Terra Preta: the Secret of Rich, Dark Soil in the Amazon

An article in the August 9, 2002 issue (Volume 297) of Science magazine (“The Real Dirt on Rainforest Fertility”) describes an exciting discovery in Amazonia in Brazil. Researchers have discovered a rich dark soil that they refer to as “terra preta do Indio” (Indian dark earth), or terra preta for short. Terra preta is very productive soil with long-lasting fertility—unusual in the tropics, where most soils are very weathered, highly acidic, and low in organic matter and essential nutrients.

One estimate suggests that terra preta might cover 10% of Amazonia. Precontact Amerindians seem to have actually improved the soil rather than degrading it. The oldest deposits of terra preta seem to date back 2000 years, and by AD 500 to 1000 they seem to have been present in almost every part of the Amazon basin. Deposits of terra preta are usually 1 to 5 hectares in size, 40 to 60 cm deep. However, plots are up to 300 hectares, and the soil in some places is 2 m deep. The plots are almost always full of broken ceramics. Locals sell the soil as potting soil.

Terra preta has been found to have more organic matter and more bioavailable phosphorus, calcium, sulfur and nitrogen than surrounding soils. The key to its long-term fertility seems to be charcoal; terra preta contains up to 70 times as much charcoal as nearby oxisols. Bruno Glaser, a chemist at the University of Bayreuth in Germany, said, “The charcoal prevents organic matter from being rapidly mineralized [Ed: broken down to basic minerals]. Over time, it partly oxidizes, which keeps providing sites for nutrients to bind to.” Charcoal contains few nutrients, so high nutrient inputs are also needed (e.g. manure and turtle, fish and animal bones). Special soil microorganisms also likely play an important role.

A few researchers have actually tried to make terra preta out of highly weathered soil, using treatments of charcoal and fertilizer. They planted rice and sorghum for three years. After the first year, there was little difference between treatments (but almost nothing grew in the control plots). In the second year, plots treated with charcoal alone grew little, those with fertilizer grew, but those treated with both charcoal and fertilizer yielded up to 880% more than fertilizer alone.

Early inhabitants of the Amazon also seem to have practiced a type of agroforestry. At one archeological site, the wood from 30 species of useful trees was found in addition to annual crops.

One major problem with terra preta plots is that weeds grow very quickly and can overwhelm crops. People who farm on ancient terra preta sites move their fields from time to timefor this reason.