Yves Lucas

On the genesis of the soil mantle of the region of Manaus, Central Amazonia, Brazil

The dynamics of the forest to the north of Manaus is tightly linked to that of the soil. The latosol that covers the plateau, which supports a dense forest, consists from top to bottom of: (a) a brown, clayey organic horizon (0.3 m), (b) a yellow horizon, very rich in clay but permeable (from 0.3 to 4 m), (c) a nodular horizon rich in Al and Fe oxides (from 4 to 9 m), and (d) a horizon which still preserves the sedimentary structures of the parent sandstone, where quartz is intensely dissolved and kaolinite crystallizes in pores. In perfectly flat areas, the clay of the organic horizon is destroyed by acidocomplexolysis, and the dissolved Al is transported vertically by the drainage water. A part of this Al is used to make the gibbsite nodules of horizon (c), and the rest is used to make kaolinite in horizon (d). Because aluminum is thus conserved within any vertical prism, the rate of destruction of horizon (a) is equivalent to the rate of advance of the kaolinization zone into the sediment: the latosol is said to be in equilibrium, the surface remains perfectly flat as it slowly sinks, the quantity of kaolinite increases with time, and the silica released by quartz dissolution in the whole profile is exported by drainage water to the water table. In contrast, near drainage axes, however small initially, the drainage becomes inclined toward the axis. Part of the Al released by acidocomplexolysis of horizon (a) is now exported to rivers, and Al is no longer conserved within any given prism. The rate of advance of the kaolinization zone (d) into the sediment now becomes less than the rate of destruction of horizon (a) and the surface sinks faster than that of the surrounding plateau. After this differential ‘podzolization’ has gone on long enough, it creates a network of ‘geochemical valleys’ characterized by convex slopes and bounded by sandy soils (campinas). The vegetation becomes sparser and sparser. At the end, only some bushes and lichens survive on the white sand.

Geochemical Processes in Tropical Landscapes: Role of the Soil Covers

The relationship between morphogenesis and pedogenesis has concerned geomorphologists and pedologists for a long time. As suggested for example by Tricart and Michel (1965), soil science relates to geomorphology as geomorphology does to structural geology. Geomorphological evolution constitutes a framework to soil formation and evolution. Soils rarely develop directly in situ at the expense of a parent rock. Much more frequent is the formation of soils within regoliths, alterites, slope debris, alluviums, colluviums or aeolian accumulations, etc.