Soil Building in the Americas

Soil in the Amazonian rain forests is notably infertile. The continual rain leaches nutrients out of the soil, carrying the runoff downstream to the ocean. A notable exception is the fertile soil—called terra preta—found all over the Amazon basin in large patches near ancient population centers. Farmers till these soils today, producing crops without fertilizers.

With Amazon rain forests being cleared, evidence is emerging that leads archeologists to think that large civilizations have managed the rain forest for the last 11,000 years. In 1542 Spanish explorer Francisco de Ornella traveled the length of the Amazon River, and reported large, sophisticated civilizations inhabiting the land all along the way. Later explorers found the river barely populated except for hunters and gathers living in primitive conditions, which cast doubt on the accuracy of Ornella’s reports. Today archaeologists are considering that the Amazon area may have supported as many as 5-6 million people in the 1500s. European diseases may have wiped out these large population centers leaving the area barely populated. They are also challenging the idea of a "pristine rain forest."

Nutrient poor soil (left) and rich terra preta soil (right) - photo by Bruno Glaser (Creative Commons) 

How could such a large population be supported on such poor soil? Fascinating to me is the discovery of black soil, or terra preta near population centers. Thought to be anthropogenic, the soil is dark and oily, and composed of organic matter, small bones, unfired pottery chards, and ash and charcoal from earth ovens (called biochar). The result is fertile soil that does not break down, and does not require modern fertilizers. The Kuikuro Tribe people, living in the upper Xingu region in Brazil, create this earth today. They toss organic waste behind their homes, and in three years the soil is very fertile. Scientists are also interested in the soil because biochar takes carbon out of the atmosphere, and puts it to work improving soil nutrition. More interesting ideas to consider from American gardening history.