Terra Preta soils are produced when charcoal is produced using the soil. The traditional way of making charcoal from wood involves high-intensity (nearly) anaerobic fires. Wood is piled neatly into rows in a hole dug from the earth. It is set aflame and is covered with other materials to keep it burning including soil. The process does not disintegrate the wood into ash but instead leaves a good amount of biological carbon intact (Professor Lehmann of Cornell University measures the bio-carbon retained at around 50%).

But what does slash-and-char really do? This study on the charcoal technology summarizes what Amazonian inhabitants did.

The study shows that at a set of easily achievable conditions, char that could be made from any plant material could be used to effectively provide safe growing conditions for many microbes. These microbes could develop into colonies that could utilize carbon dioxide from the atmosphere. And these results stand as proof as to how effective Terra Preta soil development could be!