Sulfur-coated urea (SCU) technology was developed in the 1960s and 1970s by the National Fertilizer Development Center. Sulfur was chosen as the principle coating material because of its low cost and its value as a secondary nutrient.
As the name suggests, SCUs are simply particles of urea coated with a layer of sulfur, and usually a sealant as well. SCUs are typically brown to tan or yellow, depending on the source of urea and whether a sealant is used. Soft sealants are used as a secondary coating over the sulfur coating to fill imperfections in the sulfur coating and to help keep the brittle sulfur shell intact during handling. The total N content of SCUs varies with the amount of coating applied. Many range from 30 to 40 percent 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 activity varies with temperature, the release properties of the wax-sealed SCUs are also temperature dependent.
Even though the urea from a SCU single particle is released rapidly once the coating fails, SCUs provide extended, gradual N release because some particle coatings fail sooner, others fail later, and so forth. The overall result is a gradual release of N to the turf.
The release rate of a single SCU particle is directly affected by the coating thickness and the coating quality. Particles with higher sulfur loads (thicker coatings) typically show fewer imperfections than particles with lighter coatings. You can infer, therefore, that thicker coatings may be desirable because they will extend the release rate. There is a risk, however, that particles with sulfur coatings that are too thick will exhibit lock-off, which means they may never effectively release their N.
Depending on the coating weight, N application rate and environmental conditions, SCUs can effectively provide N for 6 to 16 weeks in turf 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 turf, particularly close-cut turf such as greens, when SCU was applied during the cool-season growth period.