Garden of Eatin'

A Look at Legumes: Rhizobia and Root Nodules

Nitrogen-fixing nodules on legume roots

Legumes have long been known to be good for your garden by fixing nitrogen and improving soil fertility. These legumes come in the form of common peas and beans, as well as cover crops that act as green manure in the off season. But how exactly do they “fix” nitrogen in the soil, and what does that mean anyway?

The next time you pull up a pea or a bean plant, take a closer look at its roots. Those small white sacs are known as nodules, and they form on the roots of the legume family (Fabaceae).

Root nodules containing nitrogen-fixing rhizobia

A strain of bacteria called rhizobia lives in the soil year-round, but when specific plants are present — in this case, legumes — the rhizobia establish a symbiotic relationship with them. Legumes secrete compounds called flavonoids from their roots, which in turn trigger the secretion of nod factors in the rhizobia. Coming full circle, the nod factors spark a reaction in the legumes, causing the roots to swell and form the nodules you see here. It is within these nodules that rhizobia live in harmony with their host plant.

Nodules on the roots of legumes

In a naturally occurring process called nitrogen fixation, the rhizobia pull (or “fix”) nitrogen gas from the atmosphere and convert it into ammonium compounds, which provide essential nutrients for the plant.

When the host plant dies, the fixed nitrogen is released into the soil and made available for other plants to use. Even though there is abundant nitrogen in the air, it is present in a form that cannot be used directly by plants; therefore nitrogen fixation is an important process in the life cycle of legumes, which enrich rather than deplete the earth.

Because of this symbiosis, rhizobia always require host plants and cannot fix nitrogen independently. They can live in the soil for some time without food, but to sustain soil health, it’s important to “feed” the bacteria by rotating symbiotic crops such as peas, beans, clover, and alfalfa through different beds every season.

If you grow legumes in your garden year after year (even if you simply sow cover crops), you’ll help the rhizobia population thrive and improve soil fertility without ever using fertilizer!

Linda Ly About Author

I'm a plant lover, passionate road-tripper, and cookbook author whose expert advice and bestselling books have been featured in TIME, Outside, HGTV, and Food & Wine. The No-Waste Vegetable Cookbook is my latest book. Garden Betty is where I write about modern homesteading, farm-to-table cooking, and outdoor adventuring — all that encompass a life well-lived outdoors. After all, the secret to a good life is... Read more »


  • Avatar
    Heather MacDonald
    July 19, 2020 at 4:38 pm

    Interesting. Thank you! Do you rotate your crops and if so how do you determine what follows what? Thanks.

  • Avatar
    Kazi Nazrul
    March 12, 2013 at 5:33 pm

    When and how do you incorporate the fava plant ground cover into the soil? Is it too late once they have flowered?

    • Linda Ly
      Linda Ly
      March 13, 2013 at 2:59 am

      You’ll want to dig the plants in before they set any fruit or seed. So, shortly after flowering is okay. I just use a shovel to knock them down, break them up a bit, and plow them under the soil.


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