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Factors Affecting Urea Hydrolysis and Ammonia Volatilization in Soil¹

ABSTRACT

Laboratory incubation experiments were conducted to determine the effect of rate and depth of urea application and the rate of soil aeration on rates of urea hydrolysis and ammonia volatilization occurring with soils taken from the surface horizons of Chalmers silt loam and Plainfield sand.

Measurements were also made of the effect of urea hydrolysis on the pH of undisturbed soil in the immediate zone of urea application. The rate of urea hydrolysis was shown to be directly proportional to the rate of urea application when soils were incubated at 28C. Ammonia volatilization rates increased at an exponential rate as rates of urea application were increased. This resulted in a larger proportion of the added urea nitrogen being lost from the soil through ammonia volatilization at the higher rates of urea application. Ammonia volatilization rates were inversely proportional to the depth of urea application and decreased more rapidly with depth in wet soil than in moist soil. The pH of Chalmers soil in the zone of urea application attained a maximum pH of 8.8 with both 224 and 896 kg/ha (200 and 800 lb/acre) applications of urea nitrogen. Soils receiving a heavy surface application of urea maintained a high pH throughout a 4-week incubation period. Soil aeration rate had no significant effect on rates of urea hydrolysis, but ammonia volatilization rates increased as the gas exchange rate in the soil increased.

¹ Journal paper No. 2737. Purdue University Agr. Exp. Sta., Lafayette, Indiana. Contribution from the Department of Agronomy. This research is in part taken from the doctorate thesis submitted by the senior author and was partially supported by a fund provided cooperatively by the Allied Chemical Corp., American Cyanamid Co., Consolidated Mining and Smelting Co. of Canada, W. R. Grace and Co., John Deere Chemical Co. and Sohio Chemical Co. Presented before Div. S-3, Soil Science Society of America, Aug. 1962, at Ithaca, New York.

² Former Research Assistant, Purdue Univ., now at the Norwegian Forest Research Institute, Vollebekk, Norway, and Assistant Professor of Agronomy. Purdue Univ., respectively.

Received for publication February 21, 1966. Accepted for publication September 7, 1966.