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Terra Preta

Amazonian Dark Earth is a combination of bio-char, compost and manures. The main property of terra preta is it's high fertility which, while much higher than the surrounding soils, is bonded to the finely ground charcoal (< 50 micrometers) and not readily available to the plants. This bonding process is what makes terra preta so stable over time. Rather than being leeched away as would normally happen in most soils, nutrients are held by the terra preta until they are made available to plants by a community of microorganisms.

Characteristics of Amazonian Dark Earths as compared to surrounding soils include: higher pH, coarser soil texture, higher SOM (soil organic matter) and altered SOM. The higher pH seems to regulate the availability of phosphorous (P). Coarser soil texture allows rapid water percolation, while the high SOM aids in the retention of nutrients, and thus, the low amount of leeching. It's interesting to note that when inorganic fertilizers are added to Dark Earths, leeching is quite high due to the rapid percolation. Despite the rapid percolation of water, the coarse texture and high SOM actually increase available water content of the soil. The overall effect is that Terra Preta does not become waterlogged during heavy rainfall, yet is able to provide adequate moisture to plants during times of drought.

Surveys of various Terra Preta soils have shown that different types of Dark Earth were apparently manufactured for different purposes, presumably to meet the specific nutrient needs of certain crops. This variability was not random, as the sites showed distinct spacial patterns. Many of the sites also contain pottery artifacts, while others do not. The addition of pottery to certain plots also seems to be intentional, although it's purpose seems to be uncertain.

I have not found any information in the scientific literature to support the idea that Terra Preta is able to regenerate itself after a period of time as was reported by local Amazonians. Nor was there support for the idea that nutrients were replenished by rainwater. In fact, it was reported that Dark Earth soils do become depleted over time with continued harvesting, but the rate of depletion is much slower than conventional soils due to the slower nutrient leeching.

The percentage of bio-char in Terra Preta varies from 20-40% and comes in two types: black charcoal and brown. The two types are apparently the result of producing the charcoal at relatively low temps. The brown charcoal is much higher in plant resins and these are thought to be used by the microorganism community in binding nutrients.