Hazelnut, thanks for that link. I remember the slash and burn farming from anthropology studies. However many proponents of an anthropomorphic origin of Terra preta suggest charcoal was brought in to the gardens, others that vegetation was brought in and burned on site. Another suggestion was that the charcoal was produced by the deliberate slow burning of vegetation under soil. With low oxygen you get more charcoal. And some say that they went to the extent of making activated charcoal which was then added. Because the practice ended after the European invasion of the areas no one actually knows for sure. The thing with slash and burn was that it always had a time limit with soil fertility. Terra preta is "self renewing". And is also quite deep, more so than what slash and burn would produce.
There's a system in India still current where they use similar methods and add organic household rubbish. Then each year the smash all their clay cooking pots and add them to the garden. They never use their pots more than one year.
When I was in Papua New Guinea I saw a lot gardens out in the remote areas that were at different stages, from new to abandoned. Food was still gathered from the abandoned ones as some plants persisted for quite a while. But you could see the jungle moving back in on them. Even though abandoned, the soil in those gardens looked a thousand fold better than the soil at my place. Would have loved to have swapped some of theirs for mine.
A garden in PNG recently cleared and planted up with Taro.
An abandoned garden being taken over by Bananas and Choko before the forest moves in.
