Agricultural
Effects on Ground and Surface Waters; Research at the Edge of Science and Society
(Proceedings of a symposium held at Wageningen. October 2000). IAI IS Ptibl. no. 2 7 3 . 2002.
77
Balanced fertilization and regulating nutrient
losses from agriculture
OENE O E N E M A & G E R A R D L. V E L T H O F
Alterra, Wageningen University and Research Centre, PO Box 47, 6700 AA
The Netherlands
Wageningen,
e-mail: [email protected]
Abstract Balanced fertilization was introduced in the nineties as government
policy to decrease nutrient losses from European agriculture. However, balanced
fertilization is an ambiguous term and is not yet defined in operational terms.
This paper discusses the pros and cons of balanced fertilization as a policy tool
and suggests operational measures. Essential steps are the book-keeping of
nutrients at farm and field levels, and the evaluation of soil fertility level and
the nitrogen (N) and phosphate (P) surpluses relative to the vulnerability of the
environment. The N surplus is a proximal indicator of total N losses, but inf­
ormation about site-specific environmental conditions is needed for the partit­
ioning of the total N loss over N losses to the atmosphere, groundwater and
surface water. The P surplus is a distal indicator of total P loss whilst a soil P
test value is a proximal indicator. It is concluded that N and P surpluses comb­
ined with information about site specific environmental conditions and the agroecosystem itself, allow evaluation of the degree of balance between agriculture
and the environment, as implicitly suggested by the term balanced fertilization.
Key words conceptual framework; eutrophication; nitrogen; nutrient balances; phosphorus;
policy and measures; soil fertility
INTRODUCTION
There is a common belief that inputs and outputs of nutrients in agro-ecosystems have
to balance to make agricultural production sustainable (e.g. Smaling et al., 1999).
Outputs that exceed inputs lead to impoverishment so that agricultural production will
not sustain on the long term. And vice versa, inputs that exceed outputs lead to
enrichment of the system and in turn to nutrient losses, which make agricultural
production unsustainable from an environmental point of view. These lines of thinking
are the basis of "balanced fertilization" introduced by European policy makers in the
nineties as a policy for regulating nutrient losses from agriculture (e.g. De Walle &
Sevenster, 1998; Romstad et al., 1997). Though there is broad agreement about the
aim of this policy, there is still debate as to how to make it operational and how to
implement it in practice. It is the purpose of this paper to briefly discuss the concepts
and pros and cons of balanced fertilization as a policy tool for regulating the nutrient
losses from agriculture to groundwater and surface waters. Before doing so, we briefly
summarize the main mechanisms that control the losses of N and P from agriculture.
C O N T R O L OF N U T R I E N T L O S S E S F R O M A G R I C U L T U R E
Losses of N and P from agriculture are related to: (a) nutrient management, (b) the
agroecosystem and its management, and (c) environmental conditions (climate, hydro-
78
Oene Oenema & Gerard L. Velthof
logy, morphology and soils). The relationship between these factors and nutrient losses
is shown schematically in Fig. 1. Nutrient management has a dominant influence on
the nutrient balance (surplus), which directly affects soil fertility and nutrient losses.
The partitioning of the nutrient surplus between soil fertility and nutrient losses is
different for N and P, and is controlled by the level of soil fertility itself, and by
climate and land use. The partitioning of nutrient losses between losses to the
atmosphere, groundwater and surface waters is controlled by the type of agroecosystem and environmental conditions (climate, hydrology, soil type, morphology).
Agro-ecosystems, i.e. arable farms, vegetable farms, specialized livestock farms and
mixed livestock farms, have a dominant influence on N utilization and N loss. For
example, losses of N H into the atmosphere are associated predominantly with animal
production systems and a large fraction of the N H loss may occur in livestock housing
and manure storage systems. Morphology (slope), soil fertility, climate, and crop cover
determine N and P losses to surface waters via overland flow.
