Green manures are a cover crop grown primarily to add nutrients and organic matter to the soil. The plants are grown for a specific period, and then ploughed under and incorporated into the soil. Green manures are usually grown at times when vegetable beds would otherwise be empty or fallow, or they may be used to fill in spaces between crops. They are an excellent way of covering up bare, unused soil.

Green manures provide the organic gardener with a number of benefits, including the following:

• The root systems of some varieties of green manure grow deep in the soil, bringing up nutrient resources unavailable to shallower-rooted crops.
• Green manure smothers the soil, preventing weed growth.
• Leguminous green manures such as clover, peas and annual lupins contain nitrogen-fixing bacteria in root nodules that fix atmospheric nitrogen in a form that plants can use.
• Green manures increase the percentage of organic matter (biomass) in the soil, which improves water retention, aeration and other soil characteristics.
• They are often used on land that would otherwise be left fallow, which helps to prevent nutrients from being washed away and erosion from snow, rain and wind.
• Some green manure crops, when allowed to flower, attract useful pollinating insects.

Sow seeds of borage, rye-grass, comfrey, clover, peas or annual lupins on cleared, empty ground. When the soil is needed, cut the top growth back to ground level and dig this into the soil, leaving a minimum of two weeks before planting the next crop. The green manure's nutrients will be released and benefit the newly planted vegetables.