Nitrogen for Dry Beans
Dry beans are a legume capable of symbiotic nitrogen fixation. However, the relationship of the plant and Rhizobia bacteria in dry bean is relatively weak.
Nitrogen is a constituent of protein. Both the harvested bean and the crop residues left in the fields are high in protein. Rhizobial nitrogen fixation may not be adequate to provide the nitrogen needs of high yielding bean crops.
As an example, an 1,800-pound bean crop will leave from 1 to 2 tons of residue in the field. Calculations using 6.25 pounds of protein per pound of nitrogen, 22 percent protein content for the beans and 12 percent for the straw show the above ground portion of the plant will contain 120 pounds of nitrogen Rhizobial nitrogen fixation seldom exceeds 40 pounds per acre when the plant is stressed at any point during the vegetative growth period. Additional nitrogen for the crop must be obtained from organic matter released or added fertilizer.
Organic matter nitrogen released to the crop will average about 15 pounds/growing season month in North Dakota. Maximum nitrogen release will occur on deep, high organic matter level soils that receive timely growing season rainfall.
Current recommendations suggest that if soil test levels of NO3-N are 50 lb/acre or less, and yield goals are below 2,000 lb/acre, then relying on inoculation may be successful. If yield expectations are higher than 2,000 lb/acre or soil test levels are greater than 50 lb/acre, then inoculation is usually not recommended as the sole N source. High N levels suppress symbiosis, and adequate N from Rhizobium to support high yields in unlikely because the Rhizobium can slough off plant roots if the soil conditions are too wet, too dry or too hot. Therefore, fertilizer N is recommended at 0.05 X Yield Goal.
Some dry bean producers are reluctant to apply N because of greater vegetative growth which increases the susceptibility to white mold. However, studies have shown that regardless of the source of N, if the weather will support white mold, then white mold is possible. It is then suggested that for high yields, varieties with growth characters more resistant to white mold are grown and applications of fungicides be applied to decrease the effect of white mold.