Nitrogen Availability this Spring

After a warm fall, the potential for fall applied or residual nitrogen to be lost to leaching increased with the December rainfall amounts. Steps should be taken to evaluate and help determine if additional nitrogen may be required for the 2016 crop.

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Nitrogen (N) fertilizers contain N in one or more forms: ammonia (NH3), ammonium (NH4), nitrate (NO3), or urea (CO(NH2)2). When NH3 is applied to the soil, it is converted to NH4. The conversion helps prevent N from leaching because NH4 binds to clay and organic matter in the soil. However, the process of nitrification allows soil microorganisms to convert NH4 to NO3, the main form of N taken up by plants. Soil conditions favorable for nitrification include a soil pH of 7, soil moisture at 50% of holding capacity, and warm soils.1 Because NO3 can be subject to leaching, excessive rainfall can move it out of the root zone, into tile systems, and make it unavailable for crop production. As it moves with water into streams and rivers, it becomes an environmental issue.

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Figure 1

Source: Water and Atmospheric Resources Monitoring Program. Illinois Climate Network. (2015) Illinois State Water Survey, 2204 Griffith Drive, Champaign, IL 61820-7495. http://www.isws.illinois.edu/warm/dashboard.asp?stationID=ste&details=10&fromDate=November 1, 2015&toDate=January 26, 2016#detailedView

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When soils are waterlogged, soil organisms through a process called denitrification take oxygen (O2) from NO3 and convert it to nitrite (N2O) and N2, which are gaseous forms of N that can escape into air. This type of N loss occurs most often in fine-textured soils subjected to prolonged anaerobic (without oxygen) conditions. Nitrification inhibitors slow or delay the denitrification process. Fall applied NH3 should have had an inhibitor added; however, with the unseasonably warm weather through November and early December, some nitrification may have occurred (Figure 1).

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As planting season approaches, the amount of fall applied and residual N within reach of corn roots should be evaluated. One means of evaluation is utilization of the Climate FieldView™ Pro Nitrogen Advisor analysis program. The program continually collects environmental information such as rainfall and temperature on each field. Anhydrous application dates can then be input into the system and N loss estimated based on the environmental information. As examples, consider two scenarios based on environmental information for a Monsanto research farm near Flanagan, IL.

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Scenario 1:

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  • NH3 applied with N-Serve® on October 25.
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  • For the period from November 17 through December 1, an N loss of 14 lb/acre occurred.
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  • For the period from December 13 through December 31, a loss of 30 lb/acre occurred.
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Scenario 2:

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  • NH3 applied with N-Serve on November 10.
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  • For the period from November 17 through December 1, an N loss of 12 lb/acre occurred.
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  • For the period from December 13 through December 31, a loss of 25 lb/acre occurred.
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In both scenarios, the estimated N loss is likely from naturally occurring N from mineralized organic matter rather than from the applied NH3. Exceptions to this would be applications of NH3 without a stabilizer made too early in the fall where warm soil temperatures and excessive rainfall could have led to some N loss.

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It is important to understand that loss estimations made through the means of other methods could return different results. As mentioned, soil type, soil temperature, soil moisture, use of stabilizers, time of application, and other factors influence the potential for N loss.

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Soil testing is another method to help estimate the amount of N loss due to rainfall or flooding. Soil cores should be collected to a depth of at least one foot. In sandy soils prone to leaching, sampling at a greater depth may help to identify plant-available N deeper in the soil profile.3 If fertilizer was broadcast in the fall or early spring, collect 20 to 30 cores per sample. If previously applied fertilizer was banded, samples should contain 15 to 20 soil cores. Samples should be collected perpendicular to the direction that fertilizer was applied. Each sample should represent no more than 10 acres.3 Samples should be dried or refrigerated as soon as possible to stop soil microbial activity from changing N levels. Results indicating substantial levels of soil NH4 are more likely if NH3 was recently applied, N stabilizers were used, or soil pH is 5.5 or less. In such cases, low levels of soil NO3 may mean that little conversion of NH4 to NO3 occurred rather than loss of NO3 from the soil due to leaching or denitrification.

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The pre-plant soil nitrate test (PPNT) and the pre-sidedress soil nitrate test (PSNT) can be used to determine NO3 concentrations in soils. This makes it possible to predict the amount of N that will be available to plants during the growing season through mineralization. Most N is released from the soil in the spring when temperatures increase. The rate of N release from soils is influenced by soil temperature, moisture, and aeration. Sampling for the PSNT should be done when corn is 6 to 12 inches tall or in late May to early June. Soil cores should be taken at a depth of one foot with one sample containing 15 to 20 cores. Samples should come from field areas that are similar and no more than 10 to 20 acres in size.3 Fields that are likely to have high NO3 concentrations (manure applications, previous year in alfalfa, fine-textured, fall-tilled, south- facing slopes) should be sampled. Although some differences exist in university recommendations for interpreting PSNT results, a general rule of thumb is that if soil test results are over 23 to 25 ppm, additional N is probably not needed.3

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Summary

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Sampling and testing soil may be necessary this spring to help determine whether additional N will be needed to help maximize crop yield potential. Availability is influenced by many factors, but a study from the University of Wisconsin found that “regardless of the rate or source, the fate of fall– and spring-applied N is mostly impacted by weather conditions in early spring�?.4