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. Enhancing environmental conditions for growth Maple trees can move groundwater from their lower roots to the upper roots, where it is exuded into the soil. Herbaceous plants can use this groundwater when conditions are dry.