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Efficiency of Fertilizer Urea as Affected by Method of Application, Soil Moisture, and Lime¹

The persistence of surface-applied urea and comparative crop responses to solution and solid N sources including urea were studied on sandy soils of pH 5.4 to 6.0 and 1.7 to 2.5% organic matter. Over 80% of prilled urea applied to a soil with an air-dry surface under field conditions did not hydrolyze within 14 days, and from 42 to 72% persisted where a soil surface was only temporarily moist from recent tillage or irrigation, even though heavy dews formed nightly. With an exposed surface, continuously moist from a high water table, urea completely hydrolyzed within 7 days, with 65% of the applied N not accounted for as NH₄⁺ and assumed largely lost as NH₃ gas.

Response of pasture grasses usually showed urea to be inferior to other N sources, with ammonium nitrate being the most consistently superior. Efficiency of urea and solutions appeared to be increased by rain immediately after application. Surface liming depressed responses to urea and materials containing ammonium-N. Response of sweet corn to side dressings of urea was slightly inferior to responses to solid or solution-N materials containing nitrate-N, apparently as a result of faster penetration of NO₃^- as compared to NH₄⁺ when moisture permitted. Covering urea or applying it on a temporarily moist vs dry surface had no differentiating effect.

Persistence of urea on air-dry or temporarily moist soil surface until incorporated by tillage or rainfall vs rapid loss of NH₃ if a moist surface persists is a logical explanation of erratic responses to surface-applied urea.

² Soils Chemist.

Received for publication October 1, 1965.