Do Legumes Add Nitrogen to the Soil?

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Robert Pavlis

The legume is commonly recommended as a companion plant. It is believed that the excess nitrogen produced by the legume will help feed the companion growing next to it. In the Three Sisters agriculture system the bean provides nitrogen for the corn to grow and since corn needs a lot of nitrogen, it grows better.

Legumes are also an important ingredient in crop rotation. Grow corn one year and follow that up with beans or peas the following year to restore the level of nitrogen in the soil.

There is no doubt that legumes are able to capture atmospheric nitrogen (using bacteria) and convert it to plant usable nitrogen, but how much of this actually benefits other plants? Are legumes a good source of nitrogen for the garden?

Do Legumes Add Nitrogen to the Soil?
Do Legumes Add Nitrogen to the Soil?

Legumes and Nitrification

Legumes, including beans and peas, are able to have a symbiotic relationship with a specific family of bacteria called rhizobia. The plant roots form nodules (little bumps), which house the bacteria. The nodules provide protection for the bacteria and the root provides them with sugars as a food source. In return, the bacteria take atmospheric nitrogen, which plants can’t use, and through a process called nitrogen fixation they convert it to ammonia. The ammonia is converted to nitrate as it is absorbed by the plant.

Microbe Science for Gardeners Book, by Robert Pavlis
Nitrification root nodules on Wisteria roots (hazelnut for scale)
Nitrification root nodules on Wisteria roots (hazelnut for scale)

The plant is then able to use the nitrogen to make proteins and other molecules.

This process is well understood, and is not up for debate. However, what is much less clear is how does this fixed nitrogen become available to other plants, in what quantities and when?

Note: not all legumes make nodules and some scientists believe not all legumes are able to fix nitrogen. Others believe there is a non-nodulating way for some plants to fix nitrogen.

Most garden writers just assume that a lot of this nitrogen flows to other plants for them to use.

Nitrogen from Live Legumes

Does nitrogen move from a legume plant to a neighbor plant while the legume is still alive? This must be true if the nitrogen is a benefit to a companion plant.

The answer to this question has NOT been fully answered by science. Some research suggests that the nitrogen does move from one plant to others growing nearby. In one study broad beans were injected with radioactive urea to see where it goes. The garlic growing nearby absorbed some of the nitrogen from the bean, clearly showing it moved while both plants were alive.

Other research has shown no movement of nitrogen between plants (link no longer available). This movement may depend on environmental conditions, type of plants, type of soil, nutrient levels in soil or other unknown conditions.

We don’t have a definitive answer, but if nitrogen does move from legumes to other plants, it is almost certain that the amount is small because most studies can’t find any movement at all.

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Legumes keep most of the nitrogen so they can use it themselves.

Nitrogen in Living Legumes

Where does a legume store nitrogen? Some insights into this can be found in Designing and Maintaining Your Edible Landscape Naturally, Robert Kourick, which shows a bean plant and its relative nitrogen levels.

nitrogen in legumes

In a green plant before flowering, 60% of the fixed nitrogen is found above ground in leaves and stem and 40% below ground. The same plant with mature pods has 80% of the plant’s fixed nitrogen in the seed, 9% in leaves and stem, and the remainder in the roots.

Are Your Legumes Nodulating?

In order for legumes to form nodules and host the bacteria, the bacteria needs to be present in the soil. There are different species of rhizobia for different types of legumes. If your soil does not contain the right strain, no nodules will be formed.

Gardeners solve this problem by inoculating seed with the right bacteria at the time of planting. Little packs of bacteria can be purchased from seed companies and you can also buy seed that is already coated with the right bacteria.

Once the bacteria is in the soil, it will survive there for several years, so even a 4 year crop rotation does not need to be inoculated each time.

How do you know if you have the right bacteria in the soil? Grow the legume and have a look at the roots halfway through the summer or in fall. You can easily see the pea-size nodules if they are there and they are most visible as the plant blooms.

If the plant did not make nodules you either do not have the right bacteria in the soil, or you have too much nitrogen. Excess fertilizer will prevent the formation of the nodules since the plant simply does not need the bacteria.

