Nitrogen application practices have been changing over the years, however, fall application remains a popular practice for many Minnesota farms. Here are some things to think about if you are considering or planning on fall N application:
1. Peak N demand for next year’s crop will be in the second week of June. That is a long time from now.
Because nitrogen is transient in the environment (it moves), storing it in the soil is a tricky proposition. Have you ever considered that when you make a fall N application you are counting on it to still be available seven months later?An aspect that is frequently lost in discussion about the 4Rs is that decisions about fertilizer type, placement and application timing are usually not independent of each other. This is indeed a system, and if you make a decision to apply in the fall you have made a decision that impacts your choice of fertilizer type, which then dictates placement. Do your homework and choose practices that maximize your investment and minimize impact on the environment.
2. Anhydrous ammonia is still the best choice for fall N.
Due to the anhydrous nature (it is attracted to water) of anhydrous ammonia it wants to find soil moisture. And because it is caustic, it inhibits microbes that convert it to nitrate. Anhydrous ammonia quickly turns to ammonium, which is a cation that attaches to clay particles. There are nitrification inhibitors commercially available for use with anhydrous ammonia that further slow the conversion of ammonium to nitrate.Urea, on the other hand, is a neutral molecule that is water soluble and quickly converts to nitrate. Furthermore, research has shown the tendency of urea to volatilize and be lost to the air. University of Minnesota research over the past two decades has shown reduced yields when fall urea is used as an N source. Remember, seven months is a long time for the N to remain in place for next year’s crop and fall applied urea has shown to come up lacking in this way.
3. Consider soil characteristics, not just conditions.
The primary loss processes of N are water based and happen when the soil is saturated. Leaching happens when water moves through the soil profile to shallow ground water or field tile. Denitrification happens when soils are saturated and microbes convert nitrate to gaseous forms of N.It is important to consider not just the moisture status of the soil when making an application, but also the potential for problems. This is why fall N application is not advised on light textured (silty or sandy) soils where water moves quickly through. It is also important to realize that heavy, poorly drained soils are also at risk. If you have either of these characteristics in your fields, making N applications closer to the time the crop needs it will minimize the risk of N loss.
4. There are many application options at your disposal; put yourself in a position to take advantage of them.
There has been a proliferation of application options available for pre-plant and in-season N applications. If your primary reason for making fall applications is because you are concerned you won’t have time next year, you might want to reevaluate the situation. It is true that fertilizer is typically more expensive in the spring, and some of these application methods cost more. However, you may find that you can cut your rates because you are no longer at risk for N loss. Most farmers find that increased yields are a bigger driver for profitability than reducing input costs. You may find that increased yields from changing your fertilizer practices easily outweighs the added cost of later applications.5. For the third year in a row, carryover N is a thing.
The 2023 growing season was the third dry year in a row in Minnesota. Historically, there is a carryover of N from unused fertilizer and mineralized forms that would otherwise be lost that is still in place for the next crop following a dry year. Fall soil nitrate test data shared with the University of Minnesota from Minnesota Valley Testing Labs in New Ulm following the 2021 and 2022 growing seasons shows that roughly 25% of fields tested had a N credit of at least 155 lbs./acre, with around 15% showing a credit of over 200 lbs./acre.Situations that are likely to show a credit are corn following corn and fields that have a manure history. Remember, the Minnesota test and recommendations are calibrated for a two-foot sample. Also, the closer you can time soil testing to the point where a final rate decision needs to be made, the more accurate your decision will be. The best way to ensure the test reflects what next year’s crop will see is to wait until spring to sample. The soil test for nitrate is not calibrated to handle situations where N has already been applied, so fall N applications are not advised in these situations.
There are many considerations to be made before settling on specific management for fall N. You can visit the University of Minnesota Extension’s webpage on nitrogen at z.umn.edu/nitrogen for more information.
Additional resources:
- Fall soil sampling BMPs: Grid vs zone, timing, dry soil & more
- Everything manure: Midwest experts answer common questions heading into fall
- Nitrogen Smart’s online course adds advanced sessions on manure management, the 4Rs
- Fall fertilizer outlook: Prices, drought, N sources, soil testing, and more
- Making sense of Minnesota’s corn fertilizer guidelines: Frequently asked questions and answers
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Support for nutrient management blog posts is provided by Minnesota's fertilizer tonnage fee through the Agricultural Fertilizer Research & Education Council (AFREC). Learn more at MNsoilfertility.com.
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