About Time for Fall NH3

By Clark McGrath
ISU Extension

The last few days make it feel like fall is here to stay. Looking at soil temps, the calendar and the 6-10 day forecast, we’ll probably be in our fall NH3 application window before we know it if we aren’t already there. Having seen a few bars and tanks staged around the area, I’m not the only one thinking about this even though we have a way to go on harvest.

A common question from growers is “Should we even apply nitrogen in the fall?” As a former retail agronomy manager, I contend that fall application allows us more flexibility with equipment, generally lower nitrogen prices, more consistent supply logistics, less compaction, less seedling burn and more time for planting in the spring. Fall vs. spring pricing is a little bit murky at the moment. Working with your local dealer will give good insight on a local level, though.

As an ISU agronomist, I also have to say that fall application has to be managed correctly to even come close to the nitrogen use efficiency and economics of spring application. Also, if you are going to apply N in the fall, keep in mind that anhydrous ammonia is the only form of N recommended for fall application by ISU agronomists.

The single most important factor affecting good management of fall application is ensuring soils are the right temperature before applying NH3. Wait until soils are at least 50 degrees and trending downward, usually around the first week of November, although it looks like we may be there this week if the weather forecast is close. Soil temperatures for every Iowa county, including three-day histories and yearly trends, can be found here.

I am a proponent of N-Serve in many fall applied NH3 situations, especially as N prices rise, but one place that it doesn’t help is using it to apply earlier in the fall. Make sure those soil temps are 50 degrees and falling, N-Serve or not. Nitrification inhibitors like N-Serve are an interesting and in-depth discussion. ISU has a great resource on this topic, publication NCH 55, “Nitrification Inhibitors for Corn Production,” which can be found here. It is an older publication, but the chemistry and principles really haven’t changed for fall NH3 applications since it was revised.