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Fall Nitrogen Application

By John Lamb, Extension Soil Scientist

This year the crops have matured early and harvest is moving ahead of normal.  With a large amount of the soybeans and corn coming out, thoughts are turning to getting fertilizer applied for next year's crop.  For phosphorus and potassium, there are very few problems with an early fall application.  These nutrients are not mobile in most soils. The only big concern with a broadcast application of P and K is getting the fertilizer incorporated into the soil so it is in a place for the plant roots to utilize them next spring.  Incorporation also reduces the chances of P and K being lost through erosion.

Nitrogen is a mobile nutrient and therefore must be managed different to get the most nutrient value and the least amount of loss to the environment.  If you are in the Southeastern part of Minnesota or farm sandy ground, DO NOT apply nitrogen in the fall.  The rainfall in southeastern Minnesota along with the Karst geology will result in large losses of N from fall application.  If you farm sandy ground, N applied in the fall will not be in the soil when spring arrives.  Fall N application on sandy soil, irrigated or not, is a total waste of time and money and presents large risks of groundwater pollution.

In the South Central part of Minnesota, application of fall N is not the most efficient management option.  If your operation requires you to apply some N in the fall, there are some things you can do to get the most N out of the fertilizer application.  First, DO NOT apply nitrogen fertilizer before the soil temperature at the 6 inch depth is consistently below 50 degrees.  Second, use only an ammonium form of nitrogen.  Anhydrous ammonia would be the preferred followed by urea.  If you have a field that is consistently wet, you may want to consider the use of a nitrification inhibitor to slow the conversion of ammonium to nitrate.  The use of the inhibitor is NOT a way to allow for application before the soil temperatures are below 50 degrees.

In the Southwest, West Central, and Northwestern parts of Minnesota, fall applications of ammonia based N sources is ok if the soil temperature is less than 50 degrees.
At the time of writing this, Minnesota agricultural areas were experiencing drought at one degree or another.  The ground is hard - maybe harder than last year.  With this in mind, a late fall application of N after we receive some rain maybe the best fall option.  It will reduce the chances of loss by getting a better soil cover of the ammonia band and also save on the wear and tear of the tillage and application equipment.  If you use urea, it must be incorporated to keep it from volatilizing.  Dry soils are good candidates of urea volatilization to occur.  Research with fall N applications, has shown that anhydrous ammonia will have a lower loss of nitrogen than urea.

Also with the dry summer, it is strongly suggested that you take a soil nitrate - N test.  This is particularly true if the 2012 crop was corn.  With the dry summer, the crop may not have used all of the N fertilizer applied for the previous crop and left a large amount of residual nitrate-N that could be used by the 2013 crop.  To be useful, a soil sample for nitrate should be taken to a depth of 2 feet for corn and 4 feet for sugar beet.  The sample should be taken after the soil temperature is below 50 degrees.  A soil sample taken before the soil temperatures are below 50 degrees is a waste of money and time.  The nitrate-N soil test value will be erroneous.

If the weather conditions continue dry into winter, you should strongly consider spring application.  Spring applications result in less chance of N loss and you will also have a better idea of the crop potential in 2013.  A spring preplant soil nitrate-N test will also be helpful, similar to the fall soil test described in the above paragraph.

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  1. Dan, I have never heard of having to wait to sample until the soil temperatures get to 50 degrees. NDSU has repeatedly publicised that nitrate tests can start in Sept.
    It is understood we should not be applyng nitrogen until the soil is 50 degrees.
    Just wondering what the real story is. Thanks for the information.
    Paul Groneberg

  2. John,
    I cannot agree with these 2 sentences:
    "A soil sample taken before the soil temperatures are below 50 degrees is a waste of money and time. The nitrate-N soil test value will be erroneous."
    Your article N fertility application is good, but these 2 sentences don't make any common sense or have any practical application. In our droughty conditions, where is the soil test N going to go? How much rain will it take to move the N into the soil profile? 2 inches of rain will wet the top 12 inches.
    Are we wait until mid-October to start soil sampling for N? How'll that work for growers/samplers/applicators? Not too good.
    How much N will be mineralized in our droughty soil? Not much if any.
    I cannot agree with these 2 sentences...
    Richard Jenny

  3. Paul, that comment caused a little consternation for a few people out west. Comments from both NDSU and SDSU that I have received indicate not too large of a difference with the earlier sampling. There still are some areas I would suggest waiting until as late as possible especially the farther east you go. I would suspect that there may not be too much of a difference this year especially with the dry conditions. The biggest risk for changing concentrations should be areas that traditionally will get more rainfall or the soils remain warmer for a longer period of time.
    Thanks for the comment.


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