Depending on the specific techniques you're using, the topics of "composting," "mulching," "green manures," and "cover crops" tend to overlap. Strictly speaking, however, cover crops are crops that protect the ground from erosion after food crops have been harvested. Cover crops are normally sown, therefore, in the fall. When spring arrives, they are left to grow again for a few weeks and then dug into the soil. Cover crops also add organic material to the soil, and bring nutrients up from underground. If legumes are used, a cover crop will also add nitrogen to the soil. A cover crop is normally sown quite thickly, so another useful effect is to prevent the growth of weeds. Some commonly used species are rye, wheat, rye grass, and buckwheat. One drawback to cover crops is that they can be labor-intensive, and you may need to cut the crop with a scythe before you start digging.