Properties and Management of Kaolinitic Soils

Kaolinitic soils are the most widely occurring soils in the tropics, especially in tropical Africa. They comprise about 70% of the soils in the low-altitude tropics. Typically, these soils have a sandy, loamy sand, or sandy loam surface soil and sandy clay to clayey subsoil containing approximately 20-60% clay in the lower B horizons. Silt content is usually low throughout the profile (< 20%) with the exception of soils derived from loess materials. Soil erosion, compaction, and low nutrient- and water-holding capacities are the major constraints under intensive cropping. Ideally, kaolinitic soils in the humid and subhumid regions should be used for natural forest reserves and tree farms. In drier regions, sustainable land use includes natural grasslands and managed pastures with low stocking rates. When kaolinitic soils are used for annual crop production, crop rotation and managed fallow must be included in the farming system after a short cropping cycle to restore soil physical, chemical, and biological fertility and to sustain crop yield in the long term. Kaolinitic soils may be further distinguished into two subgroups based on inherent chemical fertility, namely, high-base-status and low-base-status kaolinitic soils. High-base-status kaolinitic soils usually have pH values of 5.3 or higher (measured in soil-water suspension), and a base saturation (BS) of 70% or higher throughout the soil profile calculated on the basis of effective cation exchange capacity (ECEC). Low-base-status kaolinitic soils generally have a pH value of 5.2 or lower, and a base saturation below 70%. The properties of high-base-status and low-base-status kaolinitic soils are given in tables 10-1 and 10-2, respectively. The common properties of these two soils are the dominance of kaolinite in the clay fraction, a low CEC, and a high bulk density in the subsoil horizon. The differences are the degree of base saturation, soil pH, degree of exchangeable Al saturation, and the quality of soil organic matter. In other words, the cation exchange sites of soil organic matter in the low-base-status soil are dominated by Al3+, whereas Ca++ and Mg++ are the dominating ions in the high base-status soil.