Scientists have discovered that inoculating legumes with nitrogen-fixing bacteria can increase crop yields.  The theory is simple --- if your plants lack the proper bacteria to team up with, they're stuck begging ammonia out of the soil rather than producing their own.

But you can't just inoculate your entire garden with one kind of bacterium and be done with it.  Most plants that team up with nitrogen-fixing bacteria are picky about the bacteria species they move in with.  Clovers share one set of bacteria species, garden and soup beans another, and alfalfa, soybeans, peanuts, clover, and peas each have their own.  You can often buy seeds already coated in the proper inoculant, or can even transplant a bit of soil from your previous pea patch to your new one to get the useful bacteria started.

As a side note, I was intrigued to learn that legumes aren't the only plants that team up with nitrogen-fixers.  The other common, nitrogen-fixing plant in our area is the shrub alder (Alnus sp.)  I've been keeping an eye out for some wild alders to transplant into my forest garden as a method of naturally boosting the area's fertility.