Prolonged cold weather this spring has led some producers to wonder if soybean planting should be delayed. Over the next few days weather forecasters are even predicting a “polar vortex” with possible daytime highs of 3°C and nighttime temperatures of minus 3. Does this mean soybean planting should be delayed until things warm up? There is an old adage that “you want to see soybeans twice in one week”. The first time is in the drill and the second time is when they emerge. Older agronomy textbooks state that soybeans should not be seeded unless soil temperatures are above 10°C. There is no question that under ideal conditions soybeans will emerge in less than a week, but this requires soil temperatures of over 23°Celsius. There are two main risks of planting into cold soils:
- Imbibitional chilling injury (ICI). First, it should be noted that ICI of soybeans is rare. Almost exclusively, it occurs during the initial rapid uptake of water into the seed which takes place during the first 24 hours after planting. At low temperatures, the re-establishment of the biological membranes (a biochemical process requiring enzyme activity) cannot occur quickly enough, which reduces vigour. A number of factors determine the extent of imbibitional chilling injury. These include the rate of imbibition, seed quality, and cultivar. In general, more rapid uptake of chilled water leads to more extensive injury. The level of ICI is much lower in soybeans when water is imbibed slowly. This means seed planting into fit (relatively dry) soils is less vulnerable compared to seed that is “shocked” with saturated conditions. Therefore, cold soils at planting are actually not a significant issue for soybeans if conditions warm up after seeding. On the other hand, most agronomists do not recommend seeding just before a cold rain, to avoid the rapid uptake of cold water within the first 24 hours. We planted soybeans at various dates this spring into cold soils. On April 22, soil temperatures were 3°C and nighttime temperatures went to minus 3. After 2 weeks in the soil the seed has shown no signs of injury (see picture). It should be noted that no heavy rains occurred after seeding. Obviously, it’s too early to determine if any true injury has occurred, but at present the seeds look healthy. Field experience has shown that planting into cold soils with good quality seed is an acceptable practice. Planting just before a significant rainfall, however, can lead to ICI as well as other issues like crusting and damping off.
- The most significant risk to planting into cold soils is that the seed takes a long time to emerge. Soybean seed can take up to 4 weeks to emerge when soils are cold. The longer they are in the ground, the greater the risk from soil-borne diseases and insect feeding. However, a good seed treatment can go a long way to protect the seed. Also keep in mind that soybean seed will rot in the field if it’s sitting in saturated soils, but it can remain viable for a long time if soils are relatively dry.
So, can soybeans be planted into cold soils? The answer is yes! If soil conditions are fit, it is not necessary to wait for warmer soil temperatures. Once the calendar switches to May, soybeans should be seeded as long as the ground is fit, the seed is treated, and there is no imminent significant rainfall predicted. We have conducted early planting experiments for over 12 years and the benefits of early planting (even into cold soils) usually outweighs the risks of slow emergence.