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Factors Related to Urea Hydrolysis in Soils¹

ABSTRACT

The use of urea as a nitrogen source for crop production has increased in recent years. Because the hydrolysis of urea to ammonia and carbon dioxide is mediated by the enzyme urease, efficient use of urea requires an understanding of factors that influence urea hydrolysis in soil. The purpose of this research was to relate the rate of urea hydrolysis to chemical, physical, and biological properties for soils in pasture and in cultivation. For each of 22 soils in Kansas, Missouri, and Oklahoma, samples were collected from paired fields, one in pasture and one in cultivation. The samples were taken from both 0 to 2.5 cm and 0 to 15 cm, and urea hydrolysis rates were measured in field-moist and air-dried samples. Urea hydrolysis rates in air-dried soils ranged from 1 to 149 µg urea g soil^–1 h^–1 and from 1 to 117 µg urea g soil^–1 h^–1 in field moist soils. For each soil, the greatest urea hydrolysis rate was in the 0 to 2.5-cm depth of pasture samples. The urea hydrolysis rate for each soil was greater in pasture samples than in samples from cultivated fields. For samples from cultivated fields, urea hydrolysis rates in both field-moist and air-dry samples were positively correlated with total nitrogen (TN) and organic carbon (OC). Only in the field moist soils were urea hydrolysis rates correlated with TN and OC in the soils from pastures. Urea hydrolysis rates in the 0 to 2.5-cm depth of pastures exhibited the greatest change from field-moist to air-dry conditions. Rates generally decreased with drying, but did remain constant or increased with drying in some cases. Both the greatest rates and the greatest variation in urea hydrolysis rates between field-moist and air-dry soils were observed in the surface 0 to 2.5 cm samples from pastures.

¹ Contribution of the Dep. of Agronomy, Univ. of Arkansas, Fayetteville, AR 72701. Research leading to this report was approved by the Director of the Arkansas Agric. Exp. Stn.

² Research Assistant and Professor of Agronomy, Univ. of Arkansas, and Director, Plant Science Research, Farmland Industries, Inc., respectively.

Received for publication August 26, 1983. Accepted for publication September 13, 1984.

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