How do soybeans respond to prolonged saturated soil conditions? When soils are saturated for a prolonged period of time, a lack of oxygen in the roots can lead to the accumulation of lactic acid and other products of anaerobic respiration. This is the underlying cause of damage to plants in waterlogged soils where only the roots are flooded.

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 how a given soybean field will react to waterlogged soils.

Growth stage factors

Research examining the influence of growth stage on the degree of injury from waterlogged soils has provided mixed results.

* 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 (2 to 3 days) at V2 to V4 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.

Duration of soil saturation

The longer the soil is saturated, the greater the injury, mortality, and consequent yield reductions. During germination, saturated conditions for 48 hours can decrease germination by 30 to 70 percent depending on the timing of the saturation -- 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. Intolerant varieties begin to show yield reductions after 2 days of saturation, but tolerant varieties can withstand up to 4 days of waterlogging with little reduction in yield. As the duration of soil saturation increases, researchers have documented greater reductions in population, height, pods per plant, yield, and leaf tissue nitrogen.

Nitrogen-fixing bacteria

Nitrogen-fixing Bradyrhizobia species need oxygen to survive in the soil, so flooding can potentially affect their abundance in the soil. Saturated conditions for less than 7 days likely will not impact Bradyrhizobia populations. If saturation persists for 7 to 10 days or longer, populations may decrease, potentially delaying or decreasing nodulation or causing uneven nodulation throughout the field.

Other factors

Soil conditions play a role in the severity of injury from waterlogging as well. Coarser textured soils will drain more quickly, minimizing the duration of oxygen deprivation to the roots. Fine-textured soils maintain saturation longer, increasing the chances of injury.

Higher levels of soil nitrates can minimize injury from flooding, but fertilizing after the soil has dried is generally not helpful. 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.

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. 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. 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.