Fall application of nitrogen is a common agronomic practice in northern Illinois, but one that requires judicious management to protect the environment and improve nitrogen effectiveness and profitability.

The soil type, climatic condition, and form of nitrogen fertilizer have a great impact on the amount of loss that may occur over winter. Nitrogen in the nitrate form is susceptible to loss by leaching, or the movement of nitrate nitrogen in soil solution below the root zone. Nitrate is also subject to loss by denitrification or the conversion of nitrate nitrogen to unavailable atmospheric nitrogen.

Fall application of nitrogen should be avoided on coarse-textured, sandy soils or shallow soils over fractured bedrock. These situations offer the greatest possibility for leaching.

Farmers planning fall nitrogen application are advised to use the ammonium form and avoid using nitrate-containing fertilizers as well as urea. Anhydrous ammonia is a preferred nitrogen source since it has a slower nitrification rate that other nitrogen sources.

Anhydrous ammonia should not be applied until the soil temperature at the 4-inch depth is below 50 degrees F. If a nitrification inhibitor is used with anhydrous ammonia, nitrogen can be applied earlier, when the soil temperature at the 4-inch depth is below 60 degrees F. Soil temperatures must be below these levels and continuing to get colder. At these temperatures, soil organisms are less active and less likely to convert ammonium to nitrate. Below 50 degrees F, nitrification slows rapidly, but it does continue until temperatures are below 32 degrees F.

The 50 degree F temperature guideline also applies to ammonium sulfate fertilizer as well as injected liquid manure with high ammonium nitrogen content.

Soil thermometers should be used to guide the fall application as average dates on which the above-mentioned temperatures are reached are not satisfactory because of year-to-year variation. Current soil temperature is available at the Illinois State Water Survey web site, http://www.sws.uiuc.edu/warm/soiltemp.asp

Nitrification inhibitors can be viewed as a nitrogen management tool. They will increase the cost of fall application, but could significantly improve the efficiency of fall-applied nitrogen.

If applying anhydrous ammonia or injecting liquid manure in the fall be sure soil temperature is at the recommended level and maintained at or below that level. Further information on nitrogen management can be found in the current Illinois Agronomy Handbook, http://iah.aces.uiuc.edu/, or in the October 5, 2007 issue of Pest Management & Crop Development Bulletin, http://www.ipm.uiuc.edu/bulletin/article.php?id=859