Labor Day has come and gone and now it is time to think about nitrogen (N) plans for next year. This news article will cover some thoughts about fall applications of N.
If you plan to use a soil nitrate-N test, you need to wait until the soil temperature is below 50°F to get a soil test value that is useful for predicting fertilizer need.
The past three years have been challenging for N management for corn. The wet springs have caused larger than normal N losses. In 2014, we saw some of the largest number of acres of N deficient corn in Minnesota in years. The current University of Minnesota N guidelines for corn were based on the use of N best management practices. Fields that had N applied at UMN guidelines may have been short of N, if the fertilizer was applied in the fall. One suggestion for fields with a history of fall N applications with N deficiency problems the last three years is to strongly consider pre-plant spring applications or a split application with some side-dress N before the V8 corn development stage.
Soon, growers in Southeastern Minnesota in soils with high N loss susceptibility and on sandy soils will be prohibited from fall applications of N. We know that applying all of the N in spring and side-dress applications, particularly in wet springs, can cause logistical problems for the dealer and grower, but it will result in better utilization of N by the crop and potentially less N loss.
What forms of N should be used?
For fall and spring pre-plant applications, only use ammonium forms of N such as anhydrous ammonia and urea. Ammonium-based N sources reduce the chances of N loss immediately after application because ammonium is held by the soil. Do not use nitrate forms in the fall or early spring. Nitrate is not held by the soil, so it can be lost via leaching, or if the soil is waterlogged and warm, it can undergo denitrification and be lost as a gas to the atmosphere. Fall applications should not be done until the soil temperatures are 50°F or below, because ammonium will transform quickly to nitrate above that temperature.
So what about using a nitrification inhibitor to reduce N losses? In cases where you know that denitrification is a problem, these products might be useful. Finally, do not use a nitrification inhibitor to apply N before the soil temperature is below 50°. It will not reduce the speed of the conversion of ammonia to nitrate enough and you will be wasting money.