Terra Preta (black earth soils) are a byproduct of swidden (slash and burn, juhm) horticulture. Juhm farming or slash and burn is a type of indegenous farming used to farm tropical forest soils which tend to be thin and nonfertile. The indigenous practice was to clear plots by burning and then planting certain crops amongst the burned stumps -- so this was incorporating the fertility of burned wood in different stages -- including charcoal into the soil. The result is a soil more suitable for farming, but also an ecological situation where fields in various stages of regrowth were maintained by the land owners. So there was not only farm land, but also other fields in various stages of being returned to a forested condition. And variability in stages of regrowth also provided forage for animals -- thus creating a much more complex ecology than would have existed without the intervention of the human farmers.
It was a way of making a not so productive environment very productive and sustainable --- if the amount of land required was available for the human population.
Slash and burn horticulture has gotten a bad rap -- mostly because outsiders have viewed its failure under extreme population pressure and attempts to continue the traditional practice on restricted acerage.
The charcoal contained in terra preta soils of South America are just one remnant of a highly productive slash and burn horticure economy.
Here is one description of the practice:
http://www.eoearth.org/article...
