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Model for the Release of Urea by Granules of Sulfur-Coated Urea Applied to Soil¹

ABSTRACT

The release of urea N from granules of sulfur-coated urea (SCU) in soil must be predictable so that efficient use can be made of the fertilizer's slow release characteristics. The objective of this study was to develop and experimentally evaluate a mathematical model which makes it possible to forecast the release of urea from SCU granules as a function of time.

Equations were formulated to describe the rate at which soil microorganisms remove coatings from initially intact granules, the rate at which holes appear in the coatings as a consequence of this removal, the rate of diffusion of urea through these holes into the soil, and the effects of temperature and soil water content on each process. These equations were incorporated into a computer program which calculated the amount of urea released on consecutive days over periods as long as 120 days.

The effects of temperature and soil water content on the growth and activity of microorganisms were formulated on the basis of literature data. Temperature corrections for the diffusion process were based on the temperature dependence of the solubility of urea and the diffusion coefficient.

The model was validated by comparing predicted rates of release with experimental results. The predictions made by using the model conformed with experimental results. The model calculations responded accurately to changes in temperature, soil water content, and differences in coating thickness and coating composition. The use of the model clearly points to the need for a more precise assessment of the factors which affect the release of urea from SCU granules.

¹ Technical Paper no. 4838, Oregon Agric. Exp. Stn., Corvallis, OR 97331. Contribution from the Dep. of Soil Science, Oregon State Univ. Research supported by TVA, Contract no. ORE-1131-86.

² Formerly Graduate Research Assistant, now Assistant Professor, Dep. of Soil & Environmental Sciences, UC Riverside, Riverside, CA 92521, and Professor, Dep. of Soil Science, Oreg. State Univ., respectively.

Received for publication May 2, 1978. Accepted for publication May 22, 1979.

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