Soil Nitrogen Loss in Flooded Corn Fields

• Soil nitrogen loss can result from denitrification, leaching, nitrification, and ammonia volatilization.
• The variables that impact nitrogen loss include nitrogen sources, soil temperature, soil texture, field drainage, and rainfall amount.
• An assessment of potential nitrogen loss should be made to determine if additional nitrogen is needed to help maximize yield potential. This can be done with testing or calculations based on estimates.

Importance of Nitrogen in Corn Production

Nitrogen (N) is a critical corn nutrient for photosynthesis and the production of amino acids, proteins, and chlorophyll. Without these crucial processes, corn yield potential may be compromised if nitrogen is lost during the growing season through denitrification, leaching, nitrification, or volatilization. Lost soil nitrogen is not only unavailable for crop growth and a potentially wasted expense, it also becomes an environmental concern when it enters the air and water—a process that continually occurs via the nitrogen cycle (Figure 1). The risk of nitrogen loss is higher in late spring or early summer when soil temperatures are higher.

Soil Nitrogen Loss by Denitrification, Leaching, Nitrification, and Ammonia Volatilization

Denitrification

This process occurs when microbes convert nitrate (NO3) and nitrite (NO2) into nitrogen gas (N2). Denitrification is common in flooded fields because the excess water forces oxygen out of the soil profile, creating oxygen-free (anaerobic) conditions that favor the growth of nitrogen-converting bacteria. Denitrification may begin after soils have been saturated for two to three days.¹ The rate of denitrification is influenced by soil temperature and accelerates above 60 °F.¹ University of Nebraska research indicated that, at 55 to 60 °F, saturated soils may lose 10% of their nitrate-N to denitrification over five days and 25% when saturated for 10 days (2 to 2.5% loss per day), and that losses increased with warmer soil temperatures.^1,2

Heavier-textured soils tend to exhibit more denitrification. Research conducted in late May and early June in Illinois indicated about four to five percent nitrate-N loss occurred by denitrification for each day that silt loam and clay loam soils were saturated at soil temperatures above 65 °F. In that study, an additional 50 lb of nitrogen per acre brought yield levels to about the same as non-saturated soil yields on silt loam and clay loam soils.^3,4

Leaching

This loss of nitrogen occurs when nitrate-N is picked up by water and washed through the soil profile into tile lines or the water table. Leaching is more prevalent in coarse-textured sandy soils. In the Illinois study above, virtually all the nitrate-N that was lost from the root zone of sandy soils was lost to leaching.^3,4 However, nitrogen loss from leaching can vary depending on precipitation. A study in Iowa found that yearly leaching loss ranged from as little as 1 lb nitrate-N per acre to 75 lb of nitrate-N per acre. Wet years following extremely dry years had the greatest loss, and the highest loss rate occurred in spring. The authors of the study estimated that in wet springs, the loss rate via tile flow would be 40 to 50 lb of nitrate-N per acre.³

Nitrification

Nitrification is defined as the conversion of ammonium (NH4) by microbes first to nitrite and then to nitrate. Nitrate is more prone to leaching and denitrification than nitrite or ammonium. Fertilizers that are ammonium-based (anhydrous ammonia, urea, or ammonium sulfate) are less prone to nitrification under saturated soil conditions than other N fertilizers. Nitrogen fertilizers with the N partly in the nitrate form (urea-ammonium nitrate [UAN] solution or ammonium nitrate) are more susceptible to loss.

Nitrification inhibitors can help manage this form of potential N loss by temporarily reducing populations of Nitrosomonas and Nitrobacter bacteria, the primary soil microbes that convert ammonium to nitrite. Reducing nitrification may also reduce N loss from both denitrification and leaching, by preventing the first step of the process that changes relatively loss-resistant ammonium-based fertilizer into forms of N more vulnerable to loss.⁵

Ammonia Volatilization

This process involves the conversion of urea [CO(NH2)2] products to ammonia (NH3) instead of ammonium (NH4) by a soil enzyme called urease. This more volatile form of nitrogen—a form more easily lost to the atmosphere as gas—is usually produced from urea by urease in high pH soils.⁵ As with the N-loss pathways mentioned above, the rate of ammonia vitalization also increases with higher soil moisture. Urease inhibitors can be used to help stall the breakdown of urea into ammonia and usually last about two weeks, depending on soil moisture and soil temperature. Products containing N-(n-butyl) thiophosphoric triamide [NBPT] and N-(n-propyl) thiophosphoric triamide [NPPT] are common urease inhibitors.⁶

