Nitrogen Fertilization
Dry beans are relatively poor N-fixers, producing less than 45% of their N requirement. Nitrogen uptake rates in dry beans range from 3.9-4.7 lbs N/cwt of seed, meaning a 2000 lbs/ac dry bean crop would require 78-90 lbs N/ac. This nitrogen may come from a combination of residual soil N, biological N fixation and N fertilizer.
As fertilizer nitrogen rate applied increases, nodulation decreases since plants become ‘lazy’ and rely on soil nitrogen alone. In 4 out of 6 on-farm trials, good to excellent nodulation was found to occur when N rates applied were below 70 lbs N/ac. In small-plot trials, nodulation has generally been less than with on-farm trials.
On-farm trial yield responses to different nitrogen rates have been inconsistent. At four trials, there was no yield response to different N rates and at two trials there were opposite responses. At one, the highest rate of N (140 lbs N/ac applied) reduced yield, while at another, the highest rate of N applied (70 lbs N/ac) increased yield. In the first scenario, yield was reduced due to prolonged vegetative growth and delayed maturity. The second scenario occurred in a drought year, where the contribution of mineralized-N was expected to have played less of a role, meaning the crop relied more heavily on applied fertilizer-N.
In small-plot trials at Carman and Portage (2017-2019), yield was increased with the highest rate of N fertilizer (140 lbs N/ac applied). When considering the return on investment, it was statistically the same for all rates of N application, meaning the economic optimum rate was to not apply any N fertilizer at all.
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