I saw a documentary on The Science Channel last week, that wondered if a Spanish explorer really did see a large city in the Amazon Basin. When the researcher found were ‘islands’ of high ground, and causeways connecting them. I don’t remember the number, but the population the evidence suggested seemed to be way too large to support. The programme mentioned ‘slash-and-burn’, and noted what Colibri noted above. How could a large population be supported?
Charcoal was found in the soil. While ash – and its nutrients – is easily washed away by the heavy rains, charcoal seems to persist. Apparently they ancient farmers may have turned their crop waste and jungle wood into charcoal and used that as a nutrient.
Another thing the programme showed was ‘black earth’. Valuable stuff, which is found around the ancient settlement sites. It’s ‘mined’ by the truckload. One farmer said that he sells 60cm of the ‘black earth’ from a plot, and then lets it sit while a similar amount is harvested from the next plot. IIRC, the narrator said that after 20 years the mined site will have regained the 60cm that was removed. Jungle litter (leaves, grasses, etc.) gather and decompose, resulting in very fertile soil. Natural compost.