Don't jump the gun on fall N

After last year, no producer wanted to be late getting into the fields this fall. While it’s great to get the crop out early, collect soil samples while it’s still nice outside, and perform tillage while soil conditions are adequate, Fabian Fernandez, University of Illinois Extension specialist in soil fertility and plant nutrition, said no one should be applying nitrogen yet.

“Last year’s harvest made it nearly impossible for many to properly fertilize their fields,” Fernandez said.

Every fall, producers who apply nitrogen worry that if they wait too long for temperatures to drop sufficiently, soils might become too wet to do the application. While the window of opportunity is small, it’s important to exercise good judgment to realize the benefit of such application.”

The management of nitrogen is important because this nutrient is both one of the most expensive inputs in today’s farming operations and one that can pose environmental concerns. Whether producers think of cost, environmental implications, or both, no one can afford poor nitrogen management.

“Being smart about nitrogen use can pay large dividends,” Fernandez said.

Knowing when to apply nitrogen is key. Wait until soil temperatures at the 4-inch depth are below 50 degrees Fahrenheit. Temperature impacts the activity of soil organisms that can mediate conversions of ammonium to nitrate.

Fernandez said it’s critical to understand that although the rate of nitrification is significantly reduced when soil temperature is below 50 degrees Fahrenheit, microbial activity continues until temperatures are below freezing..

Anhydrous ammonia is a preferred nitrogen source for fall application because it has a slower nitrification rate than other sources. Once it’s applied in the soil, ammonia reacts quickly with soil water and is converted to ammonium. Use a nitrification inhibitor with anhydrous ammonia applications. The nitrification inhibitors are chemicals that inhibit the activity of bacteria responsible for the first step in the process of nitrification, Fernandez said.

“Proper use of these inhibitors will reduce the rate of nitrification, thus maintaining for a longer period a great proportion of the applied nitrogen in the ammonium form,” he said.

Fernandez recommends ammonium sulfate because it works well for no-till fields where broadcast applications are preferred. It can also be applied on frozen ground as long as the slope of the field is less than 5% and the potential for surface water runoff is very low.

Urea should not be used in the fall because it has been shown to be less effective than fall-applied anhydrous ammonia. Urea converts to ammonia and then to ammonium within a few days of application, causing greater risk of losses before nutrient uptake by the crop the following spring.

In addition, there has been a renewed interest in using manure, poultry litter and other organic fertilizer forms to supply not only nitrogen, but also phosphorus and potassium. Before applying, be sure to analyze the source for nutrient content. Typically, if these sources are applied to meet the nitrogen needs of the crop, an overapplication of phosphorus will result. For this reason, the rate of application should typically be based to meet the crop’s phosphorus requirements rather than the nitrogen requirements for the year of application. Another possibility is to apply nitrogen requirements for the year, but remember that there is sufficient  phosphorus for a few years.

If using animal manure, Fernandez cautions producers to make sure it is incorporated into the soil, and follow the time of application guidelines discussed for commercial nitrogen management.

Determining the most economically optimal nitrogen rate can be done using the nitrogen rate calculator at Keep in mind this does not account for carryover nitrogen, he added. With this year’s wet spring, it’s unlikely that much nitrogen will be carried over. Consider applying only a portion of the total nitrogen needed in the fall and the rest in the spring.

“Because more acres are planted to corn following corn, there is increased interest in corn residue management,” Fernandez said. “Don’t apply nitrogen to increase residue breakdown. Research has shown no benefit in this.”

When making nitrogen application decisions, consider the risks and benefits of fall nitrogen application. Overall, research on nitrogen application timing has shown that application in the spring, close to the time of rapid nitrogen uptake, maximizes yield because there is less chance for leaching or denitrification. However, late-fall application of some nitrogen sources is adequate, especially for medium- to fine-textured soils where cold winter temperatures prevail and early springs are not excessively wet and warm.

“If you don’t like taking big risks, but a fall application makes sense, it may be better to apply part of the nitrogen in the fall and wait until spring to apply the rest,” Fernandez said. “This approach is like buying an insurance policy. It gives peace of mind but costs money, and you can never be certain whether the investment will pay off.”

For more information, check out The Bulletin, an online publication written by U of I Extension specialists in crop science, at

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