Louisiana: Early-Season Soybeans’ Response to Flooding

Flooded soybean crop. ©Debra L Ferguson Stock Photography

With 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 soybeans (early vegetative) to flooding depends on several factors, including 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 depends on the type of flooding.

There are two types of flooding:

1. Waterlogged soils. Water only covers the roots, but plants are emerged (Figure 1).

2. Submerged plants. Water covers the entire plant (Figure 2).

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Figure 1. Waterlogged soil.

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Figure 2. Submerged crop at the end of a field.

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

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

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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, which is detrimental to the plant.

Researchers have found the oxygen concentration can be near zero after 24 hours of flooded conditions, depending on the environment and water movement. More information is available on the Iowa State University Extension and Outreach website.

The likelihood of soybeans surviving flooded conditions and sustaining 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 reduced by 30 to 70%, depending on the time of the flooding event. More information on the impact of flooding on germination can be found on the University of Nebraska-Lincoln’s CropWatch site.

Assessment and management

Any practice to facilitate water movement off the field without damaging the crop would be good (clear obstructions in water furrows, ditches, culverts, etc.). Scout fields four to seven days after the waters have receded. Look for signs of good growth such as new buds on stems and good color. Determine plant populations to decide if replanting is necessary.

Information on varieties, seed rates, recommended planting dates and late planting can be found in “2019 Soybean Variety Yields and Production Practices,” a guide available on the LSU AgCenter website.


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