Soybeans can be at a variety of growth stages during mid-season rainstorms. Partially submerged plants have an increased chance of survival compared to completely submerged soybean plants. Water-logging, where roots are flooded without plant submergence, is also common. Several factors influence potential soybean damage:
Yield loss results from reduced root growth, shoot growth, nodulation, nitrogen (N) fixation, photosynthesis, biomass accumulation, stomatal conductance, and plant death from disease and physiological stress.
Nitrogen. Soybean growth and development depends on N availability as the nutrient is in high demand, especially during seed development. The N2 fixation process is sensitive to lack of oxygen and flooding can lead to reduced fixation. Fields with low soil organic matter experiencing dry conditions in August and September are expected to have a response to mid-season flooding. Regions in the United States with mid-season flooding and insufficient rainfall later in the season have developed low protein crops. Actual yield loss would be difficult to calculate; however, low protein levels are an indication of yield loss in fields.1
In summary, fields should be evaluated after water recedes for plant survival and disease. Fields with adequate stand counts may achieve near normal yield potential. Disease pressure and soybean product tolerance influences the health of plants for the remainder of the growing season.
1Stahl, L., Coulter, J., Naeve, S., and Fernandez, F. 2018. Flooded fields and saturated conditions impact crops. University of Minnesota Extension http://blog-crop-news.extension.umn.edu/2018/07/flooded-fields-and-saturated-conditions.html.
2Conley, S. 2018. Assessing flood damage to soybean. University of Wisconsin. Integrated Pest and Crop Management. 3 Jean, M., Thelen, K., and Steinke, K. 2017. Managing corn and soybean fields submerged by recent, heavy rains. Michigan State University.
Web sources verified 8/18/18