LOW-LEVEL slash-and-burn farming doesn’t harm rainforest. On the contrary, it
helps farmers and improves forest soils. This is the heretical view of a German
soil scientist who has shown that burnt clearings in the Amazon, dating back
more than a thousand years, helped create patches of rich, fertile soil that
farmers still benefit from today.
Most rainforest soils are thin and poor because they lack minerals and
because the heat and heavy rainfall destroy most organic matter in the soils
within four years of it reaching the forest floor. This means topsoil contains
few of the ingredients needed for long-term successful farming.
But Bruno Glaser of the Institute of Soil Science and Soil Geography at the
University of Bayreuth has studied unexpected patches of fertile soils in the
central Amazon. These soils contain lots of organic matter.
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Glaser has shown for the first time that most of this fertile organic matter
comes from “black carbon”—the detritus from smouldering camp fires and
charred wood and charcoal left over from thousands of years of slash-and-burn
farming. “The soils, known as Terra Preta, contained up to 70 times more black
carbon than the surrounding soils,” says Glaser.
Unburnt vegetation rots quickly, but black carbon persists in the soil for
many centuries. Radiocarbon dating shows that the charred wood in Terra Preta
soils is typically more than a thousand years old.
“Slash-and-burn farming can be good for soils provided it doesn’t completely
burn all the vegetation, and leaves behind charred wood,” says Glaser. “It can
be better than manure.” Burning the forest just once can leave behind enough
black carbon to keep the soils fertile for thousands of years. And rainforests
easily regrow after small-scale clearing.
Contrary to the conventional view that human activities damage the
environment, Glaser says: “Black carbon [combined with] human excrement and
residues from hunting and fishing are responsible for the richness of Terra
Preta soils.”
Terra Preta soils turn up in patches of a few hectares all over the Amazon,
where they are highly prized by farmers. All the patches fall within 500 square
kilometres in the central Amazon, between the rivers Tapajos and Curua-Una.
Glaser says the widespread presence of pottery confirms the soil’s human
origins.
The findings add weight to the theory that large areas of the Amazon have
recovered so well from past periods of agricultural use that the regrowth has
been mistaken by generations of biologists for “virgin” forest.
During the past decade, researchers such as Anna Roosevelt of the Chicago
Field Museum of Natural History have discovered hundreds of large earth works
deep in the jungle. They are up to 20 metres high and cover up to a square
kilometre. She claims that these mounds, built between AD 400 and 1400, were at
the heart of urban civilisations. Now it seems the fecundity of the Terra Preta
soils may explain how such civilisations managed to feed themselves.