Ronnie Schnell, Ph.D.
Associate Professor and Cropping Systems Specialist
Texas A&M AgriLife Extension, College Station
Tony Provin, Ph.D.
Professor and Extension Specialist – Soil Chemistry
Texas A&M AgriLife Extension, College Station
Numerous rain events have resulted in flooding or significant ponding of water in many corn and sorghum fields across Texas. While low-lying areas may be flooded, other areas of fields may be saturated for extended periods of time. How long can corn or sorghum survive under saturated or flooded conditions? What impact will these conditions have on grain yield, if any?
Oxygen is required by plants for respiration, including above ground (shoots) and below ground (roots) plant tissue. Respiration is the process where plants metabolize sugars, producing energy needed for growth and development. Soil contains about 50% pore space that is occupied by air and water. Flooding increases the amount of pore space occupied by water and reduces exchange of air between the soil and atmosphere. Deep ponding has the same effect on above ground tissue. Oxygen does not easily move through water so saturated or flooded conditions will limit oxygen availability to plant tissue, especially roots. This has detrimental affects on plants.
The growth stage of the crop will influence the plant’s ability to withstand flooded conditions. Younger plants are more susceptible to damage or death by flooding, especially when the growing point is at or below the soil surface. Younger plants are easily submerged compared to older, taller plants. Higher temperatures will exacerbate the effects of flooding. Young plants may survive for up to 48 in oxygen limited environments under cool conditions but may not survive 24 hours under warm sunny conditions (>77°F). For this reason, yield loss is typically greater when young plants (< 6 leaves) are exposed to saturated or flooding conditions. Stand losses in early growth stages is a major factor in yield loss. Similar to freeze and hail damage, look for new growth 3-5 days after conditions improve to determine surviving plant populations.
Extended periods of saturation will affect plants of all ages. Root tissue can die and new growth will be stunted or delayed under saturated conditions. Reduction in root volume will reduce the capacity for uptake of water and nutrients during later growth stages. Flooding can induce nutrient deficiency symptoms. Nitrogen will be remobilized from older (lower leaves) to younger (upper leaves) resulting in yellowing of lower leaves. Purpling of leaf tissue is possible due to accumulation of carbohydrates in the shoot tissue under flooded conditions, a symptom usually associated with phosphorus deficiency. In addition, denitrification and leaching of nitrate can result in loss of nitrogen from soil and potentially reduce yield. Damaged root systems and associated stress can increase the potential for various plant diseases, including root and stalk rot diseases. The degree of flooding will ultimately determine the potential for yield loss.
How much nitrogen is lost during extended periods of flooding or saturation? Should you apply more nitrogen fertilizer? There is no simple answer to these questions. Extended periods of saturated soil may result in loss of soil nitrate-nitrogen, through denitrification and/or leaching of the nitrate ion. Many factors will affect how much nitrogen is lost. Factors include the amount of nitrate present in soil (affected by source, rate, and timing of nitrogen fertilizer), soil temperature, and duration of saturation. Bacteria in soil will convert nitrate-nitrogen to gaseous forms (N2O and N2) when soils are saturated and soil oxygen becomes depleted. It normally takes a day or two of saturation for this to begin. From there, 1 to 5% of soil nitrate-nitrogen can be converted to gaseous forms and lost during each additional day of saturation. Considering nitrate may be deeper in the soil profile and labile carbon, the bacteria food source, may be limited compared to other regions in the U.S., nitrate-nitrogen loss from Texas soils is likely lower than levels reports from midwestern states. The other thing to consider is the growth stage of the crop. Corn or sorghum that have larger plant size (late vegetative or flowering) already contain the majority of nitrogen it will use in the plant tissue. This reduces the potential for loss.
If significant nitrogen loss is suspected or if nitrogen fertilizer applications were delayed or prevented, late applications in corn and sorghum can provide yield response. Corn has shown the ability to recover grain yield when nitrogen fertilizer is applied as late as R1 (silking) when at least 25% of the nitrogen fertilizer was applied earlier in the season (V5) (Mueller and Vyn, 2017). Some yield loss was observed when all of the nitrogen was applied at silking but severe yield loss was avoided. If fields are relatively weed free and plants have good size, rescue nitrogen fertilizer application may be warranted. Applications can be made using high clearance equipment and dribble nozzles or y-drops to apply UAN. Aerial applications of urea are also an option. Some minor burning from aerial applied urea can be expected, especially if leaves are wet during application. However, benefits of the nitrogen fertilizer will outweigh any minor burning of plant tissue. When making rescue or late applications of nitrogen fertilizer, rain will be needed to move the nitrogen into the root zone for quick uptake. If this process is delayed, it will limit the response to the late application.
A brief period of flooding will likely have minimal impact on grain yield, especially for older plants. Repeated or long-term saturation/flooding will increase the potential for yield loss due to a variety of complications. Take note of the location(s) within individual fields where flooding/saturation or other visual indicators suggest the impact to corn/sorghum are greatest. Factors including compaction, inappropriate pH, salinity, fertility limitation, shallow soils, or other factors influencing rooting depths and rooting development will cause the observed symptoms to occur first and result in greater impairments and lower yields. These field areas should be evaluated individually after the flooding and saturating conditions have subsided for rooting and fertility limitations.