How long can crop plants survive saturated or flooded soils?

Recent, significant rain events have filled drainage ditches both within and along the outside of fields. Some of this flood water has backed up further into the field from ditches that border the field (Figure 1). This article is both for those that have ponded water in fields in which their crop has already emerged and for those producers that just recently planted after scrambling in the last week to get the rest of their 2024 soybean crop seeded. 

Figure 1. Ponded water in a corn field near the drainage ditch on the field edge (Photo: Angie Peltier).

What saturated or flooded soils do to developing plants. The dangers to roots from flooded soils are many. Flooded soils quickly become devoid of oxygen - which is essential for proper root function. We know that plant leaves are able to use the sun's energy to convert CO₂ and water to oxygen and glucose through a process known as photosynthesis. Respiration is sort of the opposite of photosynthesis, where below-ground, oxygen and glucose are converted back into energy (and CO₂ and water) that is used to run the machinery of cells. On a plant’s typical day there is a balance between respiration and photosynthesis. On sunny days more photosynthesis than respiration occurs, allowing plants to make the building blocks essential for growth and development, eventually contributing to yield.

All of the organisms that live in soil need to respire to live and function. This includes many bacteria, and soil-living fungi, nematodes, insects and plant roots. Flooded soils quickly become oxygen-free (anaerobic) environments that do not support aerobic respiration. In the absence of oxygen, respiration still continues to occur in the soil and in roots, but this anaerobic respiration leads to the build-up of substances toxic to cells (ex. ethanol, organic acids). Additionally, while an anaerobic soil environment certainly does not favor normal cellular functions, root growth or development, prolonged oxygen deprivation can lead to cell death and death of roots or the whole plant.

Corn. Corn is very vulnerable to damage from flooding when it is younger than the six-leaf (V6) growth stage and the growing point is still below-ground. Only 3 to 4 days of being submerged in floodwater can be fatal to these young plants. If plants survive the flooding, root growth and function can continue to be reduced even after the flood waters recede. If root development is retarded, they may be unable to access the subsoil moisture needed to meet water and nutrient demands of plants in the reproductive growth stages.

Additionally, there can be concerns regarding soil nitrogen retention as flooding can lead to nitrogen loss through denitrification and overland flow. For a more fulsome discussion of this topic, the latest UMN Nutrient Management Podcast episode titled, 'Supplemental nitrogen fertilizer: Is it time to pull the trigger?' can be found here. 

Soybean. Flooding can also be detrimental to soybean root growth function and nodule formation and function. Without proper nitrogen fixation, soybean leaves can begin turning yellow. Research has shown that photosynthesis can be reduced by 1/3^rd with 48 hours of flooding (Oosterhuis, 1990). This reduces dry matter accumulation both during and after flooding and can reduce seed yields. One bright spot is that after water drains away, photosynthesis and dry matter accumulation can resume.

Potential for crusted soils after rainwater drains. The strong winds and sunny weather today can cause soils to dry to form a hard crust that can interfere with can cause uneven seedling emergence (Figure 2). 

Figure 2. Compaction of the soil surface, also called 'crusting' can negatively impact seedling emergence after seed germination.

Yield losses can occur when some corn plants emerge significantly earlier than their neighbors. Those plants that emerge earlier than their neighbors are often taller, have larger root systems and more photosynthetic capacity due to their later stage of development. Later emerging plants must compete with those that have already emerged for finite sunlight, moisture and nutrient resources and are often barren.

To determine how much yield loss may be associated with uneven emergence of corn plants, over seven location-years researchers in Illinois and Wisconsin (Nafziger et al., 1991) hand planted seeds at different times within rows to force plants to emerge at different times. Seeds were sown at three different times: 1) early: late April to early May, 2) middle: 10 to 12 days after early, and 3) late: 21 to 27 days after early.

Uneven emergence, 1 ½ week delay. When a portion of the plants emerged 1 ½ weeks later than others yield was reduced by 6 to 9 percent compared to a uniformly emerging early stand. Yields of these uneven stands were comparable to planting the entire stand 1 ½ weeks later.

Uneven emergence, 3 week delay. Delaying emergence of some plants in a field by 3 weeks resulted in significant yield reductions. Yield was decreased by approximately 10 percent when 25 percent of the plants emerged 3 weeks later than the rest of the stand. Yield loss reached 20 to 22 percent when 50 to 75 percent of the stand emerged late. These yield losses were greater than had the entire stand been planted 3 weeks later (Carter et al., 1989).

At this point in the growing season, replanting drowned out areas of the crop is unlikely to be an option for most.

References:

Carter, P.R., Nafziger, E.D. and Lauer, J.G. 1989. Uneven emergence in corn. NCR-344.

Nafziger, E.D., Carter, P.R., and Graham, E.E. 1991. Response of corn to uneven emergence. Crop Science. 31:811-815.

Oosterhuis, D.M. et al. 1990. Physiological responses of two soybean (Glycine max (L.) Merr) cultivars to short-term flooding. Environmental and Experimental Biology. 30:85-92.