February 21, 2011

Rhizobia and Root Nodules

You know how people say that legumes "fix" nitrogen in the soil? Well here is how they do it:

Root nodules on vetch from my garden.

Rhizobia are a type of soil bacterium which infect and form a symbiotic relationship with legumes. Together they create little root nodules where they "fix" the nitrogen (N)... that is to say they take atmospheric N (straight out of the air) and turn it into a form usable by the plant. For this service the plant trades carbohydrates.  

There are many species of nitrogen fixing bacteria in two genera (Rhizobium and Bradyrhizobium) which infect different species of legumes. The Rhizobia that formed these lovely nodules is R. leguminosarum

How you treat the crop will effect the amount of N left in the soil. The legume utilizes the N to grow itself so if you harvest the seeds and/or hay from a leguminous crop, you take most of the N with you out of the field. If you incorporate the biomass of the plant into the soil, you put the N back where it then becomes available to subsequent plants growing in that spot.

And... not all symbiotic relationships fix N equally well in the first place. Beans and peas (that's Pisum sativum the garden pea not Vigna unguiculata, the cowpea) are not great fixers of N (30-50 and 71-119 kg N/ha/yr respectively as opposed to 150-250 kg N/ha/yr for alfalfa, a fantastic nitrogen fixer... although interestingly, while vetch is a pretty good fixer of nitrogen, R. leguminosarum is also the symbiote for the entire genus Pisum which as we saw above, is kinda crap at it. Go figure.) Any N they fix they use and since you don't want to return the seed to the soil (you want to eat the seed) they don't really contribute much to the stock of soil N. 
In summary, there are three impacts Rhizobia have on agriculture:

1. The symbiotic plant utilizes the N during its life to grow better. So for example, peas inoculated with Rhizobia grow larger and produce more seed than those that aren't. 
2. Non-fixing plants in the vicinity may be able to immediately utilize fixed N. For example corn grown with beans might grow better than corn without beans. 
3. The biomass of the plant which now includes all that N can be incorporated back into the soil where it used by subsequent plants growing in that soil. Vetch, by the way, contributes 50-150 kg N/ha/yr when grown as a cover crop with sufficient quantities of R. leguminosarum.

For more info on soils and Rhizobia you might take a look at a basic soils book like Elements of the Nature and Properties of Soils (this is the one I have although it's an earlier edition) by Nyle C. Brady and Ray R Weil, Prentice-Hall 2000.

Figure 7b in this document from the University of Hawaii is a great picture of what my root nodules would look like if I sliced them in half and had a better macro on my camera.

UMass Amherst has some info on growing vetch to enhance N in the soil. In my case, vetch just grows on its own but if you're interested in using it as a cover crop many places, including Johnny's Select Seeds and Peaceful Valley Farm Supply, sell bulk vetch seed.

Another view of the Rhizobial nodules from that vetch plant.


  1. Lots of information! I haven't heard of vetch or Rhizobia. Right now I have winter rye growing as a green manure. I plan to turn it (by hand) back into the soil a little while before planting the spring garden. Is vetch something better than winter rye or clover? And is the rhizobia something I have to introduce to my garden or is it already there waiting for something to attach to?

  2. It's not better, it's just different. Often vetch and rye are planted together because rye and other non-legume cover crops add huge amounts of organic matter to the soil while legumes, like vetch, add nitrogen but not much organic matter. You need both things for a healthy soil ecosystem. A good soil test will tell you how much N and how much organic matter you have (along with the other macro and micro nutrients) which can inform you choice of cover crop.

    An appropriate Rhizobia species may or may not be in your garden. It depends on what kinds of legumes were growing there and how recently. If you're going to buy a leguminous cover crop I'd go ahead and get Rhizobial inoculant too. But, as I said in the post, you have to make sure you get the proper inoculant for the species of legume you're planting because there are many species of Rhizobia and they each infect a different set of plants.

    These are great questions! I'm about to write a post on choosing cover crops and your questions are giving me some insight into what kinds of info people need.

  3. As a nitrogen-fixing legume, vetch is similar to clover and different from winter rye (or any grass). Legumes with Rhizobia add nitrogen to the soil, while grasses add biomass and break up massive soils.
    My understanding is that many soils may already have Rhizobia, although not always the right species. Generally, not a bad idea to add the bacterium with its symbiotic plant seed.

  4. Thanks Annie. I feel lost, but yet I learned so much (hope that makes sense!) Just so much more to learn (as always!) I know I haven't done anything "wrong", but always learning more as I go along and there is always more I can do to improve my soil. So should this vetch + Rhizobial inoculant be grown the winter before I want to plant some sort of bean in a specific bed? If you are covering that in your upcoming post on cover crops you don't need to answer here. Emily

  5. Vetch, around here, grows late winter/early spring although since we're so mild this close to the coast I sometimes see it start growing earlier. You don't necessarily need to follow vetch with a bean... the nitrogen that vetch adds to the soil will be good for any vegetable you grow, particularly heavy feeders like tomatoes and corn. In fact I wouldn't grow beans immediately following a leguminous cover crop because then you're just wasting the N you put in the soil with the vetch since beans can fix their own N (which they won't do if there is plenty already in the soil). Definitely follow a leguminous cover crop with either a non-legume cover crop (if it's a garden bed that you're spending multiple seasons improving fertility in) or non-legume vegetables.

    Great questions!

  6. So it is the cowpea that would work well with the corn?

  7. I assume you're talking about growing beans up the stalk of corn so... no they won't work. Cowpeas are bush (or semi-runner) not pole plants. The reason you plant corn and beans together is because you save space (since the bean grows on the corn stalk, the plants occupy almost the same ground area), not really because the bean feeds the corn. It's more a case of the beans not needing any extra nitrogen so what's already in the soil is totally available for the corn. And only certain beans can be successfully grown with corn (it needs to grow at the right speed). If you're interested, Southern Exposure Seed Exchange has identified which varieties of bean are appropriate for this arrangement.

    Now, if you're talking about growing cowpeas as a cover crop before you grow corn to stock the soil with N then that would work, but only if you cut the cow peas down before they set seed.