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 earths", Terra preta do indio, Terra preta de indio, and "Indian black earth".

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 shards, plant residues, animal faeces, 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.

[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. The Portuguese explorer Francisco de Orellana in the 1500s reported cities running 25 kilometers along the river, and native traders who mocked the sailors' attempts to con them. While Orellana may have exaggerated the level of development among 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. Due to various diseases, a huge amount of the Latin American population died after European arrival; this is a possible way such knowledge of land cultivation was lost. 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 -- they would not have been able to enjoy the renewed soil because they would have been forced to move for safety. Slash-and-burn might have been an adaptation to these harsh colonial conditions.