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II. Brief Description of Tropics
Land Areas of Major Climatic Regions of the Tropicso
Tropical climatic regions
OO0 000 ha
Rainy climates (9.5-12 months humid)
Humid seasonal climates (7-9.5 months
Wet-dry climates (4.5-7 months humid)
Dry climates (2-4.5 months humid)
Semidesert and desert climates (less than 2
Adapted from Table 8-1 of the President’s Science Advisory Committee (1967).
Area of Principal Kinds of Soils by Major Tropical Climatic Regionse
Tropical climatic regions
Light colored soils; base rich (Desert; Semidesert),
Dark colored soils; base rock (Chernozemic,
Grumusols, Renddna, Brown Forest), Mollisols
Moderately weathered and leached soils (noncalcic
Brown, Ando), AIjisoIs and Andepts-lnceptisols
Highly weathered and leached soils (Latosols,
Lateritic, Red-Yellow Podzolic), Oxisofs and
Shallow soils and dry sands (Lithosols and
Alluvial soils, Entisols
Adapted from Table 8-2 of the President’s Science Advisory Committee (1967).
FERTILIZERS FOR USE UNDER TROPICAL CONDITIONS
variation of temperature at 50-cm depth is less than 5OC. Naturally, average temperatures vary considerably in a spatial sense because of differences
in altitude, etc.
In spite of extensive weathering of many of the soils in the humid tropics,
such soils are regarded as having promise for development. Kellogg and
Orvedal (1969) pointed out that most of the potentially arable but unused
land of the world is in the tropics. According to the President’s Science
Advisory Committee (1967), most of the unused but arable land in the
tropics is in Africa and South America. According to Revelle (1974),
much of the potentially arable land in the tropics is of poor quality and
would require major capital investment for development. Consequently,
agricultural use of these lands will be difficult, requiring a good deal of
adaptive research to determine workable systems. But, as populations continue to expand, such lands must be utilized.
The most basic division of agriculture in the tropics is between modern
agriculture and traditional agriculture (Coulter, 1972). Modern agriculture
accounts for industrial products and certain food crops on a largely plantation basis-and with a relatively efficient use of technology. Traditional
agriculture accounts for most of the food produced in the tropics, and the
technology used is relatively primitive.
Accordingly, the modern sector utilizes a disproportionately larger share
of fertilizers and other inputs consumed in the tropics, The larger scale
under which this sector operates makes adoption of such practices much
easier than by traditional farmers who usually operate with limited capital,
high risk aversion, and under the customary socioeconomic restraints.
Cropping patterns common to traditional agriculture vary so widely and
are so diverse that no attempt will be made here to list or discuss all of
them. A few of the more important types are discussed in relation to the
problems of maintaining soil fertility.
1 . Shifting Cultivation
Sanchez ( 1973) defined shifting cultivation as a continuing agricultural
system in which the cropping period is shorter than the fallow period. In
1957, F A 0 estimated that this was the predominant practice on 44% of
the potentially arable and grazing land in the tropics. While several different types of shifting cultivation are practiced, the two types discussed here
are bush fallow in forested areas and grass fallow in savanna areas.
The bush-fallow system of shifting cultivation is quite satisfactory where
population pressures are relatively low. Forest vegetation during the fallow
0. P . ENGELSTAD AND D. A. RUSSEL
period is a major buffer against leaching losses of nutrients (Coulter,
1972). Nye and Greenland (1960) indicated that the forest represents
a nearly closed nutrient cycle. The soil is also fairly well protected against
Grass fallow is a less satisfactory system for agricultural purposes than
is bush fallow. Grasses are often more difficult to eradicate than is forest;
leaching losses of nutrients in the savanna are also more severe, resulting
in lower fertility levels (Coulter, 1972). Much of the present savanna area
in the tropics has been derived from originally forested land when the bushfallow rotation became too short to allow the forest to regenerate.
2 . Continuously Grown Upland Crops ( N o Fallow Period)
Because of population pressures, the future of shifting cultivation is in
doubt in many areas (Coulter, 1972). Where the nature of the soils permits, systems need to be devised that allow cropping on a sustained basis.
Such systems must solve weed control and erosion problems and also prevent decreases in soil fertility. Organic materials must be returned to the
land where possible for soil improvement and as supplementary sources
of nutrients. Where feasible, legume crops also should be grown as a source
3. Lowland Rice
In contrast to shifting cultivation generally practiced on humid uplands
of relatively low fertility, lowland or flooded rice is usually grown on the
more fertile alluvial soils of the tropics. In Asia, the population is largely
concentrated in areas where lowland rice can be grown. Although these
soils are generally more fertile, use of fertilizers (primarily N ) for lowland
rice is quite common. This is particularly true where the new dwarf types
of rice are grown in association with good water supply and control. However, good irrigation facilities are by no means prevalent in the tropics.
Problems of effectiveness of fertilizer use in relation to water management
are discussed later.
4 . Pastoral
Permanent pastures occupy about 25% of the total land area in the
tropics, mostly in monsoon areas and in dry areas between 15O and 30°
north and south. The low rainfall in these areas provides limited opportunities for forages and pasture to respond favorably to fertilizers (Russel et
al., 1974). Within the wetter areas, food crops are grown in preference
FERTILIZERS FOR USE UNDER TROPICAL CONDITIONS
to forage crops. Use of fertilizer on any crop other than food or export
crops is not an accepted practice. Livestock, although numerous, are expected to use waste lands and these are not fertilized. Returns on a fertilizer
investment for increased forage production may not be realized for several
In many tropical countries, the infrastructure does not permit extensive
use of fertilizers. Farm-to-market roads frequently are little more than foot
paths, and secondary roads support vehicles only during the dry seasons.
