The old rule of thumb is to apply fall ammonia after soil temperatures move to 50 degrees Fahrenheit and are trending downward.  As with much science, tests and research continues to refine information. Long term soil temperature analysis over time has shown that soil temperatures in December and January may not stay below 32 degrees Fahrenheit leading to conversion of ammonium to nitrate nitrogen according to Dr. John Lory, State Nutrient Management Specialist with University of Missouri.

To completely stop this conversion during winter, soil temperatures must below 32 degrees to prevent soil microbes from converting ammonium to nitrate.

What this means for growers this year? Currently soil temperatures are high through the past winter so that heat units of approximately from an October 1 ammonia application has accumulated over 1,175 heat units so all ammonia is converted to nitrate. Once in nitrate form, it can move with soil water and subject to losses.

Applications from October 15 have accumulated approximately 900 heat units and so will also be converted. November 1 has accumulated 905 heat units which most nitrogen is also converted.

Critical values are 900 to 1,000 heat units for ammonia conversion to nitrate nitrogen. If an ammonia nitrification inhibitor was added, double the heat units.  In May, even those applications with a nitrification inhibitor, will be converted to nitrate nitrogen.

Given this, we need to monitor corn color for health. Losses can occur during wet springs especially during the month of June.

For more information, contact Wayne Flanary, Field Specialist in Agronomy, University of Missouri Extension at 816-279-1691.