I have been reading a lot recently about agriculture in Africa, and one of the big impediments in a lot of places is the poor quality of the soil. Primarily this refers to a lack of usable molecular nutrients in the soil that plants need (nitrogen, phosphorous, and potassium, primarily).
I understand there are a few different methods of soil enrichment; however, these processes are geological in nature. All of the high-population density subsistence agricultural parts of the wet-and-dry tropics are directly correlated to soils that are enriched by one or more of these mechanisms.
The first mechanism is direct deposition of nutrients by volcanism from the last hundred million years or so. This is the mechanism responsible for high population densities around the East African rift, Indonesia, Ethiopia, southern India and the Phillipines. The next mechanism is flood plain deposition of eroded materials from somewhere upstream. This is responsible for fertile, high density regions like north India, Egypt, Nigeria, and most of Southeast Asia. These areas have high population densities (of subsistence farmers) other ares in the tropics have very low densities (northern Australia, most of the Sahel, most of southern Africa).
In the temperate regions, however, there is a biological means for creating fertile soils. The plains of the American Midwest or Ukraine have had neither recent volcanism nor extensive floodplains, yet are covered in rich, black chernozem soils. Evidently, these soils are produced by the deposition and incomplete decay (due to hard winters) of grasses.
Is there a biological process that can create rich agricultural soils in either the wet-and-dry tropics?
EDIT - Despite the introduction, the purpose of this post is not about agriculture, or what we could do today to to rejuvenate poor soils. This is about natural processes that would take place on timescales of tens or hundreds of thousands of years, or longer.