Guidelines for Fall N Application in North Dakota (09/08/21)
Fall N application is a possible fertilizer management strategy if the timing is optimized. The following are timing guidelines for application of anhydrous ammonia and urea in North Dakota. The guidelines consider the delay in nitrification caused by a concentrated anhydrous ammonia band on bacteria activity immediately following application, a smaller delay caused by the ammonia gas aura surrounding a concentrated urea band, and no delay in nitrification from ammonia released from individual urea pellets as a result of broadcast application. If urea is applied at the surface or less than 2 inches from the surface, an NBPT-based urease inhibitor should be added to the urea.
Never apply anhydrous ammonia before October 1. From October 1 on, look at the NDAWN data on 4- inch depth soil temperature from 6AM-8AM, or take the measurement yourself. When the temperature drops to 50oF or less, the risk of a large percentage of ammonia transformation to nitrate, which is susceptible to leaching/denitrification loss is low. Once this temperature is reached, there will nearly always be a day or days when temperatures are greater than 50o. But in North Dakota, winter is coming, so charge ahead. A nitrification inhibitor (I dislike the marketing term ‘stabilizer’, as it is grossly misused) can be used to protect against an unusually late starting freeze-up, but it should not be used to push the date of application earlier, as the late freeze-up would erase the advantage. Nitrapyrin (N-Serve™) is the most effective nitrification inhibitor, with DCD and its relatives an effective 2nd.
Banded urea with an air-seeder, drill, or in strip-till date should be one week later than the date for anhydrous ammonia. Nitrification inhibitors for urea are less effective than nitrapyrin for ammonia, but still provide some protection.
Broadcast urea should be delayed 2 weeks following the date for anhydrous ammonia. Nitrification inhibitors provide some protection, but not as much as nitrapyrin for ammonia.
For example, if soil temperatures in Cando dropped to 50o on October 5, anhydrous ammonia application may begin in that area. October 12, banded urea application is relatively safe. October 19, broadcast urea may be applied with confidence.
The later the application of any of these options, the lower the risk of nitrate loss in the spring. The challenge is to balance the logistics of waiting with the ability to apply all the planned acres in-between rains and before freeze-up. Fall N application is never more efficient than spring application. However, the logistics of being able to apply N before or at seeding will weigh heavily on the decision to apply fall fertilizer. Although lower efficiency of fall N is a production cost consideration, a delay in seeding from waiting on fertilizer application in the spring is a greater cost in most years.
Extension Soil Specialist