Soybeans and Hot Weather
Soybeans, like many other plant species, cannot translocate enough water to keep up with the evapotranspiration demand that temperatures over about 95 degrees creates. So with temperatures over 95, soybeans will wilt in the heat of the day regardless of how much water is available to them. They close their stomates to dramatically reduce water loss from the leaf, but this also closes the stomates to carbon dioxide getting into the leaf, so little or no photosynthesis takes place while the stomates are closed.
If water is available to the soybean roots, the plants will tend to recover significantly when temperatures drop down into the 80s, and may well look pretty normal before nightfall. If water is not available to the roots, however, it will take them longer to recover. It's possible to kill soybean plants with a combination of little moisture in the root zone with high daytime temperatures, but soybeans can tolerate more of this situation than many other plant species can.
If soybeans have not started blooming, there is probably little long term effect other than having lost a few potential growing days. If soybean are blooming, or past blooming, they will tend to abort whatever reproductive structure is being developed at the time (e.g. flowers, pods, or seeds), which won't necessarily be too serious, if they have time to put on more flowers/pods/seeds.
Whether to plant with the current temperatures or not is not an easy call. If the soil has enough moisture that I expect to be able to get a crop up, I would be concerned about the soil temperature, and the expected daytime highs several days out. I'd be inclined to delay planting if soil temperatures are in the 90s at 2 inches depth, or if I expect the daytime highs to be above 95 when the plants emerge. If soil temperatures are in the 80s or below, and the daytime highs when the soybeans emerge are expected to be below 95, I'd probably go ahead and plant.
If water is available to the soybean roots, the plants will tend to recover significantly when temperatures drop down into the 80s, and may well look pretty normal before nightfall. If water is not available to the roots, however, it will take them longer to recover. It's possible to kill soybean plants with a combination of little moisture in the root zone with high daytime temperatures, but soybeans can tolerate more of this situation than many other plant species can.
If soybeans have not started blooming, there is probably little long term effect other than having lost a few potential growing days. If soybean are blooming, or past blooming, they will tend to abort whatever reproductive structure is being developed at the time (e.g. flowers, pods, or seeds), which won't necessarily be too serious, if they have time to put on more flowers/pods/seeds.
Whether to plant with the current temperatures or not is not an easy call. If the soil has enough moisture that I expect to be able to get a crop up, I would be concerned about the soil temperature, and the expected daytime highs several days out. I'd be inclined to delay planting if soil temperatures are in the 90s at 2 inches depth, or if I expect the daytime highs to be above 95 when the plants emerge. If soil temperatures are in the 80s or below, and the daytime highs when the soybeans emerge are expected to be below 95, I'd probably go ahead and plant.
-- Dr. Jim Dunphy, Crop Science Extension Specialist
