Photo Caption:USGA-sponsored field research at the University of Arkansas and University of Illinois indicate that creeping bentgrass putting greens absorb most nitrogen sources applied to the leaves within a few hours. The studies evaluated the effects of spray volume, nitrogen source, addition of adjuvants, and tank mixing with commonly applied turf care products on foliar uptake of nitrogen.
Sometimes the adoption of a new golf course management practice occurs because it simply works, and quickly, news spreads among superintendents. For example, many golf course superintendents use low spray application rates of nitrogen fertilizer on putting greens. However, how much of the nitrogen does the turf foliage actually absorb? USGA-sponsored field research at the University of Arkansas and University of Illinois indicate that creeping bentgrass can rapidly absorb most nitrogen sources applied to the leaves. The studies evaluated the effects of spray volume, nitrogen source, addition of adjuvants, and tank mixing with commonly applied turf care products on foliar uptake of nitrogen.
Previous research on foliar nitrogen applications conducted in growth chamber or greenhouse experiments suggested that nitrogen uptake efficiency ranged from 31 to 69 percent for creeping bentgrass. Environmental conditions can affect the amount of applied material that grass leaves absorb, particularly for water-soluble compounds such as fertilizers. The main environmental factors that affect foliar uptake are relative humidity and air temperature. Uptake of applied materials is minimal once spray droplets dry on leaf surfaces. Under low humidity, droplets dry more rapidly and reduce the amount of uptake regardless of air temperature.
In controlled environment studies, relative humidity was generally above 80 percent with constant temperature. The high humidity would lead to longer spray-droplet drying times and allow for greater fertilizer uptake. The urea-nitrogen uptake efficiencies for field studies in Arkansas ranged from 36 to 69 percent compared to those in Illinois that ranged from 6 to 34 percent. Both studies indicate that most of the nitrogen uptake occurred within four hours after application; however, why do the results differ? In addition to differences in climate, scientists in Arkansas used hollow cone nozzles that produce smaller droplets than the flat-fan nozzles used in Illinois. Smaller droplets would provide more uniform coverage and greater uptake. The height of cut also differed for the two field studies. In Illinois, bentgrass cut at a half-inch was higher than the eighth-inch height of cut in Arkansas.
In the University of Illinois study, adjuvants did not provide enhanced uptake of urea-nitrogen. There also was no effect on uptake when tank mixing urea-nitrogen with turf care products such as chlorothalonil (fungicide),Primo Maxx(growth regulator), Renaissance (biostimulant), and Tracker (indicator dye) alone or in combination. Lower spray volumes increased the nitrogen uptake efficiency and the Illinois researchers recommend not exceeding 40 gallons per acre with flat-fan nozzles. There were no differences observed among the nitrogen sources urea, ammonium nitrate, and calcium nitrate in the University of Illinois study. However, the University of Arkansas results indicated potassium nitrate absorption was lower than urea and ammonium sulfate. The organic, amino acid products (glycine, L-glutamic acid, and L-proline) were similar to urea in their ability to supply nitrogen to the grass foliage.
To achieve the greatest efficiency, use low application rates (0.1-pound nitrogen per 1,000 square feet); wait several hours to maximize foliar uptake, and then water-in, washing any unabsorbed nitrogen remaining on leaf surfaces into the soil for uptake by the courseCareLinksPageContent system. With the range of results reported for the studies conducted in Arkansas and Illinois, further research on the environmental and application factors that affect foliar nitrogen uptake would be valuable. The determination of the impact of humidity, temperature, nozzle type, and cutting heights will provide additional information on the foliar nitrogen uptake efficiency.