The so-called black earth, a mysterious soil in the Amazon that is much more fertile than expected for the region, was probably produced intentionally by indigenous people long before the arrival of Europeans, a new study indicates.
Combining archaeological data and analysis of the current habits of the Kuikuros, a group native to the Upper Xingu, a team that includes Brazilian and foreign scientists has assembled the most complete portrait yet of the terra preta “recipe”. The data has just been published in the academic journal Science Advances.
At least in Xingu territory, everything indicates that the art of enriching the soil in this way has not been lost in recent centuries.
The remains of organic matter produced by activities such as fishing and cassava cultivation, as well as charcoal and ashes from fires, are systematically discarded in certain parts of the villages and give rise to modern versions of terra preta, which, later, they are used to grow crops with an above-average nutrient demand.
Coordinated by Morgan Schmidt and Taylor Perron, from MIT (Massachusetts Institute of Technology, in the USA), the study also included the participation of some of the main Brazilian experts who are reconstructing the Amazon past, such as archaeologist Eduardo Neves, from USP, and the agronomist Wenceslau Teixeira, from Embrapa Solos.
The work is also signed by Yamalui Kuikuro and six other representatives of the Xingu indigenous population, something that has become common in research carried out in the region.
In addition to being much darker than the normally reddish soils in most of the Brazilian Amazon, terra preta is usually much richer in organic matter and several important nutrients for plant development. It is possible to find it along the entire Amazon channel and its main tributaries, from Ilha do Marajó to Acre (and also further west, outside the Brazilian Amazon).
There is a strong association between the presence of black earth patches and archaeological sites, with ages ranging from several millennia before Christ to close to the time of the European invasion. It is relatively common for soil to be sold for use as fertilizer in gardens and vegetable gardens.
The large discrepancy between the terra preta and the surrounding ground always suggested an unnatural origin, but doubts persisted about the possible intentionality behind its origin. One possibility is that it was just a byproduct of “waste management” in past indigenous settlements, appearing thanks to the accumulation of food scraps in certain disposal sites.
Other researchers, however, saw it as one of the key factors behind Amazonian agricultural productivity in the past, helping to explain how pre-Cabraline villages came to be five or ten times larger than today, housing thousands of inhabitants and being the stage for of monumental structures, such as large roads, moats and walls.
The Alto Having lost a large part of its population since the 18th century, it was never abandoned by indigenous groups.
This raised the possibility that the cultural continuity between the pre-Cabral occupation and the current inhabitants helped to unravel the origins of the soil.
That’s what the researchers did, analyzing samples from four archaeological sites in the region (generally aged between 1,000 years and 300 years) and villages abandoned in more recent times, as well as in current Kuikuro settlements. For comparison, archaeological sites on the Tapajós River and the Carajás region were also included in the analysis.
They found, firstly, that the patches of black earth tended to be concentrated in the central areas of the old settlements, forming mounds where organic matter was most likely discarded in the past. In Xingu, these mounds were also present alongside old roads.
The soils analyzed were much less acidic than most Amazonian soils (which is good news for cultivation), with the expected high organic matter content and a content at least ten times higher (compared to the surrounding soil). of nutrients such as phosphorus, potassium and magnesium. And the amount of terra preta was high, reaching 4,500 of this type of soil in one of the archaeological sites.
The similarity in chemical composition and location of terra preta in archaeological soils and those of current villages indicated a similar formation process, and this was what interviews with dozens of current residents of the indigenous territory revealed.
They give the black earth the name “eegepe” and associate it with their ancestors, but also with their agricultural work today.
“We sweep up the coal and ash, gather it all up, and then dump it where we’re going to plant it, so it turns into a beautiful ‘eegepe.’ That’s where we can plant sweet potatoes. When you plant it where there’s no ‘eegepe,’ the soil is weak”, declared one of the study’s informants.
“Black earth appears in disposal areas and in mounds, but it can also be more widespread,” Morgan Schmidt, the study’s first author, told Sheet.
“We observed the kuikuros directly cultivating the waste areas and also spreading ash with charcoal and many organic remains, especially derived from cassava processing, over areas of the ground to fertilize the soil. I have never observed them transporting black earth, but it is something that can happen and could have happened in the past, to some extent.”