Corn
Heavy rains over the weekend resulted in localized ponding and flooding of cornfields, mainly in southern Ohio, and especially along river bottoms. If the ponding and flooding was of a limited duration, i.e. the water drained off quickly within a few hours, the injury resulting from the saturated soil conditions should be minimal.

The extent to which ponding injures corn is determined by several factors including:
1. Plant stage of development when ponding occurs
2. Duration of ponding
3. Air/soil temperatures

Prior to the six-leaf collar stage (as measured by visible leaf collars) or when the growing point is at or below the soil surface, corn can usually survive only two to four days of flooded conditions. Since most of the corn that’s been planted so far is not beyond the VE stage, it’s especially vulnerable to damage from ponding and saturated soil conditions. The oxygen supply in the soil is depleted after about 48 hours in a flooded soil. Without oxygen, the plant cannot perform critical life sustaining functions; e.g. nutrient and water uptake is impaired, root growth is inhibited, etc. If temperatures are warm during ponding (greater than 77° F) plants may not survive 24 hours. Cooler temperatures prolong survival. Once the growing point is above the water level, the likelihood for survival improves greatly.

Even if ponding doesn't kill plants outright, it may have a long-term negative impact on crop performance. Excess moisture during the early vegetative stages retards corn root development. As a result, plants may be subject to greater injury during a dry summer because root systems are not sufficiently developed to access available subsoil water. Ponding can also result in losses of nitrogen through denitrification and leaching. Even if water drains quickly, there is the possibility of surface crusts forming as the soil dries that can impact the emergence of recently planted crops. Growers should be prepared to rotary hoe to break up the crust to promote emergence.

For corn that’s emerged, check the color of the growing point to assess plant survival after ponding. It should be white to cream colored, while a darkening and/or softening usually precedes plant death. For corn not yet emerged, evaluate the appearance and integrity of seeds or seedlings that have yet to emerge (likely rotting if discolored and softening). Look for new leaf growth three to five days after water drains from the field.

Disease problems that become greater risks due to ponding and cool temperatures include pythium, corn smut and crazy top. Fungicideseed treatments will help reduce stand loss, but the duration of protection is limited to about 10-14 days. The fungus that causes crazy top depends on saturated soil conditions to infect corn seedlings. There is limited hybrid resistance to these diseases and predicting damage from corn smut and crazy top is difficult until later in the growing season.

For more information on ponding and flooding damage to corn, check out the article written by Bob Nielsen at Purdue University, Effects of Flooding or Ponding on Young Corn.

Soybeans
Water logging and poor aeration associated with localized floods and ponding can result in significant soybean yield reduction. The extent of ponding and flood damage to soybean is related to the temperature of the water, the amount of water motion and the duration of the flooding and ponding conditions. The following information is adapted from an article by Palle Pedersen, former soybean Extension specialist at Iowa State University.

Soybean prefers adequate soil oxygen for maximum productivity. Oxygen content of water is much lower than air therefore saturated soils and flooding reduces the amount of oxygen available to the plant. Research has shown that oxygen concentration can be close to zero after 24 hours in flooded soil, depending on water movement. Without oxygen, the plant cannot perform important functions like respiration, an important function of plant growth. Soybeans can generally survive 48-96 hours when completely submersed. The actual time frame is dependent upon air temperature, cloud cover, soil moisture conditions prior to flooding and rate of soil drainage.

Temperatures influence the speed of respiration so high temperatures will be more detrimental to soybean recovery since the faster the respiration is “running” the faster the oxygen is depleted and the plants then start rotting. Cool, cloudy days and cool, clear nights increase the survival of a flooded soybean crop.

Research from Minnesota shows that flooding for six days or more may result in a significant yield loss or loss of the entire crop. With temperatures in the 80s, soybean plants may only survive a few days. Ohio researchers found that plants in flooded fields are injured from a buildup of toxins and carbon dioxide, which is up to 50 times higher in flooded soils than in non-flooded soils. They concluded that plants are more injured from the buildup of carbon dioxide than from lack of oxygen. During emergence, soybean fields subjected to flooding and saturated soil conditions are at major risk from Phytophthora and Pythium damping-off.

Reference: Pedersen, P. 2008. Effect of flooding on emerged soybeans, Department of Agronomy, ISU