Plant & Soil: Impact of Saturated Soils on Soybeans

How do soybean seeds and seedlings fare when soils become saturated or water ponds on a field?

It isn’t uncommon to get a heavy rain after planting, followed by the upper soil profile becoming saturated and staying saturated because rains continue unabated and the soil profile is full. And water can pond on a field or spots in a field for seemingly days on end if rain continues. How long can a seed, seedling or small plant survive when soils become saturated and water ponds?

Soybean seeds and 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 will begin to die. Plants can survive under water longer under cool temperatures than warm temperatures because metabolic processes are slower.

When soils are saturated for a prolonged period, a lack of oxygen in the soil leads to anaerobic respiration and production of lactic acid and other products that damage tissue. The longer the soil is saturated, the greater the injury and yield reductions. During germination, saturated conditions for 48 hours can decrease germination by 30% to 70% depending on the timing of the saturation.

For plants that have emerged, a waterlogged condition that lasts for less than 48 hours causes little or no yield reduction. But beyond 48 hours, up to 3 or 4 days, plant loss and yield reduction can be significant. However, varieties differ in their ability to tolerant ponding. Intolerant varieties begin to show yield reductions after 2 days of saturation while more tolerant varieties can withstand up to 4 days of waterlogging. However, it may be difficult to identify a variety in a seed book based on its ability tolerate ponding.

Soil conditions play a role in the severity of injury from waterlogging as well. Coarser textured soils will drain more quickly, minimizing the duration of oxygen deprivation to the roots. Fine textured soils maintain saturation longer, increasing the chances of injury. Of course, compaction can also contribute to water logging.

And remember, fields that are saturated or flooded are at greater risk of water molds like Pythium when it is cool or diseases Phytophthora and Rhizoctonia when it is warm.

If a field or spot was submerged for several days evaluate the seedlings to see if they will survive. Once water recedes, split the stem and examine the growing point at the top of several plants. A healthy growing point will be firm and cream colored. A brownish tissue indicates the growing point was damaged. However, this symptom may not show up for a few days after the water recedes so don’t get in a big hurry and make a premature assessment; give the field a few days to dry and for the crop recover before doing damage assessment.

Agronomist Dr. Daniel Davidson posts blogs on agronomy-related topics. Feel free to contact him at djdavidson@agwrite.com.