The origin of Terra Preta
Different theories have been proposed for the origin of terra preta. The current consensus is that terra preta was created by humans before the European colonization (between 3000 and 500 years BP). It is believed that terra preta is a by-product of habitation, while “terra mulata” (brownish soils) is the result of active long-term soil management (i.e. through mulching and burning) in permanent agricultural fields (Denevan 1986, Woods et al. 2000, McCann 2001). Spatially terra preta is surrounded by terra mulata, which gradually transitions into the original, infertile terra firme at its margin.
There is large variation regarding terra preta throughout the Amazon basin. To illustrate this variation we can refer to the location of terra preta in the landscape. Terra preta sites usually occur on bluffs near the major rivers (Denevan 1996). These sites can be extensive (up to a few hundred hectares, as in the vicinity of Santarém or Manaus, where they represent settlements), but large terra preta patches have also been reported in upland forests (Smith 1980, Eden et al. 1984, Heckenberger et al. 1999). Most of the terra preta patches found so far are nearby permanent water sources, although some patches near intermittent water courses are also reported (Paz-Rivera & Putz 2009).
But how did Amerindians create these fertile soils? Current evidence suggest that Amerindians incorporated charcoal and wood ash, and as a result soils increased in soil organic matter, nutrient retention, cation exchange capacity, and pH (Woods et al. 2000). The fertility of these soils was reportedly further enhanced with household wastes (e.g., food remains, shells, bones, feces, blood, and urine), and other organic material brought from the surrounding forests (e.g., palm leaves) and wetlands (e.g., aquatic plants). It is worth mentioning that the exact way in which terra preta was created still remains unknown. For a scheme on how terra preta was created see the diagram below:
Schmidt MJ, Rapp Py-Daniel A, de Paula Moraes C, Valle RBM, Caromano CF, Texeira WG, Barbosa CA, Fonseca JA, Magalhães MP, Silva do Carmo Santos D, da Silva e Silva R, Guapindaia VL, Moraes B, Lima HP, Neves EG, Heckenberger MJ (2014) Dark earths and the human built landscape in Amazonia: a widespread pattern of anthrosol formation. J Archaeol Sci 42:152-165. doi:http://dx.doi.org/10.1016/j.jas.2013.11.002