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Issue No. 10, Article 8/May 28, 2004

Predicting and Measuring Nitrogen Loss

Excessively wet soils, while still not prevalent over most of Illinois, have occurred in some areas in the last few days. The excess water has created numerous problems, not the least of them enhanced potential for nitrogen loss.

There are different techniques to estimate or measure the amount of N loss that might have occurred during this excessively wet period. No technique is a sure bet; in fact, unless used with caution, any of them may be misleading. The options along with the precautions are listed below.

Amino Sugar-N Test (Illinois N Soil Test)

This test will not predict N loss from excess water. The test is designed to predict the ability of the soil to release nitrogen through the mineralization process from organic N sources. These organic N sources will not be lost during excess water events. The amino sugar-N test is not ready for commercial use by farmers and won't likely be ready for at least another year.

Presidedress Nitrate Test (PSNT)

The presidedress nitrogen test may provide an indication of the need for additional nitrogen. However, the reliability of results from this procedure depend heavily on ensuring samples are collected, handled, and processed correctly. Even then, the reliability of the test when values are low is questionable. If values are high--greater than 25 PPM--the odds are good that no additional N will be needed for the 2004 crop. The following suggestions are derived in large part from research conducted by faculty at Iowa State University.

Collecting soil samples: Collect samples to a 1-foot depth at eight positions perpendicular to the direction of travel of the nitrogen applicator: one position in the corn row (this assumes that the ammonia was applied in the same direction as the corn rows) or knife track of the applicator (assuming you can still see where the knife track was), and the other seven sequentially 1/8, 1/4, 3/8, 1/2, 5/8, 3/4, and 7/8 the distance between the rows. All soil from the eight cores to the full 1-foot depth needs to be placed in the sample bag. Collecting any probe to less than the full 1-foot or eliminating any cores will render results suspect. Since most commercial sample bags do not hold that much soil, be sure to use a larger bag. The timing normally recommended for sampling is when corn is 6 to 12 inches tall. However, since many fields are not yet planted, we suggest that samples be collected in late May or early June, irrespective of corn height. Although this pattern should minimize sampling errors, keep in mind that there will likely be significant variation across sample areas. Collect at least one sample for each 10 acres in the field. Wait until soils have dried enough to allow obtaining a representative core.

Handling samples: If samples cannot be delivered to the laboratory the same day, freeze them and then deliver them. Another option is air-drying before delivery, but since the lab has better drying facilities than most farmers, it is best to freeze and then ship. If you do air-dry samples, spread them out on a paper, crush the cores, and set a fan on them so they dry as fast as possible.

Laboratory instructions: Be sure to indicate that you want nitrate nitrogen determined on the samples and that all soil in the sample bag must be dried and ground before a subsample is pulled. There is no way to accurately subsample from wet cores.

Interpretating results: If results from the PSNT test are 20 to 22 PPM N or higher, you need not apply any additional N. If results are less than 22 PPM N, use the calculations in "Estimation of N Loss Based on Soil Temperature" to determine whether you need to use supplemental N. The PSNT 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.

Supplemental Nitrogen Strip

Consider applying two or three strips of supplemental N across the field at a rate of 60 to 80 lb N/acre. As the season progresses, compare the color of corn in the strips with that in the remainder of the field. If corn in the supplemental strips is noticeably darker green than the rest, consider applying an additional 60 lb N/acre to the rest of the field. Our research has indicated that a yield response can be obtained by applying N as late as 2 weeks after tasseling. The risk associated with this option is that the color difference may not show up until the corn is in grain fill, a time when it is too late to apply the supplemental N. Another risk is that the late-applied N will require significant rain to be moved into the root zone. Rain during that time period is traditionally less frequent.

Estimating N Loss Based on Soil Temperature

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 depends on soil temperature since application and whether a nitrification inhibitor has been used. Table 4 lists estimates of the amount of N converted as of May 25, 2004.

Conversion of ammonium to nitrate does not mean that it has been lost from the soil system but rather that it is susceptible to loss in fields that have been saturated with water for several days. When soils are excessively wet, nitrogen is lost through the process of denitrification, or leaching.

Denitrification is the major nitrogen loss mechanism in most Illinois soils, particularly medium- to heavy-textured soils. Illinois research has shown that 4% to 5% of the amount of nitrate nitrogen present (note that this is not 4% to 5% of the total nitrogen applied) will be lost via denitrification for each day that soils are saturated when soil temperatures are above 65°F to 70°F. Below 55°F, it is estimated that denitrification will be closer to 1% to 2% of the nitrogen that is in the nitrate form and increase to 2% to 3% when temperatures are between 55°F and 65°F.

How much N loss has occurred? Loss will vary, but the following example provides a guide for determining losses from specific situations. Assume

  1. 180 lb N/acre were applied on November 1, 2003, without a nitrification inhibitor.
  2. Corn was planted on a silty clay loam soil on April 25, with a resultant stand of 25,000 plants per acre.
  3. Soils were saturated for 9 days, from May 15 to 24.
  4. The 5-year average yield for the field is 180 bushels/acre.
  5. The previous crop was soybean.
Step 1:

Calculate N present as nitrate
N applied x % in nitrate form
180 lb N/acre x 0.65 = 117 lb N/acre

Step 2:

Calculate N denitrified
N in nitrate form x % denitrified
117 x .27 (9 days x 3%/day)
31 lb N/acre lost

Will it pay to apply more N? This 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 N remaining from your earlier application is 40 to 80 lb N/acre less than you will need, apply an additional 60 lb N/acre. If the calculated need is more than 100 lb N/acre, add an additional 90 lb N.

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

  1. Inject anhydrous ammonia or UAN solutions.
  2. Broadcast ammonium nitrate or ammonium sulfate.
  3. Broadcast urea.
  4. Dribble UAN solutions between the rows.
  5. 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 it will cause severe foliar burn.

How late can I apply 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, you 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

Robert Hoeft

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