the comparative role of cations. The influence of ions on changes in chlorophyll fluorescence yields or changes in the absorbance of metallochromic indicators
has been studied and it was concluded that MgZ+ is the main counterion to proton
Table 7. Light-induced release of cations from the intrathylakoid space across the thylakoid
membrane of spinach chloroplasts to the external medium (Pfliiger [1974])
Cations
Transport,
nmoles mg chlorophyll-'
K
_.....................................
. 278
N a 2+........................................
171
M g2 ...... ...... ........ .. ....... ....... .. 75
....... ... ..... ........ ...
Ca ..............
72
138
Variability
155-469
129-286
4 3- 13 1
52-140
uptake in the intact chloroplast in situ (Krause [1973, 1977]; Barber et al. [1974];
Telfer et al. [1975]). However, these reports showed that the effect on chlorophyll
2+
fluorescence yield caused by 5 mM Mg can be mimicked with 100 mM K+ (Krause
[1974]; Barber [1977]). Hence, the main reason for the disagreement lies in the
uncertainty of the real ionic composition of intact chloroplasts in situ.+
2+
Mg 2+ , Ca
Menke [1940] first reported that chloroplasts of spinach contain K ,
and PO 43-. Subsequent work with both aqueously and non-aqucously isolated
2
chloroplasts showed a high content of K+ and Cl-, an intermediate content of Mg +
+
2+
and Ca and only a small Na content, the latter depending on plant species (Stocking and Ongun [1962]; Saltman et al. [1963]; MfacRobbie [1964]; Harvey and Brown
[1967, 1969]; Larkun [1968]).
2+
as counterion
The main argument of Barber [1977] and others who favour Mg
+
who found the
[1975],
et
al.
to H uptake is based on the analytical data of Gimnier
+
contents of Mg 2+ and K in the chloroplast to be of the same order of magnitude
2+
(about I [ nole mg chlorophyll-'), indicating a K+/Mg ratio of 1. Taking the osmotic
correspond to an internal
values
free space of the chloroplast into consideration, these
2+
concentration of about 30 mM. The values for Mg agree well with Pflaiger's [1974]
+
data, but there is a great discrepancy in the case of K contents. Estimates of the
+
K contents of chloroplasts from different plant species show that K+ is present in
concentrations of about 100 mM in Pisum sativun (Nobel [1969]), 140 mM in
Spinacia oleracea (Pfl~iger [1974]) or even about 300 mM in Beta vulgaris and
Tolypella (Larkurn [1968]). Mix and Marschner [1974] analysed non-aqeously
isolated chloroplasts of Phaseolus vulgaris, Hordeum vulgare and Beta vulgaris and
2+
+
found significantly higher K than Mg contents in all. These data show that the
+
2
K+/Mg + ratio would be 5 or more in the intact chloroplast, indicating K to be th
+
main counterion to the light dependent H uptake in the thylakoids of the chloro2+
ratio of
plast. Bulychev and Vredenberg [1976] even suggested that a K+/Mg
chloroplasts
intact
in
situation
the
for
about 50 may be more or less representative
+.
2+
is small in relation to that of K
and that the light-induced efflux of Mg
2+
In summary, it now appears that Mg is required at a low concentration for energy
+
transfer in photosynthesis but that monovalent cations (in particular K ) are needed
in the concentration range of up to 100 mM for high efficiency of energy transfer.
+
Hence, the principal role of K in the primary reactions in photosynthesis may be
+
ion transport across the thylakoid membrane (Figin
H
to serve as counterion to
ure 17).
There is good evidence that the cation content of chloroplasts is important for another
photosynthetic reaction. Light-induced electron transport in chloroplasts involves
two photosystems acting in series, requiring a balanced input of quanta to both
photosystems for optimum efficiency. Both the wavelength for the excitation and
the presence of cations are important. Evidence as to the nature of cations involved
2+
seems to be very effective (Murata [1969, 1971]; Delrieu
is contradictory. Mg
[1972]; Sun and Sauer [1972]; Barber [1977]; Jennings et al. [1978]). On the other
2+
had no effect if the chloroplasts
hand, Gross and Hess [1973] reported that Mg
were washed free of monovalent cations prior to the experiments. They postulated
a definite requirement for monovalent cations (Gross and Prasher [1974]). Hence,
Mg 2+
there is good reason to believe that there is an absolute requirement for some
(Sinclair [1972]). Also, monovalent cations are necessary at a concentration range
of 60 to 150 mM for high efficiency of energy transfer between the two photosystems
139
(Homann [1969]; Arntzen and Ditto [1976]; Davis et al. [1976]). The mechanism
of regulation is not yet clear. Presumably, cations adsorb to binding sites of the
photosystems and induce structural changes in the membrane (Marsho and Kok
[1974]; Murakami el al. [1975]). Cations thus alter the efficiency of the photosystems as well as the spillover of energy between the two photosystems.
K+ also influences photosynthetic processes by controlling chlorophyll synthesis.
When K+ is withheld from the growing medium of Chlorella vulgaris suspensions a
structural protein in the chloroplast is not filled entirely with chlorophyll but, after
K + is added to the cells, chlorophyll is formed even in the dark (Boger [1964]).
These findings have been confirmed in higher plants. K+ was shown to be an essential
factor in chlorophyll synthesis in lettuce cotyledons (Knypl and Chylinska [1972])
and in leaves of sugar beet and spinach (Marschner and Possinghain [1975]) and
could not be substituted by another cation.
Finally, K+ deficiency can cause structural changes in the chloroplast. A possible
site of action of K + is in the granas of the chloroplast formed by the thylakoid membranes. There is evidence that low salt concentrations in the reaction medium lead
to a relaxation of the grana stacks and to a more homogeneous shape of the thylakoids (Izawa and Good [1966]; Punnett [1971]). The same observation was made
with chloroplasts of K + deficient plants of Zea mays and Cucurbita sativa. They
showed poorly defined granal stacks and a more extensive system of stroma lamellae
(Hall et al. [1972]; Penny et al. [1976]). Other structural alterations in chloroplasts
of K + deficient plants were described earlier (Thomson and Weier [1962]; Marinos
[1963]; Vesk et al. [1966]). Treatments with sodium chloride or mycostatin resulted
in a removal of K+ from the tissue and the chloroplasts and in an alteration of the
fine structure of chloroplasts of Phaseolus vulgaris and, to a lesser extent, of Hordeun
vulgaris. However, these treatments had no effect in chloroplasts of Beta vulgaris
(Marschner and Mix [1974]). Such differences reflect differences in salt tolerance.
Removal of K + also caused swelling of proplastids (Hecht-Buchholz and Marschner
[1970]; Hecht-Buchholz [1971]; Hecht-Buchholz et al. [1974]).
5.2 Possible role of K + in the activity of enzymes involved in CO 2 fixation
Both ATP and NADPH are required for the reduction of CO 2 to the carbohydrate
level. Whereas photophosphorylation and photoreduction are light-dependent
reactions, the incorporation of CO2 occurs even in the dark. There are reports on
the rates of CO 2 uptake by whole leaves or leaf slices. K+ deficient plants showed
reduced CO 2 assimilation (Peaslee and Moss [1966]; Cooper et al. [1967]; Watanabe
and Yoshida [1970]; Wolf et al. [1976]). At K + contents below 1.5% in dry matter,
the rate of CO 2 fixation in Zea mays was drastically reduced (Estes et al. [1973];
Koch and Estes [1975]). In experiments with Beta vulgaris Terry and Ulrich [1973a, b]
assumed that decreased CO 2 uptake by plants of low K+ status was caused by an
increase in mesophyll resistance to the flow of CO.
Similar results have been obtained by other methods. Using isolated intact chloroplasts of Spinaciaoleracea and Viciafaba grown under various K+ nutrient conditions,
Possingham [1971] and Pflfiger and Cassier [1977] showed that CO, fixation was
reduced with more severe K+ deficiency. Additions of K+ to the test medium stimulated CO, uptake by intact chloroplasts of plants grown normally (Larkum and
140
+
Boardman [1974]). Pfliiger and Cassier [1977J demonstrated a need for K in the
suspension
the
chloroplast
entire process of CO 2 fixation. Addition of valinomycin to
resulted in a severe decrease in CO, fixation (Table 8). This ionophore is known
to increase the permeability of biological membranes to K+ (Pressman and De Guzman [1975]). Thus, in these experiments the membranes of the envelope became
permeable to K + , resulting in much lowered K+ concentration in the stroma of the
+
chloroplast. This valinomycin effect can be reversed by adding K to the test medium
fixation is restored
CO,
of
rate
at concentrations of 50 to 100 mM, when the initial
the enzymes
affects
K+
is
that
results
these
of
interpretation
general
(Table 8). The
involved in the reduction of CO,. Working with the marine diatom Phaeodacylun
tricornatum, Overnell [1975] came to the same conclusion.
Unfortunately data are scarce on the influence of K+ on individual enzymes of the
Calvin cycle, except for the activation of granular-bound starch synthase by K+
(Murata and Akazawa [1968]; Nitsos and Evans [1969j; Hawker et al. [1974J;
see Table 10).
Table 8. Influence of valinomycin and K+ on rate of CO, fixation by intact spinach chloroplasts (PflIiger and Cassier [1977])
Treatment
Rate of CO, fixation, 1
imoles mg chlorophyll ' h-
% of control
Control
100 mM K ...........................
I IM valinomycin ......................
100 mM K+ 1 ttM valinomycin .........
23.3
79.2
11.0
78.4
100
340
47
337
6. Role of potassium in transport of photosynthates
K+ deficiency decreases transfer of photosynthates from the leaf to other parts of
the plant (Table 9), as has been demonstrated in various crops, e.g. sugar cane
(Hartt [1969, 1970]), cotton (Ashley and Goodson [1972]), tomato (Mengel and
Viro [1972]), potato (Haeder et al. [1973]) and Ricinus communis (Mengel and
+
Haeder [1977]). Translocation of photosynthates is via the phloem. K appears to
be involved to some degree in photosynthate translocation in the sieve tubes, and it
is found at relatively high concentration in the sieve tubes (see 3.3 above).
Table 9. Effect of K, deficiency upon export of 14C-labeled photosynthates from the leaves of
sugar cane. 14C-data expressed as % of total counts, 90 minutes after feeding 14CO 2 to the leaf
blade (Hartt [1969])
Part of plant
14C, %
+
+K
-K +
Fed leaf blade ..................................................
Sheath of fed leaf blade .........................................
Joint of fed leaf blade ...........................................
Leaves and joints above fed leaf ..................................
Stalk below joints and fed leaf ....................................
54.3
14.2
9.7
1.9
20.1
95.4
3.9
0.6
0.1
0.04
141
A key step in the translocation process is the mechanism by which sugar is taken up
into the sieve tubes, i.e. phloem loading. Assuming that sucrose transport into the
sieve elements is dependent on pH and other parameters (Giaquinta [1977]: Konor
et al. [1977]; Hutchings [1978a]), Malek and Baker [1977, 1978] proposed a model
which includes K + (Figure 18). According to this hypothesis the uptake of sugars
free
space
sieve
(apoplast)
Sugars
r
element
_+
uug ar-H *s ymp ort
or
H~_
co-transp or
AT P ener gi sod
pump
K+
High
Low
H+
K
Low
Ht
K
High
by
an
ATP-ase,
at the plasmaFig. 18. Hypothetical scheme for a H+ pump, possibly energised
lemma of sieve tubes coupled with K+ influx. Sugars are transported into the sieve tube by a
sugar-H+-cotransport down the H- gradient resulting from the H + pump (redrawn from
Malek and Baker [1977])
into the sieve tubes is coupled to the influx of protons down a H + gradient. The H +
gradient itself is generated by a pump, possibly energised by an ATP-ase, in the
membrane of the sieve element which extrudes H± in exchange for K+. In fact,
Hutchings [1978b] was able to demonstrate that the magnitudes of H + efflux and
K+ influx were consistent with a I I stoichiometry K+: H+. As regards the participation of an ATP-ase in phloem loading, located at the plasma membrane of the
sieve tubes, the cytochemical evidence available is conflicting. While ATP-ase
activity was found in leaves of Cucurbita maxima (Gilder and Cronshaw [1973a])
and Nicotiana tabacurn (Gilder and Cronshaw (1973b, 1974]), there was no ATP-ase
activity detectable at the plasma membranes of the sieve elements in the leaves of
Pisum sativuin (Bentwood and Cronshaw [1978]).
142
7. Activation of enzymes by potassium
Evans and Sorger [1966] listed some 46 enzymes in various plant, animal and microbial species which need K+ for maximum activity. Later, Suelter [1970] expanded
this list to 58 enzymes and enzyme systems and suggested three classes of the various
enzymic reactions activated by monovalent cations. Some of the K± activated enzymes
of plants are listed in Table 10. The first class comprises enzymes transferring phosphoryl groups, e.g. pyruvate kinase (phosphoenolpyruvate + ADP-.pyruvate + ATP).
The second class includes enzymes catalysing eliminating processes, e.g. aldehyde
Table 10. Some plant enzymes activated by K (from Suelter [1970j)
Enzyme classes
Reactions catalysed
Plant sources
Pyruvate kinase
(E.C. 2.7.1.40)
phosphoenolpyruvate + ADP=
pyruvate+ATP
6-Phosphofructokinase
(E.C. 2.7.1.11)
Glutathione synthetase
(E.C. 6.3.2.3)
Succinyl-CoA synthetase
(E.C. 6.2.1.5)
Cucurbitapepo (seedlings),
Zea mays (seedlings),
and others
yeast
fructose 6-phosphate +ATP =
fructose 1,6-bisphosphate+ADP
glutamyl-cysteine+glycine+ATP= yeast
glutathione+ADP+ Pi
succinate+CoA+ATP=
Nicotiana tabacum (leaves)
succiny-CoA +ADP+ Pi
Glutamyl-cysteine
glutamate + cysteine + ATP
=
Phaseolus vulgaris (seedlings),
synthetase
(E.C. 6.3.2.2)
NADI synthetase
(E.C. 6.3.5.1)
glutamyl-cysteine + ADP + Pi
Triticum vulgare (wheat germ)
Enzymes transferring
phosphoryl groups
Formyltetrahydrofolate
synthetase
(E.C. 6.3.4.3)
deamido-NAD++glutamine+
yeast
H0O+ATP=
NAD I + glutamate + AMP + PP
formate + tetrahydrofolate + ATP= Spinacia oleracea (leaves)
10-formyltet rahydrofolate +
ADP + Pi
Enzymes catalysing
eliminating processes
Threonine dehydratase
(E.C. 4.2.1.16)
Fructose-bisphosphate
aldolase
(E.C. 4.1.2.13)
Aldehyde dehydrogenase
(NAD(P) +)
threonine +1-10 =
2-oxobutyrate + NH3 + H 20
fructose 1,6-bisphosphate =
dihydroxyacetone phosphate+
glyceraldehyde 3-phosphate
aldehyde+ NAD(P)- + H2 0
yeast
yeast
yeast
acid + NAD(P)H
(E.C. 1.2.1.5)
Unclassified enzymes
Starch synthase
(particulate)
(E.C. 2.4.1.2i)
ADP-glucose + (1.4-a-D-glucosyl)n
=(l.4--D-glucosyl)n+i +ADP
Zea mays (seeds),
Pisum sativum (seeds),
Solanum tuberosum (tubers),
and others
143
dehydrogenase (aldehyde+NAD[P]+H 2 0-acid+NAD[P]H). The third group includes enzymes which cannot be placed in either of the first two classes, e.g. starch
synthase.
Most is known about pyruvate kinase, which is a key enzyme in respiratory metabolism,
controlling the formation of pyruvate and ATP from phosphoenolpyruvate and
ADP. In recent years pyruvate kinase has been isolated and purified from many
sources (Meli and Bygrave [1972]; Duggleby and Dennis [1973a, b]; Vaccaro and
Zeldin [1974]; Turner and Turner [1975]) and an absolute requirement of K + for
maximum enzyme activity has been demonstrated. Much of the work on this enzyme
in higher plants concerned the effect of K + in relation to plant nutrition (Evans
[1963]; Besford [1975]; Wildes and Pitman [1975]). Besford [1978] even proposed
the use of pyruvate kinase activity in leaf extracts as a method of quantitative determination of K + in plant tissues. An early effect of K + deficiency in plants is the
accumulation of reducing sugars through reduced pyruvate kinase activity (Evans
and Sorger [1966]). Enzyme activity can be reduced either by low levels of the enzyme
itself or by low levels of K+ in the plant (Evans [1963]). For maximum enzyme
activation K + and a Mg2 + concentration of about 1 mM are necessary (Tomlinson
and Turner [1973]; Vaccaro and Zeldin [1974]). The Km for activation by K + is
about 2 mM, depending on the plant species used. Maximum stimulation is obtained
with 50 mM K + in the reaction medium (Miller and Evans [1957]; Wildes and Pitman [1975]). The effect of K + on pyruvate kinase can be partially substituted by
Rb + and NH 4 + (MeCollum et al. [1958]; Hess and Haeckel [1967]). However,
this is only an in vitro finding, for in the living cell Rb + is scarcely detectable and
NH, + would be toxic in the concentrations required (Mengel [1969]). Rb + also
seems to be toxic, when applied to the higher plant for a considerable time (Michael
[1959]; Maynard and Baker [1965]; Berry and Smith [1969]). K + is the only
monovalent cation responsible for stimulation and activation of pyruvate kinase
and related enzymes in vivo. In the intact plant the specificity of this effect must be
considered in relation to the delivery of K + to the site of its action and its ready
transport through plant cell membranes.
It may be assumed that more enzymes will be found to be K + dependent. K+-containing buffers are used in many biochemical experiments and may mask possible
effects of K + on the reaction.
The mode of action of K+ upon enzymic reactions remains to be elucidated. Comprehensive studies on this subject were presented by Wilson and Evans [1968], Evans
and Wildes [1971] and Wilson [1971]. There are two main concepts. One possibility
is that ions affect enzymic processes by influencing the structure of water (Miller
and Davey [1967]). At physiological conditions, e.g. normal temperatures, water
does not exist as single H 2 0 molecules but has a ring structure due to the formation
of hydrogen bridges. All the high-molecular compounds, including enzymes, are
hydrated with H 20 molecules arranged in such a ring structure. Ions are hydrated
to an extent, depending on the charge, and thus the total diameter of an ion (hydrated
diameter) is altered. The hydration water is bound relatively firmly. It follows that
strongly hydrated ions (Li + , Na +) have more influence on the ring structure of water
than less strongly hydrated ions (K + , Rb + , NHt), as outlined by Alekseev and
Aburachmanov [1966]. When this idea is applied to macromolecules, it means that
Li+ and Na + will cause dehydration and the same may also apply, though to a lesser
degree, to the weakly hydrated ions K + , Rb + and NH, + . In the latter case the de144
hydration of the macromolecule may result only in a weakening of the hydration
layer which would have beneficial effects for an enzyme-type macromolecule.
A somewhat similar explanation of the function of monovalent cations in enzymic
reactions concerns the structure of the enzyme protein (Mildvan and Cohn [1964];
Keyne and Suelter [1965]). Working with glycerol dehydrogenase from Aerobacter
(Lin and Magasanik [1960]) and formyl tetrahydrofolate synthetase from spinach
(Hiatt [1965]), it was shown that K + acts upon the enzyme by lowering the Km
values, indicating improved affinity between the enzyme and its substrate. On the
other hand, Kachmar and Boyer [1953] and Wildes and Pitman [1975] reported an
unchanged Km but an accelerated reaction (Vma,) of pyruvate kinase from roots
of barley seedlings. The same was found for acetic thiokinase of yeast (Evans et al.
[1964]) and spinach (Hiatt [1964]).
These findings may be explained by the assumption that the number of substrate
binding sites at the enzyme is increased in the presence of stimulating cations.
As already mentioned, K+, Rb+ and NH,+ have similar effects on enzymes in vitro
but Na+ and Li + are either without influence or even inhibitory. This may be attributed to their greater radii in the hydrated state. Taking this into account, Wilson
and Evans [1968] proposed a mechanism for the action of monovalent cations on
enzymes (Figure 19). Assuming that enzymes undergo conformational changes,
K+ and other activating monovalent cations with a relatively small ionic radius
of about 0.4 nmf cause a change in enzyme conformation which, in turn, leads to an
increased number of substrate binding sites. On the other hand, the larger cations,
Li + and Na+ are ineffective because they cannot open up the active sites of the enzyme
to the substrate.
S
Hydrated diameter (nm)
Li+
)NONACTrArOR
0
0
0.732
NONACTIVE
Na' 0.562
K+
Rb'
NH+
N4
0.376
0.362
0.378
0.370
0o
ACTIVATOR
CATIONS
Q
O
0
ACTIVE
00
Fig. 19. Model of enzyme activation by monovalent cations: conformational changes induced
by small activator cations (K +, Rb', NH 4+) and large nonactivator cations (Lit, Na +) (redrawn from Wilson and Evans, [1968])
145
8. Potassium and respiratory metabolism
Respiration and the reactions of the associated electron transport are directly dependent on substrates provided by photosynthesis. In heterotrophic organisms and
in cells of the green plant that are not photosynthetically active electron flow and
ATP synthesis through respiration are the energy sources. The production of ATP
in respiration thus has an effect in conserving energy, similar to the photosynthetic
process.
Moderate K+ deficiency increases the rate of respiration determined as 02 uptake
(Latzko [1965]; Botrill et al. [1970]; Walker and Ward [1975]; Besford and Hobson
[1975]). Simultaneously, CO, liberation is decreased, resulting in reduced RQ
values (Pirson et al. [1952]; Jackson and Volk [1968]). These values, however,
give no information on the efficiency of respiration, that is on the relation between
02 consumed and ATP produced. Moreover, K+ deficiency initially accelerated
respiration by taro plants but increasingly severe deficiency ultimately decreased it
(Okatnoto [1969]). This author found increased activities of the tricarboxylic acid
cycle and of 02 uptake and supposed that respiration of leaves low in K+ may be
uncoupled from oxidative phosphorylation, resulting in diminished ATP synthesis.
Similar results have been obtained with yeast (Latzko and Claus [1958]).
So far as we know, no experiments have been done using mitochondria isolated
from plants with a varying K+ supply, but there are reports of experiments with
mitochondria of K + depleted cells (Moroff and Gordon [1973]; Miyahara and
Utsuni [1975]). A certain intramitochondrial K+ concentration is needed for a
satisfactory rate of phosphorylation. ATP formation itself controls the rate of electron
flow in the respiratory chain. Under conditions where ATP synthesis is decreased,
e.g. at suboptimal K+ levels in the cell, the controlling function on the electron
transport cannot be fulfilled. This causes an uncoupled accelerated electron flow,
that is increased 02 consumption and a decreased P/O ratio. Wen this happens, the
energetic capacity of the substrate is not fully utilised. Thus, one main question as
regards the role of K+ in respiratory metabolism concerns its function in the mechanism of ATP formation.
As in the case of chloroplasts, let us first consider the topography of a mitochondrion.
It is surrounded by two membranes of which the outer one is readily permeable to
all substances of low molecular weight. The inner membrane as the functional barrier
partitions the intramitochondrial compartment (matrix) and the extramitrochondrial
space, i.e. the cytoplasm of the cell. The respiratory chain and the site of ATP synthesis
are located on the inner membrane (Figure 17). The mechanism of oxidative phosphorylation is similar to that of photophosphorylation. While electron transport
in chloroplasts is induced by absorption of light energy, redox reactions are responsible
for this process in the inner mitochondrial membrane.
The chemiosmotic theory of phosphorylation postulates an electron transportdependent flux of H + across the mitochondrial membrane (Mitchell [1961, 1966]).
Whereas in chloroplasts the protons were taken up into the intrathylakoid space,
+
mitochondria extrude H into the cytoplasm ( Wilson and Graesser [1976], Figure 17).
+
output, cations must be taken up into the matrix space or
H
of
As a consequence
anions released from there in order to maintain electrochemical neutrality. In the
steady-state of this process, the amount of extruded H+ equals that of the protons
taken up, i.e. there is no net flux of H+ in any direction. The back-flux of H+ into
146
the intramitochondrial compartment is coupled to the phosphorylation of ADP to
ATP. The other cations and possibly anions remain in the steady-state distribution
determined by the extent of the electrochemical potential.
As in the case of chloroplasts there are contradictory reports as to the kind of ions
which compensate the H+ extrusion from the mitochondrion. In addition, it is more
difficult to isolate functional plant mitochondria. Their rates of H + transport are
only small compared with chloroplasts or mitochondria of animal tissues (Wilson
and Graesser [1976]). It is known that mitochondria accumulate Ca 2 + at high rates
(Hanson and Hodges [1967]. Wilson and Minton [1974]). The accumulation of
Ca 2 + and also of K + appears to balance the H+ extrusion to a large extent (Chen
and Lehninger [1973]; Earnshaw el al. [1973]; Kirk and Hanson [1973]; Moore
and Wilson [1977]). Thus the function of K + in respiratory metabolism of plants
in effect is closely related to its role in photosynthesis (Figure 17).
As already described for chloroplasts, withholding K + from a plant results in a
modification of the structure of the cells. In the subcellular region of root tips of
Cucurbilapepo removing K+ in the growth medium for a long period (3 to 34 days)
led to destructive changes in the fine structure of the mitochondria (Kursanov and
Vyskrebenceva [1967]). This effect is possibly a secondary one and may be caused
by premature senescence resulting from K+ deficiency (Hecht-Buchholz and Marschner [1970]). In experiments with rapidly K + depleted corn root tips, the latter
authors were unable to find significant modifications in the fine structure of the
mitochondria in relation to the K+ content of the cells. A possible way to obtain
plant tissues low in K + is by treatment with Na+. Substitution of K + by Na+ resulted
in an increased number of mitochondria in the cells of corn roots and bean leaves.
As the RQ values remained unchanged in these short-term experiments, the increased
number of mitochondria in the cells may be interpreted as a compensatory reaction
of the cells to the Na+ induced damage of the metabolic machinery in the mitochondria (Table 11). In the xylem parenchyma cells of the proximal region of corn
Table 11. Effect of substitution of K+ by Na on O uptake, CO2 release and on the number
of mitochondria in the cells of corn root tips (from Hecht-Buchholz et al. [1971])
Treatment
0, uptake,
td mg fr.w.- 1 h- 1
CO, release,
RQ value,
II mg fr.w. - h - 1 CO/O,
Control .............
25 mM Na ..........
78
72
66
60
0.85
0.84
Number of
mitochondria
per cell
(cross-section)
35.1
53.1
roots and bean stems, salt stress led to partial replacement of K+ by Na+, accompanied
by large increases in the number of mitochondria (Kramer et al. [1974]; Yea et al.
[1977]). Quantitative measurements of respiration in the salt-stressed corn roots
also indicated the enhanced formation of mitochondria to be a compensatory reaction
(John and Lduchli, in preparation).
147
9. Conclusion
Potassium is an important element for plant growth. This review has attempted to
demonstrate that K + is equally important in many aspects of plant metabolism.
Information has been drawn mainly from publications of the last 25 years, with
emphasis on the most recent advances.
K+ is selectively transported through plant cell membranes and circulates readily
in the plant to the sites of its function. K+ appears to have many functions in plant
metabolism, particularly in photosynthesis, respiration and enzyme activation.
However, our knowledge of the exact mode of action of K+ in metabolism is still
+
limited. This may have to do with the fact that K does not appear to be a constitu+
ent of any plant metabolites, nor is K known to be incorporated in plant structures.
Continued study of isolated cell organelles, particularly functionally intact chloroplasts and mitochondria, and concentration on the role which K+ plays in enzymic
reactions appear promising. Another promising avenue of research is the investigation
of the significance of K+ as an osmotically active solute and its involvement in turgor
regulation of plant cells.
Acknowledgement
The authors are grateful to W.Stelter for critical suggestions during development
of this review.
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163
The Effect of Potassium on Growth and
Yield Components of Plants
O.Steineck, Institute of Plant Production and Plant Breeding, University of Agriculture,
Vienna/Austria*, and
H.E.Haeder, Landwirtschaftliche Forschungsanstalt Biintehof, Hannover/Federal Republic
of Germany"
1. Introduction
There is no denying the fact that there has been tremendous progress in crop production in all countries with highly developed agriculture over the past 25 years.
It is not difficult to find reasons for this progress. Yield and quality haye both been
markedly improved by applying the results from agricultural research. The main
contribution has been from plant breeding which, by developing new cultivars, has
placed the genetic potential of crops on an altogether higher level than applied a
quarter century ago.
Improved cultural methods and crop protection measures have made possible the
realisation of the full potential of crops appropriate to the areas in which they are
grown. Correct plant nutrition is of the greatest importance in enabling such highyielding crops to grow properly and the modern cultivars have a much higher nutrient
requirement than the low-yielding types grown formerly. The requirements of modern
crops can only be satisfied by the regular and rational use of mineral fertilisers.
Whenever genetic improvement results in higher yield, it is necessary to increase
fertiliser application. If this is not done, the full potential will not be achieved.
To illustrate how yields have increased in the past 25 years some Austrian statistics
are quoted in Table 1. Other developed countries can tell a similar story.
Because yields have more than doubled, attention must be paid to fertiliser policy
to ensure that the nutrient requirements of crops are covered by the manures and
mineral fertilisers applied. Increased yields mean higher nutrient removals. It is also
clear that further improvements which we may expect to be made in the future will
necessitate further upward revision of fertiliser recommendations for practical
farming.
The question of the part played by potassium in raising crop yield over the past 25 years
is not without interest in the context of this paper. In order to be able to give proper
fertiliser advice, it is necessary to know the function of potassium in forming yield
and how it affects crop quality.
* Prof. Dr. O.Steineck, Director, Institute of Plant Production and Plant Breeding, University of Agriculture, Gregor-Mendel-Strasse 33, A-1180 Vienna/Austria
* Dr. H.E.Haeder, Agricultural Research Station Bflntehof, P.O. Box 3209, D-3000 Hannover/Federal Republic of Germany
165
Table 1. Average yields of major crops in Austria, 1952-1976 (Anon. [1977])
Crop
1952
t/ha
1976
t/ha
W heat ....................................................
R ye ......................................................
Barley ...................................................
W heat ......................................
.............
Barley ...................................................
O ats .....................................................
Maize ................................................
Potato ...................................................
Sugar Beet ................................................
1.99
1.6 1
1.66
1.58
1.76
1.70
3.04
15.1
21.1
4.33
4.0 1
3.95
3.27
4.12
3.4 1
5.86
23.8
46.1
Regular colloquia and symposia organised by the International Potash Institute
have served to increase and spread knowledge of plant nutrition and have thus
improved and broadened the basis of recommendations for the practical use of
potash fertiliser. Drawing on the results of worldwide experimental results, we are
now in a much better position than we were 25 years ago to bring to the farmer the
nedd for correct and sufficient potash manuring.
The aim of this paper is to describe the function of potassium in some of the metabolic
processes in the plant and then to describe its influence on the yield components
of various crop plants.
2. The fundamental function of potassium in plant metabolism
The synthesis of organic matter and its distribution between different parts of the
plant during the course of growth is closely related to the yield of the crop. Research
in the international field has been largely successful in elucidating those metabolic
processes for which a continuous supply of potassium is needed. It has become
clear that its function cannot be considered in isolation. It is established that it has
a specific function as a regulator in optimising a large number of biochemical processes. In assessing the importance of potassium as a plant nutrient it is not sufficient
to base conclusions solely on its effect on yield.
2.1 The effect of potassium on plant yield under conditions of varying nutrient supply
Mitscherlich [1921] was the first, on the basis of numerous pot experiments, to
allot to potassium a numerical rank among other plant nutrients. This was based on
the effects of nutrients on the weight of the above-ground vegetative parts of the
plant or weight of reproductive organs. The intensity of the effect of a nutrient was
expressed in the constant 'c' measured in dt/ha or in g/pot. On the latter basis, 'c'
was 0.20 for N, 0.40 for K 2 0 and 0.60 for P2 0 5 (Boguslawski [1972], Sadeghian
and Linser [1967]). Thus, basing measurements on the above-ground parts of the
plant, potash was placed in a position intermediate between N and P20.
166
It was shown that yield depended upon the level of potassium supply but, by the
nature of the experiments, growing the plants in a solid medium, nutrient supply
varied throughout the growth period. Nutrient levels in solid media must be decided
at the beginning of the experiment and cannot easily be adjusted during its course,
so that they become lower as the plant grows (Kopetz and Steineck [1962], Steineck
[1962]). Conditions in pot culture are somewhat similar to those in the field under
practical conditions. However, experiments of this type give little or no information
about the specific function of K in plant metabolism.
2.2 The investigation of the effect of potassium in plant metabolism under conditions
of constant nutrient supply
The importance of potassium in affecting growth and yield depends upon its specific
function in plant nutrition. Thus it is desirable in the first place to measure the effect
of varying the K supply in isolation, to find out what part of organic matter formation
can be ascribed to potassium, and this should be done in relation to the supply of
other nutrients. It was only possible to do this by using a technique which assured
defined nutritional conditions over a period (Steineck [1963, 1963a, 1968]).
Such a method is offered by growing plants in nutrient solutions which are renewed
at frequent intervals so that nutrients are maintained at substantially constant concentration (Kopetz and Steineck [1962], Steineck [1962]).
Since 1960 a number of such experiments has been carried out with a range of crop
plants in order to elucidate the effect of potassium in comparison with that of nitrogen.
These two nutrients are taken up by the plant in much larger quantity than phosphorus, sulphur, calcium and magnesium, which gives the impression that their functions
are interdependent. To determine how each nutrient affected growth, the experiments
were designed to measure the effect of varying the ionic concentration of N at low
and high K concentration and vice versa by similarly varying K concentration at
low and high N levels.
2.2.1 fhe effect ofpotassium on total plant weight
An advantage of solution culture in comparison with using solid media is that it is
easy to measure root weight, so that the influence of nutrients on total plant weight
can be determined. Series of experiments using the same increasing levels of nitrogen
and potassium were carried out with eight different mono- and dicotyledenous crop
plants. The effect of increasing N supply is illustrated in Figure I.
Nitrogen was supplied at 8 levels in geometrical progression from 0.10 to 1.64mg atom/I.
The low rate of K supply was 0.10 mg atom K/I giving a N: K ratio of 1 :1 at the lowest
N level and a steadily widening ratio. Total plant weight increased up to the N. level,
after which further increase in N supply had no effect. At the high K level the picture
was different; plant weight increased steadily and in proportion to N supply up to
the highest level applied, at which point the N:K ratio was 1:1. Thus, at the high
level of K supply, the plants were able to utilise nitrogen fully in dry matter production. Clearly the formation of dry matter is directly dependent upon nitrogen
supply.
The effect of increasing potassium supply is shown in Figure 2.
The response of plants to increasing K supply is quite different from the N response.
167
FRESHWmGHT OF WNOLEI LANT:
--- LOWK SUPPLY- K1
HIGIK SUPPLY. K8
VIEW INCREASE
00
--.-
-------------------
90
--------
-------
%00
-_
80
Ks---
90
-___-80
70
70
-
60
50
6-0
50
40
------
3
0
40
so
2
40
N TREATMENT
67860
70
80
90
I000
Fig. 1.The effect of increasing ionic concentration of N in the nutrient solution on dry matter
production by plants at low (K,) and high (K.) levels of potassium. (Mean of 10 experiments).
FRESHWEIGHTOF WHOLE PLANT:
H N SUPPLY- NI
--- H LOW
.....HIGHNSUPPLYN8
_____________
________
INCREASE
EW
to%
100-
0Ne
O-10 N
8
-
90-
0
..
0
0 50
S
0
80
90
100
Fig.2. The effect of increasing ionic concentration of K+ in the nutrient solution at low
(N1) and high (Na) nitrogen levels. (Mean of 10 experiments).
168
At the lowest N level (0.1 mg atom N/I) increasing K supply has little effect on total
plant weight, but at high N supply (1.64 mg atom N/I) there is a different picture and
the plant responds to K as the N : K ratio narrows, right up to the K, level, relative
yield being increased by 70-100% at this level. There is no further increase from K, to
Kf. Thus there appears to be no direct connection between relative yield increase and K
supply. The fact that increasing K has no effect at the low N level, while increasing the
K supply enhances response to increasing N supply, indicates that the main function
of potassium is in controlling N metabolism. Both nutrients must be supplied in
sufficient concentration for the realisation of their optimum effects.
2.2.2 The effect ofpotassium on nitrogen utilisation in dry inatter production
The experiments measuring the effect of potassium on nitrogen response carried out
under conditions of constant nutrient supply have shown that the efficiency of N
utilisation by the plant is determined by the potassium supply. The potential of N
to increase yield is only fully realised when the necessary amount of potassium is also
supplied. The effect of potassium is best demonstrated by showing the effect of
increasing K supply on production of dry matter or fresh plant material per unit N
(0.1 mg atom N/I) applied, as is illustrated in Figure 3 showing the results of an experiment with oats in which 5 rates of each N and K were applied in factorial combination
(25 treatments in all).
At the lowest K level (K,) production per unit N decreases markedly as the N rate
is increased but as K supply is increased production per unit N increases steadily
up to the K, level. Efficient N utilisation as, for example, between N, and N4 at
levels of K supply K4 and K5 is only realised when sufficient K is available. Similar
results were obtained using the same methods with other crop plants.
/i '
35
35r--
3
30L
25
25"
20
20"
5
-0
Kfr
(,0)
(1,49)
(1,95)
( 2.4 4 )
(3,05)
S500
(ION CONCENTRATION mg atom/I of solution)
Fig.3. The effect of increasing K supply on fresh plant weight per unit N (0.1 mg atom N/I).
169
2.3 Discussion
The results of the solution culture experiments described above make it clear that
potassium has a specific regulatory function in N metabolism in the plant (Steineck
[1963]). There is a close interaction between the two nutrients. In quantitative terms,
the direct effect of potassium on yield is less marked than that of nitrogen, which
itself constitutes a large part of the organic matter synthesised during growth (Steineck [1974]). On such a basis it is understandable that Mitscherlich [1921], on the
basis of his constant 'c', ranked it as second to nitrogen in importance, but its function in yield formation is, in fact, of at least equal importance.
Mengel and Koch [1971] investigated the effect of K on N utilisation in sunflower
and found that K supply strongly influenced the effect of N. Pawlowski [1966] did
experiments in solution culture of constant composition with petunia, chrysanthemum
and begonia to determine the optimum N:K ratio and found that dry matter production per pot increased as the N:K ratio narrowed.
Mengel and Helal [1968, 1968a], in pot experiments in which N and K were applied
in various combinations, studied, with oats and spring wheat, their effects on the
composition of the soluble amino-N fraction. They found that N increased while K
decreased the content of such compounds. This is another way in which the favourable
effect of K on N utilisation is apparent. The findings of our solution culture experiments are confirmed by these findings.
The interdependence of N and K in yield formation was demonstrated with Mentha
piperita L. by Franz [1972] and with Matricaria chamomilla L. by Franz and Kirseh
[1974]. It was also shown in these experiments that N had a greater direct effect
on yield than K. Talibudeen, Page and Mitchell [1977] investigated the N-K interaction in pot experiments with Lolium perenne and Bruchholz et al. [1977] reported
field experiments on the effect of N at varying K levels on millet and groundnuts in
Tamil Nadu, showing the same close interdependence of the two nutrients under
subtropical conditions. Further investigations of the N-K interaction in field experiments have been reported by Munson [1970], Chevalier [1970, 1975], Lou [1978]
and S.C.P.A. [1970, 1975, 1978] with a wide range of crops under a variety of growing conditions.
There is ample evidence from experiments in solution culture, in pots with solid
media, and in the field of the special function of potassium in yield formation. It is
clear that the effects of nitrogen are dependent upon potassium supply and the two
nutrients should always be considered in combination.
3. The effect of potassium on photosynthesis
In the past ten years research has shown that potassium supply controls the course
of a number of physiological processes in the plant.
Amberger [1968] reviewed the function of potassium in carbohydrate metabolism
and more recent research has further elucidated the connection that has for a long
time been known between potassium nutrition and light utilisation in photosynthesis.
170
3.1 The effect of potassium on the intensity of CO 2 -assimilation
The rate of photosynthesis per unit leaf area depends upon light intensity and also
on a number of physiological and morphological factors. Potassium has an important
role in this metabolic process. The following results confirm the dependence of
assimilation rate on potassium supply.
The general effects of K supply on the efficiency of C-assimilation were shown in
solution culture experiments with spring wheat by Mengel and Haeder [1974]. Some
features of this work are shown in Table 2.
Table 2. The effect of K supply on CO, assimilation in spring wheat (Mengel and Haeder
[1974])
Stage of development and K supply
Root
(mg atom/I)
"C-concentration (Ci/g dry matter)
I. Flowering
0.4 K .................................
4.0 K ..............................
2. Milkripe
0.4 K .................................
4.0 K .................................
3. Ripe
0.4 K .................................
4.0 K .................................
Stem and leaf
Ear
0.78 (100)
1.04 (133)
1.00 (100)
1.01 (101)
1.66 (100)
1.86 (112)
0.26 (100)
0.44 (189)
0.73 (100)
0.85 (116)
1.68 (100)
2.33 (139)
0.39 (100)
0.56 (144)
0.25 (100)
0.29 (116)
0.37 (100)
0.43 (116)
Increasing the K supply tenfold increased the labelled C content of all plant parts
at all stages of growth. These results well demonstrated the effect of K on photosynthesis. Similar results were obtained by Sokod'ko [1971] with winter wheat and
rye, with sugar beet by Mikul'skaja [1972], with cabbage turnip, by Hoffinann and
Duffek [1973] who also found that increasing K hastened maturity. Estes, Koch and
Bruetsch [1973] investigated the effect of K on net CO2 intake by maize in water culture
and in the field. They found that raising the K concentration from 0.25 to 0.50 me/I
increased assimilation by 60%.
3.1.1 Dependence of the effect of light intensity on K supply
Being the energy source, light intensity is the most important environmental factor
influencing assimilation. According to Stay [1962] photosynthesis in Gaastra's
[1958, 1959, 1962] experiments increased up to light intensities between about
40 000 and 60 000 lux in maize, wheat, beet, red clover and sugar cane. The plants
were unable to utilise higher light intensity.
Staid and Peaslee [1977] grew maize in the open in sand culture at densities of
33 000, 98 800 and 118 000 per ha and light intensity was varied by artificial shading.
K was applied in solution at 15, 45, 135 and 400 microgram/cm 3. Assimilation rate
depended upon the point of insertion of the leaf. Results for the fifth leaf from the
apex at a density of 98 800/ha are given in Table 3.
171
Table 3. Interaction of light intensity and K supply on CO, assimilation by maize in sand
culture in the open with constant water supply (5th leaf). After Smid and Peaslee [1977]
Light intensity (lumen/cm 2 )
K supply
microgram/cm
3
7.5
3.2
1.6
solution
CO2 assimilation, mg/cm 2/h
15
45
135
400
22.6
28.8
32.8
35.3
.....................
.....................
.....................
.....................
(100)
(127)
(145)
(156)
20.1
25.0
27.2
28.2
K content
(100)
(124)
(135)
(140)
15.8
19.3
19.4
20.1
mg atom %
(100)
(122)
(123)
(127)
12.8
21.8
52.0
64.4
(100)
(170)
(406)
(503)
Assimilation rate fell as light intensity was reduced, slightly between 7.5 and 3.2 lumen/cm2 and more strongly thereafter. K concentration in the plant increased with
increasing K content of the nutrient solution at all light intensities. The assimilation
rate increased with increase in K content and this increase was more marked at high
light intensity as shown by the relative values in the table. At 1.6 lumen/cm2 K supply
had much less effect on assimilation rate. Thus, K was shown to improve the efficiency
of light utilisation.
Wicke [1973] worked with oats in pot culture and found that K uptake was increased
by increasing light intensity and that the increased K uptake led to better utilisation
of light energy. Haeder and Mengel [1975] and Mengel and Haeder [1976] worked
with oats in solution culture on this problem. Light intensity was reduced during the
generative phase to half the normal intensity and they found that enhanced K supply
during the change from vegetative to reproductive development largely compensated
for the reduction in light. Increasing K increased yield significantly at the mature
stage in normal daylight.
3.1.2 Influence of potassium on chlorophyll content and CO, fixation by chloroplasts
Chlorophyll content has an important influence on rate of photosynthesis. Potassium
increases photosynthetic activity of the chloroplasts. Pflaiger and Cassier [1977]
investigated the effect of varying K supply on the CO, fixing power of isolated chloroplasts (Table 4).
Table 4. Influence of increasing K concentration on CO, fixation by isolated chloroplasts.
After Pfliger and Cassier [1977]
K supply
Vicia faba
mg atom
K content
CO, fixation
K content
CO, fixation
K1 0.1 ...................
K, 0.5 ...................
K, 2.0 ...................
35.8
56.9
89.0
6.1
7.0
7.7
27.9
73.1
115.1
11.0
17.3
19.0
Spinacea oleracea
K content:
mg atom % in dry matter
CO, fixation rate: Micromol CO,/mg chloroplast/hour
172
In both Spinacia and Vicia, increasing K content increased the rate of photosynthesis
by chloroplasts, the effect being more marked in spinach than in field beans. The
experiment further demonstrated the effect of potassium in optimising phosphorylation
and photo-reduction. The effect on CO2 fixation was ascribed by the authors to the
fact that K increased the activity of enzymes concerned in CO 2 reduction.
Molotkovskij and Dzubenko [1970] and Coneva [1971] showed that K increased
the photochemical activity of chloroplasts in maize. These experiments in nutrient
solution showed that the effect of K in increasing chlorophyll content was accompanied
by increased activity of chlorophyllase. Pflfiger and Mengel [1972] reported experiments with spinach, field bean and sunflower concerning photo-reduction and photophosphorylation in chloroplasts. High K supply increased the intensity of both
parts of the photosynthetic process. Forster [1976] demonstrated the dependence of
chlorophyll content in the flag leaf of spring wheat on leaf K content - the chlorophyll concentration was increased by 88% at the highest K level.
3.1.3 The influence of K on leaf area and stomatal behaviour
Leaf area of photosynthetically active leaves is a fundamental determinant of the
rate of photosynthesis by the plant. This is particularly important so far as concerns
the leaves which are concerned in supplying assimilate to storage organs. In cereals
the laying down of starch in grain endosperm is overwhelmingly controlled by the
flag leaf, particularly in the beardless types. Bonaner and Dainbroth [1970] showed
that grain yield was a function of the area of leaf formed after ear emergence and
that the area of the older leaves had no influence. Stoy [1973] also found, using
labelled
CO 2, that 89% of carbon stored in the grain originated from the flag leaf,
14
CO being applied 3 weeks after ear emergence.
Forster's [1976] results on the influence of K supply on flag leaf area of 5 cvs of
spring wheat grown in pots are given in Table 5.
Table 5. The effect of K supply on flag leaf area in spring wheat (cm'/leaf). After Forster
[1976]
Cultivar
K
0.5 g KO
per pot
K2
1.5 g KO
per pot
K3
4.5 g K20
per pot
K olibri ...................................
Solo ..........................
...........
O pal ......................................
Janus .....................................
K leiber ....................................
25
30
39
34
24
37
39
50
46
36
36
47
72
57
39
M ean .....................................
30.4
41.5
50.2
The area of the flag leaf was measured at the milk-ripe stage and varied according
to cultivar. On the average of all cultivars, flag leaf area was increased by 2/, when
K supply was increased from 0.5 to 4.5 g KO/pot. The experiment also investigated
the effect of N and the NK interaction.
Smidand Peaslee's [1977] experiment to which reference is made in 3.1.1 above also
gave information on the effect of K nutrition on leaf area (Table 6). The effect of K
173
Table 6. Effect of K on leaf area in maize (at 98 800/ha). After Smid and Peaslee [1977]
K level
microgram/cm
Leaf area (cm')
3
No. 5
No. 7
solution
from the apex
15
45
135
400
5.7
6.6
5.9
7.2
.................................
.................................
.................................
.................................
Total
.5.0
5.6
6.1
6.6
No. 9
leaf area
cm 2
4.1
4.8
5.4
6.0
51.9
51.9
53.6
59.8
was most marked on the lower leaves and the effect on total leaf area was relatively
slight. However, only some of the leaves, essentially the upper ones, are of importance
in grain filling (Rommer and Dainbroth [1970]). Vig and Bhagvan Das [1977] in
pot experiments in India showed that K at 38 and 75 kg/ha increased the area of
the flag leaf in wheat by 18%.
Koch and Estes [1976], growing maize in solution culture, found that K did not
affect leaf area or number of stomata per unit area but that K improved the functioning of stomata so that water loss through transpiration was reduced by 30% and
CO, uptake improved by 70%. Thus CO, assimilation was enhanced.
3.1.4 The effect of potassiun on turgor
There is a mass of data, old and new, showing that K promotes water storage in the
cell and turgor of the cytoplasm and enzyme proteins, thus providing favourable
conditions for photosynthesis and the succeeding steps in metabolism. Turgor has
an important influence on leaf alignment, thus affecting light interception.
Table 7 shows results obtained in constant nutrient level solution culture of potatoes.
Increasing N at the low level of K increased water content only slightly. It increased
N content, increased K content only at N, and steadily reduced Ca content, so that
Table 7. Effect of increasing N at low and high K levels on content of H,O. N, K and Ca in
potato shoots (N applied as NH, NO3). Changing solution culture.
Level
Water content
N
g/g dry matter
dry matter %
K
J. At low K level
N, K ...................
N, K ...................
N, K ...................
N, K ...................
2. At high K level
N, K ....................
N, K ....................
N, K ... ................
N, K ..................
174
N content
K content
Ca content
mg atom %
8.43
8.62
8.68
9.18
250
284
303
338
87
78
79
97
50
43
38
39
7.68
8.77
10.25
10.93
212
245
277
302
136
146
148
155
46
40
44
39
with increasing N supply the K/Ca ratio, which is decisive in controlling turgor,
widened. In contrast, at the high K level (K4) water content was greatly increased
by increasing N. The higher K supply (K4 ) greatly increased K content and somewhat lowered Ca content in comparison with K, The effect of N on Ca uptake was
similar to that at the low K level. K/Ca ratio was throughout much higher than at
K. The much lower water content at K1 is a result of K shortage. It is only with
generous K supply (K4) that optimum turgidity is realised. The effect of K on water
content can also be seen in the results of a similar experiment with oats. Four levels
of each N and K were applied in factorial combination, the rates being kept constant
throughout growth. Figure 4 illustrates the effect of K on dry matter production
per unit N applied with the corresponding water content.
9K
K2
I
g
K3
50
z0-
30 -WATER CONTENT OF FRESH
MATTER PER mg atom N
-30
20-
-0
,¢0
r
DRY MATTER PRODUCTION
PER mg atom N
N3 (L129
0
(ION CONCENTRATION)
K,
(0,10)
K2
K3
Kz
(0,15)
(0,20)
(0.25)
0
(magatom L-' NUTRIENT SOLUTION)
Fig.4. Effect of increasing K on water content of oats (changing solution culture).
There was a close correlation between D.M. production per me N applied and water
content as affected by K application. At all N levels, increasing the K supply increased water content. Increasing N reduced water content at K, and K, but increased it at K3 and K4 , the highest degree of turgor being recorded at N, K4, N4 K3
and N4 K4.
The increase in water content brought about by increasing K supply is closely related
to the K content of dry matter. The effects of N and K on mineral content of potato
plants are shown in Figure 5.
Increasing K at each N level increased K content and slighly reduced N and Mg
contents. Ca content is hardly affected. Thus applying K closes the N/K ratio and so
175
=
II
ME %
350
MI CALCIUM
MAGNESIUM
NITROGEN
POTASSIUM
300
250
200
150
£O0
0
K,
K2
K(3
N,
1(Z
K,
K3
K2;
K4
Kt
K2
K(3
14
K,
KZ
K3
1(4
N,
N3
N2
Fig. 5. Dependence of N, K, Ca and Mg contents (mg atom %) in potato plants on K level at
different N levels (changing solution culture).
water storage in the tissues is improved. The K/Ca ratio is widened and this may
also be relevant to water storage.
Forster [1976] has also shown a close connection between succulence of the flag
leaf in wheat and K content (Table 8).
Table 8. Chlorophyll content, succulence and K content in flag leaves of spring wheat (average
of 4 cvs) as affected by K. After Forster [1976]
I. Chlorophyll content .......................
2
2. H 20/100 cm , total leaf area ...............
3. % K in dry matter ........................
KI
0.5 g K2O
per pot
K,
1.5 g K20
per pot
K3
4.5 g KO
per pot
6.4
349
0.32
7.7
380
0.86
12.0
442
1.76
There was a clear connection between flag leaf succulence and K content. K increased
chlorophyll content of the leaves, confirming other results.
Dhindsa et al. [1975] found a relationship between potassium supply, turgor pressure
and growth of fibres in cotton. Stuart and Jones [1977] found that the elongation
of isolated lettuce hypocotyls was favoured by potassium salts (KCI, KBr and KNO3)
but that increased turgor pressure was only a partial explanation.
176
3.2 Influence of potassium on translocation of assimilates
Using radioactive tracers, it has been possible to show that potassium favours the
translocation of assimilates. This indirectly increases the rate of photosynthesis,
first by removing metabolic products from the site of active photosynthesis and,
second, by enhancing the growth of further storage tissue, thus increasing the demand
for assimilate.
3.2.1 E~ftct on translocation
Viro and Haeder [1971] grew tomatoes in solution culture. They found that, with a
K supply of I mM/litre, only 8% of labelled C was found in the fruit 18 hours after
applying 1C, whereas, at 10 mMK/l, the comparable figure was 20%. This indicated
a direct influence of K on translocation.
Haeder [1977] found that K supply influenced all three stages in the translocation
process: diffusion of assimilate into the symplasma, active transport into the phloem
through the plasma membrane and passive transport in the sieve tubes. Phloem
loading, controlled by K, creates an osmotic gradient in the sieve tubes, thus increasing the flow of assimilate. In castor plants the flow of sap was much faster at a
K supply of I mg atom K/I in the nutrient solution than at 0.4 mg atom K/I.
These findings are confirmed by Haeder and Mengel [1972 with tomato and Mengel
and Haeder [1974] with spring wheat (Table 2); Viro [1974] with tomato in solution
culture and Miller [1964] in sand culture. Haeder et al. [1973], using potato in
solution culture, found that transport of assimilate to the tubers was accelerated
by K, while Anisinov and Bulatova [1976] showed that K deficiency reduced the
transport of sugar and I.A.A. in sunflower and field beans.
3.2.2 Effect on sink capacity
Growth of the sink is favoured by potassium through its influence on the efficiency
of N utilisation in metabolism. Linser [1972] said that the functioning of the photosynthetic organs is improved in proportion to the increase in sink capacity, hence
it would be expected that K would have an indirect effect on assimilation rate.
The dependence of photosynthesis rate on sink capacity was demonstrated by Bdrger
et al. [1956] who grafted the shoot of a low starch potato variety on to the lower
portion of a high starch variety. The leaves of the low starch scion produced as much
starch as the high starch variety, demonstrating the effect of the enlarged sink.
Later, using tracer techniques, it was possible to clarify the relationship between sink
capacity and assimilation rate. Such work on grain crops has been reported by Stay
[1963, 1969, 1973], Lupton et al. [1974] and Ruckenbauer [1975]. Addiscott [1974]
discussed the effect of K on the sink, pointing out that sink capacity resulted from
protein synthesis, favoured by adequate potassium while K also enhanced the activity
of starch synthetase in the sink.
3.2.3 Effect of K nutrition on the storage of assimilation products
The length of the period over which active photosynthesis and translocation continue
has much influence on grain filling. Experiments by Peaslee et al. [1971] showed
that in maize K increased the period over which assimilates were stored. Later
Peaslee [1977] showed that increased K supply lengthened the grain filling period
by 2 weeks.
177
Dyson and Watson [1971], in a field experiment on potato, found that increasing
P and K supply increased leaf duration by up to 17%, leading to a tuber yield increase of 11%.
In conclusion it can be said that much progress has been made in the 25 years since
Scheck [1953] published his work on the influence of K on carbohydrate metabolism
in various crop plants. We now have much more knowledge of the influence of
potassium on the various steps in the process.
4. Effect of potassium supply on the yield components and yield of crop plants
There has been much research in the past 25 years into ways in which yield is built up,
comprising physiological and morphological studies. As a part of such research
the part played by plant nutrition, including the effects of potassium supply, has
been much studied.
4.1 Genetic improvement
Improved understanding of the physiological and morphological basis of yield has
been of great value to plant breeders in their request for genetic improvement of crops.
The underlying aim has been to bring about increases in the proportion of valuable
assimilatory organs and to increase the size of the sink.
Particular progress has been made in wheat breeding where the breeding of short
strawed types has been accompanied by enlargement of the flag leaf, the main source
of grain filling. In dry regions yield depends much on number of ears per unit area,
in humid regions on grains per ear. In grain maize, plant density is closely controlled
by precision drilling and the aim of the breeder has been to increase grain yield per
cob. In potatoes and beet the importance of the relationship between leaf area and
storage organs has been realised.
From the cultural point of view, it is important to achieve rapidly complete ground
cover which, by shading the soil, conserves soil structure and friability. It also results
in increase in the leaf area index and, thus, improved light utilisation. Fertilisers are
important in this connection.
4.2 Shoot/root ratio
Shoot/root ratio is of primary importance in determining the way in which assimilates
are partitioned in the plant. This ratio is, in the first place, determined genetically
but can be modified by environmental factors, particularly water and nutrient supply.
The interaction of N and K supply on shoot/root ratio was investigated in solution
culture and the results are given in Table 9. It is seen that potassium had a large
effect in widening the ratio, and that this was more marked at the higher levels of N.
The optimum was recorded with the combination N3 K4 . Thus K, and N, favours
shoot growth.
178
Table 9. Influence of K on shoot/root ratio at varying levels of N (changing solution culture)
Rate
mg atom/i
N,
N,
N,
N,
(0.10)
(0.15)
(0.20)
(0.25)
............................
............................
............................
............................
K,
(0.10)
K,
(0.15)
K,
(0.20)
K,
(0.25)
1.83
2.30
2.49
2.38
2.21
2.64
3.19
3.07
2.25
3.00
3.49
3.24
2.16
3.19
3.59
3.33
4.3 Yield components
Heuser [1927128], working with grain crops, pointed out that yield was determined
by three components: plant population, number of grains per ear and thousand
grain weight. In maize the important components are population, cobs per plant,
grain number per cob and thousand corn weight. Potato yield is made up of plant
density, tubers per plant and mean tuber weight. Other crops, like beet and most
vegetables or salads, are simpler in that yield is made up of only two components,
plant population and average weight of the part harvested. The greater the number
of yield components, the more various are the possibilities of influencing yield by
modifying culture or nutrition.
4.3.1 Plant density
By plant density, Heuser [ibid.] meant the number of ear bearing shoots per m 2 .
In grain crops, tillering, which depends much on environmental conditions, is thus
very important, and nutrient supply has much influence. Figure 6 gives results of a
solution culture experiment with oats.
TtaERS PER
5TIUAS
PKR
1---s
j
3 I
-lTL-ER
-7
_
(Z
5)
I
'2
(ION CONCENTRATION) (1.0)
(1,5)
(2,0)
(2.5)
Mg atomn/I soltjon
Fig.6. The effect of N and K on filtering of oats in changing solution culture.
179
Treatments comprised all combinations of 4 rates of N and 4 rates of K. The effect
of K on tiller number was only slight at the lowest level but it was appreciable at N 3
and even greater at N 4 where increasing K supply from K, to K3 doubled the number
of tillers per plant. Similar results were obtained with 3 other grain crops.
4.3.2 Yield components in grain crops, rape andpotatoes
The interaction of N and K fertiliser supply was checked in a rotational field experiment testing 4 rates of N and K in a split plot layout at Grossenzersdorf, the field
station of our Institute. This was planted with spring barley in 1977 and results for
this crop are illustrated in Figure 7.
S
7
%
%20
5-
t5
110-
100
105"
95
0
95-
PLANT POPULATION
0 GRAIN YIELD
0
0
K0
K?
K2
93
N3
N3
N43
N43
0
W-OCORN WEIGHT
NO OF GRAIN/EAR
Fig. 7. Effect of increasing N and K on yield components in spring barley. Values relative
to NK 0 = 100% (Density 913 fertile tillers/m 2 , 18.6 grain/ear and 41.2 1000 grain weight).
Increasing K had little influence on grain/ear and thousand grain weight in this
year, but it markedly increased plant population and, through this, grain yield. This
finding is based on one season's results only, but is nevertheless important.
Different results were obtained by Forster [1976] in pot culture of 5 cvs of spring
wheat using a sandy soil low in nutrients (Table 10). In contrast to our field experiment with barley, he found no effect on plant density, number of grain per ear was
increased by 30% and 1000 corn weight by 38% at the highest rate of K.
Forster and Mengel [1974] measured the effect of K in improving yield of spring
wheat in pot experiments on a similar sandy soil. Mengel and Haeder [1974] investigated the effect of K on yield components of spring wheat in solution culture and
found only slight effects on plant density and grain per ear but a large increase in
1000 grain weight. Mengel and Forster [1968] investigated the effects of interrupting
180
Table 10. Effect of increased K supply on yield components in spring wheat (mean of 5 cvs)
Component
K,
0.5 g K20
per pot
K,
1.5 g K20
per pot
K3
4.5 g KO
per pot
J. Nb. of ears/pot ...........................
2. Grain/ear ................................
3. 1000 grain weight .........................
732
24.6
25.1
761
28.1
32.1
773
31.8
34.7
K supply at the tillering, shooting and ear emergence stages. Increasing K supply
without interruption of the supply did not affect number of fertile tillers per plant
or grain per ear but increased 1000 grain weight by 24%. Interrupting K supply at
tillering significantly reduced the number of fertile tillers and 1000 corn weight but
not number of grain per ear. Interruption of K supply at ear emergence and after
flowering had no influence but interruption at all three stages reduced all yield
components.
Forster [1973] found that interruption of K supply at tillering and reduced N supply
during grain filling reduced grain yield by reducing grain weight in wheat. There
were similar though less marked effects in oats.
Recent experiments by Forster [1976, 1977] showed with rape in sand culture that
increasing K supply increased number of grain/plant but hardly affected 1000 grain
weight. He also emphasised the importance of the N-K interaction in yield formation
in winter rape grown in pot experiments with soil, as did Steineck [1975] using solution
culture. Herlihy and Caroll [1969], reporting NPK experiments with potatoes,
found that as K was increased (73, 146, 220 kg K/ha) mean tuber weight was increased, due to a greater proportion of large tubers.
This review of experimental results shows that K can have a number of effects on the
yield components of a range of crops.
Though standing power is not a yield component in the true sense, it is nevertheless
of importance for yield under practical conditions and it is therefore of interest to
quote results of a field experiment with spring barley (Figure 8) in which a range of
N and K rates were tested. The plots were scored for lodging just before harvest,
using the scale 0= no lodging, 8= 100% lodged. In line with experience, increasing
N increased lodging. The highest K rate reduced lodging at all levels of N.
4.4 Potassium supply and yield
The effects of potassium in relation to nitrogen supply on the various stages of
metabolism in the plant find their final expression in yield of produce. Figure 9
illustrates results of a typical field experiment with spring barley. At N0 increasing
the K supply has almost no effect on yield.
At 25 kg N/ha there is some effect and this becomes more marked as N supply is
increased. Thus the findings of solution culture experiments described above are
confirmed in the field. The effect of N on yield is more obvious than that of K but
the efficiency of N utilisation is improved by increasing the K supply.
Even more impressive than results of single season trials with annual crops, as in
181
16010
W-~~
--
0I
t00-
KO
K,
K2
K3
Fig.8. Effect of increasing N and K on lodging in spring barley. Relative values (mean eyescore 4.91 = 100%). K applied at 42. 84, 168 kg/ha, N at 25, 50, 100 kg/ha.
120
5
Ito0
105
1000
9
95
K2
N2
NI
K3
NK,
Fig.9. Influence of increasing N and K fertiliser on yield of spring barley in a field experiment.
Relative yields (NK = 6.6 t/ha = 100%).
182
the above example, are those found in long term grassland experiments. Here yield
comprises virtually the whole of the fresh or dry matter production and discussion
of yield components does not arise. As an example, the results of Gruber (in press)
are illustrated in Figure 10. This experiment, in which 3 rates (including zero) of N
and K were applied, has run for 8 years. Mean D.M. yields over 8 years clearly
demonstrate the interaction of the two nutrients. At low rates of K, N response
falls off, while, when sufficient K is applied, the D.M. response to applied N is a
straight line up to the highest rate applied.
dtr/ha6lh
120
20
too0
80
.50O
0
N2
N,
NO
Fig. I0. Mean annual dry matter yield (1970-1977) of cut grass at varying rates of N and K.
N applied at 0, 00, 200 kg/ha, K at 0, 125, 250 kg/ha (Austrian Fertiliser Advisory Bureau).
Some further results which may be quoted are those of:
Schdfer and Geidel [1975] who found that K increased maize grain yield by an
average of 7.5% at various N and P levels; Mengel and Aksoy [1973] in pot experiments with a range of soils; Forster [1970] who found K increased root yield of
sugar beet in nutrient solution and that K counteracted the tendency of increasing
N to reduce sugar %; Haghparastand Mengel [1973] who reported a favourable
effect of increasing K on leaf, stem and root weights of Viciafaba in nutrient solution
as well as an increase in root nodulation; Hehl [1973] working with fodder grasses
in pot culture.
5. Summary
During the 25 years for which the International Potash Institute has existed, results
of research into the ways in which potassium is concerned in the formation of yield
have shown how the element improves nitrogen utilisation, light utilisation and how
it affects the morphological components of yield. This work has improved the basis
of fertiliser advice.
183
The important functions of potassium can be summarised in the following points:
1. Potassium supply determines the efficiency with which nitrogen, which directly
influences yield, is utilised by the plant. Nitrogen is only used efficiently when
potassium supply is adequate.
2. Dry matter production per chemical equivalent of nutrient applied is higher for
nitrogen than for potassium. This has been established by investigation in nutrient
solutions of constant composition.
3. Potassium nutrition should always be considered in relation to the supply of
nitrogen, the functions of the two elements being inextricably connected.
4. The indirect participation of potassium in growth processes rests on its controlling
functions in various synthetic processes, particularly in light utilisation in photosynthesis.
5. The improvement in total photosynthesis is the result of increase in chlorophyll
content, the CO, fixation capacity of chloroplasts, increase in leaf area and in the
turgidity of plant tissue and improved water economy.
6. Potassium favours the translocation of metabolites and thus indirectly favours
photosynthesis by removing products from the sites of active photosynthesis.
7. A further indirect effect of potassium results from its effect in increasing sink
capacity thus increasing the demand for assimilates.
8. As a result of its participation in metabolism, potassium influences the morphological yield components.
The results which have been obtained in the past quarter century have demonstrated
the necessity for a sufficient potassium supply in order that crop plants may yield
their full potential. The reasons why fertiliser policy should aim for the maintenance
of soil potassium supply at a constant high level are better understood than it was
formerly the case.
6. References
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der Kaliumversorgung; in: Landw. Zentralblatt, p. 1685 (1976)
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Stickstoff- und Kaliumdfingung auf den Ertrag sowie den Nfihrstoffgehalt des Bodens
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Mais in AbhAngigkeit von der Art der Kalidlingung. Ref. Nr 03-0334, Landw. Zentralblatt 16, Abt. 11. Pflanzenproduktion (1971)
184
Dhinsda, R.S., Beasley, C.A. and Ting, J. P.: Osmoregulation in cotton fiber. Accumulation
of potassium and malate during growth. Plant Physiology 56, 394-398 (1975)
Dyson, P. W. and Watson, D.J.: An analysis of the effects of nutrient supply on the growth
of potato crops. Ann. appl. Biology, London 69, 47-63 (1971)
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(1973)
Franz, Ch.: Der Einfluss variierter Stickstoff- und Kalidoigung auf Wachstum und Nihrstoffaufnahme von Mentha piperita L. Gartenbauwissenschaft 37, 495-509 (1972)
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varijerter Stickstoff- und Kalidiingung. Gartenbauwissenschaft 39, 9-19 (1974)
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Ertrag und die Qualitit der Zuckerrfibe. Zfichter 23, 343-346 (1970)
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Forster, H.: Einfluss der Kaliumern5hrung auf Ausbildung und Chlorophyllgehalt des
Fahnenblattes und auf die Kornertragskomponenten von Sommerweizen - Untersuchungen an fiinf verschiedenen Sommerweizensorten. Zeitschrift ftir Acker- und
Pflanzenbau 143, 169-178 (1976)
Forster, H.: Die Ertragsleistung einiger Sommerrapssorten (Brassica napus L. ssp. oleifera)
bei variierter Ernihrung mit Kalium und Stickstoff. Kali-Briefe (Baintehof) 13, Fachgebiet 3, Folge I (1976a)
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bei alten und neuen Rapssorten (Brassica napus ssp. oleifera). Kalibriefe (Bintehof)
14 (4), 249-254 (1978)
Forster, H. and Mengel, K.: Einflbsse der Kaliumernahrung auf die Ertragsbildung verschiedener Sommerweizensorten (Triticuan aestivum L.) Zeitschrift f. Acker- und Pflanzenbau 139, 146-156 (1974)
Gaastra, P.: Light energy conversion in field crops in comparison with the photosynthetic
efficiency under laboratory conditions. Mededel. Landbouwhoogsch. Wageningen 58,
1-12 (1958)
Gaastra,P.: Photosynthesis of crop plants as influenced by light, carbon dioxide, temperature
and stomatal diffusion resistance. Mededel. Landbouwhoogsch. Wageningen 59, 1-68
(1959)
Gaastra, P.: Photosynthesis of leaves and field crops. Neth. Journ. Agr. Sci. 10, 311-324
(1962)
Gruber, P.: Private communication, 1977
Haghparast, M. R. and Mengel, K.: Der Einfluss ciner gesteigerten K-Dingung auf den
Ertrag, den Gehalt an I6slichen Aminosturen und Protein bei Vicia faba. Zeitschr. f.
