Assessing Nitrogen Loss After Soil Saturation

  • Nitrogen (N) loss is a major concern after soils have become saturated from heavy rainfall. 
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  • Denitrification (microbial conversion of nitrate to nitrogen gases) and leaching are the processes involved with saturated soils that can result in significant N loss.
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  • Estimating N loss is not an exact science; however, some guidelines exist that can help with N assessment.
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Nitrogen Loss

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Denitrification.This process occurs under anaerobic (lack of soil oxygen) soil conditions. Nitrogen in the ammonium-N form (NH4+) is not subject to denitrification or leaching. Nitrogen loss can occur rapidly if nitrate-N (NO3-) is present, soils are saturated or flooded, and soil temperatures are >50° F. Studies conducted in Illinois showed that up to 5% nitrate-N loss through denitrification occurred each day soils were saturated.1 In these studies, all-nitrate fertilizer was applied when corn was in the V1 to V3 growth stage. Urea converts to nitrate quicker than anhydrous ammonia; approximately 2 weeks and 4 weeks, respectively.2University of Nebraska data (Table 1) demonstrates the potential nitrate-N loss for every day of saturation at various temperatures. While the process of denitrification is ultimately dependent on saturated soils, the potential for N loss due to denitrification also increases rapidly as soil temperature increases. In soils where saturation or ponding characteristically occurs during the year, special consideration should be made to either not apply N until the risk of soil saturation decreases, or retain N applications in the NH4+ form until the crop is able to utilize NO3-. 

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Leaching. This pathway of loss is a potential concern with soils that allow rapid downward movement of water such as sandy soils, well drained soils, and/or soils with improved drainage. Ammonium nitrate and urea ammonium nitrate (UAN) solutions are more susceptible to leaching than anhydrous ammonia, with differences due to the rate of conversion to nitrate and the N source applied in the fertilizer products. Once fertilizer N is converted to nitrate, there will be no difference in the behavior of N in the soil profile between any sources of fertilizer N. 

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 Table 1. Estimated denitrification losses as influenced by soil temperature and days of saturation. 
 Soil Temperature (F)
 Days Saturated
 Nitrate - N Loss (% of Total N Applied)
 55-60
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 10
 10
 25
 75-80
 3
 60
 Source: Shaver, T.M. and Ferguson, R.B. 2014. Nutrient management for agronomic crops in Nebraska.3
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Management

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If significant N has been lost, then more N is likely required. UAN liquid solutions can be applied as a band on the surface with drop nozzles, even on fairly large corn. To help minimize volatilization and maximize effectiveness, rainfall or irrigation is needed to move UAN and urea into the soil. Up to 30% of the urea could be lost due to volatilization if no rainfall occurs within two weeks and temperatures are warm.6

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To help optimize yield potential in corn, adequate N is critical. Purdue University provides general recommendations by N form, timing of N application prior to excessive rain, and soil type (Table 2). Assessing N loss and requirements is not an exact science, but it can help provide estimates that impact your bottomline.

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Table 2.  Estimated nitrogen applications to replace lost nitrogen, based on nitrogen form and timing of nitrogen prior to excessive rain.*
 Field Scenario
Fields where urea or UAN (28% or 32% N) was applied more than 2 weeks prior to rains.  Also, where anhydrous  ammonia was applied more than 4 weeks prior to excessive rain.
Fields where urea or UAN (28% or 32% N) was applied 1 to 2 weeks prior to rains.  Also, where anhydrous ammonia was applied 3 to 4 weeks prior to excessive rain.
Fields where N loss is estimated to be less than 30 lbs N/acre and the         projected optimum N rate or higher was used initially.  Fields where N was applied 2 to 7 days (urea, 28% or 32% UAN) or 3 weeks (anhydrous ammonia) before excessive rain.
 Should I Apply Nitrogen?
Additional N Likely Required 
Additional N May Be Required
Additional N Likely NOT Required
 What Rate of Nitrogen?
Consider 60 to 120 lbs N/acre
Consider 30 to 60 lbs N/acre
Likely None
 Source: Camberato, J., Joern, B., and Nielsen, R.L. 2008. Nitrogen loss in wet and wetter fields. 2008. Purdue University.  http://www.agry.purdue.edu.
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*For more specific recommendations based on soil type, reference the above-cited document from Purdue University.
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Is Supplemental Nitrogen Necessary?

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Applications of supplemental nitrogen may be warranted if sufficient loss has occurred. Amount of N loss is hard to quantify, as it depends on several factors, including soil type and structure, soil temperature, form of N, and days of saturation.  One method to determine if supplemental N is required is the pre-sidedress nitrogen test (PSNT). PSNT soil samples should be collected to a depth of one foot when corn is between 6 and 12 inches tall. The accuracy of the test is highly dependent upon the sampling and handling procedures. Contact your lab for proper sampling and handling techniques. Test results of over 25 ppm can indicate that no additional nitrogen will be needed for the current growing season.4Test results less than 25 ppm can indicate you may get a positive yield response from sidedress nitrogen. In theory, the lower the test level the more nitrogen you will need to apply.  Refer to local university guidelines or consult and agronomist for the amount of additional N to apply if PSNT results are under 25 ppm. 

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Estimating N Loss
1.  Calculate N present as nitrate: (N applied multiplied by % in nitrate form).
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2.  Calculate N denitrified: (lbs nitrate/acre from Step 1 multiplied by % denitrified from Table 1.
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Note that the conversion to nitrate occurs almost immediately with N applied as urea. With UAN (28% or 32% N), half of the N is in the urea form, 25% is found as ammonium, and the remaining 25% as nitrate. The nitrate is already subject to loss, and the other fractions are readily converted. Conversion of N applied as anhydrous ammonia is delayed 10 to 14 days following application, regardless of any stabilizer added.
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Soil temperature has a large influence on conversion of ammonium to nitrate. It takes approximately 2 weeks for complete conversion at 60° F and 1 week is needed at 70° F.5  
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