Organic Green Manures are grown to specifically benefit the soil. They are normally fast growing and produce masses of fine roots and foliage which hold nutrients and return them to the top few inches when dug in. This foliage can also act to smother weeds and protect the ground from the weather in the winter. Some send down large tap roots to dredge nutrients up from deep in the soil and some fix valuable nitrogen from the air. They should be used when the soil is free for some time and are particularly valuable over winter.