Organic matter and the roots of growing cover crops stimulate microorganisms, which help suppress disease organisms, improve soil structure, and digest organic matter so that nutrients can be used by plants. Steve Diver of ATTRA-The National Sustainable Agriculture Service recommends inoculating cover crops with compost tea before tilling them under, because it helps them decompose faster.
Cover crops cover the soil and prevent excess nitrogen from leaching.
Plants take up excess soil nitrogen and return it to the soil when they are turned under.
In addition, legumes, such as clover, fix atmospheric nitrogen in special nodules on their roots, in collaboration with Rhizobium bacteria.
This nitrogen can be used by the next crop you grow. In this way, cover crops act as a bank account for nutrients, returning the investment with interest when you turn them under.
"Cover crops add organic matter, which feeds the microbes that can play important roles in the garden like disease suppression," says Nancy Creamer, Ph.D., director of the Center for Environmental Farming Systems at North Carolina State University.
Cover crops also encourage beneficial insects by providing flowers for a nectar source and foliage for shelter.
This is the one I like—less weeding! Some cover crops, such as buckwheat, grow so densely that they outcompete weeds—that's why they are sometimes called smother crops.
Others, like rye, oats, and sorghum-sudangrass (I call them the cover-crop “bad boys”) have an allelopathic effect on weeds—they actually exude compounds that suppress the germination of weed seeds.
Because these compounds can last awhile in the soil, it’s important to wait three weeks after tilling them under to plant your garden.