With the NOAA predicting a 71% chance of La Niña emerging this Fall, with the cooling trend potentially lasting through January-March 2025, the odds of a wet spring delaying field operations in the spring in 2025 are increased. With such conditions, a Fall N application may be an interesting and economically effective option.

Fall applied N is a viable fertilizer management strategy in most soils of North Dakota, as long as the timing is optimized. With soil temperatures dropping to the 50’s across much of the state (Figure 1), a few factors must be taken into consideration before deciding on a Fall N fertilizer application:

Don’t guess, soil test – Always soil test before making a fertilizer application. With the soil test results in hand, refer to NDSU’s Crop Nitrogen Calculator App to calculate your crop’s needs.

Nitrogen sources - For fall application, it is advisable to use ammonium-N sources like anhydrous ammonia and urea. Fertilizers that contain nitrate, such as urea/ammonium nitrate (UAN) solutions, should be avoided as there is a high risk of nitrogen loss due to rapid nitrification.

Timing - Do not apply anhydrous ammonia before October 1. After this date, refer to the NDAWN data for soil temperatures at a 4-inch depth between 6 AM and 8 AM, or measure it yourself. When the temperature falls to 50°F or lower, the risk of a significant amount of ammonia converting to nitrate— which can be lost through leaching or denitrification— is minimal due to lower soil microbial activity.

Use a nitrification inhibitor – The use of a nitrification inhibitor can help protect ammonium-N from being transformed to nitrate in fall seasons with delayed soil freeze-up.

Nitrapyrin (N-Serve^TM) is the most effective nitrification inhibitor, with dicyandiamide (DCD, C₂H₄N₄) and its relatives being a good alternative, also.

Do not use these materials for an earlier than recommended application. Always refer to the soil temperatures.

Fertilizer placement and timing – Banded urea using an air-seeder, drill, or strip-till method should be applied at least one week after the application of anhydrous ammonia. Although nitrification inhibitors used with urea are not as effective as nitrapyrin for anhydrous ammonia, they still offer some level of protection.

If broadcasting urea is the chosen method, application should be pushed back 2 weeks following the date for anhydrous ammonia. Nitrification inhibitors provide some protection, but not as much as nitrapyrin for ammonia.

Example - If soil temperatures in Portal dropped to 50° on October 8, anhydrous ammonia application may begin in that area. Banded urea application is relatively safe on October 15. Broadcast urea may be applied safely on October 22.

Victor Gomes

Extension Cropping Systems Specialist

Dickinson Research and Extension Center