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Everything You Need to Know Before Applying N This Fall

Fabián G. Fernández, Extension nutrient management specialist

It’s that time of year again. Fall is upon us and harvest is just around the corner. There are big decisions to make when it comes to fall nitrogen. Should you apply? If so, when? What form of N should I apply? Should I use an inhibitor? We’ve rounded up our most frequently asked questions about fall applications and the answers are all right here.

When should I apply? 

Our guiding recommendation for fall applications is the 50° F rule. Soil temperature at the depth of application can have a huge impact on the efficiency of the fertilizer and the effectiveness of nitrification inhibitors. Nitrifying bacteria are active until soils freeze at 32° F, but their activity is greatly reduced once soil temperature goes below 50° F. Higher temperatures also result in faster breakdown of the molecules that provide nitrification inhibition. The 50° F rule is a good compromise between when the activity of nitrifying bacteria is low enough and there is still enough time for nitrogen applications before soils become too wet or frozen. The cooler the temperature the greater the efficiency of an inhibitor and the greater chance ammonium does not convert to nitrate.

Look for up-to-date soil temperatures here. Use these values only as a reference. Since soil temperatures can be influenced by a number of factors, like residue cover, soil color, and drainage, it is always best to monitor soil temperatures in individual fields before nitrogen application.

What source of N should I apply?

When it comes to fall application, not all nitrogen is created equal. Avoid sources with nitrate or that transform quickly to nitrate because they can be readily lost through leaching below the root zone or denitrification off to the atmosphere. In south central Minnesota, we suggest anhydrous ammonia with a nitrification inhibitor. Polymer coated urea applied late in the fall is also acceptable, but we have seen inconsistent results with that source. In the southwest, west central and northwest regions of the state, urea incorporated with tillage is another acceptable source.

Why is anhydrous ammonia a better source than urea?

Urea converts to ammonia and then to ammonium within a few days of application. Anhydrous ammonia also converts to ammonium quickly as it reacts with soil water, but it kills the nitrifying bacteria that are responsible for the transformation of ammonium to nitrate at the point of application. As ammonia reacts with water to form ammonium, the reaction creates an alkaline (high pH) environment within the ammonia retention zone. This high pH also inhibits activity of nitrifying bacteria temporarily. In order to lengthen the period of bacterial inhibition, include a nitrification inhibitor with the application.

Why should I use a nitrification inhibitor?

Many years of research show that nitrification inhibitors can protect fall nitrogen against loss and increase the amount of nitrogen present in the ammonium form the following spring. Just like with most practices, the use of a nitrification inhibitor might not pay every year. For example, if the following spring is dry and cool, the inhibitor might not be as important to enhance ammonium recovery because the potential for N loss is low. Overall, however, the use of an inhibitor will ensure the greatest chance to protect your nitrogen investment.    

When NOT to apply fall N

In Minnesota, there are certain conditions where we do NOT suggest fall nitrogen application. They are as follows:

  • In Southeast Minnesota because of the combination of high precipitation and karst soils
  • On sandy soils or soils with shallow depth to gravel/coarse materials
  • Fine-textured soils that are heavily tiled or tend to pond water for prolonged periods of time

Follow these guidelines for the greatest chance of success with fall nitrogen applications, but keep in mind that even in situations where fall applications will do well, they carry greater risk than the same application in the spring. Think carefully about the risks and rewards before making your decision to apply.

Are you planning to apply fall N? Let us know your plans in the comments!

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Support for this project was provided in part by the Agricultural Fertilizer Research & Education Council (AFREC).


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