Nitrogen Deficiency Symptoms in Corn

Nitrogen deprived plants are usually light green to yellowish in color (Figure 2). From emergence through the V6 corn growth stage (six leaf collars fully visible), corn plants uptake about five percent of their annual nitrogen requirement. Between V8 (eight leaf collars fully visible) and silking, the plants accumulate about 60% of annual nitrogen requirement. Mid- to late-season nitrogen deficiency symptoms also include a yellow to brown “V” shaped pattern starting at the leaf tip and extending to the stalk in addition to plants that are light green to yellowish (Figure 3).

Methods for Estimating Soil Nitrogen Loss

Determining exactly how much nitrogen has been lost in a large rain event or flooding situation is usually not possible; however, estimating loss can be helpful when determining a crop’s nitrogen needs. Several methods are available to help estimate the amount of nitrogen lost and the amount needed to replenish the loss.

A basic first step is to estimate the amount of applied nitrogen converted to nitrate-N, which depends on the form of nitrogen applied, the temperature, precipitation, and whether any nitrification inhibitors were used. For example, if anhydrous ammonia was applied in the fall followed by cool (reduces conversion) and wet (increases conversion) weather, about 60% would be converted to nitrate by early spring. With an early spring anhydrous ammonia application under the same conditions, about 50% would be converted to nitrate.³

The second step is to determine the rate of nitrate loss from the soil. If soils are cool, an estimate of 2 to 2.5% per day could be used. If soils are warm, a rate of 4 to 5% could be used.³ If a nitrate inhibitor was used, less conversion would likely occur. More detailed examples of how to use these methods to calculate the estimated nitrogen loss are presented below, under “Example Nitrogen Loss Scenarios.”

To make these estimates as accurate as possible, the amount of precipitation must be considered. Iowa information indicates additional nitrogen should be considered if rainfall exceeds 15.5 inches for most of Iowa from April 1 through June 30.⁷ Additional rainfall that occurs after soils are already saturated is particularly likely to increase losses to leaching.^3,4 Rainfall rates may vary for different regions or states, and the interactions between precipitation and soil types should also be considered. However, the principle is that additional nitrogen should be considered after excessive rainfall has occurred during early crop growth.

Instead of estimating conversion to nitrate, the Late Spring Nitrate Test (LSNT) can be used to help determine nitrogen loss. This test measures nitrate-N in the top foot of soil and can be used when corn plants range from six to 12 inches tall. The test accounts for the various forms of soil nitrogen including residual nitrate, mineralized organic matter, and nitrate converted from fall- or spring-applied anhydrous ammonia.⁷ The LSNT uses a nitrate-N threshold or critical value of 25 ppm. If the sampled field is above 25 ppm, no additional nitrogen is recommended. If the sampled field is below 25 ppm, additional nitrogen should be considered to help maintain corn yield potential. If spring rain has been abundant, the critical value may be adjusted downward to 20 to 22 ppm.⁸ The LSNT is not recommended for sandy soils because of their greater leaching potential.

The Pre-Sidedress Soil Nitrate Test (PSNT) may also be used but has more restrictive requirements. It should only be utilized if corn is being grown, manure or biosolids were applied in the last one to two years, a forage legume was the prior year’s crop, and less than 50 lb per acre of a commercial nitrogen fertilizer was already applied. The test is also conducted when plants are six to 12 inches tall.⁹ The critical level for the PSNT test is 21 ppm. If nitrate-N is above 21 ppm, additional nitrogen is not recommended, and if results indicate nitrate-N is below 21 ppm, applying additional nitrogen should be considered.¹⁰

Example Nitrogen Loss Scenarios

The following are example scenarios of common nitrogen-loss conditions growers may encounter. Using the calculations presented below with the information above may help when estimating nitrogen loss.

Scenario 1: UAN (urea ammonium nitrate fertilizer) was applied pre-plant and wet conditions occurred in late spring. The estimated percent of UAN converted to nitrate is 80%, 130 lb N per acre was applied, soil temperatures are warm at 65 °F, and fields have been saturated for 7 days.