3
3
Farmer
1
X-
>
Nutrient
Policies &
Management
Measures
V
i
Soil
V
Agro-
j Soil
Nutrient surplus
fertility
Climate
Nutrient losses
system
I
N
Atmosphere
Groundwater
^<-
Hydrology
x
Morphology
Surface waters
= Nutrient flow
"x*
= Controling factor
Fig. 1 Simplified diagram ofthe multiple relationships between nutrient management,
nutrient surpluses, soil fertility and nutrient losses to the atmosphere, groundwater and
surface water. Flow of nutrients = solid arrows, controlling factors = dotted arrows.
Accumulation of P in soil contributes to the build-up of soil fertility. Most soils
can store large amounts of P, but concomitant with the accumulation of P there is an
increase in soil P test values and in the risk of P loss to surface waters. As a result, the
relationship between P surplus and P loss to surface waters is indirect; the effect of P
surplus on P losses becomes apparent only after soil P test values have increased. By
contrast, most soils have little or no capacity to accumulate N, mainly because they
already store large amounts of N . As a consequence, there is a direct relationship
between N surplus and total N losses (Table 1 ).
Balanced fertilization and regulating nutrient losses from
agriculture
79
Table 1 Indicators for total N and P losses from agro-ecosystems to the atmosphere, groundwater and
surface water. Indicators are listed according to the straightforwardness of control, i.e. proximal
indicators have direct control, and distal indicators have indirect control.
Indicators
N losses
P losses
Proximal
N surplus
Environmental conditions
Type of agro-ecosystem
Soil N level
Soil P test
Environmental conditions
Type of agro-ecosystem
P surplus
f
i
Distal
Summarizing, the nutrient surplus and the level of soil fertility control the longterm nutrient losses from agro-ecosystems. Total N loss is directly related to the N
surplus, while total P loss is predominantly related to soil P test values. The pathways
of N and P losses are controlled by environmental conditions. The soil is the main site
of diffuse losses of N and P, but in the case of livestock farming systems significant N
losses may also occur from stables and manure storage systems. The nutrient surplus is
a result of nutrient management. The nutrient management strategy, in turn is defined
by the farmer, and a result of the aims of agricultural production, management skills
and possible restrictions imposed by policies and measures.
B A L A N C E D FERTILIZATION: C O N C E P T A N D DEFINITIONS
The term "balanced fertilization" was first introduced by Justig von Liebig in 1840,
who stated that farmers have to add those nutrients to the soil that have been removed
by harvested crops, to be able to sustain high crop yields (Russell, 1912). Soon there­
after, this simple concept was rejected when it became clear that the soil is a large
reserve of plant available nutrients. Hence, most of the current fertilizer recommen­
dations, developed during the second half of the 20th century, consider both soil
reserves and crop needs; recommended fertilizer applications depend on the amounts
of plant available nutrients in the soil and crop demand.
Two ministerial agreements have contributed to a revival and broadening of the
150 year old concept of balanced fertilization. In December 1993, a ministerial
meeting on the protection of the North Sea agreed on implementing "balanced
fertilization" in all participating countries by the year 2002, as a policy to reduce the
inputs of N and P from agriculture into the North Sea by 50% relative to the reference
year, 1985. A common and accepted definition of balanced fertilization did not exist
and, therefore, the meeting encouraged work towards a practical definition of the term.
In 1991, EU countries accepted the Nitrate Directive (91/676/EEC), which deals with
the protection of groundwater and surface waters from contamination by nitrate from
agriculture. The Nitrate Directive indicates that a balance is needed between the N
requirement of the crop and total N supply. The supply of N by soil, manure and
fertilizers should not exceed the demand of the crop.
Basically, balanced fertilization aims at a harmony between economy (agronomy)
and ecology (environment). In operational terms, balanced fertilization can have three
meanings:
(a) the supply of all essential plant nutrients is adjusted in the proper ratios to crop
demand;
(b) the supply of plant nutrients equals the uptake of nutrients by the crop; and
Oene Oenema & Gerard L. Velthof
80
(c) the supply of plant nutrients equals the removal of nutrients from the field via the
harvested crop.
