On average, navy and pinto beans produce less than 45 percent of their nitrogen needs through biological fixation with symbiotic rhizobia bacteria. There are few commercial inoculants available and they are not often applied, so farmers typically give their dry beans a springtime nitrogen fertilizer even though they are a legume crop.
But beans do bring some of their own lunch, which begs the question: how much N do they need to top them up?
According to a three-year study led by Kristen MacMillan at the University of Manitoba, from a dollars and cents standpoint, the optimum nitrogen scenario rate for dry beans is zero.
Read Also
Dry bean seeded acreage in Manitoba hits 20-year high
Dry bean acreage across all types reached around 207,000 acres in 2025, representing a significant increase from last year’s 182,000 acres.
A report on the study is appears in Pulse Beat: The Science Edition published by the Manitoba Pulse and Soybean Growers who also funded the research.
In dry beans, application of 70 lbs N per acre is common. Nitrogen uptake rates range from 3.9 to 4.7 lbs N required per cwt of seed. A 2,000-pounds-per-acre dry bean crop would need 78 to 90 lbs of N per acre. This nitrogen can come from a combination of residual soil N, biological fixation and fertilizer. The experiment evaluated N rates while a follow-up companion study has been evaluating inoculant options.
The study, conducted at Carman and Portage la Prairie from 2017 to 2019, applied five rates of N fertilizer (0, 35, 70, 105 and 140 lbs N/ac). Varieties in the study were Windbreaker pinto beans and T9905 navy beans. Nitrogen was applied as spring broadcast urea incorporated prior to planting. Non-inoculated dry beans were planted on 15-inch rows into tilled wheat stubble.
Residual N levels among site-years ranged from 23 to 56 lbs nitrogen per acre at the 0-24 inches in depth. The 2017 to 2019 growing seasons were dry and warm. The researchers reported that this lack of soil moisture may have influenced N dynamics, reducing mineralization, inhibiting nodule development and promoting root exploration to access deep nitrogen (that is, greater than 24 inches).
Nodulation was low overall, which is not surprising since beans were not inoculated and sites did not have recent dry bean history. At flowering, dry bean nodulation was evaluated on a scale of 0 to 4, with 4 being more than 20 nodules per plant and 0 being no nodules present. Pinto beans had slightly greater nodulation than navy beans.
Since the beans were not inoculated, any nodule development was a result of native rhizobia. As N fertilization increased, dry bean nodulation score decreased. Yield response to nitrogen rate did not vary with market class. Dry bean yield was only significantly increased over the zero N control at the greatest rate of 140 lbs N/ac, which boosted yield by 17 percent.
Yields of the other N rates were no different from the control. However, yield was maximized at the lowest rate of N applied which was 35 lbs N per acre. This was equivalent to 60-90 lbs total N per acre as a combination of N applied and soil residual N.
Which N rate was the most economical? Across multiple N cost and bean pricing scenarios, the return on investment was statistically similar for all rates of N. This indicates that the economic optimum practice in these experiments was not applying N at all.
Yield from the 0 N control was exceptional, averaging 2700 lbs per acre and resulting in 83 percent of maximum yield. Total N uptake in the zero N control was estimated to be 64 to169 lbs N/ac.
Residual soil N would have only provided 23 to 56 lbs N per acre, resulting in a deficit of eight to 131 lbs N/ac.
Post-harvest soil samples found residual N levels in the zero N control ranging from 20 to 60 lbs N per acre. This post-harvest surplus indicates the N needs of dry beans were met through a combination of biological N fixation, mineralization and accessing deep nitrogen sources.
Emerging guidelines from this research suggest full fertilization to meet N requirements may not be necessary in Manitoba and that biological N fixation is contributing to the N requirements of dry bean. In this study, non-fertilized, non-inoculated beans resulted in 83 percent of maximum yield.
Applying the highest rate of N maximized yield but was not economical. Applying N fertilizer at a rate of 35 lbs/ac to reach 70 lbs/ac of total N, including soil residual N, matched maximum yield without reducing nodulation.
Field-scale work conducted by the Manitoba Pulse and Soybean Growers On-Farm Network showed similar results, with good to excellent nodulation from native rhizobia alone.
“Even though inoculation is not common practice for dry beans in Manitoba, native soil rhizobia populations appear to be associating effectively with dry beans,” the researchers wrote. “This leads us to question how much biologically fixed N is contributing to dry bean N nutrition.”
Not applying nitrogen was the economical decision at four out of five on-farm trials conducted over three years. On-farm trials were established to determine the effects of different N fertilizer rates on dry bean nodulation and yield at the field-scale.
The research involved five trials testing a range of N fertilizer rates in non-inoculated dry bean fields. The selected fertilizer rates were specific to each farm, ranging from 0-140 lbs N per acre. Residual soil nitrate-N levels ranged from 20-70 lbs N per acre.
At one trial in 2020, yield actually went down at the greatest N rate (105 lbs N per acre), likely because the crop kept growing longer and maturity was delayed.
Despite these results, the field trials may have identified a sweet spot for supplemental N. The 2021 trials saw a significant yield increase of 151 lbs per acre when 70 lbs N per acre was applied compared to the zero N control. Chasing this yield boost can be worth it – if the cost of N is less than $1.00 per pound and bean prices are more than 50 cents per pound.
As always, there is no substitute for ground truth. The researchers recommend that farmers dig up dry bean plant samples and examine the roots for nodules. A pinkish-red colour indicates active N fixing activity and it’s the first step in making future N management decisions.
