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Fall- vs. Spring-applied Sulfur-coated Urea, Uncoated Urea, and Sodium Nitrate for Corn¹

The practice of applying fertilizer in the fall for crops grown the next summer is advantageous to the fertilizer manufacturing and marketing sector as well as saving time for farmers in the spring planting season. Sulfur-coated urea (SCU), NaNO₃, and uncoated urea were applied in field experiments during 1972-74 to compare their efficiency in fall application with spring application for the production of corn (Zea mays L.) grain on a Mountview silt loam soil (Typic Paleudult, fine silty, siliceous, thermic) at Cookeville, Tenn. Two dissolution grades of SCU were compared with NaNO₃ and uncoated urea, at rates of 112 and 224 kg N/ha in 1972–73 and 168 and 224 kg N/ha in 1974. Grain yields were measured and relative efficiency of fall application and response to rates, sources, and dates of application were calculated. Soil concentrations of ammonium and nitrate N were measured on some treatments. Soil temperature at 10 cm depth was recorded during the winter and early spring of 2 years.

There were little or no differences in corn grain yields between fall and spring applications of N at the 224 kg/ha rate. At the lower N rates, spring-applied N increased yields compared to fall-applied N. The relative efficiency of fall compared to spring application of the materials, averaged for 3 years, was 0.52 and 0.86 for NaNO₃ at 112 and 224 kg N/ha respectively, 0.49 and 0-95 for uncoated urea, 0.80 and 1.10 for SCU-1, and 0.72 and 1.03 for SCU-2. Nitrates leached to a depth of 45 cm or more during the winter and early spring in plots treated with NaNO₃ and uncoated urea. Conditions were probably favorable for a slow but significant dentrification rate at times during the winter and early spring, leading to the belief that dentrification may have caused some of the decreased efficiency of fall-applied N.

Although SCU was more effective than NaNO₃ or uncoated urea, fall-applied N from any fertilizer source would be less effective on subsequent spring and summer growth than spring-applied N under similar soil and climate conditions.

Key Words: Zea mays L. • Slow-release N • Exchangeable NH₄⁺ and NO₃^– • Leaching • Dentrification • Relative efficiency

² Assistant professor of agronomy, Univ. of Kentucky; formerly processor of plant and soil science, Tennessee Technological Univ.

Received for publication June 5, 1976.