In the 1990s, Amazonian researchers began taking soil samples in Amazonia. While most of the soil in the Amazon River basin is notoriously bad, there were patches of black earth which they called terra preta do Indio (Indian dark earth).
Local farmers respect, even revere, this black soil which they have farmed for years. Digging for samples, the scientists believe that pottery shards found in the soil proves that farmers have been continuously raising crops, on the same plots of land, for a thousand years.
The rich soil varies in depth from one foot to six feet, and plot size varies from five acres to seven hundred or more. And because its presence is not linked to any one type of ecosystem or base soil, researchers have become convinced that it is man-made. The presence of broken pottery just clinches their case.
What is this black soil composed of? It has been found to have high levels of plant-available nutrients. These nutrients include elements found in most any fertilizer or manure: phosphorus, calcium, sulfur, and nitrogen. Its properties include a high level of organic matter, good retention of moisture and nutrients, and long-term fertility. Terra preta also has a high level of microorganisms.
It also has another substance not found in our commercial fertilizers. That substance is charcoal. Charcoal by itself has few nutrients, but what it does is hold the organic matter tightly to its surfaces.
Researchers believe that terra preta is not just a byproduct of slash and burn agriculture. Most of the carbon released in slash and burn goes up into the air, and is lost to agriculture. It is believed that the ancient Indians charred the trees they wished to remove from a plot of land; that is, they burned them incompletely to make charcoal. They then worked the charcoal into the soil.
The University of Beyreuth participated in a test run at creating terra preta on rice and sorghum plots outside Manaus, Brazil. First-year results were marginal, although almost nothing grew on the control plots. But by the second year, the charcoal plots were significantly better than conventional plots. Plots treated with a combination of charcoal and fertilizer yielded nearly nine times as much as plots treated with fertilizer alone, or an improvement of 880 percent.
The Kayapo Indians in central Amazonia still burn low grade fires today. They fuel the fires with pulled-up weeds, cooking waste, crop stubble, palm fronds, and even termite mounds. They have much to teach modern agricultural experts about how to feed the world’s starving billions on modern degraded soils.