The quintessential nitrogen source plant, either growing or cut. There are several ways to use it. Alfalfa meal is expensive, either as a nitrogen source for gardens or a protein source for animals. It is easy to make your own. Our packet plants 100 sq ft.
Alfalfa makes a great companion plant for trees and vines, fixing nitrogen they can use. It is a classic pasture plant, with more protein than other forage plants. If you raise any kind of livestock, from cows and horses to chickens, rabbits and guinea pigs, this is a great way to cheaply feed them. A new use is the fertility trap--a strip downhill from your garden that traps runoff water and nutrients for you to recapture and reuse. Also a famous bee plant, and very useful as a pollinator and beneficial insect attractor. Deep roots bring up minerals from the soil that other plants can use, in addition to the substantial amount of nitrogen it puts into the soil-- about half a pound of nitrogen per 100 sq ft per year.
Because this is perennial, be aware that it will not die in the winter--the tops will, but the roots will persist. That makes it a bit difficult to use in a vegetable bed. Deep digging or tillage will kill it, but not surface till.