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Tropical 1Environments and Cropping Systems
Chapter
Chapter 1
Tropical Environments:
Climates, Soils and Cropping
Systems
The tropics have the potential to be the most productive cropping environments in
the world. Plants need heat, light and moisture to grow and all of these are available
in abundance in the tropics. Where rainfall is sufficient, crops can be grown yearround, rather than only in the warm seasons as in temperate regions. And yet, despite
these natural advantages, yields in tropical cropping systems are often pitifully small.
The unpredictability of the climate – in particular the timing of the rains – and the
lack of nutrients for plant growth in many soils, combine to limit crop production in
the tropics. Whilst we can do little to modify the climate we can use various
approaches to solve the problems of soil fertility. The most obvious solution is to
import nutrients in the form of mineral fertilizers, but for a variety of social,
economic and political reasons this is generally difficult, especially in Africa. The
alternative is to increase the biological inputs of nutrients and it is here that biological fixation of atmospheric nitrogen (N2) has a crucial role to play in increasing
the sustainability of yields with minimal external inputs. The actual and possible
contribution of biological N2-fixation in tropical cropping systems is the subject of
this book.
The tropics are precisely defined as the region between the Tropic of Cancer
(23.5°N) and the Tropic of Capricorn (23.5°S). However, in this book we will
follow the lead of many other writers and use the term ‘tropics’ loosely to encompass
the true tropics and also the subtropics, namely latitudes between 30° north or south
of the equator.
So what are the characteristics of tropical environments? At the coast on the
equator the mean temperature varies from 26 to 27°C by only 2–3°C between
months and diurnal variation is no more than 6–10°C. And yet, close to the equator
we can find the snow-capped peaks of Kilimanjaro in northern Tanzania, or the
3
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4
Chapter 1
Ruwenzori Mountains on the border between Uganda and Zaire. The seasonality of
climates and the opportunities for crop production depend largely on the rainfall and
the tropics encompass climates ranging from arid deserts to those with the highest
rainfall in the world. Tropical soils are notoriously highly leached and acid but, as we
shall see, some tropical soils are highly fertile. The environment in which crops are
produced is determined by the climate, the soil and local modifications of these
resulting from the cropping system, in which the crop itself plays an important role.
We shall consider each of these in turn.
Tropical Climates
The major factors that give rise to the diversity of climates in the tropics are the
topography, the rainfall and the winds. Total incident radiation varies seasonally with
latitude, but never by more than 15% within the tropics. Thick cloud cover can
reduce penetration of radiation to a much greater extent and therefore the potential
for production is much higher in the dry seasons without cloud as long as sufficient
irrigation water is available. However, daylength can vary with latitude in the tropics
by up to 1.5 h from the constant 12 h found on the equator and this can have an
effect. Some plants in the tropics are so sensitive to small changes in daylength that
flowering is only triggered at certain times of the year.
Topography rather than latitude is the major factor determining temperature.
The temperature falls by 0.65°C with every 100 m increase in altitude, and thus
temperatures in the highlands are substantially below those near sea level. Topography can also influence cloud cover, and thereby both temperature, through its effect
on penetration of radiation, and rainfall. Temperature regulates the rate of plant
growth and so is a primary determinant of the time crops take to reach maturity in
different zones. However, it is rainfall which is usually the most important factor in
determining the potential productivity and the major climatic zones in the tropics are
distinguished primarily by the amount and distribution of the rainfall throughout the
year. Here we will follow the classification of climates given by Norman et al. (1995).
The major climatic zones
The wet tropics
The ‘humid’ or ‘wet’ tropics have a mean monthly air temperature greater than 18°C
and rainfall above 1800 mm year-1. For at least 10 months of the year rainfall exceeds
evaporation from the soil and vegetation and so crops can be grown virtually the
whole year round. This zone includes the majority of the great river basins of the
Amazon and the Congo and much of lowland Southeast Asia.
The wet and dry tropics
The ‘wet and dry’ regions also have mean monthly air temperatures above 18°C
but have strongly seasonal rainfall patterns with a dry period of at least 2 months
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Tropical Environments and Cropping Systems
5
when crops cannot be grown without irrigation. The rainfall can have a bimodal
distribution with two main rainy seasons or there may be a single rainy season with
total rainfall between 300 and 1800 mm year-1. This large category includes what are
often referred to as the ‘sub-humid’ and the ‘semiarid’ tropics and covers most parts
of Asia dominated by monsoon season(s) and the large savannah areas of South
America and Africa. In West Africa there is a marked gradation of climates, with
rainfall decreasing further inland from the humid, tropical coast to the semiarid
climate of the Sahel. In East Africa rainfall is bimodal, with a short and a long rainy
season; in southern Africa there is only a single rainy season. The length of the
cropping season is determined not simply by the rainfall. It also depends critically
on the capacity of the soil to retain moisture and on the additional water that can be
collected by runoff from the surrounding land. Thus in some areas in this category
cropping is possible throughout the year but in others crop growth can only be
sustained for less than 3 months. The great problem of agricultural production
in such areas is the unpredictability of both the onset of the rainy season and the
distribution of rainfall during crop growth. Where the dry season is long, the farming
system must allow for provision of food for both humans and animals during the
period when crops cannot be grown.
