Nitrogen Loss in Illinois in 1999

June 4, 1999
Some areas of Illinois have experienced excessive precipitation that may have caused significant nitrogen loss this spring. Whether this loss has been 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 from 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.

Minnesota research has shown that virtually all of the ammonia applied on October 2, 1998, without N-Serve had been nitrified by the end of April 1999. Even when application was delayed until mid-October or the first week of November, most of the ammonium had been nitrified by late April. When N-Serve was included with ammonia applied November 2, 1998, the rate of nitrification was markedly reduced. Based on the Minnesota data as well as data we have collected in prior years, we have compiled the information in Table 4 to provide a guide for estimating the percentage of nitrogen applied that is now present in the nitrate form as of May 15, 1999.

Note that urea, urea-ammonium 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 is 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 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 for how to determine losses from specific situations. Assume the following:

a. 180 pounds N/acre was applied on October 25, 1998, 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 15 to 26.

d. The 5year average yield for the field is 180 bushels per acre.

e. The previous crop was soybean.

Step 1

Calculate N present as nitrate

N applied x % in nitrate form

180 pounds N/acre x 0.90 = 162 lb N/acre

Step 2

Calculate N denitrified

N in nitrate form x % denitrified

162 x .36 (9 days x 4% per day)

58 pounds 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 1foot 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 10acre area, collect eight samples 3 to 4 inches apart. This sampling pattern should minimize sampling errors.

If the results from the PSNT are at 22 ppm N or higher, you need not apply any additional N. If the results are less than 22 ppm N, use the above 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 onefoot 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 because of 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 utilize conventional equipment, the choices, in order of preference, 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 highclearance sprayer using drop nozzles that will keep the nitrogen solutions off the corn. Do not aerially apply UAN solutions; it will cause severe foliar burn.

How late can I apply the N and expect an economical response?

An economical yield response to the application of nitrogen has been obtained as late as tasseling on corn that was severely deficient. However, 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

Author: Robert Hoeft