Pflanzenernaihrung and Bodenkunde 135, 150-155 (1973)
Haeder, H.E.: Effects of potassium on phloem loading and transport. Proc. 13th Coll. Int.
Potash Inst., 115-121 (1977)
Haeder, H.E. and Mengel, K.: Translocation and respiration of assimilates in tomato plants
as influenced by K nutrition. Zeitschrift for Pflanzenerndhrung und Bodenkunde 131,
139-148 (1972)
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auf CO-Assimilation und Ertragsbildung bei Sommerweizen. Zeitschrift ffir Pflanzenernihrung und Bodenkunde 138, 573-582 (1975)
Haeder, H.E., Mengel, K. and Forster, H.: The effect of potassium on translocation of
photosynthates and yield pattern of potato plants. Jour. Sci. Food and Agric., London 24,
1479-1487 (1973)
Hehl, G.: Die Wirkung steigender Kalium- und Stickstoffgaben auf den Kohlehydratgehalt
in verschiedenen alten Futtergraisern und Futterleguminosen. Kalibriefe, Fachgebiet 2,
Pflanzenernihrung. 4. Folge (1973)
Helal, M. and Mengel, K.: Der Einfluss einer variierten N- und K-Ernahrung auf den Gehalt
an I1islichen Aminoverbindungen und auf die Ertragsbildung bei Sommerweizen. Zeitschrift fur Pflanzenernahrung und Bodenkunde 120, 89-98 (1968a)
Herlihy, M. and Carroll, P..: Effects of N, P and K and their interactions on yield, tuber
blight and quality of potatoes. J. Sci. Food Agric. 20, 513-517 (1969)
185
Heuser, W.: Die Ertragsanalyse von Getreidez0chtungen. Pflanzenbau 4, 353-357 (1927/28)
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Produktion (1974)
Koch, D. W. and Este3, G. 0.: Influence of potassium stress on growth, stomatal behaviour
and CO, assimilation in corn. Potash Review, Subject 3, Suite 56 (1976)
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Bodenkultur 13, 145-162 (1962)
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roots with localised potassium supply. Potash Review, Subject, 9, Suite 36 (1977)
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Fachveranstaltung Bd 1Il,
Studienrichtung Landwirtschaft, Studienzweig Pflanzenproduktion, Teil 1, 1-31, 1972
Louon, A.: Le potassium et ]a pomme de terre. Au Service de l'Agriculture, Dossier KO,
3-23 (1978)
Lupton, F. G. H., Oliver, R. H. and Ruckenbauer, P.: An analysis of the factors determining
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Mengel, K. and Forster, H.: Der Einfluss einer zeitlich variierten, unterbrochenen K-Erndhrung auf die Ertrags- und Qualititsmerkmale von Gerste. Zeitschr. f. Acker- und Pflanzenbau 127, 317-326 (1968)
Mengel, K. and Haeder, H.E.: Photosynthese und Assimilatetransport bei Weizen wihrend
der Kornausbildung bei unterschiedlicher Kaliumernihrung. Zeitschrift f. Acker- und
Pflanzenbau 140, 206-213 (1974)
Mengel, K. and Haeder, H.E.: The effect of potassium and light intensity of the grain yield
production of Spring wheat. Proc. 4th Int. Coll. on Control of Plant Nutrition 2, 463-475
(1976)
Mengel, K. and He/al, M.: Der Einfluss variiierter N- und K-Ernihrung auf den Gehalt an
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Mitseherlich, E.A.: Das Wirkungsgesetz der Wachstumsfaktoren. Landw. Jahrb. 56, (1921)
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Zentralblatt 15, Abt. II, Pflanzenproduktion
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Munson, R.D.: N-K Balance - An evaluation. Potash Review, Subject 16, Suite 50, I.P.T.
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Pawlowski, H.E.: Die Wirkung unterschiedlicher N:K-Relationen in der Ndhrlbsung auf
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186
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Subject 7, Suite 22 (1977)
Fig, A.C. and Bhagvan Das: Potash improves growth and yield of wheat. Potash Review,
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die Verteilung von Assimilaten in der Tomate. Kalibriefe. Fachgebiet 2, 2. Foige (1974)
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Acker- und Pflanzenbau und Bodenkunde 17, 55-63 (1973)
187
Influence of K Nutrition on the
Response to Environmental Stress
H. Beringer and G. Trolldenier, Agricultural Research Station Bantehof, Hannover/Federal
Republic of Germany *
I. Introduction
Average crop yields, even in the developed countries are well below potential, suggesting that growth and yield of crops are limited by a number of permanent or temporary
constraints.
Stress may be due to unfavourable weather conditions, to adverse soil conditions, i.e.
abiotic stress, or to attack by pest or disease (biotic stress). Response to stress implies
either tolerance and/or avoidance (Levitt [1972]). Tolerance means adaptation of the
plant to and equilibrium with stress without detrimental effect on final yield. Examples
of such tolerance are the accumulation of high salt concentrations in halophytes and
compensation for pest or disease infestation ( Wheeler [1975]). Avoidance is more a
defence mechanism, as in the exclusion, excretion or dilution of high salt concentration
or the prevention of entry by a pathogen. The latter implies real resistance.
Plants assume the temperature of the environment and cannot avoid low temperature.
Freezing can be reduced by supercooling or by lowering the freezing point through
increasing cell sap concentration but such avoidance mechanisms are not important
in the field, most crops having freezing point depressions less than 4C and herbaceous
plants seldom supercool more than 2°C (Burke et al. [1976]). Plants survive low
temperature mainly by tolerance.
Plants can avoid high temperature and heat stress by the cooling effect of transpiration,
by reduced absorption of light or by insulating, protective tissues. They can also
tolerate heat if the enzyme proteins are thermostable or if there is rapid resynthesis of
vital compounds.
Response to drought seems to be mainly by avoidance through morphological adaptation, but the response varies and Levitt [1972] classifies plants as avoiders, tolerant
avoiders and tolerant non-avoiders. Frost, drought and salinity, which are discussed
in detail in chapter 2, in principle all cause dehydration of the protoplasm. Therefore
an attempt is made in chapter 3 to look at such stress situations from some general,
*somewhat hypothetical angles.
Nutritional disorders caused by soil acidity, or alkalinity leading to micronutrient
deficiencies are stress situations which act more specifically on the plant's metabolism.
Prof. Dr. H. Beringer and Dr. G. Trolldenier, Agricultural Research Station, Binteweg 8,
D-3000 Hannover/Federal Republic of Germany
*
189
As soils can be improved if economically feasible and, as much effort is made to select
species and cultivars better adapted to infertile soils (Wright [1977]), nutritional
disorders will be discussed only briefly (2.5).
Still another stress, anaerobiosis, may be caused abiotically, for instance, poor soil
structure often associated with salinity or waterlogging. But, as anaerobiosis can be
accentuated by the activity of soil micro-organisms, we consider it justified to discuss
this factor as a biotic stress and to emphasise the role of the rhizosphere in plant health.
Finally, the growth of crops may be hampered by disease and pest attack. Host
pathogen interactions differ from host to host, from pathogen to pathogen and the
constraints on the plant can hardly be traced back to a general principle, like the
desiccation of protoplasm resulting from drought, frost or osmotic stress. Therefore,
the response of plants to pathogens and adverse microbial activity in the rhizosphere
will be treated separately from response to abiotic stress (chapter 4).
In general it can be assumed that proper plant nutrition supports the plant's response
to stress. Although avoidance mechanisms are mainly plant specific, they can be
modified by cultural practice. For example, the size and extension of the root system
can be influenced by the method and depth of fertiliser application (Glieneroth
/1953]), which would be important for drought avoidance. Additionally, the possible
exclusion of excessive Na ions by some salt sensitive glycophytes may not only be
related to specific membrane constituents (Kuiper [1968]; Stuiver et al. [1978]) but
also depend on the efficiency of the Na/K exchange pump and energy supply, both
requiring proper nutrition with K (Lauchli [1976]; Marschner and Ossenberg [1976]).
The tolerance of the plant might be improved by its nutritional status. As tolerance is
defined as adaptation and survival of plants in thermodynamic equilibrium with an
.unfavourable environment, the concentration of ions and sugars in the plant alleviates
by osmoregulation the dehydration of the protoplasm caused by drought, salinity or
freezing.
Because of the complexity of causes and response reactions to environmental stress
and the multiple functions of inorganic nutrients in metabolism, it is extremely difficult
to identify the specific role of a particular nutrient in improving plant health. Nevertheless, many observations and experimental results, although sometimes contradictory, are available indicating a positive influence of K nutrition on the tolerance
behaviour and health of plants. Consideration of existing hypotheses of stress response,
as well as consideration of how K could contribute to it, are challenging tasks. It is
hoped that the discussion of these aspects will stimulate more research, which is
needed for a transition from hypothesis to truth.
2. Abiotic stress
Metabolism relies on the existence and function of membranes separating and coordinating catabolic and anabolic reactions within an organism. If some stress acts upon a
plant, the membranes must adjust their permeability in order to maintain the necessary
regulation of metabolism and to avoid excessive dehydration and denaturation of
enzymes. Even if some structures are damaged by stress, the existence and the intensity
of a repair mechanism can improve tolerance. Adaptation and repair will be the more
efficient the better the nutritional status.
190
K has many functions in the plant. It stimulates synthesis of chemical energy, promotes
photosynthesis and the formation of high molecular compounds and largely determines
cell turgor. Thus adequate K nutrition would be expected to increase resistance to
stress.
2.1 Chilling and frost
Low temperature can cause freezing or chilling of a tissue. Long ago Mohlisch [1896]
differentiated between the two occurring below and above 00 C, respectively.
Chilling primarily occurs in plants of warm climates, if exposed to sudden temperature
drops between about +5' and + 10'C. It is now generally accepted that chilling
injuries are caused by an abrupt change in membrane permeability due to a sudden
phase change from the liquid crystalline (normal) to the solid gel state. This leads to
membrane disruption followed by leakage of solutes, by disturbed compartmentation,
by imbalances in the energy status and metabolism as well as by accumulation of toxic
metabolites (Figure 1, Lyons [1973]). Such phase changes are caused by the lipid
moiety of membranes ( Wade et al. [1974]) and are shifted to lower temperatures the
more unsaturated the fatty acids are (Lyons and Asinundson [1965]). Accordingly,
chilling tolerant species have a higher unsaturated/saturated fatty acids ratio than
chilling sensitive tissues.
CHILLING TEMPERATURE
+
~~
Inavnd
oils,
UQUID-CRYSTALUE
eusoued
ACTIVATIO
ENERGY
of nembab er
azlm
Cesotb
f popsmlc*
Wraning
Rlum to
lit
SOLO GEL
,,/
Reduced
ATP
supply
Imbolom
in
me nibs
e4 OweOIdthet,
d*~~
o
dtow, Wtc.
ProlonedI
INJURY AND DEATH OF
CELLS AND TISSUES
Fig. 1. Schematic pathway of the events leading to chilling injury in sensitive plant tissues
(Lyons [1973] reproduced by permission from Ann. Rev. Inc.)
191
High K-content reduced desiccation and accordingly water potential after three days
remained higher than in those plants with low K content of needles. Other examples of
K improving frost resistance are given by White and Finn [1965] (tulip poplar),
Gangstad et al. [1954] (Sanseveria) and Vlasishin [1972] (apple).
During autumn and adaptation to low temperatures, K content increases and this will
increase frost resistance. But, as pointed out above, stress can best be tolerated in the
range of optimal nutrition and growth. This is shown in Table 2. The treatment with
75 ppm K was optimal for growth and frost hardiness, while 675 ppm K in the irrigation water was obviously already toxic and resulted in much higher frost damage.
Table 2. Effect of K on frost hardiness of stems of Forsythia intermedia (Beattie and Flint
[1973))
ppm K in
irrigation water
g DM/plant
% K in DM
Sept.
Dec.
25
75
225
675
30.8
35.8
30.1
25.7
1.0
1.7
1.9
2.5
.......................
.......................
.......................
.......................
6.5
8.1
9.6
14.5
Index
-180 C injury
4.8
3.4
6.1
15.2
There has been much research on frost resistance of cereals and forage plants. The
effects of all major nutrients on cold resistance in wheat are variable, but general
experience suggests that correct fertiliser use is, on the whole, beneficial (Single
[1971]). Brad [1972] noticed higher K contents in frost resistant wheat varieties and
further indications of the effect of K are collected in Table 3. It is obvious that the
survival of maize seedlings is related to the amount of K fertiliser applied and that the
regrowth of Bermuda grass is the better the higher the K :N ratio of autumn applied
fertilisers. Reduced frost injury to Bermuda grass supplied with K was also reported
by Juska and Murray [1976] and by Reeves et al. [1970], who found low P: K ratios
in tissue correlated with frost hardiness. Reduction of frost damage by K was also
observed in ryegrass (Welling [1977]) and timothy (Pahnason [1971]), but with
Table 3. Influence of K application on frost resistance
Maize seedlings
(Trierbveiler [1971])
Bermudagrass
(Adams and Twersky [1959])
(Gilbert and Davis [1971])
K2O
kg/ha
% plants
injured
N 448
K2O
kg/ha
% Stand
N:P:K
Temp. at which
50% reduction
in top growth
112
204
372
677
1233
22
20
21
15
14
56
112
224
38
48
63
74
4 00
4 10
4 13
4 16
-4.8
194
-5.6
-8.1
-8.3
Pangola grass (Diglaria decumbens) K had little effect (West and Prine [1974]).
K fertiliser applied in autumn might increase K and carbohydrate content, both of
which would increase osmotic pressure, water retention and frost hardiness. But
Ruelke [1966] found the same amount of carbohydrates in the roots before commencement of regrowth in spring in sods from different K treatments.
Frost resistance of lucerne and winter hardiness of red clover were both increased by
generous application of K fertiliser, 200 and 280 kg/ha K respectively, as compared to
lower rates (Fordonski and Paprocki [1977]; Snith and Smith [1977]). Data by
Wang et al. [1953], again on lucerne, gave 50% winter kill in the PoK 0 treatment,
50% in P67 5 K. but < 20% in the PoK 6 75 (kg/ha P20 5 , K2O) plot. Purtov [1972] found
that PK fertiliser increased the disaccharide content of red clover roots during winter
and reduced the activities of invertase and peroxidase. Peroxidase activity was negatively correlated with frost hardiness, probably because cold-hardened tissues may
contain less peroxides (3.1).
2.2 Drought
Wilting of plants is a symptom suggesting possible K deficiency. Biebl [1958] cites
many papers indicating that K ions facilitate water uptake by the root and, at the
same time, reduce transpirational water loss. These results, formerly explained by the
swelling action of K on the plasmacolloids, can be interpreted by the stimulating
influence of K on water uptake via osmotic potential of root cells and xylem sap and
by the involvement of K in the regulation of stomatal movement. Further details on
osmoregulation and the mechanism of stomatal movement are described by Lduchli
and Pfiger elsewhere in this volume (page I II).
Potassium nutrition may support both tolerance and avoidance of drought. This is
shown in Figure 4, where plants adequately supplied with K respond almost immediately to a water stress induced by hot winds by reducing their transpiration, while
plants with moderate or severe K deficiency are obviously unable to close their stomata
efficiently. Thus, with adequate K nutrition, water utilisation is improved, less being
transpired per unit dry matter produced (Figure 5, Table 4).
L.
n
Vi
___~
. severe K deficienc
14
rnoderateK
deficiency
I12
12
~C08
o
04
02
adequate K/
10
30
50
70
minutes exposed
to hotwindy conditions
Fig.4. Influence of K nutrition on transpirational water loss (after Skogley [1976])
195
500
400
00
oo
1%
2%
3
4
%Kin DM
Fig.5. Relationship between K content in lucerne leaves and transpiration/g dry matter
produced (after Blancher et al. [1962])
Table 4. Influence of K nutrition on the transpiration coefficient of flax at two water regimes
(Linser and Herwig [1968])
Dry matter (g) .........................
Leaf-K (% DM) .......................
Water consumed I/pot ...................
Transpiration coefficient .................
40% field capacity
80% field capacity
-K
+K
-K
58.5
0.4
34.0
581
65.6
2.6
30.1
459
64.6
0.4
40.3
624
+-K
80.4
2.9
40.5
504
The transpiration coefficient, however, is a relative term and the question remains as
to whether K has a water saving effect or whether K stimulates growth more than
water consumption. Table 4 shows that the lower transpiration coefficient of plants
well supplied with K and water was due to higher dry matter production. But, at low
soil moisture, the +K plants also consumed less water, indicating more efficient
stomata regulation. Lower transpiration rates of wheat and peas well supplied with K
were also observed by Brag [1972]. Christersen [1976], however, observed higher
transpiration rates in plants supplied with K but, due to better water retention of the
cytoplasm and more efficient osmoregulation, these plants survived drought better.
A decrease of bound water and osmotic pressure in K deficient sugarcane was also
reported by Lal et al. [1965].
K can also contribute to drought avoidance. Fertiliser might stimulate root growth,
raising root shoot ratio and the water absorbing potential. Which might be more or
less affected by K depends on the kind and duration of the experiment (Viets [1962];
Lal et al. [1968]).
A deep root system is obviously important for drought avoidance, as seen by comparing
response to water stress in pots under controlled conditions and in the field. Leaf
extension rate of maize grown in pots dropped sharply at leaf water potentials between
-2 and -4 bars but was little affected in the field at -8 bars (Figure 6). Similarly, a
decline in stomatal conductance in cotton has been found at -16 bars leaf water
potential in growth chambers, while field grown cotton showed high stomatal conductance at -27 bars (Ackerson et al. [1977]). In the field, roots extend to the subsoil and
196
i
I
5A
4E
E3-
A&
D
I
C
0
0
0
I
I
-10
-8
-6
-4
-2
-0
leaf water potential (bars)
Fig.6. Relationship between leaf water potential and leaf growth of maize grown in controlled
conditions (A light, 30*C) and under field conditions (0) (after Begg and Turner [1976))
exploit a larger volume, while in pots water is quickly depleted and stress occurs
earlier.
Obviously, plant water relationships under controlled conditions have little relevance
to field conditions. If stomata] aperture remains high at leaf water potentials of -25
bars, the guard cells must still be turgid, indicating more efficient adaptation and
osmoregulation in field conditions, probably due to higher light intensity (Begg and
Turner [1976J) which could increase photosynthesis and photophospborylation, thus
accumulating osmotically active sugars and ions in the cells.
Potassium is most important for cell osmotic pressure, especially in early growth when
K content of the plant is highest. K determines root pressure and therefore osmotic
water flow into the root xylem (Mengel and Pfliger [1969]). Further, via its role in
phloemt loading and the transport of assimilates (Mengel and Haeder [1977]), it
could also indirectly increase root osmotic pressure and water uptake. Interesting to
note, that the higher drought resistance of sorghum and cotton as compared to maize
seems to be mainly due to efficiency of water uptake and transport to the leaves rather
than to adjustment of stomata] conductance (Ackerson and Krieg [1977]).
Though K has a distinct role in stomnatal regulation and in the general water economy
of the plant, it is difficult to interpret its physiological effects in experiments covering
growth to final maturity. Sensitivity to water stress varies between species and cultivars
and during the season (Begg and Turner [1976]; Ackerson and Krieg [1977]). Table 5
clearly shows that water stress at heading had the most severe effect on sorghum yield.
Grains/head were much reduced, but partly compensated by additional induction of
beads. Such complex regulatory steps and the multiple function of K in metabolism
make it very difficult to specify the causal effects of K on drought resistance.
Responses to water stress are sequential. First, cell enlargement and leaf area development are reduced, followed by a reduction in net photosynthesis and finally of trans197
Table 5. Influence of water stress at various growth stages of sorghum, cv. RS 610 (H-siao
et at. [1976])
Stress
Control
early
Yield, kglha ........................
Heads/plant .......................
Grains/head ........................
6080=100%
1.16
950
98%
1.09
890
late
heading
78.5%
1.93
370
95%
1.09
1010
location, which seems to be least sensitive (Boyer and McPherson [1975]). Accordingly, crops are often more sensitive to stress during vegetative growth than during seed
and fruit development. The obviously lower sensitivity of seed crops could be due to
higher mobilisation of stem reserves for yield formation, should water supply and
photosynthesis be suboptimal. Passioura [1976] varied the water supply in pot
experiments and found that up to two thirds of the grain weight could be accounted
for by mobilisation of stem reserves. Under field conditions Gallagher et al. [19761
calculated an average of 43% of the grain weight to result from stem reserves. Both
figures might be overestimates, because Bidinger et al. [1977] found much lower grain
yields of wheat and barley under post-anthesis drought, only 27 and 17% respectively
being derived from pre-anthesis assimilates. But tiller formation and the differentiation
of grains/ear, two major yield components, occur during vegetative growth and water
stress in this period will probably seriously affect the final total grain yield (Downey
[1971]; Begg and Turner [1976]), because it will reduce the number of ears and
limit their 'sink' capacity, the latter being the driving force for the intensity of assimilate
transport from leaves to ears ( Wardlaw [ 1976]).
The stimulating effect of K on drought 'resistance' in the field must also be looked at
from the aspect of nutrient availability in the soil. Concentration of P and K in the
soil solution is generally low and both nutrients have to reach the root surface by
diffusion. Temporary water shortages reduce diffusion rates, which can be compensated by increased fertiliser supply. The model experiment in Figure 7 demonstrates
this point. Growth of 6 g DM maize can be obtained under favourable moisture
conditions on soil containing only 17 mg exchangeable K/100 g, but under dry conditions the soil must be higher in exchangeable K to obtain similar growth. Higher
responses to K fertilisers under dry conditions have been reported by van der Paauw
[19581, Boulay [1976] and Huschka [1976]. Table 6 shows that generous pre-planting
dressings of K increased the yield of vines. In dry years plants in these plots remained
remarkably turgid and green.
Table 6. Influence of high preplanting dressing of K2 0 on yield of vines (Boulay [1976])
Site
kg K2O/ha
preplanting
Yield increase
(kg-degrees)
V nijan ...........................................
St. Pons-la-Calm ...................................
.. . ...................
Tornac ... ...............
Saint-Denis ........................................
800
750
600
1150
+
+
+
+
198
56%
9%
12%
28%
exch K/1O g
B.
7-
-6
4.
31
'1"
17
3+
16
20
2:4
2.7
water tension (pF)
Fig. 7. Importance of soil moisture and exchangeable K in soil for growth of maize seedlings
after Mengel and von Braunschweig [1972])
2.3 Salinity
Water stress also limits growth on saline soils, covering 2% of the world land area.
There are also areas where only saline water is available for irrigation. High salt
concentrations in the soil reduce availability of soil water and impose osmotic stress
on the plant. Symptoms of salt injury are growth reduction and, in severe cases, leaf
necrosis. Apart from the general osmotic effect, the prevalence of certain ions, such
as Na+, C-, SO4--, Mg++, borate and bicarbonate, can be toxic and the high pH
value often found in saline soils can induce Fe- and other micronutrient deficiencies.
In addition to plant characteristics, climatic factors can also modify salt stress. Inhot,
dry periods with high transpiration, more Na+ and CI- is transported to the root
surface by mass flow (Sinha and Singh [1974]) and diffusion of potassium and phosphorus could be insufficient leading to ionic imbalances in nutrient supply.
Salinity of the soil is generally defined by the electrical conductivity of the soil saturation extract. Up to a conductivity of 12 mmhos, corresponding to a 100 mM NaCI
solution, yield of barley, sugarcane or cotton, which are salt-tolerant, is only slightly
depressed, but maize, beans and lucerne are susceptible to saline conditions (Reeve and
Fireman [1967]; Meiri and Shalihevet [1973]). Such a classification is informative but
within each species there are cultivars differing in their adaptation to salt stress
(Tanaka and Tada [1975]; Epstein [1976]; Rush and Epstein [1976]; Janardhan el al.
199
[1976]). An example is given in
barley cultivar, whose yield is less
than for Na. K is taken up at a
translocation of assimilates must
salt stress depends on the growth
Table 7, showing that the tolerant, higher yielding
depressed by salt, also has a higher specificity for K
high rate, especially during ear development when
be high. These results also show that the extent of
stage.
Table 7. Ratio of K/Na uptake by the salt sensitive barley cultivar Chevron and the salt
resistant cultivar CPI 11083 from nutrient solution containing 125 mM NaCI/l (Greenway
[1965])
K/Na uptake
Days after planting
31-43
43-50
50-64
64-93
...................................................
...................................................
...................................................
...................................................
ears g/plant
Control ...................................
+ NaCI ..................................
CPI 11083
Chevron
0.59
0.33
0.45
1.02
0.39
0.30
0.16
0.14
113
61
64
21
There are two kinds of adaptive mechanism: a) salt avoidance due to salt exclusion (in
glycophytes), salt excretion or salt dilution by growth (Levitt [1972]; Greenway et al.
[1966]; Flowers et al. [1977]) and b) salt tolerance due to osmoregulation, maintaining turgor potential which is the driving force of cell elongation and growth
(Greenway [1973]; Oertli [1976]). A salt-excluding mechanism would partly explain
how the salt-sensitive shoots of beans and maize can be protected against salinity. In
Phaseolus vulgaris and maize Na+ is retained in the root (Marschner and OssenbergNeuhaus [1976]; Yeo et al. [1977]), so that leaf Na concentration remains low.
According to Mix [1973], chloroplasts of Phaseolus vulgaris and, to a lesser extent,
those of barley leaves are sensitive to NaCI concentrations of 25 and 50 mM, while
those of the salt-tolerant sugar beet are not affected at this level. In these experiments
sugar beet chloroplasts also retained most of their K+ in spite of the excess Na+. It
seems that K + has an essential function in maintaining the fine structure of chloroplasts. The degree to which K can be replaced by Na in chloroplasts is probably an
indication of the salt sensitivity of a particular plant.
Morphological evidence for the low Na translocation in salt-sensitive plants has been
presented by Lauchli [1976] and Yea et al. [1977] and is shown in Figure 8. In the
plasmalemma membrane of the xylem parenchyma cells, which surround the xylem
vessel, an energy requiring Na±/K+ exchange pump operates, reabsorbing Na+ from
the xylem, exchanging it for K+, which is translocated to the shoot. The efficiency of
such a Na/K exchange is suggested as the basis of differential salt tolerance of barley
cultivars (Hull and Epstein [1978]). This mechanism could also explain why in saltsensitive species, low salinity can have a synergistic effect on K uptake, whereas in
salt-tolerant species Na± uptake reduces the uptake of K (Heinann [1958]; Wignarajah
et al. [1975]; Hamid and Talibudeen [1976]).
Tolerance of salinity by osmotic adjustment of the plant to the reduced availability of
soil water can be achieved by increasing ion uptake or by accumulation of organic
200
A
Xylem
parenchyma
KWall
Vessel
4a
fucto
Fi..osil
ofxye
xhnginteot
anhmcelanofnK/a
for the salt tolerance of plants (after Liduchli [1976])
and retention by the cell (Greenway [1973]).
solutes in roots, favouring water uptake
Although osmotic adjustment can somewhat sustain further growth, the growth rate
is reduced by slower cell extension and because salinity inhibits cell division. Thus
salinity produces fewer and larger cells, a characteristic of succulence (Oeruli [1976]).
All ions can contribute to osmoregulation under saline conditions but in glycophytes
K+ is most efficient, as shown in Table 8. In the presence of 41 me NaCI/l, increasing
concentrations of KCI in the nutrient solution led to higher K and Cl contents in the
leaves, increasing osmotic pressure in the leaf sap, favouring water retention and
reducing water loss by transpiration.
of eave and osmotic
cn
of
Table 8. Effect of increasing KGI supply on yield, K and
pressure of the leaf sap of Plaseols vulgaris, grown in a saline medium of 41 me NaCI/
(Lagererff and Eagle [1961])
KCI
mM/I
o
t
3.5 ............................
8.8..........................
14.1..........................
in £
K cont.
me/g DM
CI cont.
me/g DM
osmot. pr.
arm.
534
1.45
1.91
11.4
544
562
1.66
1.92
2.06
2.27
12.5
14,8
The situation in the field is much more complex than in short-term experiments in a
controlled environment. Of the many interactions possible, one should be specifically
mentioned. Soil salinity due to high NaK and/or Mg++ concentrations disturbs the
nutrient balance and also causes poor soil structure and low root respiration. The
uptake of ions like K + is inhibited and root growth is reduced. Tolerance of salinity
partly to tolerance of anaerobiosis. Apart from rice,
might, therefore, also be due
i uncommon in crops, so it is important to improve soil
tolerance of anaerobiosiis
201
structure to support root metabolism and uptake of essential nutrients. The effect of a
soil conditioner in changing ionic balance in the plant by improving soil structure is
given in Table 9. At high salinity without Vama (vinylacetate-maleic acid) lucerne
growth is very poor due to high Na content and low K :Na ratio in the tops. Vama
improves aeration and root respiration, restoring ion selectivity of root membranes.
Uptake of K is increased, uptake of Na is suppressed and growth is stimulated. This
shows that anaerobiosis in the rhizosphere should be avoided if K fertiliser is to be
effective in improving salt tolerance. Soil structure, soil moisture and the rate of K
supply to the root are seldom measured during the growing season in the field, so it is
hardly surprising that field observations on the role of K in salt tolerance are not
always consistent.
Table 9. Influence of salinity and of soil conditioner (VAMA) on growth and K- and Nacontent of lucerne tops (Chang and Degrue [1955])
% exchangeable Na
VAMA
15.2 .....................
.....
.. .. ........
39.3 .....................
.....................
0.2%
0.2%
DM
g
11.0
10.9
5.4
9.8
Content of tops
me/100 g DM
K
Na
K/Na
76.5
67.0
56.9
61.1
11.8
19.4
3.94
5.67
1.12
2.23
50.8
27.4
Data shown in Figure 9 indicate that on low K soil wheat growth is markedly reduced
at 20/. salt content, whereas on two sites with better K status growth was similarly
reduced only at about 50o. On these two sites K content in the leaves was always
>3% DM and the authors concluded that, for optimal K nutrition of wheat under
plant height
(cm)
70
o0~
60
d
50
40
30
20
10
0
*-K (-)site
0-K (n) site
I
2
3
4
5
6
7
8 (%) salt in
soil (O-4Ocrn)
Fig.9. Growth of wheat in relation to soil salinity and the K status of the soil (after Schleiff
and Finck [1976])
202
saline conditions, K fertiliser rates should be 20-50% higher than under non-saline
conditions. K also improved salt tolerance in experiments by Lipman et al. [1926]. At
a K concentration of 5 me/l in the nutrient solution, enough K was available to the
plant so that even the salt-sensitive garden pea developed well at a salt concentration
up to 68 mM NaC. Both these examples indicate the importance of the absolute
amount of plant-available K.
Bernstein el a/. [1974], working with maize in nutrient solutions of 0 and -3.8 bar
osmotic potential, report lower cob yield in the saline medium but an increase in the
K concentration from 0,45 to 2 mM K/I did not compensate for the yield reduction
caused by salinity.
NPK fertiliser improved salt tolerance in several crops (Lumin and Gallatin [1965S;
Ravikovitch and Porath [1967]). In the latter case, increasing K did not improve salt
tolerance, perhaps because the soils (42% and 64% clay, K saturation of CEC 5.5
and 7.9% respectively) were well buffered and always met the K requirements of the
plants.
2.4 Ieat
Heat resistance is a consequence of genetic constitution; psychrophilic, mesophilic and
thermophilic species are known. It depends also on the physiological state, insofar as
dry seeds are more heat resistant than metabolically active cells and, finally, the
duration of exposure to heat is important. Actual, irreversible heat injury to agricultural crops, i.e. damage by enzyme denaturation, seems to occur relatively seldom,
a consequence of the selection of crops and cultivars adapted to hot climates.
Under arid conditions, where drought and high temperatures are generally associated,
temporary heat stress may cause high respiration and depletion of assimilates as well
as proteolysis, which could lead to excessive formation and accumulation of NH,
(Engeibrecht and Mothes [1964]). According to Petinov and Molotovsky [1962], heat
resistant plants respond to excessive heat by accelerated production of organic acids
which bind the NH3, thereby minimizing heat stress. Changes in membrane structure
have also been assumed to be a result of heat stress. Inhibition of photosynthetic
electron transport by high temperatures is correlated with disturbances in the thylakoid
membrane, and adaptation to high temperature involves changes of the thylakoid
membrane which render the pigment system embedded in this membrane less heatsensitive (Armond el al. [1978]).
It is not known how K improves heat tolerance, but stimulation of photosynthesis and
protein and starch synthesis would probably reduce glycolysis and proteolysis or
compensate respiratory losses. Wilted leaves are mostly a few degrees warmer than
turgid ones (Levitt [1972]) and K could improve heat tolerance via stomatal regulation. More research is needed in this area.
2.5 Acidity
In low pH soils plant growth is inhibited by too high concentrations of Al-ions in the
soil solution. Al-toxicity is the more severe the less other nutrients are available. Data
by Nemeth and Grimme [1974], Figure 10, show that, in soils with pH< 5, ryegrass
203
40-0
0
30-
O0 ph >5.0
.. ph< SO
20-
0
E
V
10
0.1
G2
da
d4
d5
d6
07
K- concentration insaturation extract (me/I)
Fig. 10. Relationship between the K concentration in the soil solution and growth of Lolium
perenne on soils differing in pH (after Nenth and Grin,,e [1974])
yield could be nearly doubled by raising K from 0.2 to 0.6 me/I soil saturation extract.
indicating that higher K supply improves the plant's tolerance of aluminium stress. It
is probable that K improved tolerance by a general stimulation of growth rather than
by a specific metabolic reaction. Similar conclusions can be drawn from experiments
by Williams and Vlamis [1957], who were able to counteract toxic effects of Mn in
barley by raising the concentration of macronutrients.
3. Basic principles of stress reactions
Frost, drought and salinity are all stress factors causing cell dehydration and reducing
availability of water for metabolism. This generalisation may be superficial and inappropriate in view of the complexity of stress and response reactions. But biochemical
similarities in the adaptation and response to temperature-, water- and salinity stress
calls for a summarising discussion.
3.1 Membrane structure and integrity
Elucidation of the structure and function of membranes in the past 20 years enables
us to say that changes in membrane functions, especially the destruction of active
transport, are the primary steps through which a stress may influence the metabolism
and growth of a plant (Palta et al. [1977]), thus modifying Stocker's [1958] view.
Research on artificial lipid membranes has shown that the fatty acid composition, the
nature of the phospho- and glycolipid, and the interaction of the lipid with sterols,
proteins and synthetic ionophores determine the permeability of a membrane (van
Deenen [1972]). An example is given in Figure I1, showing that the permeability of
liposomes for glycerol decreases with temperature. Additionally, if temperature drops,
a given permeability can be maintained the more unsaturated the fatty acids are. This
is in line with results showing an increase in percentage of unsaturated fatty acids
204
1.6
1.4
linoleic
"N 1.2
linoleic
.
/
/
°
0.8
.palmitic
oleic
palmitic
/
/I
/
/
/
I
/
I
/
/
/
02
palmitic
///
///
0.2
o10
20
30
40
50
temperature (00)
Fig. 11. Permeability of liposomes for glycerol in relation to temperature and the fatty acid
composition of tihe liposome membrane (after van Deenen [1972])
during the adaptation of plants to low temperatures (Gerloff [1966]) and with data
showing higher amounts of unsaturated fatty acids in frost-tolerant cultivars of lucerne
and wheat (Grenier et al. [1975]; de ta Roche et at. [1975]). Adaptation to frost can
also be due to an increase in polar lipids and membrane constituents (de Ie Roche et al.
[1972]; Singh et al. [1975]). It is also interesting that mitochondria, generally rich in
polyunsaturated fatty acids, are frost-damaged later than other membranes and organelles of rye coleoptile cells (Singh et al. [1977]).
Further research is needed to prove whether lipids are the cause or the effect of frost
tolerance and adaptation and to show how other substances like sugars, proteins,
electrolytes and ascorbic acid are involved in the response reactions to environmental
stress (Heber and Santarius [1976]; Levitt [1972]; Schinfiz [1969]).
Unfortunately most investigations concern only one stress factor and consider only
one of the biochemical groups just mentioned. It is, therefore, worth mentioning that
Chern et al. [1977] demonstrated the induction of frost hardiness by water stress in
stem-cortical tissues of Cornus stolonifera, suggesting that frost hardiness and drought
hardiness are inter-related. In water stress, proteins, nucleic acids and starch decreased
while sugars increased. Sugar accumulation might be associated with improved resistance of the protoplast to desiccation by drought or frost.
If membranes containing enzyme proteins and carrier molecules are the site of stress
attack, a number of substances may be altered, without evidence of a specific and
causal link of a particular substance to drought, frost and salinity stress. Conventional
biochemical methods are too insensitive to detect the immediate response reactions of
the plant to stress, and more sophisticated biophysical analysis will have to be used in
future.
Biopolymers contain hydrophilic and hydrophobic parts. Accordingly, intracellular
water or, more precisely, cytoplasmic water exists in two states, distinguished as
'bound water' (attached to hydrophilic groups) and 'bulk water' (occurring as free
water molecules). Figure 12 shows this schematically and demonstrates that free or
205
"OH
H OH
H H OH
OH H
HOH'o
O
T
H
H
HH
)HOH
H
H
HOH
HOH
H
H
\NH
H-
H
O.
NH
&0H--H
0
-
H
HHH
H OH'-..j
OH
OH
<H. HO* H
H9
hydrophobic groups
hydrophilic groups
Fig. 12. Binding and orientation of water molecules at hydrophilic and hydrophobic sites of
biopolymers
bulk water is more susceptible to desiccation by frost, drought or salinity (Kuntz
[1971]). If a long-chain, saturated fatty acid within a membrane is replaced by a
short-chain or unsaturated fatty acid, there will be fewer hydrophobic bonds with
less bulk water. Accordingly stress tolerance will be increased. This may explain why
many cases of adaptation and tolerance to frost can be correlated with an increase in
unsaturated fatty acids.
Anticipating that stress attack and stress tolerance occur in membranes, a hypothetical
combination of some groups reported to be involved in stress tolerance can be
attempted (Figure 13). The basis for this assumption is provided by Levitt [1972] in
his SH/S-S hypothesis. Dehydration by drought or by formation of ice crystals could
cause structural changes of membranes. As a result, SH-groups of membrane proteins
would no longer be protected by hydration-water and they could be oxidized to intermolecular and intramolecular S-S-bonds leading to changes in protein structure or
even denaturation. This transition is reversible if the stress is not lethal or could be
minimized if antioxidants, itirer alia ascorbic acid, were to inhibit the oxidation of
SH-groups i.e. would stabilise membrane structure.
Similarly, dehydration could facilitate the penetration of molecular oxygen into
membranes. This could, in the presence of lipid peroxidases, cause peroxidation of
membrane fatty acids, thus leading to membrane destruction and the formation of
highly reactive peroxide-radicals (Krinsky [1977]), but this could also be prevented
if enough antioxidants are present (Wang and Baker [19781).
Additionally, sugars and potassium salts can protect biopolymers against dehydration
(Hellebust [1976]). The sugar concentration of plant tissue increases in autumn
(Remeslo [1971]; Hartmann [1975]; Aronson et al. [1976]), increasing osmotic
206
stabilisation by osmoregulation
K.tAxltNa'.
Kfl
nh<SHMZ5
0-0
s-s
Fig. 13, Hypothetical scheme for the maintenance of'membrane structure by osmotic substances
and by antiodidans
pressure and water retention in the cell so that the hydration shell of sugars and
inorganic electrolytes is shared between these protecting compounds and the otherwise
dehydrated protein molecule, It can also be imagined that R-OH-groups of sugars
could replace the OH-group of H-OH and thus take part in the water structure
•around hydrophobic bonds (Figure 12), as suggested by Schobert [1977].
K+, found in the cell sap of many higher plants
at concentrations 100-300
aM,
the
highest of all ions, is often, in association with organic acids, the main osmoticum.
Sugars, naturally present in considerable concentration in glycophytes, do not seem
to be an appreciable component of osmoregulation in shoots of cultivated plants
(Wignaraja etral. [1975]; Cram [/976]). However, in onion bulb cells 80% of the
total soluble components are sugars and only 20% ions. During freezing tests 85% of
the ions diffused out of the cell, but the cells remained alive as judged by plasma
streaming (Pa/
t dat. [/977]), indicating that
quantitative contribution of ions
and sugars to osmoregulation depends on the typethe
ofbdehydration stress, on the species
and on the physiological state of" the tissue tested (Cutler e at. [977]). Also the
nutrient supply influences the relative contribution of sugars and/or ions to the osmotic
potential (Table 10). The osmotic pressure ofte two youngest leaves increases from
about 8 atn during shooting to 2 atm at panicle emergence. At the same time sugar
content, chiefly sucrose, more than doubles, while ash content increases by 50%.
Accordingly, ash content contributes relatively more to osmotic pressure in the younger
state. Higher fertiliser application also leads to higher osmotic values. As KGI was
used and C- content also increased, both ions are likely to be responsible for this
increase in osmotic pressure.
Tolerance mechanisms against dehydration of cytoplasm seem to be twofold: 1) Maintenance of hydration by osmotically-active substances and/or polyols and 2) Protection
207
Table 10. Osmotic pressure and composition of cell sap of field-grown oat at shooting and
panicle emergence (Jaeger [1966])
Date
June 12
July 9
Osmot. press. Sugars
mg/ml
atm.
NPK
kg/ha
30 30
* 120 90
30 30
120 90
60
120
60
120
.............
.............
.............
.............
7.89
7.94
11.6
12.5
21.8
20.6
58.4
45.9
Ash
mg/ml
Cl
mg/ml
12.6
13.3
18.9
19.5
1.72
2.13
1.64
2.50
of double bonds in fatty acids and of SH-groups in membranes by antioxidants. Which
compound dominates in the tolerance reaction depends on a number of experimental
and metabolic variables. Much research is still required to elucidate the structural
arrangements of membranes which are the basis for compartmentation and coordination of biochemical reactions within a cell and for the survival of an organism
under stress conditions.
3.2 Accumulation of proline
Accumulation of proline is often observed in water and salinity stress (Barnett and
Naylor [1966]; Singh et al. [1973]; Stewart and Lee [1974]: Chu et al. [1976]; BarNun and Poliakoff-Mayber [1977]). It has been suggested that proline supports osmotic
equilibrium in stressed plants. This osmoregulating function of proline might explain
why plants growing on solutions of KCI accumulate less proline than plants on isoosmotic solutions of polyethylene glycol (Chu etal.[1976]). Schobert [1977] questions
the osmoregulatory function of proline. She postulates instead that the amphiphilic
proline molecule associates via its hydrophobic part with hydrophobic side-chains.
These are thereby converted into hydrophilic, less stress-susceptible groups, by exposure of proline's carboxylic and imino group versus water molecules. Betaine, also an
amphiphilic substance which was found to be positively correlated with salt resistance
in several plants (Hellebust [1976]), might act similarly to proline, although its role
in osmoregulation cannot be excluded (Wyn Jones et al. [1977]).
3.3 Changes in phytohormones
It is not surprising that stress causes changes in the contents of phytohormones (Khan
etal. [1976]). Growth stimulating cytokinins are reduced and their antagonist abscissic
acid (ABA) accumulates (Hsiao [1973]; Loveys and Kriedemann [1973]; Doerffling et
al. [1974]; Ini and Benzioni [1976]). ABA induces closure of stomata and synthesis
of proline (Raschke [1977]; Eder and Huber [1977]) and seems to improve stress
tolerance, but probably only for short-term exposure to stress. It is also known, however, that ABA accelerates senescence of plants, i.e. seed filling is shortened and yields
will be lower. This exemplifies once more the difficulties in the transfer of short-term,
laboratory findings to field conditions. That phytohormones are involved in stress
response can also be concluded from data by Schindler [1974], who found that CCC
increased salt tolerance of Phaseolus beans and tomatoes.
208
The interaction between stress and phytohormones raises the questions of what might
be the intermediate steps between stress-induced cytoplasmic dehydration and phytohormone synthesis and what role the nutritional status of the plant may play in this
connection. It can be assumed that structural changes of membranes by dehydration
are the first response reactions to stress and that synthesis of ABA, proline and other
compounds follows, although these compounds can also affect membrane permeability.
Chloroplasts are the site of ABA synthesis in leaf cells (Loveys [1977]) and mevalonic
acid is the precursor of carotenoids as well as of ABA (Levitt [1977]). A slight stressinduced change in chloroplast membranes could shift the direction of mevalonic acid
pathway to more ABA synthesis. The transition of chloroplasts to chromoplasts and,
correspondingly, of carotenoids to xanthophylls in senescing tissues are examples of
such metabolic shifts (Lichienthaler [1977]).
Due to the involvement of potassium in osmoregulation and water economy of plants,
it could be speculated that plants well supplied with K might have a lower level of
ABA and/or a more intensive ABA degradation. This would be one explanation for
the increased leaf area duration and grain size in cereals with ample K nutrition
(Haeder and Mengel [1974]; Forster [1976]).
3.4 Energy status of the plant under stress
The contribution of nutrient ions, especially of K+, to osmotic potential and maintenance of turgor pressure, being the triggering force for cell enlargement and growth,
as well as the stimulating effect of K + on the synthesis of ATP and of proteins, reveal
the unique and universal role of K for the tolerance ability of plants (Pfliger and
Mengel [1972]).
If plants adjust osmotically to stress and continue to grow, albeit at a lower rate, protein synthesis also continues. This is shown by the incorporation of'IN into the proteins
of salt-stressed barley (Table 11). NaCI at 60 mM retards growth, which is restored by
the presence of 10 mM KCI. Uptake of '5 N and protein synthesis are very poor in the
absence of both Na and K. NaCI improves protein synthesis but the highest SN
uptake and protein synthesis was in the treatment with 10 mM KCI. In further experiments (Helal, unpublished), salinity stress was better tolerated under saturating light
intensity. Root respiration was also increased under saline conditions and an adequate
K supply provided more sugars for this stimulated respiration. Both observations
Table II. Effect of salinity and potassium on growth and '5N-nitrogen metabolism of barley
seedlings (Helal et al. [1975])
Treatment
NaCI KCI
mM
mM
-
60
60
120
120
.................
. ..............................
10 ...............................
..............................
10 ...............................
-
Growth
mg DM/plant
"N in shoots
mg/100g FW
199
165
201
161
165
3.83
4.44
9.74
3.16
5.26
% of"'N
in protein
12.7
30.4
47.8
35.2
39.0
209
suggest that better energy supply and ATP levels, both promoted by adequate K
nutrition, play a decisive role in the response reaction to environmental stress. Metabolic energy is required to preserve membrane integrity and this explains the fact that
enzymes in halophytes can tolerate higher salt concentrations than those in glycophytes, although under in vitro conditions enzymes from halo- and glycophytes are
equally salt-sensitive (Greenway and Osmond [1972]; Austenfeld [1974]; Flowers et
al. [1977]).
4. Biotic stress
Plants in the field are always associated with micro-organisms and representatives of
the micro- and mesofauna. Some such organisms are benefactors, like symbionts;
others, like certain rhizosphere organisms, are opportunists - becoming unfavourable
under some conditions and others are pathogens. Biotic stress can be exerted by the
two latter groups.
4.1 Pathogenic interactions - diseases and pests
Since the end of the 19th century it has been known that the infestation of plants by
pathogens may depend on the nutritional status of the plant. The large body of literature on the effect of nutrients on diseases has been reviewed several times, most
comprehensively by Fuchs and Grossmann [1972]. I.P.. held a colloquium 'Fertiliser
Use and Plant Health' in 1976 (LP.I. [1976]) and an extensive compilation of literature
was published recently (Perrenoud [1977]). Nitrogen and potassium are the nutrients
most involved in plant health. In general terms, while high nitrogen usually increases
susceptibility, potassium increases resistance of a wide range of hosts to fungal,
bacterial and viral diseases as well as to pests. Thus, the behaviour of plants often
varies according to their N K ratio. Table 12 summarises 1209 observations, reports
and experiments documented in the literature and indicates that K seems to be most
effective against fungal and bacterial diseases, and without clear effect on nematodes
and viruses. The influence of other minerals is usually less. Silicon has been reported
to increase the resistance of cereals to fungal diseases (Trolldenier [1969]). Boron has
Table 12. Effect of potassium on incidence of diseases and pests (Perrenoud [1977])
Number and percentages of indications
Total
Incidence
decreased
Incidence
unchanged
Incidence
increased
740
231
54
116
68
71%
59%
42%
41%
75%
11%
16%
4%
14%
12%
18%
25%
54%
45%
13%
Total ............................ 1209
65%
12%
23%
Fungal diseases ..................
Insects+-Mites ...................
Nematodes ......................
Viruses .........................
Bacteria .........................
210
increased the resistance of a number of hosts to fungal attack, as have Mn, Cu, Zn and
Li at suitable rates.
Fertiliser application may influence the response of plants to pathogens directly or
indirectly. By improving growth and closing the canopy, it changes the microclimate
and increases humidity, which increases infection, spread and severity of attack
because most fungi require high humidity during the first few hours after germination.
This unspecific effect can be caused by eliminating any limiting factor but, in practice,
the usual cause is generous use of nitrogen.
Fertiliser affects the ability of the plant to escape disease. Stomata! movement is
important when pathogens enter a plant through the stomata as do bacteria and most
rust fungi. In K-deficient plants stomata remain open longer, favouring infection
(Trolhlenier [197/a]).
For successful infection the presence of a pathogen must coincide with the susceptible
stage of growth. For many diseases the plant is only susceptible for a limited period,
e.g. in the budding or flowering stage, which can be shortened or prolonged by fertiliser.
(Nitrogen tends to prolong susceptible stages, whereas K and P favour the onset of
flowering.) The plant's ability to regenerate diseased organs may enable it to escape
the pathogen. High nitrogen increases predisposition, but also greatly enhances
regeneration, compensating the damage (Garrett [1970]).
4.2 Disease resistance mechanisms
The effect of plant nutrition on pathogenesis varies both with pathogen and genetic
properties of the plant. Gdumnann [1951] mentions that fertiliser only slightly affects
wart in potatoes (Synchytriumn endobioticun), whereas it greatly affects Phytophthora.
Smut of cereals is less affected than rust and powdery mildew.
Susceptibility to toxins, produced by pathogens, may depend on plant nutrition but
the infection density is also important (examples cited by Fuchs and Grossmann
[1972]). Differences in resistance become less or disappear completely at low or high
infection density.
Hassebrauk [1930], working on rusts of cereals, observed that fertiliser application
had the greatest effect with moderately susceptible or partially resistant varieties,
highly resistant and highly susceptible varieties being little affected by nutritional
status. This has been confirmed several times (Fuchs and Grossmann [1972]), and
recently, for bacterial leaf blight on rice (Reddy and Sridhar [1975]).
4.2.1 Histological barriers
Many pathogens penetrate the cuticle and epidermis, whose structure and thickness are
preformed histological barriers which are thought to be important defence mechanisms.
Potash fertiliser was observed to improve leaf thickness in rye at the end of the last
century. The outer epidermis walls of K-deficient plants are thin. Dentler [1958]
found that germination of powdery mildew spores was independent of K nutrition,
but that only the more vital conidia succeeded in penetrating the thicker epidermal cell
walls of plants well supplied with K. Thicker outer epidermal walls also confer resistance to rust. The penetration process has been studied recently by scanning and
transmission electron microscopy. In some pathogens the penetration peg of the germ
tube seems to be forced mechanically through the cuticle, while in others its passage is
211
facilitated by enzymatic degeneration of the cuticle. Electron micrographs indicate
that enzymatic activity is more important than mechanical force (Wheeler [1975]). It
is known that many plant pathogens cannot degrade lignin, so lignification of cell walls
bars penetration (Friend [1976]). Adequate K enhances lignification. The intricate
penetration process in variously nourished plants needs more study.
Disease resistance is often controlled by factors other than cell wall strength because
spore germination, germ tube production and appressoria formation do not differ in
resistant and susceptible plants ( Wheeler [1975]). The plant may respond to infection
by counter-reactions which restrict further penetration, for instance by lignification
(Friend [1976]).
Presumably both the preformed histological barriers and the formation of infectioninduced barriers depend on nutrient status, though there is no experimental evidence
for this. The function of K in plant metabolism suggests that it enhances lignin production as a response to invading organisms and the production of callose, cellulose
and the suberization of cells. The success of such defence reactions would depend on
the rate of production of these substances and the rate at which pathogens extend.
4.2.2 Biochemical defence
Biochemical defence mechanisms can also determine resistance, but little is known of
the influence of mineral nutrition on these mechanisms. Phenols have been known for
many years to be a source of disease resistance in many plants. Pathogens attack and
cause more damage in varieties with low phenol content.
Kir~ly [1964] has shown that increasing nitrogen decreases the total phenol content
of wheat leaves, thus increasing susceptibility to wheat stem rust (Puccinia graminis
var. tritici). Moreover, an increase in the ratio of soluble N/phenols lowers the toxicity
of polyphenols to fungal pathogens (Kirdly [1976]). Though inadequate K supply
causes phenols to accumulate, their degradation by polyphenoloxidases might also be
accelerated, because when K is inadequate soluble N compounds accumulate. But
optimal K nutrition favours protein synthesis, reducing soluble N and preserving
toxic polyphenols. This explains the effect of K in increasing resistance to Xanthomonas
oryzae in rice (Reddy and Sridhar [19751). Fuchs and Grossmann [1972] give more
evidence on the interactions between plant nutrition, amino acid and carbohydrate
content and resistance.
Often susceptible plants contain more of these compounds than resistant plants. For
instance, lupin seedlings susceptible to Fusariutn oxysporum contained more leucine,
aspartic acid, alanine and asparagine than resistant plants, and they also contained
glutamic acid which was not detectable in resistant plants. Infection decreases the
amino acid content of susceptible plants, perhaps because the amino acid is utilised
by the fungus; plants are resistant because the fungus grows poorly in the absence of
these amino acids (Whitney [1976]).
Amino-acid content is influenced by mineral nutrition, suggesting that resistance may
be related in some diseases to amino acid concentration in the tissue. The free amino
acid level is generally directly correlated with high N and inversely correlated with
high K supply level. As long ago as 1934 Gassner and Franke attributed susceptibility
to rusts of cereals with high N supply to the higher level of soluble amino acids in the
plants. Further work is necessary to find out whether this is directly connected with
resistance.
More is known of the relation between nutrition, carbohydrates and resistance. It is
212
well known that when potassium is low soluble N compounds and carbohydrates of
low molecular weight accumulate at the expense of cell constituents of higher molecular
weight, apparently because the activity of catabolic enzymes, such as amylase, saccharase, glucosidase and protease, is increased. Furthermore, K deficiency restricts
phosphorylation, so that carbohydrates of low molecular weight and soluble N compounds will accumulate (Mengel [1972]). The higher susceptibility of plants low in
potassium to most pathogens may be related to their higher content of these low
molecular food sources.
However, there are contradictory results, possibly due to the differing nutritional
requirements of various pathogens.
Horstidl and Dimond [1957] distinguish high and low sugar diseases and discuss the
effect of manipulated tissue sugar content on susceptibility. As sugar level increased,
plants became more susceptible to rusts, powdery mildews, chocolate spot of bean
(Botrytisfabae) and Verticillium wilt of potato, but less susceptible to Fusarium wilts,
Alternaria solani on tomato, Dutch elm disease, Hehninthosporiun sativin on cereals
and Hehinthosporiun victoriae on barley. Change in susceptibility with age of leaves
may be linked to carbohydrate content, old tomato leaves being more susceptible to
target spot (Alternaria solani) than young leaves with their lower sugar content.
Removing tomato fruits as carbohydrate attracting sinks decreased the susceptibility
of leaves.
Very little seems yet to be known about the in situ nutritional requirements of pathogens (Wolffgang [1975]) and it remains to be shown whether the change in carbohydrate level is causally connected with resistance. The effect of K on decrease of
sugar levels fits the decrease in susceptibility for high sugar diseases (rusts and powdery
mildews). The hypothesis of Horsfi ll and Dinond [1957] would be further supported
if the reverse held true for low sugar diseases, but almost nothing is known about this.
With a few diseases, such as rice blast (Piricularia oryzae) high K seems to be unfavourable. It would be interesting to check the hypothesis of low and high sugar
diseases in such cases. The interactions between pathogens and plants are often more
complicated than can be simply explained by carbohydrate metabolism and requirements.
A better approach may be offered by Garber [1956], who considers the host both as a
nutritional and as an inhibitory environment. Both aspects are affected simultaneously
by the nutritional status of the plant.
4.3 Non-pathogenic interactions - the rhizosphere
Saprophytic organisms in the rhizosphere can also cause stress, although this stress is
indirect, reducing growth by oxygen depletion and denitrification and contributing to
physiological disorders. Such physiological disorders in rice are known under various
names describing symptoms which are often difficult to elucidate. Akagare Type I, 11
and Ill in Japan, suffocating disease in Taiwan, Bronzing in Sri Lanka (Yoshida
[1971]), an unnamed disorder in West Java (Ismunadfi et al. [1973]) and Anaranjamiento in Colombia (CIA T [1972]). Extremely reducing soil conditions aggravate
these disorders. They may be associated with excess Fe2 + and HIS or lack of free iron
(Babaetal. [1965],; Park and Tanaka[1968]). At least in the cases of Akagare Type 1,
Bronzing and Anaranjamiento, the main cause is excessive uptake of Fe2 + resulting in
213
Fe toxicity, which is probably the most important physiological disorder limiting rice
yields on vast acid soil areas of the tropics, especially on acid sulphate soils. Injurious
bacterial metabolites are said to be related to the occurrence of such disorders.
Plant roots provide abundant organic material to the surrounding soil as exudates and
sloughed-off cell debris, stimulating the proliferation of micro-organisms in the root
region. Among other factors the nutritional status of the plant influences the amount
and nature of organic substances. As discussed elsewhere in detail, higher nitrogen
application increases, while higher phosphorus, potassium and calcium application
decrease the quantity of water-soluble root exudates (Trolldenier [1975]).
4.3.1 Oxygen depletion
Micro-organisms of the root region consume more 02 than the roots themselves,
indicating their significance. Bacterial numbers and oxygen consumption are highest
on roots of plants supplied with excessive N and insufficient K (Table 13). In a poorly
aerated medium abundant microbial activity contributes to oxygen depletion in the
rhizosphere, causing oxygen deficiency. This applies to all species relying on soil
oxygen. Once anaerobic conditions are established, root exudation accelerates and
promotes further microbial growth (Rittenhouse and Hale [1971]).
Table 13. Oxygen consumption of sterile and non-sterile roots and bacterial numbers as
related to potassium application (Trolldenier [1972])
Treatment
0, consumption
([,l O/mg FW/h)
K1 sterile ........................................
non-sterile ....................................
K 2 sterile ........................................
non-sterile ....................................
0.09
0.27
0.10
0.24
Bacteria
(106/g FW)
1124
710
4.3.2 Denitrification
When oxygen is depleted in the rhizosphere some micro-organisms use other compounds as terminal hydrogen acceptors. As long as it is available under anaerobic
conditions, nitrate is the preferred hydrogen acceptor. During denitrification, nitrate
is converted to volatile reduction products, i.e. N2, N 2 0 and NO, and this requires, as
well as oxygen deficiency, the presence of hydrogen donors supplied by root excretion.
As K deficiency favours root excretion, microbial activity and 02 consumption, it also
promotes denitrification when oxygen diffusion is too low to meet the demand of roots
and adjacent micro-organisms. In nutrient solution experiments Trol/denier [1971b]
found that adequate K reduced denitrification but, when the nutrient solution was
aerated, denitrification was low and not affected by potassium supply (Figure 14).
Recent pot experiments on soil nitrogen balances confirmed the beneficial effect of high
potassium applications in limiting nitrogen losses by denitrification (Trolldenier
[1979]).
4.3.3 Fe-toxicity
In general, rice roots have an oxidizing power which counteracts adverse soil conditions. Oxygen excreted from the roots detoxicates hydrogen sulphide and organic
214
nK,
250
n- not aerated
a- aerated
Z
Z
200
S150
A nK,
C
.9
too
50
*aK 2
aK,
0
5
10
hours
15
20
Fig. 14. Denitrification in the rhizosphere as affected by K nutrition and aeration of the
nutrient solution ( Troldenier [1971])
molecules by oxidation; also ferrous iron is oxidized and precipitated near the root
surface as ferric oxide.
Potassium deficiency reduces the oxidizing power of rice roots (Baba el al. [1965]).
This is in accordance with the finding that in solution culture potassium deficiency
caused the severest drop in redox potential (Trolldenier [1977]). There is ample evidence that excessive nitrogen decreases, while higher potassium increases the oxidation
power of rice roots. The cause for the opposite effect of the two nutrients seems to be
associated with their effect on microbial activity in the rhizosphere. When oxygen is
used up, ferric iron and sulphate are reduced and iron is absorbed in excess. Several
workers have suggested that the power of rice roots to exclude iron is related to potassium nutrition (Tanaka and Tadano [1972]). Troldenier [1977] grew rice in a low K
soil in pots. Soon after transplanting, the thin branched lateral roots turned black.
At high K concentration all roots remained a healthy brownish colour. The black
discoloration is due to precipitated ferrous sulphide. Sulphide is formed only under
strictly anaerobic conditions and the black roots had no oxidizing power, the rhizosphere apparently being even more reduced than the surrounding soil. The low K
plants took up more iron than those better supplied with K (Table 14). These results
throw some light on the causal relation between the nutritional status of the rice plant,
the microbial activity in the rhizosphere and the occurrence of physiological disorders.
215
Table 14. Potassium and iron content in the dry matter of rice shoots 57 days after transplanting (Trolldenier [1977])
Exchangeable K
(meq/100 g soil)
0.08
0.15
0.25
0.40
...................................
...................................
...................................
...................................
Shoot
dry weight
K
(% in DM)
Fe
(ppm in DM)
16.9
20.7
25.3
27.7
0.45
0.93
1.41
2.00
520
400
380
330
5. Conclusions
The nutrition of a crop obviously affects its tolerance and/or avoidance of abiotic
stress and its resistance to biotic stress. A good supply of K increases the plant's
tolerance and resistance due to its function in osmoregulation, in energy status and in
the synthesis of high molecular compounds. The universal role of K in growth and
metabolism makes it extremely difficult to trace a specific and causal relation between
K nutrition and the response mechanism to a given environmental stress. More sophisticated methods are needed, more detailed description of type and extent of stress and
of the physiological stage of the plant investigated are required to recognise the specific
action of K in this respect. Better knowledge of the role of potassium in counteracting
environmental stress may help to minimize yield constraints.
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222
25th Anniversary of the Scientific Board
11th IPI-Congress held in Bern/Switzerland
from September 4 to 8, 1978
4th Session
Potassium
Requirements of Crops
223
A Consideration of Factors which Affect the
Potassium Requirements of Various Crops
K. Mengel, Institut fur
Republic of Germany-
Pflanzenerni.hrung, Justus-Liebig-Universitat, Giessen/Federal
I. Introduction
In 1762 the Englishman Home carried out an interesting experiment in growing
barley on a sandy soil. In one treatment Home had added potassium sulphate to the
soil and this resulted in increased vigour of growth of the barley, showing that potassium, or sulphate, had a beneficial effect on plant growth. In the following century
the investigations and experiments of Theodore de Saussure, Carl Sprengel and
Justus von Liehig showed clearly that K + is an essential plant nutrient. K+ is indispensible for every plant species.
This qualitative aspect of the K + requirement of crops is interesting but, from an
agronomic point of view, we need also to know how much K+ the crop requires in
order to produce a satisfactory yield of produce of good quality. The total K+ requirement can be estimated from the total K taken up by a crop per unit area. Data
of this kind have been published by many authors and they are of direct value to
practical farming in suggesting what rates of potassium fertiliser should be applied.
However, on their own they are insufficient as, in assessing fertiliser needs, it is also
necessary to consider the rate at which this K+ must be supplied to the plant. Thus
both total requirement (quantity) and rate of supply (intensity) are equally important.
The rate (intensity) requirement can conveniently be stated in terms of the amount
of K+ required by a crop per unit area and per day. K rate requirement has received
only sporadic attention in the literature and for this reason it is a particular target
of this paper and the following contributions to deal with this intensity aspect for
the most important crop groups.
In further considering the quantity requirement, it will clearly depend upon the total
amount of plant material produced per unit area but it also depends upon the kind
of plant material, or type of plant organ, produced and harvested.
A final and most important point in assessing the requirement for K fertiliser is
that crop requirement and fertiliser requirement are not identical; the latter will
depend much on the K status and K dynamics of the soil, on the rooting pattern
of the crop and also, probably, on root metabolism.
* Prof. Dr. K. Mengel, Institut fOr Pflanzenernahrung, Justus-Liebig-Universitat, Sidanlage 6,
D-63 Giessen/Federal Republic of Germany
225
2. Total K requirement
Total K requirement may be defined as the total amount of K (kg K/ha) needed to
produce the highest possible economic yield under given conditions of growth. If
this total requirement is correctly met, the various plant tissues should contain K
in the optimum concentration; higher K concentrations than this mean that more
K has been taken up than is really required, lower than optimum concentration may
mean that too little K is taken up to satisfy the production potential of the plant.
The total K in the plant includes the K of aerial parts and that of the roots and other
organs (tubers) growing in the soil. Root K content is difficult to measure under field
conditions and most estimates of total K uptake ignore the root content. However,
in most cases, root K is only a small fraction of the total and the error thus involved
may be tolerated.
The K content of organs low in dry matter is generally especially high, provided the
K content is expressed on a dry weight basis. The difference in K content of tissues
rich or poor in water is very small when it is expressed on a fresh weight basis. Thus,
Jungk [1970] found that the K content of Sinapis alba was substantially constant
throughout the growing season provided it was calculated on a fresh weight basis.
The optimum K content found in this investigation ranged between 400 and 600 ppm
K in fresh material.
The maintenance of cell turgor requires a certain K concentration in the water of
cells and tissues. Considering that mesophyll tissues contain 85 to 90% water, of
which about 90% is found in the vacuoles, 5% in the cytoplasm and 5% in the cell
wall material, it is clear that the K content of leaf dry matter must be high. Most
of this K is in the vacuoles where it contributes to cell turgor. According to Mengel
and Arneke (unpublished results) K± is especially necessary for the turgor of young
leaves and the general finding that younger leaves are richer in K+ than older ones
+
may be related to the higher physiological K demand of young leaves. Because K
is very mobile in the whole plant, much can be translocated from the younger to
the older leaves (Greenway and Pitman [1965]). Mengel and Arneke found that
insufficient K supply resulted in suboptimal turgor (4p<6 bar) in young leaves of
Phaseolus vulgaris, which was associated with severe growth reduction, while the
turgor of the older leaves was hardly affected by K deficiency. From these findings
it is concluded that young growing tissues have an especially high K requirement
for the maintenance of optimum turgor, which is needed for cell elongation and
also, probably, for cell division. Because of the involvement of K+ in various biochemical and biophysical reactions for which it is indispensible, the reaction which
requires the highest K concentration controls the total K demand of the particular
tissue. In young leaves this 'controlling reaction' is the osmotic function of K+. In
+
natrophilic species, Na + may partially substitute for K in achieving optimum turgor
as has been shown by Marschner and Possinghain [1975] using leaf discs of sugar
beet and spinach. In terms of practical crop production this means that, whenever
the object is to produce young green plant material, much K is required per unit
dry matter production.
Other than young leaves, non lignified stems, petioles and culms are generally rich in K.
Much of these organs consists of phloem tissue and, as the most important inorganic
ion in the phloem sap is K+, this may explain why culms and stems often have higher
K contents than leaves. In comparison with other inorganic ion species, the K+
226
concentration in the phloem sap is high and, according to Hall and Baker [1972], in
the range 60 to 112 m molar. Thus, besides sugars and amino-acids, K+ contributes
substantially to the osmotic potential of the phloem sap. Turgor pressure is an
important driving force for solute movement in the phloem tissue (Geiger [1975]).
However, it seems doubtful if the beneficial effect of K + on phloem transport is
mainly the result of its effect on the turgor in sieve tubes as, according to Mengel
and Haeder [1977], K+ promotes phloem transport mainly by promoting phloem
loading.
