Both organic and conventional growers can gain many benefits from increased use of green manures. A wide range of plant species can be grown as green manures as different ones can bring a variety of benefits. Leguminous plants will fix nitrogen from the air whilst non-legumes will conserve nitrogen by preventing nitrate leaching. Green manures add organic matter to the soil, improving its physical and biological properties and they can assist with pest, disease and weed management. Some of the effects on soil physical properties may only become significant after several green manure crops have been grown over a period of perhaps five to ten years. Green manures are often categorised according to the time of year they are grown.
Winter green manures or cover crops are usually sown in the autumn and incorporated in the following spring and may be legumes (e.g. vetch) or non-legumes (e.g. rye). Summer green manures are usually annual legumes (e.g. crimson clover) which are grown to provide a short term boost for fertility. However, they could also be nonlegumes (e.g. mustard).
Longer term green manures are usually pure clover or grass/clover leys grown for two or three years. They are common in organic stockless rotations where they form the main source of nitrogen. However, in conventional farming these rotations would be harder to justify unless there were animals to graze them.
Green manures may also be used in intercropping systems, although in vegetable cropping it is important to avoid too much competition with the cash crop. Protected cropping systems offer particular challenges and opportunities for green manuring whilst fertility building in orchards can be difficult as nitrogen must be provided at the right time to ensure good fruit set and crop quality. Green manures grown as an understory can also attract beneficial insects.
Green manures are often grown to add nitrogen to the soil. In organic systems this represents the main source of nitrogen, whilst for conventional growers, it can be a way of minimising fertiliser inputs. Almost all legumes use Rhizobia bacteria to fix nitrogen from the atmosphere. Unfortunately finding out how much nitrogen is actually fixed is not easy and depends on many factors. Firstly, the correct strain of bacteria must be present. Different bacterial species interact with different groups of legumes (clovers, lucerne and trefoils, lupins, beans etc.). If the same types of plants are regularly grown then sufficient bacteria will usually be present to establish sufficient nodules. Sometimes it is worth inoculating the seeds with the correct type of bacteria. There are several types available commercially, at a modest cost.
Sometimes the nitrogen fixation still does not occur, even if the roots form a symbiosis with the bacteria. Some strains will infect the plant but not be very effective. They can even drain the plant of resources
(Source: Horticulture Development Company – http://www.organicadvice.org.uk/Factsheet%2024.10.pdf)