What is Terra Preta?
“Terra Preta” (‘black earth’ in Portuguese) are fertile anthropogenic (i.e., man-made) soils found in the Amazon basin. They are recognized by their dark colour, owing to the abundance of charcoal, by their association with archaeological artefacts (e.g., ceramic fragments, lithic material, etc.), as well as by their high nutrient content (especially phosphorous and calcium). Terra Preta are also known as Amazonian Dark Earths (ADE), "Terra Preta de Índio" (in Portuguese) or "tierras negras" (in Spanish). They were first described by Orton (1870), and were mentioned by several other Amazon naturalists in the following decades. Their discovery did not have much impact until the last two decades, when the interest for Terra Preta increased exponentially.
These soils are remnants of ancient, pre-Columbian societies and were generally created between 3000 and 500 years BP, with the earliest reports being older than 5000 years BP. It has been estimated that Terra Preta covers between 0.1 up to 3.2 % of the area of the Amazon, but these are still very rough estimations given the scarcity of archaeological information for large parts of the basin. Most known Terra Preta sites are less than 2 ha in size, but much larger sites up to 200 ha have also been identified. Terra Preta patches are usually found along bluffs of major rivers and their tributaries, and are particularly common in central and southern Amazonia.
Being the product of cultural activities in the past, patches of Terra Preta are very heterogeneous, both within and between patches, and accordingly they might also have been formed through different processes. Although the specific practices that originated Terra Preta are still not fully understood, it is relatively consensual that these soils were formed through the addition of charcoal together with other organic waste materials. Possible sources of carbonized organic matter include fires for food and ceramic preparation, charcoal kilns and low-heat burning of debris in homegardens, while nutrient sources could have come from plant residues (e.g., material used to build houses, residues of food production), mammal and fish bones, manure and human excrements (urine and faeces).
The context in which Terra Preta is found and their properties suggest that, at least initially, these soils were created unintentionally, as a by-product of household activities (the ‘kitchen-midden’ model). Whether these soils could have also been created intentionally is still a matter of debate. Some scholars argue that transitional soils with intermediate properties that occur between Terra Preta and surrounding soils (called ‘Terra mulata’) could have been created intentionally, but these claims lack strong scientific evidence. Although the scientific knowledge about Terra Preta has grown exponentially in the last decades, there are still many unanswered questions regarding their origin, distribution and properties.
Glaser, B. and Birk, J. 2012. State of the scientific knowledge on properties and genesis of Anthropogenic Dark Earths in Central Amazonia (terra preta de índio). Geochimica et Cosmochimica Acta 82: 39-51. http://www.sciencedirect.com/science/article/pii/S001670371100144X
Neves et al. 2003. Historical and socio-cultural origins of Amazonian Dark Earths. In J. Lehman (Ed.) et al., Amazonian Dark Earths: Origins, properties, management. Kluwer Academic Publishers, Dordrecht, pp. 29-50. http://link.springer.com/chapter/10.1007%2F1-4020-2597-1_3