The phloem sap is the most important source of the material which is laid down in
storage tissues such as tubers, roots, seeds and fruits and it is for this reason that,
provided their water content is relatively high, these storage tissues are rich in K+.
Fleshy storage tissues like sugar beet roots, potato tubers, tomatoes, bananas, grapes
and other fruits contain much K in proportion to their dry matter content and such
crops generally have a high K requirement because much K is needed to fill these
storage tissues properly with K+. The most important function of K + in these tissues
is probably that of maintaining optimum turgor. Much of the K + required by storage
tissues and fleshy fruits comes from the K in the leaves and stems and will thus have
been taken up by the plant before the major growth of storage tissues commenced. This
is shown in Figure I, based on data from a sand-solution experiment with sugar
beet (Mengel and Forster [1973]). It appears that K uptake rates (mag K absorbed
per plant per day) were highest during the period of vigorous leaf growth. At a later
stage, however, during September and October when root growth and sugar storage
were the major processes, the rate at which K was taken up was relatively low. Clearly
242016-
C
g12
0.
-o8
E 4
30.6 23.7
31.7
15.8
31.8
15.9
30.9
1.11
Fig.lPotassium uptake rates of sugar beet at different stages during the growing season
(Mengel and Forster[19731)
227
+
at this stage substantial amounts of K were translocated from the leaves along
with photosynthates. It remains to be investigated whether this is also true for other
crops. There is evidence that plants can take up Kt at high rates during the vegetative
stage of growth and this may be related to their phytohormone status (Cram and
+
Piftnan [1972]). It is thus supposed that only a minor part of the K required by
fruits and storage organs is taken up directly from the soil and that most of their K
requirement is taken up during vegetative growth and translocated from the leaves
to the storage organs and fruits. This question, which is relevant to the K rate requirement, needs further research. In the case of crops like cereals, oil seeds and cotton,
their fruits are low in water and hence the K content of grain or seed, on a dry matter
basis, is relatively low. As with the crops discussed above, the major part of the K
requirement of grain crops is taken up during vegetative growth, as illustrated by
results of Mengel and Forster [1966], which showed that grain production by barley
+
was not significantly affected by removing K from the nutrient solution during grain
+
filling. The physiological role of K in grain and seed is probably related to the activation of starch synthetase and protein synthesising enzymes [Evans and Wildes
[1971], Hawker et al. [1974]).
The total K requirement of a crop depends much on the growing conditions provided
by soils and climate. When much organic matter can be produced, much K is also
required. Total K requirement also clearly depends on genetic yield potential; for
example, modern rice cultivars need two or three times as much K as local traditional
varieties, as shown in Table I (Kemniler [1973]).
For many crops the total K content of the crop at maturity is lower than it is at an
earlier stage of growth. Figure 2, showing a typical K uptake curve, reflects this
behaviour (data of Sturi and Jungk [1972]). Total K taken up by the time of
flowering was as much as 200 kg K/ha whereas at maturity, the above-ground portion
of the crop contained only 125 kg K. The loss of K between flowering and maturity
+
may be the result of the secretion of a substantial amount of K from the roots into
the soil during the period of grain formation. However, experiments by Haeder
[1971] have shown that only one or two per cent of the total K content of the plant
is secreted in this way and it is suggested this loss of K from maturing cereals is
+
mainly due to leaf fall and the leaching of K from the mature plant by rainfall.
The problem of total K requirement is still more complex in perennial crops than in
annual crops. This is particularly true for fruit trees, vines and various plantation
crops since they are able to store substantial amounts of K in the wood and bark.
In deciduous trees and in vines K+ is retranslocated from the leaves to the wood
before the leaves are dropped. This K may be mobilised in the following spring
when new leaves are developed. Net production of organic matter by these crops is
Table 1. Yield level and nutrient uptake of a conventional local rice cultivar and the modern
high yielding rice cultivar 'TN I' (Kennz/er [1972])
Cultivar
Grain yield
t/ha
Nutrient uptake, kg/ha
K
P
N
Local .........................................
TN I ..........................................
2.8
8.0
82
152
228
10
37
100
270
240
200
/
200-
1
150
0-1 0-165
a. 12 0
135
CL
0.
D 80.9:
r-J
0
0.
End of 2od
er :: milk
tiltering
node emerg f
_oIestage
iotur.
I
Fig.2. Quantity of K in the upper plant parts of winter wheat during the growing season
(Sturm and Jung [19723)
especially high during their juvenile development, and it is particularly over this
period that an ample K supply is required. Thus Fritnond and Ouvrier [1971] showed
that the yield potential and also the precocity of fruiting of coconut palms depended
much on the K supply of the young palms.
Quantitative data as to how much K is required per tree or per ha during the juvenile
development of these crops until their full size is attained, how much K is mobilised
in the wood and bark in spring and translocated to the newly-developing leaves and
how much K is exported by the fruits are badly needed for a number of perennial
tree crops.
3. Potassium rate requirements
The rate requirement may be defined as the K requirement per unit time, i.e. kg K
per hectare of crop per day. A high rate requirement does not necessarily go handin-hand with a high total need for K, as exemplified by data of Nelson [1968] comparing K uptake by sugar beet and sugar cane. Both crops need about the same total
amount of K (400 kg K/ha) but, while sugar beet reaches maturity in 120-150 days,
the cycle of sugar cane is about three times as long, so that the former crop has an
average K rate requirement about three times as high as that of the latter.
The K rate requirement differs between crops and, as it is closely related to the growth
rate, climatic factors such as temperature, light intensity and water supply also come
into play. For instance, the extremes represented by grassland in cool mountainous
229
regions and the warm lowland conditions of the tropics impose very large differences
in this respect. Tropical grass requires large amounts of K and needs a high K content
of dry matter in order to achieve optimum production (Gartner [1969]); rate of
growth is also high. The example demonstrates that the question of K rate must be
considered in relation to the yield potential of a particular site.
It must be emphasised that the K rate requirement of a crop varies much throughout
the growing season and the 'maximum K rate requirement' is particularly important
in considering the ability of a soil to supply K to the growing crop in such a way as
to impose no limit on growth. An approximate idea of this maximum rate requirement
can be obtained from a K uptake curve which plots K quantity in the crop against
period of growth. For practical purposes, as mentioned earlier, root K content may
be neglected. Such a K uptake curve for maize is shown in Figure 3, after Nelson
[1968]. The maximum rate requirement AK/Atime can be obtained by drawing a
tangent through the inversion point of this sigmoidly shaped curve. The maximum K
rate requirement is found at the point where vegetative growth is at its maximum.
High growth rates demand rapid K uptake and it is during such periods of rapid
growth that plants are particularly susceptible to K deficiency.
The rate at which a crop can take up K is dependent upon the K intensity of the soil,
high rates of uptake demanding high intensity and the reverse. The maximum K
uptake rate of a crop should give a guide to the desirable level of K intensity of the
soil while its total maximum K uptake is related to the quantity of K in the soil.
100
UPTAKE OF NUTRIENTS
RELATION
so '0INDRY
WEIGHT-TO -Q
"
60
0
20
""
.leaves
0
0
DAYS AFTEREMERGENCE
Fig.3. Potassium, nitrogen and phosphorus uptake and dry matter production of maize
during the growing season (Iowa State University, quoted by Nelson [1968])
230
There has been much discussion of the concepts of intensity and quantity during the
past ten years but there is, as yet, little agreement as to the parameters which best
express these concepts. Conventionally, it is usual to state K quantity in terms of
exchangeable K, but this simple measurement is not enough as, depending on soils
and crops, soil K which is not exchangeable may make a considerable contribution
to the K taken up by the plant (Wiechens [1975]).
From the point of view of plant nutrition, the K intensity may be defined as the rate
at which K is supplied by the soil to the plant roots. This 'K supplying rate' is directly
related to the K concentration of the soil solution which controls the rate of diffusion
+
soil. K diffusive flux and K uptake are positively related as shown for
of K in the
young maize plants in Figure 4 (Mengel and von Braunschweig [1972]). It seems, then,
that the K concentration of the equilibrium soil solution is a reliable parameter for
measuring K intensity in the soil.
The relationship between K quantity and K intensity differs considerably between
various types of soil according to clay content and the type of clay minerals present
(Nentzh et al. [1970]). The slope of the curve showing the relationship between K
quantity and intensity gives direct information on the K buffering capacity of a soil,
a steep slope indicating high buffer capacity and vice versa (Mengel and Kirkhy
[1978]). Soils of high buffering capacity often have a high total K content even though
the exchangeable K content may only be medium or low. In such soils non-exchange-
500'
0
400
0.24
300
20
2420
K-i
2.7
16
16
o o
J•
K
2
0
O K2
Y24
100-
20
0
K3
r-0.94 XXA
y-3.37x - 4.37
20 D 2.7
S 16n
27
40
20
60
100
80
120
140
2
K-diffusive flux (pval/15cm /week)
Fig.4. Relationship between the K diffusion rate in the soil and K uptake by maize. The figures
associated with the symbols denote the pF of the soil at which the particular diffusion rate and
K uptake was obtained. The various symbols stand for different K fertilizer rates (Mengel
and von Braunschweig [1972])
231
able K content may contribute considerably to buffering the K concentration of the
soil solution. The quantities of non-exchangeable K can be very large and the
fact that crops growing on such soils may suffer from K deficiency is not a consequence
of too low a K quantity but because the K intensity is too low (Nemnfh [1975]).
Clearly, when a crop has a high K rate requirement it will be liable to potassium
deficiency if the soil K intensity is low even though the quantity of K in the soil is
more than adequate.
The reverse problem may be encountered on sandy or organic soils which usually
have a low K buffering capacity, insufficient to maintain soil solution K concentration
at a sufficiently high level against removal of K by the crop. In such cases it is clearly
the K quantity which is limiting and crop species having a high total K requirement
may run into severe K deficiency.
4. The relationship between plant roots and K requirement
Satisfying the K needs of a crop is not just a matter of soil K quantity and intensity;
rooting pattern, rooting depth and root metabolism are also involved in the complex
of K uptake and K requirement. Unfortunately there are few available field data
referring to this question and, for this reason, only a few major points will be discussed
in this section.
Maertens [1971] showed, in an interesting investigation, that a small portion only
of the total root system was capable of absorbing all the N, P and K required by the
plant so long as the nutrient solution was high enough in N, P and K. Obviously,
the optimum K concentration of the nutrient - or soil solution (K intensity) - which
is required must depend upon the root surface area, since a large root surface area
capable of taking up nutrient would require only a relatively low intensity and vice
versa. Jungk and Barber [1974] showed that the total uptake of P by maize roots was
related to root length. This finding also justified the conclusion that a plant with a
relatively extended root system may better exploit the soil for nutrients than one with
a low root/shoot ratio.
The critical period for the K requirement of a crop need not necessarily be at that
time when the highest K uptake rate is found. Mengel and Barber [1974] have shown
that the critical period for maize, during which the K demand of the vigorously
growing shoot may not be met by the root uptake potential, may occur at a rather
young stage. Table 2 shows calculated uptake rates of N, P and K per metre root
length in relation to plant age and it appears that the requirement per unit root
length is particularly high in the early stages of growth. More information is badly
needed as to how root/shoot ratio and K requirement are related at different growth
stages. This would give more precise information as to the particular stage of growth
at which K supply by the soil might become critical.
Rooting pattern and depth of rooting differ between the various crops. Study of
rooting patterns may indicate the extent to which soil K can be exploited by a crop.
Even cultivars of the same species may differ in their ability to 'mine' the soil for K,
as shown by a recent example for cotton (Halevy [1977]). Two cultivars, Acala
1517-C and Acala 4-42 were grown on the same soil, the former being the more
susceptible to K deficiency while the latter had a larger root system and, in particular,
more fine roots than Aca/a 1517-C.
232
Table 2. Effect of plant age on average nutrient flux into roots of maize grown under field
conditions (D. B. Mengel and Barber [1974])
Plant age
Calculated nutrient flux
days
r-moles/m root length/day
N
20
30
40
50
60
70
80
90
100
...................................................
...................................................
...................................................
...................................................
...................................................
...................................................
...................................................
...................................................
...................................................
227
32.4
18.5
11.2
5.7
1.2
0.46
2.0
4.2
P
11.3
0.90
0.86
0.66
0.37
0.17
0.08
0.10
0.23
K
53
12.4
8.0
4.8
1.6
0.15
0.06
0.37
0.16
Soils may vary in the depth of their rooting zones and it is suggested that a shallow
soil needs a higher K intensity to satisfy the plant's needs while in a deeper soil the
roots can exploit a relatively large volume of soil per unit surface area.
Root metabolism also has a crucial impact on 'mining' the soil for nutrients. Potassium uptake is closely associated with root respiration which, in turn, depends upon
the translocation of carbohydrate from the above ground parts (Pinnan [1972]).
Sugar translocation is reduced when photosynthetic activity is low or when a high
proportion of photosynthate is consumed in the upper plant parts.
Kaila [1967] suggested that the release of K + from K-bearing minerals is controlled
by the K concentration of the adjacent soil solution, low concentrations favouring
release of K + . There is a dynamic equilibrium between K+ in the soil solution and
root K 4 , the level of which is controlled by root metabolism. Drews [1978] has shown
that plants exposed to reduced light intensity cannot deplete the K+ of the soil solution to as low a level as plants in full light. This affected the release of K from clay
minerals. The K concentration of the soil solution under stressed plants remained
relatively high (40-50 l-molar K +) and hardly any K + was released form interlayer positions. But, plants exposed to full light lowered the K concentration to
about 10 zmolar K+ and at this low level K+ was released from the clay minerals
so that, in effect, these plants were feeding from the interlayer K. From this point of
view the results of Malquori et al. [1975] are interesting as they showed that wheat
plants could feed from the interlayer K of biotite while lucerne could not. It may be
worth investigating further whether differences between plant species in their ability
to exploit interlayer K are related to differences in their abilities to lower soil solution
K + concentration through plant uptake.
Legumes are not very effective in exploiting soil K, a statement which is supported
by data published by Schiln et al. [1976]. This field trial, continued for 20 years on a
loess soil containing much non-exchangeable K, showed that Vicia faba and a grassclover mixture responded most favourably to K fertiliser. K also markedly increased
the yield of potatoes, whereas it only slightly increased the grain yield of cereals
(see Table 3).
Grasses are very efficient in exploiting soil K but whether their high 'mining power'
will suffice to produce continued high yields on soils rich in non-exchangeable K
233
Table 3. Response of various crops to K fertiliser application. Average yield data from an
experiment lasting 20 years (Schdn et al. [1976])
Crop
Yield, t/ha of tubers, grains,
seeds or fresh matter
NP
No.
4.21
2.84
3.54
3.08
2.26
3
W inter wheat ........................................
5
Spring barley .........................................
3
O ats ................................................
2
W inter rye ..........................................
Spring rye ........................................... I
Broad beans .........................................
2
C lover-grass ......................................... 1
Potato .............................................. 4
1.27
38.3
23.8
NPK
4.55
3.21
3.80
3.15
2.49
(108)
(113)
(107)
(102)
(115)
2.46 (194)
45.8 (120)
32.6 (137)
In parenthesis relative values as compared with the control (NP = 100).
without the addition of K fertiliser is a question of great importance. There are
numerous examples to show that the continued removal of K in cut grass will eventually reduce soil K to the point where growth is limited by deficiency. Grinme
[1974] showed, in pot experiments, that there was a negative correlation between
grain yield of oats and the proportion of K which was taken up from non-exchangeable sources in the soil. Similar results were obtained by von Boguslawski and Lach
[1971], also in pot experiments. Instances where, under field conditions, continued
intensive cropping has led to K exhaustion are described by von Boguslawski and
Lach [ibid.] for sugar beet and De Datta and Goinez [1975] for rice. The former
work (Table 4) occupied 14 years on a soil derived from loess, which is generally
rich in illite and thus has a high K supplying power. In the course of the 14 years,
this particular soil released about 1600 kg K/ha, but as K depletion proceeded,
the rate of release of K became insufficient to maintain crop yields at a satisfactory
level. As shown in Table 4, K uptake by beet declined in a remarkable way and at
the last two harvests (1960 and 1964) amounted to only about 25 or 30% of the total
K needed for maximum sugar beet production under the climatic conditions of
W. Germany. Needless to say, such a decline in K availability resulted in severe
yield reduction.
Moisture promotes the release of interlayer K. Wiechens [1975] found that Loiun
perenne could take up very large quantities of non-exchangeable K when soil moisture
Table 4. Dry matter production and K uptake of sugar beet grown without K fertiliser for
14 consecutive years (von Boguslawski and Lach [1971])
Year
Yield, kg DM
(roots+tops)
K uptake
kg K/ha/year
1954 ..............................................
1870
424
1956 ..............................................
1960 ..............................................
1964 ..............................................
1430
1460
980
19 1
168
118
234
was satisfactory but, under drier conditions, release of non-ecxhangeable K was
very much restricted. This was shown in that, under moist soil conditions, only
occasional responses to K fertiliser were recorded, whereas when conditions were
dry, consistent K responses were obtained. This finding is of practical importance,
showing that even plant species with a high 'K exploiting power' may be less able to
extract soil K under dry conditions, in which case suboptimal K supply may result
in very much reduced yield.
Continuous cropping without K fertiliser so exhausts soil K that even Loliun perenne
known as a very potent soil K extractor, will also suffer badly from K deficiency under
optimum soil moisture conditions. This has been recently shown by Mengel and
Wiechens [1979] in pot experiments with rye-grass. During the first three cuts there
was no significant difference in yield due to potassium. At the 4th cut the K0 plants
even produced a significantly higher yield than those receiving potassium. All the K
taken up by the K0 plants of the 4th cut originated from the non-exchangeable soil K
fraction. Obviously these plants were able, under favourable weather and soil conditions, to extract the interlayer K of the clay minerals efficiently. Beginning at the
8th cut there were considerable yield depressions under the K, treatment and these
became increasingly severe in the later cuts. At the last cut the yield was only 39% of
that of the plants fertilised with K. By the I Ith cut, the K0 plants exhibited very marked
K deficiency symptoms. These plants were so weakened that even their ability to
exploit soil K was affected, as less than 50% of the K taken up originated from the
non-exchangeable soil K fraction.
Pot experiments with barley and vetch on the same soil showed remarkable yield
increases due to K fertiliser and they also showed a clear inverse relationship between
the yield level and the proportion of K which originated from the non-exchangeable
fraction (Table 5).
Table 5. Grain yield of spring barley and yield of aerial plant parts of vetch in relation to the
percentage of K taken up from the non-exchangeable soil K fraction (data from pot experiment of Wiechens [1975])
Content of exchangeable K
at the beginning of the experiment,
ppm K in dry soil
Grain yield
g/pot
DM yield
of vetch
g/pot
% K from the
non-exchangeable
soil K fraction
K0 500 (control) ..................
K, 800 ...........................
K2 1140 ...........................
77.8
85.5*
90.7**
31
36*
41
60.2
23.3
4.8
* P<5%
** P<0.1% compared with the control treatment
5. Conclusion
Crops differ in their K requirements due to differences in the physiological roles in
which K+ is involved. Where the harvested produce consists of young green material
the K requirement per unit dry matter produced is high, as in the case with young
235
grasses. If the same crop is harvested at the fully mature stage, the K requirement
per unit dry matter is substantially less. Crops producing fleshly fruits or storage
organs contrast with cereals as they require much K for filling these tissues.
The ability of a soil to supply a crop's K requirements is fairly well described by
parameters of K quantity and K intensity. K intensity is particularly relevant to the
K rate requirement, K quantity to the total K requirement. Root growth and root
metabolism probably play a major role in determining K availability but the relationships between K requirement and root growth and metabolism are not yet fully
understood.
The potassium requirement of a crop and the need for fertiliser K may differ widely
according to soil and climatic conditions. Soils well buffered for K do not generally
lose much K through leaching and when the soil K status is optimum the K fertiliser
requirement can be based on the removal of K in harvested produce. On poorly
buffered soils, especially on sandy soils under humid conditions, the K fertiliser
policy must take into account the danger of loss through leaching. In this context
it is also worth pointing out that the K in crop residues (straw, leaves and roots) can
also be leached by winter rainfall or under monsoon conditions.
6. References
Boguslawski, E. von and Lach, G.: Die K-Nachlieferung des Bodens im Pflanzenexperiment
im Vergleich mit dem austauschbaren Kalium. Z. Acker- u. Pflanzenbau 134, 135-164
(1971)
Cram, W.J. und Pitman, M.G.: The action of abscisic acid on ion uptake and water flow in
plant roots. Austr. J. Biol. Sci. 25, Nr. 6, 1125-1132 (1972)
De Datta, S.K. and Gomez, K.A.: Changes in soil fertility under intensive rice cropping with
improved varieties. Soil Sci. 120, 361-366 (1975)
Drews, J. U.: Die Aufnahme von Kalium aus der nichtaustauschbaren Kaliumfraktion des
Bodens in Abhingigkeit von der B.lichtung der Pflanzen. Dr. Thesis in the Fac. Nutritional
Sci. Justus-Liebig-University, Giessen, 1978
Evant, H.J. and Wildes, R.A.: Potassium and its role in enzyme activation, p. 13-39. In:
Potassium in Biochemistry and Physiology, International Potash Institute, Berne (1971)
Frnond, Y. and Ouvrier, M.: Importance of adequate mineral nutrition for the establishment
of a coco plantation on sandy soils. Olagineux 26, No. 10, 609-616 (1971)
Gartner, J. A.: Effect of fertilizer nitrogen on a dense sward of Kikuyu. Paspalumn and carpet
grass. 2. Interactions with phosphorus and potassium. Queensl. J. of Agric. a. Anim. Sci.
26, 365-372 (1969)
Geiger, D.R.: Phloem loading, p. 396-431. In: M.H.Zimmermann and J.A.Milburn: Transport in Plants 1, Phloem Transport. Springer Ver. Berlin, Heidelberg, New York, 1975
Greenway, H. and Pitman, M.G.: Potassium retranslocation in seedlings of Hordeum vulgare.
Aust. J. Biol. Sci. 18, 235-247 (1965)
Grimme, H.: Potassium release in relation to crop production. In: Potassium Research and
Agricultural Production, p. 131-136. Intern. Potash Institute, Berne (1974)
Haeder, H.E.: Kaliumabgabe reifender Gerste. Z. Pflanzenern~ihr. Bodenk. 129, 125-132
(1971)
Halevy, J.: Wachstumsrate und Nfhrstoffaufnahme von 2 Baumwollsorten bei Bewfisserung
Kali-Briefe, Fachgeb. 27, Nr. 5, 79. Folge (1977)
Hall, S.M. and Baker, D.A.: The chemical composition of Ricinus phloem exudate. Planta
106, 131-140 (1972)
Hawker, J.S., Marschner, H. and Downton, W.J.S.: Effects of sodium and potassium on
starch synthesis in leaves. Aust. J. Plant Physiol. 1, 491-501 (1974)
Jtungk, A.: Mineralstoff- und Wassergehalt in Abhhingigkeit von der Entwicklung von Pflanzen.
Z. Pflanzenernihr. Bodenk. 125, 119-129 (1970)
236
Jungk, A. and Barber, S.A.: Phosphate uptake of corn roots as related to the proportion of the
roots exposed to phosphate. Agron. J. 66, 554-557 (1974)
Kaila, A.: Releases of non-exchangeable potassium from Finnish mineral soils. J. Sci. Agric.
Soc., Finland 39, 107-118 (1967)
Kenunler, G.: Zur Dflngung von neuen Reis- und Weizensorten in Entwicklungslfndern. In:
Proc. 7th Fertilizer World Congress, p. 545-563, Vienna (1972)
Maertens, Al. C.: Etude exp&imentale de I'alimentation mindrale et hydrique du mais. Comparaison des besoins de la plante et des possibilitds d'absorption d'azote, de phosphore et
de potassium par les racines de Zea mnays. C. R. Acad. Sci. (Paris) 273, Sdrie D, 682-684
(1971)
Malquori, A., Ristori, G. and Vidrrich, V.: Biological weathering of potassium silicates:
I. Biotite. Agrochimica 19, 522-529 (1975)
Marschner, It. and Possinghan, J. V.: Effect of K - and Na + on growth of leaf discs of sugar
beet and spinach. Z. Pflanzenphysiol. 75, 6-16 (1975)
Mengel, D.B. and Barber, S.A.: Rate of nutrient uptake per unit of corn root under field
conditions. Agron. J. 66, 399-402 (1974)
Mengel, K. and ron Braunschweig, L. C.: The effect of soil moisture upon the availability of
potassium and its influence on the growth of young maize plants (Zea mays L.). Soil Sci.
134, No. 2, 142-148 (1972)
Mengel, K. and Forster, H.: Der Einfluss ciner zeitlich variierten, unterbrochenen K-Ernahrung auf Ertrags- und Qualititsmerkmale von Gerste. Z. Acker- und Pflanzenbau 127,
317-326 (1968)
Mengel, K. and Forster, H.: Der Einfluss der Kaliumkonzentration der <(Bodenldsungo auf
den Ertrag, den Wasserverbrauch und die K-Aufnahmeraten von Zuckerrbiben. Z. Pflanzenernihr. Bodenk. 134, 97-192 (1973)
Mengel, K. and Ilaeder, H. E.: Effect of potassium supply on the rate of phloem sap exudation
and the composition of phloem sap of Ricinus conmunis. Plant Physiol. 59, 282-284 (1977)
Mengel, K. and Kirkby, E.A.: Principles of plant nutrition, p. 67-71, Intern. Potash Institute,
Berne, 1978
Mengel, K. and Wiechens, B.: Die Bedeutung der nicht austauschbaren Kaliumfraktion des
Bodens fair die Ertragsbildung von Weidelgras. Z. Pflanzenernahr. Bodenk., im Druck
(1979).
Nelson, W. L.: Plant factors affecting potassium availability and uptake, p. 355-383. In:
V. J. Kilmer, S. E. Younis and N. C. Brady: The Role of Potassium in Agriculture. Am.
Soc. Agron. Madison, Wisconsin, USA, 1968
Nenth, K.: The effect of K fertilization and K removal by ryegrass in pot experiments on
the K concentration of the soil solution of various soils. Plant a. Soil 42, 97-107 (1975)
Nemndth, K., Mengel, K. and Griunne, H.: The concentration of K, Ca, and Mg in the saturation
extract in relation to exchangeable K, Ca, and Mg. Soil Sci. 109, 179-185 (1970)
Pitman, M.G.: Uptake and transport of ions in barley seedlings. Ill. Correlation between
transport to the shoot and relative growth rate. Aust. J. Biol. Sci. 25, 905-919 (1972)
Schian, M., Niederbudde, E.A. and Mabkorn, A.: Ergebnisse eines 20jihrigen Versuches mit
Mineral- und Stalimistdjingung im Ldssgebiet bei Landsberg (Lech). Z. Acker- u. Pflanzenbau 143, 27-37 (1976)
Sturm, I1. and Jung, J.: Naihrstoffumsatz und Eiweissertrag bei Futtergetreide in Abhlingigkeit
von der Stickstoffdiingung. Landw. Forsch., Sonderh. 28111. 264-272 (1972)
Wiechens. B.: Die Bedeutung der K-Dynamik der Keuperb6den fiir die Ertragsbildung.
Dr. Thesis in the Fac. Nutritional Sci., Justus-Liebig-University, Giessen, 1975
237
Potassium Requirements of Cereals
P. Quintanilla Rejado, Ministry of Agriculture, Madrid/Spain*
1. Introduction
Investigation of the effects of mineral nutrition on plant growth and crop yield has a
long history and, recently, increasing attention has been paid to the ways in which the
supply of mineral nutrients interacts with the other factors, such as light, water supply
and temperature, which affect growth. Research workers have been concerned with the
dynamics of nutrient uptake and the ways in which nutrients are partitioned within
the plant and how these affect successive stages of development.
Growth and yield of cereals is determined by the apical meristems of the shoots which
later in growth become reproductive meristems, by the meristems of the roots, of
tillers and adventitious roots and the intercalary meristems of the leaves and internodes. There is competition between the meristems of shoots and leaves in the early
stages of growth.
Most, if not all, of the grain dry matter is assimilated after flowering as has been
demonstrated in wheat, barley, rice and maize. Thus, yield depends upon the level
and duration of photosynthetic activity in those parts of the plant which remain green
after anthesis.
2. Root system
Despite the fact that the root is one of the fundamental organs of the plant, its physiology has been less studied than that of the rest of the plant.
The root system of granineae is made up from the seminal roots (3-5 in maize, 7-8 in
wheat) and adventitious roots. The former have a much higher efficiency per unit
weight than the latter. The roots grow over a long period, eventually reaching a length
of one and a half to two metres; this applies to both principal and adventitious roots.
Root growth depends upon carbohydrate supply which, in turn, depends on the
development of the aerial part of the plant.
Gramineae generally have a well developed root system but there is much difference
between species. Root growth is rapid during the vegetative stage and reduces when
* Prof Dr. P. Quintanilla Rejado, Subdirector General, Jefe de la VIO Divisi6n Regional
Agraria, Ministerio de Agricultura, Av. de Baviera No. 3, Madrid/Spain
239
the plant goes over to the reproductive phase and there is sometimes even a reduction
in root weight during grain ripening (Figure 1).
100
so
20
toS.
2
t
nrlpl.
- -rn
--- !100
0*5
ccms
0-
c
E
/ // "
0"1
s
."Tiller
5
0'05
20
0-02 ,1/2
0.01
10
.
15.
.
.
29.5.
.
26.6.
.
24.7.
Date
Fig.l. Full lines: dry weight of shoot (s) and root (r); dashed lines: number of crown roots
(cr, r) and of tillers.
The seminal roots never comprise more than 5% of the total root mass, often only
about 1%, but their efficiency is up to 50 times higher than that of the adventitious
roots. Actually, comparatively little is known about the physiological difference
between the two types of root.
Root/shoot ratio is usually fairly constant during vegetative growth but reduces rapidly
later. Nitrogen supply greatly influences this ratio which increases when N supply is
reduced. Potassium does not affect the root/shoot ratio.
Root growth of wheat is adversely affected by K deficiency which reduces rootlet
number. The effect can be seen at the 6th day and is still more evident at day 10. K
affects root length, both seminal and adventitious roots being affected in this way, thz
latter more so than the former (Figure 2). The seminal roots play the dominant role in
the earliest stages of development, thus we find them to be more important in spring
sown cultivars.
Soil type greatly influences root development and structure, root diameter being
greater in heavier soils and this also holds for secondary and tertiary roots. Cultivation
240
Seminal roots
' Nodal roots
.
Ad26
26
Totals-
C
A
---
0
2--------
2I;Z2
08
Area
6
Z
CK
E
z
First order branches'-
2 8
06
4
£
ond order branches
0
0'
.
I
0 2 4 6
19 21 23
8 10 12 14 16
Days after planting
26
30
Fig.2. Seminal and nodal root numbers (log,) of wheat at 23 and 30 days after planting,
respectively, in plants supplied with standard (A), nil nitrogen (U), nil phosphorus (L) and
nil potassium (0) nutrient solutions
improves apparent rooting density. A slight soil moisture deficit in the flist weeks after
sowing improves root penetration but a water deficit at tillering may reduce root
growth. Too high temperature in the early stages of growth has an adverse effect.
Poor soil aeration limits root growth. The roots reach a depth of 15-20 cm within a
few days and then extend downwards. While most of the roots are found at 20-30 cm
241
depth, at a density of 20-30 cm per cm 3, rooting density at 60-70 cm is reduced to
between 0.1 and I cm/cm 3. It has been shown that the radius over which the root is
effective is only up to 1 mm for P, while for N it exceeds I cm- K occupies an inter3
mediate position. Thus, effective exploration for K requires 8 cm root per cm while
I cm/cm3 suffices for N.
K uptake is not entirely confined to any particular zone though the root apex has
always been thought to be most important, and uptake is better correlated with total
root volume than with length or total area of root. The configuration of the root
system is important when soil K availability is low. When the uptake rate is limited
by low K concentration in the soil solution, total uptake will depend mainly upon the
extent of the root system.
It has been shown that the absorption capacity of the root may change in response to
the needs of the plant. While root growth and its capacity to take up K is much affected
by local conditions, these are not the exclusive determinant factors and the plant's
nutrient needs influence ion transport across the soil-root interface and the translocation of metabolites to that part of the root where growth is active.
3. Leaf area
The development of leaves on the shoot is closely connected with nutrient supply and
this is especially so in relation to nitrogen shortage.
The total leaf area is determined by the number of living leaves and their individual
areas. Both these parameters can be further subdivided. Leaf number is a result of
the number of tillers, the number of leaves produced by each tiller and their longevity.
Area per leaf is determined by expansion index and duration. The effect of the individual nutrients on each of these properties is not understood thoroughly but there is
no doubt that all the nutrients and their interactions do affect leaf area. Though N
has the largest effect, both P and K have positive effects at the higher levels of N.
Experiments have demonstrated that both P and K increase area per leaf and that K
improves longevity (Figure 3). The number of leaves per shoot varies little, so the
P(p
N(150p.p.n.)
iw500.
m
p
,
K10p.p.m.)
K (100 p. p. n.)
400t
' 3001
K(4 ppm)
P(0
200-
3
.pm.)
N (6
too
7
11 15
nI.p
21
3
7
11
15
21
3
7
11 15
21
Weeks
Fig.3. Effect of N, P and K on leaf area per plant in timothy (Langer [1966])
242
influence of nutrients on total leaf number must be mainly through affecting the
number of shoots.
4. Tillering
There is much experimental evidence to show that N deficiency in barley has the
greatest effect in reducing tillering and that K only has an influence at real deficiency
levels when the number of tillers is clearly reduced. Mineral nutrition also influences
the duration of tillering which ceases at shooting. Low nutrient supply shortens the
duration of tillering. The earliest formed tillers are the most important for yield, later
tillers merely compete for nutrients without producing grain and it has been found
that the removal of surplus tillers improves yield by removing competition. The
apparent interruption in growth resulting from the loss of late tillers is not reflected in
the growth of the main stem.
5. Stem elongation
At ear initiation, the stem begins to elongate by lengthening of the internodes through
the production of new cells by the intercalary meristems and through cell elongation.
Except in rice, where they grow earlier, the leaf sheaths grow at the same time as the
stem, the lower internodes growing more strongly than the upper. The interval between
germination and stem elongation varies greatly, very short for maize and up to 5 or
6 months for wheat. At this stage the plant is very sensitive to drought and there is
severe competition for nutrients between the stem and ear, the leaves and the roots, in
which the main stem has priority, leading to death of some of the lateral shoots.
6. Grain formation
Grain yield per unit area is the resultant of the following: ear number, number of
grains per ear and weight per grain. Potassium has practically no effect on ear number
since it does not affect tillering. The only reports of a positive effect of K on tillering
are those of Forster and Mengel [1975] where K deficiency reduced yield of barley,
spring wheat and oats by reducing tillering and, thus, ear number. There is, however, an
effect of K on grain number per ear in that, while the number of flowers per ear is
effectively constant, K deficiency reduces the proportion of flowers which are fertile.
This effect is significant under adverse climatic conditions and there are many references to this in the literature.
It has been widely demonstrated that K increases weight per grain (1000 grain weight).
K also increases apparent grain density by about 10% and, in certain cases, by up to
25% thus increasing flour yield. It appears that the main effect of K in winter wheat
is on grain size, while the main effect in spring wheat is on grain number.
Potassium increases grain protein content of wheat thus improving quality. It also
increases the oil content of maize grain.
It has been observed that K fertiliser increases the harvest index (grain :straw ratio)
and grain: cob ratio in maize. It increases the C assimilation index during grain filling,
increasing the carbohydrate content of ears and roots.
243
7. Carbon assimilation
Carbon assimilation depends on leaf area but also on the efficiency with which the
leaves intercept the available light. The main effect of nutrients is on leaf area. Most of
the grain carbohydrate in wheat (85%) is derived from that part of the shoot near the
flag leaf; in barley the proportion is somewhat lower; in rice it is only about 70%. In
maize, 50% comes from the 6 uppermost leaves, 35% from the next 5 and the remainder from the lowest 5 leaves. Winter wheats have a leaf area twice as great as that
of spring wheats up to flowering but produce only about 15% more grain, probably
because the total leaf area after flowering is only about 17% greater. There are similar
differences between varieties. Most of the grain carbohydrate is synthesised after
flowering, 80% of photosynthate produced at that time going to the grain with only
a minor proportion translocated to other parts of the plant. It thus appears that grain
yield depends mainly upon leaf area duration between flowering and ripening. It is
well known that K deficiency reduces photosynthetic activity, probably mainly through
its influence on the stomata but it also reduces leaf area. K has no influence on the
number of leaves.
8. Uptake of potassium and its distribution
The influence of mineral nutrition on the plant is affected by its capacity to take up
nutrient and the duration of active uptake. The young plant depends on the nutrient
concentration in the soil solution surrounding the root. If this is low, the development of the root system will be the limiting factor. A more extended root system can
compensate for low nutrient availability. Root development, in turn, is dependent
on carbohydrate translocated from the shoot.
There are four phases in nutrient uptake: movement of ions from the solid phase
into the solution, movement in the soil solution towards the root, penetration into
the root and translocation from the root within the plant. Nutrients reach the root via
three mechanisms: interception, mass flow and diffusion. Table I shows the extent
to which these mechanisms are responsible for nutrients taken up by maize.
Table 1.The relative importance of interception, mass flow and diffusion in nutrient supply
to maize from a fertile silt loam (Arnon [1974] after Barber and Olson [1968])
Nutrient
N ...........................
P ...........................
K ...........................
Ca ..........................
M g .........................
S ...........................
Cu .........................
Zn ..........................
B ..........................
Fe ..........................
M n .........................
M o .........................
244
Required for
9.5 t/ha grain
187
38
192
38
44
22
0.1
0.3
0.2
1.9
0.3
0.01
kg/ha supplied by:
Interception Mass flow
2
I
4
66
16
1
0.01
0.1
0.02
0.2
0.1
0.001
185
2
38
165
110
21
0.4
0.1
0.7
1.0
0.4
0.02
Diffusion
-
30
150
-
0.1
-
0.7
-
The roots occupy only about I% of the soil volume so the contribution of root
interception to K supplies is small. The amount supplied by mass-flow depends on
the rate of transpiration and would not exceed 20% of K requirement. The major
contribution is by diffusion.
K uptake is rapid in the early stages of growth, the curve relating uptake to time
being steeper than those for N and P and that representing growth (Figures 4 and 5).
During the time of peak uptake rate, K is taken up at a rate of 2.0-3.3 kg/ha/day K
140
K
oo
-'PP
60
Dry weight
,,
20
30
70
so
90
110 Days
Fig.4. Effect of N, P and K on nutrient uptake and dry matter production of spring wheat
(Woodford and McCalla [1936])
100
80
K'
60
DRYMATTER
40.
20.
JUNE
JULY
AUG.
SCPT.
Fig. 5. Relative rate of accumulation of dry matter, N, P and K in maize during the growing
season with vertical dashed line showing date of silking (reproduced from Hanway (1962])
245
by wheat and 3.1-6.0 kg/ha/day by maize. A sub-deficient level of K availability
over this period can be decisive for final yield, wheat being more sensitive in this
respect than barley or oats. Maximum K uptake is reached some time before maximum
dry matter production and the K content decreases as the grain matures, though
there is varietal and seasonal variation. Wheat takes up 70-75% of its K need by
anthesis with 40-60% during tillering and ear initiation.
Maize takes up 75-90% of its K requirement by flowering with 60% by the time
the 9th leaf emerges. In contrast, rice, because there is little translocation of K from
the older leaves, takes up 48% of the total K after ear formation, so it is important
to ensure good root development. It has been shown that for high yield in oats,
50% of the K should be taken up by flowering.
Most of the K taken up by maize in the period of maximum absorption accumulates
in the leaves, where K deficiency first becomes apparent. K is translocated to the
grain from all parts of the plant except the stem and, at harvest, 40% of the K is
found in the stem, 20% in leaves, 15% in sap and 20-25% in grain (Figure 6).
%of Total
too
tO0
Complete Fertiliser
75
Very N deficient A grain
A
B cob, sik, husk,
75
shank, ear, shoot.
C stalk, tassel
B
50
iD
leaf sheath
SO
F leaves
%of Total
c
D
,"1
25
25
50
r
UE
-
75 -5 P deficient
.100
75
5-A
K deficient
10
B
So
25 -CB
25
June
July
Aug.
Sept.
June
July
Aug.
Sept.
Fig. 6. Total K contents, in pounds/acre and in percent of total, of different parts of the maize
plant taken from four plots on seven dates with the date of silking shown by the vertical
dashed line (reproduced from Hanway [1962])
K uptake by sorghum follows a pattern similar to that in other cereals, except that
uptake is generally speaking rather slower (Figure 7). K uptake is rapid in early
growth, less so between flowering and the onset of grain formation, during which
it speeds up. After this, K is lost from the plant as in other cereal crops.
246
.(a)
280
-. 80.80.0
X-- 80.,80.80
240
.
,oo.
200160
0
120
;)
,I
20
80
60
40
120
100
Days from planting
(c)
(b)
*1.
-
0.0.0
0--80.80.0
80.80.10
*-.
100
100
i 00.0
.. . 80.80.0
*-..x 80,50.80
x
80
80
I,,
I.:
:
60
v
60
40
Olt"
40
xx
20
)
60
80
100
120
20
40
60
80
100
120
Days from pla nting
Fig.7. Uptake of K by sorghum (Lane and Walker [1961]) (a) K content of whole plant as %
of maximum ; (b) leaf K content as % of total K in whole plant ; (c) K in head as % of total K
in whole plant
247
Varieties and species differ considerably in their ability to take up nutrients; probably
the result of adaptation to different soils and to the effects of competition. It has been
shown that the exchange capacity of roots is negatively correlated with K uptake.
Figure 8 illustrates varietal differences in K uptake in maize; generally speaking
the newer hybrids are more K demanding than the older varieties.
165
/
150
/.-.
% A2
Inra
'
"\ Inra A1
A1
UIowa
-aZ ,
r
;
135
120
90
Iowa B
75
Iowa A2
,or
.1
A
S
0
Fig.8. Uptake of K (kg/ha K2O) by various maize cvs at 40 000 plants/ha (Lout [1963])
9. Resistance to disease and adverse conditions
Application of P and K to barley grown in Oregon reduced the effects of yellow
dwarf virus even when the soils were high in P and K. K reduces rust in winter cereals
and reduces the effects of other fungal diseases, particularly Erysiphe graminis,
Septoria avenae and S. tritici. On the other hand, net blotch (Pyrenophora teres)
in barley is reduced when K supply is lowered. K deficiency increases the visible
symptoms of Ophiobolus spp. It reduces stem and root rots in maize. With the exception of blast, the common rice diseases are reduced by K as has been shown for
Helminthosporium oryzae, Leptosphaeria salvinii, Thanatopherus cucumeris and
Rhizoctonia solani.
K has often been observed to have a beneficial effect on frost resistance. K also has
a favourable effect on drought resistance by increasing osmotic pressure in the cells,
improving the transpiration coefficient and root development.
248
10. Resistance to lodging
For high yields, a cereal must be able to tolerate high N dressings without lodging,
which may be due to insufficient mechanical strength, to disease or pests or to combinations of these and other factors. Rice, like many other crops, responds to K fertiliser by resisting lodging because K speeds up lignification of the schlerenchyma
cells and increases cell wall thickness, especially at the base.
In maize, K deficiency reduces root development especially of adventitious roots,
rendering the plant more liable to lodging. At cob formation, assimilates are translocated in great quantities from the root and stem to the cob. If total carbohydrate
production is reduced through K deficiency, these losses are the more important
and the effects are seen in the lower leaves where the parenchyma disintegrates.
This is a normal phenomenon as maize matures but is accelerated by K deficiency
making the stem liable to breaking (Figure 9). There is much variation between
genotypes in this behaviour.
60
50
4,0
o
30
20
--
N1P
N1PK2
N1 PH
N1PK3
1
N2P
N2PK2
Nj
Nf K3
N3P
NPK2
N3Pfl1 NfPK3
Fig.9. Effect of N and K treatments on lodgingi n maize (Rhodesian double hybrid) (Burkersroda [1965])
11. K deficiency symptoms
Because of the mobility of K in the plant, deficiency symptoms appear first on the
older leaves. The first sign of deficiency in maize is a reduction in size and vigour of
seedlings and young plants. Then the first formed leaves turn yellow, the edges and
249
tips become yellow and, finally, the whole leaf withers. In less extreme deficiency,
growth is retarded but symptoms only appear when the plant is fully developed.
Early deficiency can be corrected by inter-row application of K fertiliser. Leaf symptoms in mature leaves are the same: the leaves turn yellow-green, the edges turn
brown and die - necrosis of the leaf margin is a characteristic symptom. In acute
deficiency damage to leaves is obvious and growth is reduced so that the internodes
do not elongate, the cobs are small and the grain not filled.
The leaves of K deficient winter cereals may appear bluish green, through the characteristic symptoms are the same as in maize - scorching of the leaf margin and tip.
Barley may also show purplish spots on the leaves.
12. Potassium needs
Though cereals are often thought to have only a moderate K requirement as indicated
by the quantities of K removed by the mature crop, like all graminae their requirements at the peak of growth are very high. This aspect has been discussed in paragraph
8 above where it was shown that there is considerable loss of K from the plant as
it matures. In deciding on a K fertiliser policy therefore there are two distinct aspects
which have to be considered:
a) Soil fertility
Fertiliser K must replace K removed in crop or soil fertility will decline. The amount
of K fertiliser needed for this purpose will depend on whether or not the straw is
removed from the field. In general the straw contains at least twice as much K as
the grain alone. There is a good deal of variation in published figures for K removal
by the mature crop, but the values given below may be taken as typical (kg/ha K
per tonne grain).
Grain ..........
Straw ..........
Wheat
Barley
Oats
Rye
Rice
Maize
Sorghum
5
10
6
12
6
14
5
10
5
20
7
13
3.5
17
b) The needs of the crop at the period of maximum growth and K uptake
If growth and yield are not to be restricted by limitation of the K supply, it is clear
from the discussion above (paragraph 8) that soil K supply plus fertiliser K must be
much larger than the amount removed in the mature crop. Thus in the case of wheat,
for example, while applying 15 kg/ha K per tonne of grain produced might maintain
the status quo as regards soil K, larger amounts should be applied if soil K availability
is low or only moderate.
It must also be realised that the nutrient demands of the modern high yielding cultivars and hybrids are much higher than those of the traditional varieties and it is
suggested that fertiliser recommendations should, in appropriate cases, allow for
some build up in soil nutrient levels to ensure that the needs of future, higher yielding, crops can be met. The following figures for K removal in mature crops (grain +
straw) for low and high yield levels will illustrate this discussion:
250
Wheat: Germany, yield 2.8 t/ha 73 kg K; yield 5.0 t/ha 100-125 kg; India, yield
2.2 t/ha 67 kg K; yield 6.0 t/ha 126-175 kg K (Kemmler [1974]).
Rice: Improved indica 2.7 t/ha 50-60 kg/ha K; high yield varieties 7.4 t/ha 250 kg K
(von Uexkiill [1970]).
13. Critical levels
Figure 10 serves to indicate how response by cereals to applied fertiliser K varies
with the level of available soil K. It is not possible to lay down a critical value for
soil K above which response would not be expected as this would vary with soil
type, climate and, of course, variety. It might appear from Figure 10 that the critical
value for maize is somewhere between 0.10 and 0.20 meq%.
2
AY =-12.l.0.021 .0.0003KS 1 -0.14 S2.0.25KF-O.OOOSKF
-0.0006K1 KF, 5.2T-0.018Ksi T.0.095
R=0.38**
25
2
KSI=50pp2m
20
U
KSI110p~
-
5
025
s
75
100
121
Lblac K20 applied
KSi- exch. K in surface soil
K *exch. K in6-12 inch layer
S = 'O00plts/ ac
T = soil texture
Fig. 1O. Effect of soil K level on K response in maize (Hanway et al [1962])
251
Plant analysis may be expected to give more consistent results. For instance the
critical level for barley is said to be 0.92% K in the flag leaf at ear emergence and
1.01% in the stem. In rice 1% K at earing is considered critical and 2% optimum.
At the same stage 1.12% is thought critical for oats and the corresponding figure for
rye is 0.95%. Maize has been more thoroughly investigated and the results are variable;
in summary it appears that the critical leaf level lies between 1.3 and 1.8% K, hybrids
usually requiring higher levels (Figure 11). There is little published information on
RESPONSE OF CORN TO POTASSIUM AS
AT SILKING
INKLEAF
M 25 ~CKRELATED TO K%t
=0.7511.
Is
2
20
CK-/. K =IG'.5/
CK-t, K = 1.75*1
O
W
10
00
25
0
25
5
5
0
100
75
50
POUNDS OF K APPLIED PER ACRE
2
125
Fig. 11. Predicted yield increase in maize in response to applied K at various levels of % K in
leaves at silking (Hanway etal. [1962])
sorghum but it may be taken that critical values are similar to those for maize. The
following values for stems and leaves in maize are quoted from Loud [1963].
Acute deficiency
0.25-0.41%
Severe deficiency
0.41-0.62%
Deficiency without symptoms 0.62-0.91%
Normal
0.91-1.3 %
Generous
1.3 -2.1 %
K
K
K
K
K
Several authors report close correlation between leaf K and available soil K and it
appears that, when N and P are adequate, K content of the whole plant or of the
6th leaf is better correlated with yield than is soil K.
14. The influence of other factors on K nutrition
Cultural methods, climate, supply of other nutrients are all concerned here. Poor
soil aeration, to which K is more susceptible than other nutrients, reduces K uptake
252
and when soil aeration is poor differences in soil temperature affect K content of
the grain.
Long term field trials at Purdue University underline the importance of K supply in
relation to water availability. Under normal conditions of soil moisture, sufficient
soil K was mobilised to produce satisfactory yields and K fertiliser seldom gave any
response, but the response to K fertiliser under dry or excessively wet conditions
was a yield increase up to 50%. The role of K in controlling stomatal opening is
well known - plants well supplied with K utilise soil moisture more efficiently and
wasteful transpiration loss is prevented. Soil moisture content greatly influences K
uptake and drying out of the rooting zone limits the rate of diffusion of K in the soil
and mass-flow is non-existent. Deficiency of soil moisture adversely affects the K
content of young maize plants.
The effect of K on carbon assimilation and yield of spring wheat grown at varying
light intensity was that increased K availability increased yield only under full lighting, while when plants were shaded, though it increased vegetative growth, yield was
somewhat reduced.
Timing of K fertiliser application seems generally to have little effect and there is no
advantage in applying potassium at other times than in the seedbed. But, cases have
been reported where there is advantage in giving divided dressings to rice. This
applied in warm climates on light soils of low exchange capacity or at high N levels
on poorly drained sites when the first application should be at tillering and the second
4 to 6 weeks before ear emergence.
K uptake is much affected by N level; the effect of P level is more variable. In most
cases, K is the more effective the higher the level of N fertiliser and this applies
especially to modern high-yielding varieties of rice, wheat and barley (Figure 12
and Table 2).
, with 155kg N
638,5.
~
60
~~~
5959
wth.13i
592
59 59185
5B7
ma.I
o
Average
57
51 5g 566
55
with 70 kg N
539 5392.
53
I53
52.9
52
I
0
08695
I1
129
152 Rate of K (in kg K2 0/ha)
Fig. 12. Effect of K fertiliser at different levels of N applied to wheat (Loud [1977])
253
Table 2. Effect of potassium on rice yield at three levels of N fertiliser (average of 6 years).
After Singh and Singh [1978]
Grain yield (tonne/ha)
N applied
(kg/ha)
K applied (kg/ha K)
100 ............
150 ............
200 ............
0
42
4.14
4.14
4.14
4.38
62
83
125
4.54
4.81
4.91
4.84
5.15
166
187
250
5.24
5.16
5.53
A striking example of this N x K interaction is illustrated in Figure 13 which shows
that increasing N reduced maize yield in the absence of K fertiliser and increased it
when K was applied. K increases dry matter production and grain yield and reduces
the number of unproductive tillers and improves translocation of N to the grain.
Yield
100 lb/acre
38-
-
34
I
I
26 OOI
N'r lbIac
.
K 3~/ Iac
OlIac
2
jovrl
18
NIP
N1PK 2
Nt PK1 N1PK3
N2P
N2PK 2
N3P
N3PK 2
N2PKI
N 2PK3,
N3PK 1 N3 PK3
N, = 40 Iblae N
K, = 30 lblac K20
N2 =80 lblacN
N3 = 1201blac N
K2 =6OlblaC K20
(3 =1201blac K20
60 lb/lac P205 overall
Fig. 13. Effect of N and K treatment on maize yield (Burkersroda [1965])
254
15. Response to K fertiliser
In introducing this section it is appropriate to point out that the preceding sections
have shown that K supply affects several yield components. That the relative importance of these varies between cultivars is illustrated in Figure 14 showing how yield
is built up in 4 cvs of spring wheat at varying K levels.
log x
Kolibri
2.
Solo
Hohenheimer
Franken
Heines
Kolben
1.5
Ear no. I
pot
1.0
grainlear
0.5
wt. /grain
(mg)
Ki
K2
K3
K1
12
K3
K1
K2
K3
KI
K2
K3
Fig. 14. Grain yield components of 4 cvs spring wheat as affected by K supply (Forster and
Mengel [1974])
As has been pointed out above, response by cereals to K fertiliser varies greatly
with soil type, soil K level, the availability of water and plant population. The variation in response to K fertiliser between species and varieties grown on the same
site is illustrated in Figure 15.
The resultsof plant breedingand improved cultural methods in the ricecrop are shown in
that ten or twelve years ago, potash responses, when they were recorded at all, amounted to about 2 kg grain per kg K 20 applied, while now, with a variety like IR 8 the
corresponding figure is 7-13 kg grain. There are major differences in responsiveness
between the modern cultivars of rice, IR 8, IR 26 and IR 30 responding better to K
fertiliser than IR 20 and IR 22. Response by rice appears variable in other respects,
some trials having shown better K response on high than on low K soils. Rice growing
is normally a long-term project and, particularly when high yielding cultivars are
used, continued cropping without K fertiliser leads to soil depletion and declining
yield (Figure 16).
Response by wheat can also be very variable though response would always be
expected with a soil K level below 0.10 meq%. In long-term experiments, the responses
increase with time and can reach as high as 7-8 kg grain per kg K 2O applied, though
255
K 323
K233
92
88
--
-
640
*K
84
76
7
*
gA
72
K320
68
K332
4'
44
'"1q
60
56
52
1K
164
46
2
4
24
0
160
80
240
320
Fig. 15. Potash response by various species in different years at La Sauvetat/France (Loud
[1969])
g/pot dry
matter yield
2m
S bnrg~d
-,
is
Uns'knerged
16
12
10
6,
4,
.........
I
2
3
4
5
6
7th SUcceSSjVe Crop
Fig. 16. Decline in rice yield on soil receiving no K fertiliser (von Uexkall [1976])
256
a more usual level would be a little under 4 kg for dry farming and a little over 5 kg
grain per kg KO under irrigation. Responses higher than 2 kg grain/kg K 2 0 are
quite common throughout the world for both wheat and barley.
Similar variability is found in field experiments on maize. Responses as high as
.13 kg grain per kg K20 have been recorded on K fixing soils (Louj [1977]) while,
at the other extreme there may have been no response to high rates of applied K.
Because there is such variability and because responses to K usually increase with
time, cereal fertiliser trials, like those on other crops should be long-term; it is unsound
to base recommendations on single season trials.
16. References
Arnon, .: Mineral Nutrition of Maize. 452 p. Int. Potash Inst., Berne, 1974
Brouwer, R.: Root growth of cereals and grasses; in: The growth of cereals and grasses (eds.
F.L.Milhorpe and J.D.Ivins). Proc. 12th Easter Sch. Agric. Sci. Univ. Nottingham.
pp. 213-226, Butterworth, London (1966)
Burkersroda, K. W. von: Fertilising maize in Rhodes'a. Better Crops 43 (4), 6-13 (1965)
Forster, H. and Mengel, K.: Einflflsse der Kaliumernahrung auf die Ertragsbildung verschiedener Sommerweizensorten (T. aestivumn. L.). Z. Acker- u. Pflbau 139, 146-156 (1974)
Gamboa, A.: La fertilisation du mais. IPI Bulletin No.5. Int. Potash Inst. Bern, 1978
Hanway, J.J.: Corn growth and composition in relation to soil fertility. It. Uptake of N, P
and K and their distribution in different plant parts during the growing season. Agron. J.
54, 145-148 (1962)
Keninler, G.: Modern aspects of wheat manuring. IPI Bulletin No. 1. Int. Potash Inst. Bern,
1974
Lane, H.C. and Walker, H.J.: Mineral accumulation and distribution in grain sorghums.
Texas agric. Exp. Sta. M P- 533 (1961)
Langer, R.H.M.: Mineral nutrition of grasses and cereals; in: The growth of cereals and
grasses. Butterworth, London, 1966
Lou , A.: Fertilitd (1963)
Lou, A.: Etudes sur la nutrition et ]a fertilisation du mais. SCPA Mulhouse, 1968
Loua, A.: La fertilisation potassique des sols a fort pouvoir fixateur. SCPA, Mulhouse, 1977
Lou, A.: La potasse et les cdrdales. SCPA, Mulhouse.
Quinanilla,P. and Dominguez, A.: Etude de la r6ponse de la production de mats A la fertilisation et A la densitd de la semaille. VI Congr. Mondial des Fertilisants, Lisbon, October
(1968)
Singh, M. and Singh, R.K.: Agronomic and economic evaluation of response of rice to
potassium: Indian experience; in: Potassium in soils and crops. Potash Res. Inst. of India.
New Delhi, 1978
Tennant, D.: Root growth of wheat. Austral. J. agric. Res. March (1976)
Tucker, W.B. and Bennett, W.: Fertilizer use on grain sorghum; in: Changing patterns in
fertilizer use (ed. L. B. Nelson), 1968
Uexkfill, H.R. von: Aspects of fertilizer use in modern high-yield rice culture. IPl Bulletin
No.3. Int. Potash Inst. Bern, 1976
Woodford, E. K. and MeCalla, A. C.: Can. J. Res. 14, 245-266 (1936)
257
Potassium Requirements of Grain Legumes
D.Fauconnier,Direction technique, Socit Commerciale des Potasses et de l'Azote, Paris/
France-
1. Introduction
The general term 'grain legume' embraces many wild plants including trees and
shrubs but the present treatment is limited to the main cultivated species: soya
(Glycine max. L.), bean (Phaseolus vulgaris L.), pea (Pisum spp. L.), groundnut
(Arachis hypogea L.) and cowpea ( Vigna unguiculata). Their importance is in large
measure due to their high protein content.
2. Response to potassium fertiliser
Potassium usually increases the yield of grain legumes and improves quality. Symptoms of potassium deficiency have been described by many authors (Chevalier
[1976, Courpron and Tauzin [1975], Gillier (1955), Kamprath [1974], de Mooy
[1973], etc.).
The function of potassium in the metabolism and morphology of these plants is the
same as that in other species, the effects of shortage are shown in various ways:
Effect of deficiency
On stem height: Short stems, short internodes
On foliage:
Reduced photosynthesis, yellowing, necrosis
On roots:
Reduction in number and weight of nodules
On fruiting:
Fewer inflorescences per plant, fewer grain per pod, and reduction in
grain weight.
K content of the grain of certain legumes is particularly high: normally 2 to 2.5%
in soya and beans, over 3% in peas, compared with only 0.7% in groundnut and
0.4-0.45 in wheat and maize. IRA T say that soya is more sensitive to fertilisation
than groundnut.
Review of a large number of experimental results shows that K response is variable
and affected by a number of different factors.
* D. Fauconnier, Ing. agr., Direction technique, Socidtd Commerciale des Potasses et de 'Azote
(SCPA), 62, rue Jeanne d'Arc, F-75646 Paris-Cdex 13/France
259
The predominant factor appears to be the potential production of the field (Lin [1965],
Nelson [1970] etc.) resulting from the physical and chemical characteristics of the
soil, cultural methods, species and variety, climate and, most important, water supply.
The level of potential production determines the potential development of the plants,
the weight of biomass produced and, consequently, the pattern of potassium supply
required. Thus, good responses have been recorded in good crops of soya, peas,
kidney beans or groundnut yielding 2.5 to 3 tonnes of grain per hectare and smaller
responses in crops of dolichos or lentils yielding only I or 1.5 t/ha. This appears to
be an example of Liebig's Law of the Minimum in that potassium cannot exert its
full effect unless all other factors are optimal. It also illustrates the interdependence
between K and other nutrients.
The second factor is the potassium status of the soil, particularly its ability to supply
potassium rapidly at periods of peak demand. Many authors have tried to determine
the critical level of exchangeable potassium (50-80 ppm for soya in Brazil) but
generally fail to take into consideration C.E.C.
An important point for soya, beans and peas is the form of potassium fertiliser and
method of application: these plants are sensitive to salinity. Several cases of lack of
response on low potassium soils can be explained by the salt effect of KCI applied
by placement too near to the seed. It also explains why some have found a better
response when potassium is applied to the previous crop (Fouilloux [1976]), Nelson
[1975]).
3. Potassium requirements during growth
3.1 Production and K uptake
The patterns of dry matter production and potassium uptake are similar (MichaelisMenten kinetics). Maximum yield of groundnut can be obtained at a concentration
of 200 tLMK in the nutrient solution (Fageria [1976], Leggett [1970]).
Comparison of the patterns of K uptake and dry matter production well illustrates
the relations between K absorption, dry matter production and grain yield (Chevalier
[1976]) (Figures 1 and 2).
The uptake of potassium by grain legumes has also been studied by Bataglia [1976],
Courpron [1975], Egly [1975], Puech [1974], Haag [1967] and Bromfield [1973],
(Figures 3 and 4).
3.2 Daily absorption rates
The duration of vegetative growth varies with climate; for example the usual varieties
of soya mature in 100 to 150 days at 30' latitude and in 90 to 100 days in the tropics.
The maximum rate of K uptake is found between flowering and grain formation.
Theoretically, a higher rate of K uptake would be expected for equivalent biomass
production in hot climates. Some varieties are capable of forming dry matter at two
to three times the rate of other varieties of the same species (Table 1).
The mean daily rate of dry matter production of soya can vary from 15 to 50 kg/ha.
Daily K uptake rates are less well documented but must vary in the same proportion.
260
Kg/ha
6000
5000"
/000
KO
30003
l
2000leaves. stems
-.
1000-
15;7
24/7
1/8 5'1814/8
1/9
26/9 1/10
9/10
Fig. l. Production of dry matter (leaves, stems and grain) over time for soya (cv. Altona) at
3 levels of K (Chevalier [1976])
Kg/ha
K20 K
100- 83
90-.......
90-
--_.
- - -K2
-.........
-- -
8070
60
50Grain
40"
20-
,"
leaves, stems
Io0
15/7
2417
5/8 14/8
1/9
26/9
9/10
Fig.2. Uptake of potash (kg K 2 0/ha) by soya (cv. Altona) at 3 levels of K (Chevalier [1976])
261
Production tlha
10
, Total DM
(fallen leaves)
Total OM
Start of
flowering
5
*
,Recovery
Stem
"-'"-Pods
1 •
Leaves
ta
0;
0
from shed
50
100
150
Days alter emergence
Fig.3. Production of dry matter (whole plant) over time (Soya cv. Amsoy)
NITROGEN
10
PHOSPHORUS
9
POTASSIUM
9 1
100
A
F DS
60
NS
7
POD
PETIOLES
5 -STE
PETIOLES
00
PETIOLES
-_
3 V10'3
28
56
PODS
o
POD
STEM
3
LE WES
0
BEANS
_
STEM-)O
40
E
7
84
VE
112
0
28
56
84
112
0
28
56
84
112
DAYS AFTER EMERGENCE
Fig.4. Relative rate of N, P and K accumulation in soybean plant parts during the growth
262
Table 1. Comparative yields of Dolichos (Vignasinensis) cultivars (Singh and Manfhli[1975/)
Cultivar
C 19 ....................................
C 20 ....................................
C 2 .....................................
RS 9 ....................................
T 2 .....................................
Maturity
Grain yield
(days)
kg/ha
kg/ha/day
75
80
80
120
125
985
1.053
917
536
928
13.1
13.1
11.4
4.4
7.4
There are thus great differences within and between species. Maximum daily K
uptakes vary between 1.5 and 6 kg K (Table 2 and Figure 5).
More research is needed into the relations between peak K uptake, biomass production and grain yield as few results are available on this important point.
Table 2. Daily K uptake rates
Species
Period
Soya ........
50th-10th day
Daily biomass
production
Uptake rate
Source
176 kg/ha
Soya ........
4.5
1.05
n.c.
4.9
1.90
Hammond [1949]
Soya ........
Soya ........
60th-80th day
n.c.
333 kg/ha
7.7
6.6
4.60
6.0
Henderson [1970]
Mascarenhas [1973]
50th day
n.c.
Soya ........
10.0
n.c.
Beans .......
6.6
4.00
Hanway [1971]
Trocm6 [1977]
Haag [1967]
I 'N
150
Ca-..-
M9 .......
P'------"
P
//
0
10
2
30
40
50
60
70
80
Fig.5. Major nutrient uptake by beans (250 000 plants/ha)
263
3.3 Amount and distribution of K uptake (Table 3)
For dolichos, Silvestre [1965] gives 17 kg N, 7.4 kg P and 40 kg K per tonne grain.
For cowpea Nicou [1967] gives 37 kg k per tonne grain compared with only 20 kg/t
for groundnut. For groundnut, Pouzet [1974] quotes uptakes of 26.9 kg N, 2.27 kg P
and 9.04 kg K per tonne of dry matter (that is 378 kg grain, 132 kg shells and 490 kg
straw). Groundnut is thus not a very demanding crop,
It may be noted that the level of K in the soya plant declines in an approximately
linear manner after the twentieth day from emergence (Table 4).
Table 3. Amount and distribution of K uptake
Species
Soya (Ohlrogge and Kamnprath [1968]) grain ............
Soya (Bataglia et al. [1976])
Soya (Trocn# [1977])
Beans (Cobra [1967])
Beans (Haag [1967])
Groundnut (Gillier [1955])
straw ...........
stubble, root ......
grain ............
straw ...........
grain ............
whole crop ......
whole crop ......
whole crop ......
grain ............
leaves and shells .
Yield
Uptake (kg/ha)
kg/ha
N
P
4030
246
28
78
n/a
n/a
2093
3539
3500
78
39
135
11
205
480
102
201
58
25
1I
5
11
2
26
39
9
35
5
2
50
28
47
33
41
95
93
200
9
16
1278
1891
K
Table 4. K content in dry matter, soya cv. Altona at 3 K fertilizer levels (0-100-200 kg
K 20/ha) on soil containing 1.07U/00 exchangeable K2O (Chevalier [1976])
Date and growth stage
K0
K,
K,
15/7
24/7
5/8
14/8
18/8
1/9
1.40
1.27
1.34
1.10
1.05
1.12
1.97
1.91
1.88
1.49
1.24
1.24
2.49
2.22
2.07
1.46
1.00
1.32
4 trifoliate leaves ..................................
Flower differentiation ..............................
Beginning of flowering .............................
Full flower .......................................
Start of grain formation ............................
50% grain formed ................................
3.4 Comparison with other crops
It is seen that the K demand, in terms of total need and maximum daily uptake, are
very similar for a 3.5 t soya crop and a 6 t wheat crop. Soya needs more K than
maize, but the latter crop has a higher peak absorption. (Table 5).
264
Table 5. Nutrient element uptake of other crops (Trocni
[1977])
N
P
K
160
150
120
2.5
32
32
26
0.43
249
103
24.9
5.81
195
195
135
8
39
39
26
1.31
166
112
29
9.96
160
150
90
9
26
26
17.5
1.31
232
182
25
12.45
480
250
205
to
65.5
39.3
26.2
1.75
166
95.4
41.5
6.64
Wheat 6 t/ha
Maximum uptake ...............................
Content at harvest ...............................
Content of grain ................................
M aximum daily uptake ...........................
Maize 8.5 t/ha
M aximum uptake ...............................
Content at harvest ...............................
Content of grain ................................
Maximum daily uptake ...........................
Rape 2.5 t/ha
Maximum uptake ...............................
Content at harvest ...............................
Content of grain ................................
M aximum daily uptake ...........................
Soya 3.5 t/ha
M aximum uptake ...............................
Content at harvest ...............................
Content of grain ................................
Maximum daily uptake ...........................
4. Conclusion
The response of grain legumes to potassium fertiliser depends upon potential production of the crop (availability of water, levels of other nutrients, species and variety
and cultural methods).
It depends also on potassium availability in the soil both as regards intensity and
buffering capacity. The rate of K supply (replenishment of the soil solution) is critical
at periods of peak growth.
In the absence of other limiting factors, it seems that soya and peas, in which biomass
and grain yields are relatively high and in which grain K content is high, are the
most responsive to potassium.
In groundnut, where biomass production is relatively lower and the grain has a low
K content, response to K is less and only to be expected on low K soils. Consequently
it seems that, in order to satisfy the K needs of grain legumes, it is necessary that
soil and fertilisers should be able to supply K at a sufficiently high rate at all growth
stages. At peak periods this is high, in the case of soya exceeding 6 kg K/ha/day.