Under these conditions, fertilizers must be transported by headpack or,
at best, by bullock cart. Neither method is conducive to extensive use of
fertilizers. To add to the problem, fertilizers do not always arrive at a district warehouse until after the crop is planted, and those that do arrive
are not always the most suitable grade or material. Fertilizers in the warehouse may be caked, and many bags may be broken. Rebagged fertilizer
may be underweight and/or adulterated. Farmers have no way of checking
either and usually cannot resort to legal action if cheating is suspected.
Few farmers in tropical countries have machines that provide rapid, precise placement of fertilizer for maximum efficiency. Actually, few farmers
have fields large enough to justify ownership of mechanical equipment or
even rental from a cooperative or government machinery pool. Nearly all
fertilizers must be hand-broadcast upon the soil surface; subsequent incorporation is often inadequate or neglected entirely.
Educational personnel (extension service) are few and relatively immobile. Farmers must consult them in extension offices rather than in their
own fields. Many extension people may be no better informed about the
intricacies of fertilizer use than are farmers. The dealers may be even less
knowledgeable about the fertilizers they sell.
Still another major deficiency in infrastructure is the lack of a viable
marketing system. Traditionally, most food commodities are consumed in
the home or in the local area with little intention to produce surpluses
for commercial sales. Consequently, most local markets are not commercially oriented. Use of fertilizers to produce more of a commodity than
needed in the local area cannot be promoted until a new marketing system
is developed. In addition to transportation, the marketing problem involves
local grain storage in vermin- and pest-free silos until adequate transportation is available, marketing credit, marketing intelligence, and a host of
other supporting items.
In contrast to the generalized picture, however, there are many specific
examples where fertilizer use in the tropics is comparable in all respects
0. P. ENGELSTAD AND D. A. RUSSEL
to use in the temperate zones. Cash crops frequently are fertilized in accordance with the best recommendations. Farmers with large land holdings
can create large fields and justify their own mechanical equipment. In some
areas, farmers have formed cooperatives that purchase necessary inputs
(seeds, fertilizers, agricultural chemicals) and handle credit arrangements
with the local branch of the agricultural bank. These cooperatives sometimes maintain a machinery pool, but more often they provide the government with adequate justification for locating a machinery pool in the area.
The introduction of high-yielding varieties has been especially helpful in
fostering improvements in agricultural technology and local infrastructure.
History of Fertilizer Use in the Tropics
Low fertility has been a problem with most soils in the humid tropics
almost since man began to farm. Many cures have been discovered through
the centuries, such as ashes, mud, seaweed, compost, and dung, but few
have been widely adopted for a very long period of time (Fussell, 1971).
Shifting cultivation is the major exception.
Chemical fertilizers from the industrialized nations were used first on
the so-called estate crops, such as tea, coffee, rubber, and cacao, and then
on the cash crops, such as cotton and tobacco. Peterson and Frazier
(1964) pointed out that such use reflects the single-crop culture of large
plantations managed by well informed people, the easier access to credit
for cash farmers, and the orientation of cash farming to markets rather
than to home consumption. Despite these islands of fertilizer use in the
midst of vast areas devoted to traditional agriculture and despite the many
years of technical assistance programs, fertilizer generally is not used by
farmers who practice shifting cultivation. The system is too inefficient for
the imposition of modern soil management practices, and the technique
for transforming this traditional system to a more appropriate system has
not yet evolved (Coulter, 1972).
Nearly half of the land area of the world is located in the tropical zone
and so is over 40% of the world’s population. Nevertheless, in 1969 only
a little over 12% of the fertilizer consumed in the world was used in the
tropics (FAO, 1970). This is changing, however. Forecasts are that developing countries (nearly synonymous with tropical countries) will consume
20% of the nitrogen ( N ) , 18% of the phosphorus (P), and 13% of the
FERTILIZERS FOR USE UNDER TROPICAL CONDITIONS
potassium ( K ) (or 17% of the total plant nutrients) by 1980 (Harre et
al., 1974). Traditionally, the developing countries (i.e., tropical countries)
have obtained most of their fertilizer supplies from developed countries.
There was considerable fertilizer plant construction in developing countries
during the 1960s. Many of these countries sought to exploit indigenous
supplies of raw materials by processing them into intermediate and finished
fertilizers. Other countries imported virtually all raw materials needed for
fertilizer production. Thus, by 1972-1973, there were one or more N fertilizer plants in 24 tropical countries; similarly there were P plants in 18
countries and K facilities in 2 countries. As Figs. 1, 2, and 3 show, fertilizer
production in the tropics, even after a decade of construction, was not
adequate to match consumption. Fertilizer plants located in tropical countries produced only enough fertilizer to supply the equivalent of 42% of
the fertilizer actually used.
Fertilizer plants located in developing countries were designed to produce more fertilizer than was needed in these countries. All were conventional plants producing ammonium nitrate, ammonium sulfate, urea, ammonium phosphate, superphosphate, and potash. They were identical or
very similar to plants producing the same fertilizers in the developed countries; yet, they produced less than half the fertilizer that was used in tropical
countries, and only a third as much as they were designed to produce.
Although there was an apparent surplus of production in the world in
1972-1973, plants in the developed nations were operating at 85% to 95%
Fro. 1. Trends in the production and consumption of fertilizer N in the tropics.