Credited to Brazil. Dark earths are best known from the Brazilian Amazon, where the name comes from, but they also occur in other South American countries. Researchers still debate whether the soils were made on purpose or built up as a by-product of settlement.
How it began
Terra preta de indio is the local name for dark earths in the Brazilian Amazon. Cornell soil scientists write that they were most likely created by pre-Columbian peoples between 500 and 2,500 years ago and abandoned after the arrival of Europeans. Another account dates their formation to between 450 BCE and 950 CE. The soils are found at sites across the Amazon Basin and, according to the Cornell group, in several countries of South America.
Before it existed
Ordinary soils in the Amazon are poor. Once the forest is cleared, arable land is productive for only a short time before its nutrients are used up or leached away by rain and flooding. Farmers then have to move and clear a new plot with fire. On such soils fallow periods usually last 8 to 10 years, while on dark earths fallows can be as short as six months.
How it works
The soil is black because of its weathered charcoal. It was made by adding a mixture of charcoal, bones, broken pottery, compost and manure to the low fertility soil. The charcoal is stable and stays in the ground for thousands of years, binding minerals and nutrients. Dark earths hold up to 150 grams of organic carbon per kilogram, against 20 to 30 grams in the soils around them, along with high levels of phosphorus and a greater capacity to hold nutrients.
Who pushed back
The human origin of the soils was not accepted at first. Reports of dark, fertile earth reached science in 1879 and 1885. One researcher suggested in 1941 that the soils formed on volcanic ash from the Andes. Others proposed sediment from ancient lakes or recent ponds. The idea that they are a product of Indigenous soil management was put forward in 1949 and confirmed by later surveys. Whether they were made deliberately is still argued.
Where it stands today
A 2023 study in Science Advances, written with members of the Kuikuro community of the Upper Xingu, found that present-day Kuikuro soil management closely mirrors that of their ancestors and concluded that dark earth was created intentionally. Terra preta research also led to work on biochar, charcoal added to soil to hold nutrients and store carbon. One site at Acutuba was reported to have been farmed for over 40 years with no fertilizer.
