BACKGROUND ON NIGERIA SOIL TYPES
Soil types in Nigeria are influenced by and follow very broadly, the climatic and vegetational zones of the country. This is expected because the degree of available moisture in the soil is an important factor in soil reactions and fertility and productivity. The soils of the humid tropical forests are quite different from those of the drier forests and the savanna zone, which in turn are different from the savanna zone.
GENERAL CLASSIFICATION OF NIGERIA SOIL
Nigerian soils can be classified into groups made up of four (climatic) zones that are soil associations. The groups are:
(i) Northern zone of sandy soils
(ii) Interior zone of laterite soils;
(iii) Southern belt of forest soils
(iv) Zone of alluvial soils
NORTHERN ZONE OF SANDY SOILS
This area lies in the very northern parts of the country. In some areas like the Sahel savanna belt, the soils are true to type, being formed under aridity and by the deposition of sand by the wind. These soils might have been formed from wind- sorted desert sands that accumulated over long periods of time when the Sahara desert encroached several kilometres south of its present limits. The soils of this zone produce much of the groundnut crop, some of the sorghum, cowpeas and large quantities of millet. For instance in Kano, Northern Kaduna, Zamfara and Sokoto states they have fine sandy loam, friable and relatively easy to cultivate soils. The soil is little leached and therefore ideal for groundnut cultivation. Whereas in southern Kuduna is found a mixture of soils that disintegrated from local granite, and loess soils that were brought down by winds from the north. The soil is in fact not sandy. These soils are the Zaria loam that produces the largest yield of cotton in Nigeria.
INTERIOR ZONE OF LATERITE SOILS
This zone is made up of sands and clays. They are grey to black clays poorly drained and seasonally flooded forming the “fadama”. After clearing the green vegetation on them this do result into further loss in fertility, therefore rendering the available soil of little or no agricultural value. When the soil is exposed to the surface, it become as hard as brick and for this reason, the soil here is most suitable for road paving and wall construction than for farming.
Moreover, it should be noted that there are others soil types in the zone aside the laterite soils but they are at low quantity. The Biu Plateau has a very good fertile soil which is productive and gives avenue for the increase in size of the areas of cotton production.
SOUTHERN BELT OF FOREST SOILS
Soils in this zone broadly represent those of the humid, tropical forest climate zones of the south where the wet season is long, the harmattan season short and forest cover is dense. Soil types in a particular locality often depend majorly on the parent rock. So where the underlying rocks are either granite or clay, the soils is usually a rich clayey loam. The forest soils in this zone is best used to cultivate crops like cocoa, oil palm, rubber and which are of considerable importance in Nigerian agriculture.
ZONE OF ALLUVIAL SOILS
These soils are found on the flooded plains of rivers or on deltas, or along the coastal flats. This particular zone spans from the coastal inland and through the valleys of the Niger and the Benue rivers, therefore cutting across the vegetation zones. The soils found in this zone do not depend highly on climate and vegetation for their formation. The underlying parent rock is the most important factor in their formation. Soils in this zone are majorly of fresh water soil of grey to white sand type, grey clay type and sandy clay with humus topsoil.
PROBLEMATIC NATURE OF NIGERIA SOILS
Most Nigeria soils (like collapsible soil and black cotton soil) are unsuitable for construction works. In engineering, soils with characteristics which cannot be easily and economically put to use during construction without the use of some stabilization measures are often refers to as problem soils. Problem soils are expansive or swelling and collapsing soils.
To the geotechnical and highway engineers, a problem soil is one that poses problem to construction. Such problem may be as a result of instability of the soil which makes it unsuitable as a construction material in foundations, buildings, highways, water retailing structures, dams, etc.
Problem soils are the types that of low strength and high compressibility. Therefore, most of these soils have been detected in Nigeria. They include mainly; the black cotton soils which occur widely in the north-eastern part of Nigeria and the sokoto soft clay shale (attalpulgite) in the north-western Nigeria.
Therefore, if soil fails, the structure founded on it can collapse. Crack failures and collapse of light buildings founded on these problem soils and rocks due to volume changes from moisture contents or structural loading conditions have caused considerable loss of life and properties.