Urea hydrolysis in soil: Effects of urea concentration and soil pH

Abstract

When urea fertilizers are applied in bands, the concentration of urea in soil solution may range from very low at the edge of the bands to very high at the center of the bands. Thus, in order to understand the behavior of banded urea in soil, information is needed regarding the rate of urea hydrolysis under a wide range of urea concentrations. This work was conducted to study the kinetics of urea hydrolysis in samples of Kahola and Reading soils exposed to concentrations ranging from 0.01 to 10 m urea-N, and adjusted to pH values of 5.5, 6.5, 7.5, 8.5 and 9.5. The results were best described by a kinetic model involving two enzymatic reactions, both following simple Michaelis-Menten kinetics but having different affinities for urea. The V max for the high affinity reaction showed a relatively small peak at pH 6.5, followed by a decline and then a sharp increase as the pH increased from 6.5 to 9.5. The V max for the low affinity reaction and the K m values for both reactions showed maximum values at pH 6.5. The urea-N concentration at which both reactions contributed equally to the overall urease activity varied between 0.5 and 1.3 m depending on soil type and pH.