I am sure that you have read posts or heard from other gardeners that beans are great at fixing nitrogen into the soil. It is true, they do. Until recently, I thought that if I just planted beans, in fact any kind of legume for that matter, that the nitrogren fixing process would naturally occur. So for many years I thought I was doing something really good for my soil by simply rotating growing beans and peas in my garden beds.
It wasn't until the Summer of 2024 when I attended a gardening seminar on soil and learned that in order for legumes to fix nitrogen into the soil they must be innoculated. I will confess, I turned to my husband and said "are you telling me I have been doing this wrong all these years? No one ever said that the seed needs to be inoculated. Where do you buy inoculated seeds from and what are they inoculated with?" I was shocked to learn that my green beans cannot fix nitrogen on their own. To be honest, I felt like I had been let down by all the gardening sites out there because they simply just told a part of the story. While they were not technically wrong, I felt that a very important part was left out.
This started our journey into soil health. I had heard that it was really good to have beneficial fungi in my soil so I understood that process, but what I didn't know is that legumes have a symbiotic relationship with soil dwelling bacteria called Rhizobium.
How do I know if my beans are fixing nitrogen?
Rhizobia form specialized organs on the legume’s roots called nodules, which are ideal environments for the conversion of atmospheric nitrogen into a nitrogen form the plant can use. Hence the term "fixing" nitrogen.
The only way to know for sure if your plant roots have been colonized with nodules is dig up your plant and look at the root system to check for nodules.
If there are nodules and they are pink looking, and if you cut them in half and they are red inside, then they are nitrogen producing nodules.
If they are white, then this is a good indication that they are inactive and are ineffective. This means they are not providing enough N for your plants growth.
Can I inoculate my beans and help the process?
YES! But, there are 238 different species of Rhizobia so you will need to make sure that you pick the right species for what you are growing as the legume-rhizobia relationship is very specific.
If you do not choose the right rhizobia for the legume then the nodules will not form.
What is really great about Rhizobia is that it can live in the ground dormant for a very long time. So if a legume has been previously inoculated and grown in the ground where you are wanting to plant, then more than likely the Rhizobia is already there. However, if you are planting for the first time, you will need to inoculate the seed.
There is a BUT...unfortunately there is no real test to see if there is enough Rhizobia in the soil so the only way to know is to plant the beans and pull them up after harvest to see if nodules have grown.
How do I inoculate my beans?
I'm so glad you want to know this too! That's the very question I wanted answered last summer (2024). Turns out, its pretty easy. You can purchase inoculant from most online seed stores. You may even be able to purchase it at a local supply store that sells seeds. It is bacteria infused peat. While it won't look like it, there is bacteria in there and seeing as they are living organisms it is important to treat the inoculant with care.
The manufacturer will provide instructions as to how to apply the inoculant, so make sure to follow their instructions. The one I chose is for garden peas, sweet peas, lima beans, fava beans, common beans (e.g, snap beans), runner beans, tepary beans, southern peas (cowpeas), peanuts, sunn hemp, partridge peas, and vetch. This basically covers all the types of legumes that I plant in my garden.
My packet comes with an expiration date. So I plan on using it before it expires. There would be no guarantee of nodules being able to develop if the inoculant is used beyond its expiration date. The manufacturer will provide information on how to best store the inoculant.
The easiest way to inoculate legumes, is to coat the legumes prior to planting. To do this, simply put both inoculant and seeds in a bowl and follow the manufacturers instructions regarding how much to apply to the amount of seeds grown. Plant your seeds immediately.
So there you have it - the bean nitrogen secret is out! Happy Gardening!
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