Fungi, Bacteria and Beans: An Age-Old Formula for Nitrogen Fixation

While the atmosphere consists of 78% Nitrogen (N2, or di-nitrogen gas), plants and animals are unable to use it in its "free," gaseous form. Over the course of millions of years, a special relationship has developed between bacteria, fungi, and legume roots that is worth understanding a little bit more about -- especially since we're concerned with soil health and organic methods of growing food. 

Diazotrophs are the general name for the bacteria that enable nitrogen fixation to happen. Nitrogen fixation is the transformation of di-nitrogen from the air into ammonia, or amino acids, that provide food for living things in the soil (including plant roots). Rhizobia are the diazotrophs that have developed a symbiotic relationship with legumes specifically. Arbuscular mycorrhizas (AM), are the fungi that are also required for the Nitrogen magic to unfold underground.

The fava beans that were planted in the grain fields last fall turned into a sea of flowing green foliage with pretty, dark-purple and white flowers spotted throughout. When I noticed the plants showing between 10-50% of their flowers, I scythed the stalks at the base and hauled them to the compost pile, to add a beautiful and hefty layer of "greens". The roots or, more accurately, the nitrogen-fixing nodules attached to the roots, were left in the ground and will be worked thoroughly into the soil, which should hasten their decomposition. We plan to plant potatoes where this generous cover crop was living before too long. The added nitrogen from the favas will be combined with composted leaves, or leaf mold -- a nice, carbonous source of organic matter as well as a source full of the micronutrients our potato plants will appreciate.

I get jazzed about observing the nodules on bean and pea roots. In this photo I've sliced one open, revealing the pink leghemoglobin, indicating that nitrogen has been fixed by this ancient, trio of elements: bacteria, fungi, and legumes.

Of course, we are saving quite a few stands of favas for re-seeding certain areas in the gardens next Fall, as well as for enjoying both fresh, and dried, beans. Has anyone made pasta fagioli with fresh favas? Quite delizioso -- and we'll be sure to share the recipe when the time for harvesting the fresh beans rolls around.