How Rainforests Work
The very conditions of warmth and moisture that are so favorable to the profusion of life in these forests are also responsible for rapid rates of decomposition. In contrast to cold or dry climates where decomposition is dramatically slow in comparison because of lack of warmth or moisture, in rainforests decomposition sets in almost the moment a leaf falls to the forest floor.
In a stark contrast to temperate forests, there is little accumulation of leaf litter as a consequence of this rapid decomposition. In fact right below the thin leaf litter is a vast network of tiny roots actively taking up the nutrients as decay releases them. That is why rainforests generally have the vast majority of critical nutrients in the living biomass (the living mass of plants and animals) itself. A tall grass prairie in contrast has enormous amounts of nutrients stored several feet deep in the soil. So rainforests present a paradox: the greatest expression of life on earth often occurs on very poor soils. The main exceptions are soils of volcanic origin and those that get annual deposits of silt on floodplains.
Indigenous forest dwellers for millennia have known how to practice agriculture despite the poor soils. It involves making a small clearing in the forest. When the felled trees are dry enough they are burned to release the nutrients that had been in the living mass, so the ashes act as fertilizer. (In some places indigenous peoples systematically enriched forest soils by composting). This can support agriculture for three years or so and then the forest agriculturalists move on and repeat the process elsewhere, while the previous area recovers drawing on the nutrients that were washed away by heavy rains but only as far as the neighboring forest. This works well as long as populations are small. Modern industrialized agriculture in contrast clears large areas that are slow to recover.