Once soybeans are in the soil, cold stress is likely to occur in soybeans if the soil temperature was below 40 degrees F. during the first 24 hours after planting.
The germination stage of soybeans consists first of a very fast uptake of water (imbibitional phase) followed by a much slower uptake of water (osmotic phase). Chilling during the first phase can cause severe problems because the imbibed water is needed to rehydrate the cotyledons and embryo to the point that cell membranes become functional. Cold temperatures interfere with proper hydration of those membranes. The imbibitional phase is typically not very long (less than 24 hours) and can occur with relatively little soil moisture since the seed is dry.
Thus, getting a cold rain 0-24 hours after planting can lead to chilling injury in soybean and lower stands.
A study by UNL researcher W.J. Bramlage et al. showed that when the soybean seed coat was removed, imbibition injury occurred within 30 minutes. With a seed coat, imbibition is slower and a longer exposure would be needed before chilling injury occurs.
A key point to consider is that chilling injury is likely to be greater if soil temperatures were cold (less than 40° F) at planting rather than becoming cold 24 or more hours after sowing. The longer the seed is in the ground at warm soil temperatures before cold temperatures occur, the less likelihood there is of chilling injury. The bottom line is: farmers can plant soybeans if they think the soil temperatures won’t get cold (less than 40°F) for at least 24 hours. If farmers plant two or more days before cold rain, there should be no imbibitional injury due to cold temperature.
During the second phase of germination, the fully functional membranes (after imbibitional hydration) create an osmotic situation in which water diffuses into the living cells. Osmotic water uptake is slow with cold temperatures. Chilling during this phase causes little direct injury to the germinating seedling. Cold temperatures will, however, slow emergence.
More important than cold soil temperatures after imbibitional water uptake is the soil moisture content. Cold soil delays the germination to emergence process, but cold soil plus soggy soil conditions of some duration can substantively reduce soybean emergence. Why?
Soil-borne pathogens love soggy soil. Since the period of seedling germination to emergence takes longer due to the cold, those pathogens will have more time to infect the seedling cotyledons and access their carbohydrate, protein, and lipid reserves. The seedling needs those cotyledon reserves to live on until the unifoliolate leaves start photosynthesis and form more carbohydrates. This is why fungicide seed treatments are crucial if planting soybeans in April or early May and in cold, wet conditions.
Ultimately, it is soggy soil with cold temperatures that kill soybean germination.