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Estimating Nitrogen Loss

May 29, 2003

Many areas, particularly in southern Illinois, have received significantly more rain than normal. The excess water has created numerous problems, not the least of which is enhanced nitrogen (N) loss potential.

Different techniques can be used to estimate or measure the amount of N loss that might have occurred during this excessively wet period. None of these techniques are sure bets; in fact, unless used with caution, any of them may be misleading. The options along with their precautions follow.

Amino Sugar-N Test (Illinois N Soil Test)

This test will not predict N loss from excess water. It 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 likely won't 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 testing procedure is heavily dependent on making sure the samples are collected, handled, and processed correctly. Even if sampling, handling, and processing are done correctly, the reliability of this test when values are low is questionable. If the values are high--greater than 25 ppm--then the odds are good that no additional N will be needed for the 2003 crop. The following suggestions are derived in large part from research conducted by faculty at Iowa State University.

Sample collection. Collect soil samples to a 1-foot depth, at eight positions perpendicular to the direction of travel of the nitrogen applicator. The eight positions are preassigned as follows: one 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 positions sequentially 1/8, 1/4, 3/8, 1/2, 5/8, 3/4, and 7/8 the distance between the row. All of the soil from the eight cores to the full 1-foot depth needs to be placed into the sample bag. Collection of any probe to less than the full 1-foot depth or elimination of any of the cores will render the results suspect. Because most commercial soil sample bags will not hold that much soil, be sure to use a larger bag. The normal recommended time of sampling is when corn is 6 to 12 inches tall. However, because many fields have not yet been planted, we suggest that the samples be collected in late May or early June, irrespective of corn height. Although this sampling 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 you to obtain a representative core.

Sample handling. If the samples cannot be delivered to the laboratory immediately (same day), freeze them and then deliver them to the laboratory. Another option is to air-dry them before sending them to the laboratory, but since the laboratory has better drying facilities than most farmers, it is best to freeze and then ship. If you do air-dry the samples, spread them out on a paper, crush the cores, and direct a fan on them to allow them to dry as fast as possible.

Laboratory instructions. Be sure to tell the laboratory 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 that anyone can accurately subsample from wet cores.

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

As soon as all crops are planted, consider applying two or three strips of supplemental nitrogen across the field at a rate of 60 to 80 lb N/acre. As the season progresses, compare the color of the corn in the strips with the corn in the remainder of the field. This comparison can be done visually or by using a SPAD meter. The SPAD meter will predict N-deficient corn sooner than a visual inspection, but these instruments are not readily available. Check to see if your local dealer has access to a SPAD meter. If the 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 application of 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 with this option is that the late-applied N will require a significant rain after the N is applied to move the N into the root zone. Rains during that time are traditionally less frequent.

Estimation of 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 has been converted to nitrate. The rate of this conversion is dependent on soil temperature since the time of application and whether or not a nitrification inhibitor has been used. Refer to Table 1 to help you make this determination.

The 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 will be lost through the process of denitrification or leaching.

Denitrification is the major nitrogen loss mechanism in most Illinois soils, particularly in 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 temperature is above 65 to 70°F. At temperatures less than 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 and 65°F.

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 that

(a) 140 pounds of N/acre was applied on April 1 without a nitrification inhibitor in southern Illinois;

(b) corn was planted on a silty clay loam soil on May 15, with a resultant stand of 25,000 plants per acre;

(c) soils were saturated for 9 days, from April 26 to May 8;

(d) the 5-year average yield for the field is 150 bushels per acre; and

(e) the previous crop was soybean.

Step 1

Calculate N present as nitrate

N applied X % in nitrate form

140 pounds N/acre X 0.55 = 77 lb N/acre

Step 2

Calculate N denitrified

N in nitrate form X % denitrified

77 X .29 (13 days X 3% per day)

30 pounds N/acre lost

Will it pay to apply more N? That 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 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, 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

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