In rain forests as their name states, rain is a constant factor, so precipitation erodes quite easy soil, this ecosystem is characterized by very poor soils, so how is possible that very huge trees grow there and other plants. This is due to the roots, most of plants have a wide root complex that actually retains soil, also the nutrients are stored in roots and stems, that is why there are massive clearings for crops or cattle the soil degrades quickly, as the nutrients where, in the plants an there is not radicular complex strong enough that retain it.