If excessive precipitation occurs in the next 2 to 3 weeks, there may be some areas of Illinois that experience significant nitrogen loss. Whether this loss will be large enough to require supplemental application of N depends on time, rate, and form of N application; length of time soil was saturated; date of planting; and plant population. |
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 quickly converts 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.
Based on data we have collected in prior years, we have compiled the information in Table 2 to provide a guide for estimating the percent of nitrogen applied present in the nitrate form as of May 15, 2000. Note that the estimates for conversion of ammonium to nitrate are higher for the 2000 crop year than prior years because of the much warmer than normal fall, winter, and spring.
Note that urea, ureaammonium nitrate solutions, 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% of the amount of nitrate-N present (note that this is not 4 to 5% of the total N applied) will be lost for each day that soils are saturated. On the heavy-textured soils, this loss will be 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, 1999, 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 from May 25 to June 3; (d) the 5-year average yield for the field is 180 bushels per acre; and e) the previous crop was soybean.
Calculate N present as nitrate.
N applied x % in nitrate form
180 lb N/acre x 1.00 = 180 lb N/acre
Calculate N denitrified.
N in nitrate form x % denitrified
180 x .36 (9 days x 4% per day)
65 lb N/acre lost
Is there a nitrogen soil test that will indicate whether I need to apply additional nitrogen?
The presidedress nitrogen test (PSNT) may provide an indication of the need for additional nitrogen. Collect soil samples to a 1-foot depth when corn is 6 to 12 inches tall. If nitrogen was injected, collect at least 24 cores from an area no larger than 10 acres. In a pattern 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. While this sampling pattern should minimize sampling errors, keep in mind that there will likely be significant variation across sample areas.
If the results from the PSNT are 22 ppm N or higher, you need not apply any additional N. If the results are less than 22 ppm N, use the previous calculations to determine whether 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 1-foot sampling depth but still be within the rooting zone.
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 by 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 N per acre, add an additional 90 pounds N.
How do I apply the supplemental N?
If the corn is small enough that you can use 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 aerially applied or UAN solutions could be 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 they will cause severe foliar burn.
How late can I apply the N and expect an economical response?
An economical yield response has been obtained to 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 G. Hoeft