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The Effect of Cool Weather on Soybeans

Many gardeners and farmers plant their soybean crops during spring, once temperatures consistently begin warming up. A late spring frost often proves troublesome to these crops, however, since cool temperatures can wreak havoc on soybean crops. Soybean seeds and seedlings exposed to cold water during freezing temperatures face stunted growth and, at worst, may die completely if exposed to these temperatures for an extended period of time.

Optimum and Lethal Temperatures
- Optimum soil temperature refers to the minimum soil temperature required for seeds to germinate and grow. For soybeans, the optimum temperature rests at 54 degrees Fahrenheit. Lethal temperature refers to the air temperature at which plants in the developmental stage start dying. The lethal temperature for soybeans averages about 28 degrees Fahrenheit. When the air temperature drops to the lethal level for several hours, a soybean seedling often takes significant damage and may even die. Soybean seeds can survive as long as they do not face this lethal temperature, but for best results, plant soybean seeds once the soil consistently reaches the optimum soil temperature for the season.
Freezing Injury
- Freezing and chilling injury occurs when seeds take in water before germination. This process, referred to as rehydration or imbibition, mainly affects soybean seeds by causing them to dry out. Cold water from melted frost, melted snow or freezing rain seeps into the ground and causes the cell membranes of the immature seeds to stiffen and rupture. This damages the radicles of the seed -- the part of the seedling that first emerges from the seed during germination -- and causes lower germination rates and delayed growth among soybean crops. This damage also leads to weakened seedlings more likely to fall victim to diseases and pests.
Pests
- The soybean aphid, one of the most common pests plaguing soybeans, lays its eggs around the leaf buds of young plants. These eggs experience the full brunt of weather and temperature conditions, including cool temperatures. In order to survive below freezing temperatures, insects must supercool, lowering the freezing point of their body fluids to prevent death. If internal ice crystals form inside the insect, it dies. Some summer pests with weaker supercooling abilities may die off at the first signs of frost, but the eggs of soybean aphids do not die until air temperatures reach negative 29 degrees Fahrenheit. By this point, soybean plants would take significant damage, too.
Disease
- Cold temperatures during the winter can help prepare the soil for soybean planting and prevent fungal diseases from striking. Asian soybean rust, a major disease affecting soybeans, particularly in southern states like Louisiana, has less chance of developing during winters with extended periods of below freezing temperatures. Kudzu, the plant primarily responsible for carrying this fungus, dies off at below freezing temperatures. While freezing temperatures do not completely destroy this disease, these conditions help cut down on its spread, since warm, humid conditions breed Asian soybean fungus best.

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