The first meaning suggests a mutual harmony that results when the availability of all
essential nutrients is properly adjusted to crop demand (e.g. Janssen, 1999). Without
such a balance among all the essential nutrients, plants may suffer from stunted growth
through either nutrient deficiency or nutrient toxicity. The latter two meanings suggest
a steadiness that results from the input of nutrients that equals the output via crop
uptake/removal. In practical terms, the outcome of these three meanings can be very
different. The difference between meanings (a) and (b) is crop dependent; it is related
to the harvest index. When the harvested fraction of the crop is large, for example with
forage crops, the difference in outcome between the two meanings is small. The
reverse is true in the case of, for example, sugar beet and many vegetables. This is
illustrated for N in Table 2. A balance between input and output is obtained for mown
grassland when the amount of N applied is according to the N fertilizer recomm­
endation for grassland, for both meanings of balanced fertilization. However, this is
not the case for other crops.
Table 2 Relationships between fertilizer M input (according to recommendations for economic optimum
yield), total N uptake in the crop, N uptake in the harvested crop, apparent nitrogen recovery in the crop
(ANR, % ) , and Balance-1 (difference between fertilizer N input and total N uptake) and Balance-2
(difference between fertilizer N input and N uptake in the harvested crop), for average conditions in The
Netherlands. All data in kg N ha year" , except for ANR (data from Schroder & Vos, 1995).
-1
1
Crop
Fertilizer N
input
Total N
uptake
N in harvested
crop
ANR
%
Balance-1
Balance-2
Potato
Sugar beet
Winter wheat
Mown grassland
Forage maize
Leek
Spinach
Brussels sprouts
Seed onions
230
125
150
400
155
220
170
190
130
200
210
245
400
180
140
105
230
125
180
90
200
400
180
85
70
97
120
50
60
80
80
50
30
30
80
31
30
-85
-95
0
-25
80
65
50
35
-50
0
-25
135
100
93
10
5
The large differences between crops in the balance of nutrient input and nutrient
removal by the crop are related to crop specific differences in the apparent recovery of
fertilizer N (ANR) (see Table 2) and in the efficiency of N utilization (UE) in the crop
for the production of biomass, which is related to the nutrient content of the crop.
Further, there are differences between soils in nutrient supplying capacity and between
fertilizers in efficiency. All these possible effects on the relationships between
fertilizer input and nutrient removal by the crop can be analysed via a so-called fourquadrant figure as shown in Fig. 2. Such analyses can help to identify the separate
effects of soil type, crop type, fertilizer type, climate, management and interactions
between these factors on the balance between nutrient input and nutrient uptake.
As yet, there are no accepted operational procedures for "balanced fertilization" in
practice, which are applied uniformly and which would allow determination of the
degrees of balance and sustainability. There are large differences between crop types,
farming systems and in ecological conditions within and between EU countries. As
Balanced fertilization and regulating nutrient losses from
agriculture
81
Biomass produced
UE-V
II
<
Manure/fertilizer
„
P-4
UE-2
Residues
>
Input t o soil
Available in soil
Fig. 2 Relationships between nutrient input and nutrient uptake in the crop. Quadrant
III (lower left) shows the relationship between nutrient input and the amount of
available nutrients in the soil. Inputs come from residues (R) of the preceding crop and
manure or fertilizers (F), and include atmospheric deposition. The supply of nutrients
by the soil itself are indicated by S. Quadrant IV (lower right) shows the relationship
between available nutrients and nutrient uptake. The dotted line (1:1) indicates this
relationship for plant roots that take up all available nutrients without any loss.
