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Know Where And When To Apply Fall Nitrogen Fertilizer
The Minnesota Department of Agriculture (MDA) advises farmers and applicators to check soil temperature and delay fall application of anhydrous ammonia and urea fertilizer until soil temperature stays below 50 degrees F.
 
To assist tracking soil temperature, the MDA provides real-time soil temperatures at 48 locations across the state at https://app.gisdata.mn.gov/mda-soiltemp/. The website includes a map with MDA sites with soil thermometers at a six-inch depth, North Dakota Ag Weather Network sites at four-inch depths, and research sites at various depths.
 
“There are areas of the state where fall application of nitrogen fertilizer is simply not recommended due to groundwater contamination concerns,” said Bruce Montgomery, manager of the MDA Fertilizer Management Section. “Those would be areas with coarse-textured soils that drain quickly or areas underlain by fractured bedrock karst geology. In other areas of the state where fall nitrogen fertilizer application is a recommended practice, the MDA encourages delaying application until soil temperatures cool down.”
 
Map showing average date that 50 degree F soil temperature is reached in Minnesota. On average soil temperatures reach 50 degrees F during the first week in October in northern Minnesota and the fourth week of October in southern Minnesota.
 
Waiting until soil temperature stays below 50 degrees 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 or tile drainage. At cooler temperatures microbial activity in the soil slows down, slowing the conversion from ammonium to nitrate. Ammonium is stable in the soil whereas nitrate moves with water and may leach out of the root zone over winter and early spring.
 
Although the soil temperature network was established to support application of commercial fertilizer it is equally useful for those applying manure in the fall. University of Minnesota Extension recommends delaying fall manure applications until soil temperatures at six-inch depth are below 50 degrees F to prevent leaching losses. Research from the University of Minnesota at Waseca showed liquid dairy and hog manures injected in November produced yields 10 bushels per acre higher than manures injected in September and October.
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