Great article in my current Discover magazine on ecological research in the Amazon.

Normal low-income farming techniques today involve "slash and burn" of rain forests. Unfortunately, the actual soil of the rain forest sucks--the forest itself is the repository of most of the nutrients. End result: dead forest and a farm plot that craps out in a few years.

Researchers discovered ancient Indian farm sites where the soil is practically Iowa dirt. Black, rich, full of nutrients, bugs, and all the good stuff.

This is found in places where ancient natives had long-term settlements. One theory has it that instead of quick-burning forests, they cleared land with primitive tools, and slow-burning fires that created more charcoal than ash.

This charcoal remnant (biochar) holds nutrients far better than ash, and helps create good soil. It also ties up carbon better, too.

It isn't known whether or not Indians learned how to do this deliberately to enrich the soil, but it's certainly possible. It opens up a new method of farming in the area that would allow farmers to stay on one patch for many years, negating the need to clear new land.

Unfortunately, the article doesn't seem to be online yet, but you can find a lot by searching under "Indian Black Earth" or "Terra Preta."