Quadrant I (upper right) shows the relationship between nutrient uptake and biomass
production. The dotted lines indicate the efficiency of nutrient utilization; UE-\ is
representative for high efficiency crops; UE-2 for low efficiency crops. Quadrant II
(upper left) shows the relation between nutrient input and harvested biomass and crop
residues (after Van Noordwijk, 1999).
shown by for example Neeteson (1995) and Tunney et al. (1997) there are large
differences between countries in the EU in the recommendations for fertilizer N and P
application. There are differences in both procedures for soil testing and in
recommended N and P application rates, partly because of differences in the assess­
ment of the amount and timing of the N and P supply by the soil itself and by crop
residues and organic manure. Further, the focus on only appropriate fertilizer and
manure application may distract attention from losses that occur during, for example,
land use changes and during the housing of animals and storage of manure. Evidently,
the whole system needs to be considered, including possible spatial and temporal
variations in the management system.
Summarizing, balanced fertilization is an ambiguous term. It suggests harmony
between agriculture and the environment. It also suggests harmony among all essential
nutrients and between the supply and demand by the crop of these nutrients. The aims
of balanced fertilization are well understood, but implementation in practice is
hampered by the paucity of practical guidelines that would allow verification of the
degree of balance.
82
Oene Oenema & Gerard L. Velthof
N U T R I E N T B A L A N C E S : C O N C E P T A N D DEFINITIONS
Nutrient balances can make the concept of "balanced fertilization" operational, because
the book-keeping of nutrient inputs and outputs is a flexible instrument. Basically, a
nutrient input-output balance can be made of each (sub)system and at each spatial and
temporal scale in agriculture. Three basic approaches to nutrient balances are generally
considered (Watson & Atkinson, 1999; Oenema & Heinen, 1999):
Farm-gate balance or black-box approach; this records the amounts of nutrients in
all kinds of products that enter and leave the farm via the farm-gate. The balance,
i.e. the difference between inputs and outputs, is a measure of total nutrient losses
plus a possible change in the storage of nutrients in the farming system.
Soil surface balance records all nutrients that enter the soil via the surface and that
leave the soil via crop uptake. The balance, i.e. the difference between inputs and
outputs, is a measure of total nutrient losses plus a possible change in the storage
of nutrients in the soil.
Soil system balance records all nutrient inputs and nutrient outputs, including
nutrient gains and losses within and from the soil system. The balance, i.e. the
difference between inputs and outputs, is a measure of the net depletion (output >
input) or enrichment (output < input) of the system.
Proper guidelines are needed for the bookkeeping, especially when nutrient
balances are compared and used as a policy instrument. So far, all types of balance
approach have been used in practice. The farm-gate balance is being used in The
Netherlands as a policy instrument for regulating the surplus of N and P at farm level
(Van den Brandt & Smit, 1998). The surface balance approach is being used as an
indicator for the environmental performance of agriculture (e.g. OECD, 1998). The
system balance approach has been used, for example, as awareness raiser of the
nutrient depletion in sub-Saharan countries in Africa (Smaling et al, 1999).
Summarizing, nutrient balances of agro-ecosystems facilitate the understanding of
nutrient cycling and nutrient utilization within these systems. Estimates of nutrient
inputs and outputs can be made for each (sub)system and at each spatial and temporal
scale. As such, the book-keeping of nutrients can make the policy of balanced fertiliz­
ation operational. However, the differences between book-keeping approaches need to
be considered, and the neglect of soil fertility level and the bioavailability of nutrients
in manure and fertilizers (Janssen, 1999) require attention too.
DISCUSSION
So far, various policies and measures have been introduced in European countries to
improve nutrient management and to decrease nutrient losses from agroecosystems.
These various policies and measures can be categorized as: (a) guidelines for best
management practices, i.e. advice and education; (b) mandatory measures, i.e. "do's
and don'ts"; and (c) economic incentives, i.e. facilitating the implementation of
measures via levies and premiums. Evidently, the policy of balanced fertilization falls
in category (a); it recommends farmers to apply no more nutrients than demanded by
the crop. Most of the policies and measures in Europe have the character of best
management practices, i.e. guidelines for the amount, timing and method of manure
Balanced fertilization and regulating nutrient losses from
agriculture
83
and fertilizer application, and on the storage of manure (De Walle & Sevenster, 1999).
