Ways to Assess Nitrogen Loss after Soil Saturation

Wet conditions this spring have raised some questions on the potential loss of nitrogen (N). Nitrogen can be lost via denitrification (microbial conversion of nitrate to N gases) and leaching. Estimating N loss is not an exact science; however, monitoring N levels can help with N assessment.

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Denitrification. This process occurs under anaerobic (lack of oxygen) soil conditions. Nitrogen in the ammonium-N form (NH4+) is not subject to denitrification or leaching; however N 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 of soil saturation.1 In these studies, all-nitrate fertilizer was applied when corn was in the V1 to V3 growth stage. In soils where saturation or ponding characteristically occurs, 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-. Urea converts to nitrate quicker than anhydrous ammonia; approximately 2 weeks vs. 4 weeks, respectively. University 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 from denitrification also increases as soil temperature increases.

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Table 1. Estimated denitrification losses as influenced by soil temperature and days of saturation.
Soil Temperature (°F)Days SaturatedNitrate-N Loss (% of Total N Applied)
55-60510
55-601025
75-80360
Source: Ferguson, R. Nutrient management for agronomic crops in Nebraska.2
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Leaching. This pathway of loss is a concern with soils that allow rapid downward movement of water such as sandy soils or well drained soils. 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. Once fertilizer N is converted to nitrate, there is no difference in the behavior of N in the soil profile between any sources of fertilizer.

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Monitoring N

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Applications of supplemental N may be warranted if sufficient loss has occurred. The amount of N loss is hard to quantify as it depends upon several factors, including soil type/structure, soil temperature, form of N, and days of soil saturation.

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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 techniques. Test results of over 25 ppm can indicate that no additional N will be needed for the growing season. Test results under 25 ppm indicate you may get a positive yield response from sidedress N. Agronomists have different opinions on the reliability of the PSNT, but it is one more piece of data to use when trying to make a difficult decision. Refer to local university guidelines for the amount of additional N to apply if results are under 25 ppm.

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Another tool farmers can use to help evaluate N levels is the Nitrogen Advisor tool available in Climate FieldView™ Pro. Nitrogen Advisor shows you the amount of likely available N in your soil throughout the growing season. The model takes into account field soil, weather, and management conditions in order to make daily calculations of N gains, losses, and transformation. Climate FieldView Pro provides advanced tracking of N within fields to help see field-level N supplies based on application, crop stage and weather. Nitrogen Advisor tool also allows farmers to explore custom scenarios to determine an ideal amount of N to apply and the best time for application. New in 2016, a Mass Data Entry feature has been added that enables the creation of multiple N application programs at once and applies the same program across many fields. Individual fields can also be edited to customize the advisor results for a farmer’s operation to assist in monitoring for the entire year. The Nitrogen Advisor overview screen is available to help farmers identify which fields need the most attention and which fields may be at-risk for missing yield potential targets.

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Management

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Figure 1

Figure 1. Climate FieldView™ Pro Nitrogen Advisor.

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If significant N has been lost, then more N should probably be applied via sidedressing. 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. Assessing N loss and requirements is not an exact science, but it can help provide estimates that impact your bottom line.