Soya, beans and peas can be affected by the anion accompanying the K, particularly
when KCI is placed near the seed.
5. References
Bataglia, 0.C. and Mascarenhas, H.A.: Acumulo de materia seca et absorqio de macronutrientes pela soja var. Santa Rosa. Bragantia (1976)
Bromfield, A.R.: Uptake of sulphur and other nutrients by groundnuts in Northern Nigeria.
Expl. Agric. 9,55-58 (1973)
265
Chevalier, H.: Rtponse h Ia fumure potassique, rythme d'absorption des 6lments fertilisants
du soja var. Altona. Inf. Tech. CETIOM/Fr. No. 53, pp. 7-15 (1976)
Cobra Netto, A.: NutriqAo mineral do feijoeiro (Phaseolus Vulgaris L.) Piracicaba, E. S.A. Luiz
de Queiroz, 1967 (Thesis)
Courpron, C. and Tauzin, J.: Production de mati&e s&he et absorption des dldments fertilisants par une culture de soja. Inf. Tech. CETIOM, No.47, pp. 15-18 (1975)
Egly, D. B.: Rate of accumulation of dry weight in seed of soybeans and its relationship to
yields. Can. J. Plant Sci. 55, 215-219 (1975)
Fageria,N.K.: Uptake of K and its influence on growth and Mg uptake by groundnut. Biol.
Plant Tchecosl. 16, 210-214 (1974)
Fouilloux, G.: Le Haricot. Bull. Tech. d'lnform. 311, 464 (1976)
Gillier, P.: Etude des sympt6mes de carence en elements majeurs sur arachide. Olagineux 10,
479 (1955)
Gillier, P.: Les exportations en elements mindraux dans une culture d'arachide dans les
diffrentes zones du S~ndgal. Olkagineux 19, 745-746 (1964)
Haag, H.P.: Absorq;Ao de nutrients pela cultura do feijeiro. Bragancia 26, No.30, lAC
Campinas/Brdsil (1967)
Hanway, J.J. and Weyer, C.R.: Dry matter accumulation in soybean plants as influenced
by N, P and K fertilisation. Agron. J. 63, 263-266 (1971)
Henderson, J.B. and Kanprath, E. T.: Nutrient and dry matter accumulation by soybeans.
N.C. Agr. Exp. Sta. Techn. Bull. No. 197 (1970)
Kamprath, G.J.: Nutrition in relationship to soybean fertilisation in soybean production,
marketing and use. Bull. Y-69, 28-32 TVA (1974)
Leggett, J.E. and Frdre, M.H.: Growth and salt accumulation by soybean plants. Plant
Physiol. 46, Suppl. 12 (1970)
Lin, T.C.: Report on soybean potash observation in Kao-Ping area in 1964-1965. Potash
Review, Subj. 16, Suite 33 (1965)
Mascarenhas, A.A.: Acumulo de materia seca, absorio e distribuig6o de elementos, durante
o ciclo vegetativo da soja. Bul. Tecnico No.6, IAC Campinas/Brazil (1973)
de Mooy, C. J., Pesek, J.T. and Spadon, E.: Mineral nutrition, pp. 267-352; in: B. E. Caldwell,
R. W. Howell and H. W.Johnson (ed.) Soybeans. Am. Soc. ol Agron., 1973
Nelson, W.: Fertilisation of soybean. Proc. 9th Congr. Intern. Potash Inst. 161-172 (1970)
Nelson, W.: Fertilisation of soybean and protein production. Proc. I lth Coll. Intern. Potash
Inst. 229-235 (1975)
Nicou, R. and Poulain, J. F.: La fumure minrale de nikb au S~n6gal. Coil. sur la fertilitd des
sols tropicaux. Tananarive 731-754, 1967
Ohlrogge, A.J. and Kanprath, E.J.: Fertiliser use in soybeans, pp. 293-299; in: L. B. Nelson (ed.)
Changing patterns in fertiliser use. Soil Sci. Soc. Amer. Madison, 1968
Puech, J., Lencrero, P. and Hernandez, M.: R6le de quelques facteurs du milieu dans ]a
production quantitative et qualitative du soja. Ann. Agro. 25, 659-679 (1974)
Singh, A. and Manjhi, Sh.: Fertiliser effects on protein yield of tropical pulses. Proc. I Ith
Coll. Intern. Potash Inst. 179-191 (1975)
Terinan, G.L.: Yields and nutrient accumulation by determinate soybeans, as affected by
applied nutrients. Agron. J. 29, 234-238 (1977)
Trocmd, S.: Le potassium dans les plantes, pp. 75-92; in: Le potassium en agriculture,
INA Paris Grignon (ed.), 1976-1977
266
Potassium Requirements of Root Crops
Sven L. Jansson, Dept. of Soil Sciences, Swedish University of Agricultural Sciences, Uppsala/
Sweden*
1.Introduction
From a strictly botanical point of view, the root crops should be defined as crop
plants grown for their storage roots, but usually, as in this paper, they are more
generally defined as crops which develop storage organs at or below the soil surface.
Botanically these storage organs may be roots, stems, or even leaves. The storage
organs are used by the plants for propagation or as means to survive adverse climatic
conditions - drought periods or cold winters. The root crops are well distributed
throughout the plant kingdom (Table 1) and they are grown under a great variety
of soil and climatic conditions.
Table 1. A variety of root crops
Botanical family
Latin name
English name
Storage organs
Compositae
Convolvulaceae
Solanaceae
Cruciferac
Helianthus tuberosum L.
Ipomoea batatas (L.) Lam.
Solanum tuberosum L.
Brassica napobrassica (L.)
Milt.
Brassica rapa L.
Jerusalem artichoke
Sweet potato
Potato
Swede, turnip
Stem tubers
Root tubers
Stem tubers
Fleshy taproot
Rutabaga
Fleshy taproot
Raphanus sativus L.
Radish
Fleshy taproot
Umbelliferae
Manihot esculenta Crantz
Beta vulgaris L.
Beta vulgaris L.
Beta vulgaris L.
Beta vulgaris L.
Daucus carota L.
Cassava
Sugar beet
Fodder beet
Table beet
Beetroot
Carrot
Root tubers
Fleshy taproot
Fleshy taproot
Fleshy taproot
Fleshy taproot
Fleshy taproot
Parsnip
Fleshy taproot
Liliaceae
Allium cepa L.
Onion
Bulbs
Alliant porrum L.
Leek
Bulbs
Alliurn sativum L.
Colocasia spp.
Garlic
Cocoyam, taro
Bulbs
Corms, cormels
Cocoyam, tannia
Yam
Corms, cormels
Corms, cormels
Euphorbiaceac
Chenopodiaceae
Araceae
Dioscoreaceae
*
Pastinaca sativa L.
Xanthosoma sagittifolium
(L.) Schott
Dioscorea spp.
Prof. Dr. S. L. Jansson, Dept. of Soil Sciences, University of Agricultural Sciences,
S-75007 Uppsala/Sweden
267
The vegetative, waterholding and fleshy storage organs of the root crops are rich in
energy and are mainly grown for their carbohydrate content; they are used as important staple foods in most cultures of the world, in the temperate and the tropical
climatic zones. They, or preparations from them, are used as feedstuffs and for
industrial processing, being important sources of starch and sugar and of derivatives
from these basic substances.
The aerial parts of the root crops - leaves and stems - are often rich in protein and
mineral nutrients. Many of them are used as vegetable food and as feedstuffs.
2. The major root crops, general features
Only a few of the very many root crops grown throughout the world are of major
importance as sources of carbohydrate foods, feeds and raw materials. While this
paper concentrates on the few important crops, the large number of more or less
special root crops of only limited or local importance should be mentioned. They
are grown on a small scale for direct human consumption as garden vegetables or
to add savour to and to diversify otherwise monotonous basic diets. Most of such
crops have been little, if at all, developed by plant breeding and their environmental
requirements have not been scientifically investigated.
The major temperate root crops are the potato and the beets, primarily sugar beet.
The major tropical root crops (Ezeilo [1977]) are cassava, sweet potato, yams
and the cocoyams.
Modern agriculture, including the use of fertilisers, was developed in the temperate
regions during the last century; consequently much plant-breeding work has been
devoted to the temperate root crops and their nutritional and other environmental
requirements have been extensively investigated in scientific research. In contrast,
though, the major tropical root crops represent very old cultivated plants, they are
still in a primitive condition with regard to plant-breeding and their environmental
requirements have not been thoroughly investigated. However, in connection with
the general efforts to improve the agriculture of the developing world, much work
is now being directed towards genetic improvement of these crops and to improving
cultivation methods.
2.1 The potato
The potato, Solanum tuberosuin L., is a perennial plant of the Solanaceae grown as
an annual crop (Burton [1966], Brouwer [1973-1974]). The storage organs, rich
in starch, are stem tubers developed beneath the soil surface. The tubers may be
able to remain in the soil during the winter but normally they are harvested before
winter begins. The plant may develop flowers, fruits and seeds but normally it is
propagated by planting small tubers. The crop normally matures in three to six
months from planting. At the stage when the tubers begin to mature, the aerial
parts constitute 20 to 30 per cent of the total organic matter produced (von Boguslawski and von Gierke [1961], Evans [1977]). As they wilt and die rapidly, most of
the nutrient content of these parts is returned to the soil. Mature tubers normally
contain 22-25 per cent dry matter, of which 70-80 per cent is starch.
268
Though it is mainly cultivated in the temperate zones of the northern hemisphere,
the potato is by no means restricted to this area and it is cultivated all over the world
and even in the tropics. Thus, it is the most widespread and generally cultivated of
the major root crops. It is a staple food in many human diets. Besides this it is much
used as a feedstuff and for industrial starch and ethanol production. The aerial
parts of the crop are not normally harvested.
2.2 Sugar beet and root crops with similar botanical characteristics
The sugar beet, Beta vulgaris L., is a biennial plant of the Chenopodiaceae (Draycott
[1972], fP1 [1955]). The storage organ, rich in sucrose, is a taproot growing in the
soil surface. The foliage and taproot develop during the first year, the growing period
being 7 to 10 months, after which the crop is harvested. If the plant is left in the soil,
the taproot may overwinter and the following year develop a stem with flowers which
produce seed.
At harvest the foliage and the neck (the hypocotyl) of the beet are separated from
the taproot. These aerial parts of the crop (the tops) constitute a valuable feedstuff
for ruminants and are often used for this purpose, either fresh, dried, or as silage.
At harvest about one third of the total dry matter production is in the tops and two
thirds in the roots. The taproot contains about 25 per cent dry matter, of which
60-75 per cent is sucrose. The beet pulp, after the sucrose has been extracted, constitutes a valuable ruminant feedstuff. The sugar-beet crop is grown in the climatic
zones of the earth where sugar cane cannot be grown.
The garden and fodder beets are closely related to sugar beet. Normally the storage
taproots of these crops have lower dry matter and sucrose contents than the sugar
beet.
The many garden and vegetable root crops of the Unbellifrrae family have a taproot
and other characteristics quite similar to those of the beets. This is also the case
with the root crops of the Cruciferae, swedes, rutabagas and turnips (McNaughton
and Thow [1972]), which are widely used as garden and vegetable crops, but in
some temperate parts of the northern hemishpere they are also important fodder
crops.
2.3 Cassava
The cassava plant, Manihot esceulenta Crantz, is a tropical perennial shrub of the
Euphorbiaceae (Purseglove [1968], Jennings [1970], Williams [1975], Rogers and
Appan [1973], Coursey and Booth [1977], Ezeilo [1977]), though it may be cultivated
as an annual. The storage organs are tuberous roots, mostly adventitious, developed
from stem cuttings. The tubers are rich in starch. [t is normally propagated by stem
cuttings and rarely develops flowers or seed. Though the plant may remain in the
soil for many years and be sporadically stripped of tubers, the normal vegetation
period is from 7 to 15 months. 'he longer period, of course, gives the higher yields.
In good cultivars the dry matter production in tubers is about equal to the production
in aerial parts. The dry matter content of the tubers varies between 25 and 35 per
cent, about two thirds of this being starch. The protein content of the tubers is low
269
but the foliage is rich in protein, 18 to 35 per cent of the dry matter being crude
protein. The leaves are often eaten as spinach and are used as feed for cattle and pigs.
Cassava has for long been an important human food and now there is an increasing
demand for use as a feedstuff and for industrial starch production (Obigbesan [1977a]).
In terms of area planted, the cassava crop has acquired the leading position among
the tropical root crops (Ezeilo [1977]).
2.4 Sweet potato
The sweet potato, Ipomoea batatas (L.) Lam., is a perennial plant of the Convolvulaceae family. The storage organs are root tubers developed beneath the soil
surface. They are rich in starch and other carbohydrates (McDonald [1963],
Purseglove [1968], Coursey and Booth [1977], Tsuno). Normally the plant is grown
as an annual and takes 4 to 8 months to produce a crop. At the end of the year's
growth, normally with the onset of the dry season, the leaves wilt and die off and the
tubers survive in the soil.The plant, whose habit is normally twining, sometimes develops flowers and seed: in practical cultivation it is propagated both from stem cuttings
and cut tubers.
The tubers are higher in dry matter and energy than are those of the potato; their
dry matter content varies between 25 and 35 per cent. Of the total dry matter production of the crop up to 75 per cent is in the tubers. The foliage is edible. On dry
matter basis it contains 10-15 per cent protein, 3 per cent fat and about 45 per cent
carbohydrate. It is also used as animal feed.
The sweet potato seems to have the lowest yield potential of the major tropical
root crops. It is less restricted to the truly tropical regions; thus it is grown in large
parts of the United States and Japan.
2.5 The yams
The true yam, Dioscorea esculenta (Lour) Burk., and related subspecies, belong
to the Dioscoreaceae (Waitt [1963], Coursey [1967], Sobulo [1972], Obighesan et al.
[1976], Coursey and Booth [1977]). The storage organ is a subterranean stem or
corm. The yam is a climber and the vines die back at the end of the rainy season of
the tropical climate. Its cultivation is strictly confined to the true tropics. It is a very
old crop plant propagated from corm or cormel sets. This means that it seldom
flowers and produces seeds.
The dry matter content of the tuberous corms may vary between 20 and 40 per cent.
Besides the starchy carbohydrates the corms contain saponines, which are used in
cortisone manufacture. Of the total dry matter production of the crop about 40 per
cent is in the vines and foliage, 60 per cent in the corms.
2.6 The cocoyams
The cocoyam crops include two genera, Colocasia esculenta (L.) Schott and Xanthosoma sagittifolia (L.) Schott, which are closely related to each other and both belong
270
to the monocotyledonous Araceae (Plucknett et al. [1970], de la Pena and Plucknett
[1972], Coursey and Booth [1977]). Their storage organs are stems, i.e. corms and
cormels, subterranean and rich in starch. The cocoyams are strictly tropical plants.
They can be grown under waterlogged as well as upland conditions. Their vegetation
period is normally 9 to 14 months. They are propagated by setts, i.e. cuttings from
the upper parts of corms or cormels. The leaves of the cocoyams are edible and are
used as food. The entire plant can be used for stock-feeding.
3. General conditions affecting the nutrient requirements of the major root crops
Root crops have a long growing season and a higher potential yield than many other
annual crops. Even so, they can give reasonable yields under low fertility conditions.
This applies particularly to tropical root crops, especially cassava (Obigbesan [1973],
Ezeilo [1977]) which has a reputation of having small nutrient requirements. This
is because, in the primitive cultivation systems still prevailing in large parts of the
tropics, cassava is often grown on the most exhausted soils at the end of the crop
rotation. However, the idea that root crops have low nutrient requirements is entirely
false. If they are to realise their high yield potential, their nutrient requirements are
high and they have to be well fertilised (Greenland [1974], Kanwar [1974]).
Maximum yields realised in practice are of about the same size for all the major
root crops. The practical top yield level may be put at 50-60 tons per ha of fresh
storage organs, tubers, roots or corms. This yield level will be equivalent to about
15 tons of dry matter per ha which, in terms of energy, is about twice as much as is
obtainable from most other crops, for example, cereals. Most root crops contribute
a considerable amount of valuable organic matter in the form of foliage and other
aerial parts. The potential yields of the potato and sweet potato may be somewhat
lower than those of other major root crops. It is to be expected that intensified plant
breeding will considerably raise the potential yield especially of cassava, yams and
cocoyams.
Since the root crops are mainly carbohydrate producers, they have an especially
high requirement for potassium which has a special role in carbohydrate synthesis
and translocation (Mfiller [1964], Jackson and Volk [1968], Liebhart [1968],
Mengel [1977]). Abundant K supply favours the primary processes of photosynthesis.
It also regulates the balance between assimilation and respiration in a way that
improves net assimilation. This is a prerequisite for vigorous growth and formation
of reserve assimilates.
The translocation of photosynthates from the green parts of the plant is of the utmost
importance for the building up of the storage organs. An abundant supply of K is
needed for both short and long distance translocation (Haeder et al. [1973], Addiscott
[1974], Haeder [1977]). This applies to the 'push' side of the translocation - the
formation of assimilates in the green parts of the plant - as well as the 'pull' side - the
conversion of the translocates in the building up of the storage organs. The osmotic
effects of the K concentration as well as more specific effects of the K+ ion seem to be
involved in the translocation processes. An expression of this is probably the very
high and varying K content of the petioles of the root crops. The petioles are much
concerned in translocation and consequently petiole analysis is a useful indicator of
the K status of the crop (cf Table 5).
271
The total potassium uptake of the major root crops which is required for the top
yields mentioned above can be estimated at about 400 kg K per hectare which is
much higher than that of most other crops. Table 2 lists the usual potassium contents
of aerial parts and storage organs of the major root crops at harvest time.
Table 2. Potassium contents of some major root crop products at harvest time. K, per cent
of dry matter
Crop
Aerial parts
Storage organs
Reference
3.6
3.6
1.0-2.9
1.1-2.6
1.0-2.3
1.3
2.2
Vork Nielsen and Nielsen [1969]
Henkens [1970]
Loud [1977]
Carpenter [1957]
von Boguslawski and von Gierke
[1961]
Werner [1962]
von Boguslawski et al. [1961]
Draycott [1973]
Ltidecke and Nitzsche [1957]
Loud [1972]
Tsuno
Ngongi et al. [1976]
Obigbesan [1977b]
Obigbesan et al. [1976J
de la Pena et al. [1972]
Potato
Sugar beet
Sweet potato
Cassava
Yam
Cocoyam
4.6
3.2-4.3
1.7-3.9
3.5
3.5
2.7
-
2.2-4.1
1.9
0.6-0.7
0.6-1.0
0.7
0.8
1.2
0.3-0.8
0.9-1.2
1.2-1.8
The grower of root crops not only aims for high yield but also for produce of high
quality, high nutritional value, fitness for storage and processing, etc. These requirements can be summarised in the term 'quality' and vary for different crops and
according to the purpose for which the produce is destined. In most root crops and
for most purposes high dry matter and carbohydrate contents are basic requirements for high quality. The ratio root/shoot D.M. should be high. These general
quality criteria and many others are closely related to the nutrient supply to the crop,
the pattern of nutrient uptake and nutrient balance.
Yield level is primarily determined by nitrogen supply. An ample supply of nitrogen
is a must for high yields of root crops. However, the nitrogen supply also determines
the protein content and affects several other qualitative properties of the plant material.
An increased protein content entails a reduced carbohydrate content. At the same
time the dry matter content tends to decrease and the susceptibility to many fungal
and bacterial diseases is often increased.
The increased rates of assimilation and growth caused by abundant nitrogen supply
often result in an accumulation of precursors and intermediates in the processes of
carbohydrate and protein formation. Examples of this kind are accumulations of
reducing sugars in potato tuber (Miller [1964]) and of soluble amino and amide
compounds in the tubers of several root crops. These increased contents of simple
and soluble organic compounds often lower the quality of the harvest. This applies,
for example, to the increased contents of soluble organic nitrogen compounds in
272
potatoes and sugar beet. In the processing of the potatoes the soluble nitrogen (and
the reducing sugars) cause discolorations - darkening, blueing (Hesen [1964]).
In sugar beet processing the soluble nitrogen decreases juice purity and depresses
the crystallisation of the sugar from the juice (Winner [1966], Draycott and Cooke
[1966]). Thus, the soluble organic nitrogen is often called noxious nitrogen.
Several of the root crops contain unpleasant, bitter or toxic substances, for example
alkaloids, which limit or complicate their use as food and feed. The most severe
problem of this kind probably is the presence of cyanogenic glucosides in cassava.
These glucosides give rise to the highly poisonous hydrocyanic acid (De Bruijn
[1971]) which must be removed in the preparation of food from cassava tubers
and leaves. Abundant nitrogen supply to the crops often increases their contents of
these noxious constituents and thereby lowers crop quality. The qualitative disadvantages associated with the ample supply of nitrogen needed to obtain top yields
are reduced if a proper balance is maintained with the other necessary nutrient elements. A good supply of K is especially effective in controlling the qualitative drawbacks of abundant nitrogen (Winner [1966], Herlihy and Carroll [1969], Pushpadas and Aiyer [1973]). Both the absolute supply of potassium to the crops and the
nitrogen-potassium relationship and interactions have to be taken into account
(Alblas [1973], Prumnnel [1973], Steineck [1974], KOchl [1977]).
3.1 Phases of development of the root crops
When grown as annuals, the root crops have a comparatively long growing period.
Initial development is slow.
Within the total period of development there are two main phases:
1. Building up of the vegetative and assimilative apparatus of the plants, feeding
roots, stems, leaves.
2. Development and growth of the storage organs. Formation and translocation
of reserve assimilates to the storage organs.
Generally, the first phase comprises the first two thirds of the vegetation period of
the crop, the second phase the last third. The first phase has two characteristics:
initial growth is slow and there is a risk that the young crop will be suppressed by
weeds; secondly, nutrient uptake runs ahead of assimilation and growth so that the
nutrient requirements of the young crops are considerable and it is important for
later development that an ample supply of readily available nutrients, among them
potassium, are present in the soil at this early stage of growth. When fully developed,
the aerial assimilation apparatus of the root crops is rich in mineral nutrients including potassium, while the nutrient contents of the storage organs are relatively
low (Table 2). A result of this is that the nutrient content of the crop reaches a maximum by the end of the first phase of growth or at the start of the second, the building
up of the storage organs and their filling with carbohydrate assimilates.
During the second phase the assimilative apparatus is normally no longer growing
in size and some of its nutrient contents are translocated to the storage organs. This
applies to nitrogen and potassium. It may even happen that the total nutrient content
of the crop decreases during the last part of the second phase, at the end of the growing period. The running down of the assimilative apparatus is not fully counter273
balanced by the growth and nutrient accumulation of the storage organs. For example,
leaves and feeding roots may die back. Nutrients may be leached out from the tissues
and find their way back to the soil. Potassium is liable to such losses. A consequence
will be that determinations of nutrients in the crop undertaken at harvest time may
underestimate the total requirement of the crop though, of course, they give a true
measure of removal of nutrients in the harvested crop.
3.2 Growth rates and potassium rate requirements
The above considerations of different phases of crop development and nutrient
uptake point to the importance of the relation between growth rates and nutrient
(potassium) rate requirements, i.e. uptake of nutrient per unit time. For optimum
development it is not sufficient that the total nutrient requirement be fulfilled during
the growing period. It is equally important that the top rate requirements over short
periods of rapid growth can be met.
Growth rates and potassium rate requirements over individual stages in crop development have been little studied. Existing data refer to the temperate root crops, the
potato (Carolus [1973], Werner [1962], Soltanpour [1969], Mengel and Forster
[1973] Favart and Leblanc) and sugar beet ( IP[1966],von Boguslawskietal. [1961]).
Similar data for tropical root crops is almost totally lacking. In table 3 some data on
growth rates and potassium rate requirements from high-yielding sugar-beet experiments performed in Germany (Lidecke and Nitzsche [1975]) are summarised.
In table 4 the growth and potassium uptake rates for a potato crop are illustrated
in a similar manner. The experimental crop was grown in Maine, USA (Carpenter
[1957]).
Though there is a lack of data, it may be assumed that the growth rates and potassium
rate requirements follow approximately the same general pattern for all the major
root crops. There are cultivar differences but they do not upset the general scheme.
Growth rates and potassium rate requirements follow the same pattern in relation
to time whether they are low-yielding on low fertility soils or high-yielding and well
fertilised (von Boguslawski et al. [1961).
The data of Tables 3 and 4 confirm the general statements on the time relationships
between nutrient uptake and dry matter production earlier discussed. For both
sugar beet and potatoes the tables justify the following statements:
The initial development of the crops is quite slow.
The maximum K rate requirement runs ahead of the dry matter production.
The aerial assimilative apparatus of the crops is almost completed before the rapid
development of the storage organs begins. This applies to potassium uptake as
well as dry matter production.
During the last part of the vegetative period the aerial parts of the crop start to
die back, and lose potassium as well as total dry matter. Loss of potassium starts
earlier and is more obvious than that of total dry matter.
The potassium and dry matter from the aerial parts are mainly translocated to
the storage organs, though some is lost to the environment.
Both translocation of reserves and running net assimilation of the crop contribute
to growth of the storage organs.
274
Maturity of the crop is indicated by net reduction of the aerial parts in combination with a standstill in the net growth of the storage organs. This applies to
potassium as well as total dry matter.
Table 3. Growth and potassium uptake by sugar beet grown under favourable conditions.
Means of 5 experimental years and 4 cultivars (Lildecke and Nirzsche [1957]
K rate requirement
Days from
Growth rate
emergence
Dry matter, kg/ha/day
Tops
Roots
kg/ha/day
Total crop
Tops
Roots
Total crop
0- 45
10
2
12
0.49
0.05
0.54
46- 77
116
53
169
3.96
0.81
4.77
120
113
66
23
203
152
59
-9
78-107
108-138
139-167
168-198
Days from
emergence
83
39
-7
-32
Accumulated dry matter
production, kg/ha
Roots Total crop
Tops
3.92
0.84
0.06
-0.38
1.00
0.39
0.13
0.05
2.92
0.45
-0.07
-0.43
Accumulated K uptake
kg/ha
Roots Total crop
Tops
45
450
70
520
22
2
24
77
107
138
167
198
4200
6700
7900
7700
6700
1800
5400
8900
10800
11500
6000
12100
16800
18500
18200
149
236
250
248
235
28
58
70
74
76
177
294
320
322
311
Table 4. Growth and potassium uptake by potatoes. Results from field experiments in Maine,
USA (Carpenter [1957])
Days from
planting
Growth rate
Dry matter, kg/ha/day
Tops
0-28
29-41
4
34
42-55
56-74
75-98
79
57
-18
Tubers
-
26
104
178
Total crop
0.18
3.00
105
161
160
3.71
1.42
-1.62
Accumulated dry matter
planting
production, kg/ha
28
41
55
74
98
Tubers
100
540
1650
2730
2310
-
360
2340
6620
Tops
4
34
Days from
Tops
K rate requirement
kg/ha/day
Tubers
Total crop
0.18
3.00
-
-
0.71
1.74
1.87
4.42
3.16
0.25
Accumulated K uptake
kg/ha
Total crop
Tops
Tubers Total crop
100
540
2010
5070
8930
5
44
96
123
84
10
43
88
5
44
106
166
172
275
3.3 Disease and potassium nutrition
Like all other crops, the root crops are attacked by various pathogens and pests.
To some extent the nutritional status of the crop determines its susceptibility to
attack by parasitic diseases (Perrenoud [1977]).
Diseases caused by fungi and bacteria - blights, wilts, rots - are particularly dependent
on the nutritional status of the crop, and such diseases are especially important in
root crops. As well as affecting the crop during growth they are responsible for heavy
losses in store. A well-nourished crop in a well-balanced nutritional status shows
the best resistance to pathogenic attack and has greater ability to recover from the
effects of infection. Two types of unbalanced nutrition especially increase the susceptibility of root (and other) crops to fungal and bacterial diseases, namely nitrogen
excess and potassium deficiency. Generally, abundant potassium supply is to be
recommended for obtaining healthy crops (Adeniji and Obighesan [1976]). Nitrogenpotassium relationships are of special importance. The effects of surplus nitrogen;
decreased dry matter contents of the plant tissues, weakened cell walls and supporting tissues and accumulation of soluble intermediates of the assimilation processes
are apt to increase the susceptibility of the plants to fungal and bacterial attacks.
Thus, in the interests of crop health, it is advisable to remedy potassium deficiencies
by regular potassium fertilisation and to balance the intense nitrogen fertilisation
needed for high production by an ample potassium supply.
3.4 Determination of the potassium status of the root crops
As already pointed out, the root crops are heavy consumers of potassium. Their
total requirements are high, higher than for most other crops, and so are their potassium rate requirements, at least during critical periods of growth. In order to be able
to decide whether potassium is or will be a limiting factor of growth and quality,
it is necessary to be able to determine the crop's potassium status.
Fertiliser recommendations are usually, and rightly, based on soil analysis. However,
plant analysis can give direct information on the nutritional status of the crop. Both
methods should be tested in field experiments with fertiliser.
The value of plant analysis as a method of estimating the nutritional status of the
crop depends upon there being considerable variation in the contents of the individual
nutrient elements in the plant tissues and upon such variation being related to the
health, vigour and performance of the crop. From these points of view, plant analysis
may appear applicable to potassium and to root crops: Plant analysis can be carried
out on the entire plant or on some well-defined part of it. The part chosen for analysis
should be easy to define with regard to age and stage of development. There should
be a considerable range in the analytical values and these should be closely related
to the performance of the crop.
In table 5 some analytical data from root crops are assembled, showing the range of
potassium content in different parts of the plant which might be used for diagnostic
plant analysis. The storage organs are least suitable (Ulrich and Fong [1969]).
Their potassium contents are low and the range is rather limited. From the diagnostic
point of view the analysis of young petioles or young leaf blades is most attractive.
The potassium contents of these organs are high and the range from deficiency to
276
Table 5. Range of potassium contents in root crop plants
Crop
Potato
Sugar beet
Cassava
Sweet potato
Yam
Cocoyam
Part of plant
Petioles
Petioles
Petioles
Petioles
Leaf blades
Tubers
Leaves
Leaves
Stem
Tubers
Petioles
Leaf blades
Petioles
Leaf blades
Roots
Petioles
Leaf blades
Tubers
Petioles
Leaf blades
Tubers
Petioles
Leaf blades
Tubers
Petioles
Leaf blades
Tubers
Petioles
Leaf blades
Tubers
Petioles
Leaf blades
Petioles
Leaf blades
Stage of development
Early in growth season
Midseason
End of season
60-70 days after planting
60-70 days after planting
Mature
At harvest
30 days after emergence
30 days after emergence
At harvest
Fully developed
Ample Na supply
Fully developed
Na deficiency
Na deficiency
Mature
32 weeks after planting
32 weeks after planting
38 weeks after planting
40 days after planting
40 days after planting
40 days after planting
At harvest
At harvest
At harvest
Young plants
Young plants
Young plants
Old plants
Old plants
Old plants
Plants 3 months old
Plants 3 months old
Plants 9 months old
Plants 9 months old
Reference
K, % of dry matter
Deficiency
Normal
-9
-7
-4
0.4-8.0
0.4-4.0
1.6-1.7
Total span
Total span
Total span
Total span
0.2-0.6
9-11
7-9
4-6
8.0-10.0
4.0-5.0
1.7-2.0
0.3-4.3
1.0-4.2
0.9-8.9
1.1-2.0
1.0
0.3-0.6
0.5-2.0
0.4-0.5
-0.6
0.4
1.0
0.3
Total
Total
Total
Total
span
span
span
span
1.0
-
1.0
0.85
1.6-2.2
1.4-1.6
0.5-0.6
9.5
3.7
2.1
3.7
2.9
1.2
4.1
2.1
2.5
1.0
0.5
0.8
3.0-9.2
3.3-5.1
2.3-4.2
3.3-3.9
Surplus
II9610.0-12.0
5.0-6.0
2.0-
1.0- 11.0
1.0-6.0
2.5-9.0
1.0-6.0
1.02.6
1.6
0.7
Tyler el al. [1960]
Tlier et al. [1960]
Tyler et al. [1960]
Loud [1977]
Loud [1977]
Loud [1977]
Mengel and Forster [1973]
Ward[1959J
Ward [1959]
Ward[1959J
Ulrich [1961]
Ulrich [1961]
Ulrich [1961]
Ulrich [1961]
Loud [1972]
Ngongi et al. [1976]
Ngongi et al. [1976]
Ngongi et al. [1976]
Tsuno
Tsuno
Tsuno
Tsuno
Tsuno
Tsuno
Sobulo [1972]
Sobalo [1972]
Solda [1972]
Sobulo [1972]
Sobulo [1972]
Sobulo [1972]
d/e Ia Pena [1972]
de la Pena [1972]
c/e Io Pena [1972]
de Ia Pena [1972]
surplus is wide. It must be stressed, however, that the sampling of these organs must
be carefully carried out in relation to stage of development, age and position on the
plant. Otherwise unrepresentative or even erroneous analytical results may be obtained.
Values in Table 5 listed under the heading 'deficiency' indicate that the potassium
supply should be improved. 'Normal' values are considered sufficient for full yield
and quality, while 'surplus' indicates that there is luxury consumption and that the
level of K fertiliser could be reduced.
3.5 Ability to utilise soil potassium
Generally, root crops are grown on soils which have some reserve of potassium held
in non-exchangeable form. Potassium is slowly released by weathering into a plantavailable form and the crop's roots are not without influence on the release process.
Young soils may contain easily weathered minerals rich in potassium and on such
soils even root crops, with their high total potassium demand and high rate requirements, may be able to obtain their requirements from soil sources alone. However,
such conditions are very rare and in any more or less intensive cropping systems
where yields are maintained at a high level the full yield potential cannot be realised
if reliance is placed on soil potassium alone and the crops will respond to potassium
fertiliser.
Crops vary in their ability to exploit the potassium reserves of the soil. Those which
are able to deplete soil potassium in this way are said to be non-demanding. Thus the
cassava crop is often said to have a low potassium demand. This may be superficially
true if the farmer is content with low yields but if anything approaching the full
potential yield is to be achieved none of the root crops would fall into the low nutrient
demand category. In comparison with the Graminae their ability to exploit soil
potassium is weak while their total K need and rate requirement are much higher.
However, the root crops do vary in their ability to exploit soil potassium and, in
this respect, cassava may be relatively efficient, though few experimental data are
available on this point. Among the temperate root crops the potato is a particularly
ineffective exploiter of soil potassium; it requires an ample supply of soluble or
exchangeable potassium in the soil, which can only be ensured by applying potassium
fertiliser.
3.6 Potassium fertilisation of root crops
There is no difficulty in estimating the total potassium requirement of a crop from
total yield (roots+tops). Neither is it difficult to determine its rate requirements.
However, because the soil is normally able to provide some potassium to the crop,
such determinations are not accurate measures of potassium fertiliser need which
is given by the difference between total requirement and soil supply. Because the
soil is a dynamic and flexible pool of potassium, the proper level of fertiliser can only
be arrived at by taking into account the potassium balance of the cropping system
over the long term. Return of K to the soil in crop residues and farmyard manure
must be included in the balance.
On the other hand, it may sometimes - for example on potassium-fixing soils - be
278
necessary to improve soil potassium supply by applying potassium in excess of crop
requirements over an appropriate period. Normally, though, the fertiliser recommendation would be somewhat less than the total potassium requirement of the crop and,
in intensive agriculture, it would be from one quarter to one half of the total requirement.
Soil is complicated and variable and, in practice, fertiliser recommendations are
mainly based on empirical field experiments testing varying rates of fertiliser under
specific soil conditions. In a broad review of the present kind it is not possible to
delve into the vast amount of such data available and, in any case, the temptation
to generalise from such data should be resisted. In general it is true to say that the
root crops, which have a high yield potential and high potassium requirement in
terms of both total amount and rate in comparison with most other crops, require
abundant potassium fertiliser.
It is important to be aware of the cation balance needed by the root crops and to
add fertilisers containing balancing ions. In the first place this applies to magnesium.
Heavy dressing of potassium often calls for an intensification of the magnesium
supply. Sugar beet and other crops of the Chenopodiaceae require sodium, and
sodium-potassium relationships have attracted special consideration (Marschner
[1971]). To some extent the two cations are interchangeable but potassium cannot
be entirely replaced by sodium and some potassium should always be applied.
There is no great difference in efficiency between potassium chloride and potassium
sulphate so that, except in special circumstances, e.g. potatoes grown for processing,
chloride is to be preferred on grounds of cost. The chloride anion, being very mobile
in the soil solution and easily taken up by the root crops (luxury consumption), leads
to a higher potassium uptake than does the sulphate anion. This may result in luxury
consumption of chloride and also, to some extent, of potassium so that potassium
chloride fertiliser may depress the dry matter and carbohydrate contents of the root
crop and thereby lower its quality. The potato crop is known to react unfavourably
to potassium chloride fertiliser and for this crop potassium sulphate is usually recommended. It should also be stressed that sulphur is a major plant nutrient and,
especially under humid tropical conditions, the use of potassium sulphate may have
a clear beneficial effect not shown by the chloride in relieving sulphur deficiency
in the crop (Ngongi et al. [1976]).
3.7 Root crops and fertiliser policy for the rotation
While, at least in advanced agricultural systems, individual crops are monocultures
with specific requirements for fertilisers and other inputs, each individual crop is
only a unit in the agricultural ecosystem. Rotational cropping is designed to suit the
prevailing conditions of climate and soil, to satisfy biological considerations and to
be technically and economically sound. From this point of view the individual crops
must always be treated as integral parts of the whole production system.
This means that the fertilisation of the individual crop must be adapted to the nutrient
requirements and nutrient balance of the whole system. Of course, the main aim will
always be to meet the specific nutritional requirements of the individual plants
making up the crop, but this must be done within a framework constituting rational
fertilisation of the whole rotation and involves consideration of long term and residual
279
effects. The fertiliser programme must piovide the correct nutrient balance for each
crop and maintain or improve soil fertility. It must take account of nutrients in the
cycle, in crop residues and farmyard manure. It must take into account cost-benefit
relationships and the availability of labour and machinery. So far as potassium is
concerned, root crops occupy a key position in the rotation because of their high
potential productivity, their large nutrient requirements and their large contribution
to the potassium cycle via crop residues. It is thus convenient and efficient to apply a
large part of the total potassium requirement of the rotation to the root crop so
that its needs are covered from the newly added source. The less potassium-demanding
crops, mainly cereals, can then use the fertiliser residues from the root crop and may
be able largely to satisfy their potassium needs from such sources. Such a system is
easy to handle as well as labour-saving.
Normally potassium fertilisers should be worked into the soil before the planting
of the root crop. As pointed out above, the potassium uptake runs ahead of the dry
matter formation and, in the early stages of crop growth, it is important that potassium should be readily available in the top soil. Because of the slow movement of
potassium in the soil, the whole potassium dressing should be applied before planting
and top-dressing is inefficient. Especially under humid temperate conditions and
where, as in potato growing, chloride may have indesirable effects, it may be advantageous to apply potassium chloride fertiliser in the autumn before planting the crop
in the following spring so that excess Ci- plus Ca2 + ions is leached by winter rain.
Unless the soil is extremely light (poor in colloids) the potassium will remain within
reach of the root system of the spring-sown crop.
4. References
Addiscott, T.M.: Potassium in relation to transport of carbohydrate and ions in plants.
Proc. t0th Congr. Intern. Potash Inst., 205-220 (1974)
Adeniji, M.O. and Obigbesan, G.O.: The effect of potassium nutrition on the bacterial wilt
of cassava. Niger. J. Plant Proc. 2, 1-3 (1976)
Alblas, J.: Het blauwonderzoek op proefplekken in de jaren 1968-1971. Bedrijfsontwikkeling 4 (6), 585-594 (1973)
von Boguslawski, E. and von Gierke, K.: Neue Untersuchungen fiber den Nahrstoffentzug
verschiedener Kulturpflanzen: Z. Acker- u. Pflanzenbau 112, 226-252 (1961)
von Boguslawski, E., Atanasiu, N. and Zamani, R.: N~ihrstoffaufnahme und Nahrstoffver-
haltnis im Laufe der Vegetation bei ZuckerrOben. Zucker 14 (16-17) (1961)
Brouwer, W.: Handbuch des speziellen Pflanzenbaues. Bd 2. Berlin und Hamburg, 1973-74
De Bruijn, G.H.: Etude du caractre cyanog nftique du manioc (Manihot esculenta, Crantz).
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282
The Potassium Requirements of Fruit Crops
A.Malquori*, Institute of Agricultural and Forest Chemistry, University of Florence/Italy and
F. Parri**, Florence/Italy
1. General considerations
Fruit crops differ considerably from annual agricultural or horticultural crops with
regard to mineral nutrition and its effects on yield and quality. Annual crops grow
to maturity in a matter of months and it is relatively easy to carry out controlled
experiments. Most fruit crops on the other hand are perennial and their growth
cycles may be annual, biennial or poly-annual. It is much more difficult to study the
physiological effects of the various nutrients in such crops, especially so in the case
of field experiments. Among the various factors involved are: the duration of the
vegetative cycle, the physical properties of the soil as they affect nutrient and water
availability, and climatic factors. Further, the uptake of a nutrient is affected by
interactions with other nutrients. It is self-evident that tests to determine the nutrient
level requirements of fruit trees require periods of several years, in contrast with
annual crops, where results can be obtained within a season.
Experimental data obtained in the international bibliography dealing with the nutrition of fruit in the traditional growing areas such as Europe, are few and fragmentary.
In such areas, fertilisers are mainly applied according to empirical formulae established
through long practical experience. However, the increasing specialisation and commercialisation of fruit growing demands more precise information on the nutrient
needs of the different varieties. In recent years there has been much progress in
plant analysis which has proved to be the quickest and surest means of determining
nutrient level and its variation through the season, and it has been possible to establish
critical values indicating sufficiency or deficiency of a particular nutrient. Most
investigations of nutrient requirements made in the last twenty years have been based
on leaf analysis, the results of which have been correlated with soil analysis.
It has been somewhat easier to obtain more precise indications in the case of the
tropical non-woody fruits, where development is more rapid and nutrient turnover
more easily related to cropping under the conditions of high temperature and rainfall applying in the areas where they are grown.
*
Prof. Dr. A.Malquori, Head, Institute of Agricultural and Forest Chemistry, Piazzale
Cascine 28, 1-50144 Florence/Italy
** Dr. F. Parri, Via St. Tiirr 9, 1-50137 Florence/Italy
283
We shall deal first with some general considerations before describing in more detail
the individual needs of the main groups of fruit crops.
Under our climatic conditions, the effects of potassium in annual field crops are not
so easily seen as are those of nitrogen. In contrast however, potassium effects on
fruit trees are easily visible on the fruit in intensifying colour, increasing fruit size,
producing a finer peel, etc. Such effects are easily demonstrated on K deficient soils.
It is well known that potassium is an essential plant nutrient. In fruit the effect of
potassium, unlike that of nitrogen or phosphorus, is mainly shown in fruit quality
rather than in yield.
Qualitative parameters are difficult to define and measure. Potassium is particularly
valuable in compensating for the adverse effects on quality of excess of other nutrients.
This is particularly true in the case of nitrogen, which adversely affects fruit flavour,
colour, pulp strength and resistance to cold and pathogens. The beneficial effects
of potassium are especially seen when tree fruits have received too high applications
of nitrogen.
When working with annual crops, it is easy to determine accurately the amounts of
nutrient taken up because almost the whole of the aerial portion of the plant is
removed from the field every year. With fruit trees, however, it is essentially only
the fruit which is removed. Nutrients contained in the leaves and prunings are returned to the soil. Thus the nutrient requirements of fruit trees are usually related
to the production of fruit and do not take account of the amounts involved in leaves
or accumulated in the framework of the tree. The latter quantities can only be determined with difficulty.
Table I gives general mean values for the amounts of N, P and K removed per
10 tonne marketable produce per hectare for representative agricultural field crops,
tree fruits and two non-woody tropical fruits. Apart from the latter, nutrient removal
in harvested fruit is usually much lower than in the produce from annual crops especially so in the case of potassium. Even if allowance is made for removals in
leaves and prunings, the values will be only slightly increased and be less than those
in annual crops. It is to be noted, however, that more potassium than nitrogen is
contained in the fruits. This emphasises the relative importance of potassium in the
nutrition of tree fruits.
Table I. Nutrients removed (kg) in 10 tonne harvested produce of selected crops
Crop
Product
N
P
K
K:N
Wheat
Maize
Potatoes
Sugar beet
10 t
10 t
10 t
10 t
320
250
50
39
51.3
50.6
9.6
6.1
178.4
262.0
78.8
78.3
0.55
1.05
1.57
2.0
Apple
Peach
Grape
Citrus
10 t fruit
10 t fruit
10 t fruit
10 t fruit
............................
............................
............................
............................
65
78
80
25.8
8.4
9
13
2.2
68
67
82.9
30
1.05
0.85
1.03
1.16
Pineapple
Banana
10 t fruit ............................
10 t fruit ............................
40.5
64
3.8
4.6
72
185
1.78
2.89
284
grain+ 17 t straw ................
grain+ 18 t straw ................
tubers ..........................
roots+6.7 t tops ................
The pattern of K uptake by fruit trees as shown by leaf analysis is similar in all the
crops in that there is massive uptake in the pre-flowering period, following which
there is a continuous decrease in leaf K content due to translocation from the leaves
to other organs (e.g. the fruit). K is also lost from the leaves by leaching and from
the roots.
Figure 1 illustrates the pattern of K uptake as shown by leaf analysis of the vine
and accumulation in the grapes over the period May-October (Lafon et al. [1964]).
Successional analysis of leaf samples does not of course give as complete a picture
of K uptake as can be obtained in annual crops by whole plant analysis, but leaf
levels are well related to optimum and deficiency conditions in plant nutrition and to
fruit yield.
Soil analysis is essential before planting in order to determine the amounts of fertilisers which should be incorporated. This is especially important for potassium and
also for those elements such as Ca and Mg which compete with K for uptake by
the roots.
Cultural practices which affect root development are very important. Thus it is well
known that mulching promotes root development so that uptake of minerals is
improved as shown by increased leaf K content.
K bunches
kg/ha
30
20
K leaves
10
0
30/5 15/6
2n 15/7
31/7 14/8
1/9
15/9 1/10 15/10
Fig. 1. K uptake by leaves and bunches of the vine over the period May-October (Lafon et
al. [1964])
In the following more detailed consideration of fruit nutrient requirements, we shall
limit ourselves to the four most important fruit crops of the Mediterranean and
temperate regions and two tropical non-woody crops (pineapple and banana).
Because of the interdependance between the individual nutrients, potassium will
not be considered in isolation but in connection with P and, particularly, with N
nutrition285
2. Apples
This is probably the fruit crop which has been most thoroughly investigated with
regard to mineral nutrition. N and K are absorbed in approximately equal quantity
by apples (K:N ratio 1:1.05 - Table 1) though the functions of the two elements
are quite different as are their mobilities and final destinations within the plant.
K is very mobile, but leaf K content remains steady during the summer, decreasing
with the onset of fruit formation and at the end of the season due to migration and
leaching from the leaves (Boynton and Oberly [1966]). It is not easy to establish
critical leaf K levels for the apple because there is great variation between plants
regardless of their nutrient status (Cobianchi and Marro [1966]). Again, leaf analysis
can only give information about nutrient status during the summer months. There
is a lack of information on nutrient uptake and mobility during the winter rest.
Investigation of the effects of potassium fertiliser by means of leaf analysis during
the past twenty years (Cobianchi and Marro [ibid.]) have shown that K (and P)
level depends more upon nitrogen fertilisation than on K (or P) fertilisation. Thus,
there are records of K fertiliser having no effect on K content in the absence of N
fertiliser (Walker and Mason [1960]).
With regard to fruit quality, the effect of K fertiliser is easily seen in K-deficient
conditions when K fertiliser not only eliminates leaf scorch symptoms but also
improves fruit colour and flavour, by increasing acidity. K has little effect on fruit
storage of apples, though in other fruits it does improve keeping properties. A N:K
ratio of 1:1 to 1:2 in the complete fertiliser is recommended (Quidet and Richard
[1964]) and a mean leaf K content of 1% of D.M. is regarded as optimum. It is to
be noted that the leaves exhibit marked differences in nutrient status between years
with heavy and light crops (I.R.H.O. [1956]). Yield increases due to an application
of potassium as a sole fertiliser have been reported without any influence on leaf K
content, which is only increased when N is also given (Shadmi et al. [1966]).
Stock characteristics greatly influence leaf K level (Koo [1968]) which is lower
with more vigorous stocks, presumably due to increased Ca-K antagonism. Excess
K can induce deficiencies of other nutrients, notably Mg, K x Mg interaction being
very evident.
3. Peach
As seen in Table 1, peach has a relatively high N requirement. But, leaf K content is
higher in peach (mean 2.14%) than in apple (1.48%) (Lalatta and Fontana [1960]).
The nutrient ratio for the peach is (N :P:K) 1:0.1:0.8, the K:N ratio being lower than
for apple. Excess N reduces fruit colour by hindering the formation of anthocyanin,
while K increases the colour intensity and improves fruit size (Lalatta [1964]).
Adequate N is needed for growth, flower bud differentiation, fruit setting and yield,
while potassium is needed for quality, regular ripening, taste (higher acidity and
improved acid: sugar ratio) and improved resistance to adverse factors. Under Kdeficient conditions, K fertiliser increases yield and, in such cases, the effect of K on
quality is particularly marked (Koo [ibid.]). However, peach is often slow to react
to K fertiliser and its effects are usually observed only after about five years from
planting, when it increases number of fruit per tree and average weight per fruit.
286
Optimum leaf K level for peach lies between 2 and 3% of D.M., i.e. a higher level
than for apple (Balo et al. [1974]). On K-deficient soils there is good correlation
between leaf K level and K application. Also, when N fertiliser produces the main
effect, the colour of skin and pulp is better the higher the K:N ratio in the leaf.
Rootstock influences K uptake, though there is a lack of precise data in this respect.
N-K, Ca-K and Mg-K antagonisms are all important in connection with nutrient
deficiency, high K reducing uptake of the other cations.
4. Grapes
The grape, with the olive, is symbolic of Mediterranean agriculture; it is also one
of the most thoroughly investigated crops from the point of view of nutrition. K is,
for this crop, the dominant nutrient, not so much from the point of view of grape
yield but rather for its influence on sugar content (Depardon [1956]). The N:P:K
ratio shown in Table I indicates that K is predominant and that P is taken up in
larger amount than by other fruit crops. Investigations on K nutrition have confirmed
the influence of K supply on various metabolic processes. For example, Bouard
[1968] showed that increased K content of the shoot is accompanied by increased
photosynthetic activity through which the number of fruit per plant is increased.
K has a dual effect on the vine, directly influencing the quality of the grapes and
indirectly influencing the quality of the wine. Both effects are favourable, mainly
due to the element's function as an enzymatic activator. Potassium clearly improves
frost resistance.
Recent leaf analysis studies have confirmed (Crescimanno [1973]) that, during the
period May to October, K content decreases from a maximum value of 0.96% to half
this value while N and P content fall similarly. The main fall in K content is between
fruit set and maturity. The opposite trend is seen in Mg and Ca contents, the Ca
content being doubled (4%) at the end of growth. There is naturally variation in
these values between cultivars and rootstocks. Mean leaf K contents vary from
0.75-1.05% K between rootstocks, while N varies between 2.15 and 2.50 and P
between 0.39 and 0.49 (Kozma and Polyak [1964]).
5. Citrus
Most of the work on this group has been done with oranges, results for which are
regarded as being applicable, with few reservations, to grapefruits, lemons and mandarins. These crops are more demanding culturally than the preceding crops; the
K:N ratio for uptake (Table I) is slightly higher than for the other crops, though
there is some variation between crops and cultivars. The effect of potassium is very
much dependent upon nitrogen level, and K effects should always be considered in
relation to N nutrition because of the interrelation of N and K in metabolism and
hence in yield and quality. Except under conditions of K deficiency it is difficult
sometimes, even over several years, to see any effects of potassium on yields of fruit
(Funabiki and Sakanoto [1968]).
Increased K uptake by oranges results in increased fruit size due to increased hydration of the tissues (Boynton and Oberly [1966]). K favours the acid: sugar ratio
287
and this is particularly true for lemons, where K fertiliser can increase acid content
by 15% (Koo [1963]). Too high potassium can cause Mg and Ca deficiency and also
Mn and Zn deficiency. Potassium-deficient trees produce small fruit and show premature fruit drop which will be evident before K deficiency is rcognised from leaf
chlorosis and necrosis; in K deficiency fruit matures early, root development is
impeded and the trees are more susceptible to disease. K is directly correlated with
vitamin C content. Optimum K nutrition gives lemons better resistance to adverse
conditions, makes the peel thinner and gives a brighter colour (Creschinanno [1961]).
During growth, the K content of the fruit is higher than N content and both increase
steadily up to maturity.
Leaf analysis puts the critical K content at 1.1 to 1.2% below which level the tree
will respond to K fertiliser. The limiting value for grapefruit is higher (1.45-1.55%)
(Koo and Reese [1973]). For orange the optimum level is 1.2-1.7% of D.M. As in
other tree crops, leaf K content decreases steadily from flowering time to ripening
of the fruit. K absorption is at a low level in winter and takes place mostly from March
to November. During the autumn the leaves lose up to 60% of their K which migrates
to both the fruit and woody tissues (Cohen [1976])-
6. Pineapple
This non-woody tropical crop begins to yield one year after planting. The nutrient
removals in Table I refer to entire plants producing 10 tonne fruit of average size
2 kg. The crop has a high K requirement. Large amounts of K are translocated to
the fruit in line with D.M. accumulation and there stored. The root system is confined
to the surface soil layers and thus explores only a limited volume of soil. The result
is that K fertiliser is effective even on soils well supplied with potassium. It is, thus,
easy to understand that K is the fundamental nutrient for this tropical fruit. It balances
the unfavourables effects of nitrogen increasing fruit acidity which decreases at low
K:N ratios. Pulp consistency and transportability of the fruit is also improved.
On low K soils there is a direct relation between K fertiliser application and yield
and K has an effect on quality even beyond the stage at which there is no further
increase in yield (de Geus [1973]). Excessive K may cause too high acidity of the
fruit and can lead to Mg deficiency. Optimum N:K ratios in the fertiliser vary from
1:1.5 on high K soils to 1:2 on low K soils. Fertilisers have to be applied at high
rates (more than needed to replace crop removal and leaching losses) both for yield
and in order to secure fruit of good quality (Su [1969]).
Foliar analysis has been used in this crop to study patterns of nutrient uptake. Leaf K
content rises up to flowering time; when fruit are set K migrates from the leaves to
the fruit, higher amounts being translocated at high rates of fertiliser (Lacoeuilhe
[1973]).
7. Banana
This is the most widespread of tropical fruits. It is a perennial monocotyledon with
an annual fruiting cycle. Most of the roots are found in the surface layers of the soil
and the roots are little branched. Nutrient requirements are high, especially for
288
potassium, as shown in Table 1. The crop requires well-drained soil of good structure,
high in organic matter and with good levels af available nutrients. Fertiliser application rates are high and have to take into account leaching losses in the rainy season.
Nitrogen undoubtedly stimulates vegetative growth, thereby creating an additional
need for K. The desirable K:N ratio is 3, higher than for the other crops considered.
K uptake measured by leaf analysis proceeds in step with dry matter production,
reaching a peak at flowering, after which K content of the leaf falls, due to translocation to the developing fruit. Balanced nutrition is particulalry important for
this crop, which is very sensitive to excess and deficiency. Leaf development is great
and some of the earliest work on leaf analysis was done with banana (Hewitt [1955]).
It appears that the optimum K level is between 4.2 and 4.5% in D.M. (Janbulingamn
et al. [1975]), while the critical level is 2.7% (Prdvot and Ol/agnier [1958]). Leaf
analysis has been a valuable tool in controlling fertiliser application to the best effect
(Martin-Prdvel [1970]).
This crop has a high K demand (Twyford and WalImsley [1974]) and leaching losses
are high on most of the soils where it is grown - K deficiency is more usual than K
excess. Deficiency shows in yellowing of the leaf lamina with necrotic spots and a
slowing of growth. Excess of K may depress Ca and Mg uptake. At optimal rates,
K speeds up flower initiation and ripening of fruit and increases both number of
bunches and hands per bunch. It improves the acid: sugar ratio and storage properties of the fruit and improves resistance to disease. One of the effects of K deficiency
worthy of particular mention is that of premature yellowing of the fruit pulp, which
can also be due to excess Ca or Mg (Melin [1971], Echeverri-Lopez and GarciaReyes [1976]).
8. Conclusion
This short and very limited report underlines the importance of potassium in fruit
nutrition for both tree crops and non-woody plants. In the limited selection of crops
reviewed, the pattern of K uptake is remarkable similar. It can be studied by means
of leaf analysis and it would seem that the results reviewed here could be extended
to other similar crops. Potassium has a particular beneficial effect upon fruit quality.
9. References
Balo, E., Pankzel, M., Prileszky, G. and Gentischer, G.: The identification of potassium
deficiency in peach trees by leaf analysis. Proc. 10th Congr. Int. Potash Inst., 245 (1974)
Bouard,J.: The influence of the carbohydrate and nutrient element content of the canes of the
vine on the production of grapes. Potash Review, Subject 29, 6th suite (1968)
Boynton, D. and Oberly, G.H.: Fruit Nutrition. Somerset Press, Somerville N.J., p. 1-50, 1966
Cobianchi, D. and Marro, M.: Una semplice prova di concimazione del melo controllata dalla
diagnostica fogliare. Frutticoltura 28, I (1966)
Cohen, A.: Citrus fertilization. IPI Bull. No.4, 1976
Crescimanno, F.G.: Concimazione potassica degli agrumi. Agricoltura 10, 47 (1961)
Crescimanno, F.G., Sottile, 1., Averna, V. and Bazan, E.: Ricerche sulla nutrizione minerale
della vite. Variazioni del contenuto in N, P, K, Ca e Mg in viti asciutte e irrigue durante
un ciclo vegetativo. Riv. Ortofruttic. Ital., No.3, 183 (1973)
Depardon, L.: Potassic manuring of the vine. Potash Review, Subject 29, 4th suite (1956)
289
Echeverri-Lopez, M. and Garcia-Reyes, F.: Effect of potassium in the control of premature
yellowing and on the yield of plantains. Potash Review, Subject 23, 48th suite (1976)
Funabiki, S. and Sakamoto, T.: Some observations on potash fertilization in Satsuma orange
orchards. Potash Review, Subject 16, 39th suite (1968)
de Geus, J. G.: Fertilizer guide for the tropics and subtropics. Centre Etude de I'Azote, Zurich,
1973
Hewitt, C. W.: Leaf analysis as a guide to the nutrition of bananas. Emp. J. expt. Agric. 23,
11 (1955)
IRHO: Analyse des plantes et probl~mes des fumures min&ales. Paris, 310, 1956
Jambulingam, A.R., Ranzaswany, N. and Muthukrishnan, C.R.: Studies on the effect of
potassium on Robusta banana. Potash Review, Subject 27, 70th suite (1975)
Koo, R.C.J.: Potassium nutrition of tree crops; in: The role of potassium in agriculture.
Ed. Kilmer, V.J., Younts, S.E. and Brady, N.C., Am. Soc. of Agronomy, Madison, 469,
1968
Koo, R.C.J.: Nitrogen and potassium rates study on lemon. Florida Agr. Expt. Sta. Ann.,
Rept 233 (1963)
Koo, R.C.J. and Reese, R.L.: A comparison of potash sources and rates for citrus. Potash
Review, Subject 8, 24th suite (1973)
Kozina, P. and Polyak, D.: L'influence des porte-greffes sur la teneur en azote, en acide
phosphorique et en potasse des feuilles de Ia vigne. C.R. ler Coll. Europ. sur le contr6le de
Ia nutrition min&rale et de ]a fertilisation des cultures mdditerrandennes. Montpellier, 210
(1964)
Lacoeuilhe, J.J.: Rhythme d'absorption du potassium en relation avec ]a croissance: cas de
l'ananas et du bananer. Proc. 10th Coll. Int. Potash Inst., 177 (1973)
Lafon, J., Couillaud, P., Gay-Bellile, F. and Levy, J. F.: Rythme de l'absorption min(rale de
la vigne au cours d'un cycle v~gtatif. C. R. ler Coil. Europ. sur le contr6le de ]a nutrition
minrale et de la fertilisation des cultures mditerran~ennes. Montpellier, 213 (1964)
La/atta, F.: Influence de l'alimentation mindrale sur ]a qualit6 des fruits b noyau. Proc. 1st
Coll. Int. Potash Inst., 97 (1964)
Lalatta, F. and Fontana, P.: Analisi del terreno e delle foglie in impianti di pesco e di melo.
Frutticoltura 22, 137 (1960)
Martin-Prevel, P.: Aspects dynamiques des (lments minraux dans la production v~g6tale:
travaux sur bananier. Proc. 9th Congr. Int. Potash Inst., 295 (1970)
Melin, Ph.: Effects of heavy inorganic fertilizer dressings on the banana. Potash Review,
Subject 27, 52nd suite (1971)
Prevot, P. and Ollagnier, M.: La fumure potassique dans les r6gions tropicales et subtropicales.
Proc. 5th Congr. Int. Potash Inst., 277 (1958)
Quidet, M.P. and Richard, M. H.: Fifteen years of an experiment on the manuring of apples
at Baigts-de-B~arn (Basses Pyr6nees, France). Potash Review, Subject 24, 19th suite (1964)
Shadmi, A., Hofnan, M. and Hagin, J.: The effect of potassium fertilization in an apple
orchard. Potash Review, Subject 8, 17th suite (1966)
So, N. R.: Recommendations on the nutritional management of pineapple in Taiwan. Potash
Review, Subject 27, 48th suite (1969)
Twyford, I. T. and Wa/nsley, D.: The mineral composition of the banana plant. Plant and
Soil 41, 471-508 (1974)
Walker, D.R. and Mason, D.D.: Nutritional status of apple orchards in North Carolina.
Proc. Am. Soc. Hort. Sci. 75, 22 (1960)
290
Potassium Requirements of some Tropical
Tree Crops (Oil Palm, Coconut Palm,
Rubber, Coffee, Cocoa)
H.R. von Uexkull, East and South East Asia Programme of the Potash Institutes, Singapore*
and Cotton
A. Cohen, Dead Sea Works Ltd., Israel--
1. Introduction
The seed based 'Green Revolution' has received wide publicity and has been considered the 'most exciting development story of the past two decades'. By comparison,
very little is known about recent developments that have taken place in the breeding
and agronomy of tropical industrial crops such as oil palm, coconut, rubber, coffee,
cocoa, tea, cotton, etc. Unlike rice, where the yield potential, developed by the
breeder and demonstrated by the agronomist, has rarely been properly utilised by
the farmer, potential gains in genetics and agronomy have been immediately translated into actual production by most growers of commercial tree crops in the tropics.
Potassium is required by most plants in amounts greater than any other nutrient
but supply from natural sources is limited. Potash fertiliser has little practical importance in crop production as long as yields are low but it becomes a key production
component when yields are high.
Though the tree crops covered in this paper are botanically different, all tropical
tree crops have certain characteristics in common that separate them clearly from
annual crops.
1. Provided that water supply is adequate, photosynthesis, growth and production
goes on uninterrupted for 365 days of the year. (With the exception of rubber.)
2. Roots of tree crops are coarser, less evenly distributed and less efficient in utilising
nutrients in the top soil than roots of most closely spaced annual crops.
3. Roots of tree crops can make better use of water and nutrient resources of the
sub-soil.
4. Tree crops are often grown on soils not suitable for annual crops for reasons of
topography (erosion), depth of surface soil (unsuitable for ploughing), water holding
capacity of the top soil, etc. Such soils are often of low fertility.
*
Dr. H. R. von Uexkull, Head, I.P.Il/P.P.l.-Programme for South East Asia, 126 Watten
Estate Road, Singapore-I I
A. Cohen, Chief Agronomist, Dead Sea Works Ltd., P.O.B. 75, Potash House, Beer Sheba/
Israel
291
5. As soils under tree crops cannot be ploughed regularly, corrections of structural
and chemical deficiencies are more difficult than on arable land. For the same
reason, fertilisers cannot be distributed and mixed as evenly within the top soil.
6. As a general rule, tree crops are less sensitive to phosphate (and nitrogen) deficiency
and more sensitive to magnesium (and potash) deficiency than annual crops would
be on the same soil.
Potassium requirements of some tropical tree crops
2. The Oil Palm
2.1 General
In terms of oil output per unit area, modern D x P (Dura x Pisifera) oil palm hybrids
dwarf all other oil crops, though in the future hybrid coconuts will become a close
second. Current yields of various edible oil crops are shown in Table 1.
Table 1. Yields of various oil crops
Crop
Oil
kg/ha
190
Cotton seed ............................
380
Soybean ..............................
420
Rapeseed ..............................
620
Sunflower ..............................
875
Peanut ................................
700
Coconut (average) ......................
1255
Coconut - Estate average ...............
2690
Coconut - Estate recorded ...............
5200
Coconut - Modern hybrids ...............
6500
Coconut - Future potential ..............
Oil palm -Malaysian average ............ 3700
Oil palm- Malaysian recorded ........... 8295
11000
Oil palm-Future Potential ..............
Palm kernel oil
kg/ha
-
410
915
1200
Total oil
kg/ha
190
380
420
620
875
700
1255
2690
5200
6500
4110
9210
12200
Source: Adapted from Bek-Nielsen [1977]
It is safe to assume that within this century oil yields of 10-12 tons or more will be
achieved on the better estates. The current D x P hybrids will be augmented by
clonal planting material obtained through cell culture. Oil quality (and yields) will
be improved by crossing the Tenera hybrid with the South American Duels oleifera
(Melanococca). 40 years progress in oil palm breeding and agronomy is shown in
Figure 1.
The biggest gains were made between 1960 and 1980; with a further quantum jump
expected in the late nineties, once clonal planting material becomes available.
292
OILYIELD
tonslyear
CLONAL
palms
12
D x P
palnm
10
6 D . D
palmp
4
2d
1940
50
60
70 75 80
90 95
Fig. . Recorded and estimated progress in oil palm breeding and agronomy (Source: BekNielsen [1977])
2.2 Potassium requirements of young palms
The oil palm comes into bearing 3 years after field planting. It used to be assumed
that the main need in young palms was for nitrogen. Heavy potash application
usually starts only after the third year when the trees come into production.
Recent studies (Ng [1977]) showed that after the first year in the field there is a
very steep increase in the potassuim demand (Figure 2). It is now clear that, in order
kglha
280
240
K
200
N
160
/
120
80
40
P
1 2
3 4 5 6 7 8 9 10
Years from planting
Fig.2. Nutrient uptake of oil palms up to 10 years from planting (Source: Ng [1977J)
293
to obtain high yields early, the young palm must be 'loaded' with potassium in the
second year. It is not always easy to achieve this as the root system is not yet fully
developed and as K uptake may be inhibiied by lime and rock phosphate applied
in the planting hole.
Young palms have less storage capacity in the trunk than old palms and consequently
they need higher K levels in their tissues to produce a heavy crop. A dramatic example
of what can be done by a combination of breeding, good culture and correct fertiliser
application is shown in Figure 3, depicting the actual yields obtained at United
Plantations in Malaysia in the first 3 harvest years. I he 1962 plantings yielded only
5 tons of fresh fruit bunches/ha (FFB/ha) or about I ton of oil/ha. The 1974 planting
yielded over 25 tons of fresh fruit bunches/ha in the first year of harvest (over 5 tons
of oil/ha). Apart from improvements in the genetic make-up of the 1974 plantings,
a better understanding of the oil palm's early nutrient requirements has contributed
much towards this 5-fold increase in the first year's yield.
FFB/ha
tons
30 YEAR OF
PLANTING
25 1974
e
.
20 1971 r
1969
s
/
1965
10
/
5 1962
/
/
1
2
Years of harvesting
3
Fig.3. Recorded yield from D x P oil palms with harvesting commencing 36 months from
planting (Source: Bek-Nielsen [1977])
2.3 Role of K in adult palms
The oil palm appears to partition its assimilates in favour of vegetative growth. If
lack of potassium limits total photosynthesis and translocation, vegetative growth
continues at the expense of bunch production [Ng (1977]). There is very little
difference in vegetative growth between West Africa (low K soils, lower solar radiation) and Malaysia. In terms of yield per unit area, however, differences are very
substantial. A palm under stress will produce more male inflorescences, fruit bunches
294
produced will be fewer in number and smaller in size. A limited carbohydrate supply
may eventually cause abortion of fruit bunches. This feature is unfortunate as it
does not provide any early warning because damage has been done before corrective
measures can be taken (Ng [1976]).
2.4 Leaf potassium levels
As the oil palm produces leaves and fruit in a regular pattern throughout the year
(provided there is no distinct dry season), leaf K usually provides a reasonably
accurate guide to the K status of the palm. Currently, the following 'guideline' levels
are widely used (Table 2).
