Or rather, the world is running out of topsoil, the nutrient-rich soil in which we grow most of our food.
A rough calculation of current rates of soil degradation suggests we have about 60 years of topsoil left. Some 40% of soil used for agriculture around the world is classed as either degraded or seriously degraded -- the latter means that 70% of the topsoil, the layer allowing plants to grow, is gone. Because of various farming methods that strip the soil of carbon and make it less robust as well as weaker in nutrients, soil is being lost at between 10 and 40 times the rate at which it can be naturally replenished. Even the well-maintained farming land in Europe, which may look idyllic, is being lost at unsustainable rates.
We're running out of dirt. So says geologist David R. Montgomery in Charles Mann's article about the perils affecting the world's soil, including soil compaction in industrialized nations, drought in Africa, and erosion in China.1 Not that progress isn't being made. Some of the farm land in Burkina Faso has been recovered by local farming techniques.
He assembled the farmers in his area, and by 1981 they were experimenting together with techniques to restore the soil, some of them traditions that Ouédraogo had heard about in school. One of them was cordons pierreux: long lines of stones, each perhaps the size of a big fist. Snagged by the cordon, rains washing over crusty Sahelian soil pause long enough to percolate. Suspended silt falls to the bottom, along with seeds that sprout in this slightly richer environment. The line of stones becomes a line of plants that slows the water further. More seeds sprout at the upstream edge. Grasses are replaced by shrubs and trees, which enrich the soil with falling leaves. In a few years a simple line of rocks can restore an entire field.
For a time Ouédraogo worked with a farmer named Yacouba Sawadogo. Innovative and independent-minded, he wanted to stay on his farm with his three wives and 31 children. "From my grandfather's grandfather's grandfather, we were always here," he says. Sawadogo, too, laid cordons pierreux across his fields. But during the dry season he also hacked thousands of foot-deep holes in his fields-za"i, as they are called, a technique he had heard about from his parents. Sawadogo salted each pit with manure, which attracted termites. The termites digested the organic matter, making its nutrients more readily available to plants. Equally important, the insects dug channels in the soil. When the rains came, water trickled through the termite holes into the ground. In each hole Sawadogo planted trees. "Without trees, no soil," he says. The trees thrived in the looser, wetter soil in each zai. Stone by stone, hole by hole, Sawadogo turned 50 acres of wasteland into the biggest private forest for hundreds of miles.
Sawadogo's method turned out to be a little too successful. Burkina's government allows cities to annex nearby land and Sawadogo's forest was recently snatched up by a nearby town. Don't forget to check out the accompanying photos...this is National Geographic after all.
 Mann is the author of 1491, one of the most interesting books I've read in the past few years. ↩