Soil types have been classify by soil scientists by various systems, but many of these are specified to certain regions and geographical hit A broader Grouping, which was one of the first used, was divide Soils into zonal, a zonal and interzonal categories. A
zonal soil is a mature, typical soil of any given region, produced as a result of the environment in which it
occurs.
An a zonal soil is a relatively immature soil which does not relate to its environment and intrazonal soils are those which are found in more than one zone, they can include soils which have not developed as a result of environmental factors due to the presence of a single condition such as impeded drainage e.g- marshes and swamps.
SAVANNA ZONE SOILS
Many of these soils occur in areas where the rainfall is less than 110 cm per annum and often in the region of
45—100 cm. The loss of water from the soil surface and vegetation cover, which is referred to as evaporation transpiration, may be up to 120—150 cm per annum, this occurring mainly during the dry season. Leaching of mineral reserves is very limited and therefore these soils are mainly neutral or slightly aikaline, particularly in the topsoil region, due to the accumulation
of soluble salts.
Their cation exchange rate is low due
to the lack of humus, clay and a relatively high content of sand. In dry areas, the soils may contain a fairly
high level of salts, particularly those of calcium and magnesium. In extreme cases, the presence of sodium salts may limit the growth of vegetation and crops.
These soils often have ironstone or 'laterite' layers near the surface so that drainage is impeded; this also makes
them unsuitable for cultivation.
WEST AFRICAN SOIL TYPE
The following examples are from Nigeria, but many similar types soil occur in other parts of West Africa. This classification is based on the work of
H. Vine who published his descriptions in 1953 and 1954.
Zone A
This includes the savanna areas and some drier parts of the rain forest climatic zone and represents the
largest soil group in the country, covering about 858 000 km3. These soils are well-drained, with small areas which are badly drained and subject to leaching They have a low to medium humus content. The surface layers are slightly acid to neutral and the subsoil is generally acid.
The average rainfall varies from 50-200 cm, exceptionally to 250 cm per annum. The colour of the older soils is generally red-brown, more recently formed soils may be less red.
