Monday, May 20, 2024
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Legumes and Soil Mycorrhizae

We all appreciate legumes and their marvelous ability to fix atmospheric nitrogen and release it into the soil. But what happens after the legume plant releases this ammonium and nitrate?

I blithely figured that into the soil they went and the grass roots just picked them up. Hardly so! What I considered the “roots” of a plant actually represent only a tiny fraction of its actual absorptive mechanism. The real players are the soil “mycorrhizae,” beneficial fungi that colonize on plant roots and serve as a nutrient pipeline into the host plant. Not only do the mycorrhizae absorb water and mineral elements (phosphorus being a key one), they also break down other potential plant nutrients and funnel them to plant roots.

In a healthy soil structure, a single gram of soil contains over 150 feet of these fungal “hyphae.” On the average, one acre of soil that’s seven inches deep weighs two million pounds. Imagine the amount of mycorrhizal filaments in that one acre of healthy soil! They are there for us to use in our pastures.

Salt fertilizers can kill these good-guy fungi. Applying the fertilizer in small amounts multiple times throughout the growing season can lessen this mortality. However, seeking ways to promote an ever healthier and increasingly productive soil using biological processes appears to be the more futuristic and sustainable pathway. It may be profitable to use a mycorrhizal amendment to counteract the effect of salt fertilizer while we are transitioning to a more sustainable system.

Various sources of mycorrhizal amendments are available online. I am going to experiment with one this next spring. Who knows, it may be just another piece of the big puzzle.

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Dave Scott
Dave Scott
Dave Scott is a Livestock Specialist with the National Center for Appropriate Technology (NCAT). Dave has 30 years experience with intensive grazing, including dairy and sheep. He has also served as a part-time consultant in management-intensive grazing, helping ranchers design and implement grazing systems that increased their stocking rates and net profits. Currently, Dave and his wife, Jenny, operate Montana Highland Lamb, a 200-ewe enterprise that markets over 50% of their grass-based natural lamb directly to the consumers in southwest Montana.


  1. I am suspecting that our focus on direct mechanisms might weaken our ability to manage complex systems like the messy soil/plant world. Once nitrogen gets into the plant, there are lots of different ways it gets into the soil food web. It seems likely that this years soil activity is feeding off of last years nitrogen fixation. I seem to remember cases where nutrient transfer from plant to plant appear to be mediated by a fungi (using isotope tracers? I’ll have to look that up again sometime). I haven’t seen any substantive research on fungi-to-fungi exchange, and it makes my brain hurt to figure out how to isolate those mechanisms for observation.

    Perhaps the larger consideration is about how fertilizer costs are tied to fossil fuel costs, and so developing self-sustaining soil systems might be about more than whether you are into fungi.

  2. Dave -We’ve long known that legumes carry symbiotic N2 fixing bacteria in root nodules, and your post indicates that these bugs release their fixed nitrogen to microrhizae which then pass these compounds back to the plant. That begs the question whether mycorrhizae associated with legumes are functionally the same as those associated with roots of other plants (grasses) in the same location? Or are grasses fed by different mycorrhizae? I assume mycorhizae are biodiverse, right?

    • Hi, Bill,
      Good questions! I do not know the answers. Remember, I am a farmer! The soil community is so diverse and nutrient pathways so complex I would not be surprised about any configuration. Just think of the zillion times of species that are below ground vs. those above ground!

      I have not read anything concerning this specifically. It is a very interesting question. I will try and find out an answer to it.
      Give me some time and I will get back to you through On Pasture.

      Thank you very much for your comment and for reading On Pasture. I hope you have enjoyed my articles on intensive grazing!

      Yours, Dave Scott

        • Dave- You are a veritable human Google to come up with that link so fast! It does answer my question quite well enough for now, so thanks!
          But you lay yourself open to getting pestered- here’s another one. You wrote that you were considring experimenting with mycorrhizal extracts. Have you percived a shortage of MRF in your soil? How would one begin to assess that? I am aware that Elaine Ingham’s Soil Foodweb Co. deals with such questions., but have not tried her yet. There may be other ways to look.cheers, B.

          • Hi, Bill,
            Yes, we did test our soil with Earthfort’s biological analysis. Earthfort (just Google) bought Soil Food Web and is doing business. The contact there is Matt. Our soil tested low in active fungi. This is what led me to consider the mycorrhizal extracts.

            Yours, Dave

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