Question

When a tropical rain forest is cleared why does the soil usually become useless for growing crops after only a few years?

Answer 1

There are a couple of reasons, but the main reason is that soils in rainforests are typically low in minerals and nutrients to begin with.

Explanation:

Rainforest soil is typically poor in nutrients and minerals because they are stored in the plants. When plants die, they decay very quickly and their nutrients are picked up again by the living vegetation. Nutrient cycling occurs rapidly in rainforests, leading to poor soils.

Removing the rainforest also leads to soil erosion. When it rains, the roots of vegetation are no longer present to hold together the soil. The result is more soil is washed off into rivers, streams, and so forth. Thus, whatever limited nutrients remained in the soil may quickly wash away due to runoff.