Properties and Management of Oxidic Soils

Oxidic soils are deeply weathered, fine-textured, oxide-rich soils in the tropics. These soils are the second most abundant soils in the tropics. Geographically, oxidic soils are found in Latin America (Brazil, Central America), East and Central Africa (Kenya, Congo, and Cameroon), the Caribbean Basin, and the Pacific Islands. In southeastern Asia, oxidic soils are found in isolated areas of Indonesia, the Philippines, and northern Australia, usually on the volcanic and limestone-dominated geomorphic surfaces. Oxidic soils are oxide-rich, low bulk density Oxisols, Alfisols, and Ultisols according to the Soil Taxonomy classification. In other soil classification schemes, most oxidic soils are classified under Sols Ferallitiques according to the French system, and Ferralsols and Nitosols under the FAO/UNESCO system. Oxidic soils are differentiated into high-base-status and low-base-status soils on the basis of the 70% base saturation limit calculated from effective CEC. The high-base-status oxidic soils generally are enriched with Ca-saturated organic matter in the surface layer and are among the more productive upland soils in the tropics. The low-base-status oxidic soils are acidic, have a low effective CEC, and the degree of exchangeable Al saturation often exceeds 60% in the subsoil horizons. Because of their excellent soil physical properties, oxidic soils are more resistant to soil erosion and therefore better suited to large-scale mechanized agriculture than kaolinitic soils. Although the dominant clay mineral is kaolinite, the presence of moderate amounts of crystalline and amorphous Fe and Al oxides and hydrous oxides (around 5% Fe2O3 or higher) with a high specific surface area (100 m2/g or larger) gives rise to many unique chemical and physical properties, such as a variable surface charge, the formation of microaggregates, low bulk density (0.8-1.2 Mg/ m3), stable soil structure, and high permeability. Most oxidic soils are red or dark red due to the presence of clay-size hematite (Fe2O3) in the soil. The yellowish oxidic soils contain primarily goethite (FeOOH) in the clay fraction and occur in the wetter geomorphic positions of a deeply weathered landscape. Extensive areas of clayey, yellowish oxidic soils are found in the Amazon Basin. Gibbsite (A1OOH) is the major crystalline Al oxide.