As harvest begins, the Minnesota Department of Agriculture (MDA) is advising farmers and fertilizer applicators to check the soil temperature when determining the best time to apply anhydrous ammonia (NH3) and urea this fall.
Waiting until the soil temperature stays below 50º F before applying anhydrous ammonia and urea increases the availability of nitrogen to next season’s crop and decreases the amount of nitrate that could potentially leach into groundwater.
Soil temperature is measured at a six-inch depth; the same depth anhydrous ammonia is typically applied. To help farmers know when the target 50º F soil temperature has been reached, the MDA has established 21 real-time, statewide soil temperature monitoring stations, (http://gis.mda.state.mn.us/soiltemp). Station data is updated every 15 minutes with the help of satellite technology from the Minnesota Department of Natural Resources and the National Weather Service.
There are no state regulations regarding fall nitrogen application. However, for some regions of the state, fall application of anhydrous ammonia and urea is not recommended regardless of the soil temperature. This includes southeastern Minnesota and statewide on coarse-textured soils. The University of Minnesota Best Management Practices (BMPs) for Nitrogen Use offers fall nitrogen application guidelines: www.mda.state.mn.us/nitrogenbmps.
Regional BMP recommendations are given below:
- Statewide on coarse-textured soils: Fall application of nitrogen is not recommended regardless of source.
- Southeastern Minnesota: Fall application of nitrogen is not recommended regardless of source.
- South-Central Minnesota: Spring applications of anhydrous ammonia and urea are recommended. If applied in fall, use a nitrification inhibitor after six-inch soil depth temperature stays below 50º F.
- Southwestern, West-Central, and Northwestern Minnesota: It is recommended to delay fall application of anhydrous ammonia and urea until six-inch soil depth temperature stays below 50º F.
The MDA recommends fertilizer applicators check soil temperatures at a soil temperature monitoring station to their north to anticipate cooling trends and the station closest to them to determine when 50º F soil temperatures have been reached in their area.
In the coming years, the MDA plans to develop proposed restrictions of nitrogen application in the fall and on frozen ground in areas vulnerable to groundwater contamination, which includes parts of southeast Minnesota and areas of the state with coarse-textured soil. The public will have an opportunity – through a process that typically takes 2-3 years – to give input prior to the new restrictions taking effect.