Direct Measurement of Foliar-Applied Nitrogen Uptake by a Cool-Season and Warm-Season Putting Green Turfgrass Species.

The supplemental practice of foliar fertilization often comprises a significant portion of the total annual nitrogen (N) applied to putting greens.  Despite its increasingly common use among golf course superintendents, there have been relatively few research studies investigating foliar uptake by turfgrasses and no studies which document foliar uptake of nutrients in a field setting.  A ¹⁵N isotopic tracer field study was conducted to compare the seasonal uptake of foliar-applied nitrogen by a cool-season (Agrostis stolonifera var. palustris (Huds.) Farw. cv. ‘Penn A1’) and warm-season (Cynodon dactylon (L.) Pers. x C. transvaalensis Burtt-Davy cv. ‘Tifeagle’) turfgrass species managed under golf course putting green conditions.  ¹⁵N-labeled urea was applied monthly from May to September at rates of 1.25 g N/m² and 0.5 g N/m².  Sampling units of treated verdure were excised at sequential time intervals after application (1 hr, 4 hr, 8 hr, and 24 hr) and immediately rinsed to remove unabsorbed fertilizer N.  Separated, dried and ground verdure tissue was ultimately analyzed by mass spectrometry using the Rittenberg three-step technique for N isotopes.  In order to determine factors influencing foliar uptake, ¹⁵N plant recoveries were compared to hexane extractable leaf cuticle wax loads, gas chromatographic separation of cuticle wax constituents, as well as environmental and climatic data (wind speed, air temperature, leaf wetness, PAR, and relative humidity) known to affect the foliar absorption process.  Data suggest that the putting green turfgrass canopy is highly receptive to foliar urea-N uptake, as a maximum of 77 % of the N applied was absorbed at 1 hr after application at one sampling date.  However, ¹⁵N fertilizer recoveries at selected time-course intervals were highly variable among monthly application dates for both species.  Leaf cuticle characteristics and weather data parameters helped explain this seasonal uptake variability to differing degrees.