The Influence of Slow Release Nitrogen Fertilization and Application Frequency on St. Augustinegrass Response and N Leaching.

In Florida, urban fertilizer regulations limit per-application N rates to 49 kg N ha^-1 in efforts to reduce NO₃-N ground water contamination. This legislation may nullify the best features of controlled-release nitrogen sources (CRNS). A lysimeter based field study at the University of Florida compared the agronomic responses and N leaching from sources that included: polymer-coated urea (PCU), controlled-release liquid (CRL), and natural organic (BS) products applied at 49, 98, and 147 kg N ha^-1 on 60, 120, and 180 d intervals, respectively. N classes were compared by evaluating St. Augustinegrass [Stenotaphrum secundatum (Walt.) Kuntze] visual quality and clipping yields; while N release was assessed using urea as a baseline applied every 60 d at 49 kg N ha^-1. Urea as a bridging product in equal N combinations with PCU, at 49 and 98 kg N ha^-1 every 60 and 120 d, respectively, provided agronomic responses equal or greater than that of urea. Turfgrass quality and yield were significantly (P<0.01) affected by CRNS and N rate. N release from CRNS at current regulated N rates was insufficient to provide quality or yields comparable to urea. The most uniform response was obtained from PCU at 98 kg N ha^-1. At higher rates residual N release was insufficient to maintain good quality throughout the desired window. Extended N release occurred in BS plots during the winter cycle, presumably through reduced mineralization rates. Of the CRNS, only CRL resulted in detectable N leaching, conceivably contributing to less favorable agronomic responses. Urea applied solely or in combination at 49 kg N ha^-1 produced significantly (P<0.01) greater leaching with maximum losses of 20% of applied N. These findings indicate higher per-application rates of specific CRNS may not have serious environment implications; however more carriers must be evaluated to determine St. Augustinegrass rate and frequency recommendations.