National Geographic : 2008 Sep
Some believe that global warming can be slowed significantly by using carbon to reengineer bad soils. 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 num ber and variety of microorganisms than typical tropical soils-it was literally more alive. A black revolution might even help combat global warming. Agriculture accounts for more than one-eighth of humankind's production of greenhouse gases. Heavily plowed soil releases carbon dioxide as it exposes once buried organic matter. Sombroek argued that creating terra pre ta around the world would use so much carbon rich charcoal that it could more than offset the release of soil carbon into the atmosphere. According to William I. Woods, a geographer and soil scientist at the University of Kansas, charcoal-rich terra preta has 10 or 20 times more carbon than typical tropical soils, and the carbon can be buried much deeper down. Rough calculations show that "the amount of carbon we can put into the soil is stagger ing;' Woods says. Last year Cornell University soil scientist Johannes Lehmann estimated in Nature that simply converting residues from commercial forestry, fallow farm fields, and annual crops to charcoal could compensate for about a third of U.S. fossil-fuel emissions. Indeed, Lehmann and two colleagues have argued that humankind's use of fossil fuels worldwide could be wholly offset by storing carbon in terra preta nova. Such hopes will not be easy to fulfill. Identi fying the organisms associated with terra preta will be difficult. And nobody knows for sure how much carbon can be stored in soil-some studies suggest there may be a finite limit. But Woods believes that the odds of a payoff are good. "The world is going to hear a lot more about terra preta," he says. WALKING THE ROADS ON THE FARM hosting Wisconsin Farm Technology Days, it was easy for me to figure out what had worried Jethro Tull. Not Jethro Tull the 1970s rock band-Jethro Tull the agricultural reformer of the 18th century. Under my feet the prairie soil had been squashed by tractors and harvesters into a peculiar surface 106 NATIONAL GEOGRAPHIC * SEPTEMBER 2008 that felt like the poured-rubber flooring used around swimming pools. It was a modern version of a phenomenon noted by Tull: When farmers always plow in the same path, the ground becomes "trodden as hard as the High way by the Cattle that draw the Harrows." Tull knew the solution: Don't keep plowing in the same path. In fact, farmers are increasingly not using plows at all-a system called no-till farming. But their other machines continue to grow in size and weight. In Europe, soil compac tion is thought to affect almost 130,000 square miles of farmland, and one expert suggests that the reduced harvests from compaction cost midwestern farmers in the U.S. $100 million in lost revenue every year. The ultimate reason that compaction contin ues to afflict rich nations is the same reason that other forms of soil degradation afflict poor ones: Political and economic institutions are not set up to pay attention to soils. The Chinese officials who are rewarded for getting trees planted with out concern about their survival are little different from the farmers in the Midwest who continue to use huge harvesters because they can't afford the labor to run several smaller machines. Next to the compacted road on the Wisconsin farm was a demonstration of horse-drawn plow ing. The earth curling up from the moldboard was dark, moist, refulgent-perfect midwestern topsoil. Photographer Jim Richardson got on his belly to capture it. He asked me to hunker down and hold a light. Soon we drew a small, puzzled crowd. Someone explained that we were looking at the soil. "What are they doing that for?" one woman asked loudly. In her voice I could hear the thought: MEGO. When I told this story over the phone to David Montgomery, the University of Washington geologist, I could almost hear him shaking his head. "With eight billion people, we're going to have to start getting interested in soil," he said. "We're simply not going to be able to keep treat ing it like dirt." t Dig In Learn how stones, no-till farming, and composting toilets can protect soil at ngm.com.