Early season soybean response to flooding

• By Boyd Padgett •

wet fieldsWith the recent rains, I wanted to put out some information on the impact of flooding on soybean germination, as well as the impact on young plants.

Young plants: The response of young soybean (early vegetative) to flooding is dependent on several factors including but not limited to temperature, cloud cover, soil conditions prior to flooding (dry or saturated), soil type, duration of the flood and variety. In addition to these factors, plant response will depend on the type of flooding.

There are two types of flooding:

1. Waterlogged soils: water only covering roots, but plants are emerged
2. Submerged plants: water covers the entire plant

Waterlogged soils are less damaging to plants than when they are submerged. In general, soybean can tolerate submersed conditions for 48 to 96 hours without substantial injury. The amount of time plants can tolerate submergence will depend on the environmental conditions present at the time of flooding, as well as the conditions following the flood.

If the soil saturated (waterlogged) prior to flooding, then plant death will occur faster compared to flooding on dry soils.

Flooding reduces the oxygen concentration in the soil. Oxygen is essential to the plant for normal development (respiration, water uptake, root development, and other functions).

This reduction results in the buildup of toxins and carbon dioxide that are detrimental to the plant. Based on research, oxygen concentration can be near zero after 24 hours of flooded conditions depending on the environment and water movement (source: https://crops.extension.iastate.edu/cropnews/2008/06/effect-flooding-emerged-soybeans).

The likelihood soybean will survive flooded conditions and sustain minimal damage is best when days are cool (not cold) and cloudy, and nights are clear and cool during and after the flood event. Cold conditions could predispose plants to disease.

Conversely, sunny conditions and high temperatures will increase the respiration rate of the plant and demand for oxygen in the soil (which is limited); therefore, more injury is likely. Additionally, debris (soil and other residues) left on the plant after the waters have subsided can reduce the photosynthetic capacity of the plant

Germination: Flooding can adversely impact germination. When soils are saturated for 48 hours, germination can be reduce by 30-70 percent depending on time of flooding event. More information on the impact of flooding on germination can be found at: (https://cropwatch.unl.edu/early-season-flooding-and-soybean-survival).

Assessment and management: Any practice to help water movement off the field without damaging the crop would be good (clear obstructions in water furrows, ditches, culverts, etc.). Scout fields 4 to 7 days after the waters have receded. Look for signs of good growth (i.e. new buds on stems, good color).

Determine plant populations to figure out if replanting is necessary. Information on varieties, seed rates, recommended planting dates and late planting can be accessed at: https://www.lsuagcenter.com/articles/page1544459344263.

Dr. Boyd Padgett is Louisiana State University AgCenter interim Extension soybean specialist. He may be reached at BPadgett@agcenter.lsu.edu.

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