Extension Ag Update
September/October 2007
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Fall Nitrogen

Ellen Phillips, Extension Educator-Crop Systems, 708-352-0109, ephillips@uiuc.edu

Check soil temperatures to know when ammonium containing nitrogen fertilizer may be applied without excessive nitrification.  The websites below give the current and long-term average of soil temperatures across Illinois.

The Illinois Agronomy Handbook states that at  50°F and below, the rate of nitrification is reduced since cool temperatures slows  biological activity that converts nitrogen in the ammonium form to nitrate form.  At soil temperatures below 60°F, anhydrous ammonia application with a nitrification inhibitor can begin.  Inhibitors slow the conversion of ammonium to nitrate. They work best in moderately or well-drained soils or when there is excessive rainfall and frequent flooding.

Soil temperatures do fluctuate during the fall.  It is not uncommon to have a period with soil temperatures below the accepted threshold for N application followed by a time where soil temperatures are above the accepted threshold. Therefore, when making your decision of when to apply nitrogen, use the current soil temperature and short- to long-term weather forecasts.

Current Soil Temperatures for Illinois

Long-term Soil Temperature Averages