Terra preta

Topic in Gardening courses. By John Eagles.

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Around the Amazon River, there once lived a civilization of what the Spanish thought were primitive people. Due to the contagious diseases that Europeans brought with them, this civilization died out soon after, but they left a treasure for their descendants that's many times more valuable than gold: a fertile soil. The native tribes had learned how to make fertile a soil in a hot climate, which normally is very infertile because it cannot build up humus.

File:Terra Preta.jpg

Right a terra preta soil that was human-made out of an infertile soil as shown left.

The most stable humus is that formed from the slow oxidation of black carbon, after the incorporation of finely powdered charcoal into the topsoil. This process is at the origin of the formation of the fertile Amazonian dark earths or Terra preta do Indio.

Terra preta, or 'black earth' is a very dark, man-made soil found in the Amazon Basin. It has a very high charcoal content and also contains plant residues, bone and manure. It is very stable and remains in the soil for thousands of years. It is rich of plant nutrients (nitrogen, phosphorus, calcium, zinc, manganese). Its fertility may also be thanks to high levels of activities by micro-organisms.

Terra preta is more resistant against nutrient leaching, a problem in rain forests.

Terra preta soils can reach a depth of 6.6 ft or two meters. These soils are said to generate themselves. Some local farmers sell the top layer of their terra preta soil, then let this soil stay under vegetation for 20 years, after which the terra preta layer has grown back with a rate of 1 cm per year.

Processes of terra preta formation

The elements of the processes responsible for the formation of terra preta soils are:

1. Adding wood charcoal
2. Adding organic matter and nutrients for plants
3. Role of micro-organisms and animals in the soil.

Wood charcoal

Wood charcoal is made by pyrolysis, a process that rapidly decomposes organic material through anaerobic heating. Nowadays you often hear the term biochar. Biochar refers to charcoal made from any biomass waste, and may or may not have a significant bio-oil condensate component. In the broader context biochar is simply charcoal which could be used to improve soil quality.

Traditionally wood charcoal was produced in smothered fires at relatively low temperature. Wood charcoal produced in this manner has an internal layer of biological petroleum condensates that is consumed by the bacteria. Charring at high temperature loses that layer and brings little increase in soil fertility. Depending on with which temperature charcoal is produced, these condensed aromatic structures are formed.

Some researchers found that charcoal produced at temperatures of 450 C (842 F) or higher was most likely to improve soil drainage and make more water available to plants. Charcoal produced at lower temperatures could sometimes repel water. This was true for biochar produced from tree leaves, corn stalks and wood chips.

See also

• Clay-humus complex
• Compost
• Gardening courses
• Humus
• Knowledge base:Gardening
• Micro-organisms in the soil
• Soil

External links

• Terra preta Wikipedia