With the early harvest, some farmers are already starting to apply anhydrous ammonia nitrogen fertilizer to fields this fall. However, to avoid nitrogen losses, farmers need to wait until soil temperatures go lower before applying. That's the message being delivered this week by state soil and water conservation officials in Iowa and Iowa State University Extension specialists.

Anhydrous ammonia is the only type of nitrogen that should be considered for fall application, says John Sawyer, ISU Extension soil fertility specialist. Compared to other forms of N, anhydrous ammonia is less susceptible to loss. However, anhydrous should not be applied before daily soil temperatures at the 4 inch depth drop below 50 degrees Fahrenheit and continue trending lower.

Soil temperatures in central Iowa usually fall below 50 degrees and stay there sometime in the second week or so of November. Sawyer says you need to keep an eye on the 6 to 10 day weather forecast to make application decisions.

Hold off applying anhydrous ammonia until soils are colder

If anhydrous ammonia is applied before daily soil temperatures remain below 50 degrees Fahrenheit and continue trending lower, that can result in nitrogen loss. Nitrogen losses can impact crop development and it can have negative environmental impacts--such as enhanced leaching into groundwater and streams once the ammonia is converted to nitrate.

"By waiting for cold soil temperatures, the applied ammonia will have a better chance to be retained in the soil and benefit the corn crop next spring," says Barb Stewart, state agronomist with USDA's Natural Resources Conservation Service (NRCS) in Iowa. "Cooler soil temperatures slow biological activity, which slows conversion of ammonium to nitrate, therefore allowing nitrogen to stay in the ammonium (NH4) form longer." The nitrate form is mobile and is more prone to loss, while the ammonium form of N is more stable in the soil.

Heavy rains throughout 2010 caused a lot of yellow corn fields this summer due to nitrogen loss. Stewart says applying anhydrous ammonia prior to soils dropping below 50 degrees could produce similar results next year. "With high anhydrous prices this fall, consider a spring application or split spring/sidedress application to make the best use of the nutrients," she says.

Check ISU website to view daily soil temperatures in your area

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 week and third weeks of November, according to Iowa State University Extension.

Farmers and fertilizer dealers are encouraged to visit the Nitrogen and Phosphorus Knowledge web page, which is http://extension.agron.iastate.edu/NPKnowledge/ to view daily and previous day and the 3-day history of average soil temperatures in every county in Iowa.

ISU Extension research indicates lower yields can result when anhydrous ammonia is applied in the fall versus spring. Also, crop residue cover can be reduced by the tillage action of NH3 application, increasing the risk of soil erosion. "Anhydrous applicator knives can do more tillage and destroy more crop residue than you may realize," says Stewart.

Use calculators to help make better tillage and nitrogen decisions

To save energy and money, NRCS recommends farmers use online energy estimators to help make better tillage and nitrogen decisions. These calculators are at www.nrcs.usda.gov/technical/energy/index.html. 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.

Nitrogen fertilizer is one of the largest indirect uses of energy in agricultural operations. Fertilizer accounts for 29% of agriculture's energy use, according to USDA data. The energy consumption for nitrogen fertilizer manufacture and relation to application rate is outlined in an ISU Extension publication, Energy Consumption in Corn Nitrogen Fertilizer, at http://www.extension.iastate.edu/Publications/PM20891.pdf. Proper management of nitrogen fertilizer, including use of organic sources of N such as animal manure and cover crops, can save farmers energy and money, says Stewart.