• The field started with an application of 130 lb N per acre
• Estimated 80% conversion to nitrate, given that some of the initial nitrogen in the UAN was already nitrate-N, the time since the application, the warm soils, and the wet weather that occurred since the application (as described in the “Nitrification” and “Methods for Estimating Soil Nitrogen Loss” sections).³ This is only an estimate and cannot be confirmed without testing, but provides a useful starting point.
  • Calculation 1, percent of UAN converted to nitrate in the field:
  • 130 lb N per acre × 0.80 nitrate conversion = 104 lb nitrate-N per acre
• Soil temperatures are 65 °F, so nitrate is likely being lost to denitrification at approximately 4% per day while the fields are saturated–
• –and the fields have been saturated for 7 days.
  • Calculation 2, percent of previously converted nitrate now lost to denitrification:
  • 104 lb nitrate-N per acre × 0.04 daily loss × 7 days = 29 lb N per acre loss
    • Note, the two calculations can be combined as:
    • (130 lb N per acre × 0.8 nitrate conversion) × 0.04 nitrate loss per day × 7 days = 29 lb N per acre loss

Scenario 2: Anhydrous ammonia (150 lb per acre) was applied in the fall without an inhibitor. Wet conditions occurred when the soil was cool. The soil became saturated for 10 days and an estimated 70% of the anhydrous ammonia was converted to nitrate. The field is tiled and has very coarse-textured soil. Considerable additional rainfall (>4 inches) occurred on the ninth day while the soil was already saturated.

• The field started with an application of 150 lb N per acre
• Estimated 70% conversion to nitrate, given the type of nitrogen applied, the lack of an inhibitor, the time since the application, and the environmental conditions since the application (as described in the “Nitrification” and “Methods for Estimating Soil Nitrogen Loss” sections).³ As in Scenario 1, this is an estimate, but it may be a useful estimate.
  • Calculation 1, percent of anhydrous ammonia converted to nitrate in the field:
  • 150 lb N per acre × 0.70 nitrate conversion = 105 lb nitrate-N per acre
• Soil temperatures are cool, so nitrate is likely being lost to denitrification at a lower rate, perhaps 2.5% per day–
• –and denitrification has been occurring for 10 days while the soil has been saturated.
  • Calculation 2, percent of previously converted nitrate now lost to denitrification:
  • 105 lb nitrate-N per acre × 0.025 daily loss × 10 days = 26 lb N per acre loss
    • (150 lb N per acre × 0.70 nitrate conversion) × 0.025 daily loss × 10 days = 26 lb N per acre loss to denitrification
    • 105 lb nitrate-N per acre – 26 lb nitrate-N per acre loss = 79 lb nitrate-N per acre remaining
• Given the soil texture, tile lines, and additional rainfall, most or all of the remaining 79 lb nitrate-N per acre was likely lost to leaching.

Addressing Nitrogen Deficiency Due to Lost Nitrogen

Sidedressing is effective for addressing early season nitrogen losses for younger or shorter corn. Many common nitrogen fertilizers can be used for sidedress applications. From the most to least desirable, the application methods and fertilizer products are:^11,12

• Injection of anhydrous ammonia or urea-ammonium nitrate solutions between rows.
  • These should only be applied at or before the V7 growth stage because anhydrous ammonia can quickly move from the placement point into the root zone and potentially injure roots (Figure 4).
• Broadcast dry ammonium sulfate.
• Dribble band UAN.
  • Broadcasting should be avoided because UAN can burn foliage, especially on larger corn.
• Broadcast urea.

If the corn plants are too tall, it may be too late to apply extra nitrogen via standard sidedress equipment. However, Preceon^™ Smart Corn System corn products may extend the opportunity for sidedressing because of their shorter plant height. For additional sidedress information, please see Sidedress and Pre-Tassel Nitrogen Application in Corn.

For larger corn, nitrogen may be applied with high-clearance equipment that uses either a solid shank applicator or fitted drop nozzles. A split weighted hose, or “Y” drop nozzle, may be used to dribble liquid nitrogen close to the plant.¹³

Additional late season nitrogen applications may include broadcast applications of granular urea by a high-clearance spreader or airplane. Farmers with irrigation equipment can apply extra nitrogen by injecting urea-ammonium nitrate into irrigation water.¹³

In summary, losing nitrogen to excessive precipitation is always a potential problem. There are ways to help alleviate the loss should it occur. However, nitrogen best practices should always be kept in mind to help reduce the potential effect on ground, flowing, and Gulf waters.