Nitrogen from Dead Legumes

As discussed above, living legumes provide very little nitrogen to the soil or other plants. Once the legume dies, the nitrogen in the plant is returned to the soil, where decomposers (bacteria and fungi) convert the organic matter into free nitrogen ions, like nitrate, which can be used by other plants.

This all sounds great, but the story is a bit more complex.

The legume uses most of the nitrogen it obtained to produce seeds – the beans and peas. If you harvest the seeds or any other part of the plant, you are removing most of the nitrogen before it gets to the soil. In fact, “the residue from a corn crop (a non-legume) contains more nitrogen than the residue from a bean crop, simply because the corn crop has more residue“.

Forage crops that are harvested and removed from the land provide almost no nitrogen for future plants because “roots and crowns add little soil nitrogen, compared to the above ground biomass“. About 80% of a plants nitrogen is found above ground.

The other issue is one of time. Organic matter takes time to decompose. A rule of thumb is that organic matter decomposes slowly over a 5 year period with some nutrients constantly being released. This is OK for the long term gardener, but if you think that legume will provide a lot of nitrogen for next years crop, you are wrong.

There is some shorter term nitrogen released from legumes. As the plant grows it might shed the nodules as it grows new roots or matures. These discarded nodules are a quicker source of nitrogen for the soil.

Legumes and Companion Planting

For more information read: Companion Planting – Truth or Myth?

Legumes are highly touted as a great companion plant because they add nitrogen to the soil for the partner plants. This is simply not true.

They may provide a reasonable amount of nitrogen in future years, provided you don’t harvest a crop from them, but as a companion plant they fail to meet expectations.

Legumes and Crop Rotation

Legumes can provide a good source of nitrogen in future years provided you don’t harvest a crop from them. So a legume cover crop makes a lot of sense since all of the nitrogen is returned to the soil.

Keep in mind this is a good long term strategy for building up nutrients in soil – not a short term strategy.

Clover in Lawns

Clover is a legume and many gardeners suggest growing it in a lawn because it ‘adds nitrogen’ for the grass. The idea that clover is synonymous with lots of nitrogen is a myth. Cutting the clover will add some green plant material to the lawn, but the same happens when grass is cut. Both plants add some future nitrogen as the material decomposes in the lawn.

Clover does not add significantly more nitrogen than grass, however, some of that nitrogen did came from the air as fixed nitrogen, whereas all the nitrogen in grass is from the soil. Leaving the clover in a lawn is a good idea, just don’t expect it to add a lot of nitrogen.

Does the Three Sisters Agriculture Work?

I discussed this in more detail in Three Sisters Agriculture. The part of the story where the bean provides nitrogen for the corn is a myth.

References:

  1. Image of root nodules; by Rowan Adams

 

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Robert Pavlis

I have been gardening my whole life and have a science background. Besides writing and speaking about gardening, I own and operate a 6 acre private garden called Aspen Grove Gardens which now has over 3,000 perennials, grasses, shrubs and trees. Yes--I am a plantaholic!

44 thoughts on “Do Legumes Add Nitrogen to the Soil?”

  1. “Leaving the clover in a lawn is a good idea, just don’t expect it to add a lot of nitrogen.”

    You don’t say why not. If you have a bed of clover that is not cut and is just left for the bees – over the years some of the clover plants will die and decay while more grows. Why wouldn’t a long term planting of uncut clover be adding significant nitrogen to the soil over the years?

    Reply
  2. Thanks for all the information you share on here. I saw someone mention in another comment section on your site about using soybean meal for potted plants as a slow release nitrogen source.

    It got me thinking that maybe pulses/pulse meal could be used as an organic slow release nitrogen source.

    I couldn’t find any sources for this. My initial thoughts are that as pulses are high protein they would have some nitrogen attached in the amino acids, as well as carbon and they would break down by microbial action eventually.

    I can assume the nitrogen would then be used to create more bacteria and then be available to the plants at some point but that is just speculation, or perhaps the nitrogen would escape, I really have no idea.

    I was wondering what your thoughts would be? Ty.

