Impacts on Crop Plants from Flooding and Standing Water

Mother Nature has not been kind to Missouri Agriculture in 2021. Our state is diverse in crops grown and the weather they experience, but general statements can be made. Spring rains delayed corn and, especially, soybean planting in much of Missouri. Many soybean acres were planted well after the optimum date for yield. Some corn acres were replanted in areas of the state with poorly drained soils or had experienced heavy rainfalls. Then, rain in large areas of the state stopped and was replaced by hot, dry weather. Corn and soybean growth were impacted. Recently, amazingly heavy precipitation has occurred. Rain events of over four inches have been common and frequent.

To the victims of flooding, definitions of their flood aren't important. And, each flood event has characteristics that vary across the landscape. But understanding the various types of floods may help those of us not directly impacted understand future warnings and severity of the effects on agriculture.

The National Weather Service uses the terms river flood, flash flood and areal flood. River flood relates to a specific place on a river or large stream where a flood gauge is monitored. The rise in the river can be slow and can involve precipitation far upstream from the flood gauge. River floods can usually be predicted several days ahead of the event. Precipitation amounts and river drainage characteristics can be modeled, so predictions are reasonably accurate. Rivers may stay above flood stage for multiple days or weeks.

As the name flash flood indicates, water level rises quickly in a flash flood, within 6 hours or often sooner after heavy rain. Other causes of flash floods exist e.g. dam breaks, but thunderstorms that produce heavy rain in a short period of time are the main culprits for agriculture. Although the water rise is fast, the water drop is not always fast. If water levels are high in the rivers or streams into which water must drain, water from a flash flood may remain for long periods after the rain event.

Areal floods occur after more gradual and prolonged rain events. The word "areal" is derived from "area". Persistent rain means that water accumulates in low-lying areas. Although similar to flash flood, an areal flood may build over more than 6 hours.

Unfortunately, Missouri agriculture is currently experiencing all of these flood categories. And, large areas of cropland may not experience a defined flood, but ponding and water-logged soils will affect crop growth and survival.

Both moving and standing water have the potential to do great damage to farmland, crop plants, and the livelihood of farmers. Moving water is often confined to relatively small land areas, but the damage can be intense. The ability of water to dislodge, carry, and deposit material is related to speed of the water and amount of turbulence. Several problems are caused by this ability, but soil erosion may be the most harmful to current and future productivity of the soil. Erosion removes all or part of the most productive layer of soil. Moving water can also carry plant residues from fields. As water movement slows, deposition of soil and residues can block water drainage pathways and cover vegetation. Another damage from moving water is common when floods occur during summer when plants are large. Water is heavy and the force of moving water can break or lodge crop plants.

The primary cause of damage from standing water is its effect on soil oxygen content. As water accumulates in soil pore space, oxygen is excluded. Plant roots and many beneficial soil microorganisms need oxygen to survive and thrive. Air holds 25 times the amount of oxygen as water. So, standing water stops movements of oxygen into the soil and carbon dioxide out of the soil. The result is hypoxia – oxygen content below what is needed for plants to grow and produce normally.

Oxygen is so important to plants that completely submerged plants will usually die within 2 or 3 days. This is usually a threat to seedlings and young plants because they are short. This year, late planted soybean and replanted corn fields are especially vulnerable. Larger plants that have leaves and stems that extend above the water may survive longer than 3 days, but plant functions and yield potential can be greatly impacted. Regardless of plant height, roots cannot function properly without oxygen. Crops and varieties of crops differ for susceptibility to flooding, but no plant (except rice) will escape damage if standing water occurs longer than several days. The longer oxygen is excluded from soil the greater the impact on plant productivity.

Physical and chemical changes in the soil will result from standing water and the low oxygen content that results. Soil aggregates responsible for good soil structure may break apart in standing water. Soil structure will degrade if soil aggregates break apart. Individual clay and silt particles may settle from water into soil pores further impacting soil structure. Some soil microbes continue to live and use soil minerals instead of oxygen to drive their energy production. A common reaction is the reduction of nitrates to gaseous forms of nitrogen. This is called denitrification and can quickly decrease nitrogen content of soils. Nitrogen in expensive fertilizers may be lost and cannot be regained. Other mineral nutrients such as magnesium, iron and sulfur are changed by microbes and these changes affect their availability to plants.

Leaf stomates close if roots are in standing water. Closed stomates means water flow through the plant stops. Several mineral nutrients, e.g. nitrogen, enter and move within the plant with the water. One noticeable symptom of corn plants in standing water is yellow leaves caused by N deficiency. Beneficial microbes for soybean include rhizobia. These bacteria fix nitrogen by changing gaseous forms into plant available ammonium. Lack of oxygen inhibits nodulation and reduced fixation in nodules already present. So, soybean leaves will also exhibit symptoms of nitrogen deficiency.

Without adequate oxygen, plant roots are not able to produce energy through normal respiration. Instead, they use reactions that do not require oxygen. Unfortunately, several toxins, including ethanol and formaldehyde, are produced in these reactions and these can damage plant tissues.

There is very little that farmers can do to reverse the damages from flood and standing water except be patient. After water drains from soil and oxygen is once more in soil pores at least some of the damage will reverse. Plant respiration will return to normal. Nitrogen uptake and use will take place. Stomates will open and normal water flow through the plant along with absorbed nutrients will happen. Plant loss is permanent, but plants that survived will likely produce at least some yield.