A green manure is a crop you plant to rejuvenate the soil. Nitrogen fixing plants are the best. They take Nitrogen from the air and add it to the soil by way of their roots. Tiny nodules grow on the roots and are left in the soil when you chop them down. It's best to do this just before they flower.
Good plants to grow as a "green manure" include the legume family, such as broad beans, peas, lupins, vetches or annual sub-clovers.
The important thing is that the Nitrogen nodules are left below the soil surface.
Most green manures are sown to cover the soil during the winter months, when there are less plants in the ground. This has the advantage of adding more nutrients and organic matter to the soil and keeping the weeds at bay.
Cereal crops have a mass of fine roots that benefit the soil when they decompose. They also create substances called mucins that hold the soil in large aggregates called crumbs, that improve drainage and aeration.
Legumes fix atmospheric nitrogen via their root bacteria , so they improve the fertility of the soil for the following crop. Beans and lupins fix more nitrogen than peas.
An advantage to growing green manures is that they suppress weed growth, both by competing for nutrients and by shading. A cover crop also helps prevent soil erosion.
Some people like to till the crop into the ground at the point of flowering, or just before. However, as I prefer the no-dig method, I cut the crop and leave it on the soil surface as a mulch or I put them through the shredder and add them to the compost heap.
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