So you got your beans planted—then it rained and rained and now you’re wondering how your crop “stands?” Or how soybean seeds and emerging seedlings fare when soils become saturated or ponded?
It isn’t uncommon to get heavy rain or frequent rainfall events after planting. The soil profile becomes temporarily saturated for several days—water can actually pond on a field surface for as much as a week if rain continues. So just how long can a seed or your seedling plant survive when soils become saturated?
Long periods of rain or large rain events can saturate the soil, excluding oxygen, resulting in anaerobic conditions. Seed viability is impacted after several days of saturation—especially when soil temperatures are about 50° F.
During the germination phase, uniform soybean emergence may be reduced when fields are saturated with water from heavy rains and poor drainage. Saturation and its duration will never be consistent across a field. Some areas will drain because of topography while other areas will drain because of porosity and presence of tile lines. Saturation is not necessarily the issue—the duration of saturation is.
A study was conducted at Iowa State University 15 years ago that looked at the impact on germination of flooding immediately after planting. The scientists reported that soybean seeds are most vulnerable to flooding during the first 72 hours of the early germination process (absorbing water and beginning growth). Seedling damage was greater the longer flooding occurred. The longer the soil is saturated, two or three days or more, the greater the injury and yield reduction. Depending on the timing of the saturation during germination, saturated conditions for 48 hours can decrease germination by 30% to 70%.
Established seedlings are fairly tolerant of saturated soils but submerged plants aren’t so lucky. If soybean plants remain submerged for longer than 48 hours, some begin to die. Plants can survive under water longer under cool temperatures than warm temperatures because metabolism is slower. For plants that have emerged, a waterlogged condition that lasts for less than 48 hours causes little damage. But if it’s greater than three days, plant loss and yield reduction can be significant.
Soil conditions naturally play a role in the extent of water logging. Coarser textured soils will drain more quickly, minimizing the duration of oxygen deprivation. Fine textured soils remain saturated longer, increasing chances of injury. Of course compaction layers or tight soils (greater bulk densities) contribute to water logging.
And fields that are saturated are at greater risk of water molds like Pythium when it is cool or diseases like Phytophthora and Rhizoctonia when it is warm.
Once water recedes, split the stem and examine the growing point at the top of the plant. A healthy growing point will be firm and cream colored. A brownish color indicates the growing point is damaged and the plant won’t survive. However, this symptom may not show up for a few days after the water recedes, so don’t rush out and make a premature assessment. Give the field a few days to dry and let the crop recover before doing damage assessment.
Agronomist Dr. Daniel Davidson posts blogs on agronomy-related topics. Feel free to contact him at email@example.com.