Last November I bought a pound of fava beans at Claudio’s in the Italian Market for less than $2.00, soaked them overnight and planted them everywhere in my plot as a winter cover crop. Now it is May, and my plot is a blossoming sea of fava beans beginning to set pod.
Fresh fava are always a treat, but what I really wanted was an over-wintering green manure. Fava is one of the few beans that prefer cold weather and can overwinter in our area. Because they are legumes, they form mutually beneficial relationships with special bacteria in the soil which can take nitrogen from the atmosphere and supply the plant (and your garden) with its own supply of fertilizer. What could be better?
Every living cell on earth requires nitrogen. It is an essential component of protein-based amino acids and DNA. But although more than three-quarters of the atmosphere is composed of nitrogen, it is chemically non-reactive and unavailable except to special bacteria which can break the tough triple-bonds of free nitrogen and covert it to ammonium which plants use to create the first layer in the food chain that supports all life.
These bacteria usually team with plants to perform this feat. The finely-tuned communication between plant and bacteria begins when the plant exudes flavonoids from its roots to attract symbiotic rhizobial bacteria living freely in the soil. The attracted bacteria respond by producing an enzyme called “nod factor” which causes root hairs on the bean to curl up and develop into little round nodules which provides a home and food for the bacteria. The bacteria quickly colonize these nodes and begin transforming nitrogen from the air into ammonium, the form of nitrogen fertilizer that the plant can use.

The small round objects on the roots are nodules where the bacteria do their work

Fritz Haber
Constructing a nitrogen fertilizer plant costs hundreds of millions of dollars, yet nitrogen-fixing bacteria manage to do it under normal temperature using only the energy their plant host gets from the sun.
The enzyme the bacteria use to convert atmospheric nitrogen to ammonia works best in the absence of oxygen, so the plant has devised a helpful strategy for keeping oxygen out of the way of the bacteria. Root cells produce a red protein similar to the hemoglobin in our own blood. The red protein that legumes produce is called leghemoglobin. It colors the inside of root nodules actively fixing nitrogen to a pinkish or reddish color and protects the bacteria from unwanted oxygen.

Cut-away view: An active nitrogen-fixing nodule is red or pink inside
The downside of this relationship is that supporting the bacteria’s substantial energy requirements during nitrogen fixation is very expensive for the plant [costing up to 20% of all the carbohydrates it produces], so during pod-fill the plant will cut off food supply to the bacteria and the nodules will lose their red color and may even be discarded. Similarly, when high amounts of ammonium are already present in the soil, the genes controlling nodulation and nitrogen synthesis are turned off and the plant simply uses what is already there rather than having bacteria manufacture it anew.
So how do you get the nitrogen from the plant into your soil? Just cut the plant off at soil level and leave the roots in the ground (unless, of course, you would like to pull up a few to admire the nodules). Their massive root system and nitrogen-rich nodules penetrate the soil deeply and will break down over time to enrich your soil to a depth of 1 or 2 feet. You could compost the tops (they make fabulous compost material) but I prefer to simply chop them up and scatter them on top of the soil where they dry out and become a rich mulch in a couple of weeks.
If you are only interested in green manure and don’t care about beans, cut the plants just before they start setting pod to get the maximum amount of nitrogen for your soil.
NOTE: All photos are of plants I actually grew in my plot during the winter of 2011-12. Photos were taken in May 2012.
- Barbara