Know Where and When to Apply Fall Nitrogen Fertilizer

Avoid Areas with High Leaching Potential and Apply When Soil is Cooler Than 50 Degrees F

ST. PAUL, Minn. – According to the Minnesota Department of Agriculture (MDA), the recent cold-weather snap has cooled soil temperatures quickly in the northern regions of the state to the point where it may be safe to apply fall fertilizer. The southern areas of the state are still a bit warmer and delaying fall application is advised.

The MDA is urging farmers and applicators to check soil temperature and delay fall application of anhydrous ammonia and urea fertilizer until soil temperatures stay below 50 degrees F.

To assist tracking soil temperature, the MDA provides real-time soil temperatures at 25 locations across the state. 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’s 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.”

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. Microbial activity in the soil slows down at cooler temperatures, therefore 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 the 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.

Here are some other regional recommendations according the University of Minnesota Extension.

• Western MN (non-coarse textured soils): Fall application of either anhydrous ammonia or urea are recommended practices.
• Southeastern MN: Fall application of nitrogen fertilizer is not recommended regardless of soil temperature because of the karst geology.
• South Central MN: Use a nitrification inhibitor when fall applying anhydrous ammonia and not to apply urea in the fall.
• Statewide: Fall application of nitrogen fertilizer is not recommended regardless of soil temperature on any course-textured soil. Also, fall application of 28% liquid nitrogen is not recommended anywhere in the state due to its high leaching potential.

Additional specific nitrogen fertilizer use recommendations by region are available on the MDA website.

The MDA has proposed the Groundwater Protection Rule that if approved will restrict fall nitrogen fertilizer application in areas vulnerable to groundwater contamination beginning in January 2020. The rule helps improve Minnesota’s water quality by limiting the potential for nitrate leaching. The Groundwater Protection Rule is part of the state’s Nitrogen Fertilizer Management Plan.

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