The dry tropics
The ‘dry’ tropics are regions with rainfall of less than 300 mm year-1 in which crop
production is possible only with irrigation. Such areas include most of tropical Africa
north of 15°N and Australia south of 15°S. In the absence of irrigation, the only
agricultural production feasible in these regions is extensive grazing.
The cool tropics
The final category, that of the ‘cool’ tropics, encompasses areas where the mean
monthly temperature falls below 18°C but stays above -3°C. It is made up of regions
at higher altitudes and these have a marked variability in rainfall. Crops are produced
at altitudes up to 3000 m above sea level on the equator in the Andes, although we
would more readily associate many of the crop species grown there with temperate
climates.
Tropical Soils
Soils of the tropics are extremely diverse. The single common characteristic of all
tropical soils is the constancy of soil temperatures throughout the year. The widely
held misconception that all tropical soils are highly leached and infertile originated
from the early writings of rather ill-travelled scientists from northern, temperate
regions (Sanchez, 1976). The term ‘tropical soil’ became synonymous with ‘lateritic
soil’, used for those soils with layers rich in iron oxides that harden irreversibly on
exposure to air. In reality, soils of the tropics vary from young volcanic or alluvial soils
to some of the oldest, most highly weathered and leached soils in the world. Two
classification systems that divide soils into groups on the basis of their physical and
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6
Chapter 1
chemical structure are now widely used: the World Reference Base for Soil Resources
(ISSS/ISRIC/FAO, 1998), which replaces the FAO/UNESCO Legend (FAO/
UNESCO, 1974); and the USDA Soil Taxonomy (Soil Survey Staff, 1999). The
major groups of each classification, and how the two can be related to each other, are
summarized in Table 1.1. The USDA Soil Taxonomy will be used here in further
discussion.
Not all of the groups of soils can be readily interrelated between the two systems
as some of the criteria used to separate groups differ, and many generalizations are
made at this broad ‘order’ scale of description. Both systems further subdivide the
major orders of soils into many subgroupings, which give somewhat more detailed
information as to their moisture, temperature and nutrient status. A detailed
discussion of these subdivisions is beyond the scope of this book.
An analysis of the relative frequency of different soil groupings indicates that
highly weathered, leached soils (Oxisols, Ultisols and the less-leached Alfisols) cover
more than half of the land area of the tropics (von Uexküll and Mutert, 1995). Desert
soils (Aridisols) occupy some 16% of the tropics, leaving only 30% of the land area
covered by younger soil formations (Table 1.2). Soil classification is not necessarily
a good guide to soil fertility, as classification depends more on the characteristics of
the subsoil, whilst the ability of soils to support crop growth, at least in the short
term, is dependent on the surface soil horizons. However, classifications do provide a
useful framework within which to discuss the distribution and uses of soils and
generalizations can be made concerning the advantages and problems for agriculture
in the different soil types.
Table 1.1. The major soil orders of the USDA Soil Taxonomy and their
approximate equivalent in the World Reference Base for Soil Resources.
(Updated from Norman et al., 1995.)
USDA Soil
Taxonomy
Oxisols
Ultisols
Entisols
Alfisols
Inceptisols
Vertisols
Aridisols
Mollisols
Andisols
Histosols
Spodosols
aItalics
World Reference Basea
Ferralsols, Gleysols
Acrisols, Nitisols
Fluvisols, Plinthosols, Durosols, Regosols, Arenosols, Gleysols
Luvisols, Alisols, Planosols, Albeluvisols, Solonetz
Cambisols, Gleysols
Vertisols
Yermosols, Xerosols, Cambisols, Solonetz, Solonchaks,
Gleysols, Rendzinas, Albeluvisols, Phaeozems, Nitisols
Chernozems, Phaeozems, Kastanozems, Umbrisols, Rendzinas
Andosols
Histosols
Podzols
indicate the predominant corresponding group.