Table 2. Guideline K-levels in young and mature oil palms (% K in leaflets from frond No. 17)
Type of soil
Young mature palm
Mature palm
(Rich) Alluvial clays and clay Ioams ..............
Non-alluvial clays and clay earns ................
0.95-1.05
1.10-1.20
0.85-0.95
1.00-1.10
'Critical' leaf K levels vary with the age of the palm, the type of soil, the climate,
the levels of other nutrients present, etc., and it requires long practical experience to
'read' nutrient levels correctly. Privot and Ollagnier [1961] showed that there was
a positive correlation between K levels and yield only when leaf N levels had reached
2.70%. There are similar relationships between K and other nutrients (Hartlev [1977]).
Ruer [1966] showed that the 'effective sunshine factor' and other climatic factors
affected 'critical' K levels (Table 3).
Table 3. K-levels as affected by effective sunshine hours
% K in frond No. 17 ........................
Effective sunshine hours .....................
Malaysia
Ivory Coast Benin
1.2-1.3
Over 1800
1.0
1600
0.7-0.8
Dry climate
It can be assumed from existing data that higher K levels will be needed in areas
where climatic conditions permit high yields (no or little moisture stress) and adequate
sunshine (2000 or more hours/year). Apart from the 'climatic yield potential factor',
'critical' K levels will depend on the pool of readily available K the plant can draw
upon. This includes K in the trunk, K in the soil solution, soil K intensity and quantity
factors and fertiliser K. The smaller the total pool, the higher the 'critical' K level is
expected to be.
295
2.5 Lowering of leaf K by K fertiliser application
On certain soils (usually oxysols derived from volcanic material) it is very difficult
to raise leaf K. It has often been observed that application of potassium chloride
significantly decreased leaf K while increasing leaf Ca and leaf CI (Ollagnier [1973]).
Ollagnier [1973] and Ochs and Olivin [1976] suggested that the decrease in frond K
following the application of muriate of potash may be related to the mobilisation
of soil calcium. In Breure and Rosenquist's (Table 4) experiment application of KCI
increased exchangeable soil K and decreased soil Ca. Yet K in the level decreased
while Ca increased.
Table 4. Effect of potassium chloride on yield, leaf K, Ca, Cl and B on a young volcanic soil
(Breure and Rosenquist) (Frond 17, April 1975)
Treatment
(kg KCI/palm)
Yield, FFB
10.71-R. 75
tons/ha/year
Nutrients in % or ppm OM
K
Ca
Cl
B
Frond No. 17, April 1975
0 ..................................
1.50 ...............................
3.00 ...............................
4.50 ...............................
MSD 5% ...........................
17.48
17.83
19.18*
19.08**
1.07
0.811
0.773**
0.766**
0.751**
0.03
0.92
0.98*
0.96
1.05"*
0.06
0.34
11.3
0.48*** 10.5*
0.53*** 10.2**
0.56*** 9.8**
0.05
0.8
2.6 Potassium fertiliser application rates
Rates of potash fertiliser application will of course vary widely according to soil,
climate (sunshine hours and water supply), and genetic quality of the plants. Application rates for high yield normally range as follows:
kg K/ha
Years from planting in the field (143 palms/ha)
1
2
3
37-50
83-200
166-250
Over 3
166-290
On deficient soils much higher rates are often used. In extreme cases, up to 800 kg
K/ha have been applied in order fully to realise production potential. Such heavy
rates are sometimes needed to correct deficiencies and/or imbalances but they should
not be maintained for long as it is likely that such high rates of nutrient input may
induce nutrient imbalances (especially N, P, Mg, B and Ca).
The higher the yield the more important nutrient balance becomes. Potassium applied
at more than 210 kg K/ha, normally more than the removal, may appear excessive
but the active root surface does not usually expand in proportion to the yield potential.
The rate of nutrient uptake per unit root surface must therefore increase and this is
possible only if the nutrient concentration in the soil solution around the roots is
increased. It is inevitable that leaching losses will also increase.
296
For low to medium yields, on the other hand, it is not sound practice to replace
all the nutrients removed by the crop. The relationships between nutrient uptake,
nutrient removal and nutrient replacement needs are shown in Figure 4.
Nutrient Replacement
Needs
0t.
iNutrient
Remoal
Soil derived nutrients
Yield
Fig.4. Nutrient uptake, removal and replacement needs as affected by increasing yield levels
3. The Coconut
3.1 General
Until recently the coconut was a 'sleeping beauty', usually grown without any care
as the 'lazy man's crop'. Although potassium has long been recognised as the most
widely needed element for coconuts (Habord [1913], Lyke [1915], Georgi and Teik
[1932], Foster [1937], Salgado [1946], Menon et aL [1958], Frentond et al. [1966]).
the use of potash fertiliser remained low.
The traditional tall coconut is a slow maturing tree that is also slow to respond to
improvements in its environment. A new chapter for the coconut has begun with
the rapid spread of hybrids. Though hybrid vigour in coconuts was recognised a
long time ago (Patel [1937, 1938], Rao et al [1952]), it is only the recent breeding
work pioneered by IRHO that has resulted in worldwide recognition of the superiority of the hybrids.
While the traditional tall coconut tree usually comes into bearing 6-9 years after
field planing and reaches full bearing age at 12-15 years, modern hybrids commence
bearing in the 4th year and reach full bearing in the 7th year. Moreover, they produce
2-3 times more nuts than the unimproved tails. In the very near future, copra yields
of over 10 tonnes/ha appear to be well within reach (Frimond [1978]). It is obvious
that for such high yield nutrient requirements will be very high. There is still little
research data on hybrid coconuts and the following discussion is based mainly on
results with the traditional type of tree.
297
3.2 Nutrient removal
Nutrient removal by coconuts has been estimated by Georgi and Teik [1932] as
follows (Table 5):
Table S. Removal of nutrient, kg/ha (tall type, 7400 nuts/ha)
Part of palm
N
P
K
Ca
Mg
Fronds ........................
Inflorescence ........................
Nuts ...............................
30-48
2-3
31-39
4.4-11
0.4
5.7-8
15-65
7-16
60-170
2-18
0.7
0.7-2.9
2.4-25
0.6-2
2.4-5.4
Total ...............................
63-90
10.5-19
81-250
3.5-22
5.4-32
Very similar removal figures have been reported from India (Thampan [1970]) and
from East Africa (Copland [1931]) and many others, (Ramadasan and Lal [1966]).
Recently a very comprehensive study on the nutrient removal of high-yielding 'MAWA'
hybrids has been made in the Ivory Coast (Ouvrier and Ochs [1978]). For the first
time chlorine was included (Table 6).
Table 6. Nutrient removal of high-yielding hybrid coconuts, kg/ha (138 bearing trees yielding
6700 kg copra/ha)
Component
Dry weight
kg
N
Spikelet ...................
Stalk ......................
Husk ......................
Shell ......................
Albumen ..................
0,492
0.349
7.843
3.849
6.375
3
1
19
5
80
18.908
108
Total ......................
P
K
Ca
Mg
Cl
S
0.4
14.0
0.13
7.0
1.0 116.1
0.13
9.0
13.0
47.0
2.0
0.3
5.0
1.0
1.0
2.0
1.0
4.0
0.4
8.0
I1
6
92
4
12
1.0
tr.
1.0
0.5
6.0
14.7
9.3
15.4
125
8.5
193.1
Our own data suggest that, as a rule of thumb, it can be assumed that around 10.5 g K
are lost and immobilised for every nut harvested. On this basis K removal and replacement needs have been calculated as follows (Table 7):
It will be noted that low yields (below 40 nuts/tree/year) can usually be sustained
without the need to replace the potassium removed. At about 150 nuts/tree yields
can usually only be maintained if the total amount removed is replaced. At the
highest yield levels application should exceed actual removal because applied K is
used less efficiently at that level and K concentration in the soil solution should be
raised.
298
Table 7. Potassium removal and replacement needs at different yield levels
Number of nuts
per tree/year
Number of
nuts/ha*
Copra yield
kg/ha**
K removal
kg/ha***
K replacement
need, kg/ha****
kg K
per tree
25
50
100
Ordinary
tall
3 500
7000
14000
980
1960
3920
37
75
145
33
100
0.23
0.70
150
200
250
Hybrid
21000
28 000
35 000
5880
7840
9800
216
290
365
208
324
456
1.5
2.3
3.2
-
140 palms/ha (9 x 9 triangular)
** 280 g of copra/nut
** 10.4 g K per harvested nut
Assuming 42 kg K/ha continuous supply from an 'average' soil and decreasing efficiency
in utilisation of applied fertiliser K as replacement needs increase
3.3 Effects of potassium on young trees
As in the oil palm, vegetative production appears to have priority over dry matter
storage in the inflorescence and nuts. A slight K shortage will therefore first affect
nut size, then nut number and finally vegetative growth. With severe K deficiency the
rate of new leaf production is reduced, leaves are smaller (fewer and shorter leaflets)
and leaf duration is less. Young palms ill supplied with K take much longer to develop
a trunk and to come into bearing.
The trunk of a palm grown under conditions of K deficiency is slender with the
leaf scars close to each other, a sign of slow growth. Palms will never fully recover
from early K deficiency even if it is corrected later. Fhnmond and Ouvrier [1972] in
the Ivory Coast tested the effect of withholding potassium on the development of
young palms (Table 8).
Table 8. Effects of time of first potash fertiliser application on the performance of young
coconut palms (Fremond and Ouvrier [1972])
Year
Characteristics observed
1956
1958
1959
1960
1962
1966
1970
1961-1970
Number of fronds
Length of frond (cm)
Circumference of trunk
Number of fronds in one year
Kg of copra/ha
Kg of copra/ha
Kg of copra/ha
Cumulative yield, kg/ha
Time of KCI application
B as %
A from field
B from bearing of A
planting
age only
8.89-256
*
124.1 **
11.7 **
944 *
2560 **
2480
*
17344 *
7.69
223
105.4
10.7
272
2272
2096
12704
86.5
87.1
69.9
91.4
18.7
88.8
84.5
73.2
299
Similar data were obtained in the Philippines, where the effect of fertilisers on vegetative growth and on early yield was tested (Table 9).
Table 9. Effect of different fertiliser treatments on vegetative growth and number and size
of nuts
Grams of nutrient per tree and year:
N: 300 g. N as ammonium sulphate
P: 600 g. P2 0, as superphosphate
K,: 500 g. K2 0 as muriate of potash
K2 : 1000 g. K2 0 as muriate of potash
Fertilizer Number Girth Height Number Number
Copra
Cumulative
treatment of leaves at 4
of
of nuts in of nuts in average weight/nut copra yield
in 4 years years trunk 5th year 9th year 5th year 9th year per tree, kg
Control
N
NP
NK
NPK
56
55.
58
63
63
142
134
142
166
169
20.2
21.2
31.7
41.1
47.4
5.5
0.0
5.3
14.6
15.9
23.6
29.3
30.7
49.6
46.2
96
0
65
104
160
228
204
217
314
354
17.4
16.7
20.7
54.6
60.0
Source: Calculated from Prudente et al. [1978]
The effect of varying fertiliser treatment on cumulative yield is shown in Figure 5.
Hybrid coconuts that commence bearing in the 4th year or even earlier should be
'loaded' with K (and other nutrients) before heavy fruiting starts. Leaf K dropped
within one year from 1.8% K to 0.8% (frond No. 14) when hybrid coconuts came into
bearing.
LONG-TERM COCONUT EXPERIMENT
mt./ha
10
IN THE PHILIPPINES
NPK 2
p N-KI
.9
6
St
*NP
2[
1977
1976
1975
1974
1973
Fertiliser treatments (grams of nutrient per tree and year:
N: 300 g. N as ammonium sulphate
P: 600 g. P2O, as superphosphate
K1 : 500 g. K 20 as muriate of potash
K,: 1000 g. K 20 as muriate of potash
Fig.5. Cumulative copra yields per ha as affected by different fertiliser treatment (after
Mendoza et al. [1978])
300
3.4 'Critical' nutrient levels
For adult trees the 'reference' leaf is the leaf or rank 14 and 'critical' levels currently
used (IRHO) are:
N
P
1.8-2.0
0.12
K
Ca
% in dry matter
Mg
Cl
Fe
Mn
ppm
B
0.8-1.0 0.5
0.3
0.4
50
60
10
Similary to the oil palm, 'critical' K levels are higher for young than for old palms.
It should be higher if the pool of available K is low and vice versa.
3.5 The role of chlorine
Ollagnier [1971], von Uexkull [1972], Mendoza and Prudente [1972] strongly
suggest that chloride must be considered an essential macro-nutrient for coconuts.
Chlorine deficiency appears to be widespread in many inland areas, especially on
well-drained soils (volcanic soils). Many responses to applied potassium chloride
may have been responses to both potassium and chloride. Chloride deficiency affects
nut size, copra yield, nitrogen uptake and the water economy of the plant (Figure 6).
30
KCI-2
25
KCI-2
KCH
S- t1
S20
KCH-_
0l
0
05
Fertiliser
N: 300
P: 600
K,: 500
K,: 1000
,10
,15
.20
.25
treatments (grams of nutrient per tree and year:
g. N as ammonium sulphate
g. P20 5 as superphosphate
g. K 2O as muriate of potash
g. K 20 as muriate of potash
Fig.6. Effect of three levels of potassium chloride on chlorine levels in frond No. 14 and on
copra yield (lUexkull [1972])
301
3.6 The future
With the recent introduction of early maturing, high yielding hybrid coconuts, a
new chapter in the production of vegetable oils has begun. Hybrid coconuts have
the potential to produce 6 tons of oil/ha and plantation scale yields of 3-4 tons of
oil/ha are within easy reach. Few other oil crops will be able to compete with the
coconut and the oil palm and most of the future gains in vegetable oil production
will come from these two crops. Potassium is the most important plant nutrient for
both coconut and oil palm and for both, it is important to ensure good K nutrition
in the early years.
4. Rubber
4.1 General
Latex is essentially a hydrocarbon compound containing only very small quantities
of inorganic ingredients. The direct nutrient removal was therefore trifling in the early
days of low-yielding seedling rubber. Early estimates put the nutrient removal at
3 kg/ha N, 0.5 kg P and 1.8 kg K/ha (De Vries [1921]).
As with most crops, potassium became a most important factor only when, as a
result of a combination of breeding, agronomy (more intensive tapping) and crop
physiology (stimulation), the potential and actual yields increased. They shot up from
about 650 kg/ha dry rubber in the 1920's to over 5000 kg/ha today.
Early fertiliser trials showed little response to K and frequently even negative responses
were reported (Akhurst and Owen [1950], Owen et al. [1957]).
On the basis of the low K removal in the latex and poor responses observed in the
early experiments, it was concluded that:
a) rubber had a low K requirement (Rhines et al. /1952]); and
b) most mineral soils (in Malaysia) were adequately supplied with K (Bc/ton [1966]).
Early practice was to apply rock phosphate in the planting hole (and to the cover
crop) and small dressings of N (ammonium sulphate) up to the early mature stage.
Modern recommandations give the main emphasis to K.
Many factors have contributed to the rapid change in thinking about the proper
potassium nutrition of the crop. They fall into 3 different groups:
Group 1.
a)
b)
c)
d)
e)
f)
Group 2.
a)
b)
302
'Those that increase yield and nutrient requirements.
Changes from seedling to clonal rubber.
Better clones.
Better and more complex buddings.
Better upkeep.
More intensive tapping systems.
Yield stimulation.
Those that decrease soil K availability.
Replanting.
Rock phosphate and ammonium sulphate only applied in the past.
Group 3. The correction of other nutrient deficiencies.
a) Mg deficiency.
b)B deficiency.
The combined effect of the above is that, especially for wind-prone clones and for
intensive exploitation with Ethrel stimulation, potassium has become the most
critical element (Chan et al. [1972], Puddy and Warrior [1960], Sivanadyan et al.
[1972], Pushparajah et al. [1971]).
The latest recommendations for smallholder's rubber in Malaysia range from 23-47 kg
N, 0-57 kg P20 5 and 25-59 kg K20 for wind-resistent clones and 15-22 kg N, 0-56 kg
P20 5 and 30-70 kg K20 for wind-prone clones (Chan et al. [1972]).
4.2 Sources of nutrient demand
Unless grown on very fertile soils (where normally more demanding crops like oil
palm or cocoa are preferred), high-yielding rubber has a fertiliser demand that far
exceeds the amount of nutrients removed with the latex. There are 4 reasons for this.
a. Nutrients immobilised in the trees
Very substantial quantities of nutrients are immobilised in the trunks and branches
of rubber trees as shown below (Table 10):
Table 10. Nutrients immobilized in clone RRIM 600 (Lim [1974])
Age of trees
month
Number of trees
per ha
Nutrients immobilized
P
K
Mg
N
33
79
190
420 .......................................
420 .......................................
335 .......................................
140
635
656
19
73
134
75
365
874
9
103
149
Large quantities are immobilised in the tree and less than 10% of the nutrients are
contained in the green branches and leaves, Shorrocks [1965], and this explains why
potassium often becomes a critical factor on replanting.
b. Nutrients leached from leaves
Frequent heavy rains in the tropics leach considerable quantities of nutrients from
the leaves. In Malaysia it has been found that with 2,540 mm of rain per annum,
about 20 kg of K/ha can be leached out of the foliage of mature rubber (Lim [1974]).
c. Nutrients drained with the latex
Under 'normal' conditions the nutrient drain in the latex is small. Even with the
yields of 2000 kg of dry rubber/ha, removal would be below 20 kg of K/ha, but nutrient
removal increases steeply under yield stimulation. In extreme cases, where with
stimulation yields of 5796 kg of dry rubber have been obtained in 10 months of
tapping, removal in the latex reached 63 kg/ha (Pushparajah et al. [1971]). Stimula303
tion decreases the D.R.C. (Dry Rubber Content) in the latex. As practically all
potassium is contained in the serum, any yield increase as a result of stimulation
will cause a very large increase in the removal of K as shown in Table 11.
Table 11. Nutrients drained on stimulation, clone RRIM 605, pannel B (Pushparajah et aL
[1971])
Treatment
No stimulation ....
2,4, 5-T (1%) ......
Ethrel* (10%) ....
Yield
kg/ha
relative
Nutrients drained, kg/ha and relative
N
P
K
Mg
1454
1716
2269
100
118
156
7.6 100
12.7 167
19.3 254
2.1 100
3.6 171
4.8 229
1.7 100
2.4 141
4.7 276
5.1 100
8.7 171
15.8 310
* 2-chloro-ethylphosphoric acid
d. Inhibitedfeeder root proliferation as a result of exploitation (tapping)
Tapping interferes with the normal flow of assimilates to the roots and thus increases
the nutrient drain and, at the same time, decreases the active absorbing root surface
area. Feeder root proliferation is particularly inhibited by Ethrel stimulation (Haridas
et al. [1975]). To compensate for the poorer efficiency of the root system caused by
intensive tapping, the K concentration in the soil solution must be increased.
4.3 Roles of potassium
4.3.1 Effects on early growth
Lack of K during early growth limits the active leaf area and reduces the photosynthetic activity of the foliage. As a result girth increases slowly and it takes the tree
much longer to reach tapping age. Good management and proper fertiliser use can
reduce the time to come into tapping to less than 3/ years (Sivanadyan etal. [1975]).
Properly fertilised trees can be opened up at a smaller diameter as they continue to
put on girth, even under tapping (Table 12).
Table 12. Effect of tapping and fertiliser on girth increments (Sivanadyan el al. [1975])
Treatment
Girth increment in cm (Oct. 71-May 75)
Tapped
Untapped
M anured ..................................
Unmanured ................................
1.2
1.8
100
150
2.0
2.4
100
120
4.3.2 Effects on bark thickness and quality
As latex is produced in the bark, good bark 'quality' is most important for sustained
high yield. Recent work by Pushparajah[1969], Pushparajahet al. [1974] and Samsidar Hamzah [1975] have shown that K significantly improved bark thickness (bark
304
regeneration), phloem thickness, cell size, latex vessel size and number of latex vessels
per unit bark.
4.3.3 Effect of potassium on latex flow and latex stability (latex quality)
By improving bark quality K also increases the flow rate of latex on tapping. It has
also been found that K helps to prevent pre-coagulation of latex in the cup or on the
tapping cut. Improvements in latex stability could be a direct effect of K or might be
caused by lower Ca + + and Mg+ + levels and relatively higher P levels in the latex. High
Ca++ and Mg +± values are closely associated with unstable latex. Where precoagulation of latex occurred due to excessive application of magnesium or due to high soil
Mg content, application of potassium has been shown to overcome this and to increase
the yield. Potassium together with phosphorus has also been shown to improve the
stability of stored concentrated latex.
4.3.4 Potassium and wind damage
Rosenquist [1960] was the first to suggest that severe wind damage might be associated
with K deficiency. He showed that:
a) Nitrogen increased losses and this effect was related to leaf N content.
b) Rock phosphate increased losses, but this effect was not correlated with leaf P
content.
c) Low leaf K was correlated with heavy losses. This does not necessarily imply that
low potash was the cause of the losses.
Middelton et al. [1965], on the other hand, found that potassium reduced wood
strength. Today, it is an accepted field practice to reduce nitrogen and increase potash
for wind-prone clones.
4.3.5 Potassium and seed production
Watson et al. [1965] found that, when K increased yield, less seed was, produced.
There was an indication that this was associated with widening of the N/K ratio in
the leaves. Heavy fruiting is often triggered by a stress situation. Low K and high
nitrogen could cause (temporary) moisture stress or a stress in available carbohydrates;
both might induce more profuse flowering and fruiting.
K-deficient trees tend to shed their leaves later and the 'wintering' period is longer
than when K is adequate. Refoliation of trees with a good K-status is faster and more
uniform and this could possibly affect flowering and fruiting.
4.3.6 Potassium and latex yield
As K has a pronounced positive effect on bark quality and latex stability, it follows
that it also affects yield. Table 13 shows the effect of nitrogen and potash on girth and
yield of young mature rubber grown on a Rengam series soil in Malaysia (Pushparajah
[1969]).
Increasing the leaf nitrogen level from 3.19% to 3.46 at a K level of 1.35 increased the
yield. Increasing leaf N from 3.22 to 3.38 with a leaf K of 0.8% decreased yield.
Recent investigations of the nutrient requirements of ethrel stimulated hevea have
shown the need for adequate manuring under such intensive exploitation. Figure 7
shows the effect of supplementing the regular maintenance estate manuring with extra
K on the response to ethrel stimulation.
305
Table 13. Girth and yield response of young rubber to N and K in West Malaysia (Experiment
Se 1/21)
Treatment
Girth increment % K in
(cm in 6 years) leaves
Mean yield/tree/tapping (g) 6 year
1st year 3rd year 6th year average
N0 K ................
No K2 ...............
N2 K0 ...............
N2 K ................
S.E ................
Min.sig.diff. (P <0.05)
17.7
20.4
15.8
20.7
1.22
3.7
26.3
26.0
26.2
23.2
1.51
0.19
0.83
1.48
0.79
1.25
0.006
0.19
47.4
48.2
40.3
50.5
4.48
13.5
53.8
55.1
40.4
66.0
5.32
16.0
49.4
52.2
42.0
57.3
0-o NoK
28
Stimulant applied
26
.24
2 22
5 201,
18
> 16
MAR. APR. MAY
JUN. JUL. AUG. SEPT. OCT. NOV. DEC. 1971
Fig.7. Effect of potassium application on response to ethrel stimulation. (Expt. S 484/2,
clone tjir I, seedl. panel B) (Sivanadyan et al. [1972])
N and K are the main requirements of mature rubber which receives normal (NPK Mg)
maintenance fertiliser during the immature period. The effectiveness of N depends
largely on adequacy of K and vice versa.
Current recommended maintenance dressings for young mature rubber on average
Rengam series soil are 130 g N, 40 g P, 160 g K and 26 g Mg/tree and year. With a
stand of 280 trees/ha, this would come to about 35 kg N, 11 kg P, 46 kg K and 7 kg
Mg/ha.
4.4 Critical leaf - K levels
Leaf analysis is widely used to assess the nutritional status and fertiliser requirements
of rubber (Beaufils [1955], Cocci [1960], Shorrocks [1965], Guha [1969], Pushparajah
et al. [1972]).
The Rubber Research Institute of Malaya [1963] suggested the following 'critical'
values for the major nutrients (Table 14).
306
Table 14. 'Critical' leaf nutrient contents of Hevea (expressed as percentage of oven-dry
sample)
Nutrient
Nutrient level below which
response likely
Leaves exposed Leaves in shade
to sunlight
of canopy
Nitrogen .......
Phosphorus .....
Potassium ......
Magnesium .....
3.20
0.19
1.00
0.23
3.30
0.21
1.30
0.25
Nutrient level above which
response unlikely
Leaves exposed Leaves in shade
to sunlight
of canopy
3.70
0.27
1.50
0.28
3.60
0.25
1.40
-
Later, the above levels were found to be unsatisfactory for certain newer clones.
PB 5/51, RRIM 600, GTI responded to K even when leaf K ranged from 1.5-1.8%
(Table 15).
On the basis of such findings, fresh criteria have been adopted (Table 16).
Table 15. Response of clone PBS/5I to potassium in areas high in leaf potassium (Pushparajah
and Tan [1972])
K-level
kg K 2O/ha/year
% K in low shade leaves
1967
1970
0 .............................
54 .............................
102 .............................
156 .............................
1.71
1.70
1.76
1.72
1.90
1.97
2.15
2.14
5 year cumulative yield,
dry rubber, kg/ha
6585
6890
7290
7780
Table 16. Range of K content in leaves at optimum age- in the shade of canopy (% K in dry
matter)
Clone group"
Low
Medium
High
Very high
I ................................
II ...............................
1.25
1.35
1.26-1.50
1.36-16.5
1.51-1.65
1.66-1.85
1.66
1.85
About 100 days old
I : 'Normal' clones
Group 11: RRIM 600, PB 86, PB 5151, GTI
** Group
4.5 Conclusion
The natural rubber industry has made tremendous advances in the past twenty years.
These have made natural rubber more competitive. Over the same period, potassium
has also become the most prominent major nutrient. Its importance will continue to
grow as further progress is made in rubber breeding and agronomy.
307
5. Coffee
5.1 General
The need for potassium in coffee production was recognised as early as 1879 (Hughes)
and fertilisers with high K content are used worldwide. Anstead and Pittock [1913]
estimated that uptake appropriate to a crop of 5000 kg fresh cherries (about I tonne
clean coffee) were about 100 kg N, 10.5 kg P and 125 kg K. Mehlich [1966] calculated
the total nutrient requirements of 3 year old coffee (1330 trees/ha) yielding about
1250 kg dry beans/ha at 140 kg N, 14 kg P, 157 kg K and 20 kg Mg, and that about
35% of the K was contained in the fruit.
In contrast to the oil palm or the coconut, partition of assimilates is clearly in favour
of the generative phase. Coffee tends to overbear, especially when grown without
shade and with insufficient fertiliser. Such overbearing may cause patterns of alternate
bearing, dieback or even death of the tree.
5.2 Potassium and physiological dieback
Heavy bearing branches carry a bundle of fruit of 30 cherries or more for every pair of
leaves. Roelofsen and Coolhaas [1940] found that over 75% of all K, 60% of N, P
and Mg of a fruit-bearing branch was accumulated in the ripening cherries.
Schweizer [1940] found that in non-bearing trees much of the K in the canopy moved
into the branch tissue before the leaves became senile and were shed. Presence of fruits
in healthy trees tended to increase K in the leaves of fruiting branches. During early
ripening the fruit would draw heavily on the K contained in the leaves. On ripening
some of the K would be returned to the leaves; the leaves would then remain green and
healthy and would not be shed early. This is a precondition for a good following crop.
In contrast, heavily laden branches with low leaf K would draw so much K from the
leaves that photosynthesis and translocation of carbohydrates was impaired, causing
early senescence and premature shedding of the older leaves and fruit. The drain of
potassium into the fruits results in a narrowing of the K/Ca ratio in the leaves, causing
them to age prematurely, to lose moisture, to develop marginal scorch and to drop
The rate of photosynthesis per unit leaf area decreases while the respiration rate
increases, thus sharply reducing net assimilation (von Uexkall [1968]).
In extreme cases the carbohydrate exhaustion goes to the extent that the branch tissue
collapses and dies. Before that happens, energy translocation into the roots will be
impaired and parts of the roots may die. Trees whose root systems have been damaged
as a result of overbearing (caused by potassium and nitrogen shortage) recover only
slowly and often show as a secondary effect trace element deficiency symptoms
(manganese, iron and zinc in particular) on their new flush.
K starvation in coffee can set in very quickly with a heavy crop and seemingly healthy
trees may look miserable one month later. Because its effects on branches and roots
are so severe, often causing death, severe K deficiency is not easily cured, will always
cause severe crop losses and the trees may need two or more years to recover.
Potassium uptake consumes energy and the intensity of K uptake is closely related to
the amount of 'available' carbohydrates in the plant. To be most beneficial, potash
fertilisers should be applied when the leaf K is highest (Busch [1956]). According to
Wellman [1961] 'the secret of coffee production is obviously related to the encourage308
ment of absorption of potassium and nitrogen into the leaves aiding in photosynthetic
activities and starch accumulation of the leaves...'
Beaunont [1939] found that the relative length of terminal growth was an accurate
index of crop expectation in the following year and Cooil et al. [1948] showed that in
Hawaii the length of terminal growth was highly positively correlated with leaf K
concentration during August/September. Problems of overbearing and physiological
dieback rarely occur where coffee is grown under shade - and where yields are much
lower.
5.3 Potassium and yield
As with most other crops, potassium grows in importance as yields increase. Adequate
K is essential not only to prevent dieback or alternate bearing patterns but also for
sustained yield and here potassium is considered to be a dominant factor (van Direndonk [1959]).
A progressive plantation in Papua New Guinea, aiming at 3.7-4.9 t/ha made coffee,
recently started a fertiliser programme consisting of 5 alternate applications of 16-0-24
and 12-12-17-2 compound fertiliser, supplying a total of about 685 kg N, 95 kg P
and 790 kg K. Additional magnesium may be needed to balance the high levels of N
and K input. In Papua New Guinea the most popular coffee 'mix' is a 10-3-20-4
(NPKMg fertiliser). Other common formulas used are 10-5-20, 12-12-17-2, 16-0-24,
15-10-20, 12-10 20 (N :P2 0 5 :K 2 0 :MgO) etc.
High yields are only possible when coffee is grown unshaded or under very light shade.
The combination of high photosynthetic activity and temporary moisture stress seems
to stimulate fruiting to the extent of overbearing. Cultivation of coffee without shade
requires skilful soil management (water conservation) and constant attention to
nutrition. But the rewards are continued heavy crops.
5.4 Leaf potassium and yield
Leaf K level and yield have been found to be closely related, provided other factors
are not limiting. Leaf K content can vary from below 0.3% to over 3%. The optimum
cation balance for Robusta coffee appears to be between 46:42:12 and 50:38 :12
(K: Ca: Mg) (Lou [1958]). The sum of the 3 cations is fairly constant at 3.8% dry
matter so the optimum K concentration would be in the range of 1.75-1.90%. Similar
figures were given by Haag and Malavolta [1960], though many workers still consider
K levels of 2.3-2.7 to be 'medium', 'normal' or 'desirable'.
K-deficiency symptoms may also show up when leaf K drops below 1.1% but the
appearance of symptoms much depends on the presence of the other nutrients, on
water stress and other factors. Malavolta et al. [1962] showed that when N was
applied alone K-deficiency symptoms appeared at much higher leaf K values than
with a N-P treatment (Table 17).
This shows that care is needed in establishing 'critical' leaf nutrient values. A coffee
tree may look perfectly healthy and may not respond to potassium at leaf K levels
of 1.2%. Under other conditions, responses may be obtained at leaf K levels of
2.5 or more (Figure 8) which many would consider excessive.
309
Table 17. K and Mg content of coffee leaves associated with deficiency symptoms under
different fertiliser treatment
Treatment
Symptom
K
Mg
Nitrogen
None
Slight marginal chlorosis
Brownish rim
Necrosis
1.38
1.07
0.98
0.88
0.28
0.29
0.30
0.31
Nitrogen + Phosphorus
None
Slight marginal chlorosis
Brownish rim
Necrosis
0.63
0.56
0.56
0.29
0.37
0.39
0.40
0.44
2000
C
1500
21
1000
500
.5
1.0
1.5
2.0 2.5
% K in Dry Matter
Fig.& Relationship between leaf K and yield of Arabica coffee (from Medcalf et al.[1955])
5.5 Conclusion
Potassium (fertiliser) will become increasingly important in the future for a number
of reasons. The following factors will contribute to the rapidly growing role of
potassium in coffee production.
a) The rapid disappearance of fertile, virgin soils suitable for coffee cultivation.
b) Increasing land and labour costs that make coffee cultivation profitable only
if high yields are obtained.
c) Changes from shaded to unshaded cultivation.
d) Introduction of high-yielding, fertiliser-demanding clones and hybrids (arabicax
robusta for example).
e) A growing worldwide demand for coffee.
310
6. Cocoa
6.1 General
Cacao's natural habitat is the tropical rain forest of Central and South America
where it grows along rivers under the canopy of taller trees. The tree is physiologically very fragile, having a shallow root system only moderately efficient in utilising
soil moisture and nutrients, and leaves with a rather high transpiration quotient.
Lemie [1965] showed that photosynthesis, growth and transpiration are markedly
reduced when the availability of soil moisture drops to 60-70%. If cocoa leaves
lose about '/6 of their water content, necrotic areas resembling potassium deficiency develop (Alvin [1965]). According to Murray and Maliphant [1965] the
tolerable range in nutrient levels is more restricted in cocoa than in other crops.
Small imbalances may lead to premature leaf fall.
Cocoa leaves are also sensitive to high temperature and can tolerate leaf surface
temperatures of 500 for only very short periods (Mainstone [1972]). To make use
of sunlight, leaves must transpire water, not only for photosynthesis and uptake
of nutrients, but also in order to remain cool.
6.2 Fertiliser and shade
Cocoa nutritional problems cannot be discussed without at the same time discussing
shade. Shade is absolutely essential for young trees. As the trees grow taller and the
canopy closes, there is often enough 'self-shading' and no additional 'overhead
shade' may be required. The question whether and how much to shade depends on a
number of factors, such as:
a) The amount of 'climatic shade'.
b) The relative humidity (when dry periods are long, shade may be needed).
c) Planting material. (Upper Amazon cocoas require less shade than Trinitarios,
Amelanados or Criollos).
d) Soil fertility and the adequate and skilful application of fertiliser. The less shade
the higher the demand on soil fertility and proper fertiliser.
e) The surrounding vegetation and insect fauna. (In certain cases a 'shade' vegetation
is essential to reduce insect damage and to maintain an insect population needed
for pollination).
Most of the world's cocoa is grown under rather heavy shade, sacrificing yield for
health. Average yields of shade-grown cocoa range from 200400 kg dry beans/ha.
Shadeless cocoa may be able to produce yields of over 5000 kg of dry beans/ha.
Like coffee, cocoa trees respond to the combination of moisture stress and increased
rate of photosynthesis that follows the removal of shade with an impulse for generative reproduction. Unless the increased demand for water and nutrients is met,
removal of shade usually results in a steep, short-lived increase in production followed
by severe leaf-fall and dieback.
Under reduced light intensity, it is much easier to maintain a healthy balance between
water uptake and transpiration and between nutrient uptake, dry matter production
and nutrient removal (von Uexkull [1968]). Without fertiliser maximum yields
311
are usually obtained at about 40-60% light intensity, whereas with fertilisers the
best results are obtained at 75-100% light intensity (Figure 9).
Many of the early fertiliser esperiments with cocoa were conducted tinder heavy
shade where fertilisers had little effect.
1000
0
-
NKapplied
t
0F
o50erti z er
100
75
50
25
0
Degree of Shading,(*,/ of Full Day-light
Transmitted) (after Wood [1975])
Fig.9. Interaction between light intensity and fertiliser in cocoa
6.3 The role of potassium
In all crops potassium gains importance as yields increase. Cocoa is no exception.
(Fertiliser) potassium is of little importance if cocoa is grown under heavy shade
and yields are low. Cocoa pods contain 4.2-5.5% K in dry matter and cocoa beans
contain usually 2.2-2.4% K, whereas healthy leaves contain about 1.2-2.2% K.
This means that a heavy crop of cocoa drains much K from the leaves and branches.
A good crop of 3 tons of dry beans would remove over 170 kg K/ha. Even good soils
will not be able to supply K at such rates (plus the K immobilised in the living tree)
for long (Figure 10).
Mainstone and Thong [1978] reviewed fertiliser responses over 6 years from planting
of monocrop cocoa on a Bungor series soil in Malaysia. For the whole 6 year period
reviewed potassium treatment had the largest effect among all nutrients (NPKMg).
It increased the height of jorquetting, the lateral spread of the canopy and the crosssection area of the trunk. In the first year of harvest K boosted the crop yield by 53%
but this boost fell to 14% in the 4th year. The response to applied nitrogen was
largely dependent on adequate K.
K-deficient trees lose more moisture per unit leaf area, use up more carbohydrates
for respiration and translocate less energy to the roots. Extended potassium deficiency
therefore results in a weakened root system and poor water utilisation, which
312
kg/ha
kg Iha
300
K
3000
'I / \
oE
2000Z
rm200
C
_00
ioo
*00
o
"
1
5 6 7
2 3 4
Years after planting
8
9
10
Fig.10. Annual nutrient immobilization and removal of high-yielding cocoa over a period of
10 years
causes severe dieback or total exhaustion of the tree. Same as in coffee, advanced K
deficiency is difficult to correct because of damage to the root system.
The relationship between cocoa yield, K-removal and K-fertiliser needs is shown in
Figure 11.
kg ha
kg/ha
500
400
WW
Z
_
o
L
i
0
2 300
200-_
100
50
-O
1.0
2.0
3.0
4.0
Yield, Dry Beans/m.t./ha
5.0
Fig. I/. Cocoa: K2O removal and K2O fertiliser needs at different yield levels
313
6.4 Leaf analysis
In contrast to other tropical tree crops, leaf analysis has not so far proved to be
much help in determining fertiliser needs.
It is very difficult to get representative and reproducible leaf samples. Leaf nutrient
values are influenced by position on the tree, the amount of shading, the number of
flushes and ripening pods on a branch, etc.
'Critical' values may also vary widely, depending on nutrient capacity/intensity
factors, soil moisture, eva-transpiration, the amount of climatic and artificial shade,
etc. The more favourable the conditions in the rhizosphere, the lower the tolerable
'critical' levels will be. As a general guideline, the following levels are currently
considered as 'deficient' (associated with symptoms), 'low' and 'normal' (Table 18).
Table 18. Content of nutrients for normal cocoa leaves, leaves without definite deficiency
symptoms and leaves showing deficiency symptoms
Nutrient
Deficient
Low
Normal
Percent dry matter
N .........................................
P .........................................
K ........................................
Ca ........................................
Mg .......................................
< 1.80
< 0.13
< 1.20
<0.30
<0.20
1.80-2.00
0.13-0.20
1.20-2.00
0.30-0.40
0.20-0.45
> 2.00
> 0.20
> 2.00
> 0.40
> 0.45
6.5 Rates of fertiliser used for cocoa
Current cocoa yields rarely exceed I t/ha dry beans and fertiliser rates are correspondingly low. Rates recommended by different authors are shown in Table 19.
On the basis of our current knowledge we would estimate the following fertiliser
rates for young, mature, unshaded cocoa (Table 20).
Table 19. Rates of application of nutrients to cocoa (after Wirley-Birch [1972])
Author
Wirley-Birch [1972]
Maliphant [1965]
Cunningham
and Smith [1963]
Quartey-Papafio
and Edwards [1963]
Nutrient rates (kg/ha)
N
P
K
Remarks
37
41
100
105
30
167
-
50
-
Shaded Upper Amazon cocoa
Smallholder cocoa in Ghana
58
33
-
Cunningham [1963]
115
26
198
Verliere [1967]
Wessel [1967]
132
37
13
133
314
-
Light shade, yield 450 kg of dry
beans. East Malaysia
Unshaded cocoa
Unshaded cocoa producing
2.5 t/ha dry beans
P applied as rock phosphate
Smallholder cocoa in Nigeria
Table 19. Rates of application of nutrients to cocoa (after Wirley-Birch [1972])
Author
Nutrient rates (kg/ha)
N
P
K
Remarks
Jacob and
von Uexkull [1963]
22-34
34-68
100-156
or
150-233
97
195
16
23
47
31
16-37
van Dierendonck
[1959]
*
18-22
13-26
28-37
56-84
Cocoa up to 3 years old
Mature cocoa over 3 years old
44-68
83-129
Planting density 1100 trees/ha
25
50
68
6
12
12
5-11
61
123
69
19
37
75
11-28
Young trees
Trinidad
Old trees
Bahia, Brazil, Shaded cocoa
1st and 2nd years
Jamaica
3rd year
shaded
4th year
J cocoa
Ivory Coast. Young trees
2-6 years. P as rock phosphate
In addition to these recommendations, magnesium as dolomite or kieserite is applied where
magnesium deficiency is likely. Where high rates of potassium are used, application of
magnesium should always be recommended.
Table 20
Targeted
yield, dry beans, ha
kg/ha
N
P2 0,
KO
MgO
1.0
2.0
3.0
4.0
40
80
130
190
40
60
80
110
40
120
250
385
10
30
60
100
.......................................
.......................................
.......................................
.......................................
7. Discussion - Tree crops
Most earlyworkwith tropical tree crops did not produce spectacular fertiliser response.
Responses to potassium in particular were rare and limited to soils of lowest fertility
(coastal sands and peats). Today, potassium plays a central role in the nutrition of
tropical tree crops and the importance of potassium in absolute terms as well as relative
to other nutrients is going to grow considerably in the future.
The main factors that are responsible for this trend are:
a)
b)
c)
d)
e)
A rapid improvement of the genetic base.
A better understanding of crop physiology.
Better agronomic techniques from raising the seedlings to upkeep in the field.
Depletion of accumulated fertility through continuous intensive cropping.
The need to obtain high yield in order to keep production costs per unit as low as
possible.
The future trend will be towards:
a) Smaller trees.
b) Higher densities/unit area.
315
c) Earlier maturity.
d) Shorter lifetime of the tree (faster turnover).
e) Higher yield.
f) Higher fertiliser rates.
g) Higher rates of K in fertiliser.
High yield invariably means partition of assimilates (and nutrients) in favour of the
harvested (and removed) portion of the total dry matter production. This in turn
means that less nutrients (and carbohydrates) will be available for vegetative growth
and for root expansion. This in turn means that a higher nutrient concentration must
be offered to meet the higher needs of high-yielding tree crops.
Absorption of potassium is an energy-requiring process and, at the same time potassium is essential for energy transformation and transfer in the plant. To be effectively
absorbed and metabolised, potassium should never be permitted to become deficient
in high-yielding tropical tree crops where dangers of exhaustion are much larger than
in temperate tree crops.
8. Cotton
(by: A. Cohen)
8.1 General
Cotton does not appear to be an exhaustive crop (Bassett el al. [1970], Brand and
Dubernard[1971], Christidis and Harrison[1955]), since only the lint and seeds which
contain small amounts of mineral nutrients are removed, while the rest of the plant
(roots, stems, leaves and burrs) remains in the field. However, when grown intensively,
high-yielding crops need abundant supplies of available nutrients over a relatively
short period.
8.2 Nutrient uptake and dry matter production
Nitrogen, phosphorus, potassium and magnesium are the major nutrients essential for
cotton, while sulphur, zinc and other minor element deficiencies have been reportedResults relating dry matter production to nutrient uptake have accumulated since the
beginnung of the century but early studies referred to yields low in comparison to
those obtainable today with improved cultivars, more precise management and irrigation.
Basset et al. [1970] measured dry matter production and nutrient uptake by cotton
at various places in the irrigated San Joaquin Valley of California. Lint yield was
relatively high, ranging from 1178 to 1628 kg/ha with average total dry matter production from 6900 to 8900 kg/ha. In Israel, Halevy [1965] studied dry matter production and nutrient uptake of two cultivars which differ in their response to K fertiliser
under irrigation. The results, summarised in Table 21, show that irrigated cotton
makes a higher proportion of its total growth in the later stages than does the raingrown crop.
316
Table 21. Dry matter production by cotton at different stages
% of total dry matter
Stage
Seeding
Georgia ...........
California .........
Israel:
Acala 15-176 .....
Acala 4-42 ......
Days from emergence
Early square
Early boll
Maturity
37
66
51
8.0
7.0
3.1
2.0
5.6
3.4
0-57
13.3
11.3
57-72
25.5
21.4
72-84
26.5
24.6
84-98
22.3
28.8
98-112
6.8
10.5
112-15
8.2.1 Potassium uptake
In general, nutrient uptake by cotton proceeds more rapidly than dry matter production. Cotton grown in humid areas (Olsen and Bledsoe [1942]) absorbed 22% and
30% of the total N and K respectively by the time only about 11% of total dry matter
had accumulated, while in California, under irrigation (Basset et al. [1970]) the crop
took up about 15% of the total N, P and K before 10% of total dry matter production.
Seasonal uptakes of N, P and K by cotton and their partition among plant organs was
reported in detail by Basset et al. [1970] and Halevy [1965].
Potassium uptake continues up to 120 days from planting (Figure 12) after which the
amount of K in the plant diminishes.
According to Halevy (Figure 13) the shape of the K uptake curve is similar to those
for dry matter production and N and P uptake, but there are two differences:
a) Maximum K accumulation in the plant was reached at 112 days, after which the K
content diminished.
b) There were changes within the plant due to translocation of K from leaves and
stems to the reproductive organs. The decrease in total plant K content after
112 days may be due to the movement of K back to the soil, as has been reported
for other plants (Eaton and Engle [1957], Halevy [1976]).
oUPTAKE
DRY MATTER
110
10000
8000
7
60006O00
70
20000r
0
40 80 120 160
DAYS AFTER PLANTING
Fig. /2, K uptake and dry matter accumulation (after Kamprath and Welch [1968])
317
100
o DRY MATTER PRODUCTION
AK UPTAKE
,
75 -
O
/o
50
A
25
01
157
57 72 84 98 112
DAYS FROM EMERGENCE
Fig.13. Rates of dry matter production and K uptake in cotton ev Acala 1517 C (Halevy
[1976])
8.2.2 Rate of K uptake
Table 22 summarises results obtained in Georgia, California and Israel.
The rate of K uptake, slow at the beginning, increases rapidly at flowering, reaching its
maximum of 4.6 kg K per day between 72 and 84 days (Halevy [1969]) and 2.1-3.4 kg
K/day between day 90 and day 127 (Basset et al. [1970]). In Israel more than 30%
of the K was taken up in 12 days.
Table 22. K uptake in various experiments
Olson and
Bledsoe [1942]
Basset et al.
[1970]
Halevy [1976]
Cultivar
Upland cotton
Acala 4-42
Location
Georgia (humid) California
(irrigated)
2250 seed cotton 1178-1628
= 750 lint
10900
6900-8900
111
127
2.8
2.6
day 90-105 (15) day 90-105 (15)
1.9
2.1-3.4
day 90-135 (45) day 90-125 (35)
260
day 90-105 (15)
Acala 1517 C
4-42
Israel (irrigated)
Lint yield (kg/ha)
D.M. production (kg/ha)
Total K uptake (kg/ha)
Max. K uptake rate
(kg/ha/day)
Max. D.M. production
rate (kg/ha/day)
K removed in'seed cotton
(kg/ha)
318
1700
220
day 90-135 (45)
110-140
day 90-136 (46)
12 200-13 500
164-185
4.6
day 72-84 (12)
3.0-3.4
day 57-98 (41)
260-280
A. 1517 C day 72-84 (12)
A. 4-42 day 98-112 (14)
230-250
day 72-112 (40)
22.5
16-24
47-43
8.3 K requirements in relation to other factors
8.3.1 Management
Compared with early studies in humid climates, the results of Basset and Halevy
generally showed a greater proportion of dry matter production and K uptake in midto late season (80-120 days from emergence). The differences may be mainly due to
the marked increase in total boll set resulting from a combination of better water
control through irrigation, better pest control and other improved practices.
These findings explain the differences in the response of cotton to K status of the soil
and to K fertiliser under different growing conditions. When conditions are suitable
for early and rapid flowering with reduced shedding of bolls, there is a heavy demand
for K over a very short period during which K relase from soil minerals may be too
slow. If the soil is low in available K, this demand will not be fulfilled and early leaf
browning (cotton rust) will appear, with a decrease in photosynthesis and reduced
yield. If the prevailing conditions lead to more extended boll setting, the demand for
K will be slower and will also extend over a longer period so that the crop may yield
well even if leaf browning appears at a later stage.
8.3.2 Other nutrients
K requirements are closely correlated with the availability of other nutrients, especially
nitrogen. A field experiment (Halevy [1970]), in a region where K deficiency occurred,
was conducted to determine the relationships between potassium, nitrogen and phosphorus. Two varieties, Acala 1517 C and Acala 4-42 were compared at two levels of
each of the nutrients:
N: 0 and 180 kg/ha
P: 0 and 60 kg/ha
K: 0 and 300 kg/ha
Both varieties responded in lint yield to nitrogen and Acala 4-42 to phosphorus. There
was a large and positive N x K interaction in Acala 1517 C, K slightly reducing yield
at N. and increasing it at N (Figure 14).
1600
} AcaLa 1517 c
1500
1400
]A
Acala 4-42
1300
N.S.
1200
180
0
N- FERTILIZATION, kg/ha
Fig. 14. N x K interaction effect on lint yield (Ha/evy [1970])
319
8.3.3 Irrigation
The relation between potassium, other nutrients and water use of crops has been
studied by many authors. In the Upper Volta, Braud [1975] found that the K level in
the plant was positively related to the water supply when complete (NPKS) fertiliser
was applied. Halevy [1965] compared the K uptakes by cotton plants grown in pols
at four irrigation levels and found a positive interaction between medium to high
irrigation levels and high and very high K applications (Figure 15).
800
SF .9.8
600
0
2K2
a 400
-
~K,
C
_
5- 200 -K
0
I
L .
LOW MEDIUM HIGH VERY HIGH
IRRIGATION LEVEL
Fig. 15. Effect of irrigation on K uptake in pot experiment (Halevy [1965])
8.4 Discussion
K uptake by the cotton plant is closely related to the growth rate. Maximum K rate
requirements occur over a period of six weeks, ranging from day 90-135 to day 72-112
from planting, according to cultural conditions, climate and cultivar.
Response to K fertiliser will be obtained if, during the critical period, the rate of K
uptake demand exceeds the rate at which K can be released by the soil.
The total K requirements of cotton are closely related to N supply and to the water
regime, maximum K rate requirements being found at high rates of N and under
intensive cropping with irrigation.
Response to potassium fertiliser differs between cultivars, which have varying potential
to exploit soil K because of differences in root system development and K uptake rate
at critical periods of growth.
320
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324
The Potassium Requirements of Crops
Harvested Green, with Special
Reference to Grassland
G. de Beaucorps, Socidt& Commerciale des Potasses et de l'Azote, Paris/France*
1. Introduction
The crops discussed in this paper are those whose useful produce is in the form of
vegetative material (leaves and stems). Such vegetative material has a lower content
of useful material (carbohydrate, lipids and protein) than the storage organs or
fruits of the crops dealt with elsewhere in this volume. Some of the plants falling
within this general definition form an accessory part of the human diet as appetisers
and suppliers of vitamins - salads, spinaches, etc. - but they supply only a negligible
proportion of our calorie requirement. More important in the present context are
the following two categories of crops:
Foragecrops: Herbivorous animals, in particular the ruminants, have digestive systems
which enable them to use such low concentrate foods.
Certain industrialcrops: Among these, sugar cane, stimulants such as tea and tobacco,
and fibres (flax, hemp, etc.) should be mentioned.
2. The distribution of potassium in the plant
Potassium is extremely mobile in plant tissue. It has important functions in metabolism and this as well as its mobility results in its being found in particulary high
concentration in the young tissues. This can be demonstrated in two ways:
a) By determining the composition of entire plants of different ages. This was done,
for example by Mengel [1972] using oat seedlings; whole plants were analysed every
three to six days. The highest concentration of K in dry matter was found one or two
weeks after germination; later K concentration in the plant (still in terms of dry
matter) decreased rapidly eventually reaching a value only one third of the maximum.
Figure 1 shows change in K content in relative values; in absolute values, the young
seedlings have K contents above 4.5 to 5.0% in D.M. for almost a month, with a
maximum of 6.2% then the value falls due to dilution by rapid growth (Mengel
[1972]; Scharrer and Mengel [1959]).
G. de Beaucorps, Director, Direction technique S.C.P.A., 62, rue Jeanne d'Arc, F-75646
Paris-C~dex 13/France
325
Percent of
maximum
uptake
Shooting
100
Start of shooting
90-
End of shooting
Maturation starts
80
70
Mature
60
xwx
x 0
50"a
,
Ca
40,
30'
20"
10
24.4
K
+
5.5
16.5
28.5
9.6
21.6
3.7
12.7
21.7 30.7 Date
Fig. 1. Changes in concentration of oats seedlings with age. Percent of maximum uptake
Generally, plants have taken up their total potassium requirements by the time
dry matter production by the plant has reached 50-70% of its maximum and 50%
of the total K by the time 10-20% of dry matter has been formed. This means that
K uptake must proceed very rapidly at critical stages of growth. Thus, for example,
a crop of maize which takes up a total of 125 kg K/ha has a peak rate of uptake,
just before shooting of 6 kg K/ha/day.
b) By comparing, at any given stage of growth, the composition of tissues of varying
age as in the following two examples:
A model of this kind of analysis was carried out by Ayres [1935] using sugar cane.
He showed how K content varied with age and variety and gave exact figures for
composition of each part of the plant. He showed that K content varied between
0.3% in the base of the stem (reserve and mechanical tissue) to over 6% in the
growing point. This led the author to write 'With regard to the large amounts of
potassium which one usually finds in meristems and active plant organs, it is not
particulary surprising to find this one element accounting for more than 6% of the
total dry matter weight (or 45% of total ash) in this region of intense cellular activity.'
Such extreme variation (I to 20) in K content between different parts of the same
plant is perhaps a little exceptional but serves well to illustrate the general point.
Even comparing the same organs of different ages, the differences in K content can
be very large - the youngest leaves are always the richest in K. Malavolta [1962]
found that the K content of individual leaves on the same stem increased steadily
from the oldest to the youngest leaf and that the K content of dry matter was at a
maximum in the terminal leaf. To take another example, as is well known, tobacco
leaves are harvested and graded according to their height on the plant (in other
words their age) since quality depends upon this. The higher, younger leaves always
have the highest K content. As an example Lou 's [1970] results are given in Table 1.
It is clear from Table I that when the supply of K is low, the younger leaves have
priority for the limited K supply.
326
Table 1. Effect of K fertiliser on K content of tobacco leaves of varying age
Upper leaves ....................
M iddle leaves ....................
Lower leaves ....................
No K applied
150 kg K2 0/ha
300 kg K 2O/ha
2.40
2.25
1.75
3.55
3.48
3.28
4.73
4.78
4.78
The above general principles are of importance in relation to the potassium requirements of leafy crops.
3. Grassland
3.1 Economic importance
Grassland is by far the most widespread crop in the World. This is shown by the fact
that it occupies very large areas, but its importance as a source of agricultural revenue
is similary large. Grassland comprises about 50% of the 32 million hectares of agricultural land in France, 50% of 520 million hectares in the USA and 60% of
2.2 million hectares agricultural land in the Netherlands. But it is New Zealand which
holds the record with 20 million hectares more or less intensively managed grassland
alongside only 4 million hectares of arable land, of which more than half grows
forage crops'.
The general term 'grassland' covers a very wide range from both the botanical and
management points of view so that it is not easy to generalise on management or
fertiliser treatment, or on the economics of the enterprise. For most arable crops
there is a lower threshold below which it is not worth growing the crop, all arable
entreprises are to some, albeit low, degree intensive and it is comparatively easy to
arrive at fertiliser recommendations suited to the agricultural system and the region.
This is not the case with grassland where, in practice, production may vary from a
few hundred kg dry matter per hectare per year in a subsistence system to very intensive
production with high yields obtainable under suitable climatic and soil conditions,
with adequate water and fertiliser. Maximum yield levels attainable are of the order
of 20 tonnes dry matter at 450 latitude and around 50 tonnes in the tropics. This
extreme variability is illustrated in Table 2 taken from Burton [1972].
Table 2. Sward productivity on sandy soils in S. Georgia after Burton [1972]
Year
Sward type
1860
N atural prairie ..........................................
1900
Carpet grass (Axonopus officinis) ..........................
1930
1948
1972
Berm uda grass ..........................................
Coastal Bermuda grass+ 157 kg N+P+K/ha ...............
Coastcross+ 673 kg N+P+K/ha ..........................
Liveweight gain
(kg/ha/year)
9
34
90
543
2240
327
3.2 Composition
In discussing the potassium nutrition of herbage plants, it is best to proceed from
the particular to the general. We are essentially concerned with two groups of plants:
legumes exemplified by lucerne (Medicago spp.) and grasses exemplified by ryegrasses (Lolin perenne and Lolium italicum). In both cases the principles discussed
in Section 2 apply, that the younger the tissues the richer they are in mineral elements
and this is particulary the case for N and K, concentration of both of which decreases
very rapidly with increasing age. In cells which are actively multiplying K concentration
can reach 8% of dry matter while in mature tissue (e.g. straw or grass hay after
flowering) it will be of the order of only 0.6-1.2%. As the plant matures the proportion
of juvenile tissue diminishes in favour of older tissues in which carbohydrates have
accumulated. Total ash content decreases with time, K content decreases more
rapidly (Table 3).
Table 3. Effect of age on K content of lucerne and ryegrass after Thomas et al. [1952], % K
in dry matter
Stage
Lucerne
Ryegrass
Stage
Early vegetative ........
Late vegetative .........
Flowering .............
Mature ................
Decline ...............
4.0
3.3
2.3
2.0
1.5
3.00
2.60
2.08
1.80
1.20
Young herbage
Ear emergence
Anthesis
End of flowering
Hay
Total nitrogen and protein content decline similarly and since protein is an important
and costly item in the animal diet the tendency over the past thirty years has been
to cut earlier and earlier. However, earliness of cutting is limited in two ways. The
first is nutritional in that if cut too early the carbohydrate content of the herbage
is too low, because the plant has not had time to accumulate sufficiently, and the
energy needs of the animal will not be met, while in extreme cases the low energy
content of the diet can lead to animal disorders (grass tetany). Secondly, there is
a technical limit because the younger the grass the higher the water content and
content of non-protein nitrogen and the lower the cellulose content, making conservation difficult. Developments in pre-wilting, ensiling technique and grass drying
have made it possible to cut at the optimum stage.
A consequence ofearlycutting is that the herbage hasa higherKcontent so that potassium
removal per unit weight of herbage harvested is greatly increased.
3.3 Potassium uptake
It is very difficult to study the pattern of potassium uptake in plants which are harvested green. Uptake is the product of two factors: production of dry matter and its
K content (and variation of the latter) over the period under consideration. As we
have seen, the pattern of dry matter production is very variable, while K content at
the time of harvesting may vary widely according to age. In contrast, crops harvested
328
at the mature stage have a relatively constant composition. The pattern of potassium
uptake is most importantly influenced by soil conditions affecting potassium availability.
3.3.1. Soil reserves
The inherent potassium supply of the soil greatly affects the K content of herbage
at any particular stage of growth. For example in France, analyses of ryegrass herbage
cut at the shooting stage showed a range of values from 1.2 to 5% K in dry matter,
which reflected soil differences. Rochet [1978] surveyed mineral composition of
Italian ryegrass in Normandy and found a mean value of 3.51% K at the shooting
stage with a maximum of 6.61. The K content of the grass was correlated (p= 0.05)
with the exchangeable K content of the soils.
The capacity of the graminae to exploit soil potassium has been used to measure
the level of potassium which soils can deliver from non-exchangeable sources, for
example Italian ryegrass (Chaminade [1960]; Garaudeaux et al. [1965]) and barley
(de Meit et al. [1959]; Qu~mener and Roland [1970]). Though the grasses are very
efficient in extracting soil potassium, soil potassium supplies are only very exceptionally sufficient to provide sufficient potassium for full growth, when other conditions
are favourable, over a period of several years, and potash fertiliser must be applied.
Thus Vicente-Chandler [1972] used Pangola grass (Digitaria decumbens) to measure
the potassium reserves of Puerto-Rican soils. There was a five-fold variation between
soils in the quantity of K extracted in the first year and even in the richest soils available K reserves were very much reduced by one year's growth of grass.
Natural soil fertility is never sufficient to provide sufficient potassium for unrestricted
growth of grass under intensive management.
3.3.2. Effect of potassium fertiliser
Applying potash fertiliser to herbage has a dual effect. It increases yield and also
increases herbage K content. Consequently under intensive conditions, very high
potash dressings are required to make up for K removal in crops. Figure 2 illustrates
results obtained with lucerne. The hatched area represents conditions generally
applying in temperate regions without irrigation and shows that under such conditions
from 170 to 300 kg K per hectare annually are required to compensate for crop
removal of K.
The situation with grasses is more complex because K uptake and removal is much
affected by the level of N fertiliser applied and it would be necessary to construct a
similar figure for each level of N. Further, the cumulative effects of K removals
and additions in previous years must he taken into account.
Cutting and removing grass from the field greatly lowers soil potassium, especially
under intensive management. The problem is difficult because when high levels of
potassium are given, to compensate for removal, such a large proportion of the
added potassium goes to increasing the K content of the herbage. This effect is
illustrated by the results of Hopper and Clement [1966] which are quoted in Table 4.
Even when high levels of potassium are given to intensively managed cut grass (for
silage or drying) the soil is depleted of K and, in alternate husbandry systems this
can have serious consequences for the following arable crop. Indeed, the rational
use of potash fertiliser under such, not uncommon, conditions poses a very difficult
problem.
329
Uptake and application
Inbalance
400
o'
H
______Gerwig
and Ahlgren
------Parks and Chapman
*+**444* Blaser
3
Z
Annual application: kg K.01(ha.
i
Fig.:.Potassium uptake by
~lucerne
in relation to K fertiliser application
Table 4. Output of potassium harvested in herbage from a cut grass sward receiving 314 kg
N/ha/year and varying rates of K, 4 year totals
Input
kg K/ha
Output
kg K/ha
o........................ 660
250 .........................
840
500........................
1030
1000........................ 1290
±41.6
Balance
kg K/ha
660
590
530
290
Yield
ton dry
matter/ha
36
38
39
40
+1.02
%K
in dry matter
1.9
2.2
2.6
3.2
(After Clement and Hopper [1966])
3.3.3. The relationship between plant composition, yield andpotassium balance in the soil
It will be readily appreciated that the factors discussed above will have cumulative
effects on soil potassium. It is well known that the potassium content of herbage
will decrease from year to year if sufficient potash is not applied to compensate for
that removed in crops. Chevalier and Qumener [1977] studied over eight years
yield and composition of cocksfoot and the effects on K balance in the soil in a
4 x 2 x 4 (N x P x K) experiment at the Aspach Station (France). The grass was cut
on a system to simulate grazing management. They found that, in general, herbage
K content reflected soil K supply as indicated by the K balance of the soil (Figure 3).
They also found a relation between dry matter yield and the mean annual K content
of the herbage at each of the four levels of N applied. The more the soils were impoverished the higher the correlation and r attained values of from 0.90 to 0.96 in the
three last years of the trial at the higher N levels (Figure 4). This indicated that at
that stage potassium had become limiting. Similar results were obtained with ryegrass (Chevalier [1978]).
330
N2
Ni
N3
K
N4
DM
*.2=0.03
r2
/
0.87
RESULTS
Ni
N2
0.86
0.90/
N3
N4
0.90
+
+
2 049
A
3,
+
+
0.92
0
19
+.
*
*.
,1-
*
,.
Kg/h. K 2 0
- 151O
-boo
0
-doo
+1
+560
+0019o
Fig.3. Relation between soil K balance and mean annual herbage K content
68-70
.......
_
71.73
N3
_ 7 4 -7 6
.N
2
Yied•
"10
N4
.
.
":
.....
.
A;,,,..,"
•
.~
*
A
//
KA DM
'.52
I.s33.5 I1152
2.5 3
1 1.52
3.5
25 3
3Z5
115
2
25
3.5
Correhtion coefficien
1968-70
1971-73
1974-76
N1
- 0.51
0.13
0.30
-
N2
0.32
0.54
0.73
N3
0.89
0.90
0.96
N4
0.89
0.82
0.90
Fig.4. Relation between mean annual herbage K content and dry matter yield at varying N
levels
3.4 Grass-legume mixtures
Grass legume mixtures are much used particulary in temperate areas. Natural grassland usually contains both grasses and legumes. Such mixtures have several advantages.
331
From the point of view of animal nutrition, the legumes have much higher Ca content
than grasses and often also of Mg. It is preferable to supply the minerals in the herbage
rather than in the form of mineral supplements. Seasonal production of the mixtures
is more even than that of either pure grasses or pure legumes. Finally, in comparison
with pure grass, the mixtures enable savings to be made in nitrogen fertiliser.
It is always difficult to maintain the proper balance between legume and grass in
such mixtures and apart from a number of other factors (level and pattern of N
fertiliser, stage of cutting [or grazing], soil moisture,etc.) potash fertiliser plays an
important part. Grasses are more efficient in exploiting soil potassium than legumes
and when grown in association the latter will often be unable to obtain sufficient K
in competition with the grass. Generous use of nitrogen which stimulates growth
and K uptake by the grass, worsens the position. When grown in pure stands, legumes
have K contents comparable with those of grasses but when they are associated
the legumes always have lower K contents. This is demonstrated in Table 5 after
Balser and Kimbrough [1968].
Table 5. Herbage K content in a grass-legume mixture
K applied
kg K2O/ha
% K in dry matter
grass
legume
K in legume/
K in grass, %
0 .....................................
46.5 ..................................
93 ....................................
372 ...................................
2.71
3.46
4.01
3.85
26
35
42
92
0.70
1.21
1.78
3.53
(After Blaser and Kimbrough [1968])
This behaviour is partly explained by the difference in cation exchange capacity of
the roots of grasses and legumes, that of the latter often being as much as double
the former. Various workers have explained the difference and the fact that legumes
take up monovalent cations such as potassium only with difficulty on the basis of a
'membrane' theory or the Donnan equilibrium.
In practical terms the result of this behaviour is that adequate potassium fertiliser
is more important for a mixed sward than for a pure stand of grass and there is an
advantage in applying the potash fertiliser in repeated small dressings. Slow acting
potassium fertiliser such as sulphur coated KCI would be useful.
3.5 Grass in the tropics and sub-tropics
As comra-ed with the temperate zones, the manuring of grass has received little
attention in the tropics, and if fertiliser is used at all, the rates applied are nowhere
near those which the grass could utilise profitably. Provided there are no other
nutrient deficiencies and there is sufficient water dry matter production increases
linearly with increasing nitrogen fertiliser up to as much as 800 kg/ha N. According
to Salette [1971] the average response by tropical grass is 20-30 kg dry matter per
kg N applied but it can be as high as 80 kg. The response to applied N is linear up to a
332
yield of 20-30 tons DM/ha above which the rate of response falls off, and nitrogen
may be applied at up to 1500 kg N/ha. The most demanding and most productive
grasses are Cynodon (Bermuda grass), PaspaluIn (Bahia) and particulary elephant
grass (Pennisetnn purpureun) which has yielded over 50 tons DM/ha when N
was applied at 1500-2000 kg/ha. Normally the K content of such herbage would be
between 1.5 and 2.5% resulting in the removal of 500-700 kg K 2 0/ha at 30 tons
DM. Thus heavy K applications are needed to maintain soil K status.
Tropical grass responds to K fertiliser whenever it is intensively managed. Response
naturally varies according to soil fertility, rate of N and P applied and type of grass.
Under the most intensive management, response normally continues up to a level
of 500 kg K 2 0/ha and experience shows that there will be a marked response to K
whenever K content of the herbage at cutting is below 1% in dry matter. In most
cases response will continue until herbage K content reaches 1.5% K. As was done
above (Figure 2) a curve can be constructed showing K removal against K application
and it will found that for tropical grasses removal and application will be in balance
at about 500 kg K2 0/ha. Obviously at such high rates, divided dressings must be
used (Adamns et al. [1967]).
3.6 Effect of potassium on herbage composition
Potassium content has no direct influence on animal health. Animals fed on rations
composed solely of grain may suffer K deficiency when the K content is below 0.5% K,
but no grass would grow under such conditions so that the animals' potassium
requirement (even of very high yielding dairy cows) will be satisfied if grass is included in the ration. Conversely there is no evidence that high K level in the herbage
adversely affects the animal. Kemnp [1970] showed that K excreted is proportional
to K ingested. Even when intake exceeds 600 g K/day there are no ill effects on cattle
(Hendriks [1964]).
However, potassium has indirect effects on both mineral and organic composition
of the herbage. In both cases the effects on pure stands are discussed, the effects on
grass legume balance having been briefly treated in section 3.4 above.
