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Each year thousands of Minnesota soybean acres succumb to stresses from excess water. Typically and primarily, this occurs in small, low-lying areas of farm fields that "drown out" relatively often. This spring, however, whole fields will be lost in northwest Minnesota. Eight to twelve inch rains that fell over large areas earlier this week have devastated farmers, business owners, and homeowners in more than 12 northwest Minnesota counties. In many areas the waters have not yet receded, and many farmers are seriously concerned about their crops.

Although soybean is generally rather sensitive to excess water, soybeans can survive underwater for a week or more under ideal conditions. Generally, soybeans tolerate 48 hours under water quite well, but flooding for 4 to 6 days can reduce stands, vigor, and eventually seed yield. Many factors determine how well a soybean crop will tolerate flooding.

First, a few important definitions must be adressed. Field flooding may occur by either stream flooding or by lowland flooding. The former results from creeks or rivers overflowing their banks onto a flood plain. In this case silt and sand is often deposited in the field and plant tissues are covered with soil material. The latter case results from water accumulation in depressional areas due to inadequate soil permeability or surface drainage. These flooding types can be further divided into soil waterlogging, where the soil is merely saturated, or complete submergence where the crop is entirely submerged.

The type of flooding occurring on a farm directly impacts the re-growth and yield potential of that field. Although stream flooding can quickly recede, the resulting silt deposits can burry the crop and cover leaf tissue with thick layers of soil. Without rains to wash silt from soybean leaves, recovery is greatly slowed. Fortunately, soil types prone to stream flooding are often course textured. These sandy soils drain well, allowing quick crop recovery or replanting. Flooding in depressional areas can last many days. Soils in depressional areas often retain water so that as the water disappears from the surface, the soil profile may remain waterlogged for several more days. Most of the flooded acreage in the Red River Valley, this year, fits into this category.

The most important factors that determine the fate of flooded soybean fields are: 1) duration of the flooding, 2) temperature during the flood, and 3) rate of drying after the flooding event, and 4) growth stage of the crop during the flood. Yield losses are seldom noted in fields flooded for 48 hours or less. Four days or more of flooding stresses the crop, delays the plants’ growth, and causes the plants be shorter with fewer nodes. Flooding for six days or more can depress yields significantly, while flooding for a week or more may result in significant (or entire) losses of stand.

Temperature during the flooding event plays a large role in determining the fate of a submerged soybean field. Higher temperatures cause the soybean plant to more quickly deplete its stored energy. Additionally, soybean plants appear to be very sensitive to high CO₂ levels in the soil. Higher temperatures cause plants and soil microbes to respire at high rates that quickly deplete the water of oxygen and increase CO₂ levels. Cool, cloudy days and cool clear nights greatly increase the survivability of a submerged soybean crop.

The rate of field drying after a flooding event also plays a large role in soybean survival. Researchers have found yield reductions to be much greater on flooded clay soils than on silt loam soils when flooded for the same period. These researchers reported a 1.8-bushel per acre yield loss per day of flooding on a clay soil and 0.8-bushel yield loss per day on a silt loam soil. These losses were reported for flooding at the V4 growth stage. Flooding is more detrimental to soybean yields in the early reproductive phases of development. Flooding at the R1 stage caused losses of 2.3 and 1.5 bushel per acre yield losses on clay and silt loam soils. Larger yield losses would be expected in soybeans at the R3 to R5 stages.

Some of the main indirect effects of flooding on soybean yields are; 1) root diseases, 2) N deficiency, 3) and other plant nutrient imbalances. Caring for recuperating soybean stands should focus on reducing stresses to the plant where possible. Cultivation should be considered to increase soil aeration, and post emergence herbicides should be applied to conventional soybean crops judiciously. Herbicide stress should be minimized and postponed where possible.

References:

Sullivan, M., T VanTooai, N. Fausey, J. Beuerlein, J. Parkinson, A Soboyejo. 2001. Evaluating on-farm flooding impacts on soybean. Crop Sci. 41:93-100

Scott, H.D., J. DeAngulo, M.B. Daniels, L.S. Wood. 1989. Flood duration effects on soybean growth and yield. Crop Sci. 81:631-636