Soil is a mixture of organic matter, minerals, gases, liquids, and organisms that together support life. (Definition from Wikipedia)
Main characteristics
Sandy soils
These occur in many parts of West Africa and are characteristic of many areas in the savanna zone of northern Nigeria. These soils are free-draining and do not retain any appreciable amount of soil-water, which rapidly drains down to the lower horizons, leaching any soluble minerals which may be present in the upper layers. Sandy soils contain gravel, coarse sand,
fine sand and limited amounts of silt.
They may be termed sandy clay loams (approximately 20-35% clay, 40-50% sand and 30-40% silt), sandy loams (approximately 15-20% clay, 50-60% sand and 35-40% silt), loamy sands (10% clay, 85-90% sand and 5-15%
silt). The approximate identification of any particular type of soil can be obtained from Fig. 2.5. These soils are only suitable for crop growth when
they contain adequate amounts of organic matter and minerals and can be supplied with water during dry
periods. They are suitable, when these requirements are met, for short-lived annual crops such as groundnuts and also tree crops such as the cashew, which are relatively drought-resistant.
Clay soils
These soils are suitable for crop production if the clay content is not high enough to cause excessive cracking and hardening of the soils during dry periods. During wet weather, heavy clay soils can retain a great deal of water which The capacity of clay soils to retain soluble minerals
which are essential for plant growth makes them potentially fertile, the addition of organic material improves the structure of clay soils, making them less retentive of water.
Applications of materials which contain calcium, such as lime (calcium hydroxide) and dolomitic limestone (calcium carbonate and magnesium carbonate) also assist in the improvement of the structure of clay soils, but their use in West Africa is very limited due to the fact that they generally have to be imported may
also make them difficult to cultivate.
Humic soils
This term is used to describe soils which contain a high level of organic material which has been broken down by the action of bacteria and other soil organisms. Humus is often the final product of the decom position of woody material which has been buried in
Mthe soil and it is most commonly found in the upper layers of the soil. In West Africa, as in many other tropical areas, the high soil temperatures promote rapid bacterial activity and therefore any material which is either placed on the soil surface as a mulch or
buried by hoeing or ploughing will rapidly decay. The level of organic material in West African soils is
therefore relatively low and requires constant replacement if crops are grown on an intensive scale.
Humus particles have the property of attracting to their surface soil minerals, in the form of cations, in a similar manner to clay particles and therefore soils with a relatively high content of humus are considered to be potentially fertile.
