Soil Temperature Maps Help with Timing of Nitrogen Fertilizer Application, Illinois State Water Survey

Press Release

For Immediate Release October 2, 2002
Soil Temperature Maps Help with Timing of Nitrogen Fertilizer Application
Bob Scott - (217) 333-4966, Fax: (217) 244-0777,
Eva Kingston - (217) 244-7270, Fax: (217) 333-6540,
Illinois farmers have a tool to help them determine when to apply post-harvest nitrogen fertilizer with daily maps of soil temperatures across Illinois, available on the Web ( from the Illinois State Water Survey (ISWS), a division of the Illinois Department of Natural Resources.

The maps are based on continuous observations of soil temperatures at selected Illinois Climate Network sites and are updated by 4 a.m. each day. "These maps serve as a guide to general soil temperatures within a given region and reflect existing temperature trends across the state as the fall season progresses. Given the variability of temperatures within a local area, it is important that farmers and applicators verify the soil temperature of each field before applying nitrogen fertilizer in the fall," says ISWS meteorologist Bob Scott.

The maps represent soil temperatures observed between 9:00 a.m. and 10:00 a.m. (Central Standard Time) the previous day across Illinois at a depth of 4 inches below a bare soil surface and daily (midnight to midnight) 4-inch bare soil maximum and minimum temperatures at the same locations.

The Illinois Agronomy Handbook, available from the University of Illinois College of ACES, states that fall soil temperatures determine when ammonium-containing nitrogen fertilizer may be applied without the occurrence of excessive nitrification. The rate of nitrification is reduced at soil temperatures of 50°F and below. Application of anhydrous ammonia with a nitrification inhibitor can begin at soil temperatures below 60°F. The soil temperature at 10:00 a.m. each day is used in making the determination.

Map users should be aware of current soil temperatures and short- to long-term weather forecasts. According to Scott, "soil temperature fluctuations during fall may result in periods with soil temperatures below the accepted threshold for nitrogen application followed by an extended period with soil temperatures above the accepted threshold. The date of the first soil temperature below 50°F each fall is often several weeks before the last soil temperature above that value.”

Other maps on the Web site show average dates when soil temperatures drop and remain below 60°F and 50°F, respectively. Due to occasional periods of warm weather in winter, the handbook does not recommend fall application of nitrogen south of Illinois Route 16, roughly the southern third of the state.

The Web site was created through a grant from the Illinois Department of Agriculture Fertilizer Research and Education program and also includes soil temperatures at 4 and 8 inches under sod, air temperatures, dew point temperatures, wind speed and direction, solar radiation, potential evaporation, and precipitation in map and table formats.

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