3.6.1. Mineral composition
Both K+ and C- ions are very easily taken up by the plant and they significantly
affect mineral composition. Increased K uptake is necessarily accompanied by
decreased uptake of other cations. In fact one can only speak of true antagonism in
the case of sodium. Apart from evidence from experiments, Nielsen [1969] has
studied Na content of herbage throughout Denmark. The depression of Na content
is not practically important as salt is normally included in the ration of dairy cows,
even when no potash is applied to the grass. Additionally, while Na content is affected
by potassium, it also depends on species and cultivar (ap Griffith and Walters [1966];
Garaudeaux [1959]).
Potash fertiliser slightly decreases the Mg content of herbage and this has led to
statements that potash fertiliser is responsible for herbage tetany. A number of
experiments were reported showing that this is not a simple problem and that it is
bound up in a complex way with the assimilation by the animal of ingested Mg and
that potassium has no effect on this (Werk and Rosenberger [1969]).
333
3.6.2. Organic composition
Nitrogen is found in plants in three forms: mineral N, soluble N compounds (amines)
and insoluble compounds (proteins). It is now well established that potassium fertiliser in general increases protein N content and reduces soluble N content (Nowakowski [1964]; Demarquilly [1977]). Potassium usually decreases the content of
mineral N but in a less regular manner (Figure 5).
Ferlilser kg/K/ha
Total N %In dry matter
0
2,30
.
200
2,31
2.
71,1
Protein N
Mineral N
74.6
76.8
404.0
Soluble organic N
.
%of total N
7
K %In dry matter
Dry matter yield
t/ha
f10
24.1
OM
26.0
21.4
0.95
28'S
1 11,7
129
33,0
Fig.5. Variation in composition of Italian ryegrass with potassium manuring (2nd cut). After
Nowakowski [1964]
It is well known that potassium has important functions in photosynthesis and glucoside metabolism. Lowering of leaf K content always results in the accumulation of
soluble glucosides to the detriment of the higher polysaccharides (Evans and Sorger
[1966]).
These analytical studies show that while potassium fertiliser can influence the quality
of herbage the tendency is usually favourable and generally speaking it has little
effect on the nutritive value. In most cases the effects of potash fertiliser can be
completely described in terms of the effect on dry matter and digestible protein
production. It has been shown that potassium has little influence on digestibility
even when applied at high rates (Martz et at. [1967]; Reid and Jung [1965]; Calder
and McLeod [1968]).
3.7 Effect on vigour
Adams and Twersky [1959] have shown that potassium fertiliser reduces winter
kill, and Evans and Sorger [1966] showed that it had a favourable effect on disease
resistance.
334
3.8 Conclusion
It has been shown that the potassium requirements of herbage plants are very high
and that large amounts of K are involved in the nutrient cycle, that potash fertiliser
produces large increases in yield which are of benefit to the animal. Experience
shows that, under intensive management. in grass drying for example, yields can
only be maintained if potassium removed is made good by fertiliser.
The problem of the potash fertilisation of grazed grass is difficult and is discussed
in chapter 5 of this volume, suffice it to say here that the recycling of potassium is
often over-estimated.
4. Sugar cane
The area planted to this crop has almost doubled (to nearly 13 million hectares)
during the past twenty five years. During the same period average yield has moved
up from 42 to 54 t/ha cane. Sugar cane production has kept up with the increase in
World population. But, compared with the yields of 120 or 150 tons which are
obtained in some countries the 54 tons average is still very low. The three largest
producers together representing half the cultivated area in the World had, in 1975,
only the following average yields: India 51.2 t/ha on 2.79 million ha, Brazil 46.1 tons
on 2.24 million ha, and Cuba 44.3 tons on 1.15 m/ha. Because in most countries
there is little possibility to increase the area planted with cane for lack of suitable
land and sufficient water, further production increase can only be achieved by the
improvement of cultural methods, improved varieties and the use of more fertiliser.
Humbert [1958] has illustrated in classic fashion the relationship between usage of
potash fertiliser and sugar yield (Figure 6).
..
i.
1100
,,
i.,.7=
nnnnnn'Sawdn.
700
N.'O
20
M
O
250No
50
4.1 Uptake of nutrients
As a converter of solar energy into food calories the sugar cane is unequalled, so it
is not surprising that its nutrient requirements, for the realisation of full yield, are
very large. High yields can only be obtained on fertile soils which will satisfy these
needs. This was shown many years ago in measurements of nutrient uptake made
in Java on seven plantations with yields ranging from 107 to 174 tonnes/ha (Honig
[1934]) results are summarised in Table 6 which also summarises more recent
results obtained in Argentina (Fogliata [1975]) in which total removal over three
years (new cane cv N.A. 56-79/129 t/ha, ratoon of the same cv (84 t/ha and ratoon
of ev N.A. 56-30 [75 t/ha]) was measured.
Table 6. Nutrient uptake by sugar cane after lonig [1934]- and Fogliata [1975J**
Mean cane yield
(ton/ha)
Uptake kg/ha/year
N
K2 0
P205
S
MgO
CaO
SiO
148" ..........................
96** ..........................
71
56
39
I1
34
53
708
365
178
59
I1
-
-
4.2 Fertilizer use
Because of its enormous potential for growth - a world record yield of 424 tons
has been quoted! - sugar cane needs generous supplies of nutrients and water throughout growth right up to maturity. The harvest, which is a most demanding time can be
extended by late application of nitrogen and potassium during the days immediately
preceding cutting which favours translocation of sugar from leaf to stem. As with
other intensive crops, supplementary application of N should always be balanced
with potash fertiliser - potassium deficiency reduces cane yield and sugar content.
According to Hawaian experiments leaf K content below 1.25 to 1.7 % causes the
accumulation of low molecular weight N compounds, delaying maturity and adversely
affecting sugar extraction in the factory. An example of the effects of increasing
potassium levels on yield and the main quality factors in Taiwan with the cultivar
P.O.J. 28-83 receiving uniform N and P at 160 kg N/ha and 100 kg P 2Oj/ha is shown
in Figure 7.
Potash was applied at rates up to 200 kg K 2 0. This is only one of many cases were
potassium improved yield, sugar percentage and juice purity. K response is variable
but always positive as regards all these factors. Very often, sugar content and juice
purity are further improved when potash is applied at rates above that which gives
maximum cane yield.
The following is a brief summary of some world-wide results:
Rtunion: Work is reported in 'L'Agronomie Tropicale', Vol. 29 (1975), showing
that cane yield and sugar content were increased by potash applied at up to 200 kg/ha
K 2 0.
Thailand: The Annual Report of the Department of Agriculture, Bangkok for 1966
reports that cane yields were much increased by potash at 500 kg/ha K 2 O and that
336
102.U0
0
Cane yield
(kg/ha)
51.800
I7.185
'5.010
10000
6000
11.65
(kg/ha)
7.910
16.79
17
Sugar
1 6%
15
Sugar yield
15.27
15,5B
97,20
5
Juice purity
(Sugar/Other
solubles %)
95,32
0
100 150 200
KO kg/ha.
Fig. 7. Effect of potassium on cane yield, sugar yield and quality
efficiency was improved by applying the potash in three equal dressings at planting,
after 45 and 90 days.
India: Under optimum conditions sugar yield increases at as much as 20 kg sugar
per kg K 2O applied; the optimum rate is about 200 kg K 20/ha. Gupta and Shukla
[1973] showed the crucial importance of N : K ratio in the fertiliser - increase of the
former must be accompanied by increased potash.
Venezuela: Segura [1971] recommends a base dressing of NPK with additional
top-dressings of N and K.
Philippines: Kunarajah [1971] reviewed a large number of experimental results
which indicate average optimum fertilisation at 220 kg N, 100 kg P20 5 and 200 kg
K 2O per hectare, with additional amounts to compensate for soil deficiencies.
South Africa: Meyer [1975] showed in factorial trials that yield is strongly influenced
by each of the major elements (N, P, K). These factorial experiments showed interaction between K and the other elements in that potassium was particulary effective
as regards both yield and quality at high rates of N and P, which tended to affect
quality adversely.
5. Tea
Tea has been cultivated in S.E. Asia since ancient times. World production has more
than doubled in the past twenty five years to 1.6 million tonnes in 1976. During the
337
same period the planted area increased by 54% to 1.545 million ha. There has thus
been a great increase in production per unit area through improved planting material,
better plant protection and mostly through increased fertiliser use. Fertilisers are
more important when tea is grown at low altitude under high rainfall, the soils often
being acid and with a low content of exchangeable bases.
5.1 Special aspects of potassium fertilisation
In comparison with many other crops, and particulary with other crops grown in
the tropics, mineral uptake by tea is relatively low as shown by Dierendonck's [1959]
figures quoted in Table 7.
Table 7. Tea. Nutrients removed in 6 tonne fresh leaves (I tonne made tea)
Country
Sri Lanka ...................................................
Indonesia ...................................................
East Africa .................................................
N
kg
P2 0
K2 0
45
47.5
42
8
9.5
6.8
21
26
24
The young shoots have quite high contents of N, P and K, particulary of N. Growth
is stimulated by N but its one sided use adversely affects soil fertility. Potassium is
important in promoting the translocation of carbohydrate to the young shoots which
are harvested. The best quality teas are produced at high altitude and under these
conditions potassium is valuable in preventing frost injury.
Yield is mainly stimulated by N fertiliser and the N x K interaction is often positive
as we shall see in reviewing results obtained in some of the producing countries.
Soviet Union: Nitrogen fertiliser levels depend upon yield potential of the locality
that is about 150 kg N/ha for a yield of 1000 kg leaf and 300 kg for a yield of 2000 kg.
P and K recommendations are based on soil analysis. No potash is recommended
when soil exchangeable K 2 0 is over 25 mg per cent and on poor soils annual application of 250 kg K 2O is recommended. Potassium deficiency is often seen when soil
potassium is low (less than 15 mg per cent exchangeable). Oniani [1971, 1972] has
evaluated and published the results of many NPK experiments studying effects on
yield and quality. Very high rates of nitrogen (500 kg N and above) cause quality
to deteriorate and such effects can be partially counterbalanced by increasing potassium
application (Table 8).
Sri Lanka: A long term experiment was started in 1938 by the Tea Research Institute
of Ceylon. This was a 3 x 3 x 3 factorial NPK experiment and showed that lack of
potassium produced spectacular results. Mean yields in the last years of the experiment at the highest N level were:
761 kg tea
K0
K,, 1667 kg tea
K.6 1929 kg tea
338
Table 8. Effect of potassium on yield and quality of tea
Fertiliser, kg/ha
N
P2 0, K 20
0
300
300
300
0
150
150
150
0
0
60
120
...........................
...........................
...........................
...........................
Crude leaf yield
kg/ha
% in dry matter
tannin
extracts
2050
4483
6034
7409
22.4
20.0
21.9
22.3
44.0
43.1
44.2
45.4
Under these conditions, the cost of K fertiliser was repaid more than 20 fold. Standard
recommendations in Sri Lanka are for potash dressings from 60-200 kg K 2 O/ha
depending on the productivity of the plantation.
Similar results have been obtained in the other main producing countries - India,
Japan, East Africa, etc., and usual recommendations vary between 60 and 160 kg
K2 O/ha/year.
6. Tobacco
This crop grows rapidly and most of the useful yield is formed in the last two months
of growth. Chouteau /1969] estimated the daily uptake rate at about 1.8 kg K2 0/ha.
Consequently yields are always higher on soils well supplied with K or where K
fertiliser has been applied. K does not only affect yield. It is well known that high
leaf K content improves quality and that high CI content has the reverse effect.
These effects have been known since long ago and nowadays most of the tobacco
treated in Europe has leaf content of over 5% K2 O. This would indicate that they
have been grown on soils properly supplied with potassium. Chouteau [1969] found
a relationship between soil K and leaf K content.
Cl content should be kept as low as possible. In practice up to 1% CI in dry matter
can be tolerated, quality falls rapidly between I and 2% above which level the leaf
is useless. Chloride uptake is almost proportional to the Cl concentration in the soil.
The main sources of Cl are fertiliser and irrigation water. Potash should always be
applied either as sulphate or nitrate with the highest degree of purity practically
obtainable. The use of chloride should also be avoided on the preceding crop and
care should be taken that CI is not recycled in farmyard manure. Where tobacco occurs
frequently in the rotation it is best to avoid Cl containing fertiliser altogether. Water
for irrigation should contain less than 25 ppm Cl.
The influence of potassium is not only seen on yield and burning properties but also
on other quality criteria such as leaf size, specific weight, colour, texture, etc. Table 9
shows effects on leaf size measured by Bowling et al. [1947].
Research workers in Maryland conclude that moderate K dressings are needed for
maximum yield but that the value of the product is greatly increased by applying
much higher rates. In a long term experiment it was found that while 80 kg K 2 0/ha
was sufficient for maximum yield, three times this rate was justified to attain optimum
quality, greatly improving the price received by the grower. Loue [1978] gives results
of French experiments where the optimum return (a combination of yield and price)
was obtained at 300 kg/ha K 2 0 applied as sulphate (Table 10).
339
Table 9. Effect of potassium on leaf area in tobacco (Maryland Medium Broadleaf) (Bowling
et al. [1947])
K2O applied
kg/ha
0
34
135
270
.......................
.......................
.......................
.......................
Upper leaves
Mean leaf area (cm2)
Lower leaves
Middle leaves
97.3
94.4
116.8
113.4
147.4
150.3
182.9
183.0
Mean
135.7
171.0
204.5
220.0
162.5
171.0
204.5
220.0
Table 10. Effect of rate and source of K fertiliser on yield and crop value in tobacco
Yield
kg/ha
Source
Rate
kg K 2 0/ha
S/ha
Price
cents/kg
889
161
18
27
54
81
134
188
296
sulphate
sulphate
sulphate
sulphate
sulphate
sulphate
..............................
..............................
..............................
..............................
..............................
..............................
1002
1014
1081
1056
1045
1070
291
395
492
536
546
586
29
40
46
51
53
55
403
27
188
sulphate ..............................
chloride ...............................
chloride ...............................
1033
1079
1085
563
333
287
55
31
26
0
-....................... ..............
7. General conclusions
The list of plants treated here could easily be enlarged by including fibres, forages,
other grasses and legumes and leafy vegetables and one would arrive at the same
general conclusion, namely that the leafy crops take up and remove from the soil
large amounts of potassium. Potassium therefore needs particular attention not only
to see that the needs of the crop under consideration are met but also to see that
fertility is maintained for the rotation as a whole.
I wish to thank Mr. Hobt of Agricultural Research Station Bdntehof/Hannover for
helping me to prepare this paper, notably through his extensive knowledge of the manuring of tropical crops.
8. References
Adams, W.E. and Twersky, M.: Effect of soil fertility on winter killing of Coastal Bermuda-
grass. Agron. J. 52, 325-326 (1959)
Adams, W.E., White, A.W., Mc Creery, R.A., and Dawson, RN.: Coastal Bermudagrass
forage production and chemical composition as influenced by potassium source, rate and
frequency of application. Agron. J. 59, 247-249 (1967)
Ayres, A.: Variation of mineral content of sugar cane with age and season. Haw. Plant.
Record 37, 197 (1933)
340
Blaser, R.E. and Kimbrough, E.L.: Potassium nutrition of forage crops with perennials. In:
The role of potassium in agriculture. Amer. Soc. Agron. Madison Wisc., 1968
Bowling, J.D. and Brown, D.E.: Tech. Bull. No. 933, USDA (1947)
Burton, G. W.: Can the South become one of the world great greenlands? Progressive Farmer
87 (3), 22-24 (1972)
Calder, F.N. and Me Lead, L.B.: In vitro digestibility of forage species as affected by fertilizer
application, stage of development and harvest dates. Can. J. Plant. Sci. 48, 17-24 (1968)
Chaminade, R.: Experimentation en petits vases de vdgdtation. Ann. Agro. 11, 121-134 (1960)
Chevalier, H.: Influence de la fertilisation azote et potassique sur la r~partition des produc-
tions annuelles d'un ray-grass anglais exploitd en simulation de patures. Ass. Eur. Prod.
Four. Gent, 1978
Chevalier, H. and Quemener, J.: Role of potassium nutrition on the production and composition of cocksfoot. Int. Meet. Animal Production Dublin, 1977
Chouteau: Lectures on tobacco manuring. Inst. Experimental du Tabac. Bergerac, 1969
Demarquilly, C.: Fertilisation et qualit6 du fourrage. Fourrage No. 68, p. 61 (1977)
De Ment, Stanford and Bradford: Soil Sci. Soc. Amer. Proc. 23, 47-50 (1959)
Dierendonek, F.J.E. van: The manuring of coffee, cocoa, tea and tobacco, CEA, Geneva,
1959
Evans, H.J. and Sorger, G.J.: Role of mineral elements with emphasis on univalent cations.
Ann. R. Plant. Phys. 17, 47-76 (1966)
Evans, Rouse and Gudavskas: Low soil potassium sets up coastal for leafspot disease. Auburn
University Agr. Exp. St. Ala., 1959
Fogliara,F. A.: La Industria Azucarera. Vol. LXXXI, No. 944, p. 26-29. Buenos Aires (1975)
Garaudeaux,J.: Etude de ]a composition mindrale de quelques souches de gramindes. Congr&s
Mondial de l'Expdrimentation Agricole Rome, p. 593-597, 1959
Garaudeaux, J., Barry, L. and Quidet, P.: Etude sur la dynamique de la potasse des sols par
microcultures successives de ray-grass d'Italie. Vie Congrs Int. Sc. du Sol Paris IV, p. 20
(1956)
Gartner, J.A.: Queensland J. Agric. Anim. Sci. 26, 365-372 (1969)
Gupta, A.P. and Shuklas, P.: 39th Proc. of the ST.A. of India, Kanpur (1973)
Griffith, G. and Walters, R.J.K.: The sodium and potassium content of some grass genera,
species and varieties. J. agri. Sci. Camb. 67, 81-89 (1966)
Honig, P.: Java - Suikerindustrie No. 10 (1934)
Hopper, M.J. and Clement, C.R.: The supply of potassium to grassland: An integration of
field, pot and laboratory investigations. Trans. Comm. It & IV Int. Soc. Soil Sei. 237-246.
Aberdeen (1966)
Humbert, R.P.: Potash fertilization in the Hawaian sugar industry. Proc. 6th Congr. Int.
Potash Inst., Berne (1958)
Held, G.: Oiber die Wirkung steigender K- und N-Gaben auf den Kohlenhydratgehalt. KaliBriefe, June (1973)
Hendricks, H.J.: Der Einfluss der KCI-Zufiitterung auf das S~iure-Base-Gleichgewicht der
Kiihe. Tijdschr. Diergeneeskunde 86, 1264-1277 (1964)
Kemp, A.: The effects of K and N dressings on the mineral supply of grazing animals. Colloquium Proceedings No. I. The Potassium Institute Henley-on-Thames (England), (1970)
Kunarajah, S.: Hort. Abstr. 41, No. 2, p. 622 (1971)
Load, A.: Le sulfate de potasse, Dossier K2O, SCPA Mulhouse No 11 (1978)
Loud, A.: Compte rendu des essais de fertilisation SCPA, Mulhouse, 1970
Malavolta, E., Haag, H.P., Mello, F. A. F., Brasil Sobr., M. O. C.: On the mineral nutrition of
some tropical crops. Intern. Potash Inst., Berne, 1962
Martz, F. A., Brown, J. R., Das, B. K. and Padgit, D.: Effect of top-dressed nitrogen and potas-
sium on the feeding value of orchardgrass hay for lactating dairy cows. Agron. J. 59,
599-602 (1967)
Mengel, K.: Der Einfluss der D(Ingung auf die Qualit~t von Wiesen und Weidefutter. Bontehof, Hannover, 1970
Mengel, K.: Ernihrung und Stoffwechsel der Pflanze. Gustav-Fischer-Verlag, Stuttgart, 1972
Meyer, J. H.: South African Sugar Journ. 59, No. I1, p. 569 (1975)
Nielsen, J.M.: On the potassium and sodium concentrations in plants. Royal Vet. and Agric.
Coll. Yrbk, Copenhagen (1969)
Nowakowski, T.Z.: Mineral fertilization and organic composition of herbage. Proc. Congr.
Int. Potash Inst., Berne (1964)
341
Oniani, O.G.: Proc. 2nd Interregional Fert. Symp. UNIDO, Kiev (1971)
Oniani, O.G.: Proc. Vllth Fert. World Congr. Vienna and Baden (1972)
Quemener, J.: Annales agronomiques 21, 819-844 (1970)
Reid, R.L. and Jung, G.A.: Influence of fertilizer treatment on the intake digestibility and
palability of tall fescue hay. J. Anim. Sci. 2, 615-625 (1965)
Rochet, M.: Enquete sur les oligo-6l6ments, E.D.E. Alenon 61/France, 1978
Salette, J.F.: Intensification prospects of forage production in the tropics. Colloque AntillesGuyane, 1971
Scharrer, K. and Mengel, K.: fiber das voribergehende Auftreten sichtbaren Magnesiummangels bei Hafer. Agrochimica IV, No. 1 (1959)
Segura, L.G.: Agronomia Tropical 21, No. 5, Maracay (1971)
Thomas, B., Thompson, A., Menuga, V.A. and Armstrong, R.H.: The ash constituents of some
herbage plants at different stages or maturity. Emp. J. Exp. Agric. 20, 10-22 (1952)
Van den Hende, A. and Cottenie, A.: Influence de ]a fumure potassique sur ]a composition
chimique et ]a valeur fourragre des herbages. Proc. 7th Congr. Int. Potash Inst., Berne
(1957)
Vicente Chandler, J.F, Pearson, R. W., Abruna, F. and Silva, S.: Potassium fertilization of
intensively managed grasses under humid tropical conditions. Agron. J. 54, 450-453 (1962)
Werk, D. and Rosenberger: Untersuchungen zum Mineralstoffwechsel des Rindes. Z. Tierphysiol. Tiererndhr. und Futtermittelkd. 25, 125-188 (1969)
342
25th Anniversary of the Scientific Board
11th IPI-Congress held in Bern/Switzerland
from September 4 to 8, 1978
5th Session
Potassium
Fertilization in
Agricultural Practice
343
Potassium Effects in Long-Term
Experiments
G.Drouineau, National Institute of Agricultural Research (I.N.R.A.), Paris/France-
I. Introduction
Knowing what we now do about the behaviour of potassium in soils and plants, about
its role in yield formation and about the varying requirements of different crops, it is
easy to understand that the research worker in the field is confronted with some very
involved problems. The fact that response in terms of yield increase may not be
recorded in a particular year does not necessarily mean that crops do not need potassium on that site. Conversely, an isolated response might be due to the peculiarity of
one season or of the crop under test and should not, by itself, be used as the basis of a
long-term fertiliser policy. Such considerations are particularly relevant in the developing countries or in the low productivity regions of southern Europe where, understandably, there is a great temptation to rely on information derived from experiments of
short duration carried out on a large number of sites but, often, not continued for
more than one season. Such short term work cannot yield reliable information on the
potassium requirements of soils and crops.
Though lacking the fundamental information which is at our disposal today, agronomists in Europe and the USA long ago embarked upon very long term experiments,
though the objectives of these experiments were more general and somewhat different
from those which concern us here. It is not proposed to refer to these experiments in
this paper as they were fully discussed in a recent Colloquium at Grignon (Amberger
and Gutser [1976]).
This paper concentrates more on the effects of potassium in medium term experiments,
lasting 5 to 10 years or so, covering more than one rotation for annual crops. We
shall discuss the conclusions which can be drawn from such experiments, emphasising
the importance of this kind of approach. In doing this, we are not breaking new
ground for IPI who published a Bulletin on the subject (Kemnler and Malicornet
[1976]) in 1976.
* G. Drouineau, former Inspector General of Research (I.N.R.A.), Honorary Director of
Research, 34, rue Debordes Valmore, F-75016 Paris/France
345
2. Recent long-term experiments
2.1 Annual crops
Long-term experiments with annual crops have been concerned with crops grown in
rotation for periods of from 4 to 18 years. Often the experiments have covered several
rotations. One of the aims of such experiments has been to check the value of chemical
analytical methods for evaluating soil potassium status and their value in showing
how available soil potassium levels decline with time on plots where no K fertiliser is
applied. A particular concern has been to find out how it is that certain soils, while not
markedly deficient in K, as shown by analysis, and not responding to K fertiliser, still
yield at a low level. The following publications are interesting:
Barbier et al. [1957] published the results of an 11 year experiment on a range of
crops grown on a loam low in available K at Versailles. Responses to K were not very
large in the early years but increased later. The authors' discussion of K balance is
interesting.
Gething [1971] described an experiment in England in which variable rates of K
fertiliser, in combination with two N levels, were applied to a 5 year rotation (kale,
barley, cut grass, wheat and potatoes) and in which the residual effect of K was tested
on wheat in the sixth year. The results confirmed the common experience on the
variability of K response between crops and showed that K response by potatoes
increased with the passage of time.
Heathcote [1973], at the 10th Colloquium of IPI at Abidjan, gave results obtained over
five years in Nigeria on a rotation including cotton, maize, sorghum and groundnut.
He emphasised the risks incurred in basing recommendations for potash fertiliser on
the results of annual experiments.
At the same Colloquium, Richard [1973] dealt with the effect of various factors on
response to K by cotton, sorghum and groundnut grown in rotation from 1965-1972.
Foster [1972] carried out experiments in Uganda from 1959 to 1968 on 12 different
sites on experimental stations. The crops which were grown in a number of different
rotations suited to the different sites comprised: beans, maize, sweet potato, millet,
sorghum, groundnut and cotton. There was little K response in the early stages except
on very acid soils but the size of the response increased with the passage of time.
Long-term experiments in Sweden on 4 course rotations at 12 sites, started in 1957
and reported by Jansson [1975] should also be mentioned.
The Indian Councilfor Agricultural Research [1977] was well aware of the problem
and has commenced a programme of long term experiments on various crops. Results
have varied with season and differed between crops but, despite the low rate applied,
40 kg/ha K, response by wheat increased between 1972 and 1976.
Brucholz [1976] published results from a complex experiment investigating the N x K
interaction at a number of sites in the German Democratic Republic between 1967
and 1973. The results showed great variability and contributed to a better understanding of fertiliser needs.
346
2.2 Rice
Datta and Gomez [1975] and von Uexkiill [1975] have descr ibed long term NPK
experiments on high yielding rice cultivars which were laid down in the Philippines
in 1968. The results refer to a total of 10 harvests (wet and dry season crops) at 3
stations. K response depended on the levels of N and P applied and increased with the
passage of time. There was considerable difference between cultivars in response to K
and higher responses were recorded on the dry season crops. There was no advantage
in applying K fertiliser in divided dressings.
Velly [1973] describing a N x K experiment in Madagascar reported absence of K
response in the first 2 years though response developed later.
2.3 Herbage crops
Many experiments have been done on permanent grassland and these have demonstrated the effects of K on botanical composition. All here will be aware of the results
obtained at Rothamsted on the Park Grass Experiment. While there is a wealth of
data on K response by cut grass, there is much less information on response by grazed
grass on account of the difficulty of investigating fertiliser effects when the grazing
animal is involved.
Schmitt [1963] described to the Scientific Board of IP at its 10th Anniversary results
obtained in classical experiments on natural grassland in Germany. There was a
marked effect of K by the end of the first 10 years and the response per kg of K applied
increased further during the second 12 years. His evaluation of residual effects was
illuminating.
Drake and Colby [1970] reporting on experiments in the USA and Japan with
grass/legume mixtures (cocksfoot-lucerne and cocksfoot-clover) with PK fertiliser
applied over 4 years said that response to K was slight in the first year but increased
greatly from year to year.
Hanotiaux and Leblanc [1969] carried out NPK experiments for 4 years on an oldestablished ley. They recorded yields, K removals in cut herbage and changes in soil K
content. Significant differences were only recorded after 2 years. Both exchangeable
and reserve soil K declined progressively, where no K was given, and differences
between treatments became significant in the 4th year.
Behaeghe and Cottenie [1976] published results from a longer term experiment at
Gand. K removals were recorded from 1967 to 1971 and correlated with soil analysis.
Plots not receiving K fertiliser became seriously impoverished.
Comparatively little work has been done on tropical forage crops which yield heavily.
Panicum, Pennisetm and Digitaria spp., the main forage grasses in the tropics, give
high yields (33-56 t/ha DM) when adequately fertilised. In a 3 year experiment in
Puerto Rico, Chandler et al. [1962] found immediate and very large K response only
for elephant and Guatemala grasses.
The above are just a few examples from the world literature but they are sufficient to
draw preliminary conclusions about the behaviour of annual crops and grasses in
response to K fertiliser. Response varies with the degree of intensification of agriculture
and with other nutrients applied. Often there is no response in the first year of an
347
experiment and it is dangerous to draw conclusions from 1 year's results which may
be affected by seasonal factors.
2.4 Forestry
There has been much progress in the use of fertiliser in forests in recent years, in which
connection we may recall the INI Colloquium in Finland in 1967. In this area, experimentation is, of necessity, long term, though fertiliser effects are more apparent on
young trees. Reaction to N and P fertilisers are most common but there are references
to positive and progressive effects of potassium also.
Franz and Baule [1962] studied the effect of K application to Picea communis growing
on a reclaimed bog soil. K markedly affected girth and height from the fourth year,
though the rate applied was low. In 1971 the same authors published the results of
a 40 year experiment on Pinus silvestris grown on a sandy podsol; timber weights were
recorded on felling. Franz and Baule also compared the treatments P, PK and PKMg:
on Picea communis and P. sitchensis on a mesotrophic peat soil at Eglof Allgau and
found that K increased the height at 8 years by 150-180%.
De Coignac et al. [1973] gave the results of experiments on leached ferralitic soils in
Madagascar. P had an immediate effect and the effect of K became appreciable from
the fourth year with pines (P. patula and P. khasya).
3. Year to year variation in French experiments
There are very few examples in the literature to illustrate graphically year to year
variation in response to K and the development of K response over time but some
French results, on which discussion can be based, are available. A selection of data
obtained at the Aspach-le-Bas Experiment Station near Mulhouse and from experiments
conducted by SCPA (Soci t& commerciale des Potasses et de l'Azote) at various sites
throughout France is presented diagrammatically in Figures 1-7.
Fertiliser experiments on the N x K interaction were started at Aspach in 1951 when
it was realised that ideas on the potassium requirements of crops might require
modification to suit the increasing levels of N fertiliser which were then coming into
use. The soil is a deep clay-loam, medium for total N and low in both P and K. Details
of the treatments are given by Garaudeaux and Chevalier [1975]. The rotation included
winter wheat, barley, sugar beet, maize, potatoes and Italian ryegrass/red clover Icy.
P was applied at a constant high rate; N varied between 50 and 150 kg/ha and potassium from 0-160 kg/ha K.
The way in which mean annual yields and fertiliser effects varied from year to year in
accordance with weather and other possible factors is illustrated in Figures 1 and 2
for a very K responsive crop (potatoes) and in Figures 3 and 4 for a less responsive crop
(maize); the latter also show the difference in behaviour between 2 cultivars. These
figures amply illustrate the difficulty in drawing generalised conclusions regarding the
real trend of yield and response over the period of a long term experiment and how
behaviour in one, perhaps exceptional, season may be at variance with average behaviour as shown by the treatment yields meaned over the whole life of the experiment
or by the frequency with which a particular effect occurs.
348
tlha
Level N,
40
\K2
K
KO
20
1195455
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70Years
Fig.). Effects of nitrogen and potassium on potatoes (annual yields - tonne/ha tubers)
50 t/ha
L
ALevel N,
I
JAI
40'
41954 55
56
N
A
It
I
57
58
59
60
61
62
63
64
5
6
67
8
9 70Years
Fig.2. Elffcts of nitrogen and potassium on potatoes
(annual yields - tonne/ha tubers)
349
75
t/ha
Level N,
(I
It
W240
1955 56
57
58
INRA 200
59
60
61
6
6364 65
6
6768 69
70 Years
Fig.3. Effect of potassium on maize receiving moderate rates of N (annual yields - tonne/ha
grain [85% DM])
3.1 Potatoes
Figures 1 and 2 show yields obtained at the 3 levels of K under test at high (N,) and
low (NI) N fertiliser levels. Year to year variation in yield and response is great and
in some years, the response to K even by this very responsive crop is virtually nil.
There is a tendency for the yield under the K0 treatment to decline over the period and
for the response to K to become larger - more evidently so at the high level of N. An
experiment on a loamy soil in the Paris area, which had received differential K
dressings for some time before the potato yields were recorded, showed similar variation though K responses were rather more consistent than at Aspach. Mean yield
over the whole experiment tended to increase from 1955 to 1966.
3.2 Maize
Figures 3 and 4 show that the 2 cultivars used for 1961-1966 and 1962-1970 respectively
reacted differently to treatment. In the first period with W 240, mean yields and season
to season variation were small and K did not increase yield at N 1. At the higher N
level there was no yield increase beyond K 2. With INRA 200, yields were higher but
more erratic. Elsewhere, on a soil derived from old alluvium near Pau in the south-west,
maize grown in a rotation of 4 years maize followed by 4 years grass yielded well.
Yields appeared to be maintained by 40 kg/ha/year K, to decline seriously without K
fertiliser and to increase from year to year when 80 kg/ha K or more was applied. The
crop responded up to 160 kg/ha K. In the south-east at Moirans (Isere) on soil with a
high K-fixation capacity (95% by van der Marel's method) and with a high potential
350
tlha
t/ ha
01
7.5
'
Level N,
57 58
59 6b
61
62 63
KO
67 68 6
70 Years
200
,INRA
240
30W
1955 56
•
6&. 65
i6
Fig.4. Effect of potassium on maize receiving high rates of N (annual yields - tonne/ha
grain [85% DMA)
apart from its lack of K, potash fiertiliser was effective from the start, but, unless
fertiliser was applied in quantities far exceeding removal of K in harvested crops (i.e
at 190 kg/ha K) the yield tended to decline due to fixation of applied K (Lotu5 [1977]).
3.3 Wheat and barley
Capelle was the cultivar grown at Aspach. N increased the yield appreciably but the
year to year variation was very much greater at the high N level. Winter barley (Rika)
gave results similar in all respects. The effect of K was relatively slight but significant
on yield meaned for the whole period. It should be noted that in these experiments,
cereal straw was ploughed in, thus minimnising the drain on soil K. At the site in the
Paris area referred to in 3.1, response to K by wheat was more consistent though there
was considerable year to year variation in yield.
Garaudeauxand Chevalier [1967, 1975] carried out a full statistical evaluation of the
results of the Aspach experiments and have discussed the results thoroughly, particularly as concerns the N x K interaction and its effects on yield and crop quality. This
type of long term work leads to conclusions of real practical value.
3.4 Grassland
Results obtained at Aspach from a long-term experiment with cut grass (cocksfoot)
where all herbage was cut and removed demonstrate that it is essential to measure
351
effects over a long period and that the effect of K is very much bound up with the level
of N fertiliser used. In this N x P x K factorial experiment, grass was cut on a routine
designed to simulate grazing and yields were recorded from 1968 until 1975. A uniform
fertiliser dressing was applied at sowing down the Icy, and thereafter, 4 rates of each
N and K were applied. N raised yield significantly from the start of the trial; the effect
of K became significant in the second year. When no K fertiliser was applied, yields
declined from year to year, the more rapidly the more N fertiliser was applied. K
content of herbage moved in parallel with yield (Chevalier and Qutmener [1977];
Garaudeaux and Chevalier [1976]). The fertiliser treatment affected the seasonal
distribution of yield. Similar results were obtained with perennial ryegrass which is
less well adapted than cocksfoot to local conditions in Alsace. Environmental factors
affected the grass at different physiological stages in different years making the interpretation of yields and responses in individual years difficult.
It is not easy to confirm results obtained under simulated grazing by grazing experiments involving real animals and the difficulty involved in this type of investigation
accounts for the paucity of results. Suffice it to say that 80 or 90% of K ingested in
herbage is returned to the soil in urine. However, this return of K is very uneven, being
concentrated at points and at rates corresponding to I or 2 tonnes of K 2 0 per hectare
which, except on very well buffered soils, is conducive to loss by leaching (Andri
[1974]). Microplot experiments estimated the efficiency of urine K at only about 50%
of that of fertiliser K (Garadeaux el al. [1975]).
4. Allocation of K fertiliser among the crops of the rotation
It takes a considerable time - at least 3 rotations of 3 crops - to compare the efficiency
of K applied in a single dressing for all the crops as compared with annual application.
Results from an experiment of this type at Sancourt comparing the same total amount
of K applied either at the beginning of the 3 year rotation or to each of the crops in
3 equal dressings are illustrated in Figure 5. It would be difficult to draw any firm
conclusion even though the trial continued for 15 years.
5. Perennial crops
For these, as remarked by Kemmler and Malicornet [1976] it is senseless and, in any
case, impossible to base fertiliser recommendations on results from a single season.
5.1 Vine
An experiment was laid down on 3 year old vines (S~millon de l'Entre-Deux-Mers) at
Targon in the Bordelais. It continued for 13 years up to 1967 and the results were
published by Lou [1968]. Up to the end of the fifth year K had only small and
insignificant effects on yield of wine but after that it increased yield considerably. There
were significant effects on leaf K content from an early stage (Figure 6). Another
experiment was laid down in the south at St. Gilles du Gard in 1962 on 1 year old
352
Relative
120 yield
1
118
---
116
R, -3/3 0 0
R, - 1/3 1/3 1/3
I'
112
108
J
106
104
102
Fig.5. Alternative systems of applying potash to a rotation
-R,
=
3/ 3
R2 ='/,
140 -Nhina
0
0
V
V/
191
130
120
110
NF'K
100
90
80
60
40
30
20
10
195556 57 58 59 60 61 62 63 64. 65 66 67
Fig.6. Effect of potassium on vines at Targon
353
vines planted in the previous year on a soil low in available K to test the effects of K
applied at rates from 0 to 400 kg/ha on yield and quality. Yield varied greatly from
year to year and it was only after 5 years that K began to have a marked effect on
yield. The yield increase was achieved without any adverse effect on quality.
5.2 Fruit trees
An experiment at Aspach on apples (Belle de Boskoop) measured the effect of K
applied annually at 0, 150 and 300 kg/ha K with N and P applied uniformly. It was
found that the originally chosen rate of N fertiliser was too low. Care was taken to
ensure uniformity of the stand, the scions being all of the same clone and the rootstocks were marcots all from the same tree. The orchard was planted in 1949 and
yield recorded up to 1969 after which the trees were grubbed out. K increased fruit
yield in I t out of 15 years but varying weather between years altered the size of the
response. Response was large up to the 12th year but decreased in the last years of
the trial. Complete fertiliser increased fruit size and decreased the tendency to biennial
bearing (Figure 7).
5.3 Tropical tree crops
I am indebted to Ochs and his colleagues [1976] for data on variation in annual yield
in coconut and oilpalm. Figure 8 illustrates results from a factorial experiment designed
to study K-Mg antagonism in adult coconuts which received uniform fertiliser high
in K at 2 kg/tree/year up to 1968 when the experiment started. K increased copra yield
up to the highest rate tested (3 kg/tree/year KC); there was considerable year to
year variation in mean yield and response. Similarly large variation was found in
oilpalm (Figure 9). Rates of K fertiliser applied are given in the figure; there was no
zero rate of K which, on the coastal sands of the Ivory Coast, would have no practical
meaning.
6. Potassium balance
Nutrient balance sheets are discussed alsewhere in this publication (Dam Kofoed
[1978]; Cooke and Gething [1978]) but, since the study of such balance sheets is an
important aspect of long term field experiments, this paper would be incomplete
without some mention of this topic. The potassium balance is particularly important
since large quantities of this element are involved in the nutrient cycle, and the establishment of K balance sheets is of great interest to the agronomist and has important
practical implications for advisory work.
Quite frequently, discussion of nutrient balance is confined to consideration only of
the amount of nutrient applied as fertiliser and the amount removed in harvested crop.
Such an over-simplified approach can lead to false conclusions. Sometimes calculations
have been based on average nutrient contents of crops rather than on careful analysis
of crop samples. Losses in drainage may be ignored as may leaching of nutrient from
the mature crop. Account must be taken of nutrient returned to the soil in crop
354
t/ha
i
40
I
I
N
,
Ki
\
\
.
.:'I
30
K2
--. _
20
54
55
56
57
58
59
6061
62
63
65
64
66 67
68
69 Years
Fig. 7. Potash manuring of apples (Belle de Boskoop) (annual yields 1954-1969 [tonne/ha])
kg ccraltree/ann
28
27
26
0
25
24
,\\
22~Ii
23/
K,<'""
2//
KCIappd prtrnam
18
69-7D
10
70-7131CO
11
I
. K2
* K3
1,50
2,25
Is
"
x K1
075
17
.I*%
A'
12
!I
72:-n
77
74-75
472
75-76
13
14
15
16
7-7
17yr:
octio n
s
Ae of lte
Fig.8. Effect of potassium on coconut (Local W.African), IRHO (C6te d'lvoire)
355
140 kg bunches/ treelann(1)
-
130
/
.\
,/z/K,
120
110
--'
\
•
100
90
kg KCltreeiann
so
1968/72
0,5
1973177
0,750
*Ki
1,50
1,50
K2
3,00
3,00
70.
xKO
60
50-o
4030
71-72
67-68 68-69 69-70 70-71
7
6
5
4
3
(1)
Trees considered adult at 6years
72-73
6
75-76
11
74-75
10
73-74
9
Production season
76-7
12years: Age of trees
Fig.9. Effect of potassium on oil palm, IRHO (CCte d'lvoire)
Year Crop
K applied - K removed (cumuLative)
1955 BarLey
1956 Potato
1957 Wheat
1958 RG/R.Clverley Istyr.
1959. Bary
1960 S.Beet
1961 Maize
1962 RG/RC ley not cut
- Level N.
.----Level Ni
ff
1963 RG/RC ley 2r" yr.
1964 Potato
1965
1966
1967
1968
Wheat
Rape/maize forage
Maize
• /
S. Beet
•
1
1
'
,"
--.
/
$'
1969 RG/RC1Pt yr
1970 RG/RC
1971 Bar,"
(/
KO-1174 KO-1031
kg K2O/ha
c
/
-1000
-500
K2.835 K2.1111
Ki.59
K1-290
0
500
Fig. 10. Effect of N and K fertilisers on K balance over a series of crops
356
1000
residues and in animal manures. There are difficulties where livestock are involved.
Account must also be taken of soil heterogeneity and of differences in the rooting
habits of the various crops. Soil analysis in long-term studies is frequently confined to
measuring only exchangeable K; more complete analysis is necessary, but this presents
difficulty.
However, despite the difficulties involved and the large amount of detailed work which
is called for, the study of balance sheets is well worthwhile and there is available a mass
of data from experiments extending over several years where crop and soil analysis has
been carried out year by year on individual plots. It has for long been known (Barbier
et al. [1957]) that crop K content increases as K supply is improved, thus K removal
in the crop increases regularly and strongly up to a limiting value as K fertiliser application is increased. For this reason, the K balance in experiments on forage crops is
sometimes negative over all rates of K applied, while, where insufficient K is applied,
the loss of K from the soil may amount to hundreds of kilogrammes per hectare.
The rate of N usage greatly influences the K balance since by raising yield N also
increases K removal in harvested crops. The resulting effect on K balance is illustrated
in Figure 10 showing the development of the K balance on an experiment carrying a
range of crops over a period of 16 years; while K, was sufficient to maintain the K
balance at N 1, at the higher N level there was a K deficit at the end of the series of
crops unless more K (K,) was applied.
7. Conclusion
The evaluation of results from long term experiments presents some problems. The
main data can be summarised by calculating average effects and for this it is useful, in
order to be able to state the results concisely, to use a mathematical treatment which
will express the results obtained by testing fertiliser applied in discrete steps (for
experimental convenience) as a continuous function of the amount of nutrient applied.
Such a method allows the calculation of optimum fertiliser rates in terms of both yield
and economic return. LouS [1978] discusses the use of the parabolic function, which
has been widely used by SCPA, elsewhere in this volume. Some workers have been
critical of the use of such simple mathematical functions to describe average responses
and have suggested alternatives, and some aspects of the problem are discussed by
Cooke and Gething [1978] in this volume. It will be clear from the discussion above
of the great year to year variability of fertiliser effects that statements of average
response derived from long term experiments, while they are useful and, probably,
the best basis for advisory recommendations which we have at this time, conceal much.
There is a need to search for causes of such variation in yield and fertiliser response.
Greater knowledge of the causes of variability should be one of the aims of long term
experimentation and would enable us to improve fertiliser advice. It should also be
pointed out that changes in market conditions (fertiliser and produce prices) greatly
affect the profitability of fertiliser and such changes must be taken into account in
evaluating long term work.
Clearly, the effects of a nutrient like potassium should not be considered in isolation
(i.e. on the basis of main effects only) but they should be related to the levels of other
factors. LouS [1978] discusses interaction effects. Clearly, the N x K interaction is of
357
the greatest importance in long term studies because the enhancement of yield through
N fertiliser brings in its train an increased demand on soil K reserves and, consequently,
a risk of soil impoverishment. Long term factorial experiments have been carried out
in a period conspicuous for spectacular improvement in crop yield and agricultural
techniques and have contributed much in showing how higher yields increase the
demand for potassium. Only long term experiments could have achieved this result. The
fact that such experiments are particularly relevant to the farmer who farms the same
piece of land for at least a generation hardly needs emphasis.
Long term experiments have proved to be of great practical value over the past
25 years or so. It is only to be regretted that they have been too few and insufficiently
widely spread over different soils and different cropping systems. Knowing what we
do about how potassium behaves in soils and the varying needs of different crops we
should be well aware that widely scattered annual experiments which can take no
account of the effects of treatments on soil fertility or of the intensity of the farming
system, are no substitute for the patient long term investigation.
It is deplorable that the officials of Ministries of Agriculture, of government research
services and even those professionally occupied in the agricultural industry have so
frequently recoiled in the face of the effort needed to undertake and maintain long
term work. Admittedly the work is expensive and demands continuity and these may
be reasons why the present-day research worker lacks some enthusiasm for it. It may
be thought that the rapid advances being achieved in plant breeding, which result in
the adoption of new varieties of greatly increased potential and which are more
demanding for nutrients, introduce a further difficulty. But, we should be well aware
that there is no substitute for the type of work we advocate. Results achieved so far
may not give the final answer and may not be universally regarded as being of great
fundamental consequence but they will be of immense practical value in establishing
fertiliser policies for the future which are well adapted to current crop varieties and
farming systems. It would be tragic indeed if those countries which have madeefforts
in this field were not to continue their endeavours. It is understandable that many
countries may lack the means to carry out a full experimental programme and they
need help to do so. The experience we have so far acquired points the way to overcoming the difficulties inherent in this type of work.
8. References
Andr, J.P.: Ann. Agron. 25, 425-438 (1974)
Anonymous: C.R. Experiences de tr6s longue durde. Ann. Agron. (1976)
Barbier, G., Tendille, C. and Trocm, S.: C.R. Acad. agr. Fr. (1957)
Behaeghe, T. and Cottenie, A.: Potash Review, Subject 4, No. 60 (1976)
Brucholz, H.: Kali Mitteilungen, Sondershausen, 21-1 (1976)
Chandler, J. V., Pearson, R.W., Abruna, F. and Silva, S.: Agron. J. 54, 450-453 (1962)
Cooke, G.W.and Gething, P.A.: Changing concepts on the use of potash. Proc, 11 th Congress
Intern. Potash Inst., Berne (1978)
Chevalier, H. and Quemener, J.: International meeting on animal production from temperate
grassland, Dublin, 1977
Coignac, G.B. de, Bailly, C. and Malvos, C.: .U.F.R.O. Paris, 1973
358
Datta, S.K. de and Gomez, K. A.: Soil Science 120 (1975)
Drake, M. and Colby, W.G.: Potash Review, Subject 16, No. 47 (1970)
Foster, H.L.: Potash Review, Subject 4, No. 52 (1972)
Franz, F. and Baule, H.: A.F.Z. (1962)
Franz, F. and Baule, H.: Der Forst, 4 (1971)
Garaudeaux, J. and Chevalier, H.: Pub. Services agronomiques, S.C.P.A., Mulhouse, 1967
Garaudeaux,J. and Chevalier, H.: C.R. Acad. Agron. Fr. 746-749 (1975)
Garaudeaux, J. and Chevalier, H.: C.R. Acad. Agron. Fr. (1976)
Garaudeaux, J., Chevalier, H. and Pfitzenmeyer, C.: C.R. Acad. Agron. Fr. (1975)
Gething, P.A.: J. Sci. Fd. Agric. 23, 501-508 (1972)
Hanotiaux, G. and Leblanc, W.: Bull. Rech. agron. Gembloux, N.S. 4, No. 3-4 (1969)
Heathrote, R.G.: Proc. 10th Colloq. Intern. Potash Inst., Berne (1973)
Indian Council of Agricultural Research: New Delhi, 1977
Jansson, S.L.: Potash Review, Subject 4, No. 59 (1975)
Kemmler, G. and Malicornet, H.: Fertilizer experiments- the need for long term trials. IPI-Res.
Topics, No. 1. Intern. Potash Inst., Berne, 1976
Loud, A.: Dossier K.O. S.C.P.A., Mulhouse, 1977
Loud, A.: The interaction of potassium with other growth factors. Proc. 11 th Congress Intern.
Potash Inst., Berne (1978)
Ochs, R. and Ollagnier, M.: Proc. 13th Colloquium Intern. Potash Inst., Berne (1977)
Richard, L.: Proc. 10th Colloquium Intern. Potash Inst., Berne (1973)
Schmitz, L.: Potash Review, Subject 30, No. 18 (1963)
UexkII, H.R. von: Potash Review, Subject 30, No. II (1975)
Velly, J.: Proc. 10th Colloquium Intern. Potash Inst., Berne (1973)
359
Changing Concepts on the Use of Potash
G. W.Cooke, Agricultural Research Council, London*, and P.A. Gething, Potassium Institute
Ltd., Henley-on-Thames/United Kingdom**
1. Introduction
In 1976 the World's farmers used four times as much potassium as in 1953. While much
of this increase was to meet a crude, and clearly justified, need for much K to be
applied where little or none was used before, scientific work on potassium relationships
in crops and soils has played a vital part in showing where and how K fertilisers should
be used. At our first Congress in 1954, we recognised shortcomings in analytical
methods, in knowledge of the relationships of K in soils and its functions in plants,
and in our concepts of the role of K in farming systems. Research has filled many of the
gaps and we are now better able to apply logical principles to crop nutrition and to
use costly fertilisers more efficiently. This paper outlines some of the advances in
knowledge and in concepts that have been particularly useful.
The application of thermodynamics to the problems posed by the behaviour of nutrients
in soils and uptake by plants has given us improved understanding and has led to the
abandonment of empirical methods of analysis. Crop nutrition has been increasingly
recognised as a dynamic process and the need has been realised in making fertiliser
recommendations to take into account the overall effects on soil fertility and to consider
the effects of fertiliser policy and cropping system on the nutrient cycle. Increased
attention is now paid to the results of long-term experimentation and there is less of
a tendency to think in terms only of immediate gain when making fertiliser recommendations. Fertiliser recommendations continue, however, to be largely based on the
results of field experiments; there have been significant advances in the interpretation
of such experimental results which are resulting in more reliable fertiliser advice.
Nowhere in the world are presently realised average crop yields anywhere near the
yields which physiologists and agronomists have proved to be possible. The greatest
immediate contribution of agricultural science to world food production must be from
narrowing the gap between achievement and potential.
Scientific advance since 1953 has done much to equip today's farmer to make better
use of fertiliser but there is still much to be done. It seems likely that further advances
of the greatest significance for world farming will come from the development of
*
Dr. G. W. Cooke, Chief Scientific Officer, Agricultural Research Council, 160 Great Portland Street, London WIN 6DT/United Kingdom
SP.A.Gething, Dip. Agric., AICTA, Head, The Potassium Institute Ltd., The Old Post
Office, Nuffield, Henley-on-Thames, Oxon RG9 5SS/United Kingdom
361
dynamic mathematical models which can fully exploit our improved knowledge of the
processes which govern the behaviour of potassium in soils and in plant nutrition and
which can forecast not only the effect of fertiliser treatment on the immediate crop
but its effects on the future potential of the whole production system. The purpose of
such models will be not only to ensure the best possible returns from using fertilisers
but also to indicate how the nutrients in the whole crop-soil system may be conserved
and used to the best advantage.
2. The effects of fertilisers on plant growth and yield
2.1 The interpretation of fertiliser experiments before 1953
Until 1953 the basis of British fertiliser recommendations was Crowther and Yates'
[1941] paper which used the Mitscherlich equation to summarise data from large
numbers of experiments done in Britain and Europe. Recommendations for wheat
and potatoes emerging from this work are listed in Table 1. Crowther and Yates
were unable to take account of interactions as most of the experiments they used did
not measure them.
Table 1. Most profitable fertiliser dressings, the crop responses expected and the amounts of
fertiliser actually used, in 1941-1942
Cereals
N
Most profitable dressing ..............
Response expected from crop .........
Dressings used by farmers ............
kg/ha
78
kg/ha
730
kg/ha
20
Potatoes
P2 05
K20
30
13
75
25
23
0
N
P20 5
kg/ha
166
114
t/ha
5.0
3.8
kg/ha
61
78
K20
181
5.0
51
2.2 Nutrient interactions
So long as farmers used less fertiliser than was recommended, deficiencies in the
mathematical treatment were unimportant but, as farmers' dressings increased, it
became more important to take account of local conditions and of interactions between
the major nutrients.
Boyd [1956] drew attention to the importance of interaction effects in describing the
results of early factorial experiments. The effects were complex, not related to the
magnitude of the main effects and, most important, they did not fall off when fertiliser
rates were increased as did main effects. This is demonstrated by a summary from
131 experiments on sugar beet:
362
kg/ha sugar
Response to:
At lower rates ...............................
At higher rates ..............................
Interaction
N
K
NxK
406
580
163
303
45
129
Interactions were small at low rates but very important at high rates and the best
returns from one nutrient were obtained at high rates of the others. As Figure 1 shows,
interactions altered the shapes of the response curves.
Yield of sugar
t/ha
x K2
50
a K,
x
a
o
4 .5
kg k/ha
I
o--o
Ko
0
A-A
K1
62
x K2
124
x-
0
4.0'
50
100
N applied, kg/ha
Fig.I. Effects of increasing K on the response of sugar beet to N
363
2.3 Response curves
Boyd [1956] pointed out that the constant 'k' in the Mitscherlich equation was not
constant and became smaller as the rates of other nutrients were increased. Testing
low rates of fertilisers in experiments and fitting results to standard response curves
gave misleading information on responses; optimum dressings were probably higher
than Crowther and Yates had suggested.
In 1961, Boyd again discussed fertiliser response patterns. He had found that relationships were often parabolic, the form of the response being affected by other nutrients,
previous cropping, plant population and disease. He suggested that experimental
design should be altered to explore interaction effects more thoroughly and that higher
rates of fertiliser should be tested.
2.4 Linear response relationships
At Antibes in 1970 Boyd discussed the results of work done in the sixties, coming to
the conclusion that the curvilinear form of response was the result of averaging
together results from large numbers of experiments done on different soil types and
under varying cultural conditions. However, when experiments were grouped by soil
series or (for N) by crop rotation, the response was best represented by an almost
linear steeply rising portion with a sharp transition to a second linear portion where
yield changed little or slowly decreased. The results of the most recent experiments on
response relationships have been discussed by Boyd et al. [1976] and confirm that
response is best represented by two intersecting straight lines and that where smooth
curves offer the best fit, the crops were affected by disease.
This work has profound implications for fertiliser recommendations. Forcing the data
to fit exponential relationships consistently overestimated the amount of fertiliser
needed. Testing few levels, and fitting a quadratic equation to the results, as is commonly done, may overestimate both the N needed for maximum yield and the likely
loss from exceeding this amount.
The idea that crop responses have the form of linear segments is not new; it was
proposed by Liebig in 1855 and repeated by Blackman in 1905. Liebig's 'Law of the
Minimum' supposed that dressings of the most limiting factor (A) were used with
maximum (and constant) efficiency to a point where factor B limited growth. After
applying B growth continued until a third factor, C, set the limit. In some circumstances giving B does not alter response to A, in others applying B increases response
to A. In Figure 2 the upper part (2a) shows the general relationship for factors AB,
and C where response to A is at constant rate. Figure 2b shows a specific example
quoted by Boyd et al. [1976] where response to N was increased by giving K to
potatoes.
The practical importance of ascertaining the correct response relationship is shown
by Figure 3 (adapted from Boydet al. [1976]) which relates yield of barley to amount
of N applied. If the quadratic form of response is fitted yields near the turning point,
and from the largest dressing, are underestimated, those without N and beyond the
turning point are overestimated.
364
Fig. 2a
C limiting
Yield
limiting
/B
Factor A
18
With 100 kg/ha of K
Fig. 2 b
16
Yield of 14
potatoes
t/ha
12
Without K
fertiliser
10
8
50
25
N applied kg/ha
75
Fig. 2a. Theoretical example
Fig. 2b. Results of potato experiment
Fig. 2. Illustrations of the law of limiting factors
Yield of
barley (tlha)
95
138
4.5[
4.0[
3.5
3.0
[_________________________
0
150
100
50
N applied (kg/ha)
200
Fig.3. Effiect on optimal dressing of N fertiliser or fitting two straight lines (V) or quadratic
equation (y) in barley experiments discussed by Boyd et at. [1976]
365
2.5 Experimental design
Boyd [1973] outlined the changes that should be made in experimental designs to
provide adequate information on the needs of crops for fertilisers on different soils
and in different farming systems:
1. Small blocks of few treatments should be replaced by fewer large blocks with many
treatments.
2. Less emphasis should be given to fully factorial arrangements, to confounding and
to high replication.
3. Many factors should be tested at many levels.
4. Experiments should be repeated at many sites in several years to discover why
responses to fertilisers vary from site to site, and from year to year.
5. Fractional replication of only a proportion of all possible treatment combinations
is often justified.
6. Factors that influence estimates of error should be assessed and experiments
modified accordingly.
Yields are built up by interaction effects which must be explored in multifactorial
experiments, levels tested must reach the peak of response relationships. Viets [1977]
has commented that although the principles of factorial design were worked out in
the 1930's arguments about their use persisted into the 1950's. It is only in the last
twenty years that they have been accepted all over the World. Mistakes made in the
trial and error development of experimental designs have been:
1. Indiscriminate averaging of inaccurate experiments.
2. Testing too few rates of fertiliser.
3. Too much attention to the Law of Diminishing Returns.
4. 'Undue reverence for statistics' (Boyd [1973]).
To distinguish between models for crop growth needs accurate experiments testing
many fertiliser rates on responsive crops.
The subject is still expanding and references to ideas now being developed are given
by Boyd et al. [1976]. Further studies on response relationships are essential for
planning fertiliser recommendations. They are even more important for planning
research to achieve in practice the yields for which we know our crops have the
potential.
3. Measuring the soil's ability to supply potassium
3.1 Historical
Though factors affecting the availability of soil potassium to crops are discussed in
an earlier paper in this volume (van Diest [1979]) it is appropriate to mention briefly
here how advances in this field have affected fertiliser advice.
When I.P.f. was formed in 1953, methods of soil analysis in use were largely empirical
even though years before Hoagland and Martin [1933] had formulated many of the
ideas which we still recognise. They concluded that 'plants derive their supply (of K)
366
directly from the solution phase of the soil, which is in dynamic equilibrium with the
solid phase of the soil on the one hand and with the plant on the other.' They emphasised the dynamics of soil K relationships and the need to consider crop physiology
for the understanding of the uptake of nutrients. In 1953 it was realised that the
arbitrary methods led to poor predictions of fertiliser response and much time and
effort had been wasted in efforts to improve correlations by adjusting the details of
the methods. It later became apparent (Williams and Cooke [1962]) that good
mathematical relationships between chemically soluble K in soil and responses to
fertilisers could not be expected. At the 1954 Congress of I.P.I., Wiklander and Morani
discussed the problems involved in determining soil K as they were then understood.
Later research has confirmed exchangeable K as the most useful single measurement
and empirical methods have been abandoned. The analytical problems of measuring
K in dilute solutions have been resolved by the development of rapid instrumental
methods.
3.2 New concepts of availability
Notable advances were made in the ten years following the first I.P.I. Congress. The
concepts of'intensity' (1) (measured by the chemical potential of the ion) and 'quantity'
(Q) (amount in the exchangeable pool) originated in Schofield's [1955] work with
phosphate and Woodruff's [1955] with potassium. Ion mobility was stressed by Bray
[1954] and he emphasised that the processes of solution of ions, their movement
towards the root and the rate of growth of the latter were all important; that the
system was dynamic and, being so, never came to the equilibrium corresponding to
that of chemical extraction of the soil. Barber [1962] showed that ions move to the
root by both mass flow and diffusion and Olsen et al. [1961, 1962] described the
diffusion process mathematically. In recent years, these new concepts have been
integrated in models of ion movement in soil and to roots (Section 7).
It became accepted that soil K status was better defined by studying Q/I relationships
and this development has been well described by Russell [1973]. Beckett [1964]
developed the use of Q/I relationships in which the slope of the relationship between
Q and I expresses the amount of K which must be added to the soil to increase the
activity of K + in the soil solution by one unit. This is often referred to as the 'buffer
capacity' or 'buffer power' (Nye and Tinker [1977]). At the ninth Colloquium of
I.P.I., these concepts were discussed and Schroeder [1974] described the application
of the three parameters, now accepted as describing soil K status - Q, I and 'mobility'
(rate of transport to a given root area), emphasising that they matched the dynamic
processes of nutrient uptake.
Good relationships between free energy of ion exchange and K uptake have been
obtained in short-term pot experiments but there was no gain over using exchangeable
soil K in field conditions. Johnston and Addiscott [1971] examined equilibrium activity
ratios, equilibrium K potentials and buffer capacities of soils from long-term experiments at Rothamsted (clay loams) and Woburn (loamy sands). All were related,
non-linearly, to differences in exchangeable K. There was no gain from using the new
parameters and 'simple quantity measurements such as exchangeable K should be
adequate to give advice on K availability.'
367
3.3 Non-exchangeable potassium
Though ideas on the mechanism of fixation and release of K by clays have clarified,
there are still some intractable problems. There is no agreed method, soundly based
in theory, to measure fixed but potentially useful K. Arnold [1964] suggested that the
size of clay particles was important but this was not confirmed except by Darab [1974]
who found that useful K reserves in Hungarian soils were correlated with particle size
distribution of clay. Grimme [1974] emphasised the importance of rate of release in
maintaining K flux to a high yielding quick growing crop. Moderate dressings of K
fertiliser applied to some soils may only fill spaces in clay mineral interlayers. Fractionation of non-exchangeable K with tetraphenyl boron has been proposed. Talibudeen
[1972] used cumulative extraction with Ca-saturated resin, but, as yet, no practicable
suggestion has been made for routine use of such measurements, and the only means
of assessing potentially useful reserves that are not exchangeable is by extraction with
strong acids, by exhaustive pot culture or by growing crops in the field.
3.4 Tropical soils
These were discussed at the 10th Colloquium of f.P.f. at Ahidjan in 1973. While young
soils contain primary minerals which weather quickly enough to supply much K to
plants, tropical soils generally lack 2 :1 minerals in their secondary clays so that they
lack a mechanism for fixing and releasing K: thus K fertilisers are rapidly leached from
uncropped land. Acquaye [1973] and other workers found that exchangeable K was
best related to crop uptake of K and the use of activity ratios; Q/[ curves and buffer
capacities, even when Al concentration was taken into account in very acid soils, were
of no greater practical value. Severe deficiency was likely when exchangeable K was
40 mg per kg or less; limits for response to K fertiliser varied with crop and soil from
60 to 140 mg per kg and little response was expected if exchangeable K was above
200 mg/kg. (Similar limits to those set for cereals in temperate soils).
In measuring the value of non-exchangeable reserve K, Dabin [1973] found that
exchange resins and boiling nitric acid were useful but the best values were given by
continuous cropping in pots. Sobulo [1973] found the new parameters of little use
and relied on combining data for exchangeable K with K extracted by hot nitric acid.
For short term crops, exchangeable K alone was satisfactory.
3.5 The present position
New concepts developed in the past 25 years have improved understanding but have
contributed little to improving fertiliser advice for practical farming. Walsh [1972]
gave a pessimistic assessment of practical progress but we have in fact advanced, if
only in that empirical methods have been largely abandoned. Exchangeable K gives
the best measure of the amount of K immediately available to the next crop and
accurately reflects changes caused by the balance between losses and gains in cropping
systems; conclusions which are applicable alike to temperate and tropical soils.
Unfortunately we do not yet have an accepted laboratory method with a sound theoretical basis for forecasting the rate of release of K from non-exchangeable reserves.
368
Rate of release is particularly important in determining whether exchangeable K is
replaced as it is used, so that flux of K is maintained, or whether it will collapse after
a few crops have been taken. Further work on Q/J curves, and on 'chemical cropping'
with exchange resins, related to soil type, may yet be rewarding. For the present it
seems that we must be content in the laboratory to estimate fixed K that is potentially
useful by hot nitric acid extractions. Nemith's [1972] electro-ultrafiltration technique
may prove to be useful.
3.6 The future
Probably the practical returns from the basic work resulting from applying thermodynamic principles to soil systems, begun 25 years ago, will come from the development
of models that lead to more efficient fertiliser recommendations. The concepts of
intensity and quantity, and the relationships between these parameters, provide data
of the kind needed for making use of models of ion movement in soil, and of uptake
by plants. Fuller rewards by wider extension of models to all important soils, will
come from establishing relationships between Q, I and Q/t curves, and soil classes.
4. Deficiency symptoms and plant analyses
4.1 Historical
Visual symptoms were comprehensively illustrated by Wallace [1951] who described his work to the 1956 Congress of J.P.I. When such symptoms in annual plants
were well developed, the crop was already seriously damaged and it was too late to
give fertiliser. For a permanent crop there is opportunity to remedy the deficiency and
apply K for future years' production. In intensive agriculture shortage of K should
never be allowed to become so acute that deficiency symptoms occur, if they do large
losses are inevitable. Walsh [1954] concluded that deficiency symptoms help advisers
where fertiliser use is very small, or with special problems of nutrient unbalance. This
assessment is still true and leaf symptoms are of most use to those who advise on
intensifying agriculture in areas where there is little scientific information on crop
requirements.
Plant analyses were discussed by Walsh in 1954; he said that the adviser must recognise
that nutrient concentrations indicate conditions within the plant which may or may
not reflect soil characteristics.
4.2 Perennial crops
Plant analyses are particularly useful in guiding the use of fertilisers on perennial tree
crops where few long-term field experiments can be done and where it is difficult to
interpret soil analyses. They are also useful for assessing the nutrient status of quickgrowing leafy crops such as sugar cane. Several papers to the 1970 Congress assessed
the current status of leaf analysis. Malavolta [1970] was critical of its value for
advising on fertilising coffee in Brazil, though it was useful for sugar cane. Successful
369
use of leaf analyses as advisory aids was described by Bellis [1970] for rubber, and
by Ng [1970] for oil palm.
4.3 Annual crops
Several papers in our Proceedings show that leaf analyses are much less useful for
annual crops than for perennials. Bar-Akiva [1970] considered that, although satisfactory for aiding the intensification of under-developed agriculture, plant analysis
was too insensitive to improve fertiliser efficiency in intensive agriculture. Assays for
a component of an enzyme system may offer a new approach and Bar-Akiva described
the use of the nitrate reductase test as an example.
Plant analyses can be important when they are used to guide work that is planned to
increase productivity. For example Widdowson et al. [1974, 1975] showed how
increasing leaf K content was related to increasing yield of potatoes; upper leaves in
early July must contain at least 3.5% K in leaf dry matter if yields are not to be
limited by K-deficiency. It is notoriously difficult to plan K-fertilising of grassland;
supply of K must be sufficient to avoid losses in yield, but luxury uptake must be
avoided. Soil analyses are not sensitive enough to indicate when K-fertiliser is essential
and systems have been developed to use % K in grass as a guide (Section 9).
4.4 The present position
Visual symptoms are a reliable guide to abnormal nutrition but they refer to severe
deficiencies which should not be allowed to occur in intensive farming. Much progress
has been made in the last 25 years in the empirical application of leaf analyses to guide
manuring of perennial crops; but we have probably reached the limit of the advances
that are possible by these simple methods. If we are to progress further it must be by
detailed work to establish quantitative relationships showing how genetic and environmental factors affect plant composition, paying particular attention to complex
interactions occurring in multi-nutrient deficiencies. A better understanding of the
physiological and biochemical processes involved in plant nutrition may provide a
basis for new methods of assessing nutrient status.
5. The role of nutrient cycles in planning fertiliser use
5.1 Historical
At the time of the first Congress in 1954 most advice took little account of the effect
of the fertiliser regime on reserves in the soil; this information can only come from a
study of the whole nutrient cycle in the farming system. The topic was first treated in
our Colloquium in Israel in 1969 which had the theme of transition from extensive to
intensive agriculture; the subject of nutrient balances was an essential component
because intensification means that extra nutrients must be introduced to grow larger
crops. Balance sheets for a country, or for the World, such as those in Table 2 show
how much larger are the contributions of K from soil than from fertilisers and how
much we rely on K recycled through crop and animal wastes.