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Tropical Environments and Cropping Systems
7
Soils are formed by the chemical and physical weathering (or breakdown) of
parent materials and the high temperatures and rainfall of many parts of the tropics
ensure that weathering can be very rapid. High rates of leaching (that is, the removal
of nutrients in water percolating through the soil) go hand-in-hand with rapid
weathering. This, coupled with the fact that large areas of soils are developed from
rocks such as granites which contain small amounts of weatherable bases, means
that many inherently infertile soils occur in the tropics. The rapid weathering of
exposed rocks also means that some deposits of volcanic origin, which are often
young in geological terms, can also be highly leached. Likewise, alluvial soils will not
necessarily be fertile if they are formed by deposition of particles originating from
erosion of old, weathered surfaces. When considering the processes that brought
about the formation of a particular type of soil it is important to remember that the
climate prevalent at the time when the soil was formed may have differed markedly
from the present climate.
Younger, more fertile soil formations are characterized by the presence of
unweathered minerals, in which the fertility of the soil is maintained by the release
of nutrients by weathering. The more fertile soils therefore tend to occur in areas
where there has been relatively recent (in geological terms) addition of volcanic ash or
alluvium containing weatherable minerals, or in climates where a long dry season
slows down the rate of weathering and leaching. In areas subject to tectonic activity,
repeated landslides can restrict soil development so that the soils remain shallow and
unweathered minerals remain within the reach of plant roots.
Table 1.2. Area and distribution of soils in the tropics. (Based on Sanchez and
Salinas, 1981; Norman et al., 1995 with modifications from Soil Survey Staff,
1999.)
Major soil
associations
Oxisols
Ultisols
Entisols
Alfisols
Inceptisols
Vertisols
Aridisols
Mollisols
Andisols
Histosols
Spodosols
Total
Tropical
America
(Mha)
Tropical
Africa
(Mha)
Tropical
Asia
(Mha)
Tropical
Australia
(Mha)
Total
(Mha)
% of the
tropics
452
325
126
96
232
18
7
32
32
4
3
1328
495
137
305
289
178
42
166
0
1
4
1
1618
14
291
76
65
193
60
5
4
12
24
2
745
0
8
95
29
3
28
8
0
0
0
0
171
961
761
602
478
606
149
186
36
45
32
6
3862
25
20
16
12
16
4
5
1
1
1
<1
100
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Chapter 1
Soils with low activity clays
Highly weathered soils, which used to be called latosols, are now classified as the
Oxisols, Ultisols and Alfisols with low activity clays. These are deep, well-drained
soils with low activity clays, characteristic of old landscapes with very high rainfall.
They also occur in parts of West Africa and Australia that are now dry but used to be
much wetter. Oxisols are soils with weathered horizons of kaolinite, iron oxides and
sand with a small capacity for cation exchange (< 16 mmol 100 g-1 of clay). They are
usually very deep, well-drained, red or yellow soils with poor fertility but excellent
physical structure. Oxisols cover a huge area of the Amazon basin, the South
American savannahs and Central Africa. There are smaller areas of Oxisols in
Southeast Asia. Ultisols can be distinguished from Oxisols on the basis of a distinct
horizon enriched in clay with less than 35% base saturation (see below). They
generally contain more weatherable minerals than Oxisols but have a poorer structure
and are infertile. Most of the uplands of Southeast Asia are dominated by Ultisols
and these soils also cover large areas of South America and Africa. The low activity
clay Alfisols are soils with a distinct horizon enriched in clay with a base saturation
greater than 35%. They are similar to Ultisols but generally less acid and more fertile.
Soils with high activity clays
Many Alfisols have high activity 2 : 1 clays, are fertile and have no major management problems. Some Alfisols with a subsoil rich in laterite can become uncultivatable if the topsoil is eroded, and these are found mainly in West Africa, India and
Sri Lanka.
Most Alfisols, Entisols and Inceptisols are young soils with little differentiation.
Inceptisols have a moderately weathered subsoil (cambic horizon) but no other diagnostic horizons. An important group of Inceptisols, the Sulfaquepts or ‘acid-sulphate
soils’, occur in coastal plains and river deltas where iron-rich soils have been
inundated with sea water. Sea water contains a lot of sulphate, which is reduced to
H2S and then forms pyrite (FeS2). On exposure to air the pyrite is oxidized to ferric
sulphate and sulphuric acid, giving soil pH values as low as pH 2 in some cases.
