Terra preta
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Terra preta (which means "dark soil" in Portuguese), or Terra prêta do índio, refers to expanses of very dark soils found in the Amazon Basin. They are also known as Amazonian dark earths.
Overview
Terra preta is distinguished from "terra comum", or "common soil", which refers to the reddish or yellowish ferralsols that predominate under tropical soils. Terra preta is believed to be the result of past indigenous activities in the forest and is thus considered to be anthropic and/or anthropogenic, possibly from a pre-Columbian civilization. It seems to be comprised of low temperature charcoal, pottery sherds, plant residues, animal feces, fish bones, and perhaps some biological agent like a special ecosystem of healthy microorganisms. Because they are very rich in nutrients they are explored and used as compost by Amazonian local farmers and "caboclos".The origin of the Amazonian dark earths is not entirely clear and several theories have been considered. These soils were originally postulated to result from ashfall from volcanoes in the Andes, since they occur more frequently on the brows of higher terraces. Another theory considered formation as a result of sedimentation in Tertiary lakes or in recent ponds. Because of their elevated charcoal content, it is now widely accepted that these soils are a product of indigenous soil management involving a labor intensive technique termed slash-and-char. The technique is differentiated from slash and burn by a lower temperature burn and in being a tool for soil improvement. Amending soil with low temprature charcoal produced from a mix of wood and leafy biomass (termed bio-char) has been observeded to increase the activity of arbuscular mycorrhizal fungi.
About 10% of the original terra comum appears to have converted to terra preta. Whether all Amazonian dark earth was intentionally created for soil improvement or whether the lightest variants are a by-product of habitation is not clear at present time. This is in part due to the varied features of the dark earths throughout the Amazon Basin.
References
Further reading
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