Terra Preta, a Solution Buried in the Dirt

With so many nutrients caught up in the life cycles of lush forest plants, most soil in the Amazon Basin is nutrient-poor clay. But throughout the region, pockets of fertile, dark soil (“terra preta” in Portuguese) can be found.

Terra preta is the product of slash-and-char agriculture practiced centuries ago by indigenous farmers who baked wood to charcoal and worked it into the soil. Centuries later, the soil they created remains the region’s most fertile farmland: productivity is higher, far less fertilizer and fallow time are required, and soils support a larger variety of more nutritious crops.

But the real reason that terra preta is big news today is that these dark soils represent the possibility of natural, long-term carbon sequestration, a way to return some of the carbon once stored in fossil fuels back to the ground.

Carbon in charcoal does not reoxidize to become CO2, as does the carbon stored in plants when they decompose. Nearly half of the carbon content of biomass that is charred and returned to the soil can remain stable for hundreds and even thousands of years—keeping carbon out of the atmosphere far longer than other, more cyclical biological sinks like no-till farming or planting trees.

The challenge with agrichar (as modern bio-charcoal is called) is to avoid the negative side effects that plague many biofuels. Producing biomass for agrichar doesn’t have to come at the cost of deforestation or increased industrial farming. Instead, the millions of tons of agricultural waste discarded every year—plus municipal and yard waste, manure, and even, potentially, sewage—can be used, much of their carbon sequestered for centuries rather than returning quickly to the atmosphere as CO2.

In addition to reducing the amount of carbon circulating into the atmosphere, agrichar also creates some handy byproducts: bio-oil and gases that can be used to generate heat and electricity.

Brooke Jarvis wrote this article as part of Stop Global Warming Cold, the Spring 2008 issue of YES! Magazine. Brooke is a YES! editorial assistant. Photo of Brooke Jarvis
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