Ferralsols and other Soils of the Hot Regions - Major Soil Groups of the World - page 171

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).

Transition of Rhodic Ferralsol to Xanthic Ferralsol

Fig. 5.11 Extract from the map titled 'Distribution of red and beige soils on plateau and
interfl uve reliefs of middle Casamance' (Chauvel and Pédro 1978). The rock is sandstone
of the Continental Terminal. The red soils of breaks-in-slope are shown as dotted areas,
the plateau beige soils by horizontal hachuring and the beige soils of slopes by oblique
hachuring. The immature soils on materials from the dismantling of cuirasses are represented
by pebbles, and mangroves and Gleysols are in grey. T he contours outline the general relief
(plateau, slopes, valleys).