The current issue of National Geographic has dirt all over its cover, and rightly so: The fate of the earth's soil is a major story, and an unjustly neglected one at that. After all, just look at Haiti, which has been razing its forests for centuries to make way for coffee and indigo plantations. Most of the country's topsoil has now been eroded away—in many places right down to the bedrock—and local agriculture has collapsed, a major reason why the country was hit especially hard by the recent spike in global food prices.

It's a common story the world over—since 1991, some 7.5 million square miles of arable land, an area the size of the United States and Canada, has suffered some degree of soil degradation due to misuse, and it's not an exaggeration to say the world's running out of good dirt. Charles Mann does a fantastic job of telling that tale in this piece, visiting various parts of the world, from China to the Sahel, that are trying to reverse longstanding erosion trends and nurture the local soil back to health, with varying degrees of success. Especially useful was his treatment of terra preta, a technique recently discovered in the Amazon that could, potentially, be used to rejuvenate degraded soil:

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….

Sombroek had wondered if modern farmers might create their own terra preta—terra preta nova, as he dubbed it. Much as the green revolution dramatically improved the developing world's crops, terra preta could unleash what the scientific journal Nature has called a "black revolution" across the broad arc of impoverished soil from Southeast Asia to Africa.

Key to terra preta is charcoal, made by burning plants and refuse at low temperatures. In March a research team led by Christoph Steiner, then of the University of Bayreuth, reported that simply adding crumbled charcoal and condensed smoke to typically bad tropical soils caused an "exponential increase" in the microbial population—kick-starting the underground ecosystem that is critical to fertility. Tropical soils quickly lose microbial richness when converted to agriculture. Charcoal seems to provide habitat for microbes—making a kind of artificial soil within the soil—partly because nutrients bind to the charcoal rather than being washed away. Tests by a U.S.-Brazilian team in 2006 found that terra preta had a far greater number and variety of microorganisms than typical tropical soils—it was literally more alive.

Better yet, terra preta might offer an easy way to sequester lots and lots of carbon. Agriculture, after all, accounts for about one-eighth of manmade greenhouse gases—especially as the soil gets plowed and carbon from inorganic material churns up into the air—but using charcoal to create terra preta could end up putting more carbon back into the ground than is released. (Some scientists have gone so far as to argue, probably optimistically, that nearly all of our fossil-fuel use could be offset by storing carbon in terra preta.) Still, that's getting a bit ahead of ourselves: As Mann notes, we still don't know have a great handle on creating terra preta, and no one knows for sure how much carbon can actually be stored in the soil.

--Bradford Plumer