Vertisols or ‘black cotton soils’ are deep soils with a high proportion of 2 : 1
clays (see below) which swell on wetting and crack on drying, such that a selfmulching effect occurs. They cover large areas of India, Java, Ethiopia and the Sudan,
and lower topographic positions throughout ‘wet-and-dry’ climates in Africa, but
a small part of the tropics as a whole. Spodosols are usually developed on sandy
materials and characteristically have an ‘iron pan’ formed below a bleached horizon
with a surface organic layer, usually as a result of a fluctuating high water table.
Histosols are soils generally developed in wet conditions in which more than half of
the top 80 cm is organic matter. Neither of these last two groups covers large areas of
the tropics but both are of local importance.
Andisols are usually black soils developed from volcanic deposits with a high
organic matter content; they are usually found in mountainous regions. Although
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Andisols are often relatively young soils, with a large amount of weatherable minerals, they are not all fertile as they can become rapidly leached and can have a very high
capacity to fix phosphorus. They do tend to have good physical properties and
although they only cover some 2% of the land surface of the earth they support
roughly 10% of the world’s population – indicating their capacity for agriculture.
Mollisols are soils with a soft surface horizon rich in organic matter, and base
saturation > 50%; they are found in northern India, Mexico and Paraguay. Aridisols
are soils of dry regions which have little importance for agriculture, unless irrigated,
but cover a large area of the African tropics (Table 1.2).
Chemical characteristics of leached soils
Leaching removes large amounts of nutrients from the soil. The cation exchange
capacity (CEC) is a measure of the net negative charge of a soil, and this determines
the soil’s ability to retain positively charged ions or cations. The CEC results from
negative charges on the surface of clays and on the soil organic matter. In most of
the highly leached tropical soils, iron and aluminium oxides and hydroxides are
abundant and the dominant clay fraction is kaolinite. Kaolinite has a structure of
1 : 1 silica : alumina layers and carries an inherently small negative charge compared
with the 2 : 1 clay minerals (such as smectites, illites or vermiculites) that are
predominant in soils of temperate regions. Further, whilst the 2 : 1 clays carry a
permanent charge, the negative charge on kaolinite and on organic matter varies
depending on the pH and the ionic strength of the soil solution. At low soil pH the
CEC is small compared with that at high pH, rendering the capacity to protect
cations from leaching even less. In some tropical soils a net positive charge can
develop so that the rate of movement of anions (such as nitrate) through the soil can
be retarded, but this is fairly rare (Wong et al., 1990). This process may assist in
enabling deep-rooting species, especially trees, to capture and recycle nitrate from the
subsoils of low activity clay soils (Buresh and Tian, 1997).
In soils where the parent material contains much aluminium it can become the
predominant cation when the soils have been leached of other cations. Thus the
proportion of the CEC occupied by aluminium ions (the % aluminium saturation)
can be as high as 80–90% and the base saturation (i.e. the proportion of the CEC
occupied by the cations that predominate in most soils: Ca2+, Mg2+ and K+) is low.
Acidity per se is not harmful to plants, except in extreme cases, and the problems of
plant growth on acid soils are largely due to the large amounts of aluminium, and in
some soils iron and manganese, that come into solution under acid conditions and
are highly toxic.
Warm, wet conditions in soil are ideal for the rapid decomposition of organic
matter added to soil. This can be an advantage as nutrients are released rapidly but it
also means that little organic matter generally accumulates. Organic matter provides
an important component of the CEC where the contribution of the clay fraction is
small and it also contributes to the physical properties of the soil by helping to hold
soil particles together in large aggregates, which are important for aeration of the soil
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Chapter 1
and infiltration of water. There is often a marked seasonality of organic matter
decomposition in the wet and dry tropics, due to a flush of decomposition associated
with the rewetting of very dry soils – known as the ‘Birch effect’ (Birch, 1964). This
can lead to a pronounced flush of nitrate in the soil at the onset of the rainy season
that is susceptible to leaching in cultivated soils, as the high concentrations of nitrate
occur before the roots of crops are sufficiently well developed to absorb.
Deficiencies of many essential nutrients are common in leached soils, and soils
that have developed over parent materials that contain small amounts of particular
elements will be especially prone to problems. In the highly acid Oxisols and Ultisols
it is not uncommon to have an inadequate supply of nitrogen, phosphorus, sulphur,
calcium, magnesium, zinc, boron and copper. Phosphorus tends to be chemically
bound or ‘fixed’ in a form not available for plant uptake in acid soils rich in iron and
aluminium hydroxides. Molybdenum, which occurs in soils as the molybdate ion
(MoO42-), is held in the same way so that deficiencies in plants may be acute even
in soils that are not inherently depleted of phosphorus or molybdenum. These
deficiencies, coupled with the toxicity problems described above, may make one
marvel that agriculture can be practised on such soils at all, but with proper management continuous cultivation is possible (Sanchez and Salinas, 1981; von Uexküll and
Mutert, 1995).
