Text : Marie Le Breton
Even a hostile green environment can harbor little corners of paradise! The veterans of the War of Secession who settled with their families on the banks of the Amazon in the region of Santarém, Brazil, had the good fortune to discover a veritable treasure under the guidance of local farmers: terra preta do indio, or Indian dark earth. Living off the land in this part of the world is a challenge. Once the land has been cleared, the poor-quality soil of the Amazon is scorched by the sun, and equatorial rains quickly leach nutrients out of the soil. The only exception is terra preta: its yield is up to 880 times higher than that of a standard Amazonian soil! According to various estimates, patches of terra preta may be found in 0.1 to 1% of the area of the Amazonian rainforest. This rare resource owes its black color to its high carbon content coupled with intense microbial activity, which together account for the enduring fertility of the soil. The carbon is a vestige of the cooking fires of once densely-populated Indian villages (the indigenous population of the Amazon once numbered in the millions) dating to 450 BC. These low-intensity smoldering fires are very different from the fires used to clear land. Low-intensity fires and incomplete combustion produce charcoal, whereas slash-and-burn practices generate ash and carbon dioxide. Terra preta has recycled the charcoal, becoming a carbon sink at the same time. And what if this same soil could save the sky? Scientists all over the world are investigating terra preta as a potential weapon for combating the Greenhouse Effect, and a pilot bio-refinery is being developed by the University of Georgia and the US company, Eprida. Designed for small-scale operation, the unit uses heat to convert agricultural residues into vegetable oils (used to make biodiesel fuels) and hydrogen, a possible vector of tomorrow’s clean energy. The pyrolysis process used also produces plant char, which in turn can be used as a fertilizer. The same sort of recycling is observed in terra preta, a resource that scientists would like to replicate as a model of sustainable agriculture for other continents as well.