Late-fall nitrogen (N) fertilization of cool-season grasses is sort of like taking zinc when you think you’re getting a cold. You’ve heard it helps, lots of people do it, and it fits within the basic idea of, “Well, it won’t hurt.” But is this true? Do we really know what late-fall fertilization does to creeping bentgrass? And is it of any benefit? Alternatively, could late-fall fertilization have negative consequences? These are questions (except for that cold and zinc one) that faculty at the University of Minnesota, University of Wisconsin and Ohio State University set out to answer.
To find those answers, they used two 7-year-old L-93 creeping bentgrass research greens, one in Minnesota and the other in Wisconsin. Granular ammonium sulfate ((NH4)2SO4) was dissolved in water and applied under five different treatment regimes. Nitrogen rates were either low (0.5 pound N/1,000 square feet; 25 kg N/hectare) or high (1 pound N/1,000 square feet; 50 kg N/hectare), and were applied on either Oct. 15 or Nov. 15, 2009. A spoon-feeding treatment was also included, with that ammonium sulfate applied at 0.2 pound N/1,000 square feet (10 kg N/hectare) biweekly, to total 1 pound N with five applications from September to November. An unfertilized control was also included. To track fertilizer N through the bentgrass, the fertilizer was labeled using 15N, an isotopic tagging technique that allows researchers to separate applied N from N present in the soil, organic matter and microbial populations.
Collected data included eight to nine clipping collections in fall and spring, all of which were analyzed for N content. The distribution of the applied N in the verdure and thatch was determined in fall and spring, and turfgrass color (via use of a chlorophyll meter) and quality were also evaluated whenever there was no snow cover.
In both locations, spoon-feeding N at 0.2 pound N/1,000 square feet every other week from September through November produced the highest color readings and quality ratings in both the fall and spring rating periods. The high rate of N applied in October produced the second-highest-quality bentgrass in the spring and fall. However, when N was applied in November, bentgrass color and quality were reduced, and were equal to that in the unfertilized plots. The highest rate of N was needed to achieve the highest quality and color, whether applied in October or as a spoon-feeding.
Applying the highest rate of N meant that the bentgrass kept growing, and the greatest clipping yields were measured in those treatments. The timing of application did not matter as much here, because the amount of N applied most affected clipping yield. Any benefit of N applied in November was not observed until the following spring, when the November-applied N was detected in spring clippings. In the fall, 40% to 50% of applied N was found in the verdure and thatch, and by spring, 30% to 44% of that fall-applied N was still in the verdure and thatch.
Overall, the most efficient method of providing late-fall N to creeping bentgrass was through spoon-feeding N (0.2 pound N/1,000 square feet) every two weeks from September through November. Increases in clipping N content showed that fall-applied N was being moved to shoots before the onset of winter. Best quality, color and clipping growth of creeping bentgrass were observed when N was applied at 1 pound N/1,000 square feet — either biweekly in split applications, or as a single application in mid-October, but not in mid-November.
Source: Bauer, S.J., B.P. Horgan, D.J. Soldat, D.T. Lloyd and D.S. Gardner. 2017. Effects of low temperatures on nitrogen uptake, partitioning, and use in creeping bentgrass putting greens. Crop Science 57:1001-1009.
Beth Guertal, Ph.D., is a professor in the Department of Crop, Soil and Environmental Sciences at Auburn University in Auburn, Ala., editor-in chief for the American Society of Agronomy, and president-elect of the Crop Science Society of America. She is a 20-year member of GCSAA.