    Reply
    • Pulses are the edible seeds of plants in the legume family. So they are plant material. All plant material contains nitrogen, much of it in the form of proteins (amino acids). The proteins would be decomposed by microbes and eventually the nitrogen is released in plant available forms. This is true of all plant material.

      I suspect pulses have a higher nitrogen content than many other plant materials. So they would be a good source of sloe release nitrogen.

      Since they are usually used as a food product, they might be an expensive source of nitrogen.

      Reply
  3. The Role of the external mycelial network of arbuscular mycorrhizal fungi: III. a study of nitrogen transfer between plants interconnected by a common mycelium
    Andre Cruz
    1998, Revista de Microbiologia
    50 Views
    12 Pages
    1 File ▾
    Arbuscular mycorrhizal fungi,
    Nitrogen

    Reply
  4.  
     The relative contribution of each pathway to the total amount of
    15
    N transferred to maize shoots mediated by AMF, including the direct (DT) and indirect process (ITM), was lower (21.2 + 9.6 = 30.8%) than the contribution not mediated by the fungi (ITNM = 69.2%) (Table 1). Thus the main route by which
    15
    N was transferred from cowpea to maize was via the indirect transfer not involving the AMF, where the
    15
    N may have been exudated from the cowpea root into soil solution being subsequently uptaken by the roots of maize plants (citation to follow)

    Reply
  5. You write “Legumes are highly touted as a great companion plant because they add nitrogen to the soil for the partner plants. This is simply not true.”

    Can you provide some sources with empirical evidence? I seem to find only studies that DO find evidence.
    E.g.

    Dahmardeh M, Ghanbari A, Syahsar BA, Ramrodi M (2010) The role of intercropping maize (Zea
    mays L.) and cowpea (Vigna unguiculata L.) on yield and soil chemical properties. Afr J Agric
    Res 5(8):631–636

    Ghaley BB, Hauggaard-Nielsen H, Hogh-Jensen H, Jensen ES (2005) Intercropping of wheat and
    pea as influenced by nitrogen fertilization. Nutr Cycling Agroecosyst 73:201–212

    Reply
    • my links are in the article.
      Your first reference does not make any conclusion about nitrogen transfer from the legume to other crops. Synthetic nitrogen was added twice – no explanation how this affected things?
      Some of the data makes no sense to me. how do you plant “1 M:100% maize + 100% cowpea”?
      If you compare 2007 1 M:100% maize + 100% cowpea to “2 ،M: 100% maize + 50% cowpea” – so fewer cowpeas, nitrogen is down slightly, phosphorus is up 400% and potassium is down by 50%. how do you explain such drastic differences?

      Since they did not measure transfer of nitrogen, nor did they conclude anything about nitrogen transfer and some of the results are suspect – I don’t think this helps the current discussion.

      your second reference concludes that “The study suggests that pea–wheat intercropping is a cropping strategy that use N sources efficiently due to its spatial self-regulating dynamics where pea improve its interspecific competitive ability in areas with lower soil N levels, and vice versa for wheat” – not sure what that means? It does not look like they measured nitrogen transfer between the crops either?

      Reply
      • Unfortunatelly, the links in your article either just do not work, or they link to the same article. Therefore, it’s impossible to read the research that you are basing conclusions on. Can you fix those or put the correct links (or just article titles) in the comments box?

        Even if there is no evidence on the actual transfer of N between plants, don’t you think that experiments comparing intercropping with legumes (treatment) and just planting without legumes (control) are useful as well? That is, if plants in treatment have higher N than those in control, we can assume that is because of the proximity of the legume?

        Reply
          • True, but I don’t agree with your conclusion “Legumes are highly touted as a great companion plant because they add nitrogen to the soil for the partner plants. This is simply not true” then either.
            Again, if there’s a couple of studies that conclude there’s no transfer, I understand your statement.
            But as I said before, your links do not work. It would be very helpful (and more convincing) if you could fix them or provide them in the comments section.

  6. Hi Robert, I found the following statement in Jessica Walliser’s ‘Plant Partners’:
    “nitrogen fixed by a legume can be shared with neighboring plants via the mycorrhizal fungal network, which accounts for 20 to 50 percent of nitrogen transfer from legumes to nonlegumes. One study found that, in total, leguminous plants can transfer between 30 and 50 pounds of nitrogen per acre.”
    The book is heavily referenced and I would love to know your take on it.