Additional Reading
Corn Injury from Anhydrous Ammonia Fertilizer
Fall Fertilizer Application in Corn
Nitrogen Availability in the Spring
Effects of Flooding or High Rain Events on Corn Diseases

Sources

¹Murdock, L.W. 2011. Estimating nitrogen losses from wet soils. University of Kentucky, Grain Crops Update. https://graincrops.blogspot.com/2011/04/estimating-nitrogen-losses-from-wet.html
²Ferguson, R.B. 2008. Assessing nitrogen loss due to saturated soils. University of Nebraska–Lincoln, CropWatch. https://cropwatch.unl.edu/assessing-nitrogen-loss-due-saturated-soils/
³Sawyer, J. 2008. Estimating nitrogen losses. Iowa State University Extension, ICM News. https://crops.extension.iastate.edu/cropnews/2008/06/estimating-nitrogen-losses
⁴Torbert, H.A., Hoeft, R.G., Vanden Heuvel, R.M., Mulvaney, R.L., and Hollinger, S.E. 1993. Short-term excess water impact on corn yield and nitrogen recovery. Journal of Production Agriculture. 6(3): 337–344. https://www.ars.usda.gov/ARSUserFiles/60100500/csr/ResearchPubs/torbert/torbert_93b.pdf
⁵Ferguson, R., Maharjan, B., Wortmann, C., Krienke, B. 2019. Nitrogen inhibitors for improved fertilizer use efficiency. University of Nebraska–Lincoln. https://cropwatch.unl.edu/2019/nitrogen-inhibitors-improved-fertilizer-use-efficiency/
⁶Franzen, D.W. Volatilization losses from urea. Procedures of the 2013 Wisconsin Crop Management Conference. 52: 139–155. https://extension.soils.wisc.edu/wp-content/uploads/sites/68/2016/07/Franzen_urea-1.pdf
⁷Roth, R.T. 2024. Navigating nitrogen management in wet spring conditions – estimating nitrogen losses. Iowa State University Extension and Outreach, Integrated Crop Management. https://crops.extension.iastate.edu/post/navigating-nitrogen-management-wet-spring-conditions-estimating-nitrogen-losses
⁸Sawyer, J.E. and Mallarino, A.P. 2017. Use of the late-spring soil nitrate test in Iowa corn production. Crop 3140. Iowa State University Extension and Outreach. https://shop.iastate.edu/crop3140.html
⁹Pre-sidedress soil nitrate test (PSNT). 2021. University of Maryland Extension. https://extension.umd.edu/resource/pre-sidedress-soil-nitrate-test-psnt/
¹⁰Coale, F.J., Meisinger, J.J., Steinhilber, P.M., and Shipley, P. 2010. Soil fertility management. SFM-2. University of Maryland Extension. https://extension.umd.edu/sites/extension.umd.edu/files/2021-02/SFM-2_April%202010.pdf
¹¹Fernandez, F. 2015. 3 tips for sidedressing nitrogen on your corn crop. FarmProgress. https://www.farmprogress.com/crop-protection/3-tips-for-sidedressing-nitrogen-on-your-corn-crop
¹²Sawyer, J. 2001. Post-plant nitrogen applications on corn. Iowa State University Extension and Outreach, Integrated Crop Management. https://crops.extension.iastate.edu/encyclopedia/post-plant-nitrogen-applications-corn
¹³Camberato, J. 2018. Late-season nitrogen application for corn. Purdue University Department of Agronomy, Pest and Crop Newsletter https:/extension.entm.purdue.edu/newsletters/pestandcrop/article/late-season-nitrogen-application-for-corn/

Additional Resources
Coulter, J., Naeve, S., Malvick, D., and Fernandez, F. 2021. Flooded corn. University of Minnesota Extension. University of Minnesota. https://extension.umn.edu/growing-corn/flooded-corn
Heiniger, R. 2025. Impact of persistent and heavy rainfall on corn. North Carolina State Extension. https://corn.ces.ncsu.edu/news/impact-of-persistent-and-heavy-rainfall-on-corn/
Clark, K. and Beegle, D. Nutrient management to improve nitrogen efficiency and reduce environmental loss. Penn State Extension. Pennsylvania State University. https://extension.psu.edu/nutrient-management-to-improve-nitrogen-efficiency-and-reduce-environmental-loss
Web sources verified 7/16/26. 1213_121222