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FERTILIZER NITROGEN APPLICATION THIS FALL
John Sawyer, Department of Agronomy, Iowa State
October 14, 2015
Fall nitrogen fertilizer application
Much of the primary fertilizer nitrogen (N) for corn is applied in the spring as preplant or sidedress, where efficiency of N use should be greatest. Fall N fertilizer application success can be enhanced by following several suggestions: only use anhydrous ammonia; apply in late fall after soils cool to 50 degrees F (4-inch depth) and are trending cooler (the colder the better, and most years in Iowa this is sometime in November); consider a nitrification inhibitor to further slow nitrification to nitrate; and avoid fall application to soils that are more prone to wetness or leaching (poorly or excessively drained soils). Fall-applied ammonia can work; however, wet springtime conditions will reduce the efficiency and success in achieving corn yield potential. Other N fertilizers, like urea and UAN (urea-ammonium nitrate solution), nitrify too quickly which increases chance for loss and therefore should not be fall applied. For example, research across many years at the ISU Northern Research Farm at Kanawha has documented lower corn yield with fall incorporated urea compared to spring incorporated urea. The co-nitrogen application in DAP and MAP can be at risk with fall application due to rapid nitrification (N in DAP and MAP is in the ammonium form), and with the often early fall application. Read on.
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