Abstract
The chemical characteristics of two soil types, an Alfisol and an Ultisol, were studied in the humid tropical zone of southern Nigeria. The objective was to investigate the contribution of soil organic matter (SOM) to nutrient retention characteristics and to identify those SOM pools that contribute most to the ion exchange properties of the soils. Ion exchange studies involving potassium–calcium (K–Ca) and magnesium–calcium (Mg–Ca) systems in whole soil, organic fraction (53–250 µm), and clay fraction (<2 µm) showed a preference for Ca relative to K in the soil and organic fraction. For the clay fraction in Alfisol, the isotherm in the K–Ca system suggested a preference for K at low K saturation, but, from about 50% K saturation, the isotherm inferred a preference for Ca. However, the overall selectivity at constant temperature and pressure as expressed by Gibb's free energy change suggested a stronger binding of K than Ca by the fraction.
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