THE EFFECT OF SOIL ENVIRONMENT AND FERTILIZER MODIFICATIONS ON THE RATE OF UREA HYDROLYSIS

Abstract

As part of an effort to model the behavior of urea by computer simulation, we studied the problems associated with modeling urea hydrolysis. Incubation experiments with a variety of soils were used to analyze the kinetics of urea hydrolysis. We studied the effects of urea disposition, mode of application, soil temperature, and moisture, as well as soil preincubation, on the hydrolysis rate. The experiments showed that hydrolysis of urea uniformly distributed throughout the soil is adequately described by zero-order equations and for some soils might even be considered instantaneous if model time steps of 24 hours are used. However, application of prilled urea creates a heterogeneous system in which urea hydrolysis was best described by first-order kinetics following a substantial lag phase. The first-order phase is followed by a rapid increase in hydrolysis rate, possibly due to a shift to zero-order kinetics. Modeling of urea hydrolysis is further complicated by the effects of the soil environment. The hydrolysis rate decreased linearly with decreasing temperature, and moisture rapidly reduced the hydrolysis rate above the permanent wilting point (PWP). Moreover, hydrolysis rates were possibly affected by waterlogging or excessive temperatures. Thus, full characterization of the behavior of urea hydrolysis in each soil used for modeling purposes seems a prerequisite to computer simulation of urea-N in soil.