1491_ New Revelations of the Americas Before Columbus - Charles C. Mann [176]
Because careful surveys of Amazon soils have never been taken, nobody knows the amount and distribution of terra preta. Woods has guessed that terra preta might represent as much as 10 percent of the Amazon basin, an area the size of France. A recent, much more conservative estimate is that it covers .1 to .3 percent of the basin, a few thousand square miles. The big difference between these numbers matters less than one might expect: a few thousand square miles of farmland was enough to feed the millions in the Maya heartland.
Most big terra preta sites are on low bluffs at the edge of the floodplain. Typically, they cover five to fifteen acres, but some encompass seven hundred or more. The layer of black soil is generally one to two feet deep but can reach more than six feet. According to a recent study led by Dirse Kern, of the Museu Goeldi in Belém, terra preta is “not associated with a particular parent soil type or environmental condition,” suggesting that it was not produced by natural processes. Another clue to its human origin is the broken ceramics with which it is usually mixed. “They practiced agriculture here for centuries,” Glaser told me. “But instead of destroying the soil, they improved it, and that is something we don’t know how to do today” in tropical soils.
As a rule, terra preta has more “plant-available” phosphorus, calcium, sulfur, and nitrogen than is common in the rain forest; it also has much more organic matter, better retains moisture and nutrients, and is not rapidly exhausted by agricultural use when managed well. The key to terra preta’s long-term fertility, Glaser says, is charcoal: terra preta contains up to sixty-four times more of it than surrounding red earth. Organic matter “sticks” to charcoal, rather than being washed away or attaching to other, nonavailable compounds. “Over time, it partly oxidizes, which keeps providing sites for nutrients to bind to.” But simply mixing charcoal into the ground is not enough to create terra preta. Because charcoal contains few nutrients, Glaser argued, “high-nutrient inputs—excrement and waste such as turtle, fish, and animal bones—are necessary.” Special soil microorganisms are also likely to play a role in its persistent fertility, in the view of Janice Thies, a soil ecologist who is part of a Cornell University team studying terra preta. “There are indications that microbial biomass is higher in terra preta than in other forest soils,” she told me, which raises the possibility that scientists might be able to create a “package” of charcoal, nutrients, and microfauna that could be used to transform bad tropical soil into terra preta.
Despite the charcoal, terra preta is not a by-product of slash-and-burn agriculture. To begin with, slash-and-burn simply does not produce enough charcoal to make terra preta—the carbon mostly goes into the air in the form of carbon dioxide. Instead, Indians apparently made terra preta by a process that Christoph Steiner, a University of Bayreuth soil scientist, has dubbed “slash-and-char.” Instead of completely burning organic matter to ash, ancient farmers burned it incompletely to make charcoal, then stirred the charcoal into the soil. In addition to its benefits to the soil, slash-and-char releases much less carbon into the air than slash-and-burn, which has large potential implications for climate change. Trees store vast amounts of carbon in their trunks, branches, and leaves. When they die or people cut them down, the carbon is usually released into the atmosphere, driving global warming. Experiments by Makoto Ogawa of the Kansai Environmental Engineering Center, near Kyoto, Japan, demonstrated that charcoal retains its carbon in the soil for up