Characteristics, Classification And Management Of Inland Valley Bottom Soils For Crop Production In Subhumid Southwestern Nigeria

Abstract

Inland valley bottoms (IVBs) on basement complex in sub-humid southwestern Nigeria were studied for water-table (WT) dynamics, and physical and chemical characteristics for their classification and management needs for crop production. The IVBs are generally small in size. About 64% of the IVBs averaged 0.78 ha while 36% averaged 1.69 ha. The dry season WT was 0-40 cm deep in 22%, 40-70 cm in 27% and 70-120 cm in 50% of the IVBs. These, corresponding to fluxial, high WT and very poorly drained IVBs (WT class 1), phreatic, medium and poorly drained IVBs (WT class II), and pluvial, low WT and imperfectly drained IVBs (WT class III), respectively. Clay and silt fractions are irregularly distributed in the soils but generally higher in the topsoil. Soil texture is highly variable both within and among the WT regimes, but while WTI and WTIII IVBs have loam or clay loam over sandy clay loam, WT II IVBs has silt loam over clay loam or sandy clay loam. Hydraulic conductivity (Ks) averaged 2.43 cmh-1 in the top 60 cm and 0.59 cmh-1 at lower depth in WT1 IVBs. The corresponding average values in WTII are 4.43 cmh-1 and 1.16 cmh-1, and 4.14 cmh-1 and 0.77 cmh-1 in WT III. Organic C and effective CEC are moderately high, and the basic cations comprised more than 90% of the adsorption complex. The soils were classified as Aquic Endoaquepts, (WTI), Typic Endoaquepts and Aeric Typic Endoaquepts (WT III). Low input technologies for intensive uses of the IVBs consist in adapting farming to the wet condition by adopting the rice culture, or adopting a system of shallow drain-ditches with mound-tillage to simultaneously lower the WT and increase the rooting depth during the dry season. Equally, mound-tillage can be used to grow a toposequence of crops with decreasing tolerance for excessive wetness from the base to the apex. Occasional fertilizer supplementation and mulching will be needed to conserve the soils and sustain high crop yields when the IVBs are transformed into intensive agro ecosystem.