ABSTRACT
Transition of Rhodic Ferralsol to Xanthic Ferralsol ƒ In West Africa, the red soils evolve into ‘beige’ soils (Chauvel et al. 1977; Chauvel and Pédro 1978). The transformation is characterized by yellowing, collapse of micronodular structure, diminution of porosity and a corresponding compaction of the soil layers among the vertical axis. The iron is separated from the clay and crystallizes as nodules. This evolution is noticed on the northern fringe of the intertropical zone, in situations where the soil is subjected to alternate wet and dry seasons. In the rainy season the iron is solubilized. In the period of extreme dryness, ultradesiccation takes place (Chaussidon and Pédro 1979), characterized by a reduction in the thickness of the water film that surrounds the clay particles. This film concentrates the H+-ions and has a low pH, as low as 1.5-2.0, values at which iron is solubilized and mobilized (Chap. 3, Fig. 3.7). Topography is also involved. Beige soils occur where the drainage is poor, at least in the rainy season: interfluves and slope bottoms. On the contrary, red soils are preserved in the better drained breaks-in-slope (Fig. 5.11).