370
Table 2. Potassium cycles in UK and World farming
UK
World
Thousands
of tonnes of K
Inputs
Fertilisers ..............................................
Imported animal feeds .....................................
Recycled
Animal and crop wastes ..................................
Output (annually)
In arable crops ..........................................
In grassland .............................................
350
100
17 000
?
690
118 000
500
1000
75000
200000
5.2 Comparisons of cropping systems
Warington [1886] used the results of the Agdell Rotation Experiment at Rothamsted
to calculate nutrient balance for a four-crop system. Wheat and barley grain were sold,
straw was kept to make farmyard manure (FYM), clover and swedes were fed to
cattle which also received a little purchased feed. Grain and meat were the only products
sold, crop wastes and animal manures were returned to the soil. Table 3 shows how
well the system conserved K in contrast to two modern 4-year rotations growing
average yields. Rotation I grows potatoes, barley, peas and wheat and all produce is
sold. Rotation 11 grows kale, barley, ryegrass and wheat; cereals are sold, kale and
grass is used off the field. There is a marked contrast between large losses of N, P and
K in such modern farming systems and conservation in the old systems. Fertilisers
currently recommended (M.A.F.F. [1973J) in England are also shown in Table 3.
Potatoes respond well to P and K and so large dressings are recommended in Rotation 1: P applied is much more than P removed, but K applied is less than K removed.
None of the crops grown in Rotation 11 normally give large responses to P and K and
little is recommended as fertiliser in England. The result of applying recommended
manuring for both rotations is a loss of soil K, particularly rapid in Rotation I1. The
risk that yields will be diminished by K-deficiency in such cash-cropping systems is
only revealed by a nutrient balance study.
Table 3. Comparisons of losses of plant nutrients in old and modern farming systems
Losses over 4 years of
N
P
K
kg/ha
Nineteenth Century feeding rotation .........................
Modern rotations
N o. I .................................................
N o. 2 .................................................
Total fertilisers recommended for modern rotations
Rotation No. I ........................................
Rotation No. 2 ........................................
64
9
7
437
450
63
58
405
348
480
360
150
80
310
150
371
5.3 Planning for increased yields
The 1954 Congress heard Russell [1954] describe the value of nutrient balance studies
on individual crops. He pointed out that Chambers [1953] showed that large amounts
of K were found in wheat at flowering in mid-June which are not found in either grain
or straw at harvest (much is returned to soil through the roots, by leaching from straw,
or in leaf-fall). Such studies show that wheat yielding 10 t/ha of grain needs to take up
from soil 200 kg K/ha and this quantity must be available in the nutrient cycle. If
cereals are regarded as unresponsive crops, and little attention is paid to their need
for K, large yields may never be attained.
Experimental results, and advice based on them, may be outdated when improved
varieties, or control of pests and diseases, or improved cultural methods make larger
yields possible. Russell [1954] quoted the results of a six-crop experiment at Woburn
in Bedfordshire where the soil had been able to supply enough K to grow moderate
(20 t/ha) crops of potatoes for 20 years. In the experiment there were only small
responses to fertiliser K and the advice was that little was needed on this soil. Another
rotation experiment began on the same soil on a nearby site in 1960; much better
cultivations were given and large yields were grown which removed much K. Widdowson et al. [1967] reported that there was little response to K-fertiliser in the first year.
However, the reserves of K were soon exhausted: by the third year all crops responded
to K-fertilisers and by the fifth year yields of grass, potatoes and sugar beet were greatly
increased by applying potash.
5.4 Farming systems
The need for a knowledge of nutrient cycles and balance is much greater when grass
and arable crops rotate, and when management contrasts such as cutting or grazing
grass, or ploughing-in or removing crop wastes (e.g. straw), are introduced. These
facts are well illustrated by Warren and Johnston's [1963] studies on Rothamsted and
Woburn Ley-Arable experiments. When these experiments began, small uniform
dressings of K-fertilisers were given to all plots and K reserves were not balanced.
Some treatments used continuous grazing of grass-clover leys which conserved K by
return through urine; others involved cutting and removing grass for the same period,
or growing and removing arable crops - when all K was removed. K-balance was
further complicated by large amounts of FYM (rich in K) given to some plots to
provide organic matter. Table 4 shows the potassium balance for this experiment at
Woburn. The first three years of the contrasted rotations were:
1. Potatoes, rye, grass-clover ley cut for hay
2. Potatoes, rye, carrots
3. Lucerne, cut for hay
4. 3-year ley grazed by sheep
The fourth and fifth years were always the same in all rotations - test crops of sugar
beet (tops removed), then barley. The large difference in balance between removing
arable crops for three years, and grazing by sheep, was 168 kg/ha of K. Yields of test
crops were diminished by K-deficiency where FYM was not applied; with FYM a
surplus of K accumulated. To measure the true effects of rotations, or of adding
372
Table 4. K applied in fertilisers and FYM minus K removed by the five crops of each rotation
in the Ley-Arable Experiment at Woburn
Rotation
Without FYM
(to sugar beet)
kg/ha of K
With FYM
I Arable crops (hay removed) ..........................
2 Arable crops (with carrots) ..........................
3 Lucerne (cut and removed) ..........................
4 G razed ley .........................................
-90
-62
+28
+ 78
106
145
247
370
organic matter, nutrient balance complications must be avoided. This has been done
in recent work on rotations in Britain by calculating losses and gains of K and, where
necessary, applying extra amounts as fertilisers. Similar balance calculations must be
made for crop rotations on ordinary farms, particularly when farming systems are
changed.
5.5 Forestry
Natural plant communities conserve nutrients and established forests do the same. This
is important in planning to use fertilisers on forests since the extra nutrients given to
increase production must be retained on the site. A good example of conservation of
potassium in a Pinus resinosa ecosystem was reported by Stone and Kszystyniak [1977].
The plantations investigated were on sandy soil in northern New York State. Increases
in tree height, diameter and canopy from applying one dressing of 112 kg K/ha have
lasted 25 years. This extra K was efficiently recycled; after 23 years 45 kg K/ha was
still in surface soil, 25 kg K was in foliage and litter, leaving only 40 kg K to be
accounted for in wood, bark and branches, plus any leaching. This conservation is
the more remarkable as the soil is a freely-drained very acid sand.
6. The special value of long-term experiments
Studies of the effects of fertilisers and manures, and cropping systems, on nutrient
reserves can only be investigated in continuous experiments. This is particularly
important for potassium because arable crops remove up to 200 kg/ha, forage crops
even more. Results of long-term experiments done in many countries were reviewed
at an international Conference at Grignon in 1976 under the Presidency of G. Drouineau
[1976] who discusses the subject further in this Session. Here we use Rothamsted
results to discuss some important problems that cannot be resolved by other kinds of
work.
6.1 Accumulation of soil potassium
Table 5 shows how exchangeable K is affected by fertilisers and FYM applied over a
100-year period in the Rothamsted experiments. Exchangeable-K has become stable
373
Table 5. Amounts of exchangeable potassium in soils of long-term experiments at Rothamsted
and Saxmundham
Site
Broadbalk Hoosfield
Crop grown
Barley
Roots
Wheat
Exchangeable K, mg/kg
Treatment
None ...............
PK .................
NPK ...............
FYM (35 t/ha, about
165 kg of K/ha) ......
Fertiliser K
applied/ha/year .......
Period of manuring . ..
Barnfield
Exhaustion
Land
Barley
Saxmundham*
137
180
154
4 crops
102
364
341
87
433
362
180
620
480
74
131
655
758
540
106
268*
90
1843-1966
90
1852-1965
225
1876-1958
110
1856-1901
53
1898-1957
-
* 15 t/ha of FYM at Saxmundham
at values depending on soil type, organic matter, amounts of K applied and crops
grown. These experiments afford unique opportunities for investigating fixation and
release of K, the equilibrium between 'fixed' and exchangeable K, and leaching of K.
Although much has been done on these topics, the old experiments are a valuable
resource which has not yet been fully exploited.
6.2 Movement of K into subsoils
In all of the classical experiments at Rothamsted some of the K applied has penetrated
at least 60 cm deep in a century and some may have been lost in drainage. Warren
[1956] pointed out that Dyer [1894] found extra K in the subsoil of K-treated plots
of the Broadbalk experiment down to a depth of 45-68 cm; his figures for these soils
in 1944, after a century of fertilising, are in Table 6. Similar information was given by
Warren and Johnston [1962, 1964] for the Barnfield (arable) and Park Grass (grassland) experiments at Rothamsted, and by Johnston [1975] for the Market Garden
Experiment at Woburn. Some data are in Table 7.
Grass cut for hay each year in the Park Grass experiment received 224 kg K/ha
annually for over 100 years, Table 7 shows much K had moved into the subsoil. The
same rate of K applied to root crops in the Barnfield experiment also resulted in much
K moving into the subsoil. On sandy loam soil at Woburn none of the 921 kg/ha of K
applied as fertiliser from 1942 to 1962 had moved into the subsoil, all was taken up by
the crops grown. In this period FYM supplied 11 618 kg K/ha - twelve times as much
as fertiliser had done and the soil was uniformly enriched (about 220 mg/kg of
exchangeable K) down to the limit of sampling. FYM dressings stopped at Woburn
in 1967 and larger fertiliser dressings were then given to all plots. Soil samples in 1974
showed that little fertiliser-K has moved into subsoil but FYM-treated soil was still
rich in exchangeable K to the full depth sampled - seven years after the last application.
374
Table 6. Movement of potassium into subsoils of the Broadbalk experiment
Annual treatment
Surface soil
(0-230 mm)
FY M .............................................
N one .............................................
PK fertilisers .....................................
N PK fertilisers .....................................
Subsoil
(230-450 mm)
Exchangeable K, mg/kg
480
340
100
290
100
220
220
210
Table 7. Movement of potassium into the subsoil of long-term experiments at Rothamsted
and Woburn
Depth (mm)
0-225 ...
.............
225-300 .................
300-450 .................
450-600 .................
Amounts of exchangeable K, mg/kg
Park Grass
Woburn
Market Garden Experiment
Without K
fertilizer
With K
Without K
With
fertilizer K
With
FYM
70
70
70
80
670
520
520
460
109
196
138
137
100
296
216
206
199
-
120
100
6.3 Release of soil potassium
All the long-term experiments that have plots where no K is applied provide unambiguous measurements of the K that is released annually from the reserves that are nonexchangeable. After a century of cropping, analyses of produce show that Rothamsted
soils release only about 20 kg K/ha to wheat or permanent grass each year. Release
from Saxmundham soil after 60 years of a four-crop rotation receiving no K-fertiliser
is still at the level of 80-90 kg K/ha.
6.4 The value of residues of K from fertilisers and manures
The residual effect of one dressing of K (or P) fertiliser is usually too small to be
measured accurately. Where many dressings given in long-term experiments have
accumulated large residues, the value of these reserves may be investigated accurately
by unambiguous methods. However, it is not possible to distinguish between the
release of fertiliser-K which has been fixed by clay minerals, and the release of 'native'
K from primary or secondary minerals. As Viets [1977] has pointed out we went
through a period in the 1920s-1940s of regarding fixed P and K as being, for practical
purposes, lost. He states - 'unwarranted emphasis was placed on special methods and
equipment for fertiliser placement to increase short-term recovery'. We now recognise
that residues are taken up only slowly because most of the absorbed nutrient ions are
inaccessible to the root system of a particular crop. One of the achievements of the
last 25 years has been the proof, through the use of the long-term experiments controlled by Rothamsted, that much of the P, and probably all the K, applied as fertiliser
375
can be recovered by crops if the experiments continue for long enough. An outline of
some of this work is given below.
6.4.1 The Exhaustion Land has supplied continuous proof that very old residues of P
and K can increase crop yields. Certain plots received each year from 1856 to 1901
fertilisers supplying totals of 1410 kg P and 5070 kg K per hectare; FYM applied to
other plots only from 1876 to 1901 supplied totals of 1260 kg P and 3920 kg K/ha.
Some plots received no P or K in this period; the whole area received no P or K
after 1901. Analyses of soil samples taken in 1951 and again in 1974 showed soluble P
was 3-4 times as big, and exchangeable K was twice as big, on plots with residues as on
those where no P or K was supplied from 1856-1901. Yields of barley grown between
1951 and 1974 on plots with residues from the old manuring were 2%/ times larger
than yields from plots without residues. These results were obtained 70 years after the
last fertiliser dressings were applied! (Johnston and Mattingly [1976]).
The value of the residues in terms of dressings of fresh fertilisers was measured in
1957-1958 by Johnston, Warren and Penny [1970]. Barley, wheat, potatoes, sugar
beet, swedes and kale all gave larger yields from plots which had been treated from
1856-1901; 56 kg P/ha applied as fresh superphosphate was needed to give yields from
poor soil equal to those from soil with residues but without fresh P. Residues of
potassium raised yields in a similar way. Figure 4 illustrates an important result from
these experiments. Residues of K fertilisers applied from 1856-1901 so improved the
soil for potatoes that larger yields were obtained than could be grown on poorer soils
however much fresh fertiliser-K was applied. The work was continued on other sites
and results were reported by Cooke [1976], Mattingly and Johnston [1976] and
Johnston and Mattingly [1976].
6.4.2 The Agdell Rotation Experiment began in 1852 and manuring stopped 100 years
later so that the accumulated residues could be investigated. Again it was shown that
soil containing P residues produced larger yields than soil untreated for a century,
however much fresh P was applied. Rates of uptake of K were measured by growing
grass;soil unfertilised for 100 years still supplied 60 kg K/ha each year. It was estimated that two-thirds of the K residues had been recovered, the fate of the remainder
was then uncertain.
6.4.3 The Saxmundham and Rothamsted Rotation Experiments were compared. The
four crop experiment at Saxmundham ended in 1969 and grass and lucerne were grown
from 1970. Where no fertiliser K was applied from 1898 to 1970 grass removed 180 kg
K/ha each year - three times as much as in the Agdell experiment; lucerne removed
115 kg K/ha annually. Yields of both these crops were increased by applying fresh K
fertiliser, grass recovered 70% of that applied, lucerne 85%. Grass was grown on the
Agdell experiment at Rothamsted for 12 years; in this period it removed 810 kg K/ha
from soil without residues of previous fertilisers; as exchangeable soil-K remained
constant, all must have come from 'native' reserves in minerals. Grass removed
1430 kg K/ha from soil with residues; exchangeable K diminished by 34 mg K/kg,
equivalent to 89 kg K/ha, or only 6% of the total K removed; therefore it appears
that 94% of the K recovered by grass on these plots on the Agdell experiment was
released from 'fixed' residues.
When K removed in crops was just balanced by K added by fertilisers or FYM,
Rothamsted and Saxmundham soils, which are both clays, contained 150 mg/kg of
exchangeable K. At Woburn, on loamy-sand soil, balance was at a much lower
figure - 80 mg K/kg.
376
40
With residues
15
10
15
10I
0
62
31
New K given, kg/ha
124
Fig.4. The effects of fertiliser K on yields of potatoes grown on soils with and without K
fertiliser residues
6.4.4 The present position is that it is certain that residues of K applied by fertilisers
or organic manures can be recovered from soil. Rate of recovery depends on total
amount present, on soil type and on crop; it cannot be hurried. If sufficient residues
are present fresh K fertiliser will not increase yields. There is no proof that the whole
amount of K applied cannot be recovered if cropping continues for long enough. In
some soils, particularly where bad physical conditions prevent good root growth,
soil enriched with residues gives larger yields, however much fresh K-fertiliser is
applied. These results justify fertiliser policies which allow residues of K (and P) to
accumulate when poor soils are being improved, this is in striking contrast to the
traditional policy of working only for immediate effects.
377
7. Models of nutrient relationships
The Oxford English Dictionary defines a model as 'representation of structure...
showing the proportion and arrangement of its parts'. Within our subject the purpose
of constructing models is to show the relationships between the factors that affect
crop nutrition and growth and to express them in mathematical forms which are
generally applicable. It is not practicable to make sufficient fertiliser experiments to
cover all combinations of crop, soil and climate and the purpose of deriving model
relationships is to provide a basis for extending information from experiments to
other sites and situations where local conditions can be defined mathematically.
Considerable success has been achieved in modelling growth in the above ground parts
of plants and Thornley [1976] has described recent work. Corresponding work on
root growth and solute uptake has been much more difficult. No doubt this is because
the 'tops' of a plant, and its environment, are accessible and measurements may be
made of plant parts and their rate of growth and composition as well as of atmospheric
factors relevant to plant performance. Plant roots are not accessible and unambiguous
measurements on them and on the soil solution and air are very difficult, consequently
the verification of model relationships that may be proposed is much more uncertain.
Nevertheless considerable progress has been made in the last 20 years to establish
models that co-ordinate and quantify scattered information with the object of controlling soil productivity. The models that have been proposed range from simple
ones for fertiliser responses, to more complex situations involving solute movement in
soils, and then to attempts to account for all the processes involved in nutrient uptake.
On the practical side models have been developed to aid the logical application of
animal manures to land so that the nutrients are efficiently used and pollution is
avoided.
7.1 Fertiliser responses by arable crops
Boyd [1976] discussing results of multifactorial experiments, found two types of
interaction illustrated in Figure 5; both were similar to Liebig's Law of Limiting
Factors. In Type I relationships, illustrated here for N and K, when N deficiency is
made good by fertiliser, crop response increases linearly until supplies of K become
limiting. When K is applied, response to N continues to a second plateau. In Type 2
relationships the actual rate of response to N increases as K is applied, so the optimum
N needed for maximum yield is diminished by applying K as well. (Boyd states that
with barley and potatoes some experiments show that adding more K can so increase
the efficiency of N that yield is doubled and the N needed is halved.) Maximising
production calls for large inputs and it is important that all limiting factors are
identified and corrected so that inputs are fully efficient. If a factor has an effect on
yield that increases with a second factor, curvilinear relationships are obtained; for
example when leaf disease is accentuated by giving N (in experiments, controlling
cereal disease has increased yield and restored linear relationships). When soil is
deficient in two or more nutrients optimal fertiliser dressings depend on the form of
interactions; complex experi rents and careful mathematical treatment are needed to
discover correct optima (see also Section 2).
378
Yield
=
N needed for maximum yield
YedType 1 f-
With K
Without K
Type 2
With K
Without K
N applied
Fig.5. Two kinds of relationships between yield and N fertiliser, with and without K (after
Boyd [1976])
7.2 Fertiliser responses by vegetable crops
Much less is known about the best ways of using fertilisers for vegetable crops than
for arable crops. More than 20 types of vegetables are grown on many kinds of soil
and in different climates in UK and it is not practicable to do enough fertiliser experiments to cover even the major combinations of crop, soil and weather. Therefore
there is a great need to establish general relationships between the factors affecting
crop growth from which predictions about fertiliser responses can be made. D. J. Greenwood and his colleagues at the National Vegetable Research Station (England) have
done much towards solutions to these problems. Their work has ranged widely from
the development of a theory for fertiliser response [1971] to practical proposals for
379
forecasting the fertiliser needs of different kinds of vegetables [1974a]. Comment
here is mainly on work that relates to the use of potassium.
An early paper (Greenwood et al. [1971]) gave a simple static model that related
yield to levels of applied fertiliser in terms of soil and crop parameters that could be
measured; although the theory had limitations it broadly agreed with the results of
field experiments. Dynamic models were introduced in later papers (Scaife and Smith
[1973]). A dynamic model for (he effects of K and N fertilisers on the growth and
nutrient uptake of crops was given by Barnes et al. [1976]. It was able to forecast the
effects of different weather conditions on crop response and the interactions between
the effects of N and K fertilisers on the growth and chemical composition of plants.
A specific application of an early model to predict the fertiliser needs of vegetables
was made by Greenwood, Cleaver and Turner [1974a]. They used information from
experiments on agricultural crops which had been done more widely than experiments
on vegetables, finding that the responsiveness of one crop relative to another varied
much less than the response of a particular crop on different kinds of soil. Therefore
experiments were done to relate the nutrient requirements of vegetables to the responses
of agricultural crops. Models were derived for each nutrient which took account of the
amounts of nutrients in soil before fertiliser was applied, adsorption isotherms for
these nutrients, transport of ions to plant roots, influence of osmotic pressure and the
effects of nutrients on crop growth. A good fit with the data from most experiments
was given by simplified equations which, for response to K, became:
A
Y
+±A(
I
Ks + KF)
Y =observed yield
A =maximum theoretical yield
A is a measure of crop responsiveness to K
Ks = quantity of exchangeable K in rooting depth of soil
KF=quantity of fertiliser K applied
From this treatment fertiliser responses of vegetables were predicted for a range of
soil and crop conditions. The simple model above took no account of the effect of
weather on crop or fertiliser response; in later papers more flexible models have been
successful in showing how weather and time of year alter response relationships.
These developments are important. Greenwood et al. [1974a] state 'computer simulation offers a powerful means of applying existing knowledge to forecast how levels
and methods should be adjusted to allow for differences in weather patterns and soil
conditions that exist in the UK. With this technique it may be possible to show how
fertilisers could be used more effectively and thus reduce the considerable wastage of
fertiliser which occurs at the present time'. In a general review of crop nutrition and
computer simulation Scaife [1975] stresses that there will be an increasing use of
basic principles in the models; he anticipates the emergence of a fairly generalised
growth model which can be used to predict interactions between fertilisers, species and
varieties, irrigation, planting dates and other practical factors. He considers that
simulation will bridge the gap between empirical field experimentation and basic
laboratory studies.
380
7.3 Efficient use of nutrients in livestock slurries
Computer modelling has been useful in planning to apply animal wastes to agricultural
land. It is important to use slurries so that water pollution is avoided and nutrients are
used efficiently by the crops grown. Parkes and O'Callaghan [1977] and O'Callaghan,
Doddand Pollock [1973] have described a computer model developed for this purpose.
Factors taken into account are (a) type of crop and its use, (b)crop requirements for
nutrients, (c) slurry characteristics, (d) soil characteristics, particularly nutrient content,
hydraulic properties and drainage. Particular attention is given to matching the K
supplied by slurry to the needs of the crop.
7.4 Movement of nutrients to roots
Any development of crop nutrition and fertiliser use to incorporate new concepts
must take account of the ways ions move in soil to plant roots. This subject has
been well discussed by Scott Russell [1977] and by Nye and Tinker [1977]. Barber
et at. [1963] illustrated the relative roles of mass flow of ions in water moving to
roots, and of diffusion of ions through the soil solution. Mass flow was well able to
supply all the Ca and Mg and much of the NO-N needed by the crop; but this
process could supply only a small proportion of the K needed, and even less of the P.
Potassium and phosphate ions are both absorbed by charged surfaces of soils and
therefore diffuse only slowly and maintain small concentrations in the soil solution.
Scott Russell [1977] gave these figures for diffusion in solution and in moist soil:
Diffusion coefficient cm s-1x 10'
In solution
In soil
N O . ................... ........................
C I+ ..............................................
K .............................................
H PO4- ........................................
1.92
2.03
1.98
0.89
0.5
0.5
0.01 -0.24
0.0005-0.001
Mobile ions move much more slowly in soil than in solution because of the tortuosity
and narrowness of the channels. Mobile ions are depleted by mass flow from a large
bulk of soil, but immobile ions must come from soil very close to the root. Thus
Scott Russell gives an instance where it was shown that 94% of the P absorbed by a
root had come from soil within 0.1 mm. The amount of K absorbed by root systems
depends on (a) surface area and length of roots, (b) physical and chemical properties
that determine diffusion rates, and (c) water content of soil. If practical progress is
to be made ways must be found of expressing information on these factors in a
mathematical expression.
Eleven years ago Friedand Broeshart [1967] wrote that there was no fundamental theoretical difficulty in applying modern concepts to highly heterogeneous systems such as soil
and its solution but 'the experimental difficulties in determining the many parameters
that occur in the equations are so great that this approach is of more theoretical than
practical significance'. In spite of these very real difficulties progress has been made
and models for movement of ions from soil to root, and for vertical transport in
381
soil, were discussed at the 9th Colloquium of IPI. Recently Baldwin [1975] described
ion uptake by a root by the equation:
U = (1- eCY)Q
Q= amount of accessible nutrient (in labile and solution forms)
y =complex function to take account of root density and nutrient mobility expressed by
diffusion coefficient (D)
D depends on buffer capacity (b), volumetric moisture content (v), tortuosity factor
(f)and equalsD, vf(I4-) where D, is the diffusion coefficient of the ion in pure solution.
The value of such a model is that it shows factors that we need to evaluate to forecast
uptake in a particular season; our inability to supply the data is a measure of the
task ahead.
7.5 Modelling in the soil-plant system
The ultimate aim of modelling the soil-root interface must be a mathematical system
that will predict the effect of changes in both environment and crop management.
Scott Russell [1977] has pointed out that attempts to synthesise the system are
usually hindered by lack of knowledge of the rate-limiting factors of the processes
involved. Nye and Tinker [1977] have followed a synthetic approach by analysing
the working of simple small-scale systems. They dealt with water movement and its
effect on solute movement, then with distribution of solutes between the phases of
the soil. Plant roots, and the changes taking place around them were then described,
together with interactions within the root zone of one plant. Finally the treatment
was built into a study of a plant community in a field.
The components of the system which Nye and Tinker considered must be studied
were:
(i)
(ii)
(iii)
(iv)
(v)
Amount of roots and their distribution
Water transfer and uptake
Transfer of nutrients in the profile
Uptake by field crops
Competition between plants
They discussed problems, and inadequacies of current knowledge, for each of these
topics. Uptake of nutrients by field crops is, perhaps, best documented; nevertheless
many problems remain - such as the difficulty of measuring uptake from a specified
part of the root volume. They mention a study of a dynamic model for the effects of
soil and weather conditions on nitrogen response by Greenwood, Wood and Cleaver
[1974b] which incorporates many of the essential principles; this model had 'fair
success' in predicting the nitrogen needs of lettuce.
7.6 The future
It is clear that computer simulation will provide the means of unifying the new
concepts, developed in the last 25 years, on nutrient transformation and movement in
382
soil, and of uptake by plants. The present position, and future possibilities, are well
discussed by Nye and Tinker [1977]. They see plant nutrition as a dynamic process
rather than a static comparison of plant demand and soil supply; effective work must
cross the boundaries of traditional subjects. The concepts now developing integrate
plant physiology and soil science: 'it remains to be seen whether they can deal in
detail with the truly formidable physical and biological complexity of natural vegetation and soils'. Mathematical techniques can, of course, predict any process we
wish, but the process (and its model) must be correctly chosen. Nye and Tinker
conclude: 'The major factors impeding further progress are the inaccuracy of soil
physical measurements, the natural heterogeneity of soils, and the difficulty of predicting root development and properties, when these are so closely linked with the
growth of a whole plant or community in a fluctuating environment'.
8. Potassium fertilisers
8.1 Straight fertilisers
Kaputsa [1968] described the production processes used to extract potassium salts
from ores. The flotation and beneficiation, and recrystallisation processes developed
in the last 30 years have led to purer salts being available as fertilisers. Muriate of
potash (KCI) used to 'contain' 50% K 2 0, current products are equivalent to 60.062.5% K20 (pure KCI is equivalent to 63.2% K20). Eliminating most of the other
salts, formerly present as impurities, has led to muriate of potash with much improved
physical properties. Developments in making mixtures, and in bulk blending, have
required materials with larger particle size: Large crystal products have been produced
by solution and recrystallisation; other granular products have been made by fusion
and compaction processes. Current 'granular' products are in the size range 6-14
(Tyler) mesh sizes (3.3 to 1.17 mm); there is now a considerable demand for these
'sized' potash fertilisers in North America.
8.2 Mixed fertilisers
Mixed fertilisers ('compounds' in U.K.) are convenient to farmers because they
supply 2 or 3 of the nutrients N, P and K; Mg may be added, micronutrients also,
and sodium may be added for crops such as sugar beet. There is no merit in using mixed
fertilisers except that separate applications of straights, or farm mixing, is avoided.
It is easier for farmers to follow recommendations made in terms of one mixture
than in terms of three straight fertilisers. It has often been claimed that particular
mixed fertilisers are 'scientifically' or 'properly' balanced - these terms have no
validity. True balance is the correct proportioning of nutrients to suit particular
combinations of soil, climate, crop and farming system; because seasons vary these
can rarely be forecast accurately for outdoor crops. The subject was well discussed
by Crowther [1954] at the time IPI was formed. He pointed out the large and random
variations in fertilisers offered in 1954 in USA and UK. Eleven most popular mixtures
accounted for 75% of sales in USA, but 1000 other mixtures were on sale. Nutrient
383
ratios could vary considerably even within the limits in analysis allowed by legislation;
Crowther wrote 'if the precise balancing of nutrient ratios is such a difficult and
precarious act, I think it may be more suitable for the privacy of Committees than
for public display'. He proposed a set of 14 ratios to serve the needs of UK farmers;
these are in Table 8.
8.2.1 Britain: Crowther's discussion of mixed fertiliser ratios was an important
contribution to the rationalisation of mixtures and to methods of advising on their
use (Cooke [1967]). It led to a limited range of well-granulated products that serve
all needs of British farmers. The main ratios available in UK at three times in our
period are given in Table 9. Farmers have tended to increase the use of N more than
Table 8. A set of standard mixed fertilisers proposed by Crowther [1954] for use in UK in
1954, stated as ratios of percentages of elements
%N
: %P
%K
0
0
1
1
4
2
2
1
V
4
4
2
2
1
1
/
'A
1
I,
1
1
1
1
1
1
1
1
I
0
0
0
2
4
2
4
1
2
1
2
0
1
1
Table 9. Types of mixed fertilisers sold in UK, 1962-1976
Description
Ratio
Percentages
of all mixtures sold
General purpose ........
High-N ................
Low-N ................
%N
% P2O,
% K 2O
1
1
1
(2,
1
2
1
1
1
:
1
Low-P ................
I
I
High-K ................
I
Low-K ...............
N K ..................
I
1
I
1
0
N P .............. :.... .2
PK
l01
4 ...................
384
1976
4
8
6
22
49
55
2
-
10
9
1
7
14
3
I/
3
2
5
41
23
15
1
I
2/
1
1970
1
1V
:
High-P ................
1962
:
I1
k
2/,
2/ 3
0
1
J
I
-
J
-5
-
0.4
0.1
7
of P and K and the 'High-N' types (e.g. 2-1-1) have come to dominate the market
(over 50% of sales). The other striking change is that 'High-K' types, which were
formerly dominant, now only take 15% of sales; other types have altered little in
popularity. These changes in the mixtures used should be seen against the total sales
of all nutrients in UK shown in Table 10.
Table 10. Fertilisers used in the UK, 1953-1976 (F.M.A. [1976])
1939
1953
1966
1978
...........................................
...........................................
...........................................
...........................................
N
P2 0'
Millions of tonnes
K2 0
0.06
0.23
0.59
1.16
0.08
0.25
0.43
0.41
0.17
0.40
0.46
0.41
8.2.2 USA: TVA [1976] reports the 10 leading grades shown in Table 11. In contrast
to the mixtures sold in UK (Table 10) - which are now dominated by High-N types,
those sold in USA are rich in P and K. In 1960 the products listed represented 50%
of sales of mixtures, in 1975 the materials shown represented 40% of sales. In USA
roughly equal amounts of nutrients were applied as mixtures and as straight materials.
In 1970 the amounts sold in bulk equalled the amounts of bagged fertilisers; since
then bulk materials have increased, as have liquids. Potash for direct application in
USA has increased faster than K sold in mixtures; in 1975 mixtures supplied 55%
of the total market in USA. In UK mixtures have supplied over 95% of all K for
many years, they supplied 47% of N and 83% of P used in 1976.
Table II. Most popular mixed fertilisers sold in USA
1960
%N
% P'0'
% KO
1975
%N
% P2 O,
% K2 0
5
4
5
12
10
6
3
3
5
4
10
12
20
12
10
12
12
9
10
16
10
12
20
12
10
12
12
9
5
16
18
10
5
6
5
13
12
5
3
3
46
10
10
24
20
13
12
10
9
9
0
10
15
24
20
13
12
10
9
18
8.2.3 Concentration of mixtures has increased in most countries due to increasing use
of ammonium nitrate and urea to supply N, to greater use of concentrated superphosphate and ammonium phosphates, and to purer potash salts. Changes in concentrations of British mixtures are shown in Table 12.
385
Table 12. Concentrations of mixed fertilisers sold in UK
Total of % N+ % P2O, + % K20
1948
1959
1967
1973
1976
............................................
............................................
............................................
............................................
............................................
24
32.4
40.2
4 1.8
4 1.4
8.3 Complex fertilisers
This name is usually given to products in which the nutrients are in chemical combination rather than in mixtures. Examples are potassium nitrate and potassium orthophosphates, these are high cost products and are only for special purposes such as
some of the liquid or solid fertilisers sold for horticultural crops.
Potassium metaphosphate is perhaps the most interesting of the new fertilisers developed in the last 25 years. The products are polyphosphates-polymers with molecular
weight up to 120,000 - properly described as (KPO3 ) ; their analyses are equivalent
to about 60% P2 0 5 and 40% K 2 0. Although easily made, no large scale and continuous
production has been established in any country. Potassium metaphosphates contain
no soluble ions to increase soil solution concentrations which may damageyoung
plants. Both P and K are more slowly available than P and K in conventional fertilisers,
but hydrolysis in the soil solution during the growing season converts all P and K to
orthophosphates and potassium ions, British developments of this fertiliser were
described by Harris [1963]; satisfactory availability to crops was reported by
Mattingly [1963]. Agronomic evaluation was discussed by Engelstad [1968]. Potassium polyphosphates are worth further examination as fertilisers; absence of soluble
salts makes them suitable for placing close to seed; they may have advantages in
very humid areas, such as the tropics, where soluble salts are quickly leached.
8.4 Granulation
Bad physical condition in powdered fertilisers common in 1953 has been universally
overcome by granulation. In about 1930 ICI Ltd. introduced granulated concentrated
compounds based on ammonium phosphate; although granulated products became
popular in England and other European countries many producers were slow to
follow. Russell and Williams [1977] record the first production in USA in 1935.
Bulk blending became possible when well-granulated fertilisers became common.
Detailed fertiliser recommendations to fit individual fields are difficult to meet with a
limited range of mixtures; blending of granulated ammonium nitrate, concentrated
superphosphate or ammonium phosphates, with granulated potash on the scale to
suit an individual farmer was introduced to meet these needs.
386
8.5 Fluid fertilisers
The earliest application of liquid fertilisers was the use of anhydrous ammonia and
nitrogen solutions in USA. Subsequently solutions supplying N and P were developed.
Russell and Williams [1977] record that the first liquids to contain K were made in
USA in 1953. The low solubility of the K salts used limited the concentrations that
could be obtained and the first grade made was 4% N, 10% P2O5 , 10% K 2 0. Low
analyses hindered acceptance. Purer crystalline KCI was an improvement but satisfactory concentrations of fluids containing K were only obtained when suspension
fertilisers were introduced. The development of superphosphoric acid by TVA gave
higher analysis liquids and in 1958 they introduced a 11-33-0 base solution. An
important discovery was that polyphosphates in superphosphoric acid sequestered
metal ions (such as Fe, Al, Mg) in solution and prevented precipitates forming which
caused difficulties in processing and handling. By 1975 there were 2,800 liquid fertiliser
plants in USA and grades such as 7-21-7 and 4-11-11 were common.
The difficulty of low concentration was overcome by using certain clays as suspending
agents. Suspensions of superphosphoric acid, ammonia, urea, and ammonium nitrate
solutions, and potassium chloride, give high-K grades such as 15-15-30 and 7-21-21
which can be stored. Suspension and liquid fertilisers have advantages over bulk
blends because added micronutrients or pesticides can be completely mixed to uniform
composition. To save energy fluid fertilisers are now moved long distance by pipeline
in USA.
Slack and Achorn [1973] have reviewed developments in the manufacture and use of
liquid fertilisers, most of which have taken place in USA. The consumption of solids
and liquids in USA is shown in Table 13. Growth since 1955 has been so rapid that
Table 13. Consumption of fertilisers in USA (from Slack and Achorn [1973])
Millions of short tons
1955
1965
Solid ....................................
Anhydrous and aqueous ammonia .................
Nitrogen solutions ...............................
Liquid mixed fertilisers ...........................
21.8
0.4
0.1
26.1
1.8
1.9
1.0
1971
28.5
4.1
3.5
4.5
liquid mixed fertilisers supplied over 4.5 million (short) tons in 1971 (21% of all
fertilisers); all fluids (including nitrogen solutions and ammonia) supplied 30% of the
total tonnage. Few other countries use large amounts of liquids; the largest users in
1970 were:
Thousands
of metric tonnes
U SA .........................................................
F rance ........................................................
U K ..........................................................
C anada ........................................................
6090
4 14
2 19
74
387
8.6 Future developments
Future developments in fertilisers will be towards materials with special physical or
chemical properties that make them more suitable to local crop and soil conditions,
and make them more efficient producers of crops.
9. Changes in the practical use of fertilisers
9.1 Changes in recommendations for arable crops
In most developed countries schemes for using fertilisers rationally have improved
greatly in the last 25 years; recommendations in England and Wales are discussed as
an example.
In 1954 Crowther was able to summarise the advice available for fertilising arable
crops in a simple diagram (see Table 14) which gave the average 'most profitable
dressings'; small adjustments for changing conditions of soil and climate, and for the
use of FYM, were proposed. Table 15 gives the (smaller) amounts that surveys
Table 14. Fertiliser recommendations for arable crops in 1954 (Crowther [1954])
P2 O5
K 20
150
110
100
200
125
50
225
125
50
-25
+ 25
-25
-50 -
N
kg/ha
Recommendations for average conditions
Potatoes .....................................
Sugar beet ...................................
Cereals .....................................
Adjustments for local conditions
(increase [+I or decrease [-])
Wet areas ....................................
D ry areas ...................................
FYM applied ...................................
-
-75
Table 15. Amounts of fertilisers applied to crops in England and Wales, 1952 to 1977 (Church
and Hills [1977] and Church and Lewis [1977])
1977
1952
N
kg/ha
P2O5
K2 0
N
kg/ha
P 20
5
K2 0
Potatoes ...................
117
124
166
181
186
250
Sugar beet .................
113
115
138
141
71
157
33
25
16
6
28
30
35
24
15
20
15
4
115
82
165
88
40
36
33
21
Wheat (winter) .............
Barley (spring) .............
Grass (temporary) ..........
Grass (permanent) ..........
388
-
33
37
32
16
showed were used by farmers in 1952. The amounts used in 1977, also in Table 15,
are, for arable crops, as large as average recommendations; under these conditions it
is important that farmers modify average dressings to allow for differences in fertility
or farming system on individual fields. To provide this advice the results of many
hundreds of field experiments done in the post-war period were correlated with soil
types and analyses, climate and farming system. A comprehensive series of recommendations that covered most practical conditions was published by Ministry of Agriculture, Fisheries and Food (MAFF[1973]). Variations to standard recommendations
for K are based on soil types and soil analyses, interpreted by the indices in Table 16.
Table 17 gives examples of recommendations for field crops, similar tables are used
for vegetables and protected crops.
Table 16. Interpretation of values for exchangeable potassium in soils used by MAFF [1973]
Index
Exchangeable K*
mg/kg
0
0-
l
2
3
4
5
61121245405604-
6
7
8
9
Comment
60 ....................................
Arable crops fail
120 ....................................
240
400
600
900 ....................................
Glasshouse crops fail
I Reduce
905-1500
.............................
1510-2400 ....................................
2410-3600 ....................................
> 3600 ....................................
f
K fertilizers for
potatoes and vegetables
No K for glasshouse crops
K excessive, yields of many
crops will be reduced
* soluble in M NHNO3 solution
Table 17. Recommended amounts of potassium fertilisers for crops grown on soils with
differing amounts of soluble K (MAFF [1973])
Index* for exchangeable K in soil
0
1
2
kg K2 0/ha recommended*Potatoes (loamy soils) ....................
Sugar beet ..............................
W heat ..................................
M aize .................................
Beans ..................................
Cauliflowers ............................
Apples .................................
*
250
200
60
100
90
240
200
200
100
30
60
40
240
150
150
50
30
60
30
180
75
>2
>3
100
50
0
40
0
180
25
-
120
0
Indices from Table 16
** Approximate
389
9.2 Recommendations for using potassium on grassland
In 1953 very little fertiliser was used on grassland in UK (Table 15), Crowther [1954]
could make no general recommendations. Large increases in yield resulted from
applying nitrogen but there was little evidence on which to base sound recommendations for using P and K fertilisers. Fertiliser use on grassland was the topic of the
4th IPI-Congress in Vienna in 1957. Progress since then is reviewed by reference to
the 1st Colloquium of IPI held in 1963 and the 1st Colloquium of the Potassium
Institute Ltd. in 1976.
9.2.1 First Colloquium of IPI, 1963. The following essential factors governing the
use of K on grass, which were far from clear when IPI was formed, emerged from the
discussions.
(1) Large yields of herbage depend on the supply of much N (from fertilisers+ soil+
legumes) and of equally large amounts of K (from fertilisers+soil+organic
manures).
(2) Large yields of leguminous herbage crops depend more on large supplies of K
than do large yields of grasses.
(3) Where grassland is cut continuously for hay, silage or direct feeding, the amounts
of K removed in herbage must be replaced to prevent a diminution in soil K and,
ultimately, loss in yield.
(4) Where grassland is grazed, most of the K in herbage ingested is returned in excreta.
Although this return in urine patches is irregular in any one year, over a number
of years the patches join and soil K status is then maintained in grazing systems
by relatively small dressings of K fertilisers.
(5) The development of grass tetany (hypomagnesaemia) in grazing animals is a
complex condition involving weather and other components of diet as well as
concentration of Mg in herbage. Large uptake of K by grass depresses uptake of
Mg, particularly in spring; therefore it is important to supply no more K than is
needed for maximum yield of grass. 'Luxury uptake' by spring grass should be
prevented by careful control of amounts of K fertilisers and timing of the dressings.
(6) In ley farming systems where arable crops and herbage crops are grown in rotation,
special attention must be given to maintaining soil-K status since arable crops
are more sensitive to K-deficiency than are grasses.
9.2.2 1st Colloquium of the PotassiumInstitute Ltd., 1970. The most important problem
was still that of supplying sufficient K to secure maximum yield of grass without
permitting luxury uptake. Hooper [1970] proposed that 2% K in herbage dry matter
was a 'target' figure which should not be exceeded, but which would provide sufficient
K for grass without depressing Mg uptake. Hooper said that exchangeable K in soil
was not a good guide to fertiliser use since one heavy cut of grass altered the value.
Simple replacement of the K removed was also a bad basis because grass uses much
non-exchangeable K from soil and, together with exchangeable soil K, this leads to
luxury uptake if replacement dressings are applied. Hooper concluded that the procedure proposed by Clement and Hopper [1968], based on herbage analysis, could be
used as a basis for applying K to herbage.
Brockman [1970] discussed intensive grassland management, saying that the extremes
of entirely cut and entirely grazed grass had such different K requirements that they
should be regarded as different crops. He found that, although grass can remove as
390
much K as N, in the early stages of experiments there was rarely a response to applying
more than 0.5 kg K per 1.0 kg N. As soil K was depleted in continuous cutting
systems, the need for K rose, eventually reaching 1.0 kg K per 1.0 kg N, some part of
these larger quantities should be applied in late summer. In intensive grazing systems
recirculation of K through urine was efficient; soils poor in K required one annual
dressing to maintain balance, soils rich in K needed no fertiliser-K in continuous
grazing.
9.2.3 Nutrient balance on livestock farms. McAllister [1970] said at Hurley in 1970
that on farms where livestock are produced intensively, the large amounts of nutrients
imported in purchased feeds increase nutrients in soil by as much as 100 kg P and
135 kg K per hectare per year even where no PK fertiliser is applied. Smilde [1972]
has discussed these problems for Netherlands grassland farming; there too, intensive
animal production using concentrated feeds, may result in manure supplying more P
and K than the system can absorb without waste.
9.2.4 Manuring grass leys in ley-arable systems was discussed by Johnston [1970] at
Hurley. The grass of short-duration leys removes soil K very efficiently; in doing so
it depletes reserves of K so that yields of following arable crops are diminished. If the
extra K needed is applied during the grass period, it is taken up 'in luxury'; therefore
the additional K to balance the system must be applied for the arable crops only.
Gething [1972] has also described experiments showing the need for much K of crops
grown in rotation.
9.3 Progress in fertiliser use in England and Wales
Table 15 shows the average amounts of fertilisers used by farmers in 1952 and 25 years
later (Church and Hills [1977] and Church and Lewis [1977]). Both potatoes and
sugar beet were already well-fertilised in 1952, other crops received much less. By 1977
all arable crops received amounts similar to average recommendations. Grassland
still receives less fertiliser than is justified; the surveys show that farmers do not make
sufficient difference in the amounts of P and K they give to cut, and to grazed, grass.
9.4 Vegetable crops
Greenwood, Cleaver and Turner [1974a] attacked the special problems of forecasting
individual fertiliser requirements of the many vegetable crops grown on widely
differing soils in England. The models they developed have been discussed in Section 7
and the 'predicted optimum levels' of K fertiliser derived from the models are in
Table 18. Only a few vegetables (and notably spinach) were expected to respond
appreciably to K-fertiliser on soil containing more than 200 mg/kg of exchangeable K.
Independent experiments were made to test the predictions; in 85 per cent of these
the values for K needed which were predicted from the model agreed with those
measured in the field. The amounts of K recommended by MAFF [1973] are also in
Table 18; they are very much larger than Greenwood et al. recommend, as are the
amounts Church and Hills [1977] found farmers used in 1977. Greenwood et al.
conclude that the levels of P and K maintained in many vegetable soils in UK are
far higher than is needed, in consequence much fertiliser is wasted. This conflict
391
Table 18. Fertiliser potassium dressings for vegetables, amounts needed predicted from model,
amounts recommended and amounts used by farmers
Soil K index ........
Brussels sprouts .....
Carrots ............
French beans ........
Lettuce .............
Onions .............
......
Spinach
Cabbage ...........
Swedes ............
Brussels sprouts .....
Carrots ............
Beans ..............
Lettuce .............
Onions .............
Spinach ............
Cabbage ...........
Swedes ............
Exchangeable K in soil, mg/kg
300
150
90
45
2
3
1
0
Amounts of K needed, predicted from modelkg/ha of K2 0
0
0
10
40
0
0
20
60
20
50
80
130
0
10
30
60
10
40
60
110
90
150
200
290
0
0
0
30
0
0
0
30
K used on average
Amounts of K recommended**
by farmers***
kg/ha of K2 O
kg/ha K2O
136
180
180
240
240
50
50
100
100
71
90
90
150
150
120
120
180
180
156
180
180
240
240
120
120
180
180
104
180
180
240
240
71
60
60
150
150
Greenwood et at. [1974a]
** MAFF [1973]
** Church and Hills [1977]
between old advice and new recommendations from models remains to be resolved.
However, we must remember that Greenwood et al. were predicting dressings needed
for immediate effect as they were working from a static model. To allow for the
factors involved in nutrient movement in soil and uptake by crops, a dynamic model
must be used such as Greenwood et al. are now developing [1974b]. A complete
model must take account of potassium economy in producing the whole crop, and
the use made of its components (the latter determines how much K is returned to
soil in wastes) and how much is lost in sales of crops such as brussels sprouts, cabbage,
swedes or carrots, all of which remove much K.
9.5 Forest fertilisation
During the 25 years since IPI was formed fertiliser use on forests has been established,
notably in Scandinavia; the 1967 Colloquium in Finland discussed early experience.
Production of tree seedlings was firmly established by taking account of their nutritional needs on different types of soil. The former prejudice for organic manures in
tree nurseries had been resolved by experiments showing that the composts and
other organic manures used were rich sources of plant nutrients. When amounts of
392
N, P and K, and micronutrients needed were measured, and soil pH preference of
species established, it became possible to grow seedlings with fertilisers more reliably
than with composts.
Fertiliser use on established trees is still on a very empirical basis; the Colloquium
showed that it was necessary to apply appropriate concepts developed for perennial
agricultural crops. In particular it is important that all nutrients applied should be
retained in the soil-vegetation cycle (as discussed in Section 5). New multifactorial
long-term experiments are still needed to measure tree responses and nutrient interactions and to provide a basis for laboratory work.
A recent report (1FC [1977]) states that at the end of 1976 8.5-9 million hectares
of the World's forests had received fertiliser (an increase of 15% in a year). In Finland
2 m ha had been fertilised, 1.3 m ha had been treated in Sweden, 1.2 m ha in Japan,
1.0 m ha in Poland and 0.6 m ha in North America.
9.6 New methods of applying fertilisers
Fertiliser applications to arable land have, historically been tied to cultivations
given to prepare soil for planting. Changes in the old methods of broadcasting and
mixing fertiliser with soil have been made for several reasons. An early change was
when machines became available to sow fertiliser in bands. These were used to
minimise contact between fertiliser and soil, and so lessen fixation to useless forms.
The technique also gave crops a good start on poor soils. Split applications were
similarly used to improve fertiliser efficiency. Later modifications have included
deep placement, introduced to try to avoid nutrient stress in dry periods. Within the
last ten years minimum cultivation techniques, and 'direct-drilling' have been introduced and fertiliser application methods have had to be considered afresh. These
new methods of application are discussed in succeeding sections.
9.6.1 Placement in seedbeds
In the 20 years before IPI was formed much research in North America and Europe
had shown that fertilisers placed near seeds were often more efficient than broadcast
fertiliser. A typical review (Cooke [1954]) published in 1954 stated that half dressings of P and K fertilisers drilled with the seed of cereals gave as large yields as full
dressings broadcast. Gains in yield and efficiency were obtained from placing NPK
fertilisers for potatoes and vegetables, and from placing PK fertilisers for peas and
beans, as compared with broadcasting. Gains from placement were greatest on poor
soils, for short-season crops, and when fertiliser dressings were small. Placed fertiliser stimulated rapid early growth of most crops, however those with a long growing
season (e.g. sugar beet) often gave similar yields from broadcast and placed fertiliser.
Similar gains from placement methods used for crops in other countries have been
reported (Prummel [1957, 1962]). Soluble salts placed too close to the seed damaged
germination and safe methods of placing bands of N and K fertilisers to the side
and below the seed were devised for all important crops. Gains from ploughing
fertiliser deeply into the soil had also been demonstrated for some crops by 1954.
This early work also showed (Cooke [1954]) that root growth was greatly stimulated
by fertilisers placed in bands. This response of roots to high local concentrations
was found with all crops examined. The extra root growth in bands of N and K
393
fertilisers was coarse and spread through a large volume of soil, indicating diffusion
of ions from the band through the soil. When phosphate (which diffuses little) was
banded the roots were densely concentrated in a small volume and were fine. Scott
Russell [1977] has recently discussed the implications of such work for root growth.
Placement methods have been widely adopted in many countries, and for many
crops, in the last 25 years. Often application has been limited by lack of satisfactory
placement machines for sowing seed and fertiliser correctly. Much of the early work
was done with mixed NPK fertilisers, but later experiments have confirmed the gains
from placement of potassium fertilisers. For example Widdowson et al. [1959]
showed that 30 kg K 2 0/ha placed produced more barley grain than 60 kg K 2 0/ha
broadcast. In other experiments (Widdowson and Cooke [1958]) placed K was
superior to broadcast K for peas, beans and maize but not for kale.
9.6.2 Deep placement of fertilisers
Later work on deep placement has been stimulated by results from long-term experiments which showed that continued application of fertilisers or manures results
in movement of K into subsoils (Section 6). Where subsoils have become deeply
enriched with P and K yields have often been larger than could be obtained from
poorer soils of the same kind, however much fresh fertiliser was given to the topsoil.
McEwan [1975] has investigated the value of nutrients deeply incorporated in
subsoils; fertilisers were dug by hand into the subsoil, topsoil being temporarily
removed; subsoil digging without fertiliser was also tested. In 1973 1930 kg P20/ha
and 460 kg K 2 0/ha were dug into the subsoil. All crops also received normal dressings of NPK fertilisers applied to the topsoil. Yields for 1974 crops are in Table 19.
Deep placement gave considerable increases in yields of barley, potatoes and sugar
beet; similar results were obtained with potatoes and barley in 1975 and with potatoes
and sugar beet in the very dry year of 1976.
Table 19. Tests of PK fertilisers placed in subsoil for arable crops (McEwan [1975])
No treatment
Yields, t/ha
Wheat, grain .......
5.6
Barley, grain .......
4.9
Potatoes, tubers .... 68.8
. . 4.9
Sugar beet sugar
tops ... 29.2
Subsoiled only
PK to topsoil
PK to subsoil
6.5
5.2
65.1
5.0
32.6
5.2
4.5
72.1
5.1
27.1
5.9
6.2
78.5
5.5
39.0
These interesting results will have important implications if they are obtained on
other sites. There are good grounds for expecting deeply-placed K to be particularly
effective whenever a moisture deficit in topsoils diminishes uptake of K. There is
at present no standard equipment in regular use for deep placement of P and K but
recently developed attachments to subsoilers may prove useful. Difficulties associated
with gravity feed of solid fertiliser into subsoils can be overcome by using liquids
pumped through narrow tubes behind subsoiling tines, a method developed for maize
in USA.
394
9.6.3 Split applications offertiliser
Splitting of a dressing of N fertiliser is recommended in humid climates to diminish
losses by leaching or denitrification; parts of the total amount are applied during
the season to suit the growth of the crop. Splitting dressings of K fertilisers for grassland lessens the risk of luxury uptake. Apart from these examples split fertiliser
dressings have no advantage in temperate regions.
Many tropical soils are devoid of the 2 :1 clay minerals that 'fix' K and potassium
fertilisers used on cultivated crops are easily leached. Losses of K from tropical
soils were discussed in the 10th IPI Colloquium in 1973. Lacoeuilhe [1973] reported
that in growing bananas and pineapples 50 to 60% of the K applied for these crops
may be lost by leaching. He recommended applying split dressings at times when
the crop needed K most. Many examples of gains from this practice are published;
for example a recent report (I.F.C. [1977]) showed that applying 120 kg K 2O/ha
in four split applications gave 4034 kg/ha of tea wheareas the full dressing applied
wholly before the monsoon rains produced only 3664/ha.
9.7 New cultivation systems
Scott Russell [1977] has stated that until the early part of this century no aspect of
agricultural practice was more guided by tradition, and less by scientific work, than
soil cultivation. The tradition of thorough cultivation, established long before agricultural science, appeared so correct that it was never critically examined until it
was seen that exporting the cultivation practices of temperate countries to other
regions often led to disastrous soil erosion. By 1938 Russell and Keen at Rothamsted
had shown that cultivating more than was needed to prepare a seedbed, or to kill
weeds, was a waste and extra cultivations often diminished crop growth. From the
time of IPI's formation in the 1950's scarcity of labour, and increasing mechanisation
led to a reassessment of this earlier work and the need for cultivation was questioned.
The practical application of these ideas was greatly aided by the discovery in the late
1950's of the bipyridyl herbicides, and particularly paraquat; these materials could
be used to kill weeds and surface vegetation without damage to a crop that was
sown immediately afterwards.
The development of reduced cultivation techniques, and direct drilling (into uncultivated soil) which followed has been described (Pereira [1975]) and Scott Russell
[1977] discussed the implications of these advances for soil management and crop
nutrition. Absence of cultivation has been beneficial; soil organic matter has accumulated in the undisturbed soils, their structure has improved, earthworms are more
common and the soil carries wheeled traffic better. Because it is no longer possible
to cultivate broadcast fertiliser into topsoils, methods of application of P and K for
direct-drilled crops must be reassessed. Higher concentrations of P and K have
been found in direct-drilled topsoils than in cultivated topsoils as Scott Russell
[1977] and Baeumer and Bakermans [1973] report. Russell [1975] assessed the
experimental evidence and decided that it was too little to decide whether P and K
must be drilled beneath the surface or whether surface broadcasting was adequate;
earthworms may assist in the downward movement of nutrients.
395
10. Future developments
Our immediate task is to exploit the concepts that soil-crop relationships are dynamic
and that production systems have a much greater potential than is now being achieved.
Production in the last 25 years has increased greatly, largely due to improved varieties
fed by increased amounts of fertilisers, and better protected from weeds, pests and
diseases. But most planning of fertiliser use has been for the immediate task of
growing this year's crops. The new ideas and methods discussed in previous sections
have already been useful; but we are now at the important stage where these concepts
must be integrated in production systems if the research done is to be used fully.
Mathematical systems must be developed that are capable of predicting the effects
of changes in variables associated with the-environment, agricultural inputs (particularly fertilisers), crops, and their management. Attempts to synthesise the whole
system will indicate the gaps where more research is needed. When successful we
will have the means to exploit modern data handling systems for the benefit of agriculture.
New systems of crop production must have targets for yields to be achieved - for
these determine the size of the nutrient inputs that will be essential. We must plan
to achieve yields much nearer to known potentials of crop, or farming system. Yield
potentials were first stressed in the IPI Colloquium in Israel in 1969 when Warren
Wilson [1969] described a physiological basis for modelling maximum crop growth.
Maximum yield was considered again in later meetings. In 1970 Chandler discussed
how physiological barriers to higher yield are being overcome by plant breeders.
More recently [1976] he described the difference between experimental yields of
rice, and farmer's yields as 'the greatest challenge that rice research workers face in
1975'. This will be the theme for the discussions that begin our second quarter century.
10.1 Yield potentials of crops
Physiologists have calculated the yields that solar energy, water and carbon dioxide
make possible when healthy crops have sufficient nutrients for maximum growth.
Cooper's [1970, 1975] data in Table 20 are examples of grasses grown in climates
ranging from cool temperate to tropical; his 'calculated potentials' assume that 3%
of incoming radiation is converted to yield. 'Proved potentials' from experiments
show that 3% conversion of radiation can be achieved in Wales, and exceeded in
the tropics. Cooper's 'immediate targets' in Table 20 are realistic aims for the immediate future which must now be built into production models.
Accepted potentials are rarely achieved in experiments and average farmer's yields
are much less. Furthermore the differences between yields obtained by the 'best'
farmers and the country averages are not easily explained. Table 21 (FAO [1970-1976])
shows the need for the present world-wide interest in research and development to
achieve, in practice, yields much nearer to known potentials - which are five to seven
times larger than present averages. Yields in developed countries are two or three
times larger than those in developing countries. But even the best country averages
in Table 21 are less than half of the 'practical potentials' - already achieved by some
farmers.
396
Table 20. Calculated and proved potential yields of forage grasses (from Cooper [1970])
Climate
Yields of dry matter (t/ha)
Immediate
Proved potential
Calculated
Yield
(% conversion target
potential
of energy)
t/ha
(for 3%
t/ha
conversion
of radiation)
t/ha
Cool temperate ............... 35
Warm temperate .............. 50
Dry summer (Mediterranean)
and dry winter ............... 503
Tropical .................... 50
New Zealand
2 Aberystwyth (Wales)
Assuming irrigation available
2T
32
(2.2)
(1.8)
20
35
27
854
(1.6)
(4.9)
403
50
4 Puerto Rico
Table 21. Average yields (in t/ha) of product of important crops compared with potential
yields
Established potential (experiments or farmers).
Wheat
Rice
Maize
Potatoes
12
14
13
90
Average yields (FAO data [1976])
W orld ..................................
A frica ...................................
North America ..........................
A sia ....................................
Europe .................................
All developed regions .....................
All developing regions ....................
Centrally-planned economies ...............
Best average yield in country with large
production ..............................
1.7
1.0
2.0
1.2
3.0
2.2
1.2
1.7
2.4
1.8
3.7
2.4
4.8
5.7
1.9
3.1
2.8
1.1
4.5
1.9
3.8
5.0
1.3
3.0
14.4
8.3
23.0
9.8
19.3
21.7
8.5
13.6
5.2*
6.0**
5.7***
37*
* Netherlands
Japan
* USA
It is not sufficient to admire potentials, we have to devise the systems that give farmers
the possibility of achieving them. It will not be sufficient to use average recommendations- for these are intended to produce average crops. The much larger quantities
of nutrients needed by high-yielding crops must be recognised and built into production models. If we have an immediate target of 20 t/ha of dry matter from a forage
crop containing 2% K, we have to plan to provide 400 kg/ha of K (as much as is
exchangeable in the surface layer of good farmland in temperate countries). A wheat
397
crop yielding 10 t/ha of grain (a potential proved possible by farmers) must contain
200 kg K/ha at flowering. A potato crop yielding 80 t/ha of tubers (also proved by
farmers) will remove 300-400 kg/ha of K, as will a large sugar beet crop. The results
of experiments testing the fertiliser responses of average crops are of little help in
the task of planning for these larger yields. The whole nutrient cycle must be examined;
we must forecast correctly the K contributed by soil and subsoil, and we must place
the supplement of fertiliser-K correctly in the soil, it may be inefficient if confined to
the surface zone.
The fertiliser demands of high-yielding cereals have been discussed by Russell et at.
[1970] in a study made for USA ID. Most of their work was with N, but they discussed
K (and P). They stressed that there had been much less work on K than on N and
that response to K was variable and infrequent with rice. This was surprising as in
Philippines a rice crop producing 4 t/ha absorbed 219 kg K/ha, but only 20 kg P
and 90 kg N. 80-90% of the K was in the straw and would be recirculated if plant
residues were returned to the soil - said to be uncommon. They also drew attention
to the large amounts of K needed by high-yielding wheats and hybrid maize and said
that K concentrations in fertilisers had been increased to meet this need in USA
and Rhodesia.
Russell et al. [1970] stress the need to study the potassium cycle as affected by the
use of crop wastes, and also the risk of leaching. They say 'only constant surveillance,
either through field experiments or by means of soil tests, will provide adequate
warning of impending needs for potash (and phosphate) fertilisers'. They instance
change in overall fertiliser use in the Far East which has resulted in more K being
supplied relative to N.
We must allow for the future advances that will be made by plant breeders and
physiologists: Their work will make present 'targets' out of date and we must work
with them to ensure that advances in other branches of agricultural science are fully
exploited by providing adequate nutrition for crops of greater potential. An example
of the kind of work that will be important to us in future is given by recent investigations by Smid and Peaslee [1976] who found assimilation of CO2 by maize to be
highly correlated with concentrations of K in tissue as affected by differences in
leaf position and light received. There were complex interactions between the four
factors of CO, assimilation, K concentration, plant population density and position
of particular leaves. Work of Koch and Mengel [1977] is also important in this context. Up to now most modelling of above-ground portions of plants, and of soilroot systems has proceeded separately. The aim of multidisciplinary studies planned to
achieve maximum yield must be to construct whole-plant models.
10.2 Fertilisers in packages of improved practices
In early work on the value of fertilisers in developing countries they were usually
tested on farmers' crops grown in traditional ways; often the gains were relatively
small and sometimes the increased yield did not pay for the fertiliser. Experiments
done under FAO's scheme (FAO [1974]) provide many examples. It is now recognised
that large improvements in agricultural productivity may only be obtained by simultaneously improving nutrition and cultural practices, controlling pests and diseases,
and growing an improved variety of crop. There are large interactions between these
398
factors which combine to give large increases in yield. The concept of applying
a 'package' of improved practices is best seen in the exploitation of the new highyielding varieties of cereals which have led to the so-called 'green revolution'; the
large potential of the new material is only achieved when crops are adequately
fertilised, water control is good and they are protected from pests and diseases.
An example of the development of a package for maize in Nigeria has been given
by Taylor [1977]. His complete package contained (a) an improved variety, (b) high
fertiliser (125 kg N, 50kg P2 0 5 , 50kg K 2O/ha, compared with half as much), (c) herbicide, (d) control of Striga (a parasitic weed), (e) insecticide, (f) high plant population
and (g) early sowing. The value of these factors was measured by yields obtained
when one input was removed from the package. Improved variety was most important,
increasing package yield by 86%; full fertiliser consistently gave 30% more yield
than the half rate. Early sowing and high plant populations were not so important
in all experiments. Such investigations give little information on interactions and
on their relation to size of input.
Packages (often called 'blueprints') for growing better crops are now being developed
for many systems. Evans [1977] described to the York Colloquium a successful
blueprint for potatoes and the development of one for cereals. Blueprints use our
current knowledge by applying all inputs that are likely to improve yield. They are
designed to help farmers and are not a substitute for the scientific work that must
be done to define the inputs that will secure maximum interactions between factors
affecting growth. Packages of improved practices will be adopted most readily and
successfully by well-educated farmers who have capital enough to pay for the necessary inputs including fertilisers, and extra labour if needed. They will have to be
devised and applied very carefully in developing countries, improvements suggested
must suit the farmer's social and economic conditions and he must be able to put
the whole of a package into practice. It is of little use to buy improved seed and
fertiliser if the extra crop is lost by an unforeseen pest attack, or the family labour
is not sufficient to weed the land when necessary.
10.3 Developments in work on soil potassium
Some of the new information we need for constructing production models will only
be obtained by direct field experimentation. An example is the benefit likely from
deep placement of K in the subsoil under varying moisture regimes. Other advances
will come from combined theoretical and laboratory work. Methods that measure
capacity, intensity and rate factors without ambiguity, and in ways that allow them
to be incorporated in fundamental studies of plant-soil relationships, are needed.
In particular we must work more on methods of measuring the 'fixed' K that may be
released to crops; our present approach is too empirical and advances will only
come from studies of the soil, climate and crop factors that determine rate of release
by minerals.
The aim must be to write the equation
K (soil)
K (solution)
! K (root)
t K (plant top)
so that we can handle the whole system and identify the rate-limiting steps that
hinder nutrient uptake and use. The principles of soil science and crop nutrition are
399
the same in temperate and tropical agriculture; it is the rate constants that vary due
to the effect of temperature and moisture regimes on weathering processes, nutrient
transformations and movement, and on crop growth. (P1's interest is in potassium
which has, until recently, been neglected in efforts to improve tropical agriculture.
Attention has concentrated on the common deficiency of phosphate and on the
large gains from using nitrogen fertilisers. The supply of potassium may be sufficient
for a first round of improvement, but the total present is often small and is quickly
exhausted. Papers to the Abidjan Colloquium (e.g. Heathcote [1973]) showed how
essential potassium studies will be in crop improvement schemes in the tropics.
There is a great need to increase work on potassium in soils of tropical countries
such as those in South America and Africa where there is a vast potential for increased
food production. Studies of the amounts of K present and their availability, and of
cropping cycles involving potassium, are all needed.
10.4 Progress against constraints
A constraint to our progress towards larger production and greater efficiency is the
subdivision of agricultural science into subjects that are still sharply separated by
disciplinary boundaries. Most advances in crop production are now being achieved
by exploiting the interaction between improved plants and better control of inputs
and other factors affecting their growth. Most successful research that has practical
purposes therefore involves multidisciplinary work. For example a recent Conference
(Wright [1976]) was wholly devoted to work on adapting plants to tolerate stress,
nutrient deficiencies or toxicities. There is room for much more work to investigate
genotype-environment interactions, and the term 'environment' includes supply of
potassium! It is essential that those concerned with fertiliser use should cross boundaries to join in the work of other crop scientists.
We have to be aware of the possibility of public constraints on the use of fertilisers.
Twenty-five years ago fertilisers were attacked by those who believed that all plant
nutrients should have an organic origin. Studies of nutrient balance in farming
systems showed that the closed cycle of organic enthusiasts can never lead to large
productivity from poor soils, more nutrients must be injected into the system. This
subject has been replaced by public debates on the amounts of energy needed to
make fertilisers and on their role in polluting natural waters. These criticisms are
mainly directed against the use of N and P, K fertilisers need little energy in manufacture and K is not involved in eutrophication. Nevertheless any general move to
restrict N and P fertilisers would lessen the K applied, and make it less efficient.
The result would be a serious loss in crop yields without any certain gain in health
or welfare of the community.
The value of our work is not sufficiently recognised, perhaps we are too modest in
the presence of other scientists, or the public. Fifteen years ago G. V. Jacks [1963]
wrote 'the invention of chemical fertilisers has, however, enabled us to raise the productivity of soil far above that obtained under the climax association... the philosophical
implications of this tremendous breakthrough have, as yet, scarcely penetrated into
the realms of soil science...' We must stress that fertilisers are essential and that
our work is vital for using them efficiently. Our future research on soil fertility will
be needed to exploit the potentials created by other advances in agricultural science.
400
Better crops will be grown in new and more productive systems of agriculture that
are profitable, use energy efficiently, conserve other resources, and are ecologically
clean.
