Green manuring is the deliberate cultivation of specific plant species to increase the quality of the soil. Green manure crops can improve soil structure by producing humus, enriching the soil with nitrogen or make bonded nitrogen available again, loosen and aerate the soil or suppress weed growth. Furthermore, green manure serves to prevent erosion on fallow land and can be used as a means of disease and pest prevention.
Of course you can not expect one single plant species to do all these jobs at once - it's all in the mix. Typical green manure plants are legumes such as lupins, alfalfa, vetch and clover. Their specialty is primarily nitrogen enrichment, as their roots are a good host for so-called rhizobia, bacteria able to accumulate nitrogen from the air.
Often phacelia, buckwheat, radish, yellow mustard, white mustard, sunflowers or marigolds are used as well. And mixed green manure isn't only useful: cropland in bloom is a very beautiful and colorful sight.