Some of the following article on why Terra Preta works is speculation. Connecting some dots if you wish so take it with a grain of salt until someone does some definitive research on the subject.
Fairly recently fertile zones along the Amazon river were discovered. These are places which are rich in charred organic material and are fertile zones in an area of very poor soils. The depth of black soil is typically half a metre. In case this sounds strange to you; that the soils of the amazon jungle are poor, perhaps some explanation is in order.
You would think at first glance that the soils of the Amazon must be very rich to support such a large biomass of such rich flora and fauna; a veritable jungle. Apparently not so. If you cut down the jungle or burn a patch of it, you can plant some crops and if you are lucky you will get a couple of crops before you have to move to a new location. If you did the same thing in the forests of Eastern North America as the Europeans did when they arrived, you could plant crop after crop in the rich deep dark soil before you had to start to fertilize. So what is the explanation for the incredible quantity and richness of flora and fauna in the jungle. The explanation is partially in the ability of the trees and plants to recycle all the nutrients that fall on the soil. Animals and plants die, Animals defecate and urinate on the forest floor and with the high rainfall, humidity and temperature. all this material is mineralized (changed back into phosphates, nitrates and all the other ates) and is taken up by the roots of the growing flora. So why isn't there an accumulation of rich dark soil as there is in temperate zones. The answer apparently is in the relative temperatures of the two areas.
When the temperature of the soil is above 25degrees centigrade, in the presence of moisture, humus breaks down. Humus is the refractory material that is left in temperate soil when organic material breaks down. The humus is the part that doesn't break down. It is physically sticky and helps in the formation of the crumb structure (peds) of soil which allows paths for aeration and water penetration. It holds large quantities of water which plants can draw upon. Of great interest, it chealates (binds loosely) a variety of plant nutrients. If there is a source of nutrients coming from, for instance the breakdown of plants or animals, the humus will hold these nutrients in the upper layers of the soil and keep them from being washed into the subsoil. Humus is a little like the haemoglobin in our red blood cells. Haemoglobin can hold a lot of Oxygen but not very strongly. In the lungs where the Oxygen concentration is high, it absorbs oxygen and in the body where the oxygen partial pressure is low, it releases it. Humus does the same with water and nutrients. below 25 degrees, humus is very stable.
So now we come to Terra preta and why it works. Terra preta has been formed by generations of humans charring organic material and incorporating it into the soil. Along the Amazon, where these soils exist, the black layer is often about half a metre deep. Any of you who have done organic chemistry know how charcoal is used to remove odours and colours from liquids. Charcoal is very good at absorbing molecules on to its surface and releasing them. This is apparently the explanation for why all this char makes the soil so rich. It is not that it has much in the way of nutrients itself but it can hold nutrients just as humus does. If the farmers along the Amazon, for instance, net a bunch of fish and dig them into their terra preta, they will break down and the nutrients will be held by the soil instead of being leached out by the rain. And charcoal is very stable at high temperatures unlike humus. One wonders how they arrived at the idea. Perhaps they observed good growth of their yams or whatever in a place where there had been a fire that was put out by the rain. It could be that char also has some of the other properties of humus such as water retention or improvement of soil structure. Some careful work is necessary to tease out the finer details of how charcoal works in warm soils. In a way, char (charcoal) is the humus of the tropics.