    Reply
    • I have reviewed her book on companion planting and although it does have a lot of references, they don’t seem to support the contents that I looked at. When I asked Jessica for the references to support her companion planting claims – she did not supply them – I am convinced they don’t exist.

      What about the nitrogen. Plants do provide nutrients to mycorrhizal fungi in exchange for other nutrients. So I am sure some nitrogen is also transferred. So what does the fungi do with the nitrogen it gets? It uses it to grow. It’s not going to provide it to plants growing near the legume.

      Ask the author for a reference to support her claim.

      Reply
      • Why is it not possible that nitrogen be shared by the mycorrhizal network? I’m just recalling Suzanne Simard’s work with douglas fir and birch.
        Thanks. Your website is really interesting.

        Reply
        • Some will be shared with fungi, if there is a benefit to the plant, the amount is not significant. If you have a reference that shows something different – please post the link.

          Reply
          • Thanks Robert. Anyone interested can see works referenced on the wikipedia entry for Suzanne Simard. She showed carbon shared in significant amounts between trees. I thought nitrogen and water too though I must be wrong. I think the sharing worked in source-sink type way and was net benefit to both trees.

          • Research shows that nitrogen can flow from one plant to another through mycorrhizal networks (references below). As can water, carbon, chemical messages, and many other molecules and compounds. In many though not all cases, these transfers do indeed benefit neighboring plants. However, precisely how and why this happens, to what extent it happens, and how much it matters for healthy adult plants are all active areas of investigation. And the particulars will always of course vary based on the species and their environment. Nonetheless it is established fact that nitrogen can transfer between plants via mycorrhizal links.

            References:
            https://www.tandfonline.com/doi/abs/10.1080/713608315

            https://academic.oup.com/jpe/advance-article/doi/10.1093/jpe/rtac058/6569392

            https://pubmed.ncbi.nlm.nih.gov/30712202/

            https://www.sciencedirect.com/science/article/pii/S2351989420308933

          • You are correct that there is evidence of nitrogen moving between plants through their roots.
            Does this happen through the fugal network?
            Your references certainly seem to indicate that, but at least two of them are using special containers and not field plants. They are also not very certain with statements like “The lack of convincing data underlines the need for creative, careful experimental manipulations.” and “These findings suggest that CMNs could potentially regulate N-transfer from a donor to recipient plant depending upon the strength of individual plant carbon sink”.

            A meta study, dated 2021, looked at the research and concluded that movement of chemicals between plants seems to happen but that there is no solid evidence that the fungi is responsible for the transfer. The results can be explained by direct transfer between plants via the soil. The full story won’t be exposed until there is further work done.
            ref https://www.frontiersin.org/articles/10.3389/ffunb.2021.735299/full

            The second important question deals with amount. How much nitrogen moves from a legume to other plants?

            Most studies looking at nitrogen movement from plant to plant are talking about small amounts. None of the four references make claims that significant amounts of nitrogen are moving.

            There is no doubt that some nitrogen moves between plants. Roots lose root hairs all the time and they lose cells at the root cap. Almost certainly some nitrogen is released as exudates. These are digested by microbes who cycle the nitrogen and make it available to other plants. It is very possible that fungi also move some nitrogen. But there seems to be little evidence that the amount is large enough to make a difference to the gardener.

      • Hi Robert,
        Thanks for making such a useful website.
        You say “It’s [mycorrhizal fungi] not going to provide it [nitrogen] to plants growing near the legume.” Why not? I’m a beginner in this subject but I thought suzanne simard showed Nitrogen can be transferred via a mycorrhizal network? Is this just in certain species of trees perhaps? Has it been shown not to occur between notrogen fixing legumes and other plants?
        As a novice gardener should I always assume plants will compete rather than cooperate? Many thanks

        Reply
        • Nitrogen is precious to a plant – it is usually the nutrient needed most. A plant will not give it up to fungi unless there is a benefit to the plant. Some may very well be passed to fungi, but most is kept by the plant.

          Plants don’t cooperate. They take of themselves.

