Edible Bean School: Giving beans more credit for fixing nitrogen


Edible beans are usually viewed as one of the weaker performers among legume crops when it comes to fixing nitrogen, but new research shows they likely deserve more credit than they’ve been given.

“We’ve really regarded them as non-legumes and standard practice in most areas has been to fertilize to their full nitrogen requirements,” notes Kristen MacMillan, research agronomist with the University of Manitoba, in this Edible Bean School episode focusing on her research looking at how beans respond to different N rates and new inoculant products.

In the first study, MacMillan compared N fertilizer rates from zero to 140 pounds per acre in pinto and navy beans over three years.

“We only found a yield response to the high rate of nitrogen — 140 pounds. And when we applied the economics to that there was actually no difference. There was no response economically to nitrogen fertilizer, which was really surprising because here we’ve been treating dry beans as a non-legume,” she explains.

So where are the plants that received no nitrogen fertilizer getting the N to produce a statistically similar yield to those that received a regular rate?

“One of the biggest observations that I’ve made in the in the past little while has been that we are seeing nodulation in dry beans in non-fertilized, non-inoculated beans, and not just nodulation, but actually fairly good nodulation,” says MacMillan. “And so it begs the question, are we missing out on some of the credit that we could be giving to their nitrogen fixation capability? And with that question, also comes, do we need to inoculate dry beans?”

Kristen MacMillan discusses her findings when it comes to nitrogen rates and inoculants in edible beans at the Manitoba Pulse and Soybean Growers’ 2022 field tour at Agriculture and Agri-Food Canada’s Morden Research Centre (article continues below video):

The rhizobia that form N-fixing nodules in edible beans are native to Prairie soils, but several inoculant products have also become available in the last few years. MacMillan is currently researching whether these inoculants result in improved nodulation and yield.

“I’m looking at several inoculant products that have become available in four market classes — pinto, navy, black beans, and kidney beans across — across three sites. This will be the fourth year that I’ve been doing that. And so what I’m asking is, are the inoculant products increasing nodulation in our beans? And is it consistent across market classes and environments?”

Ultimately, she expects there will be changes to the nitrogen recommendations for beans to account for higher N-fixation than previously thought.

“What we’re trying to do is understand the nitrogen budget of dry beans. If we have a 2000 pound bean crop, that crop needs about 90 pounds of nitrogen. So if we have 30 to 40 pounds residual nitrogen, where is the 40 to 60 pounds coming from? Normally, we’ve been adding supplemental nitrogen, but now the question is, can we be meeting that through nitrogen fixation, and other processes like mineralization and deep nitrogen?”

She recommends digging up those roots and checking for nodulation to try to understand how much N-fixation might be happening in a specific field, and if possible, try comparing zero N treatment strips in the field.

“It really comes down to your soil, environment, scouting, checking for nodules, making sure that they’re pink inside, that they’re working effectively, looking at your soil test and just asking yourself, how are you coming up with that nitrogen budget for your for your bean crop next year?”

Wake up with RealAgriculture

Subscribe to our daily newsletters to keep you up-to-date with our latest coverage every morning.

Wake up with RealAgriculture

Please register to read and comment.


Register for a RealAgriculture account to manage your Shortcut menu instead of the default.