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Fahrenheit 50: Fall weather and nitrogen loss

I arrived in Minnesota during a December that was extremely wet and extremely warm (at least by Minnesota standards).  Being accustomed to more southern climates, it made me feel right at home, but I have to say- it isn’t very good for water quality, and it isn’t good for farmers.  Especially if you applied manure last fall here in southeastern Minnesota.

The microorganisms that are responsible for nitrification, the process that converts ammonia or ammonium nitrogen into the nitrate form are more active at higher temperatures.  While most nitrogen in manure is in organic forms, much of the organic nitrogen will convert to ammonia, and then to nitrate by the nitrification process.  Nitrification slows to a crawl when soil temperatures fall below 50° F.  This year that did not happen until mid-November in southeastern Minnesota; in an average year it would be closer to mid-October.  That difference can be critical because most of the rains we get that leach down through the soil into groundwater happen in late fall and in the spring, and nitrate is the only persistent form of nitrogen that moves easily with water.  So, if the soil temperatures stay warm for a while after manure application, nitrogen can be rapidly nitrified and lost from the soil when there isn’t a living crop there to use the nitrogen. 

Think about it this way: if you apply manure in the fall when soil temperatures are still fairly warm, a decent amount of the urea and ammonia in that manure could convert to nitrate before the soils freeze up, and then that nitrate could get leached below the rooting zone with spring rains.  That same manure applied after the soil cools below 50° F would not form much nitrate before the soils froze.  In the spring, it would take two or three weeks of warm soil temperatures before the urea and ammonia in that manure would convert to nitrate, and so any rain coming in the intervening time would not be leaching the nitrogen down through the soil- you have a lower risk of nitrogen loss.  Small differences in timing your manure applications can lead to big differences in how much nitrogen sticks around for your crop.

Weather conditions from fall 2015 to spring 2016 have definitely made a good case for using nitrification inhibitors, which slow the conversion of ammonia or ammonium nitrogen forms into nitrate.  Nitrification inhibitors lessen nitrate losses and improve yields in many years, especially those that are warmer and wetter during the late fall through spring.  I think it’s a no-brainer with fall-applied manure.

So, if you applied fall manure this year, particularly before mid-November or late October without a nitrification inhibitor, it’s probably worth asking how much of the nitrogen you applied is still around for your crop this spring.  Fortunately, there is a well-validated spring soil nitrate test that can help you answer that question- U of M extension has a good publication on it here.  I would highly recommend it if you are relying on fall-applied manure as a part of, or all of, your nitrogen fertilizer source.  Feel free to contact me if you have questions or comments.

Note: I talked about nitrogen from manure here, but the same challenges occur with fall-applied nitrogen fertilizer.  Because of the high potential for leaching losses, fertilizing in the fall is not recommended in southeastern Minnesota.  More information can be found in the Nitrogen Best Management Practices publication found here.

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