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Wim Sombroek learned about soil as a child, during the hongerwinter—the Dutch wartime famine of 1944-45, in which 20,000 or more people died. His family survived on the harvest from a minute plot of plaggen soil: land enriched by generations of careful fertilization. If his ancestors hadn't taken care of their land, he once told me, the whole family might have died.

In the 1950s, early in his career as a soil scientist, Sombroek journeyed to Amazonia. To his amazement, he found pockets of rich, fertile soil. Every Ecology 101 student knows that Amazonian rain forest soils are fragile and impoverished. If farmers cut down the canopy of trees overhead to clear cropland, they expose the earth to the pummeling rain and sun, which quickly wash away its small store of minerals and nutrients and bake what remains into something resembling brick—a "wet desert," as these ruined areas are sometimes called. The certainty of wrecking the land, environmentalists argue, makes large-scale agriculture impossible in the tropics. Nevertheless, scattered along the Amazon River, Sombroek discovered big patches of (black Indian earth). As lush and dark as the plaggen of his childhood, it formed a rich base for agriculture in a land where it was not supposed to exist. Naturally, Sombroek paid attention. His 1966 book, Amazon Soils, included the first sustained study of terra preta.

Later Sombroek worked across the globe, eventually becoming director of ISRIC and secretary general of the International Society of Soil Science (now International Union of Soil Sciences), positions he used to convene the first ever world survey of human-induced soil degradation. All the while he never forgot the strange black earth in Brazil. Most restoration programs, like those in China and the Sahel, try to restore degraded soil to its previous condition. But in much of the tropics, its natural state is marginal—one reason so many tropical countries are poor. Sombroek came to believe that terra preta might show scientists how to make land richer than it ever had been, and thus help the world's most impoverished nations feed themselves.

Sombroek will never see his dream fulfilled—he died in 2003. But he helped to assemble a multinational research collaboration to investigate the origin and function of terra preta. Among its members is Eduardo Göes Neves, a University of São Paulo archaeologist whom I visited not long ago at a papaya plantation about a thousand miles up the Amazon, across the river from the city of Manaus. Beneath the trees was the unmistakable spoor of archaeological investigation: precisely squared off trenches, some of them seven feet deep. In the pits the terra preta, blacker than the blackest coffee, extended from the surface down as much as six feet. Top to bottom, the soil was filled with broken pre-Columbian pottery. It was as if the river's first inhabitants had thrown a huge, rowdy frat party, smashing every plate in sight, then buried the evidence.

Terra preta is found only where people lived, which means that it is an artificial, human-made soil, dating from before the arrival of Europeans. Neves and his colleagues have been trying to find out how the Amazon's peoples made it, and why. The soil is rich in vital minerals such as phosphorus, calcium, zinc, and manganese, which are scarce in most tropical soils. But its most striking ingredient is charcoal—vast quantities of it, the source of terra preta's color. Neves isn't sure whether Indians had stirred the charcoal into the soil deliberately, if they had done it accidentally while disposing of household trash, or even if the terra preta created by charcoal initially had been used for farming. Ultimately, though, it became a resource that could sustain entire settlements; indeed, Neves said, a thousand years ago two Indian groups may have gone to war over control of this terra preta.

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