Organic carbon, meaning carbon-based molecules that have their origin in anything that was once living, makes topsoil black or brown in color. It is commonly known that fertile soils are black, and perhaps less commonly known that this fertility arises from the presence of organic carbon. In tropical and subtropical regions of the world, organic carbon does not tend to accumulate in soils, and so they are generally white, yellow or red in color. The reason is that because of the heat and / or humidity, organic matter decomposes rapidly and directly to CO2, and hence little to no organic carbon is present in the soil, only the sand, silt, clay, stones and minerals.
In more temperate regions of the world, organic matter decomposes slowly enough that it remains on and within the top layer of soil as it breaks down into smaller and smaller molecules. Some of this organic carbon decomposes fully to CO2, while some of these so called humic molecules bind to minerals and become relatively stable, enduring for hundreds to thousands of years.
Terra Preta becomes all the more remarkable when you understand that whether it was created intentionally or not, the Amazonian Indians have shown us a way to rapidly create and regenerate fertile soils.