Anhydrous Application Amounts Aren't Determined by Soil's CEC

U of I soil fertility specialist says a rumor is going around that you should apply 10 lbs of N per unit of CEC.

Published on: Apr 18, 2011

Despite popular belief, corn growers should not worry about the cation exchange capacity of soil when determining how much anhydrous ammonia to apply, says University of Illinois Extension specialist in soil fertility and plant nutrition Fabian Fernandez.

"A false concept has been circulating this winter that anhydrous ammonia applications should not exceed 10 pounds of nitrogen per unit of CEC," Fernandez adds. "This concept has no scientific foundation."

CEC is a soil property that is important in determining liming application rates to correct pH or to determine the capacity of a soil to supply plant nutrients such as Ca++, Mg++, K+, and NH4+. When anhydrous ammonia (NH3) is applied in the soil, ammonia reacts with organic matter; clay; and most important, it dissolves in water.

Anhydrous means "no water." When anhydrous ammonia is applied in the soil, it reacts with water to form ammonium. In essence, NH3 takes a hydrogen ion from water to form the positively charged NH4+ ion. Once the NH4+ ion is formed, it attaches to the soil exchange complex, which keeps it from moving with water.

"The initial reactions with water, organic matter and clays limit the mobility of ammonia and help retain nitrogen that otherwise could be lost by ammonia volatilizing to the atmosphere," Fernandez says. "However, CEC does not have a direct relationship to how much ammonia a soil can hold at the time of application."

Other factors that are important in ammonia retention include soil texture, soil structure and method of application (including depth of injection and proper closure of the knife track). In some U of I trials, after ensuring adequate soil moisture conditions and proper application depth, researchers have successfully applied more than 200 pounds of nitrogen per acre in sandy-textured soils.

For more information about when and how to apply nitrogen this spring, read The Bulletin at http://bulletin.ipm.illinois.edu.