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Fall application of 82-0-0 is a subject that needs to be reviewed each fall in Minnesota.   The Best Management Practices (BMP's) for use of nitrogen were developed about 10 years and provide research-based guidelines for nitrogen application.  These BMP's were developed to provide for optimum profit while reducing the potential impact of fertilizer nitrogen on water quality.

Using the BMP's as a guide, fall application of fertilizer N is not recommended for sandy soils and soils in southeastern Minnesota that have a silt loam texture.  Fall application of 82-0-0 is satisfactory  in south central, southwestern, west central and northwestern after the soil temperature has dropped below 50° F at a depth of 4 inches.

For crop producers who want to apply fertilizer nitrogen in the fall, it's usually difficult to wait until the soil temperature drops to 50ºF.  Is it important to wait?  Can I use the nitrification inhibitor, N-Serve and apply 82-0-0 earlier?   If I use N-Serve, with fall applied 82-0-0 can I expect higher yields?  How does 82-0-0 applied in the fall with N-Serve compare to spring-applied 82-0-0?  These are common questions that revolve around the fall application of fertilizer N.

The goal for all fall applications of fertilizer N should be to keep as much N as possible in the NH₄ (ammonium) form going into the winter.  With this strategy, only small amounts are present in the NO₃ (nitrate) form and subject to spring losses caused by either leaching or  denitrification.   The importance of soil temperature in achieving this goal is illustrated in the following table.  The research was conducted at the Southern Research and Outreach Center at Waseca. 

For this research, 82-0-0 with or without N-Serve was applied at 3 different dates in October and November and soil temperature was measured.  Soil samples collected in the following spring from a depth of 0 to 10 inches were analyzed for NH₄ -N and NO₃ -N.  The results are reported as a percentage of NH₄ -N to NH₄ -N + No₃ -N.  High percentage values are desired.   The results are summarized in the following table.

The effect of soil temperature at time of application and use of N-Serve on the amount of NH₄ - N and NO₃ - N found in the following spring.

Following the 1999 application, soil sampling were collected in late April of 2000.   Samples were collected in late March of 2001 following the application of 82-0-0 in the fall of 2000.

These results show that N-Serve was effective in keeping most of the applied N  in the NH₄ form through the winter.  Time of application of 82-0-0 had little effect on the % NH₄ -N measured in the spring of 2000.  Results from the fall applied 82-0-0 in 2000, however, were different.  The delayed application resulted in a higher % of NH₄ -N found in the spring of 2001 both with and   without the use of N-Serve.  Thus, the effect of time of application is not consistent over years.  However, it is very important that the  risk of N loss be kept to a minimum  if crop production is to be profitable.  The delayed application of 82-0-0 is one way to minimize the risk of N loss.

Considering corn yields, 14 years of research from the Southern Research and Outreach Center at Waseca show that average yields from spring applied 82-0-0 and fall applied 82-0-0 with N-Serve were equal.  The yield of fall applied N without N-Serve was lower in 6 of the 14 years.  Therefore, there is no guarantee that use of N-Serve with 82-0-0 in the fall will increase yields.  The use of N-Serve must be viewed as an  insurance policy - not a guarantee.

It is also important to understand that the choice to use N-Serve  does not   mean that 82-0-0 can be applied at soil temperatures higher than 50ºF.  There is no research which suggests that N-Serve use allows application at warmer soil temperatures.

Thinking back to the fall of 2000, it was warm and very dry.  Growers were advised to wait until soil temperatures dropped.  Many growers did wait and that was a good decision.  Soil samples were collected in mid-June of 2001 following excessive rainfall.  In those fields where N application was delayed, the amount of N measured to a depth of 2 feet was 80% to 100% of the amount of N applied in the fall.

It's difficult to wait until that soil temperature reaches 50ºF.  But, it is important.  In the southern two tiers of counties, wait until after October 25th.   From Morris north wait until after October 15.