Soybean seeds experience their own form of hypothermia just like mammals do. Cold soils and cold water impact the seed more than you might think.
A technical seed biologist would write: “Soybean germination 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 and the physiological process starts. Cold temperatures can interfere with proper hydration of those membranes.”
This imbibitional phase isn’t long and generally less than 24 hours. The pull for water is so strong initially that it requires relatively little soil moisture around the seed. Thus, a cold rain just after planting can bring extremely cold water into the seed, causing chilling injury and seed mortality.
Chilling injury is greater if soil temperatures are cold and less than 50 F at planting, compared with weather turning cold one, two or three days later. Chilling injury occurs when temperatures drop below 50 F within 24 hours of planting. The longer the seed is in the soil at above 50 F soil temperature the less chance there is for chilling injury. Remember it is the uptake of cold water or water when the soil is cold during the first 24 hours that causes chilling injury.
What is chilling injury? Cell walls are comprised of fluid-like and flexible lipids. When they super chill, their cell wall membranes begin to disintegrate, leading to cellular death and eventually seed death.
During the second osmotic phase, cell walls are fluid and functional and uptake depends on thermodynamic laws. In other words, osmotic water uptake is slow with cold temperatures and chilling injury risk is minimal. Cold temperatures will slow emergence which always puts the seed and seedling at risk.
Saturated soil that is cold but not super cold reduces germination rate and allows more time for pathogens to infect. Growers should apply seed treatments when planting in April or early May to protect the seed during that crucial germination period.
Be aware of chilling risk. Always check the forecast of temperature and rainfall when you plant. The first 24- and 48-hour periods after planting is the most critical, especially when soil temperatures are in low 50s and can drop into the high 40s.
Plant soybeans if soil temperatures won’t drop below 50 F for at least the first 24 hours. If you planted two or more days before a cold rain, chilling injury risk will be minimal.
Soybean agronomist Daniel Davidson, Ph.D. posts blogs on agronomy-related topics. Feel free to contact him at firstname.lastname@example.org or ring him at 402-649-5919.