Terra preta soil, or dark earth, has a high carbon concentration. It is found in the Amazon basin, suggesting that pre-Columbian societies were among the first civilizations to add charcoal to soils to improve their fertility.  From analysis of supremely fertile ancient Amazonian soils, scientists have discovered the reason for their enduring fertility to be a very high organic carbon content. This is the result of leaving smoldered crop residue in place and tilling it back into the soil, much the same as when biochar is added to soil.
The following link gives an overview of Terra Preta. More information can be found on the pull down menu page on the biochar tab entitled “Links/Additional Informationâ€.
http://en.wikipedia.org/wiki/Terra_preta
