Was watching a NatGeo show on ancient civilization in the Amazon and found out something interesting. Seems the soil there is generally very acidic and nutrient poor and our scientists didn't think it was possible to maintain a large population because they could not have fed themselves. That is except we now know that the ancient peoples modified their soil to a state we now calle Terra Preta. The mixed lots of charcoal from low temperature burns of wood into the soil. The Terra Preta holds nutrients extremely well and supports a healthy microbial population. The microbial population is able to rebuild the terra preta at at rate of 1 cm per year by converting dead plant matter. The show stated that the area converted to Terra Preta in Brazil is roughly the size of France.
http://en.wikipedia.org/wiki/Terra_preta
Terra Preta
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