On a warm September day, farmers from all over the state gather around the enormous machines. Combines, balers, rippers, cultivators, diskers, tractors of every variety—all can be found at the annual Wisconsin Farm Technology Days show. But the stars of the show are the great harvesters, looming over the crowd. They have names like hot rods—the Claas Jaguar 970, the Krone BiG X 1000—and are painted with colors bright as fireworks. The machines weigh 15 tons apiece and have tires tall as a tall man. When I visited Wisconsin Farm Technology Days last year, John Deere was letting visitors test its 8530 tractor, an electromechanical marvel so sophisticated that I had no idea how to operate it. Not to worry: The tractor drove itself, navigating by satellite. I sat high and happy in the air-conditioned bridge, while beneath my feet vast wheels rolled over the earth.

The farmers grin as they watch the machines thunder through the cornfields. In the long run, though, they may be destroying their livelihoods. Midwestern topsoil, some of the finest cropland in the world, is made up of loose, heterogeneous clumps with plenty of air pockets between them. Big, heavy machines like the harvesters mash wet soil into an undifferentiated, nigh impenetrable slab—a process called compaction. Roots can't penetrate compacted ground; water can't drain into the earth and instead runs off, causing erosion. And because compaction can occur deep in the ground, it can take decades to reverse. Farm-equipment companies, aware of the problem, put huge tires on their machines to spread out the impact. And farmers are using satellite navigation to confine vehicles to specific paths, leaving the rest of the soil untouched. Nonetheless, this kind of compaction remains a serious issue—at least in nations where farmers can afford $400,000 harvesters.

Unfortunately, compaction is just one, relatively small piece in a mosaic of interrelated problems afflicting soils all over the planet. In the developing world, far more arable land is being lost to human-induced erosion and desertification, directly affecting the lives of 250 million people. In the first—and still the most comprehensive—study of global soil misuse, scientists at the International Soil Reference and Information Centre (ISRIC) in the Netherlands estimated in 1991 that humankind has degraded more than 7.5 million square miles of land. Our species, in other words, is rapidly trashing an area the size of the United States and Canada combined.