ABSTRACT
Nitisols (earlier Nitosols) are found in very dry climates. They are less mature than Ferralsols and do not exhibit thick mottled altérites at depth, so characteristic of Ferralsols. They are very red and form on basic rocks (volcanic rocks, limestone). In the field, we often see intergrades between Ferralsols and Nitisols and, sometimes, sequences mixing the two types. Nitisols are very abundant in East Africa, in Central America, in the Caribbean islands and in part of Asia. They contain a small amount of 2/1 clay minerals. Alisols also have 2/1 clay minerals, but are strongly desaturated. Aluminium freed by the acidity can be trapped in the interlayer position (2/1/1 clay mineral). When they are red, they can be grouped with the degraded red Mediterranean soils of France. Lixisols have 1/1 clay minerals, but a higher base saturation. They can be grouped with soils earlier classified as Tropical Ferruginous soils in the French classification. Acrisols contain 1/1 clay minerals, often large quantities of gibbsite and low base saturation. They are acid and depleted of clay, at least in the surface layer. They are close to the Ultisols of USA and correspond to the ‘Yellow Latosols’ of the Brazilian classification. Thus they represent a very mature extreme of tropical soils. They are found in high-rainfall environments (more than 2500 mm y-1), are widespread in South America, Amazonia and Colombia for example (Gaviria 1993), and are also seen in Southeast Asia. They do not have a cuirass, but it could have
existed, because ferruginous and gibbsitic nodules are often present, and seem to be slowly disappearing by dissolution (Boulet et al. 1993). In the soil mass, iron is chiefly in the form of goethite. Sometimes, in their middle horizons, the soils are more or less orange or light red (Yellow-Red Latosols). Haematite and goethite are present together.
