Many legumes (plants in the pea and bean family) possess a cool skill.  In an energy-intensive process that involves partnering with soil biology at almost every stage, they take nitrogen from the air and convert it into plant-available form.  While other types of plants rely on soil biology to convert sources of nitrogen below ground into a form that they can use (or on a human to anticipate and supply fertilizer requirements if soil biology is no longer around), legumes can take advantage of the 80% of the earth’s nitrogen that is floating above-ground. 

By partnering with rhizobia – a type of bacteria of which there are many different species indigenous to different areas of the world – legumes form nodules on their roots.  To find out if your legumes are actually fixing nitrogen, gently pull up a plant (close to blooming time).  The nodules should be outwardly pink and about the size of a small marble.  No nodules, no rhizobia!  Or perhaps nitrogen fertilizer being is used.  Why would the plant expend energy on an age-old partnership if it is being artificially fed?  If you find nodules, use your thumbnail to split one open.  If the plant is really fixing nitrogen, you should see red inside. 

If rhizobia are missing, they can be re-introduced to your soil.  You can buy inoculant, although by now you may know that I am not a bug-in-a-bag kinda girl, unless you can be sure that you are purchasing a species that is local to this area (and thus better adapted to our conditions, and less likely to become invasive).  Instead you can go hunting for legumes in the valley that are already fixing nitrogen. Take some of the nodules, dry and grind them, mix with some decent finished compost, and coat your seeds.  

Do legumes share this hard-won nitrogen with plants nearby?  The “three sisters” indigenous approach to growing corn, beans and squash together, as well as much permaculture thinking, certainly suggests it.  But whether and how nitrogen is truly being shared outside the living roots of the legume remains a scientific mystery.  “Chopping and dropping” legumes seems your best bet of ensuring plants nearby get a benefit over time, as soil biology incorporates the nitrogen in their tissues into the system through decomposition.