Nitrogen is one of the most important food sources for our plants. Plants are built to draw nitrogen from the soil/water where it exists in several forms – some available for plants to use and others not. But floating above their oblivious green heads there’s an entire world of nitrogen. About 90% of the earth’s nitrogen exists in the atmosphere and not in the soil. In fact the atmosphere is just about 79% nitrogen, 21% oxygen, and eleventy billion percent filth.
Nitrogen in the soil comes from a few nicely evolved systems. Most comes from a slightly freaky-sci-fi-slightly-symbiotic force called fixation. In fixation, several species of bacteria can absorb atmospheric dinotrogen gas which they then convert into ammonium. Ammonium is structurally NH4 and in this form, plants can (and happily do) grab the nitrogen bit.
Happy bacteria and stuff do a few other things in the soil to produce or make use of nitrogen via nitrification and mineralization. Mineralization happens when stuff breaks down in the soil and the organisms turn that nitrogen into ammonium and nitrification is sort of the reverse process.
The cool thing is that all of these happy little bacteria guys sit around in the soil with specialized jobs. Many plants require the nitrogen to be absorbed via water, but legumes like peas and beans have a deal with the bacteria. They let the bacteria live in nodules within their root structure, they provide the bacteria with the fuel to fix nitrogen right from the air and in turn the legumes draw on the nitrogen. It’s a contained fertilizing system! (Incidentally, reproducing this system in a factory requires tons of energy… it’s why making fertilizer blows.)
So, if the system occurs naturally why are we told to use inoculant on our legumes? And is the inoculant “natural”? It sounds sinister.
Inoculant is usually a powdery substance and it’s nothing more than a grouping of a specific type of nitrogen-fixing bacteria. Each kind of legume will require a certain type of bacteria. For peas it’s usually R. leguminosarium. Beans need R. Phaseoli. Lasagna needs Chef Boyardeeoli. These bacteria are natural. I have seen or heard of some kinds of peas and beans that are pre-treated with inoculant and that sounds a little unnatural to me. It’s a bit like seeds that are pre-treated with chemicals to resist rotting or freezing or being good for you.
Using an inoculant that is right for your crop is a good idea. It’s not a mandatory, but it helps ensure that your plants will be boosted along with the right bacteria. These buggies exist in the soil, but sometimes not where you are planting or perhaps not in a big enough way. Using it is easy. You can make a slurry out of the powder by adding some water, drop your seeds in, coat them and plant them within 24 hours. If you’re curious, you’ll know it worked later by taking a look at the roots. If they’ve got gnarly looking white nodules, that’s your little nitrogen factories hard at work!
Incidentally, if you’re using an inoculant, don’t add your own nitrogen fertilzer to your plants. Peas and beans, like me, are gorgeous, productive and truly, truly lazy. If you give them an easy buffet of food, they won’t work to create their own.
Happy inoculanting!
Compostings 
Thanks for the timely post! I’ll be growing veggies for the first time in several years, including some peas and beans. I’ve always innoculated them in the past, but am pondering whether I should or not in my new bed.
Last fall I built the raised bed and layered it with several different kinds of compost, shredded leaves, and grass clippings. I’m think it will be very microbially active, and am not sure whether to innoculate the legumes. Probably wouldn’t hurt, right?
My pleasure Linda! Can’t wait to see how your veggies do this year. Keep me updated and I’ll be checking your blog… It definitely won’t hurt to use the inoculant. Once you’ve done it this year and grown some peas/beans you’ll be much more certain that next year you’ve got the microbes you need.
Thanks for the good advice! Having a veggie bed again after 5 years, tiny though it is, feels like being a beginner again. Hopefully it’s like riding a bike and I haven’t forgotten how to grow veggies!
Such a great mixture of humor and information. But I’ve got a question: I know inoculant makes sense if you’re planting where peas and beans have never been planted, because it’s possible the soil doesn’t contain the necessary bacteria. But if you’re planting where they have been grown recently (say within the past three years), what purpose does the inoculant serve? I actually used some last year for the first time, and didn’t notice any difference in sprout rate or vigor. Do you recommend inoculant even when planting in beds that already contain the required bacteria?
Thanks– Kate
A great question Kate! The inoculant won’t hurt in a situation where you’ve planted nitrogen fixers before, but it’s definitely not as helpful. Even in my small garden I try and rotate the locations of my beans (not so much my peas) to avoid disease and a hit of inoculant is very helpful. But once your legumes have done their thing, you can skip the inoculant without much impact. One of the reasons I like to plant and move my legumes is that they are about as good as it gets for building up the nitrogen in the soil. Whatever goes in their spot next planting will have some replenished soil to chew on.
Thank you! That’s what I suspected. But you’ve raised another question: I have long wondered whether the nitrogen fixed by legumes remains in the soil if the plants and roots are removed. As I understand it, little nitrogen-rich nodules form on the roots as the plants grow. If one just pulls up the plants, do enough of the nodules remain in the soil to enrich it, or has the benefit perhaps moved beyond the nodules, or what? (I haven’t gotten around to looking this one up yet, so if you know, please enlighten me!)
–Kate
It does remain in the soil. One of the nice cover crops to plant is clover. Clover does this some nitrogen thing. Once you’ve got these guys planted (legumes or clover) they supply the soil with the organisms that handle nitrogen and through one of a few processes keep the nitrogen in place within the soil. It’s not a perfect system as the nitrogen is consumed or rendered ungrabbable (just made that word up) or is pulled away over time, but it sticks around. Just as a nitrogen fertilizer that you’d add would keep some of it’s nitrogen within the soil so too does the nitrogen and organisms from the plants – they just do it better than fertilizer.
THANK YOU. Most sites just say that growing legumes adds nitrogen, and even those that explain the process of fixation don’t go into any details about how and to what extent the nitrogen remains if the plants are pulled rather than dug in. So again, Thank you.
–Kate
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I am establish my garden plot this week behind our apartment. Clearing of weeds and vines that was overgrown caused by flash flood of Cagayan de Oro River left from Tropical Sendong last December 16, 2011.
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