Tropical Cropping Systems
In parts of the forests of Southeast Asia and the Amazon, indigenous tribes can still
obtain a large amount of their food by hunting and gathering, but the area of forest
that can support this is rapidly diminishing. All other societies derive the major part
of their food from the cultivation of crops or from animal production in grazing
systems (Ruthenberg, 1980).
Shifting cultivation
The oldest form of crop production known is that of shifting cultivation or ‘swidden’
agriculture. Crops are produced on land from which the native vegetation (most
often forest) has been cleared, and usually burned, and after the cropping period the
land is abandoned and the vegetation allowed to regenerate. The lengths of the different phases of the cycle of shifting cultivation – clearance and burning, cropping, and
finally abandonment and regeneration – vary enormously between different regions.
The cropping phase is usually short, only 1 or 2 years, and when land is plentiful is
followed by a regeneration or fallow period of 15 years or more. Shifting cultivation
can be a sustainable form of agriculture provided that sufficient time is allowed for
the store of nutrients in the soil and the vegetation to be fully replenished, but only a
limited population can be supported. If the land is brought back into cultivation too
soon for the land to regenerate fully then the balance of the cycle will be upset and the
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11
organic matter of the soils, the key to soil fertility, will be gradually depleted (Nye
and Greenland, 1960).
Fallow systems
An intensified form of truly ‘shifting’ cultivation, in which settlements are moved
slowly as new areas are brought into production, occurs through various rotation
systems (Ruthenberg, 1980). Fallow systems are those in which crops are grown with
short, intervening fallow periods during which the land is left to revegetate, but they
tend to give way to continuous cultivation as population pressure increases. As with
most artificial classification schemes, the boundaries between different forms of
cropping system are often not clear, but fallow systems can be identified as systems
with one-third to two-thirds of the land under cultivation at any one time, whereas
continuous or ‘permanent’ cultivation systems are those where more than two-thirds
of the land is cultivated at any one time. Various types of fallow systems can be
identified: bush-fallow systems, in which the ‘bush’ – usually grasses, shrubs and trees
– regrows during the fallow; savannah-fallow systems, in which the fallow comprises
grasses; or ley systems, in which the land is also dominated by grasses during the
fallow and is used for grazing.
As the fallow periods are not long enough to restore the fertility of the soil fully,
productivity decreases with the intensity of soil use on all but the most fertile soils,
unless nutrients are imported to replace those leached or removed in the crops. In
most of the tropics the supply of organic manures is sufficient to sustain only a
moderate output, and mineral fertilizers are often beyond the means of smallholders.
Production thus continues by mining the reserves of the soil.
Permanent farming
Classifications of permanent agricultural production systems are generally based on
one of three criteria: the nature of the crop rotation; whether crops or animals are the
major outputs; or on the water supply, whether rainfed or irrigated. In the tropics
there are few farmers who do not have any animals for milk or meat production
and few pastoralists who grow no crops, particularly when we consider small-scale
production. This means that no classifications can account properly for the diversity
of crop rotations and combinations likely to be encountered. The common features
of permanent farming that tend to distinguish them from fallow systems are: a
permanent division of land between that used for arable crops and that used for
grazing; clearly defined fields; and a predominance of annual and biennial food crops
(Ruthenberg, 1980).
A feature common to agriculture in most regions of the tropics is the widespread
use of multiple cropping in which several crops are grown in the same field, either in
rotation within a year, or in combination in various forms of intercropping. Most of
these cropping systems contain some component of perennial crops, but a separate
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Chapter 1
form of system can be recognized where perennial crops are planted as the main
source of income in plantations.
Conclusions
The major determinants of agricultural productivity in the tropics are climate and
soil fertility. A number of climatic zones can be identified and these change more
with topography than with latitude. The fertility of soils in the tropics varies widely.
Soils derived from geologically recent deposits, such as unweathered volcanic deposits or alluvial material, are the most fertile. Many tropical soils are derived from
ancient parent rock poor in bases and are highly weathered and leached, with
consequent problems of nutrient deficiencies, or of acidity and associated toxicities.
A number of different cropping systems are practised in the tropics, ranging from
shifting cultivation through fallow systems to permanent agriculture. With increasing population pressures there is a tendency towards permanent agriculture and a
serious danger of a steady depletion of soil fertility. This book addresses the present
and possible future contribution of N2-fixation in the maintenance of soil fertility in
tropical cropping systems.
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