Evaluation of Slow-Release Nitrogen Sources on Baron Kentucky Bluegrass

As new nitrogen (N) sources are introduced, it is important that they be evaluated and their release characteristics determined in order to provide us with a basis on which to make fertilizer recommendations. In this study, several N sources were evaluated for maintenance fertilization of Kentucky bluegrass (Poa pratensis L.) turf. Treatments included isobutylidene diurea (IBDU) materials of two particle-size ranges, sulfur-coated ureas (SCU) in two size ranges from Canadian Industries Limited (CIL), SCU from the Tennessee Valley Authority (TVA), ureaformaldehyde (UF), activated sewage sludges, Organiform, soluble N sources, and combinations of slow-release and soluble N. All N sources were applied at a rate of 197 kg N·ha⁻¹·yr⁻¹, divided into equal fall and spring applications for 3 consecutive years. Weekly clipping yields, color ratings, and annual N recovery were the response criteria. Sulfur-coated urea produced a more uniform growth and had higher N recovery than IBDU, which was characterized by a delayed response following fertilization. Turfgrass response to the CIL and TVA SCU materials applied alone was similar. Particle-size effects with CIL SCU were slight; however, N release was quicker from fine IBDU than coarse IBDU. Slow-release characteristics were also observed for ureaform, Organiform, and the sludges, but turf quality was generally poor for these treatments. Substituting soluble N for a portion of the SCU N gave inferior turfgrass response as compared to SCU applied alone. When soluble N was used in conjunction with ureaform, Organiform LT, and IBDU, turfgrass response was improved over that obtained when these N sources were used alone. Recovery of N in the clippings was greatest for soluble N sources and SCU treatments (48 to 52%), while lowest values (15 to 29%) were associated with ureaform, Organiform, and sludges.