13. Efficient fertilizer use in acid upland soils of the humid tropics.
FAO Fertilizer and Plant Nutrition Bulletin, 10, ISBN 92-5-102387-5,
1986, 51 pp. + references
The largest reserves of potential arable land still available in the world are located in the humid tropics.
Significant advances have been made in the characterisation and management of acid soils in the uplands of the humid tropics. The aim of this bulletin is to review the experience already acquired and to summarise the research findings which have recently become available.
On the uplands, acid soils predominate and agriculture at a low level of inputs is only possible through shifting cultivation, in which the land is cropped for a few years in alternation with long periods of fallow.
In most cases crop growth in acid soils can be directly correlated with Al saturation or Al concentration in the soil solution. High H+ concentrations in the soil solution, however, favour weathering of soil minerals, resulting in the release of Al3+ and the leaching of ions such as K+, Mg2+, Ca2+ and Mn2+.
Poor crop growth on acid soils is usually caused by aluminium and/or manganese toxicity and/or by deficiencies of phosphorus, calcium and magnesium.
Most of these soils are at present under virgin rain forest, with smaller areas under savanna, tree crops and shifting cultivation. The main reason for lack of development of these soils is that a high standard of management and costly inputs are needed to bring them into permanent arable cropping.
Most of the acid upland soils of the humid tropics are classified as Oxisols (Ferrasols) and Ultisols (Acrisols). Both groups are very acid with low base status, their mineral horizons containing small amounts of most nutrients.
When cropped without proper management, most acid soils of the humid tropics deteriorate, chemically and physically, so quickly after clearing that after a few years no crop can be grown on them. With adequate inputs and proper care, the annual productivity of these soils can far exceed the productivity of most fertile soils in temperate regions.
Where population pressure is low, shifting cultivation is often still the most appropriate land use system.
Better techniques of forest clearing are being developed. Zero-burn techniques in which the felled forest biomass is broken down under a short term leguminous cover crop followed by moderate applications of lime and P fertilizer show considerable promise.
Three levels of intensity can be distinguished and are discussed in this book:
- shifting cultivation with no lime or fertilizer inputs, relying on long fallow periods for regeneration;
- continuous cultivation with moderate applications of lime and P fertilizer, using leguminous cover crops or alley crops to provide biologically fixed nitrogen and organic matter;
- intensive continous cropping with large and continued inputs of NPK fertilizer, lime and other nutrients, a system that is capable of reaching and maintaining very high levels of productivity.
With good management once infertile acid tropical soils can produce annually the equivalent of 15 to 20 t/ha of grain. The availability of high yielding and disease resistant cultivars means that maize yields in excess of 10 t/ha per crop are now obtainable, while acid tolerant and disease resistant soybean cultivars can yield 2.5 to 3 t/ha per crop. It will usually take several years of good management to achieve these yield levels.
High rates of fertilizer are recommended by the author to maintain production when three crops are taken.
The rates are very similar to the rates used by many temperate region farmers aiming for comparable yields.
When high inputs are used the risks involved must be minimised. For acid upland soils this can only be done by conserving organic matter.
Organic matter, lime, and P are the three main factors on which a successful soil management and crop productivity programme for acid tropical soils can be built.
The agronomic practices adopted must supply a correct balance of organic manure and additional mineral fertilizer.
1250 92 - 12/76
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