That is, they release a compounds into the soil which are toxic to certain other plants and animals. All green manures have benefits to the soil that extend beyond the chemicals they may release. Cover crops can reduce erosion from rain or wind, especially in the winter or early spring. When green manures are incorporated into the soil, they build up the organic matter content, which improves water retention, reduces compaction, and makes the soil a healthy habitat for beneficial organisms. Pests have to compete with the beneficial organisms in the soil, so their populations are unlikely to rise to damaging levels.
Legumes are important green manures for any organic farms.
They have a symbiotic relationship with bacteria that can fix nitrogen into a form that plants can use. When legumes are incorporated into the soil prior to a potato crop, the nitrogen is made available to the potatoes. It is released more gradually than chemical fertilizers, so potatoes benefit throughout the growing season and the nitrogen does not leach into the groundwater. Austrian peas and Nitro alfalfa are used most commonly by organic potato farmers. The nutrients provided by legume green manures can be supplemented by animal manures or kelp.