Black Gold of the Amazon

Fertile, charred soil created by pre-Columbian peoples sustained surprisingly large settlements in the rain forest. Secrets of that ancient "dark earth" could help solve the Amazon's ecological problems today.

04.30.2007
by Michael Tennesen

A pot from the pre-Columbian Guarita culture.(Courtesy of Fernando Chaves)

But around 1200, the Guarita people from the east threatened to attack, and the Pared�o built defensive structures. This phase lasted into the time of European occupation. Over the same period, the Pared�o vanished. The Guarita apparently moved in from areas near the mouth of the lower Amazon, which have even more terra preta soils than the Pared�o, and brought with them wilder, multicolored styles of pottery.

"They are like the barbarians attacking the Romans," Neves speculates. He suspects that the newcomers may also have had a valuable possession-corn. This new, more nutritious staple requires better soils, and it is not unreasonable to suspect that the Guarita drove the Pared�o out to take over their valuable cropland, built on terra preta.

Analysis of buried human remains suggests that the inhabitants of all four occupations were robust-a well-being that extended even after death. In the site's remains, Anne Rapp and her husband, Claide Moraes, both students at the University of S�o Paulo, find evidence that hints at ceremonial procedures, priests, and perhaps a cottage industry of funerary artisans as well.

Although the central Amazon may not have been the heart of the massive empire that Donald Lathrap envisioned, these cultural traces suggest that the Amazonians managed to flourish in a formidable environment-and terra preta may have been an important component of that success.

As thrilling as this evidence is to archaeologists, it may also have very practical importance as a modern weapon against some of our most urgent ecological problems. Soil scientist Johannes Lehmann of Cornell University believes that the mysterious dark earth holds clues to creating sustainable farming practices and even to combating global warming.

Lehmann explains that nutrients from plant and animal remains-like nitrogen, phosphate, and potassium-bind to charcoal or biochar, drastically reducing how much is washed away by the constant rains. It is a gradual process that begins with the charcoal breaking down in the soil over time. 

Tiny pores in the charcoal, along with changes in its chemistry, provide more surfaces for nutrients to adhere to, which in turn encourages microorganisms to colonize the soil. "With a handful of biochar you can keep many more nutrients in the soil than with a handful of mulch or compost. It is like mopping up nutrients with a magnet that looks like a sponge-that is, it has high surface area like a sponge but can attract a thin layer of material like a magnet," Lehmann says.

Although the complete transformation of soil ingredients into true terra preta may take several years, soil scientists have shown that the mixture can have immediate benefits when added to nutrient-poor soils. Experiments outside of Manaus have shown that the yield in plots treated with charcoal and fertilizer (whichs contains plant nutrients), a mix similar in composition to terra preta, was double the yield of plots treated with fertilizer alone.

The dark tera preta earth (background) contrasted with the pale, nutrient-poor soil that is common in
the Amazon basin.

The mysterious dark earth holds clues to creating sustainable farming practices and even to combating global warming

In 2001 Petersen published a paper reporting one example of a farmer who had cultivated crops on terra preta soils near A�utuba for 40 years without ever adding any fertilizer. "That's incredible," Woods says. "We don't get that in Iowa." A few years of Amazonian rains will wash away the nutrient-laden ash from land that was cleared by slash-and-burn techniques, but the charcoal in the terra preta soils persists. The terra preta soils at the Central Amazon Project goes back in many places as much as 2,500 years.

Creating new terra preta in the Amazon today would have several advantages, Lehmann says. First, because the enriched soil remains fertile for a long time, its use would discourage farmers from moving on and burning more forest to open up new fields. Second, because of the added charcoal, terra preta holds up to 10 times as much carbon as unaltered soils. The late Wim Sombroek-a legendary soil scientist whose long interest in terra preta earned him the epithet "the godfather of dark earth"-began to wonder if dark earth could be used to sequester carbon. Lehmann's studies have shown that it can: Fifty percent of the original carbon in plants and trees used to make biochar remains in the terra preta soils after the conversion.