University of Illinois

No. 8/May 15, 1998

Nitrogen Loss in Illinois in 1998

A warmer-than-normal winter plus excessively wet soils this spring may add up to significant nitrogen loss in some fields. Nitrogen loss associated with excessively wet soils will occur only from that portion of the fertilizer N that was in the nitrate form when soils became saturated. Because most fertilizers are applied as ammonium or a form that quicklyconverts to ammonium, you must first determine how much of the applied nitrogen had been converted to nitrate. The rate of this conversion is dependent on soil temperature, length of time between application and flooding, use of nitrification inhibitor, and form of nitrogen applied.

Analysis of soil samples collected at Urbana indicated that virtually all the ammonia that had been applied on October 16 and 23, 1997, had been nitrified by April 27, 1998, when no inhibitor had been included with the ammonia. When N-Serve was added to the fall-applied ammonia, from 35 to 40 percent of the applied nitrogen was still in the ammonium form onApril 27. Based on that study, we have compiled the information in Table 3 to provide a guide for estimating the percent of nitrogen applied present in the nitrate form as of May 1, 1998.

Table 3.Effect of N material, time of application, and time between application and saturated soils on the rate of conversion of ammonium to nitrate nitrogen in soil

Time after application before soil became saturated
Form of NTime of applicationNitrification inhibitor4+ weeks2 weeks
Percent of applied N in Nitrate form
Anhydrous ammoniaFallNo97--
Anhydrous ammoniaFallYes60--
Ammonium sulfateFallNo97--
Ammonium sulfateWinterNo95--
Anhydrous ammoniaSpringNo6540
Anhydrous ammoniaSpringYes5025
UAN solution SpringNo8060
Ammonium sulfateSpringNo7550

Note that urea, urea-ammonium nitrate solutions (UAN), and ammonium sulfate nitrify (convert from ammonium to nitrate) more rapidly because they do not build an ammonia concentration that inhibits nitrification. In addition, a portion of the UAN solution is present in the nitrate form when it is applied and thus susceptible to loss immediately.

The fact that ammonium was converted to nitrate does not mean that it was lost but rather that it was susceptible to loss in those fields that remained saturated with water for more than 4 to 5 days. Illinois research has indicated that 4 to 5 percent of the amount of nitrate-N present (note that this is not 4 to 5 percent of the total N applied) will be lost for eachday that soils are saturated. On the heavy-textured soils, this loss is through the process of denitrification. On sandy soils, the losses are primarily from leaching.

How much N loss has occurred? The loss will vary, but the following example provides a guide on how to determine losses from specific situations. Assume

a) 180 pounds N/acre was applied on October 25, 1997, without a nitrification inhibitor;

b) corn was planted on a silty clay loam soil on April 25, with a resultant stand of 25,000 plants per acre;

c) soils were saturated for 9 days, starting May 3;

d) the 5-year average yield for the field is 180 bushel per acre; and

e) the previous crop was soybean.

Step 1
Calculate N present as nitrate.
N applied x % in nitrate form
180 pounds N/acre x 0.97 = 175 lb N/acre

Step 2
Calculate N denitrified.
N in nitrate form x % denitrified
175 x .36 (9 days x 4% per day)
63 pounds N/acre

Is there a nitrogen soil test that will indicate whether I need to apply additional nitrogen? The pre-sidedress nitrogen test (PSNT) may provide an indication of additional nitrogen needed. Collect soil samples to a one foot depth when corn is 6 to 12 inches tall. If nitrogen was injected, collect at least 24 cores from an area of no larger than 10 acres. In apattern perpendicular to the direction of travel of the fertilizer applicator in at least three areas within the 10-acre area, collect eight samples 3 to 4 inches apart. This sampling pattern should minimize sampling errors.

If PSNT results are 22 ppm N or higher, you need not apply any additional N. If the results are less than 22 ppm N, use the preceding calculations to determine if you need to use supplemental N. This test may underestimate the soil's capacity to supply N this year because some of the N may have leached below the one-foot sampling depth but still be within the rooting zone; the cold, wet soils experienced this spring may have resulted in slow mineralization rates, but improved weather conditions later may result in rapid mineralization; and the test will not measure any of the nitrogen that has not been converted to nitrate.

Will it pay to apply more N? Whether or not it will pay to apply more N depends on how much was lost and what the yield potential will be. If yield potential is reduced due to delayed planting or poor stands, the remaining N may be adequate.

If you calculate that the nitrogen remaining from your earlier application is 40 to 80 pounds N per acre less than you will need, apply an additional 60 pounds N per acre. If the calculated need is over 100 pounds of N per acre, add an additional 90 pounds of N.

How do I apply the supplemental N? If the corn is small enough that you can utilize conventional equipment, the choices in rank order would be the following:

  • Inject anhydrous ammonia or UAN solutions.

  • Broadcast ammonium nitrate or ammonium sulfate.

  • Broadcast urea.

  • Dribble UAN solutions between the rows.

  • Broadcast UAN solutions.

If the corn is too large for conventional ground equipment, urea could be applied aerially or UAN solutions applied with a high-clearance sprayer using drop nozzles that will keep the nitrogen solutions off the corn. Do not aerially apply UAN solutions as it will cause severe foliar burn.

How late can I apply the N and expect an economical response? An economical yield response has been obtained from the application of nitrogen as late as tasseling on corn that was severely deficient. However, one must keep in mind that a rain will be required to move nitrogen that was surface-applied into the active rooting zone. If rain is not received, the supplemental application will be of no value.

Robert Hoeft (, Department of Crop Sciences, (217)333-4424