Ponding Impacts on Soybean Growth & Development

                Spring ponding, flooding, and saturated soil conditions can be very problematic for soybean seeds and young plants. There are several factors that influence how much damage can result for wet conditions such as temperature, duration, and stage of soybean growth.

EFFECT ON THE PLANT

Flooding, ponding, and saturated soils can very quickly result in low oxygen availability (anaerobic conditions). Within 24 to 48 hours of saturated conditions, oxygen levels in the soil can be reduced almost to zero which will reduce or even stop critical plant functions such as respiration, nutrient uptake, and water uptake. Cool, cloudy days with cool, clear nights increase the survivability of flooded and ponded soybean plants. High temperatures increase the amount of respiration that occurs and therefore depletes oxygen levels faster. Research from Ohio found flooded fields result in plant injury due to a buildup of toxins and carbon dioxide which is 50 times higher in flooded versus non-flooded soils. This work concluded that carbon dioxide levels were more detrimental than low levels of oxygen. Flooding and ponding of young plants can result in leave surfaces being covered with soil that can greatly reduce photosynthetic processes. If silt deposition is too great or the silt is not ‘washed’ off the leaves by subsequent rainfall, survival is greatly reduced.

Short-term saturation of the seed following water inbibition can result in a 20% reduction in germination while longer-tern saturation can reduce germination by up to 70% (Table 1). Seeds soaked for 48 hours at 77^oF had lost 60% germination compared to the control whereas at 59^oF germination was only reduced by 39%. The length of saturation at cooler temperatures had little impact whereas at higher temperatures shorter durations of saturation reduced germination loss.

After emergence, impact on the plant depends on how tall the plant is and what the depth of water is. Short-term saturated soil of 2-3 days can result in up to 50% yield loss. Plant survival is much more likely when a portion of the plant remains above the water level. Saturated soils will reduce the ability for nodulation by Bradyrhizobia to occur. This is especially of concern if saturated conditions last longer than 7-10 days.

Table 1. Survival of flooded soybean plants. Adapted from Crop Watch Newsletter (University of Nebraska), 13 May 2014.

MANAGEMENT OPTIONS

It is recommended to wait about three to five days after flood waters recede before assessing the damage done to the soybean crop. Examine the apical meristem and axillary buds (soybean growing points). If the growing points show no signs of growth, assessing a surviving plant density will be essential to determine next steps. A decision to replant or thicken up the existing stand should be made only after assessing stands and considering the economics of replanting. Generally a soybean plant density of greater then 70,000 plants per acre will have higher yield potential then a full stand planted too late. If stands are extremely poor, replanting would be a good option; although, be aware that conditions can quickly change with several good days of weather.

Consider that soybean that have had flooding, ponding, or saturated conditions may be susceptible to Pythium seedling disease (cooler weather) or Phytophthora or Rhizoctonia root rots (warmer weather). Understand both the weather conditions and variety characteristics to determine how much risk seedling diseases may be. If saturated conditions occur more than two weeks following planting, be aware that seed treatment effective will be greatly reduced. In situations were poor nodulation has occurred, application of supplemental nitrogen is not recommended because it is rarely economical to apply enough nitrogen to fully meet the need of a soybean crop. For crusting on soil surfaces, cultivating between rows or using a rotary hoe should be done as soon as possible to allow water and oxygen to get to the roots, or allowing new seedling to emerge.

SOURCES

Elmore, R and J Specht. 13 May 2014. Early-season flooding and soybean survival. CropWatch, University of Nebraska – Lincoln, Lincoln, NE. https://cropwatch.unl.edu/early-season-flooding-and-soybean-survival

Pasley, H, I Huber, M Castellano, and S Archontoulis. 2020. Modeling flood-induced stress in soybeans. Frontiers in Plant Science. DOI: 10.3389/fpls.2020.00062

Pedersen, P. 1 June 2008. Effect of flooding on emerged soybeans. ICM News, Iowa State University, Ames, IA. https://crops.extension.iastate.edu/cropnews/2008/06/effect-flooding-emerged-soybeans

Wuebker, E, R Mullen, and K Koehler. 2001. Flooding and temperature effects on soybean germination. Crop Science. 41:1857-1861.