Information from the Illinois Agronomy Handbook, available from the University of Illinois College of ACES, states that fall soil temperatures determine when ammonium-containing N fertilizer may be applied without excessive nitrification. The nitrification rate is reduced at temperatures of 50°F and below. Anhydrous ammonia application with a nitrification inhibitor may begin at soil temperatures below 60°F. The 4-inch bare soil temperature at 10:00 a.m. each day is used to make this determination.

With a grant from the Illinois Department of Agriculture Fertilizer Research and Education Program, the ISWS’ Water and Atmospheric Resources Monitoring (WARM) Program (http://www.sws.uiuc.edu/warm/) developed daily maps of soil temperatures at 19 sites across Illinois 4 inches below a bare soil surface observed between 9:00 a.m. and 10:00 a.m. (Central Standard Time) on the previous day. Daily (midnight to midnight) maximum and minimum temperatures at the same depth also are provided for each location.

According to Scott, “soil temperature fluctuations during the fall may result in days with temperatures below the accepted threshold for N application followed by an extended period of time when soil temperatures once again exceed the accepted threshold. Thus, users need to be aware of both current soil temperatures and short- to long-term weather forecasts.”

The WARM Web site provides mean dates when soil temperatures drop and remain below 60°F and 50°F, respectively, as well as daily values of soil temperatures under a sod surface at depths of both 4 and 8 inches. In addition, daily observations on air temperature, dewpoint temperature, wind speed and direction, solar radiation, potential evapotranspiration, precipitation, and growing and pest degree-day information across Illinois are presented for the most recent 7 days.

Map data, updated by 4 a.m. each day, represent conditions at actual locations where observations are taken. Elsewhere, the information should be viewed as a guide to general soil temperatures within a given region and to current temperature trends progressing across the state.

“Farmers and applicators are encouraged monitor the soil temperature of each field to be treated before fall application of N fertilizer. The Illinois Agronomy Handbook recommends that no fall N application south of Illinois Route 16, roughly the southern third of Illinois,” reminds Scott.