2.3 Soil characteristics

The soil is the upper part of the earth, extending cat 1 metre in depth, where most of the plant roots are found. Its properties are determined by age, geology, climate, and topography, as well as the occurrence of plants, animals, and micro-organisms. There is great variation among the types of soil found in the tropics. The term "tropical soil" expresses few common qualities apart from geographical location. The oldest civilizations in tropical areas developed on the very fertile soils of river plains or volcanic materials. Such soils consist of relatively young materials and are characterized by a steady supply of nutrients either through mud (the flooding of river plains) or through weathering (volcanic materials). On the other hand, surfaces which are old in geological terms have strongly weathered and leached soils with low content of nutrients, limited ability to retain added supplies of nutrition, and a tendency to form compact layers as a result of drying out and compaction. A common denominator for soils in tropical and sub-tropical areas is the rapid turnover (absorption and release) of nutrients. Upon the removal of vegetation, nutrients can rapidly be leached from the soil. Soil which is deficient in nutrients is more vulnerable to erosion and compaction as vegetation is established relatively slowly and remains more sparse.

The content of organic matter is of great importance to the soil's fertility or productive ability. The term humus partly refers to the relatively stable organic components of the soil, and partly to the total content of organic matter. For agricultural soil, the terms mould and mould content are often used. The content of organic matter in the soil will depend on supplies from plant residue, roots and organisms, and on the speed at which decomposition takes place. The latter is determined by the climate, and by the temperature in particular. Too much water may obstruct the air, and the decomposition is inhibited (bog formation). There are also examples of periodic water shortages lowering the speed of the turnover. In tropical areas with a lot of rain, there will be a rich supply of organic matter, but this is counterbalanced by rapid turnover. In rainforests, the organic matter is found in the vegetation. The rapid turnover in tropical and sub-tropical rainforests causes quick changes in the mould content of the soil, and in a matter of only a few years the cultivation of field crops will bring the content down to a level where plant production becomes very hard to sustain. Agroforestry (see booklet 3 "Forestry") represents new systems and methods with the objective of utilizing qualities about trees (deep-penetrating roots) and agricultural crops (shallow roots) in order to achieve a more sustainable plant production.