Green manures are plants grown specifically to benefit the soil - replacing soil nutrients, improving soil structure and increasing humus content. They tend to be quick growing, producing a mass of weed smothering foliage. Some are legumes which have the ability to take up nitrogen from the air, tapping a free source of soil fertility. Nutrients which would otherwise be washed away are taken into the plants and then released when the green manure is cut down and turned in to the top six inches of soil.
A green manure crop should be considered whenever an area of ground is to be left free for six weeks or more, and is of particular value through the winter.