The basic components of soil are; minerals, organic matter, water and air. The typical soil consists of approximately 45% minerals, 5% organic matter, 20%-30% water and 20%-30% air. These percentages are only generalization at its best. In reality, the soil is very complex and dynamic. The composition of the soil can fluctuate on a daily basis depending on numerous factors such as; water supply, cultivation practices, and/or soil type.
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 (Oyenuga, 1967).
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; and (iv) Zone of alluvial soils (Oyenuga, 1967; Iloeje, 2001).
(i) 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 Kaduna 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.
(ii) 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”. Soil in this zone is deeply corroded, generally sticky and impervious to water and has low fertility. When the virgin forest on them is cleared it reduces the fertility further, thus making available soil of little 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. However, not only laterite soils are found in this zone. The Biu Plateau has rich soil that is productive and offers prospects for the expansion of the areas of cotton production.
(iii) 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. Local soil types depend largely on parent rock. Where the underlying rocks are granite or clay, the soils is a rich clayey loam. The forest soils yield cocoa, oil palm, rubber and they are of considerable importance in Nigerian agriculture.
(iv) Zone of alluvial soils: These soils are found on the flooded plains of rivers or on deltas, or along the coastal flats. This zone extends from the coastal inland and runs along the valleys of the Niger and the Benue rivers, thus cutting across the vegetational 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 characteristic of fresh-water soil of grey to white sand, grey clay and sandy clay with humid topsoil. Another group consists of brownish to black saline mangrove soils, with a mat of rootlets.
Therefore, importance of soil test can’t be over emphasized to farmers. It’s advised to do a soil test before embarking on production in the soil to know if a particular or specific crop or vegetable can thrive on the soil and avoid losing investments. The good news for farmer is the federal ministry of agriculture and rural development will be introducing soil test kit to farmers nationwide as part of its effort to boost food production. This will enable farmers to conduct instant test on the soil to determine its strength and weaknesses. The technology which is referred to as “soil doctor” analyses the fertility of the soil and instantly, result comes immediately stating the fertilizer to be used for a particular soil. This is an improvement as opposed to the previous soil test method where fields samples are taken to the laboratory for analysis and takes a couple of weeks to get the result.
The maintenance of good soil quality is vital for the environmental and economic sustainability of annual cropping. A decline in soil quality has a marked impact on plant growth and yield, grain quality, production costs and the increased risk of soil erosion. Farmers need improve their cultivation practices, select fertilizer inputs, and ultimately improve crop production while avoiding soil degradation.
Food and Agriculture Organisation (World Bank)