Learning From Ancient Gardeners

In the upland areas of the Amazon basin, the soil is acidic and highly weathered, and the constant rain washes nutrients out of the upper soil layers. In the forests, the trees have deep roots to collect nutrients. But in areas used for agriculture, the topsoil cannot retain the soluble nutrients needed to grow corn, beans, or other crops.

However, some 2000 years ago, Amazonian farmers developed a way of building exceptionally rich soils called Terra Preta do Indio, that are remarkably good at retaining phosphorus, potassium, calcium, zinc, and copper, and have been found to be better than surrounding soils, even those with added phosphate fertilizers.

Throughout the Amazon region, large islands of this rich soil can be found, surrounded by the natural nutrient-poor soils of the region, called oxisols.

The secret is charcoal.

Charcoal forms when wood is burned slowly in low oxygen environments, such as when it is buried. The charcoal does not break down as easily as unburned organic matter, and can remain in the soil for centuries. Charcoal is used in charcoal filters because it is very good at trapping many chemicals, such as the phosphorus and potassium that plants need to grow. That same quality helps charcoal keep nutrients in the topsoil from leaching out during heavy rainfall.

The soil also contains calcium and phosphorus from fish bones, and has a lot of organic matter in it, making it an excellent soil for earthworms and termites, which turn the soil and work the nutrients deep into the earth.

The benefits of storing large amounts of carbon in the soil extend beyond making the soils fabulously more productive. By sequestering carbon in stable soils for centuries, we can reduce the amount of carbon dioxide in the atmosphere, and reduce global warming.