The soil is primarily tropical ans subtropical terra rosa (red earth). Amazônia, the valley of the Amazon and its tributaries, is a vast alluvial plain in which flooding continually washes away and replenishes topsoil. A number of low alluvial plateaus, however, can be found above normal floor levels. Some inland regions of the northeast are semiarid. In lowland areas, the soil supports dense rain forests. The state of São Paulo is marked by fertile, almost purple, terra rosa, because of basalt decomposition accelerated by heat and humidity.
