Anytime heavy rains occur early in the growing season, a legitimate concern arises regarding nitrogen (N) loss from fertilizers that were applied prior to the rain. The major N loss processes of concern are leaching of available N below the crop root zone and gaseous loss of available N through denitrification in saturated or flooded soils.  The exact extent of N losses through leaching and/or denitrification following the heavy rains is unknown. Both of these loss processes occur through the nitrate form of N, so the potential for significant loss is determined by the amount of the crop N supply that was in the nitrate form when the excess rainfall occurred. Losses depend on many factors such as when the N was applied, the forms of N applied or expected to provide N for the crop, soil characteristics, and how wet the soil is/was.  In general, leaching losses are more likely on sandy soils where water can move through the profile quickly.  Denitrification is more likely on medium and fine textured soils that are not well drained.  These soils tend to become saturated with water and/or retain flooded areas for several days following excess rain. 

        Where fertilizer N was applied before planting, the timing of the application and the form of N used are important in determining the risk of loss.  Keeping in mind that losses occur through the nitrate form of N, the timing of nitrate formation is an important consideration in evaluating potential losses.  For spring preplant applications, ammonium forms of N such as anhydrous ammonia or urea are converted to nitrate-N in about 4 to 6 weeks.  Urea usually is converted to nitrate more rapidly than anhydrous ammonia.  Nitrogen solutions (28% UAN) contain half of the N as urea and the remainder as ammonium nitrate. Essentially, this fertilizer contains 75% of the N as ammonium and 25% as nitrate when it is applied.  Urea-containing fertilizers are converted to ammonium-N in 3 to 5 days after application, and this conversion occurs even in saturated or flooded soils. 

        Denitrification losses can occur within a few days if the soil remains saturated or flooded and nitrate-N is present.  Warm temperatures and extended periods of saturated conditions favor high losses.  Work in Illinois suggests that 4 to 5% of the nitrate-N present can be lost each day the soil remains saturated.  Table 1 from the University of Nebraska provides some estimates of denitrification losses at various temperatures and times of saturated soil conditions. 

        If part or all of the crop N requirement is expected to be provided from organic sources such as manure and/or previous legume crops, losses through leaching and denitrification are expected to be relatively small at this point in the growing season.  This is because most of the N from organic sources has not yet been converted to nitrate.  Rapid nitrate production from manures and previous legumes usually begins in mid-June and continues for several weeks during the growing season. The availability of N from organic sources can be checked using the preplant soil nitrate test (PSNT) described below. 

Determining the need for additional N

        A key decision for corn producers is whether additional N should be applied to compensate for N losses that may have occurred.  The best method to assess N loss is to perform a presidedress nitrogen test (PSNT).  The PSNT offers a diagnostic method for evaluating the N supply for the crop.  The test is particularly useful for those cases where previous legumes or manure applications are providing part or all of the crop’s N need.  It can also be used to confirm the extent of fertilizer N loss in the soil depth sampled.  Since soil samples for the PSNT are taken to a 1-ft depth, nitrogen that has leached below this depth will not be reflected in the test.  To use the PSNT, soil samples should be collected to a 1-ft depth when corn plants are between 6 and 12 inches tall.   The details of performing the PSNT can be seen at the following web site.   http://ipcm.wisc.edu/pubs/cards/a3630.htm

Other considerations

            Options for applying supplemental N when it is needed include traditional sidedressing with anhydrous ammonia or N solutions.  Where the entire crop N requirement has not yet been applied, sidedress or other postemergence applications should contain the balance of the crop N requirement plus 25-50% of the N that was already applied.  Urea-ammonium nitrate solutions (28%) can also be applied as a surface band or as a broadcast spray over the growing crop.  Dry N fertilizers such as urea can be broadcast.  Leaf burning from solution or dry broadcast applications should be expected.  Applying the dry materials when foliage is dry will help to minimize burning.  Several precautions should be followed in making broadcast fertilizer applications over growing corn.  Basically, broadcast N rates should be limited to 90 lb N/acre for corn with 4 to 5 leaves and to 60 lb N/ acre for corn at the 8-leaf stage. Under N deficient conditions, corn will respond to supplemental N applications through the tassel stage of development if the N can be applied.