Green manure is a quick growing crop, grown to improve the soil to help the following crop grow. It can be used to cover bare soil, and may be called a cover crop. The crop is grown, and then dug into the ground, left to rot before the ground is planted with usually your vegetable crop. It can be helpful if you do not have ready access to manure but has a role to play even if you do have manure. Green manures have several uses it can be used to increase the organic content of soil, perhaps its most important use. Bare soil has a higher erosion rate than soil which is covered, losing top soil due to wind, rain and weathering. Soil is very slow to form, leading to a net loss of the fertile top soil. Using a cover crop reduces this erosion. Another use is to help suppress weeds by out-competing the weeds for resources like light. Lastly green manures can be used to retain, accumulate minerals or convert nitrogen to a useful plant food. During the winter nitrogen is washed away by winter rain as nitrogen is not tightly bound to soil particles in the UK. During the summer the bacteria in the soil become more active capturing nitrogen from the air in the soil converting it into nitrates which a plant can use. Green manures can capture that nitrogen before it’s washed away, storing it in the plant over winter. The nitrogen is released when the green manure rots down. The roots of some green manures can also pull from deep down in the soil minerals storing them in the plant itself, being released after the plant dies. Some green manures capture nitrogen from the air. Their roots are covered in nodules. The nodules contain bacteria that capture nitrogen from the air converting them into nitrates. The bacteria will not be active in the winter; they are only active above 8 degrees centigrade. This means that this only happens during the summer months. Some green manures also are more suited to improving the soil structure their roots penetrating deeply into the soil. Their roots break up deep compacted soil, and when the plant dies leave airways in the soil and traces of organic matter where their roots where.
As green manures are living plants when they rot they add organic matter to the soil. Organic matter in the soil this both improves drainage and water retention. Also humus formed from organic matter can hang onto nutrients (increasing the cationic exchange capacity). This allows your plants to have access to a larger reserve of nutrients when actively growing.
Personally I have not used green manure, solely because I have access to large amounts of manure to increase my organic matter and nutrients, plus I spread my manure on the surface of my soil acting as a barrier to erosion and weeds. So I cannot give an opinion on how effective they are. The green manure plants will provide organic matter from top growth and its roots. As the roots rot drainage will be improved. Some green manures are annuals killed by frost and others are biennials and frost hardy, yet others are perennials. Downside of green manure is it is hard to dig in and it may encourage slugs. You also have to be careful to use the correct green manure if you have club root as some like mustard are susceptible to this disease, making the infestation worse. Green manures are usually sown in the summer or autumn, you need to harvest them before they flower and set seed.
Some Green Manures:
Green manures used to capture nitrogen from air in the soil
Annuals Crimson Clover Yellow Trefoil Vetch Persian Clover
Bi annuals Sweet Clover
Perennials White Clover Red Clover Lucerne Sainfoin
Green manures to improve soil structure and biology in the soil
Annuals Mustard Buckwheat Radish Phacelic
Perennials Chicory
Green Manures used to capture minerals and nitrogen already in the soil
Annuals Westwolds Ryegrass Cereal Rye
Perennials Perennial Ryegrass Timothy Cocksfoot
Green manures can be used over winter to stop minerals leaching from the soil as well as capturing carbon from the air to build organic matter. In the summer they can be used as a quick growing crop taking as little as 4 weeks to grow on some bare soil, capturing nitrogen from the air. They can also be interplanted with some larger vegetables. You can grow different green manures that are in the different groups above together this can give you an even bigger boost to the soil getting up to 49% additional benefits compared to growing the manures as separate crops. When your green manure crop is finished you cut the crop down and dig it into the soil. Leave for a few weeks to rot down and then plant or sow your vegetable crop. Nitrogen levels after an initial drop will peak about three months after the crop is dug in.
November. You can sow: Broad Beans. Sow a winter hardy variety e.g. Aquadulce Claudia
You can plant: Fruit trees and bushes Garlic Rhubarb Other Jobs Dig and manure your plot before the ground is unworkable due to the weather. Lime ground if it’s too acidic. Do not lime freshly manured ground Remove bean poles and store in a dry place Prune apples, pears, vines, currents and gooseberries . Take hardwood cuttings of Vines, currents and gooseberries Figs: Remove fruit that is bigger than a cherry, leave embryo fruit alone, these are next years figs Collect leaves to make leaf mould Net Brassicas against pigeons