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Characterization of the Thickness and Uniformity of the Coatings of Sulfur-coated Urea¹

ABSTRACT

The objective of this study was to develop a test which can accurately predict the contribution of coating characteristics to the control of rates of release of urea from SCU granules. It was assumed that product quality is a function of average coating thickness and uniformity of coating thickness.

To achieve the stated objective, concentric layers of coating were removed in successive increments by dipping granules in benzene and determining at each step the percentage of granules rendered available to water penetration due to exposure of cracks or pinholes. Five SCU materials were used with a range of coating characteristics. For materials which were coated with both sulfur and sealant, the results could be explained by postulating the existence of three classes of granules: Class I granules, which have unobstructed holes through the coating, Class II granules, with holes through the sulfur coating which are plugged with sealant, and Class III granules, which have sulfur coatings with no holes through them. Class I granules begin to release urea as soon as they are wetted, Class II granules begin to release urea as soon as the sealant is broken down or removed, and Class III granules begin to release urea as soon as the sulfur coating is penetrated by removal of parts of it.

¹ Technical Paper no. 4837, Oregon Agr. Exp. Stn., Corvallis, OR 97331. Contribution from the Dept. of Soil Science, Oregon State Univ. Research supported in part by TVA, Contract No. ORE-1131-86.

² Formerly Graduate Research Assistant, now Assistant Professor, Dept. of Soil & Environmental Sciences, UC-Riverside, Riverside, CA 92521; formerly Graduate Research Assistant, now Research Associate, Dept. of Agronomy, Iowa State Univ., Ames, IA 50011; and Professor, Dept. of Soil Science, Oreg. State Univ., respectively.

Received for publication April 12, 1978. Accepted for publication February 19, 1979.