          Reply
  7. Hi Robert, really appreciate this and other articles on the site. I’m considering buying both your books so that I have the same information in hardcopy. Before I do, I wanted to ask whether the books contain full references/bibliographies pointing to the relevant scientific papers, as seen in the hypertext links on this post.

    Another question specifically about nitrogen, though admittedly not in leguminous plants\. A friend of mine has been talking up Italian alder as a nitrogen fixer. It’s apparently recommended by Martin Crawford, of UK forest gardening fame. The idea is that the deciduous tree drops its nitrogen-rich leaves each autumn and adds the nitrogen to the soil that way. I imagine the same would be true of, for instance, mulberry which is known to have high-protein leaves. But is that nitrogen still present in the falling leaves, or has the nutrient been drawn back into the core of the tree before it drops them?

    Since I live in Australia where Acacias are everywhere, I also wonder about the effect of mycorrhizal networks on nitrogen sharing by perennial leguminous shrubs and trees. What you describe above seems right for annual peas and beans in a traditional veggie garden, but I wonder if it’s still true for edible perennial plants like licorice or carob or pigeon pea, or for that matter ornamentals like Russell lupins or wisteria?

    Reply
    • 1) the myths in the books are not necessarily the ones on the blog – and vice versa.
      2) the book has much fewer references – designed more for the general public.
      3) The nitrogen content of deciduous plants is usually reduced before the plant releases the leaves. That is why brown leaves have a high C:N ratio and green ones have a lower one. If the Italian alder drops green leaves (I don’t know the plant) it might contain more nitrogen. All brown leaves contain some nitrogen. Anyone who promotes this idea should be able to provide a link to a scientific study that analyzed the nitrogen content – then you know the facts.
      4) Don’t know a specific answer for your listed plants. Plants are stingy about their nitrogen and don’t give it up easily. They will pass some along to mycorrhizal fungi, if it is a benefit to the plant.

      Reply
  8. I did a little trial planting last summer-I planted a block of sweet corn, but in 4 rows, I immediately over seeded with white clover after planting the corn. The corn ended up stunted and it browned up earlier than the adjacent rows. It was so bad, I mowed it down….I did end up with a nice patch of clover….about 4 rows wide!

    Reply
  9. Sir, I really enjoyed your post. Question, I see many people advocating planting nitrogen fixing trees such as black or honey locust near fruit trees. They claim that if the nitrogen fixer is pruned heavily often it will cause root shedding of nitrogen to feed the fruit trees. Some go so far as to plan out an orchard where every other tree in the rows is a honey locust or other nitrogen fixing plant for example.

    My question to you sir, does this make sense? Has it actually been scientifically tested to work? Is it just people saying, well clover can fix nitrogen for wheat, so lets try locust for peaches, that should work.

    I see so many people saying things like this, but there does not seem to be evidence for the claims??? If you or someone else know of evidence or study I would LOVE to hear it.

    Reply
    • 1) I don’t know if it was tested. Ask the people who suggest it – if they don’t have a study showing this works – it probably doesn’t.
      2) I doubt that pruning trees would cause them to release nitrogen. Would pruning not trigger the tree to grow more so it needs the extra nitrogen for itself?
      3) honey locust are big trees – they would compete with fruit trees for sun and soil. Seems like a really dumb idea.

      Reply
      • Hello, speaking from personal experience, cutting alders on the east coast ( which I believe have an association with frankia [ similar to legumes and rhizobia ] , there IS a release of nutrients to some degree. The alder does vigorously regrow and so does the surrounding vegetation, that’s negating the concept of additional sunlight.
        As far as the amount of nitrogen a pea plant may produce at maturity, at an average crop yield of approximately 2500lbs per acre, average of 23% protein, 6.25 pounds of protein per pound of nitrogen … if my math is correct, that’s about 90 lbs of nitrogen.
        As far as “sharing nitrogen between plants” .. in a pasture situation, sure seems there is more “ sharing” done after it runs through the animal. Perhaps the competition of plants give the illusion of more growth, perhaps it’s root exudates and soil biota.
        With the current price of fertilizer I’m sure there will be more studies done in the near future !!

        Reply

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