Fall Nitrogen Application Reminders

If considering fall application of nitrogen, it is economically beneficial to follow best management practices.

Applying nitrogen (N) in the fall can have some advantages including economic incentives relative to fertilizer costs, reduced soil compaction, and spreading out the workload. However, there are some potential disadvantages including the loss of N between application and crop use and the potential environmental concern of nitrates leaching into streams, lakes, and groundwater.

Anhydrous ammonia is the recommended N source for fall applications; however, fall applications should not occur until daily maximum soil temperatures are 50 ºF at a 4-inch depth and trending downward. The nitrification rate (bacterial activity) is significantly reduced at 50 ºF, but microbial activity still occurs until soil temperatures are below 32 ºF.1 Long periods of time at soil temperatures above 32 ºF after application can still result in a significant portion of the ammonium being converted to nitrate. Consider the use of a nitrification inhibitor with fall application. Nitrification inhibitors do not “tie-up ammoniu,, but inhibit the bacteria responsible for converting ammonium to nitrate. Also, fall N application should not occur on soils that are poorly drained or have excessive drainage, sandy soils, or soils that rarely freeze.1

Some of the disadvantages with all N requirements applied in the fall or spring pre-plant can be reduced by applying a lower rate in both the fall and spring. Fall-applied N can provide what the crop needs to get started in the spring. The remainder applied in the spring, closer to when the plant needs N, contributes to increased nitrogen use efficiency and reduces the chance of N loss by leaching and denitrification.