I get a package or two of this “magic fairy dust” as I like to call it, and keep it stored in a cool, dark place for the season. When you plant peas or beans, simply wet the seed prior to planting (if you’re like me, you will soak your seed overnight prior to planting anyway) and lightly dust with the powder. You don’t need a lot, so a package or two should last you all season, and the package should tell you how much seed it will treat, anyhow.
The “fairy dust” is actually a dormant form of Rhizobium bacteria, the little buggers that have mastered the trick of capturing nitrogen from the air and making it available to leguminous plants (pea, bean, clover, vetch, fenugreek, garbanzos, lentils, tamarind, peanuts, etc). Although Rhizobium bacteria are naturally occurring in the soil, by inoculating seeds you insure that pea and bean roots get colonized by Rhizobia immediately and thus you gain free nitrogen for your plants and for your garden. Look up nitrogen fixation if you’re curious about how this works. Usually a “garden variety” package of inoculant will work for peas, beans and fava beans, but if you jump ship to another legume species, such a clover or vetch, you’ll need a species-specific inoculant. Apparently the bacteria are picky.
Packages generally are viable for one year. You can see the date clearly stamped on my package, reminding me that I, too, need to stock my garden pantry with some fresh innoculant.