Fields in some areas of Kansas have recently been flooded or waterlogged. Some of these fields had only recently been planted to soybeans, and producers may be concerned about the effects of the wet conditions on these fields, said Kraig Roozeboom, K-State Research and Extension crop production specialist.
It is hard to know exactly how a given soybean field will reach the point of flooded or waterlogged soil conditions, he said. Many factors are involved.
“Injury can depend on variety, growth stage, duration of waterlogging, soil texture, fertility levels, and diseases present. Interactions of these factors make it hard to predict the effect on soybeans,” Roozeboom said.
Research examining the influence of growth stage on the degree of injury from waterlogged soils has provided mixed results, but some general trends have become evident, the K-State agronomist explained.
•Germination. Saturated conditions during germination can reduce successful germination by up to 40 percent and can inhibit seedling growth. Seeds that are further in the germination process at the time of saturation sustain more injury.
•Vegetative growth stages. Excess water during vegetative stages usually causes less injury than waterlogging during the reproductive and grain filling stages. Short-term waterlogging (two to three days) at the V2 to V4 stages can cause yield reductions of 0 to 50 percent, depending on soil texture, variety, and subsequent weather. Yield reductions from waterlogging during the early vegetative stages have been attributed to reduced plant population and shorter plants with reduced branching and fewer pods per plant.
The longer the soil is saturated, the greater the injury, mortality, and consequent yield reductions, Roozeboom added.
“During germination, saturated conditions for 48 hours can decrease germination by 30 to 70 percent depending on the timing of the saturation,” he said. “This is nearly twice the yield decrease resulting from durations of 24 hours or less.”
For plants that have emerged, a waterlogged condition that lasts for less than two days often causes little or no noticeable yield reduction, he added.
Intolerant varieties begin to show yield reductions after two days of saturation, but tolerant varieties can withstand up to four days with little reduction in yield, he said.
As the duration of soil saturation increases, researchers have documented greater reductions in population, height, pods per plant, yield, and leaf tissue nitrogen, Roozeboom said.
Nitrogen-fixing Bradyrhizobia species need oxygen to survive in the soil, so flooding can potentially affect their abundance in the soil, said Chuck Rice, K-State Research and Extension soil microbiologist.
“Saturated conditions for less than seven days likely will not impact Bradyrhizobia populations. If saturation persists for seven to 10 days or longer, populations may decrease, potentially delaying or decreasing nodulation or causing uneven nodulation throughout the field,” Rice explained.
Higher levels of soil nitrates can minimize injury from flooding, but fertilizing after the soil has dried is generally not helpful, said Dorivar Ruiz Diaz, K-State Research and Extension nutrient management specialist.
“Most Kansas soils mineralize enough nitrogen during the season to maintain the young soybean plants until nitrogen fixation becomes well established. Fertilizer nitrogen will ultimately inhibit nodule nitrogen fixation,” Ruiz Diaz said.
Fields that are flooded, or are at or above the water-holding capacity of the soil, will be more likely to develop root rot problems, added Doug Jardine, K-State Research and Extension plant pathology specialist.
“Flooding accompanied by cooler temperatures would be favorable to Pythium root rot whereas as warmer temperatures would favor Phytophthora and Rhizoctonia root rots. Whether Phytophthora root rot develops often depends on the tolerance or resistance of the variety used,” he said.
“If the flooding occurs beyond the first week or two after emergence, any seed treatment fungicides that may have been used will no longer be effective,” Jardine said.