Vegetation:Tropical rain forest   Savannas   Sahel Zone  Deserts

The tropical rain forest suffers  from nutrient scarcity - despite sumptuous growth and rapid decomposition. How come?
In the Tropics the precipitation and temperatures have been high for millions of years. Therefore, the soils have undergone more rapid decomposition of the minerals (acceleration of the chemical processes by higher temperature) and seepage of nutrients (by high precipitation). The soils are therefore predominantly poor in nutrients.
Nutrients, which are used for plant growth, are to a large extent contained in the existing plant mass of the rain forest. By the high temperatures, growth as well as decomposition processes are accelerated. If plant parts die (a branch breaks, leaves fall off, roots rot), these nutrients are rapidly taken up by other plants.  This cycle keeps the nutrients "home" in the rain forest and "shields" them from being washed away.
Problems arise, if this cycle is interrupted. This may occur through human intervention, by clearing the land for wood or food production, or mark territorial claims in the rain forest. More harmful than the withdrawal of individual plants is the destruction of the ground cover: the missing " Filter" , which takes up nutrients rapidly through their roots rapidly again. Until these " Filter" plant regenerate, nutrients may are already be washed away by the plentiful rain into lakes, rivers and into the sea - unreachable for the plants.
This happens possibly through clearing. Wood is a valuable good and therfore the export of tropical woods may be a profitable business. The nutrients contained in the trees are now missing in the rain forest.
The second, still larger large problem is the slashing and burning.The rising number of inhabitants of tropical countries seek land in the rain forest to grow their food. The surfaces are partly cleared by fire. Since the soils contain so few nutrients, and since nutrients are more rapidly washed away when plant cover is thin, the soils become useless for farming after few years. Then, the inhabitants move further. In order to feed themselves, they clear new surfaces in the forests.
The burning down of the rain forest causes the loss of an important part of the nutrients, which were present in the plants, as well as the ability to protect the freed nutrients with the roots from rapid washing -away. Thus, a rapid loss of the soil fertility is causedm, which makes the soils almost infertile after few decades of such management. On such soils, plants find it increasingly hard to grow. A gradual formation of savannahs or even deserts may occur.

Example: Manaus - the capital of the state of Amazonas