Q:

How can poor tropical soils be enriched by biological means?

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. 

A:

There are two main problems that "wear out" soil, and people working on this deal with both of them.
A soil might be low in organic matter and nitrogen, perhaps because all the crops were sold away and the topsoil eroded off. This would also happen on a new sandbank, or where a landslide had revealed a lot of subsoil. Naturally what happens is that the site is colonized by "primary succession" plants that are good at surviving there. These plants photosynthesize carbon compounds, and have bacteriological associates that fix nitrogen out of the atmosphere. The plants and bacteria live and die and leave detritus that slowly turns into richer soil, and somewhat more complex "secondary succession" plants move in. Reclamation agriculture or reforestry helps this along, and agroforestry tries to forestall soil loss by growing trees instead of arable crops. 
But C and N are not all that plants and bacteria are made of -- just about everything else, eg P and K and Mg and Ca, comes from the geological parent material of the soil. When as anions and cations these elements are being traded and fought over by plants and microbes, they are soluble, and over time they will weather out of the soil. Over geological time they weather out of the underlying rock, which is the problem in the oldest parts of the Earth, and then it's difficult for even primary succession plants to grow. 
Hawaii is a beautiful natural laboratory for this, as the islands are made of the same lava but are of very different ages. The classic work on this is by Vitousek; here's an open access paper following up on it: Primary Succession on a Hawaiian Dryland Chronosequence. 
There's a ecology-economy group trying to fix this problem in Haiti. Haiti has exported so much, for historical reasons, that their soil is terrible and they have to import a lot of food although they were recently a very NPP-productive place. SOIL works on designing and introducing composting toilets so that the elements imported as food are incorporated into agricultural soil instead of running off (causing public health and fisheries problems as they go!). It's a very nice virtuous-circle, though it takes a lot of work to keep it going. 

A:

The greatest asset of soils in tropical regions is the organic matter rich 'O-horizon'. Check any reference on soil profiles, they will tell you a lot. Yes, the tropical climate is conducive to excessive rain and leaching of mineral nutrients and salts in upper layers of soil profile that can be observed in a well developed 'E-horizon'. Depending on the local hydrology, not all the nutrients will be lost, just displaced deeper in the soil. Some trees have adapted taproots to access the nutrients deeper in the soil and effectively act as pumps fixing the nutrients to organic matter such as leaves which then fall to the forest floor and deposit the organic form of nutrients ready to be remineralized by detritovores and form a rich surface O-horizon. With that said, an efficient means to trap nutrients in this organic matter cycle is important when dealing with highly weathered mineral soils. Sustaining a heterogeneous forest is important in preserving nutrients and I believe that surface water runoff and erosion can wash away the structure of the nutrient cycling machine. Robust forest structure enhances water infiltration deterring surface water flow and erosion. 

