Terra preta

From eagle-rock.org
Topic in Gardening courses. By John Eagles.
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 crop.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.[1]

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.[2]

Processes of terra preta formation

The elements of the processes responsible for the formation of terra preta soils are:[2]

  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.[3] Nowadays you often hear the term biochar.[4] 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 (according to this Wikipedia article).[2] 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.[5]

Woody charcoal has a layer of bio-oil condensates that microbes consume and is equal to glucose in its effect on microbial growth. High temperature char loses this layer and consequently may not promote soil fertility as well.[6]

What are higher and lower temperatures with respect to producing biochar? It's unclear to me at which temperatures biochar loses its biological oil condensates (which are apparently necessary to promote the growth of microbes), but 120 C is the lowest temperature at which wood will char, and pyrolysis of char coal seldom occurs at temperatures above 600 C.[6]

Organic matter and nutrients

Charcoal has a porous structure and can retain organic molecules, water and dissolved nutrients. It can also bind pesticides.

Because terra preta has a great quantity of charcoal it contains on average three times more organic matter than the soil it was originally made of. It can contain up to 150 g organic matter per kg soil.

Terra preta soils also have higher quantities of nutrients and a better retention of these nutrients than the surrounding infertile soils. More details can be found here.[7]

Micro-organisms and animals

Many micro-organisms, bacteria and fungi, live within the pores of charcoal. For example, there is evidence that the fungus Aspergillus niger produces black carbon especially under moist tropical conditions.[2]

Earthworms ingest pieces of charcoal and mix them with the mineral soil.[2]

It seems that some ents are repelled from fresh terra preta.[2]

See also


External links