It's hard to find a bare spot on the more than 800 acres farmed by Cory Atkins of Seaford, Del. Even in the dead of winter, a carpet of ankle-high ryegrass blankets the soil where he plans to grow soybeans in the spring. In other fields, wheat and barley sown last fall poke through the dirt next to remnants of sunflowers, clover, and radishes. "These cover crops hold the dirt in place and put nutrients back in the ground," Atkins says. They also increase soil organic matter-the dark material, called humus, in the top layer of dirt. That layer contains cellulose, starch, lignin, and other molecules from the decomposition of plants and animal residues, plus a slew of biochemicals produced by earthworms and other organisms that live in soil.