No. 7 Article 8/May 8, 2009

Have I Lost My Nitrogen This Spring?

Wet soil conditions this spring are a reason for concern that some of the nitrogen (N) applied last fall for the 2009 corn crop might be lost. When soils become saturated, the potential for N loss is directly related to the amount of N present in the nitrate (NO3-) form. When soil temperatures start to increase and water-saturated conditions exist, nitrate is most likely to be lost through denitrification in fine-textured soils. In coarse-textured soils, leaching below the root zone is the biggest concern. Most of the fall-applied N is either ammonium (NH4+) or a form that transforms rapidly into ammonium. Nitrification, or the conversion of ammonium to nitrate, is a bacteria-mediated transformation. The bacterium Nitrosomonas converts NH4+ into nitrite (NO2-), while the bacterium Nitrobacter converts NO2- to NO3-. The activity of these bacteria is minimal at temperatures below 50°F. The bacteria also need aerobic (unsaturated soil water) conditions to nitrify ammonium. Thus, the amount of nitrification that occurs in the soil depends largely on soil temperature and the time elapsed from application until the soil becomes saturated with water. Further, the nitrification process can be reduced with the use of inhibitors that will lower the activity of these bacteria and allow N to stay in the ammonium form for a longer period. Since the time elapsed between application and crop uptake is short for spring applications, the use of inhibitors is most useful in fall.

What is important to remember is that only the portion of the applied N that is in nitrate form is subject to denitrification or leaching. The fact that N is in the nitrate form does not mean that N is lost; it means rather that is susceptible to loss. Table 2 shows the percentage of ammonium that was transformed to nitrate by the end of May in three Illinois locations depending on whether and when a nitrification inhibitor was used. Once you determine how much of your N is in the nitrate form, you can estimate how much N is potentially lost through denitrification by taking into account soil temperature and the number of days the soil has been saturated. Research has shown that for each day the soil is saturated with water, 4% to 5% of the N in the nitrate form is lost via denitrification when temperatures are above 65 to 70°F. When temperatures are between 55 and 65°F the loss is 2% to 3%, and when temperatures are below 55°F losses are estimated to be 1% to 2%. Soil temperatures at the 4-inch depth can be found at www.sws.uiuc.edu/warm. Again, these losses are not for the total N applied, but rather for the portion that is in the nitrate form. Losses will vary depending on various factors, but these values offer an estimate.

The following calculation is a hypothetical situation given as an example using the data in Table 2:

Let's assume that 180 pounds N per acre were applied in early November with a nitrification inhibitor in a silty clay loam soil in DeKalb, and soils were saturated for the last 5 days in April.

First, calculate N present as nitrate: N applied x % in nitrate form

180 lb N/acre x 0.55 = 99 lb N/acre

Second, calculate N denitrified: N in nitrate form x % denitrified

99 x .20 (5 days x 4%/day) = 20 lb N/acre lost

At this point it might be too early to decide if additional N is really necessary. However, if you determined that you lost nitrogen you will need to decide whether it will be economically advantageous to apply more. If the yield potential is reduced because of late planting or poor stands, there might not be need to apply additional N. If in the calculation you find that you will be around 40 to 80 pounds of N per acre below what you need for the crop, it would be appropriate to apply an additional 50 to 60 pounds per acre; if you are more than 100 pounds per acre below what is needed, an additional 90 pounds per acre can be applied. If the field is already planted, the best way to apply the additional N would be--in order of preference--injected anhydrous ammonia or UAN solutions, broadcast ammoniated products (ammonium nitrate or ammonium sulfate), broadcast urea, UAN solution dribbled between rows, and broadcast UAN solution. If you have not planted your field yet, plant now and apply the additional N later so planting is not delayed further.

Another way to determine if additional N will be needed is to establish a couple strips with supplemental N across the field at a rate between 60 and 80 pounds acre and to compare the crop colors as the season progresses. If you find that those strips are substantially greener than the rest of the field, you can apply 60 pounds of N per acre to the rest of the field, as long as you do it before tasseling. Potential drawbacks to this approach are that color differences could develop too late for a timely application or there might not be sufficient rain to move the late-applied N into the root zone.

Another way to determine the need for additional N is to use the pre-sidedress nitrate test (PSNT). However, the test reliability is heavily influenced by the field and by the way samples are collected. Typically the test works best in fields with high potential for N mineralization, such as fields that have been manured in the last 2 to 3 years. A sample to 12 inches deep is collected when corn plants are 6 to 12 inches tall (V4 to V6 development stage), or in late May to early June when planting is delayed. If the field had a history of broadcast applications, randomly collect 20 to 25 samples from an area no greater than 10 acres. If band applications of fertilizer or manure were used to fertilize the previous crops, collect at least 10 sets of three cores each from two corn rows.

Collect the first core 3 inches to the right of the right-hand row, the second core between the two rows, and the third core 3 inches to the left of the left-hand row. In all cases, place all the cores in a bucket and obtain a subsample after the cores have been thoroughly mixed. If mixing the entire sample to produce a representative subsample is too difficult, it is better to use a large sample bag and keep the entire sample. Collecting a sample less than the full 12 inches or not collecting all the cores will produce unreliable results. If the samples cannot be delivered to the laboratory the same day, either freeze or air-dry the soil. If you air-dry samples, dry them as fast as possible by spreading them out on a paper, crushing the cores, and blowing air with a fan. Since drying can be difficult without proper facilities, freezing samples is likely the best option for most people. Make sure to tell the laboratory that you want to measure NO3- nitrogen. If the entire sample is sent, request that the whole sample be dried and ground before a subsample is taken. If the test results come back above 25 parts per million, no additional nitrogen will be necessary for the 2009 crop.--Fabián G. Fernández

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