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Serving: IA

Guidelines for Fall-Applying Nitrogen

TAGS: Extension

To decrease the risk of polluting groundwater and streams, and to prevent soil erosion and help save money on energy costs, conservationists are encouraging Iowa farmers to wait before applying anhydrous ammonia this fall.

"You should wait until daily soil temperatures drop to below 50 degrees Fahrenheit at the 4-inch depth, and the temperatures are trending lower," says Steve Brinkman, a nutrient management specialist with USDA's Natural Resources Conservation Service in Iowa. "Better yet, it would be best to wait until next spring to apply anhydrous ammonia to reduce the risk of loss."

Brinkman says anhydrous ammonia applied in the fall tends to have a higher potential of leaching into streams than spring-applied NH3. "When soil temperatures are too warm, it increases the risk of nitrates polluting groundwater and streams," he says.

Don't go until it's 50-degrees or below

According to the Iowa State University Extension agronomists, applications of anhydrous ammonia should not start until mid-day soil temperatures, at a 4-inch depth, are below 50 degrees Fahrenheit and trending lower.

Historically, soil temperatures at a 4-inch depth cool below 50 degrees in the northern third of the state during the first week of November. In central and southern Iowa, soil temperatures cool below 50 degrees during the second and third weeks of November, according to ISU Extension agronomist John Sawyer.

"Cooler soil temperatures slow biological activity," says Brinkman, "allowing nitrogen to stay in the ammonium (NH4) form longer. Then it has a better chance of being retained in the soil. The ammonium form of nitrogen is more stable, while the nitrate form is mobile and is more apt to leach away."

Farmers and fertilizer dealers are encouraged to visit the Nitrogen and Phosphorus Knowledge Web page,, to view daily average soil temperatures for every county in Iowa.

Fall NH3 can increase soil erosion

ISU Extension research indicates lower yields can result when anhydrous ammonia is applied in the fall vs spring, and crop residue cover can be reduced by the tillage action of anhydrous application, increasing the risk of soil erosion, notes Brinkman.

Fred Wilson of Clearfield, Iowa, has continuously no-tilled for more than 20 years. He quit applying fall anhydrous three years ago and went to a spring-only application. He applies only the dry form of nitrogen fertilizer now. "I don't think I will ever go back to anhydrous," he says. "I had more soil erosion from anhydrous a year ago than I've had in 20 years, so I just gave up. I've switched to using dry nitrogen. It costs a little more, but it's worth it."

Farmers urged to use "Energy Estimators"

To save energy and money, the NRCS recommends farmers use online energy estimators for tillage and nitrogen, at

The energy estimator for tillage estimates diesel fuel use and costs in the production of key crops. It compares potential energy savings between conventional tillage and alternative tillage systems. The energy estimator for nitrogen enables farmers to calculate the cost of nitrogen product use. It also evaluates options based on user input.

According to USDA, nitrogen fertilizer is one of the largest indirect uses of energy on an agricultural operation. Fertilizer accounts for 29% of agriculture's energy use, according to USDA research. Proper management of nitrogen fertilizer, including the use of organic sources of nitrogen such as animal manure and cover crops, can save producers energy and money.

Brinkman offers the following example for Jasper County. He plugged numbers into the NRCS energy estimator for nitrogen. The results show that if you live in Jasper County and apply 180 pounds of anhydrous ammonia per acre on 500 acres of corn, grain or silage in the fall at $400 per ton for fertilizer, the final cost would be $4,390. With all things being equal, including yields, a split spring application would require less nitrogen (147 pounds of N per acre) at a total cost of $3,587--for a savings of $803.

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