In fields where farmers plow away crop stubble before turning over the ground, erosion occurs relatively quickly. When the stubble is left and turned over with a disk, that rate dramatically drops, according to a new study in this week's online edition of PNAS. You might say, who cares about losing a little dirt? A lot of civilizations probably should have. David Montgomery, an author of the study who also penned Dirt: The Erosion of Civilizations, has linked the demise of civilizations to how long it took them to deplete their soil. We don't have the best history ourselves. We took out prairie plants in the Midwest that had roots 18 feet deep to plant corn: a shallow rooted vegetable that let top soil blow into the wind and run off into the Mississippi. Aside from all the civilization decline talk, there's another good reason for farmer's to keep their earthly stubble: It not only prevents the loss of dirt, more carbon in the earth means less in the air.