ABSTRACT

We shall begin the review of the great soil groups with Ferralsols because they are the oldest and most mature soils of the world. They correspond to the intertropical zone. They owe their exceptional depth to three factors: (i) high rainfall and temperature that accelerate rock weathering; (ii) feeble surface erosion because of their topographic position on stable, nearly horizontal shields under forest vegetation which slows down removal of surface soil; (iii) absence of glaciations that, in higher latitudes, have scraped off old soils. The mechanisms of formation of Ferralsols will clarify the concept of the weathering-front model. They will also help understand the great pedological diversity that passage of time results in! Ferralsols cover nearly 750 million hectares in the world, mainly in Amazonia, Central Africa and Oceania.