Evaluation of Urease Inhibitors as Urea Amendments for Use on Kentucky Bluegrass Turf

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Urea applied to turf areas can result in loss of urea N through NH3 volatilization. One approach to decrease N loss is the use of urease inhibitors. The objectives of this study were to evaluate the urease inhibitors N‐(n‐butyl) thiophosphoric triamide (NBPT) and phenylphosphorodiamidate (PPD) as fertilizer amendments for increasing N‐use efficiency of urea applied to Kentucky bluegrass (Poa pratensis L.) turf in both the laboratory and the field. A forced‐draft system used to measure NH3 volatilization in the laboratory incorporated four levels of NBPT and three levels of PPD combined with 49 kg urea N ha−1. Cumulative loss of urea N as NH3 without inhibitors was 49.9% over 7d. Percentage reduction of NH3 volatilization in 7 d was 34.4, 46.5, and 50.1% for 1, 2, and 3% (w/w) PPD treatments, respectively; and 41.8, 50.7, 54.2, and 59.2% for 0.125, 0.25, 0.5, and 1.0% (w/w) NBPT treatments, respectively. A 2‐yr field experiment was also conducted that measured turf response to urea N at 0 to 73.5 kg ha−1 with NBPT at 0, 0.25, and 0.5% of the weight of N. There was little quality or clipping yield response to NBPT in the 1988 and the 1989 field trials. When responses were measured, they ranged from negative, due to tip burn, to a positive clipping response at the high rate of NBPT late in the 1989 season. More work will be necessary to further assess the usefulness of urease inhibitors for reduction of NH3‐N loss under field conditions.