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Topdressing nitrogen on wheat? Keep these 4 factors in mind

Agriculture.com Staff 02/24/2009 @ 7:25am

Timing is everything. And, the time to topdress this year's winter wheat crop is now.

The key thing to watch for is jointing. Applied nitrogen needs to be at the root zone level by the time the crop is jointing, says Kansas State University agronomist Dave Mengel.

"The nitrogen in topdress applications should be moved into the root zone with precipitation well before jointing begins," Mengel says. "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 says there are four factors important to good nitrogen management when topdressing wheat.

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.

"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 says.

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.

Timing is everything. And, the time to topdress this year's winter wheat crop is now.

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 says.

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