Where and when Fall Nitrogen applications should be considered for Minnesota Farmers
By Dave Nicolai, Extension Educator, Crops, UMN Extension
Minnesota field crops are nearing completion for 2017. The USDA Minnesota Crop Progress and Condition Report for the week ending October 1st reported that nearly all of the corn for grain crop had reached the dent stage. Fifty-five percent had reached maturity, one week behind average. Corn for grain harvest was just getting started. Corn condition was unchanged with eighty one percent good to excellent.
As corn grower’s attentions turn to this year’s harvest they should be considering a number of key questions for this fall and next year since ninety seven percent of the soybean crop was turning color with eighty two percent dropping leaves and soybeans were thirteen percent harvested.
Should I consider applying nitrogen this fall for next year’s corn crop?
In Minnesota, there are certain conditions where we do NOT suggest fall nitrogen application. They are as follows such as inn Southeast Minnesota because of the combination of high precipitation and karst soils or on sandy soils or soils with shallow depth to gravel/coarse materials and finally on fine-textured soils that are heavily tiled or tend to pond water for prolonged periods of time.
If I can apply fall nitrogen, when is the best time?
The 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 at https://app.gisdata.mn.gov/mda-soiltemp/ 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 advantages does anhydrous ammonia have compared to applying urea fertilizer in the fall?
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.