In his book The Green Gardener : Sustainable Gardening in Your Own Backyard Josh Byrne explains… “Green Manures are annual plant crops that are grown specifically to improve the soil. Leguminous species such as vetch, peas and beans are commonly used because they naturally take nitrogen from the air and fix it to their roots in tiny nodules with the help of beneficial bacteria. Green manure crops are usually sown in vegetable beds during winter, and they should be dug into soil just before they go to flower in early spring, when they contain the maximum amount of nitrogen. The crop will break down very quickly and the bed can be planted after two or 3 weeks.”