The benefits of Green Manure include:-
Improving Soil Structure
Some green manures have deep penetrative roots that as they grow open up the soil. This is an advantage on heavy soils as allows drainage to occur more freely and organic matter to be left in the soil and on lighter soils the particles of soil can bind together better so they can hold water better and leaves organic matter in the soil.
Green manures crops grow quickly and their very leafy growth smothers weeds. It is like a living mulch as it suppresses weeds and retains moisture in the soil. It is good practice to make sure the soil is weed free first. That is why they are very important when areas are left fallow in winter.
Certain varieties bring to the surface minerals they would be unusable to plants and leguminous green manures absorb nitrogen from the air and fix it in root nodules on their roots so that when it is dug in it becomes available to the following crop. Specific soil bacteria are required to be present but they are usually present in healthy soil. Nitrogen is required by plants as it encourages healthy stem and leaf growth.
As a living mulch it helps to protect the soil from compaction due to heavy rainfall, prevents the leaching of nutrients, and helps hold the soil together. In the summer it will protect the soil from the drying effects of the sun and wind.
Some soils need a rest to recover from constant cultivation and by planting with a green manure it will help soil fertility and structure with very little effort. Green manures can be left in for a year or more, but in the case of most domestic gardens and allotments it is generally a winter thing.
Green manures can be left to grow and then periodically cut down before flowering so as to prevent seeds growing. The plant material can be composted in a compost bin.
They can be allowed to grow and then dug in and left to decompose – allow 30 days before planting the next crop.
No dig systems can still use green manures the crop is simply cut down, the foliage is left on the ground to decompose, and is treated as mulch and planted through this layer or just move it to one side to sow seeds. The foliage can also be removed and composted.
Taken from Green Manures website
What are they and which one to use when?
The Royal Horticultural Society lists the common ones and their uses here. Some of the suggested items are easy to obtain either from your local garden centre but all of them can be purchased through the well known on line seed retailers. Decide which one you need and then decide how much you need before you go happily off to purchase too much, too little or the wrong thing of course.