First, there are no canned answers for this problem. When flood waters cover fields, oxygen concentration can be reduced to zero in 24 hours. Without oxygen, soybean plants cannot perform essential processes (respiration, water uptake, root growth and nodulation) to remain alive. According to the literature, soybeans can survive flooded conditions up to 96 hours. However, this is dependent on a number of factors like ambient and water temperature, standing verses moving water, soil type and crop growth stage.
Temperature: High temperatures result in increased respiration which reduces the time the plant can survive the flooded condition. Under these conditions, the plant requires more oxygen. Since the oxygen levels are depleted, the plant will die sooner compared to survival during cooler temperatures. Ideally, if flooding does occur, cool temperatures (ambient and water) coupled with cloudy weather will allow for the best opportunity for recovery.
Water standing vs. moving: Flood waters are bad; however, moderate water movement will allow some oxygen to get to the plant’s root system and buy a little more time for survival.
Soil type: The faster the soil dries, the less likely the damage will be. Flooding is potentially worse on clay soils because these soils dry slower than other soil types.
Growth Stage: Certainly, flood waters can negatively impact soybeans at any growth stage. However, the most susceptible growth stage is R1-R2. Generally speaking, larger plants need more oxygen to survive.
Something to keep in mind… In addition to retarding growth and reducing yield potential, flood waters and saturated soils will increase the risk to soil born the pathogens Pythium and Phytophthora. These fungi are referred to as ‘water-loving’ fungi.