Certain cover crops concentrate specific nutrients in their tissues. Deeply rooted plants move nutrients from the subsoil to the aboveground parts, and when the plants decompose, the nutrients become available for subsequent crops. Potassium levels can be increased significantly by selecting a good preplant cover crop. Buckwheat, grain rye, and sudangrass are good preplant covers. Plants in the legume family are capable of gathering unusable nitrogen from the air and converting it into usable nitrogen in root nodules, with the help of special bacteria.
Legumes increase soil fertility as they decompose, thus releasing the stored nitrogen.
In fact, any cover crop that is plowed under will release nitrogen as the crop decomposes. Many plants produce substances that are toxic to other plants. Varro's observation was explained by the discovery of a substance called juglone - a natural herbicide produced by the roots of walnut trees. Many plants have allelopathic effects including sunflowers, cucumbers, oats, alfalfa, rye and tobacco. When these crops are planted prior to other crops, weed pressure is reduced.