Throughout the tropics, and especially in the Amazon, soil is remarkably fragile. Countless millennia of torrential rains and desiccating sun have left most tropical soils poor to the point of sterility. As Wade Davis describes in One River, "the tropical rainforest, though home to tens of thousands of species, is in a sense a counterfeit paradise, a castle of immense biological sophistication built quite literally on a foundation of sand." Some scientists estimate that in the Amazon, well over eighty percent of all nutrients are bound up in the living bodies of plants, animals, and fungi.
The forest itself is a closed circuit for the extraordinarily efficient recycling of any and all available supplies. In The Burning Season, Andrew Revkin notes, "The Amazon forests must be self-sufficient, because the ancient soils beneath most of the basin have long ago been leached clean of minerals and nutrients. Only 4 percent of the Amazon has what agronomists consider fertile soils." Destroy the forest, as in slash-and-burn farming, and the exposed ground quickly loses any remaining semblance of fertility. Enriched by the ash of burnt organic matter, a typical farm can support a year or two of annual crops like corn or rice before the plot must be abandoned and new forests cleared; the vicious circle continues.
In the second half of the twentieth century, scientists rediscovered terra preta (simply, "black earth"), also known as Amazonian Dark Earth, an anomalously rich soil with a deep black color. It was soon suggested that terra preta is an anthrosol, or human-made soil, a hypothesis that has been confirmed with further research. Certain groups of Amazonian Indians, likely living in organized communities that were more structured and stratified than previously thought, created this fertile soil through the incorporation of organic matter and, importantly, charcoal.
Whereas slash-and-burn farming provides a quick fix of nutrient-laden ash that rapidly leaches from the exposed ground, the charcoal in terra preta acts in much the same way as activated charcoal in water filters: it tenaciously clings to particles and nutrients, preventing them from being pummeled out of the ground by rain and sun. Though containing few available nutrients of its own, the charcoal's surface grabs onto minerals and nitrogen-based compounds and makes these available to plants. The result is a phenomenally stable soil; the oldest plots of terra preta discovered date to two thousand years ago and continue to be productive.
Without a doubt, terra preta represents a potential paradigm shift in notoriously fragile tropical agriculture. Interest in the use of "biochar" (organic matter carbonized into charcoal through a low-oxygen burn) as a soil amendment has sprouted up everywhere from Brazil to Cameroon to India. The stability of charcoal in the ground has also created interest in terra preta as a significant carbon sink. Recently, terra preta has attracted the attention of mainstream media, thanks in part to the research and efforts of Charles Mann, National Geographic contributor and author of the popular science bestseller 1491. For further information, please follow the links below to articles in The Financial Times (http://www.ft.com/cms/s/2/67843ec0-020b-11de-8199-000077b07658.html) and