Visit our website for workshops and many free resources for growing food organically at http://www.livingwebfarms.org
Soil scientist Jon Nilsson is joined by Pat Battle for a hands on workshop at the Mills River Educational Farm teaching about the soil food web and ways to increase fertility from the rich beginnings of well made compost and other methods. The foundational principles soil fertility are expertly explained with a plethora of resources available on the topic. See the Dr. Schonbeck cover crop series for Fertility Systems II.
With regards to the Amazonian middens, my theory is that the charcoal in these densely populated tropical areas was originally added to reduce odors (just as it is today). It was then discovered, over the course of time, that old midden sites in former village locations (which were abandoned and reoccupied on a rotation to keep game abundant and close at hand), were extremely fertile and populated with useful plants. Once the benefits of this addition was learned, the land in productive agriculture increased dramatically fueling an increase in population, specialization and culture culminating in the fantastic agricultural communities that the Spaniards originally witnessed.