Bacteria that fix nitrogen on peas, clover and other legumes will not necessarily fix nitrogen on alfalfa. Be sure that the commercial inoculant you use is specific for alfalfa. Peat-based inoculant are the most popular because they assure good Rhizobia survival in the package and on the seed. Be sure to follow package directions and check the expiration date of the inoculant. If past the expiration date, discard the old inoculant and treat with new.
Some seed is now available in a 'pre-inoculated' form.
This seed is coated with an inert carrier, sometimes containing nutrients. The inoculant is placed inside the coating on the seed. High temperatures and freezing rapidly kill the bacteria. There are nearly 200 different varieties of alfalfa available in the United States. Not all are adapted to Montana.