Terra Preta

The technique of using charcoal to improve the fertility of soils originated in the Amazon basin at least 2500 years ago. The native Indians of the region would create charcoal, mix it with organic matter and broken pottery, and incorporate it in small plots of land from 1 - 80 hectares in size. Terra Preta, as it is known in this area of Brazil, remains highly fertile until today, even with little or no application of fertilizers. And this is in a region of the world known for its highly infertile tropical soils.1

Local inhabitants have always known Terra Preta to be uniquely fertile, but it was largely unknown worldwide until a scientist named Wim Sombroek began to research it intensively. It was from Wim's passion for Terra Preta soils that interest in creating Terra Preta Nuova arose, which eventually was called biochar. In addition to their high char content as mentioned above, Terra Preta soils are characterized by high phosphorus content reaching 200-400 mg P/kg, and higher cation exchange capacity, pH and base saturation than surrounding soils.

Terra preta Wim Sombroek
Wim Sombroek explaining Terra Preta characteristics in Manaus, Brazil.

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.

Terra preta soil profile
Cross sectional profiles 1 meter deep comparing Terra Preta on the left, with nearby Oxisol on the right of the type that is normally found in the Amazon basin. [Glaser, B., Haumaier, L., Guggenberger, G., and Zech, W. (2001). The Terra Preta phenomenon: a model for sustainable agriculture in the humid tropics. Naturwissenschaften 88, 37-41.]

Like most aspects of our natural world that we absolutely depend on to survive, we take fertile soils rich in organic carbon for granted. We call it dirt. We don't want it on our clothes or hands or anything else we own or touch, in our homes, on our shoes, in our cars. We have at least some respect for oxygen and water, but very little for dirt.

Soil organic carbon is the foundation of wealth and well being in our world. Without it, what is left is only the minerals; sand, clay and rocks. Generally speaking, the regions of the world that have inherited a geological legacy of fertile soils rich in organic carbon have developed economies and stable societies. Those regions that lack organic carbon in their soils tend to be plagued with poverty, malnourishment, instable societies and governments, and a lack of development, education and health care.

Modern agricultural techniques, including tilling and abundant use of chemical fertilizers, pesticides and herbicides, have caused most of the soil organic carbon in developed regions of the world to decompose to CO2. One third of the excess carbon dioxide in the atmosphere is from our soils.

Goat head
Regions of the world devoid of organic soil carbon have very limited food chains, and little opportunity for the few human inhabitants they can support to develop beyond occupations such as goat herding.