Chemical Characterization of Soils of a Tropical Humid Forest Zone: A Methodology

A methodology, based on a combination of routinely performed analyses and investigation of fundamental charge and anion sorption properties, was used to characterize the soils of the humid forest zone of Cameroon. In general, the soils have about 2 cmol kg⁻¹ permanent negative charge, with about 1 cmol kg⁻¹ from variable-charge sources at current soil pH values. Furthermore, they are improverished with respect to Ca, Mg, and K, while Al frequently dominates the exchange complex. Thus, the ability of these soils to retain base cations is more limited than is suggested by the cation-exchange capacity (CEC). Therefore we propose the concept of a degradation index (DI) defined as: DI = 100(CEC₅.₅ − sum of basic cations)/CEC₅.₅, where CEC₅.₅ is the CEC measured at pH 5.5. This index encompasses degradation a soil may have experienced from natural or man-made causes. Extractable PO₄ concentrations are considered very low and the soils have a moderate to high capacity to fix added PO₄. Surface soil SO₄ concentrations are considered marginal to deficient for plant growth, though adequate reserves of SO₄ are held in the subsoil by SO₄ sorption. The approach used demonstrated that the five morphologically different soil profile classes identified in the zone have similar chemical characteristics. Thus, the results of experimentation conducted on one of the soil profile classes will be applicable throughout the zone. Furthermore, the approach has provided a means of identifying comparable soil types in other parts of the world and will guide technology transfer. The analytical methods used in this study are relatively simple and require no specialized equipment, and are therefore within the capabilities of many laboratories in the developing world. Contribution from the International Institute of Tropical Agriculture, Humid Forest Station, Yaounde, Cameroon.