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In a series of studies over four years, researchers examined how N and K fertilization should be adjusted on aging ultradwarf bermudagrass greens.


In the mid- to late 1990s, ultradwarf bermudagrass became the new go-to cultivars for bermudagrass putting greens. A lower-growing alternative to Tifgreen or Tifdwarf, these cultivars were often installed as a first step in a putting green renovation or new construction.

However, as these grasses matured, research was needed to address the longer-term management strategies for them. What kinds of cultivation were needed? Fertilization strategies? In a series of studies, researchers in South Florida tackled these issues, with one study examining four years of nitrogen (N) and potassium (K) fertilization of ultradwarf bermudagrass putting greens.

Researchers started with a newly constructed USGA-type (90/10 sand/peat v/v) putting green, where the ultradwarf bermudagrass cultivars Champion, Floradwarf and TifEagle had been established. In year one of the study, nitrogen was applied as urea at either 261 or 523 pounds/acre/year (293 or 586 kilograms/hectare/year). These nitrogen rates were combined with potassium chloride to achieve N:K ratios of 1:1, 2:1 or 1:2.

In year two, it was determined that more nitrogen and potassium were needed, so the nitrogen rate treatments were changed to 523, 784 or 1,046 pounds/acre/year (586, 879 or 1,172 kilograms/hectare/year) with N:K ratios of 1:1 or 2:1. Fertility treatments did not change after year two. The nitrogen and potassium were applied weekly as a liquid formulation.

Over the four years, differences in the effects of cultivar on turfgrass performance were variable, never associated with just one cultivar, and quality for all cultivars was above the acceptable threshold. During the wet season (May to October), TifEagle consistently had the best quality, whereas in the dry season (November to April), Champion had better quality. Champion typically had higher shoot densities than those measured in TifEagle, and dry root weight was unaffected by cultivar. Ball roll was also rarely affected by cultivar, except in year three only, when TifEagle had shorter ball roll.

As the greens aged, their reaction to nitrogen fertilization changed. In year one, increases in nitrogen always increased turfgrass quality. In year two, the middle nitrogen rate of 784 pounds N/acre/year was needed to obtain the best quality, regardless of the wet or dry season. However, by year three, that same rate (784 pounds N/acre/year) was only needed in the dry season, while in the wet season, nitrogen could be reduced to 523 pounds/acre/year. Finally, by year four, as the greens aged, 523 pounds N/acre/year was enough for quality turfgrass, regardless of the season.

And what about the added potassium? Except for the wet season in year two, N:K ratios did not affect bermudagrass quality. In that wet season of the second year, bermudagrass receiving N:K ratios of 2:1 had higher quality than bermudagrass receiving N:K at a 1:1 ratio. In fact, regardless of the N:K ratios applied, the N:K ratio in bermudagrass leaf blades was consistent at about 2:1 throughout the study.

So, as ultradwarf putting greens age, adjustments to the amount of applied nitrogen should be considered. This study was performed in a subtropical environment with year-round growth, and after three years of growth, it was found that the bermudagrass could be adequately maintained with a yearly nitrogen rate of 523 pounds N/acre/year (for a 52-week year, that is about 0.2 pound N/1,000 square feet/week), less than the amount needed in the first three years. Increasing the N:K ratio from 1:1 to 2:1 also did not impart any agronomic benefits. Thus, the authors provided concrete data to support the practical idea that fertility should be adjusted over time to account for the age of the putting green.

Source: Park, D.M., J.L. Cisar, M.A. Fidanza, E.J. Nangle, G.H. Snyder and K.E. Williams. 2017. Seasonal cultural management practices for aging ultradwarf bermudagrass greens in the subtropics: I. Nitrogen and potassium fertilization. International Turfgrass Society Research Journal 13:280-290.

Editor’s note: Read all of Beth Guertal’s recent Verdure columns.

Beth Guertal, Ph.D., is the Rowe Professor of Soil Fertility in the Department of Crop, Soil and Environmental Sciences at Auburn University in Auburn, Ala., and past president of the Crop Science Society of America. She is a 21-year member of GCSAA.