Written by Anne Struffert and Greg Klinger
University of Minnesota Extension Educators in Agricultural Water Quality Protection
Despite delays due to wet weather, harvest is in progress or already finished for most Minnesota row crop acres. Now an important decision for many farmers is whether or not, and when, to apply fall nitrogen fertilizer. Fall nitrogen fertilizer is only recommended in the northern and western part of the state where climate and soils make nitrogen loss less likely. (More information on where fall nitrogen is appropriate can be found here).
University of Minnesota Best Management Practices (BMPs) recommend waiting until the top six inches of the soil are consistently at or below 50°F. This is because, regardless of the form of nitrogen fertilizer applied, nitrifying bacteria in the soil can convert it to the nitrate form. Nitrate is the form of nitrogen most susceptible to loss. Nitrifying bacteria remain somewhat active until soils freeze, but their ability to convert other forms of nitrogen to nitrate is minimal at soil temperatures below 50°F. In order to prevent nitrogen losses, the timing of fall nitrogen applications should be decided by soil temperature, not by date or other considerations.
As of November 1st, soil temperatures in most of central and southern Minnesota have yet to stabilize at 50°F or less. If you’re in an area of the state where this is the case, hopefully you have the option to wait another week or two to apply fertilizer or manure. You might not be able to prevent all nitrogen loss, but by waiting the extra time for the soils to cool you have a better chance of protecting the investment you have made in your fall fertilizer.
Recently, there have been many questions about the effectiveness of nitrification inhibitors. These chemicals, which prevent the conversion of other nitrogen forms into nitrate, are recommended as a BMP in areas where fall nitrogen application is acceptable. Although research has shown that chemicals such as nitrapyrin and diciandyamide (DCD) can slow the conversion of ammonium to nitrate, these products are only active during a short window of time. They are generally effective for around 10 days, but temperature can impact this quite a bit. High soil temperatures will speed up breakdown of these products, making them much less effective, while cooler soil temperatures will allow them to be effective for longer periods of time. Thus, the cooler the soil temperature the greater the chance that your nitrogen fertilizer application will stick around in the soil until the spring, when you need it. Long-term studies in south-central Minnesota have shown that using nitrapyrin as an inhibitor with fall-applied nitrogen fertilizer added an average of 10 bushels of corn yield.
Up-to-date soil temperatures can be accessed at https://app.gisdata.mn.gov/mda-soiltemp. However, these values should be used as a reference. Since soil temperatures can be influenced by a number of factors (such as residue cover, soil color, and drainage), it is always best to monitor temperatures in individual fields prior to nitrogen application.