The Nitrates Directive includes guidelines for best management practices and
mandatoiy measures. The manure policy in The Netherlands includes guidelines for
best management practices and mandatory measures but the policy is enforced by
levies to be paid by individual fanners when targets at the farm level have not been
achieved (Van den Brandt & Smit, 1998).
To be successful, policies and measures have to be both effective, i.e. decrease
nutrient losses, and efficient, i.e. they have no side effects. Furthermore, measures
have to be acceptable, i.e. fanners are ready to implement the measures, and control­
lable, i.e. the implementation of the measure can be verified. De Walle & Sevenster
(1999) provide an overview of current policies and measures in European countries,
and indicate that the success of these policies has been limited so far. There are a
number of reasons for the limited success. Firstly, there are many assessments but very
few direct and accurate measurements of the loading of surface waters, especially with
N and P from agricultural land. Secondly, there is a complex of interacting factors and
multiple pathways that control the loss of N and P to groundwater, surface waters and
the atmosphere; the farmer's role is not always clear. Third, there are still many
unknowns, especially with regard to the managerial steps required to decrease nutrient
losses effectively and efficiently. Fourth, economic impulses to further decrease costs
and to intensify agricultural production have exerted a strong antagonistic effect
against any policy aiming to decrease nutrient losses from agriculture. Fifth, relatively
large losses are associated with intensive livestock fanning systems in the farmyard,
which have not been well addressed so far. Sixth, many policies and measures have not
been based on a proper mechanistic understanding of the processes and interactive
nature of the controlling factors involved. Seventh, most policies and measures have
been rather pennissive and lack easy control. In summary, fanners have not
satisfactorily been convinced of the nutrient problem, they lack proper tools and are
marginally encouraged to implement measures, and they face economic side effects
and loss of competitiveness when measures are implemented.
Evidently, to become successful, the balanced fertilization policy has to be translated
into operational measures that allow evaluation of the degree of balance and control of
its effectiveness in practice. As discussed above, the N surplus is a proper indicator for
total N loss, whilst the partitioning of the N surplus over the various N loss pathways
is mainly controlled by site-specific environmental conditions. This suggests that the
target or acceptable N surplus differs between agro-ecosystems and that site-specific
environmental conditions must be taken into account (Fig. 3). Further, the P surplus is
a distal indicator of total P loss (e.g. Table 1); for proper evaluation of the environ­
mental impact, additional information is needed about soil P test values and, for example,
the morphology and hydrology of the site (Fig. 3). The evaluation of the N and P
surpluses can be easily extended further to a complete framework by including other
indicators. Making this framework quantitative is a major challenge for the future.
CONCLUDING REMARKS
Balanced fertilization can be made operational via nutrient balances at farm and field
level. However, the N and P surpluses provide only indirect estimates of the (long-term
84
Oene Oenema & Gerard L. Velthof
N surplus, kg per ha
P surplus, kg per ha
Vulnerability of environment
••
Soil P test
•
Fig. 3 Schematic diagram that allows evaluation of the "degree of balance between
agriculture and the environment". The N surplus of agro-ecosystems 1 (one major N
loss pathway) and 2 (many N loss pathways) are plotted against the vulnerability of
the environment for N (left figure). The P surplus for systems on steep (slope 1) and
flat (slope 2) land are plotted against the soil P test values (right figure). Target N and
P surpluses are derived from the solid curves.
term) N and P losses. Additional information is needed about the type and management
of the agro-ecosystem and about environmental conditions, i.e. climate, morphology,
soil and hydrology, to make the evaluation of N and P surpluses sensible (Fig. 1).
Enforcement of balanced fertilization in practice requires proper information exchange
and economic incentives, as the Dutch story tells (Van den Brandt & Smit, 1998).
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