ST. PAUL -- Farmers who want to apply fertilizer this fall are being asked to be patient.

The Minnesota Department of Agriculture says farmers should delay application of anhydrous ammonia and urea fertilizer, as well as manure, untilaverage soil temperatures reach 50 degrees F or less.

The ag department provides real-time 6-inch soil temperatures at 25 locations in the state. The interactive map of the current 6-inch soil temperature and the past week’s history can be found at www.mda.state.mn.us/soiltemp.

Fall fertilizer application is not recommended in all areas of the state due to groundwater contamination concerns. Regions with coarse-textured soils, areas over shallow bedrock, and regions with karst geology are the most vulnerable to nitrate loss and groundwater contamination.

Beginning in the fall of 2020, fall fertilizer application in these regions will not be allowed. A map of the vulnerable regions is available at www.mda.state.mn.us/vulnerableareamap.

In areas of the state where fall nitrogen fertilizer application is allowed, delaying application until soil cools to 50 degrees of less helps prevent loss, protect swater quality, and ensures more nitrogen will be available for next season’s crop.

At cooler soil temperatures, the fertilizer is less likely to be converted to water soluble nitrate. This occurs because the microbes in the soil that convert ammonium to nitrate are less active in cooler temperatures. Less activity and less conversion mean more ammonium will remain in the soil. Once converted to nitrate, it can be moved by water percolating through the soil, leaching out of the root zone. This occurs in early spring when soils thaw.

The same temperature guideline works for those applying manure in the fall. 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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