Turf and Landscape - Slow Release Fertilizers: Sulfur-Coated Fertilizers

This is the third in a series on slow release fertilizers for turf and landscapes. This post contains information on sulfur coated fertilizers.

Sulfur-Coated Fertilizers.

Sulfur-coated urea (SCU) technology was developed in the 1960s and 1970s by the Tennessee Valley Authority. Sulfur was chosen as the principal coating material because of its low cost and its value as a secondary nutrient.

Sulfur-coated ureas (SCUs) are typically brown to tan to yellow depending on the source of urea, whether or not a sealant is used, and the type sealant employed. Soft sealants are typically used as a secondary coating over the sulfur coating to fill in imperfections in the sulfur coating and to provide handling integrity to the brittle sulfur coat. The total N content of SCUs varies with the amount of coating applied. SCUs available in the early 1990s ranged from 30 to 40% N.

Agronomic Properties and Nutrient Release Mechanisms of SCU :

The mechanism of N release from SCU is by water penetration through micropores and imperfections, i.e., cracks or incomplete sulfur coverage, in the coating. This is followed by a rapid release of the dissolved urea from the core of the particle. When wax sealants are used, a dual release mechanism is created. Microbes in the soil environment must attack the sealant to reveal the imperfections in the sulfur coating. Because microbial populations vary with temperature, the release properties of the wax-sealed SCUs are also temperature dependent.

The release rate of a SCU particle is directly affected by the coating thickness and the coating quality. Particles with higher sulfur loads, i.e., thick sulfur coatings, typically show fewer imperfections than particles with lighter coatings. There is a risk, however, that particles with too-thick sulfur coatings will exhibit lock-off, i.e., they may never effectively release their N.

Depending on the coating weight, N application rate, and environmental conditions, SCUs can have residual characteristics which provide agronomic response from 6 to 16 weeks in turfgrass applications. Because of the differential release of N due to the lack of uniformity in coating thickness and the influence of temperature on N release, severe mottling has been observed in turfgrass when SCU was applied during the cool-season growth period.

Reprinted from Selected Fertilizers Used in Turfgrass Fertilization by J. B. Sartain and J. K. Kruse, University of Florida Extension.