Terra preta

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Terra preta (which means "dark soil" in Portuguese), refers to expanses of very dark soils found in the Amazon Basin. It is also known as "Amazonian dark earth" or "Indian black earth". In Portuguese its full name is Terra preta do indio or Terra preta de indio.

Terra preta is distinguished from terra comum, or "common soil", which refers to the reddish or yellowish ferralsols that predominate under tropical soils. The soil 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.

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[edit] Composition of terra preta

Terra preta consists of low temperature charcoal, pottery shards, plant residues, animal faeces, fish bones, and perhaps some biological agent like a special ecosystem of healthy microorganisms. As it is very rich in nutrients to this day local farmers and caboclos in Amazonian basin seek it out for use as compost.

[edit] Origin of terra preta

For a long time, the origins of the Amazonian dark earths were not immediately clear and several theories were considered. One idea was that they resulted 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.

However, because of their elevated charcoal content and the common presence of pottery remains, 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 temperature charcoal produced from a mix of wood and leafy biomass (termed biochar) has been observed to increase the activity of arbuscular mycorrhizal fungi. It is theorized that terra preta self-propagates via this mechanism; a virtuous cycle established as the fungus spreads from the charcoal, fixing additional carbon and stabilizing the soil with glomalin, and increasing nutrient availability for nearby plants.

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. Thus suggesting the existence of an extensive ancient native civilization dating back 500 to 2500 years bp.

[edit] Product of an advanced pre-Columbian civilization?

The Spanish explorer Francisco de Orellana, the 16th C explorer who was the first European to transverse the Amazon River, reported densely populated regions running 100's of kilometers along the river, suggesting population levels exceeding even those of today. While it is possible Orellana may have exaggerated the level of development among the Amazonians, their semi-nomadic descendants have the odd distinction among primitive societies of a hereditary, yet landless, aristocracy, a historical anomaly for a society without a sedentary, agrarian culture. This suggests they were once more civilized and agrarian but after the demographic collapse of the 16th and 17th century due to European introduced diseases they reverted to less complex modes of existence but maintained certain traditions. Moreover, many indigenous people were forced to adapt to a more mobile lifestyle in order to protect themselves against colonialism. This might have made the benefits of terra preta, such as its self-renewing capacity, less attractive — farmers would not have been able to enjoy the use of renewed soil because they would have been forced to move for safety. Slash-and-burn might have been an adaptation to these conditions.

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