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Nitrogen Fixing Root Nodules

September 23, 2008
Fresh-pulled Nodule-filled Green Bean Roots

Fresh-pulled Nodule-filled Green Bean Roots

I several previous posts I’ve talked about fixing nitrogen. This symbiotic work between plants, fungi and soil is a great way to easily help your garden. And, its inexpensive!

Yesterday, as I was clearing out beds of summer vegies to make way for fall plantings, including nitrogen-fixing fava beans, I pulled out spent green bean plants. Green beans provide nitrogen fixation, doing the work in tiny bean-like nodules in their roots. When I pulled the plants, so came several with well-formed nodules.

Not every nitrogen fixer has nodules, but green beans are one of them. The nodules aren’t exactly part of the plant but are actually the home for a bacterium that works symbiotically to exchanging carbs from the plant for the bacterium for ammonium for the plant from the bacterium. (You can read more here¬†about the science of the exchange.)

Close-up of Green Bean Nodules

Close-up of Green Bean Nodules

So, here again we see symbiosis at work in the soil. There are fungi living in the soil that work with plant roots to share carbohydrates (from plant to fungus) in exchange for water and soil nutrients (from fungus to plants). This is an important reminder that although it can be easy to forget about what goes on within our gardens below the soil, this area of a plant’s world is one of the most important. Again and again we are reminded that our investment in soil health is critical to a healthy garden. And, the relationships that have evolved among plants, fungi and animals in the soil goes beyond what humans have developed in chemical labs over the last 100 years or so. The plants and their soil buddies have formed alliances like nitrogen fixation, carbohydrate sharing and water acquisition and supply that out-wit humanity’s short-term fixes that often create long-term problems such as toxic chemical fertilization run-off poisioning water supplies. But…that’s a story for another time.

Get out there and fix some nitrogen with some cover crops for winter. A few beans will save you a bundle in nitrogen fertilizer (and related issues) down the road!


  1. […] summer in a 4-by-8 foot raised bed that will be given over entirely to beans. The reason: Beans are nitrogen fixers — that is, they take nitrogen from the air and put it in the soil, with the help of some […]

  2. […] to potential high ozone days. Although this study itself is interesting, it makes me wonder if other nitrogen fixing plants like green beans (in the legume group like Kudzu) are also potential […]

  3. Nitrogen fixation says:

    You can check how active the nodules are by splitting them open. If the nodules are red to dark red they are active. If they are light pink to white then the rhizobia are not active and not fixing significant amounts of Nitrogen

  4. Thanks for sharing Brett. This is fascinating stuff!!

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