Wetland rice cultivation on the volcanic slopes of Java and in parts of Sumatra traditionally depended on this source of nutrients. Volcanoes become extinct eventually, however, and then natural fertility is no longer replenished - though leaching, laterisation and so on continue. So in the end volcanic soils can become just as poor as most other soils in the humid tropics. There are, of course, differences in the geology beneath rainforests. These variations create many different, complicated soil types.
Soil type is second only to climate in determining the nature of the forest.
Of special interest are the soils of the flooded forest in Amazonia, those under campina and heath forest, and the limestone-based soils of Asia. Poor soils affect the forest's system for processing nutrients. For its nutrient requirements, the forest depends almost entirely on a small amount of recycled nutrients. Decomposing organic debris is the major source of plant nutrients. If one examines a fallen tree or the decaying leaf mass on top of the mineral soil, one often finds a dense network of fine roots.