Pasture legumes form a symbiotic (mutually beneficial) association with specific soil bacteria (rhizobia) to meet their nitrogen requirements. Nodules develop on the plant roots and house millions of rhizobia that convert nitrogen from the air into a form the plant can use in a process known as nitrogen fixation.
The association between the host plant and its rhizobia is very specific and pasture legumes must be inoculated with the correct rhizobia strain (or Group) for maximum nitrogen fixation. Techniques to successfully inoculate pasture legume seeds are described here.