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Posted 16 February 2009. Crop Management. Soil Specialist Gives Tips on Nitrogen Topdressing of Wheat Crop Kansas State University. www.ag.ksu.edu Manhattan, Kansas (February 11, 2009)--Timing is everything – so the saying goes – and the time for topdressing the winter wheat crop is now, said Kansas State University agronomist Dave Mengel.
The key consideration is to make sure the plants have enough nitrogen at the right time, said Mengel, who is a soil fertility specialist with K-State Research and Extension. “The nitrogen in topdress applications should be moved into the root zone with precipitation well before jointing begins. Ideally, the nitrogen should be available to the wheat when head differentiation occurs and head size is being determined, which can be about two weeks before jointing,” Mengel said. The four main factors involved in good nitrogen management when topdressing wheat , he said, are timing, source, application method, and rate. Timing It is best to apply topdress nitrogen early – preferably before the end of February – to have the best chance of receiving enough moisture to move the nitrogen into the root zone. This is especially true in the western half of Kansas. “While some producers wait until spring just prior to jointing, this can be too late in some years. For well-drained medium-fine textured soils that dominate our wheat acres, the odds of losing much of the nitrogen that is topdress-applied in the fall or winter is low since we typically don’t get enough precipitation over the winter to cause significant denitrification or leaching. For these soils, topdressing should begin anytime now. Usually, the earlier the better,” Mengel said. For wheat grown on sandier soils, it is better to wait until closer to spring green-up to make topdress nitrogen applications, he added. On these soils, there is a greater chance that nitrogen applied in the fall or early winter could leach completely out of the root zone if precipitation is unusually heavy during the winter. It is also better to wait closer to spring green-up on poorly drained and/or shallow clay pan soils, he said. Nitrogen applied in the fall or early winter would have a significant risk of denitrification nitrogen loss on these soils. Nitrogen should not be applied to the soil surface when the ground is deeply frozen. This will help prevent runoff losses. Application Method Most topdressing is broadcast applied. In high-residue situations, this can result in some immobilization of nitrogen, especially where liquid UAN (urea ammonium nitrogen) is used, Mengel said. If no herbicides are applied with the nitrogen, producers can get some benefit from applying the nitrogen in a dribble band on 15- to 18-inch centers. This can help avoid immobilization and maybe provide for a little more consistent crop response. The ideal application method would be to subsurface place the nitrogen into the soil. Source The typical sources of nitrogen used for topdressing wheat are UAN solution and dry urea. Numerous trials by K-State over the years have shown that both are equally effective. In no-till situations there may be some slight advantage to applying dry urea since it falls to the soil surface and may be less affected by immobilization than broadcast liquid UAN, which tends to get hung up on surface residues. Dribble UAN applications would avoid much of this tie-up on surface crop residues as well. Rate Producers should have started the season with a certain nitrogen recommendation in hand, ideally based on a profile nitrogen soil test done before the crop was planted and before any nitrogen was applied. If some nitrogen has already been applied to the wheat crop, it is too late to use the profile nitrogen soil test since it is not reliable in measuring recently applied nitrogen. One new alternative for selecting the optimum nitrogen rate is to use an active sensor such as a GreenSeeker, Mengel added. “The ideal way to use this tool would be to establish a series of reference strips, or high nitrogen areas in the field, before the crop breaks dormancy, and then use the sensor to determine the overall nitrogen rate for the field at jointing,” he said. A nitrogen rate calculator and instructions for establishing reference strips for wheat using the sensor is available on the K-State Soil Testing Laboratory Web site at www.agronomy.ksu.edu/soiltest. Wheat used for pasture needs a little extra nitrogen. “If the wheat was grazed this fall and winter, producers should add an additional 30 to 40 pounds of nitrogen per acre for every 100 pounds of beef weight gain removed from the field. For heavy grazing, it may be necessary to make an additional nitrogen application in late winter to compensate for nitrogen removed by grazing, depending on how much nitrogen was applied earlier,” Mengel said. Contact: |
