Terra preta (literally “black earth” in Portuguese) is a type of very dark, fertile anthropogenic soil 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 índio” or “Terra preta de índio”. Terra mulata is lighter or brownish in color.
Terra preta owes its name to its very high charcoal content, and was indeed made by adding a mixture of charcoal, bone, and manure to the otherwise relatively infertile Amazonian soil over many years.
Terra preta is characterized by the presence of low-temperature charcoal in high concentrations; of high quantities of pottery sherds; of organic matter such as plant residues, animal feces, fish and animal bones and other material; and of nutrients such as nitrogen (N), phosphorus (P), calcium (Ca), zinc (Zn), manganese (Mn).
It also shows high levels of microorganic activities and other specific characteristics within its particular ecosystem. It is less prone to nutrient leaching, which is a major problem in most rain forests.
Terra preta zones are generally surrounded by terra comum, or "common soil"; these are infertile soils, mainly acrisols, but also ferralsols and arenosols.
Terra preta soils are of pre-Columbian nature and were created by humans between 450 BC and AD 950.
Thousands of years after its creation it has been reported to regenerate itself at the rate of 1 centimeters (0.4 in) per year by the local farmers and caboclos in Brazil's Amazonian basin, who seek it for use and for sale as valuable compost.
Terra preta soils are found mainly in Amazonia, estimate that they cover at least 0.1 to 0.3%, or 6,300 to 18,900 square kilometres (2,400 to 7,300 sq mi) of low forested Amazonia; but others estimate this surface at 10.0% or more (twice the area of Great Britain).
Terra preta sites are also known in Ecuador, Peru, Guyana, in Benin, Liberia), and on the South African savannas. Similar soil was found in late Roman Britain.
Biochar is low temperature charcoal produced from a biomass of wood and leafy plant materials.
Amending soil with biochar has been observed to increase the activity of arbuscular mycorrhizal fungi.
Tests of high porosity materials such as zeolite, activated carbon and charcoal show that microbial growth substantially improves with charcoal.
It may be that small pieces of charcoal migrate within the soil, providing a habitat for bacteria that decompose the biomass in the surface ground cover.
This process may have an essential role in Terra preta's self-propagation; a virtuous cycle develops as the fungus spreads from the charcoal, fixing additional carbon, stabilizing the soil with glomalin, and increasing nutrient availability for nearby plants. Many other agents contribute, from earthworms to humans as well as the charring process.
If biochar becomes widely used for soil improvement, it will involve globally significant amounts of carbon sequestration, helping remediate global warming.
Links:
Lew Godfrey
Green Harlow Productions
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