11. References
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10th Colloquium Intern. Potash Institute, 51-69 (1973)
Anderson, G. D.: Potassium responses of various crops in East Africa. Proc. 10th Colloquium
Intern. Potash Institute, 413-437 (1973)
Arnold, P. W.: The potassium status of some English soils considered as a problem of energy
relationships. Proc. Fertil. Soc. No. 72 (1962)
Bauemer, K. and Bakermans, W.A.P.: Zero tillage. Adv. Agron. 25: 77-123 (1973)
Baldwin, J. P.: A quantitative analysis of the factors affecting plant nutrient uptake from some
soils. J. Soil Sci. 26, 195-206 (1975)
Bar-Akiva, A.: Chemical and biochemical measurements on peanuts as a means of controlling
yield and plant performance. Proc. 9th Congress Intern. Potash Institute, 211-220 (1970)
Barber, S. A., Walker, J. M. and Vasey, E. H.: Mechanisms for the movement of plant nutrients
from the soil and fertilizer to the plant root. J. agric. Fd. Chem. II, 204-207 (1963)
Barnes, A., Greenwood, D.J. and Cleaver, T.J.: A dynamic model for the effects of potassium
and nitrogen fertilizers on the growth and nutrient uptake of crops. J. agric. Sci. Camb.
86, 225-244 (1976)
Beckett, P. H. T.: Studies on soil potassium I, 11. J. Soil Sci. 15, 1-23 (1964)
Beckett, P. H. T. and Nafady, M. H. M.: Studies on soil potassium VI. J. Soil Sci. 18, 244-262
(1967)
Bellis, E.: Intensification of plantation rubber production by Hevea brasiliensis through
manuring. Proc. 9th Congress Intern. Potash Institute, 345-356 (1970)
Blackman, F.F.: Optima and limiting factors. Ann. Bot. 19, 281-295 (1905)
Boyd, D. A.: The effect of potassium on crop yield. Proc. 3rd Congress Intern. Potash Institute,
143-154 (1956)
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403
The Interaction of Potassium with Other
Growth Factors, Particularly
with Other Nutrients
A. Loud, Societd Commerciale des Potasses et de I'Azote, Mulhouse/France*
1. Introduction
Some of the interactions which affect the way in which crops respond to a nutrient
such as potassium are with qualitative factors such as form of fertiliser, method and
date of application, crop variety, etc. The occurence of such interactions may lead
to changes in detail of the ways in which K fertilisers are used. The more important
type of interaction is with quantitative variables such as level of other nutrients
applied, rate of irrigation, plant spacing, etc. Among these, the interaction of potassium with other nutrients, particularly with nitrogen, are the most important. Emphasis
is given in this paper to effects on crop yield. Effects of interactions between cations
on nutrient uptake and plant composition have received much attention elsewhere,
e.g. Lehmann [1974] and the subject was extensively reviewed by Munson [1970].
Most attention is given to results obtained in work carried out by the Socijt Commerciale des Potasses et de I'Azote (S.C.P.A.) at the Research Station at Aspach-leBas and throughout France, which are discussed in relation to published findings.
Because change in the level of one factor may alter the way in which a crop responds
to another factor, it is desirable to investigate the effects of changing the level of one
in combination with increasing levels of the other. For this purpose factorial experimental designs are used where the treatments comprise all possible combinations
of the two or more factors under test, though, if large numbers of combinations are
involved, some of the possible combinations may be discarded without sacrificing
important information.
In computing the results of such experiments, it is often convenient to fit equations
to the experimental yields obtained so that the effect of factor (x) may be expressed
in a continuous manner rather than in tables of means. In the present work the
equation y=a+bx+cx2 is used to describe the effect of a single nutrient, and the
combined effects of two nutrients, e.g. N and K, can be described by the equation:
y=a+ bN+cN+dK+eK+fNK
where a=yield at the lowest levels tested and b to f are constants, characterising
the form of the response. The latter equation represents a response surface showing
A.Lou , Chef du Ddpartement d'Agronomie S.C.P.A., 2, place du Gdn6ral de Gaulle,
F-68053 Mulhouse/France
*
407
Table 2. Calculated optimum rates of N and K and N x K interaction at these levels. Outside
experiments
Crop
Optimum rate
(kg/ha)
N
K
Potatoes ........................
Sugar beet ......................
M aize ..........................
W heat ...........................
Barley ..........................
168
142
127
104
88
224
213
132
95
94
N x K interaction
t/ha
1.70
2.60
0.30
0.18
0.18
Number of
experiments
11
16
56
65
24
2.1.1. Potato
It would be expected that this crop, which responds strongly to both N and K, would
show strong N x K interaction. Boyd [1961] reported results of 100 experiments in
which the effect of K averaged over all other treatments was to increase yield by
4.8 t/ha, but by 10.4 t/ha when N and P were also applied. Inkson and Reith /1966]
used quadratic expressions to describe the results of 43 experiments extending over
10 years. Other, similar, experiments were described by Simpson and Crooks [1961]
and Widdowson and Penny [1961b].
Figure 2 shows mean (17 years) yields achieved in the Aspach experiment and clearly
indicates a strong interaction effect. This varied much from year to year (2 years
Tub.
36
t/ha
N150
35
34
N10 0
33
N50
32
31
kg/haK
25
0
83
166
Fig.2. Potatoes. Response to increasing K at 50, 100 and 150 kg/ha N. The shaded area
shows how the response to N increases as K level is raised (mean of 17 crops)
410
negative, 7 years positive but less than 1.25 t/ha, 5 years between 1.25 and 2.5 t and
3 years over 2.5 t/ha). Calculation indicates a maximum yield of nearly 36 t/ha at
185 kg N and 226 kg K per ha and optimum profitability, based on current prices,
of French Frs. 2808/ha at 171 kg N and 211 kg K, but these lie outside the range
tested and can only be taken as an indication that more generous fertiliser application
than that used would have been justified.
The outside experiments have been described by Lou [1977] and are summarised
in Table 2.
Taken together, the results from all these experiments show that while the N x K
interaction is variable it is rarely negligible or negative and would be expected to be
of the order of at least 1-1 '/ t/ha between the extremes of treatment tested and that
for potatoes the fertiliser dressings should be from 150-170 kg N and 210-225 kg K
per hectare.
2.1.2 Sugar Beet
Positive N x K interaction has been reported by Draycoft. [1972], McDonell et al.
[1966], Tinker [1965], Widdowson and Penny [1967]. As well as affecting yield, the
interaction also affects sugar content and juice purity.
The large interaction effect on root yield recorded at Aspach is illustrated in Figure 3.
Roots
45 -N150
N 100
44
43
42
41
N50
40
39
38
,
0
kg/ha K
83
166
Fig.3. Sugar beet. Response to K at 50, 100 and 150 kg/ha N (mean of 18 crops)
As with potato there was great annual variation (5 years negative, 3 years 1.5 t,
4 years 1.5-2.5 t, 4 years 2.5-5.0 t and 2 years above 5 t/ha). Calculated maximum
yield (Table 1) (49.6 t/ha) lay well outside the rates of fertiliser tested, the highest
yield actually recorded (45.1 t at the highest rates of N and K) gave a profit of
FFrs. 694/ha. The results obtained suggest that further investigation is needed using
more than 3 rates of N and K and taking applications to much higher levels. In outside
experiments (Table 2), the interation on the basis of calculated optima was somewhat
411
larger (2.6 t/ha) than at Aspach and 1.5 t on the basis of differences between extreme
treatments actually tested.
Sugar content is usually reduced by increasing N which favours leaf development at
the expense of storage root and this effect can be partially counteracted by potassium.
At Aspach, however, the N x K interaction increased sugar content by only 0.1%,
much lower than is usually recorded, but sufficient to increase the interaction effect
when yields are expressed as weight of sugar.
K, Na and, especially, noxious N content all adversely affect sugar extraction rate.
While increasing K application slightly increases juice K content, it reduces Na and
amino-N contents so that it generally has a favourable effect on sugar extraction.
N :K balance of the fertiliser is therefore important. Von Muller et al. [1962], in
sand culture, and Heistermann [1968] indicate that the optimum N :K 2 0 ratio in
fertiliser is between 1 :2 and 1 :3 and that a closer ratio increases noxious N content.
Kichl [1977], on the basis of solution culture, recommends the ratio 1:2.35 for
optimum yield and quality.
Thus yield of extractable sugar is affected by N x K interaction effects on all 3 components, root yield, sugar % and juice purity.
2.1.3 Maize
Arnon [1975] has observed that N x K interactions are frequent in field experiments,
showing that the N : K balance is particularly important for this crop. N and K are
taken up by the maize plant in similar quantities. The N :K balance is important
from the earliest stages of crop development and has much influence on lodging
caused by parasitic fungi (e.g. Giberella). There are numerous references (Burkers• Grain
6.6
N9 9
tiha
6.5
6.4
6.3
N6 6
6.2
6.1
6.0
5.9
5.8
5.7
5.6
r
5.5
kgha K
5.4
5.3
33
.
0
62
i
125
Fig.4. Maize. Response to K at 33, 66 and 99 kg/ha N (mean of 9 crops)
412
roda 11965]; Canard [1967]; Fisher [1960]; Krantz and Chandler [1951]; Liebhardt
and Murdoch [1965]) showing that high N with low K favours lodging. The N :K
balance affects all aspects of quality in maize: protein content, silage quality, 1000 grain
weight (Burkersroda [ibid.]; Stangel [1965]).
In the Aspach experiments, as shown in Figure 4, the interaction effect on grain
yield was very marked. It was strongly positive in 5 years out of 9 and slightly negative
only once. Calculated maximum yield (6.75 t/ha) corresponded to application of
133 kg N and 150 kg K/ha, and maximum profit (FFrs. 628) to 115 kg N and 115 kg
K, only slightly outside the limits of the experiment.
The overall result of outside experiments (Table 2) showed a calculated value for
the interaction of 0.3 I/ha, while the mean value for the extremes of treatment actually
tested was 0.19 t. The means were made up from 56 individual results, 36 showing
positive interaction (mean 0.39 t/ha) and 20 negative (-0. 14).
The N x K interaction in maize is increasingly important as cultural conditions are
improved and intensified. As N fertiliser is increased in quest of maximum yield, it is
important to increase K application correspondingly.
2.1.4 Wheat
Few workers have recorded strong N x K interaction effects and some of the available results are mutually contradictory; however, the interaction does assume importance as N rates are increased, as Talibudeen et al. [1976] have shown. Figure 5
shows that the interaction in the Aspach experiments was appreciable (0.075 t/ha)
on the average, though this effect was not statistically significant. It was positive
only in 5 years out of 13. Calculated maximum yield (4.92 t) at 114 kg K and 150 kg K
per ha was only just outside the limits of the experiment. Maximum profit was
achieved at 95 kg N and 66 kg K/ha (FFrs. 278/ha). In outside experiments the calGrain
4.8
_mmt
125
t/hN95
4,7
4.6
4.5
4.4
4.3
4.2
"N 65
kg/ha K
4.1
0
67
134
Fig.5. Wheat. Response to K at 65, 95 and 125 kg/ha N (mean of 13 crops)
413
culated interaction was 0.18 t/ha, lower than that in maize but higher than in wheat
at Aspach. The mean of observed interactions between the extreme rates tested in
the various 65 experiments was 0.11 t/ha, positive in 43 cases.
The N x K interaction also affects quality in wheat. Increasing nitrogen tends to
decrease mean grain weight, but the tendency is lessened when sufficient K is applied
as illustrated by results from an experiment in the Dr6me where increasing N from
40 to 120 kg/ha reduced 1000 grain weight from 76.2 to 74.3 g while at 167 kg K/ha
it was reduced only from 79.0 to 78.3 g. Interaction is also shown in baking quality
of grain where potassium counteracts the unfavourable effects of N on gluten quality.
Effects on dough swelling have been investigated in N x K factorial experiments
and it was shown that the swelling coefficient could be maintained by judicious
adjustment of the N :K ratio in the fertiliser (Chevalier [1975]).
Wheat appears to be less affected by the N x K interaction than maize but nevertheless the effect on yield and quality is frequently positive.
2.1.5 Barley
The crop behaves similarly to wheat. In the case of malting barley, the N x K interaction is of no great importance as N rate is usually kept down to avoid unfavourable
effects on malting quality, but N is used liberally on the feeding crop both to increase
grain yield and protein content. Widdowson and Penny [1961a] and McLeod [1969
a and b] have reported investigations. In the Aspach experiment the interaction on
the mean of 14 crops was negligible (Figure 6), being slightly positive in 6 years.
Calculated maximum yield was at 72 kg N and 108 kg K/ha (4.17 t/ha) and maximum
profit FFrs. 405/ha at 68 kg N and 68 kg K. In 24 outside experiments, interaction
at the calculated maximum yield was +0.18 t/ha, similar to that in wheat. Again
there was considerable variation from trial to trial.
4.2
. Grain
t/ha
N70
4.0
N50
3.9
3.8
3.7
3.6
N30
3.5
3.4
kg /ha
K
3.3
0
67
134
Fig.6. Barley. Response to K at 30, 50 and 70 kg/ha N (mean of 14 crops)
414
It thus appears that barley behaves similarly to wheat but it is possible that high
yielding N responsive varieties may show a larger interaction effect.
2.1.6 Grass and forage crops
Crops within this group are botanically and culturally diverse. Where forage crops
are included in the rotation, they intensify the N x K interaction shown in other
crops because they are exhaustive of soil K, with consequent effect on the following
crops.
Adams and Twersky [1960] demonstrated the effect of the interaction on winter kill
in Cynodon dactylon. Welch et al. [1963] studied the NK response surface and
economic returns from the same crop. McLeod [1965] found in factorial experiments
that lucerne, bromegrass, cocksfoot and timothy showed N x K interaction on yield
increasing in that order. Monroe et al. [1969] worked with meadow grass and blue
grass in sand culture. Talibudeen [1976] studied yield and nutrient uptake by perennial rye grass in relation to the N: K ratio in the soil.
The N x K interaction plays a part not only in total dry matter production but also
in seasonal pattern of production, longevity of the sward and forage quality. Teel
[1966] showed that the N : K ratio was an important determinant of forage quality.
The N x K interaction on temporary grass has been studied at Aspach in experiments
in which the grass was cut at the grazing stage, under factorial fertiliser treatments
(4N x 2P x4K) which started in 1967. N rates were 40, 60, 80 and 100 kg N/ha in
early spring and 20, 40, 60, 80 kg after each cut. Results have been fully reported by
Garaudeaux and Chevalier [1976]. Table 3 shows the results obtained with cocksfoot.
Table 3. Effect of N and K fertiliser applied annually on yield of cocksfoot (average of
10 years) - t/ha dry matter
N, kg/ha
100
220
330
440
Mean
K, kg/ha
250
0
125
8.32
9.13
8.70
9.27
8.85
8.40
10.49
10.49
10.57
9.99
8.80
10.83
12.25
11.95
10.96
375
Mean
8.41
11.18
12.74
12.96
11.32
8.48
10.40
11.04
11.19
10.28
The N x K interaction grew larger from year to year, amounting to 0.3, 0.42, 0.42,
1.54, 2.46, 2.66, 2.81, 2.71, 2.49 and 2.18 t D.M./ha in successive years, the mean
for the 10 years being 1.80. Most of this effect was due to the response to N at K0
falling off with the passage of time; (N 4 -N 0 ) declined from 4.6 t to -0.07, partly due
to frost damage. At K3 the corresponding effect decreased from 5.21 t to 4.28 t
D.M./ha. Similar results were obtained with perennial ryegrass (Chevalier [1978]).
In one of the outside experiments in which ryegrass/red clover mixture was grown
for 2 years for hay as part of a rotation including also potato, wheat and barley, the
results (Table 4) showed no N x K interaction, due to the antagonistic action of N
and K on the proportion of clover, even though the experiment was sited on a low
potash soil (Kch= 6 mg/100 g C.E.C.= 8 meq.) Response continued up to 105 kg/ha
N and 165 kg K.
415
Table 7.Effect of N and K fertilisers on yields of sorghum and maize (kg/ha dry grain) (after
Heathcote [1972])
N, kg/ha
1971, sorghum
K, 45 kg/ha
No K
1970, maize
K, 45 kg/ha
No K
58
116
1062
923
1071
1179
1666
2111
1388
2596
To summarise, positive N x K interaction is the rule in tropical plantation crops,
where cultivation is intensive and yields high. Though large interaction cannot be
expected in food crops grown in the traditional manner, the introduction of higher
yielding crops and the general intensification which is taking place in some areas
has brought the need for potassium fertiliser to the fore, as it has been found that
lack of potassium has increasingly limited the response to N fertiliser.
2.1.10 Discussion
The extent to which crop yield and quality are affected by the interaction of N and
K depends upon environmental conditions of soil and climate, on type of crop and
variety and upon the degree of intensification, particularly the level of fertiliser use.
The improvements which have taken place in all these areas in the past 25 years
have pointed the need to take fuller account of interactions between nutrients. As
successive barriers to high yield are eliminated, so the nutrient requirements of crops
increase and at the high rates at which fertilisers are now used the interaction effects
become more important. As Steineck ([1974] and in this volume) has shown, using
his nutrient solution technique, there is a close relationship between N and K in
their physiological functions and the main effect of potassium is to improve the
efficiency of N utilisation. Increased K uptake results in increased N uptake and
vice versa and Steineck's opinion is that the plant takes up only that amount of K
which it needs for full utilisation of N.
Plant analysis is useful in illuminating the results of factorial experiments. Some
examples of N x K interaction effects on plant K content are given in Table 8, which
shows K contents of plants under the extreme treatments in a number of experiments
in France. The effect of increasing N at low or medium rate of K is to reduce K content,
while at high K rates it is increased. The effects of the nutrients on both plant composition and yield have important consequences in the nutrient cycle, particularly
where crops which take up large amounts of K, like the forage crops, are concerned.
The effect of treatment on K removal largely explains why such crops show marked
N x K interaction, while the effects on K balance in the soil are important for the
following crops in the rotation. An example showing removals of K in a cocksfoot
experiment at Aspach is given in Table 9. The mean response to nitrogen over the
10 years increased as K application was raised: 0.95 t at K1,2.17 at K1 , 3.15 at K,
and 4.55 at K3 , while the K balance corresponding was -72, -81, -522 and -1318 kg
K/ha.
Many interaction effects can be explained by the operation of the law of limiting
factors. As yields are raised by the introduction of more responsive cultivars and more
418
Table 8. Effect of N x K interaction on K content of plants
Site
1976 Coincy
Rampillon
Aigues-Mortes
1975 Puch
Aigues-Mortes
1974 Omicourt
Elliant
1973 St.-Etienne-en-Bresse
1970 Omi6court
Le Chesnoy
Tauxigny
1966 Pont-St.-Martin
Crop
Wheat
Wheat
Vine
Wheat
Vine
Spinach
Peas
Wheat
Beans
Barley
Barley
Wheat
Organ
Leaf
Leaf
Petiole
Leaf
Petiole
Leaf
Vines
Leaf
Runners
Leaf
Leaf
Leaf
% K in dry matter
At lowest N
At highest N
rate
rate
Interaction
No K
HighK
No K
High K
NxK
1.41
2.02
2.31
2.48
2.63
3.53
1.06
1.99
2.15
2.78
3.43
4.05
+0.32
+0.41
+0.34
2.09
1.94
4.31
2.06
1.60
2.07
1.40
1.56
0.85
3.45
3.62
6.52
2.38
2.12
2.71
1.64
1.74
2.20
0.93
1.79
4.22
1.82
1.07
1.21
1.35
1.69
0.55
3.30
4.06
7.92
2.61
2.50
2.73
1.96
2.12
2.45
+0.48
+0.30
+0.75
+0.28
+0.45
+0.44
+0.18
±0.12
+0.28
Table 9. Effect of N and K applied to cocksfoot on the K balance kg/ha after 10 years
(Total K applied - total K removed)
N applied annually
(kg/ha)
K applied annually, kg/ha
0
125
250
375
110
-1591
220
330
- 939
-
-1452
-1459
-1234
-1054
- 628
-1022
440
+ 158
- 464
-1659
-1019
- 739
-
219
+1030
283
N fertiliser, soil supply of K is increasingly likely to be limiting. However, many of
the interaction effects, particularly as they affect crop quality, are more complex
and concerned with the specific interacting functions of N and K in plant nutrition.
2.2 Phosphorus
Generally speaking, P x K interactions are smaller than those between N and K
and they have attracted less attention, agronomists often preferring to investigate
P and K effects separately. There does not appear to be a close connection between
the functions of P and K in plant nutrition and, while N and K are both taken up in
large quantity by the plant, uptake of P is relatively small. It seems that P x K interaction is only marked when soils are poorly supplied with P and K; on reasonably
fertile soils where fertiliser is regularly applied there is little interaction. Some experimental results from a range of sites in France (Table 10) may serve to demonstrate
this point. It is seen that the P x K interaction increased, as did the separate responses
to P and K, with increasing poverty of the soils. In the last example in Table 10, on
soil reclaimed from pine forest and extremely deficient in both P and K, the interaction assumed very large proportions.
419
Table 10. Responses to P and to K and the P x K interactions at various sites in France
Site
Crops
(No. of
Effects
Fertiliser applied*
(tonne/ha)
years)
P, kg/ha K, kg/ha
Ozoir-le-Breuil
(1964/76)
Calc. loam.
P 9, K 21 mg/100 g
maize
(13)
wheat
hard wheat
Tauxigny (1966/73)
Sandy loam.
P 3, K 12 mg/100 g
maize
wheat
hard wheat
barley
Laval (1971/74)
Loam.
P 6, K 6 mg/100 g
Pocancy-Marne
(1957/69)
Poor chalk cleared
from pine forest
pH 8.3
(71% CaCO 3)
*
P2-K 0 K2-K 0 P x K
72
138
0.07
0.40
0.04
(8)
55
112
0.48
0.33
0.06
maize
wheat
barley
(4)
73
150
1.20
1.92
0.96
sugar beet
wheat
barley
+ oats
lucerne hay
(4)
(4)
(8)
(1)
(3)
83
65
250
124
19.3
2.10
14.8
0.48
12.20
0.63
61
76
116
200
2.10
4.3
0.60
3.8
0.29
3.2
Average annual rate of highest dressing; Control No P No K
On very poor soils and in areas, such as much of the developing world, where agriculture is primitive, the first problem is to detect by simple experiments the main
nutrient deficiency. Most frequently this is P, and though such soil may contain only
moderate amounts of K, it is only when the P deficiency has been corrected that any
need for K fertiliser will be found. In such conditions the P x K interaction is important.
In advanced agricultural areas, the aim of fertiliser policy it to build up and maintain
in the soil adequate levels of P and K and, under such circumstances, appreciable
interaction is rare, as indeed are spectacular responses to P or K fertiliser.
2.3 Magnesium
The K x Mg interaction has been studied mainly from the point of view of the mutual
antagonism the two cations show in their uptake by the plant from the soil. According
to Salmon [1963, 1964] magnesium deficiency does not result solely from low soil
content of exchangeable Mg but from ion antagonism in very acid soils and those
+
of high exchangeable K content. K ions are easily taken up by the plant and so
++
ions.
reduce the diffusion of the heavier hydrated Mg
2.3.1 Plant analysis
There is considerable interest in plant analysis in connection with K: Mg antagonism.
Experiments investigating the effects of K or K x Mg on maize yield have given
420
information on the relationships between yield and cation (K, Ca, Mg) composition
of the plant (Louae [1962. 1963, 1965]). Generally there was negative correlation
between leaf K content and Ca and Mg contents. As Figure 7 demonstrates, the
K :Mg antagonism is most evident over the range where K supply is deficient or
low; as the K content increases to the level where K supply no longer limits growth,
the depressive effect on Mg content diminishes, which suggests that the dressings
of potassium fertiliser used in practical maize cultivation are very unlikely to induce
magnesium deficiency. The K:Mg ratio is important in viticulture and much work
has been done on this in France (Loud [1968]). Drying of the stalk ('Dess&hement
de ]a rafle') is associated with low Mg content rather than with high K:Mg ratio
(Delas et al. [1976]). In vine, the most sensitive indicator of nutritional status is the
petiole. It is found that when K content is below optimum, increasing K content depresses Mg but when K content is adequate, further change in K level has little effect.
Ca and Mg*l
1.4
1.2
Ca
1.0*
0.8
0.6""",
"4
"
o
0.4
.
."
0.2
0.4 0.6 0.8 1.0
1.2 1.4 1.6
1,8 2.0 2.2 2.4 K,
Fig. 7. Effect of K content on % Ca, and % Mg in leaf dry matter
2.3.2 Effects on yield
Comparatively little experimental work on the K x Mg interaction has yet been done
in field experiments and this has been mainly on tree fruits, vines, protected and
special crops. Usually leaf analysis has been done as in the case for vine quoted above.
Much attention has been given to K :Mg antagonism in herbage crops, in view of
the significance of low herbage magnesium contents in relation to animal nutrition.
Even though low magnesium content is now less blamed for the occurence of hypomagnesaemia it is nevertheless desirable to aim for adequate herbage Mg content.
Laughlin [1966]; Simpson and Crooks [1973]; Walsh and O'Donohoe [1945];
Hossner and Doll [1970] and Birch et al. [1966] have reported work on temperate
arable crops, mainly potatoes. In the tropics, the interaction has been reported on
coconuts in the Ivory Coast by Coomans [1977] and Ochs and Ollagnier [1977].
Some results are available from field experiments of S.C.P.A. carried out in France
421
in areas where magnesium problems are experienced (Lout [1977]). Results from
one experiment at Pau on a very acid (pH 5) soil deficient in K, Ca and Mg, formerly
under bracken and planted to maize, are given in Table 11. For the second three
years the magnesium dressings were applied either in equal amount annually or in
one dressing for the three years. There was positive interaction in all years when Mg
was applied in annual dressings. Where the heavy dressing of Mg was applied, Mg
depressed yield at the lowest rate of K, while having little effect at higher K levels.
Results for wheat and beet on a calcareous soil (pH 8) very low in Mg (exch. Mg 0.030I,0)
showed strong positive interaction. Applying Mg at 60 kg/ha to sugar beet increased
yield by nearly l It/ha. K decreased yield in the absence of Mg but increased it when
Mg supply was adequate. With wheat, interaction was still positive but much smaller;
while there was no response to K applied at more than 80 kg K/ha when no Mg was
applied, doubling the rate of K when Mg was also given raised yield by about 0.08 t/ha.
The mean response to Mg at 50 kg/ha was 0.4 t/ha.
Table I1. Effect of K and Mg on yield of maize at Pau (t/ha dry grain)
Yield of
Control
Main effects and interaction between highest
and lowest nutrient rates*
1963-1965
Mg applied each year
1.40
K-K
2.67
Mg 60-Mg0
0,76
K x Mg
0.21
1966-1968
Mg applied each year
2.55
K17-K27
2.66
Mg 12 -Mg 20
0.29
0.28
1966-1968
All Mg applied
in 1966
K,,-K 42
Mg3 6.- Mgs
1.68
-0.81
*K
4.55
0.35
and Mg, kg/ha
Table 12. Effect of K and Mg on yields of sugar beet and wheat at St.-Jean-sur-Moivre
Yield of
Control
Roots (16% sugar) and grain, t/ha
(mean of 2 crops)
Effects
Sugar beet
35.0
2.2
Mg 60-Mg 0
10.7
K × Mg
3.4
Wheat
4.25
K167-K83
0.09
Mg 30 -Mg.
0.41
0.03
K 3 3 4 -KI
67
2.3.3 Discussion
Low magnesium content in crops is, in general, caused by low soil Mg rather than
by application of potassium fertiliser and, if low, the Mg status of the soil should be
improved to a level which will allow K fertiliser to be applied without causing depression of Mg content. To decrease K fertiliser in such conditions of Mg deficiency
422
solves nothing. Welte and Werner [1963] came to similar conclusions. It is interesting
to compare the interactions in Figures 8 and 9, the former showing the effect of
increasing Mg at 2 levels of K supply in solution culture of Sterignatocystic nigra
and the latter similar effects in a field experiment with sugar beet at St. Jean sur
Moivre. Clearly the response patterns are very similar; when Mg is low, increasing
K has unfavourable effects, while when Mg is sufficiently high, increasing K supply
improves yield.
Weight harvested
B2 K.500mg/I
0 3
10
Mg(mg/l)
Fig.8. Effect or increasing Mg at different K levels for Srerignmatocysris nigra
.Roots
46
44
42
32
h
B2
30
~
h
Mg
601
Fig.9. Effect of K level on response to Mg by sugar beet (St. Jean sur Moivre)
423
2.4 Sodium
In many crops sodium can have toxic effects, while in a few (halophytes) it is beneficial. In either case there can be considerable K x Na interaction. Toxic effects are
found on saline soils or where irrigation water has an appreciable Na content and
Heimann [1958] and Heimann and Ratner [1962 a and b] have studied this problem
showing that the two elements can have synergistic and antagonistic effects, according to the amounts of each present.
While Na is not considered generally to be an essential nutrient required by plants
in large amounts, except in halophytic species, it is a secondary nutrient. Lehr [1953]
tentatively classified crops on the basis of the extent to which Na could replace K.
Marschner [1971] has discussed K-Na relationships in a number of crop plants.
Under practical conditions, Na-K relationships are important in sugar beet, carrots
and, to some extent, cotton. Generally speaking, response to sodium is greatest
when K is in short supply; its effects are less as K level in the soil increases. Investigations have been made with a range of crops: potatoes, barley, oats, maize and
cotton, but usually in pot culture. Larson and Pierre [1953] worked with red beet,
oats and maize. Lancaster et al. [1953] showed that Na could increase cotton yield
on low K soils but that it had little or no effect when K was also applied. McEvoy
[1955] found a strong negative interaction in tobacco, showing that Na could substitute for K in some of its physiological roles.
2.4.1 Sugar Beet
A 50 tonne crop of beet removes 85 to 95 kg Na from the soil, of which less than
10 kg is in the roots, and there has traditionally been a preference for Na-containing
fertilisers like kainit, sylvinite and nitrate of soda. The increasing purity of modern
fertilisers has entailed a reduction in the amounts of Na applied indirectly, giving
an additional reason for paying attention to the K x Na interaction. These problems
have been dealt with in particular by Draycott [1970] and Holmes et al. [1961].
Results show that while both elements increase yield, the effect of one decreases
as the level of the other is raised and that K can only be partially replaced by Na;
even when large amounts of K are applied, sodium will further increase yield, showing that Na should always be applied. The interaction is also relevant to juice purity.
Heimann and Ratner [1962a] showed that on high sodium soils application of K
reduced Na uptake, improving juice purity, but on normal soils effects of the K x Na
interaction on K and Na content of the roots are not marked (Draycott [1970]).
Fodder beet behaves similarly, showing negative interaction between Na and K.
Lehr [1953] reported on 20 field experiments in the Netherlands and Masterson
[1958] tested combinations of 0, 250 and 500 kg/ha KCI with the same rates of
NaCI; the beet responded strongly to both nutrients and, though there was a marked
negative interaction on the basis of the four extreme treatments, it was clear that
both nutrients were needed and there was little difference in yield between the three
most generous treatments (500 kg KCI + 500 kg NaCI, 250 kg KCI + 500 kg NaCI
and 500 kg KCI + 250 kg NaCI).
2.4.2 Grass and forage crops
The K x Na interaction here is important from the point of view of herbage composition rather than yield (see de Beaucorps in this volume). There are large differences
424
between grass species in Na uptake (cocksfoot and ryegrass>timothy>fescue).
Hylton ei al. [1967] worked in solution culture and found little interaction as regards
yield but strong effects on Na and K content. It is desirable to aim at a content of
0.2% Na in dry matter.
Nowakowski [1971] studied the effect of replacement of K by Na on the content of
soluble carbohydrates and nitrogenous compounds in ryegrass.
2.4.3 Discussion
In conclusion, it can be said that the K x Na interaction is always significant in plants
which respond to sodium, halophytes, and the interaction effect is then negative.
Nevertheless, even in such cases, both nutrients are required for maximum yield.
According to Marschner [1971] Na is less effective in activating enzymes than is K,
except in certain species. When Na has favourable effects enzyme activity is optimised
when K Na ratio in the plant is appropriate.
2.5 Trace elements
General reviews have been made by Munson [1968] and Arnon [1975]. There appear
to have been few field experiments, most of the work having been done in pots or
the laboratory.
2.5.1 Molybdenum
Jones [1965] found that leaf Mo content in K deficient maize was much affected
by the anion accompanying K. Positive interaction has been found particularly in
legumes and Baroeio [1962] found K x Mo synergism in lucerne and wheat, Mo
stimulating K uptake.
2.5.2 Boron
Potassium boron relationships were studied by Reeve and Shive [1944]. There are
several reports in the literature of high K inducing B deficiency, notably in soyabean
(Woodruff et al. [1960]). Recently, Holevas [1976], growing olives in water culture,
found that K deficiency caused the accumulation of B in the leaves and vice versa;
B deficiency was evident when K supply was very high.
2.5.3 Iron
Many experiments in the field have demonstrated the effect of K fertiliser in reducing
iron toxicity in rice; Tanaka et al. [1972] found that high K content reduced iron
uptake and that high Fe content reduced K uptake.
2.5.4 Zinc
Interaction with K has been less investigated than that with P. According to Arnon
[ibid.], the tendency of P to reduce Zn content in maize is lessened by K, K and Zn
both being concerned in the activation of pyruvate kinase. Citing Thompson, he says
that Zn deficiency can be aggravated both by too low and too high K supply. We,
in S.C.P.A., have been concerned about K×Zn interaction in maize for several
years, and some of the K treatments in long-term K and N x P x K experiments on
soils where there is a risk of Zn deficiency have been split to study the effect of Zn
425
application (0 vs 6 kg/ha Zn). In one of our experiments, for example, where K at
150 kg/ha produced a large increase in grain yield (1.47 t/ha) and zinc a response
of 0.47 t/ha, there was indication of positive K x Zn interaction at 0.23 t/ha.
3. Interaction of potassium with other growth factors
Among all the interactions which are possible between potassium supply and other
factors, two which appear to be of great practical significance are: between K fertiliser and the return to the soil of farm residues (crop residues and farmyard manure)
where the reason for the interaction is found in the nutrient, principally potassium,
content of such materials, and between K and the availability of water to the crop
(irrigation). In the latter case the effects of water on growth are responsible for large
changes in the soil content of available potassium. Many other factors may be expected to interact with K supply but space does not permit all to be discussed here.
3.1 Irrigation
Achitov [1961], Hifner [1971] and Hudson [1958] have discussed general aspects
of the effect of water supply to crops (rainfall or irrigation) on response to K fertiliser which can often be explained by the effect of improved water relations in lengthening the growing period.
Irrigation improves growth, raising yield and increasing the need for K; the availability of soil potassium is modified; leaching may redistribute K in the soil profile:
- In well-aerated soils increasing water content increases the amount of K in solution
and improves its mobility so that uptake by the plant is facilitated, and so long as
the soil K reserve is adequate K nutrition will be good.
- In poorly-aerated soils, increasing the water supply may affect K nutrition adversely, through creating anaerobic conditions in which, because of lack of oxygen,
potassium availability is low (Larson [1954]),
- In very dry years, low water limits K uptake and, under these conditions, potassium fertiliser may be particularly effective in increasing K availability. In such cases,
response to K fertiliser often arises because the control yield is abnormally low.
Thus irrigation can have variable, not to say contradictory, effects on K response,
as illustrated in Younts' [1971] results with maize (Table 13). Improving the water
supply from deficient to optimum improved yield under the K0 treatment by increasing availability of soil K (strong negative interaction). Increasing water supply
beyond the optimum reduced the yield at K0. K fertiliser was more effective in improvTable 13. Effect of water supply on K response by maize (after Younts [1971])
Water supply
Grain yield (t/ha)
Without K
83 kg/ha K
Deficient (202 mm) .....................
Optimum (448 am) ....................
Excessive (655 mm) .....................
5.56
9.30
5.71
426
8.10
9.80
8.73
4. Conclusion
In 1956 Boyd said that most experiments up to that time had tested fertilisers at
relatively low rates of application and that too much reliance had been placed on the
main effect of nutrients ignoring interactions. Giving full weight to interactions
would indicate that the range of fertiliser dressing which would be considered optimal
or near optimal would be larger than had been suspected. Since then there has been
progress and the significance of interactions is now more widely appreciated.
Where, as in much of the developing world, fertilisers are little used, interactions
with potassium occur as successive limiting factors are eliminated. As yields increase
so will the need for potassium fertiliser.
Future improvements in crop varieties, in water utilisation and the general improvement of cultural techniques will enhance interaction effects. It is important to take
into account the effects of increasing yield on the potassium cycle in the soil. It has
been shown that important interaction effects arise in this way.
The study of interactions is important alike for the practising farmer, the research
agronomist and the mineral nutrition specialist. There is still a need for experimentation and for the interpretation of experimental results by interdisciplinary cooperation.
In practical terms, interactions are important because K improves the response to
other nutrients, increasing profitability which can only be fully assessed when interactions are taken into account.
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Arnon, I.: Mineral nutrition of maize. Intern. Potash Inst., Berne, 1975
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429
Boyd, D.A.: Fertilizer responses of maincrop potatoes; a reexamination of the experimental
evidence. J.Sci. Food Agric. 12, 493-502 (1961)
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Chevalier, H.: The influence of nitrogen and potassium dressings on wheat quality. Proc.
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Chevalier, H.: Fertilisation azote, phosphatde et potassique de la prairie temporaire exploitde au rythme de ]a pature. Fourrages No 62, 133-159 (1975)
Chevalier, H.: Influence de ]a fertilisation azotde et potassique sur la r6partition des productions annuelles d'un ray-grass anglais exploit6 en simualtion de pAture. 7th General
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Coomans, P.: Premiers rdsultats expdrimentaux sur ]a fertilisation des cocotiers hybrides en
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Delas, J., Dumartin, P., Molot, C. and Boniface, J.C.: Le dess&chement de ]a rafle dans le
vignoble bordelais. Connaissance Vigne et Vins, 10, No 3, 227-247 (1976)
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Agron. J. 54, 276 (1962)
Draycott, A. P., Marsh, J.A.P. and Tinker, P.B.1.: Sodium and potassium relationships in
sugar beet. J. agric. Sci. Camb. 74, 567-573 (1970)
Draycott, A.P.: Sugar beet nutrition; interactions between nitrogen and other fertilizer
elements, 28-29 and 64-66. Applied Science Publishers, London (1972)
Fisher, F. L.: The influence of nutrient balance on yield and lodging of corn. Agron. J. 58,
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Forster, H.: Influence of K and N fertilizers on the quality and yield of oil from old and
new varieties of rapeseed. Proc. 13th Colloquium Intern. Potash Inst., 305-310 (1977)
Gaillard, J. P.: Recherche d'un 6quilibre K/N dans la production de 'ananas frais au Cameroun. Rdsultats agronomiques: Fruits 25, No 1, 11-24 (1970)
Garaudeaux, J. and Chevalier, H.: Etude des interactions entre fumures azot~es et potassiques. R6sultats globaux obtenus dans les essais de longue durde de ]a Station Agronomique d'Aspach-le-Bas. Compte Rend. Acad. Agric. Fr. No 12, 746-759 (1975)
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Heathcote, R.G.: Potassium fertilization in the Savanna zone of Nigeria. Potash Review,
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the sugar contents of beets and on their processing. Potash Review, Subject 11, 17th suite,
(1962a)
Heimann, H. and Raner, R.: The influence of potassium on sodium absorption by plants
under saline conditions. Potash Review, Subject 24, 14th suite (1962b)
Heistermann, P.: Yield and quality of sugar beet and potatoes as effected by fertilizer N/K
ratio. Zesz. probl. Postep. Nauk, roln 84, 273-288 (1968)
Hbifner, W.: Influence of potassium on water economy, in: Proc. 1 th Colloquium Intern.
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Holevas, C.D.: Potassium-boron relationships in olive nutrition. 4e Colloque Int. sur le
contr6le de I'alimentation des plantes cultivdes, Gand, 11, 167-173 (1976)
Holmes, J.C. and Gill, W.C., Rodger, J.B., White, G.R. and Lawley, D.N.: Experiments
with salt and potash on sugar beet in South-East Scotland, Expl. Husb. 6, 1-7 (1961)
Hossner, L. R. and Doll, E. C.: Magnesium fertilization of potatoes as related to liming and
potassium. Soil Sci. Soc. Amer. Proc. 34, 772-774 (1970)
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and mineral content of italian Ryegrass. Agron. J. 59, 311-314 (1967)
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Jungk, A.: Influence of nitrogen and potassium concentration of nutrient solutions on
yield. Proc. 6th Colloquium Intern. Potash Inst., 310-319 (1968)
Kanwar, J.S.: Assessment of potassium fertilization in the tropics and subtropics of Asia.
Proc. 10th Congress Intern. Potash Inst., 261-282 (1974)
Kychl, A.: The effects of nitrogen and potassium nutrition on yield and quality of sugar
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Krantz, B.A. and Chandler, W. V.: Lodging, leaf composition and yield of corn as influenced
by heavy applications of nitrogen and potash. Agron. J., 43, 547-552 (1951)
Lancaster, J.D., Andrews, W.B. and Jones, U.S.: Influence of sodium on yield and quality
of cotton lint and seed. Soil Science 76, 29-40 (1953)
Landi, R.: La fertilisation des plantes potag~res de plein champ et les rapports d'interaction
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Larson, W.E. and Pierre, W.H.: Interaction of sodium and potassium on yield and cation
composition of selected crops. Soil Science 76, 51-64 (1953)
Larson, W.E.: Response of sugar beet to potassium fertilization in relation to soil physical
and moisture conditions. Proc. Soil Soc. Amer. 18, 313-317 (1954)
Laughlin, W. M.: Effect of soil applications of potassium, magnesium sulfate and magnesium
sulfate spray on potato yield, composition and nutrient uptake. Amer. Potato J. 43,
403-411 (1966)
Lavalleye, M. and Steppe, H.M.: The effects of potash on pea growth and quality. Proc.
8th Congress Intern. Potash Inst., 235-248 (1966)
Lehmann, K.: Interaction of potassium, magnesium and calcium concentrations and forms
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Lehr, J..: Sodium as a plant nutrient. J. Sci. Food Agric. 4, 460-471 (1953)
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Load, A.: La nutrition cationique du mais et le diagnostic foliaire Ann. Physiol. Vdg. 4 (2),
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431
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432
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433
The Potassium Cycle in Cropping Systems
A. Dam Kofoed. Askov Experiment Station/Denmark*
1. Introduction
Yields in modern agriculture are considerably higher than they were in the past.
High yields demand supplies of adequate amounts of plant nutrients. The sources
of these nutrients are mineral fertilisers and plant and mineral waste products.
Fertiliser policy must take account of the direct and residual effects of fertilisers
applied, of the needs of individual crops, of soil and weather conditions, of animal
and human needs. Study of nutrient cycles among crops is valuable in planning
fertiliser use. We need to know the inputs and removals of the nutrient and the nutrient
requirements of the crop being grown. Such balance sheet data should be considered
along with soil analysis.
Study of balance sheets is particularly interesting in the case of potassium because
it is turned over in large amounts. The gains and losses can be greatly affected by a
change in the farming system.
2. Components of the potassium cycle
In the potassium cycle, gains to the system from fertilisers, animal and plaht rmsiddes
and other waste products, from rainfall and from mineralisation are set against
losses due to removal in harvested crops, by fixation and by leaching. The components
of the system can be fairly precisely identified for a single farm; regional balance
sheets present greater difficulty.
2.1 Gains
2.1.1 Fertiliser
Various types of compound fertiliser (PK, NK and NPK combinations) are used by
Danish farmers. 4% of the total K consumed is applied as straight K fertiliser (mainly
chloride). Fertilisers purchased in the year 1976/77 supplied 139 100 tonne K. It is
* Forstander A. D. Kofoed, Askov Forsogsstation, DK-6600 Vjen/Denmark
435
estimated that, in the same period, animal manures supplied 115 000 tonne K. Applied
to 2 912 000 ha, the average rate of use of K was thus 87 kg/ha.
2.1.2 Animal and other organic manures
In any animal or mixed husbandry system supplies of potassium in manures are
large and these amounts should be returned to the soil. In some districts animal
manure plays a dominant role and it is estimated that in Denmark such manures are
responsible for producing ten millions tonnes dry matter.
Table I details mineral composition of a number of animal manures. Solid manure
from cattle or pigs contains 0.3-0.4% K and slurry has a similar content. Poultry
manure contains more K - 1.1 to 1.9% and also much more N and P.
Table I Content of N, P and K in solid manure, slurry and liquid manure
D.M.
% of fresh weight
P
N
21.6
23.8
38.4
51.5
0.55
0.75
1.53
2.74
0.18
0.34
1.46
1.12
0.30
0.41
1.07
1.87
9.2
6.8
8.3
15.1
0.45
0.68
0.44
1.07
0.08
0.16
0.08
0.49
0.40
0.27
0.37
0.43
2.5
0.37
-
0.66
K
Solid manure after storing
Lindhard [1977]
Cattle (20). ...........................
Pig (19) ...............................
Laying hen (18) ........................
Broiler (II) ............................
Slurry
Kjellerup [1975]
Cattle (120) ...........................
Pig (21) ...............................
Cattle + pig (33) ......................
Laying hen (6) .........................
Liquid manure (208)
Olesen et al. [1961]
....................
* Number of samples
Table 2 lists the quantities of manure produced by various classes of livestock and
Table 3 details the nutrient contents of other farm wastes and sewage used in agriculture and which contribute to the K cycle.
Root crop tops are relatively rich in potassium and this is also true of silage effluents.
On the other hand sewage sludge is very low in K. K content of straw is variable,
oat straw containing much more K than wheat or barley.
2.1.3 Rainfall
The nutrient supply in precipitation is modest in quantity and significant only for
forests and uncultivated land or for crops of very low nutrient requirement. The K
supply from rainfall in N.W. Europe may amount to from I to 8 kg K/ha/year
(Cooke [1969]) and Danish measurements (Jorgensen [1972]) show from I to 10 kg,
mainly in the autumn, but there is much variation and some locations (Ronhave,
Svinninge, Askov) have shown high values.
436
Table 2. Quantities of waste from housed stock (Christensen [1976]; Lindhard[1977])
tonnes/unitjyear
Unit
Cattle
I dairy cow .......................
1 young beast .....................
slurry
f.u. per
unit/year
solids
liquids
4600
1550
13.5
5.0
7.5
2.1
21.0
7.0
2100
2250
2.9
3.0
2.6
2.7
5.5
Pigs
I sow .............................
10 pigs* ...........................
Poultry
2.4
100 layers ..........................
100 -..............................
...................
5000 broilers"
*
7.9
5.5
equivalent to 3.3 pigs at 50 kg in total in the year.
in 5 periods of 6 weeks/year.
Table 3. Content of N, P and K in crop wastes and organic manures (L.I.K. [1974], Jensen
[1954])
Beet tops (116) .........................
Swede tops (32) ........................
Silage effluent (beet) .....................
Sludge, Sewage plant (3) ................
Barley straw (129) ......................
Oat straw (50) .........................
W heat straw (7) ........................
D.M.
% in fresh material
P
N
K
13.4
16.7
5.0
23.8
78.3
71.8
80.0
0.35
0.49
0.15
0.77
0.54
0.39
0.45
0.40
0.43
0.41
0.03
0.64
1.08
0.63
0.04
0.07
0.02
0.48
0.07
0.09
0.05
2.1.4 Release of soil potassium
Arable soils of N.W. Europe contain large total amounts of potassium, 25-50 tonnes
K per hectare in the plough layer, with the highest figures in clay and marsh soils.
But the major part of this quantity is tied up in feldspar which mineralises only
very slowly and does not make a significant contribution to the day-to-day requirements of the growing crop. Micas and similar minerals are of some importance
but such reserves can be exhausted in the long run. Soils with high clay content,
for example marsh soils, can liberate much potassium over a number of years but,
in considering fertiliser use on such soils, it must be borne in mind that, when such
soils are exhausted, the micaceous minerals will fix a proportion of the potassium
applied.
437
The total potassium in the soil is generally thought to be partitioned into different
forms according to the following scheme:
Non-exchangeable K
Kg K/ha
Exchangeable K
kg K/ha
25,000-50,000 -----------
Solution K
kg K/ha
200
0
4
4
Slow process ---------
I
2-3
Relatively rapid process 4
The quantities of K mentioned in this diagram would correspond approximately
to the contents in the plough layer of an average Danish soil with total K at about
1.6% and exchangeable K content of 8 mg/100 g soil.
Very long-term experiments at Askov have thrown light on the release of potassium
from soils, by comparing uptakes of K by crops on manured and unmanured plots.
The experiments started in 1893 and the results are given in Table 4.
Unmanured plots on the light land released 15 kg K/ha to crops annually; when
58 kg K/ha/year was applied in manure only 5 kg was liberated and when fertiliser
was used to supply 66 kg K, no K was released from the soil. Larger quantities were
supplied from the loamy soil. The low apparent K release by the manured sandy
soil is probably explained by leaching losses. Heavy clay soils may release much larger
quantities, for instance 27 kg K/ha per year at Rothamsted and 45 kg at Saxmundham (Cooke [1972]).
Table 4. Removal of potassium in long-term field experiments Askov, average 1949-1972
(Kofoed and Nemming [1976])
Unmanured
since 1893
loam
sand
FYM
since 1893
loam
sand
NPK
since 1893
loam
sand
Yield c.u./hat
yearly average all crops
Potassium, kg/ha
Removed in crops
Supplied in fertiliser
22.0
13.2
56.4
39.1
60.2
37.3
25
0
15
0
83
58
63
58
90
66
66
66
Removed - supplied
25
15
25
5
24
0
* crop unit (c.u.)= 100 kg barley
2.2 Removals
2.2.1 Crops
For the construction of a balance sheet it is essential to know the amounts taken up
by crops and such measurements have been made all over the world. The uptake
438
depends on the size of the crop and varies with soil K availability, varying from, say,
15 kg K/ha for a small crop to three or four hundred kg for large crops on loamy
soils.
2.2.2 Fixation
This plays an important part in the cycle because fertiliser applied surplus to the
needs of a crop is stored for a succeeding crop if clay with 2 : 1 minerals is present.
But rapid methods for estimating how easily this fixed potassium can be liberated
again are not yet available. Fixation capacity is naturally limited and heavy fertilisation over a long period may result in the equilibrium being exceeded and surplus
K is then lost by leaching.
The K fixation capacity of a soil is determined by a number of factors. Thus the
diffusion of ions influences the speed of the process. Drying and rewetting of the soil
results in stronger fixation. Soil pH also plays a part; decreasing pH of the soil
solution decreases K-fixation. The rate of potassium fixation is increased when its
concentration in the soil solution increases. Sandy and humus soils have little ability
to fix K but in loam and clay soils fixation regulates the supply, reducing the likehood
of luxury uptake.
2.2.3 Leaching
Leaching of potassium depends upon factors such as soil type, precipitation, evaporation, temperature, absorption capacity of the soil and degree of base saturation,
pH value and plant cover. In the short term potassium leaches from the water soluble
fractions (Jensen [1973]). K is more easily leached from humus or sandy soils, as
compared with loams, due to their lower absorption capacity which depends upon
clay content. Leaching losses of up to 25 kg K/ha/year have been measured in lysimeters. Analyses of drainage water from 15 Danish loam soils showed a loss from
the soil of 1 kg K/ha/year (Hansen and Pedersen [1975]) but it must be noted that
only a part of the leaching water is intercepted by the drains.
3. The need for and use of potassium fertilisers
We now discuss in some detail factors which have to be taken into account in formulating a fertiliser policy and their importance in connection with the potassium cycle.
3.1 The effect of fertiliser potassium
Experiments to investigate the effects of potassium fertiliser have been carried out
all over the world but the present review is confined to Danish work. Its effects have
been studied under Danish conditions in many series of field experiments (Skriver
[1978]).
The results in Table 5 show that it is necessary to take into account the availability
of soil potassium before applying fertiliser. In Danish advisory practice exchangeable K
is determined by extraction with 0.5 N ammonium acetate and expressed as mg K
per 100 g soil. Results by this method correlate well with Egn&r's [1960] ammonium
439
Table 5. Effect of potassium on yield of barley, beet and grass-clover 1969/1977 (Skriver
[1978])
t/ha
Soil K value
K applied
(kg/ha)
<7 7.0-9.9 >9.9
Barley grain
<8 8.0-11.4 >11.4
Beet dry matter
<6 6.0-9.9 >9.9
Grass-clover dry
matter
0
50
100
3.88
4.01
4.04
8.5
8.65
8.82
8.5
9.09
9.27
4.26
4.33
4.35
4.42
4.47
4.49
9.2
9.35
9.50
10.0
10.12
10.11
11.1
11.42
11.49
10.7
10.84
10.85
lactate method (Henriksen and Jensen [1969]). A value for exchangeable K (Kexch)
above 8 is usually taken to mean that maintenance dressings only will be needed.
Response to K is not expected at Kexch values of 8-10 or more or when the K content
of grass is 2.5% of dry matter. On soils with low exchangeable K extra potassium
should be applied over and above the maintenance dressing.
Table 6 details results from potato experiments testing K fertiliser on soils with
varying K content. These observations must be accepted with some reserve: the
Table 6. Yield and response to potassium by potatoes on soils of varying K content
t/ha tubers
Number of
experiments
Soil K
mg/100 g
No K
Response to K
11
36
27
14
8
9
below 2
2-4
4-6
6-8
8-10
above 10
14.4
19.3
22.9
24.3
30.0
36.2
8.5
3.0
2.0
1.4
1.2
0.2
soils may have differed in respects other than K content and the different classes
are made up from different numbers of experiments. Nevertheless, they do indicate
that there was generally less response to K fertiliser on the soils richer in K. Though
there was great variation between individual results there was invariably response
to K fertiliser when Kcxch was below 8. At Kexch between 8 and 10 there was no K
response on a few sites while when Kexch was over 10 small responses in half the
experiments were balanced by small yield reductions in the others. It should be noted,
however, that soils with high Kexch values yielded better than low K soils to which
fertiliser had been applied, though this may have been partly due to their higher
clay content.
3.2 Effects on soil potassium
Lysimeter experiments have often been used in calculating nutrient balances. Such an
experiment in which increasing rates of N, P and K fertilisers were applied to a
440
four-course rotation was carried out at Askov between 1974 and 1978. P.S.Klausen
[1978] has used the results to prepare the K balance. Soil analyses when the experiment was started were:
0- 20cm .............
20- 40 cm .............
40-100 cm .............
pH
Soil K
Clay
Silt
Fine
sand
Coarse
sand
6.8
6.3
6.0
15.7
4.0
4.5
9.3
6.9
9.1
11.2
6.6
6.9
37.3
29.8
31.4
42.2
56.7
52.6
The results of the experiment are shown in Table 7.
Table 7. Supply and removal of potassium in crop rotation in lysimeters. Askov 1975/77
g K/m 2/year
N,*
N.
K,
K,
K2
0
5.1
2.7
10
5.7
2.8
20
5.5
2.8
- 7.8
1.5
11.7
0
30.8
2.0
22.5
38.0
2.4
45.0
38.9
2.7
N2
K,
K,
K2
10
8.8
2.5
20
9.3
2.5
- 1.3
8.2
22.5
54.0
1.8
45.0
64.4
1.9
K,
K,
K2
10
10.8
2.2
20
11.9
2.3
Wheat
K, supplied
K, removed in crop
K, removed in water
Gain-losses
0
8.5
2.1
-10.6
0
8.8
2.0
-10.8
- 3.0
5.8
22.5
55.5
1.7
45.0
72.1
1.9
Beet
K, supplied
K, removed in crop
K, removed in water
Gain-losses
Barley
K, supplied
K, removed in crop
K, removed in water
Gain-losses
Italian ryegraks
K, supplied
K, removed in crop
K, removed in water
Gain-losses
l g
*N, = 22.5 g
7.5 + 2.5 g i
-32.8
0
5.7
2.3
- 8.0
-17.9
3.4
7.3
6.9
2.5
14.6
6.4
2.8
-2.1
5.4
0
43.3
1.7
-45.0 -33.3 -21.3
0
8.4
2.1
-10.5
0
7.9
2.7
22.5
8.4
3.3
45.0
9.1
3.9
0
19.7
2.2
-10.6
10.8
32.0
-21.9
N/m2 for
-
-
7.3
10.0
2.3
14.6
11.6
2.5
5.0
0.5
22.5
26.6
2.7
45.0
30.8
3.0
6.8
11.2
0
42.8
1.6
-44.4 -34.7 -29.0
0
10.9
2.0
-12.9
0
23.9
2.3
7.3
12.4
2.1
14.6
14.5
2.4
- 7.2 - 2.3
22.5
35.5
2.5
-26.2 -15.5
45.0
40.7
3.0
1.3
fwheat
beet and grass
barley
10 g K/m 2 to wheat was sufficient to maintain the K balance except perhaps at N 2.
Beet, which takes up large amounts of K, drew on soil potassium reserves, the more
so the more nitrogen was given. 10 g K to barley nearly maintained the balance
at No but not at N, and N2 . The grass crop was very small without N fertiliser and
441
only small amounts of K were drawn from soil reserves. Increasing N fertiliser
resulted in larger crops and thus in greater loss of potassium.
Leaching losses were from 1.5-3.5 g K/m 2 and were greatest where crops were small,
i.e. where no nitrogen was used.
The effects of the interaction between N and K on K balance and exchangeable K
averaged over the four years are shown in Table 8.
Table 8. Gains- losses of potassium in Askov lysimeter experiment
Treatment
applied (g/m)
0 N
16.3 N
32.5 N
Average of 4 crops K, g/m'/year
15.6 K
0 K
-14.8
-22.0
-23.6
- 1.9
-11.6
-15.1
31.2 K
13.1
- 0.3
- 6.1
Increasing N fertiliser markedly increased K removal. When no K was applied a
great deal of K was drawn from soil reserves. K, failed to maintain the balance at
N, and N 2 while K2 maintained the balance at N, and resulted in only a small deficit
at N. Soil K values also reflected potassium removed but the removals recorded
would be expected to result in greater differences in soil K than were actually observed.
It may be suggested that, in addition, plants mobilised significant amounts of K
from the subsoil.
A survey to investigate the effects on soil K status of applying potash fertiliser over a
number of years has been carried out by the Farmers Unions (Olesen and Hedegdrd
[1968]). Farm management data were collected from 318 farms and soil analyses
were made twice at an interval of 6-10 years: see Table 9.
Table 9. Classification of farms by consumption of potassium fertiliser
Number of farms ....................................
Consumption kg K/ha/year ............................
Annual rate of increase in soil K (mg K/100 g)* ..........
Consumption
low
medium
high
106
101
37
55
0.13
0.26
111
84
0.38
* average of 8 years
When much K was used, K content of the soil increased but the increase was only
modest in relation to the amounts applied - about 10% of the supply on an eight
year basis.
It appears that soil exchangeable K content is affected mainly by recently applied
potassium fertiliser and that fertiliser applied more than, say, 4 years earlier has
442
little influence. Soil K determinations would not be expected to remain valid for
more than 2 years or so.
I am indebted to C.Andersen [1978] for the information in Table 10 which is derived
from the results of routine advisory soil analysis carried out over a long period. There
Table 10. Changes in K content of Danish soils
Percentage of samples in soil K* classes
Jutland
Soil K
0 - 6.2
6.3-12.4
12.5-18.6
above 18.6
1939
31
48
11
10
1945-49
31
50
13
6
Soil K
0 - 5.9
5.0- 9.9
10.0-13.9
above 13.9
1960-64
17
36
25
22
1975-77
16
34
26
24
Seeland
Soil K
1945-49
Soil K
1964-69
1975-77
0 - 6.2
6.3-12.4
12.5-18.6
above 18.6
20
62
13
5
0 - 5.9
6.0-11.9
12.0-17.9
above 17.9
9
62
21
8
5
51
30
14
Funen
Soil K
0 - 6.2
6.3-12.4
above 12.4
1955-59
17
59
24
Soil K
0 - 5.9
6.0-11.9
12.0-17.9
above 17.9
1963-64
30
57
24
9
1975-77
4
46
32
18
Lolland-Falster
Soil K
0 - 6.2
6.3- 9.3
1955-59
22
40
Soil K
0 - 7.9
8.0-11.9
1965-69
28
33
1975-77
10
34
9.0-12.4
23
12.0-17.9
25
36
above 12.4
15
above 17.9
14
20
Bornholm
Soil K
0 - 6.2
6.3- 9.3
9.4-12.4
1951-57
18
35
25
Soil K
0 - 7.0
7.1-10.0
10.1-14.0
1961-65
I1
36
30
Soil K
0 - 5.9
6.0- 9.9
10.0-13.9
1975-77
1
18
34
above 12.4
22
above 14.0
23
above 13.9
47
* Exchangeable K content (mg/100 g)
are some variations between provinces in the way in which the soils are classified
and records are available for varying periods, but the general picture in all provinces
is of an increase in soil K values since the 1950s. The average increase was about
2 mg K % in Jutland, 3 mg in Funen, Seeland and Lolland-Falster and a little over
4 in Bornholm.
443
3.3 Potassium content of crops
Potassium affects crop composition as well as affecting yield. Table II lists K content
and K removal in crops in relation to exchangeable soil K (Olesen et al. [1971J).
Table 11. Potassium content of crops in relation to soil K
Soil K (mg/100 g)
6.0
12.0
per cent K in dry matter
18.0
Barley, grain* .......................
Barley, straw* ........................
Oats, grain* ..........................
Oats, straw ..........................
Grass-clover, I cut ....................
Grass-clover, all cut ..................
Beet, roots ...........................
Beet, tops ............................
Swedes, roots ........................
Swedes, tops .........................
0.44
0.79
0.42
1.28
2.77
2.70
1.42
2.66
1.57
2.40
0.45
0.85
0.44
1.46
3.12
2.95
1.53
3.03
1.97
2.79
0.46
0.90
0.45
1.65
3.47
3.20
1.64
3.29
2.36
3.19
Removal of K, kg/ha
Barley ..............................
O ats ................................
Grass-clover .........................
Beet
.............................
Swedes .............................
43
69
276
241
202
47
78
302
274
258
52
88
328
307
314
* No significant increase due to increase in soil K
K content of crops was significantly increased by higher soil K in all crops except
barley and oat grain. All crops remove more potassium as exchangeable K increases.
It must be borne in mind that plant nutrients can be leached by rain from growing
crops and when they are leached into the soil they are again available. Table 12
shows the effect of harvest date on potassium content of barley and oats (Olesen et al.
11971]). In both crops, potassium content is lower the later the harvest, due to
removal of potassium, mainly from the straw. Removal of K in harvested crops is,
of course, approximately proportional to yield.
Table 12. Effect of harvest date on potassium content in cereals
Barley harvest date
% K in grain ........................
% K in straw ........................
K removed, kg/ha ....................
21 August
0.49
1.01
56
31 August
0.45
0.84
47
10 September
0.41
0.66
38
Oats harvest date
% K in grain ........................
% K in straw ........................
K removed, kg/ha ....................
26 August
0.49
1.69
89
3 September
0.45
1.52
81
11 September
0.40
1.35
73
444
Too high dressings of potassium may raise crop K content to unnecessarily high
levels. This is particularly liable to occur in crops which are harvested green. This
effect, for a grass-clover sward, is illustrated in Table 13.
Table 13. Influence of K fertiliser on content of K, Na and Mg in grass-clover
(Average Askov, LundgArd and Hojer)
K applied
(kg/ha)
% in dry matter
1st cut
Na
K
Mg
4th cut
Na
K
0 .......................
150 .......................
300 .......................
2.80
3.24
3.38
0.17
0.17
0.16
2.66
2.91
3.11
0.10
0.07
0.07
0.26
0.21
0.18
Mg
0.21
0.20
0.20
The table also shows the effects of K on the content of other cations (Na and Mg)
and such questions of mineral content and cation balance are relevant to some
problems of animal health. These effects are particularly marked at the first cut of
the year and it is advisable to avoid applying maintenance K dressings to grass in
the spring when, in any case, available soil K is at its peak, and instead to apply
it in repeated small dressings throughout the season.
4. Farming system and the potassium cycle
The ways in which the farming system affects the balance between K inputs and
withdrawals and the quantities of K involved in the cycle can be illustrated by considering two hypothztical farms, each of 20 ha under contrasting systems of management. The first is a dairy farm with a large area under grass and in which virtually
all the crop produce is fed to the stock and in which it is necessary to buy in feedstuffs which contain appreciable amounts of K. The second is a cash cropping system
without livestock where all crop produce except straw and sugar beet tops is sold
off the farm. Table 14 sets out the overall K balances of the two farms. Neither
farmer buys K fertiliser.
The first, dairy, farm is approximately in balance, gaining about I kg/ha K. The cash
cropping farm, on the other hand, loses 26 kg/ha K each year. If the dairy farmer
wished to increase his stocking level, he could do so if he bought in more feed. This
would result in substantial additions of K to the farm, indeed if he were to
double his stocking rate it would be difficult to utilise efficiently the extra 125 kg/ha
K contained in the additional feed needed. Information on which these models are
based is derived from Wohlbier [1960], Studstrup [1977] and from Iversen and
Dorph-Petersen [1949].
The overall balance of the farm does not tell the whole story and it is instructive
to consider in some detail the quantities of K which are involved in the cycle and
445
Table 14. Overall potassium balance in different methods of farm management, calculated
for two farms each of 20 ha
K input (kg)
K in produce sold (kg)
1. Animal husbandry 26 dairy cows and 29 young stock - 40 livestock units
30,173 kg concentrates
241 kg
136,110 kg milk
Seeds
6 kg
I I calves at 350 kg
5 culled cows
gains to the farm
211 kg
12 kg
9 kg
15 kg
Total
247 kg
247 kg
2. Arable crops only
Seeds
Losses from the farm
10 kg
526 kg
Total
536 kg
Barley
Sugar beet
Rapeseed
296 kg
190 kg
50 kg
536 kg
this is done in Table 15 for the same two hypothetical farms. Table 16 shows data
upon which the calculations are based. On the dairy farm a substantial area is under
grass and the amount of K in the cycle is very large in comparison with the cash
cropping farm. However, because in the former case almost the whole of the crop
produce is fed back to the animals, the loss of K is very small whereas on the stockless farm the loss is large because most of the crop produce is sold off the farm. In
fact, even larger quantities of K are involved in the cycle. The table is based on crop
K content at harvest; at the time of maximum uptake the standing crops contain
much more K.
Similar assessments of the K cycle can be made for any combinations of crops and
stock.
In planning fertiliser policy on the farm, the construction of balance sheets must
be taken a stage further and it is necessary to construct balances for individual
fields in order to make the best use of purchased fertiliser.
Theoretically, when all the crop produce is fed to stock, there is no loss of K from
the farm other than in milk and other animal products sold. There will, however,
be losses in drainage from housed stock and from stored manures which good management can keep at a low level. Slurrly and manure must be applied in a systematic
manner so that each field receives its due share of nutrients. Frequently, for instance
where the farm layout is not compact, this poses difficulties and there is a tendency
for fields near the steading to receive more than their fair share, resulting in a build
up of soil K to unnecessarily high levels on the nearby fields and K starvation on
the outlying areas.
The return of nutrients through the grazing animal is uneven. Gross unevenness
occurs as a result of topography (animals avoiding frost pockets at night, etc.) or
siting of drinking troughs. Return is uneven on the micro scale also. K is applied
in urine patches at very high rates to a small part of the total field area; though the
field as a whole receives enough K, a large part of it may be K deficient and fertiliser
K will still be required to correct this. The degree of unevenness arising in this way
will depend on stocking density; the higher this is the more even will be the return
446
Table 15. Internal potassium balance for 2 farms of 20 ha each and with different forms of
management
Cattle
(number)
Crops only
Animals:
Dairy cows ............................................
Young cattle ..........................................
26
29
Area used for:
hectares
hectares
B arley ................................................
B eet ..................................................
Arable grass ...........................................
Sugar beet ............................................
R ape .................................................
7.3
3.1
9.6
14.6
Total, ha
20.0
20.0
Supply of K in:
kg K
kg K
Slurry ................................................
2996
Burned straw...........................................
.....................
Beet tops ploughed in .......
2.5
2.9
732
398
.......
2996
1130
427
Barley grain and straw ..................................
Beet .................................................... 803
1900
A rable grass ...........................................
Sugar beet ..... .......................................
Rape grain and straw ...................................
854
3130
1666
134
536
Total supply K, kg
K contained in crops, kg
Total removed; kg K
Losses from the soil; kg K
588
224
7
Losses/ha; kg K
27
Table 16. Basis for calculations in Table 15
Crops
Barley
Beet
Sugar beet
Grass
Rape
Dry matter - yield, t/ha
K, % in dry matter
Grain Straw Roots Tops Grass
Grain Straw Roots Tops Grass
4.5
0.45
4.5
10.0
9.5
2.0
4.0
3.5
5.0
0.85
1.53
0.80
9.0
0.86
1.50
3.03
3.18
2.20
Sources: Olesen et al. [1971]; Christensen [1976]; Studstrup [1977]
447
25th Anniversary of the Scientific Board
11th IPI-Congress held in Bern/Switzerland
from September 4 to 8, 1978
6th Session
Translation of
Research into Practice
451
Fertiliser Use as a Lead Practice in
Modernising Agriculture
1. Arnon, Settlement Study Centre, Rehovot/Israel
I. Introduction: Transformation of subsistence agriculture - essential prelude to
development
Until recently, agriculture has been largely neglected by policymakers, economists
and planners in many developing countries on the assumption that all, or most,