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The availability of residual, fall, and spring applied nitrogen (N) is a concern for some farmers because of the amount of April and May rainfall. Corn plants that are now showing N stress should be evaluated for compaction and possible water shortage stress.
There is debate among agronomists whether fall or spring applied N was lost because of the amount of spring-time precipitation. Farmers who used an N inhibitor with fall applied anhydrous should have benefited from its use.
The University of Illinois has indicated that according to their sampling, most of the applied N remains available.1 However, N stress may be appearing in some fields as a result of ponding, compaction, or dry soils. In each of these situations N may be available, but plant uptake is hindered by poor root growth and/or lack of water to facilitate N uptake.
One means of evaluation is the utilization of Climate FieldView™ N monitoring offered by The Climate Corporation. 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.
Another method to help estimate the amount of N loss due to rainfall or flooding is soil sampling. However, soil sampling is likely too late for this year’s crop unless the field was planted very late. Sampling procedures are available from the University of Illinois or Purdue University.
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.
For future years or late-planted corn, the pre-plant soil nitrate test (PPNT) and the pre-sidedress soil nitrate test (PSNT) can be used to help determine NO3 concentrations in soils. This makes it possible to predict the amount of N that may 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 N is released from soils is influenced by soil temperature, moisture, and aeration. Sampling for the PSNT should occur when corn plants are 6 to 12 inches tall or in late May to early June. Soil cores should be taken at a depth of 12-inches 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.2 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.2
Some fields may be short on N depending on how much water passed through the soil (Figure 1). However, monitoring and sampling methods should be utilized to help determine if additional N would be beneficial to help maintain 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�?.3
Figure 1. Nitrogen deficiency symptom on mature corn leaf.13;10;13;10;
1Nafziger, E. 2017. The corn crop and sidedress nitrogen. The Bulletin. University of Illinois. http://bulletin.ipm.illinois.edu/?p=3912.
2Shapiro, C., Hergert, G. and Ferguson, R. 2012. Using the PSNT for spring testing of nitrogen availability. CropWatch. University of Nebraska-Lincoln.
3Fernandez, F.G., Hoeft, R.G., and Randall, G.W. 2011. How much nitrogen is there in the spring from fall-applied MAP, DAP, and ammonium sulfate? Proc. of the 2011 Wisconsin Crop Management Conference, Vol. 50. www.soils.wisc.edu.