Green Manures: Let’s Look at Legumes

by D.A. Salazar, MG Class of 2009

Young faba beans.

Young faba beans.

We’ve all heard that legumes are “nitrogen fixers”, and can do great things for poor, tired soil. It being the season for planting cover crops (a.k.a. green manures), and as legumes figure large in that capacity, let’s take a look at how legumes work their magic.

First a refresher: Legumes are of the family Fabaceae, also called Leguminosae, and their fruit takes the form of a dry seed or pulse that grows in a pod. Grain legumes include chickpeas, faba beans, lentils, field peas and winter or Austrian field peas. Forage legumes include alfalfa, clovers like red and sweetclover, and the vetches. Legumes are plants high in protein — and proteins are full of nitrogenous amino acids, which means legumes require a high intake of nitrogen.

Also known as “N.”

And where do legumes score all that N? Remember in soil biology class when you studied up on good bacteria and microscopic air pockets in soil pores? (Of course you do.) Well, that knowledge is about to become handy at last!

Root nodules on a bean plant.

Rhizobia nodules on bean roots.

Legumes enjoy a symbiotic relationship with a specific type of bacteria in the soil called rhizobia; rhizobia cannot work without a plant host. As legume seeds germinate, rhizobia “infect” the young roots, causing nodules to form on the roots near the crown. The pores in the surrounding soil are full of air, which in turn is full of nitrogen. The nitrogen binds to those rooty nodules, thus supplying the plant with the N that it craves.

In return, the plant delivers carbohydrates manufactured through photosynthesis back to those nodules, providing energy for the rhizobia. It’s a well synchronized give and take, and once things get going, the cycle continues until the plant fruits. Legumes can get 50%-90% of their N in this way, depending on the variety.

Which is all well and good for the legume, but how does this help the soil?

True, most of the fixed N is removed when the legume’s fruit is harvested, packed into all that marvelous protein that legumes are famous for. (That conversion of nitrogen into a compound, like protein, is the “fixation” part.) However, during the plant’s growth high levels of N are leaked back into the soil through the plant’s root ball. Post-harvest legume residues are also rich in nitrogen, and break down easily, quickly releasing more nutritious N into the soil; the same process happens naturally with legumes that grow and die in a wilderness or meadow. Which is why turning under those leftover leaves, roots and stems after harvest make legumes such a valuable green manure.

To ensure the success of your winter legumes, you may need the assistance of an inoculant. An inoculant is rhizobia made available through a natural carrier (like peat moss or clay) in powdered, liquid or granular form, and used as an additive when you sow those seeds. Legumes are particular about which rhizobia they’ll work with; while there are probably rhizobia in your soil, it’s hard to know whether you have the right ones for the legumes you’re planting. Nurseries that sell seeds for cover crops should also have available the correct inoculant to use with yours.

Healthy rhizobia nodule.

Pink & healthy rhizobia nodules.

How will you know your legumes are fixing nitrogen? Gently dig up a few plants from different spots and check the crowns for clusters of nodules. Cut a few nodules open; if they’re successful, they should be pink to bright red in color on the inside.

Get your cover crops planted now through early fall — download our Green Manure Planting Schedule to help you plan. And review our earlier posts on using green manures in your garden here, and here.

Austrian field peas near Sisters, Oregon.

Austrian field peas near Sisters, Oregon.

We’ve barely scratched the surface here!
Get all the fascinating facts and science on cover crops and green manures by visiting our sources:
Saskatchewan Dept. of Agriculture
WSU Vegetable Research and Extension Center
Purdue University Center for New Crops


Photo credits:
Young faba beans from
Root with nodules from Science Photo Library
Healthy nodules by Jennifer Dean at The Compost Gardener
Austrian Field Peas from Rainshadow Organics


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