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Crop Science 42:507-512 (2002)
© 2002 Crop Science Society of America

TURFGRASS SCIENCE

Tifway Bermudagrass Response To Potassium Fertilization

J. B. Sartain*

Soil and Water Science Dep., Univ. of Florida, P.O. Box 110510, Gainesville, FL 32611

* Corresponding author (jbs{at}gnv.ifas.ufl.edu)


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Potassium fertilization rates used for ‘Tifway’ bermudagrass [Cynodon dactylon (L.) Pers. x C. transvaalensis Burtt-Davy] vary greatly among turfgrass managers and there is no consensus as to the actual quantity required or as to the best Potassium (K) source to use. Effects of K source and rate on Tifway bermudagrass shoot and root growth, quality and tissue K concentration were studied in a 3-yr field study in Central Florida. Turf was grown on an Arredondo fine sand (loamy, siliceous, hyperthermic, Grossarenic Paleudult). Two K sources (KCl and K2SO4) were applied at 8 rates (0, 3.7, 7.4, 9.8, 14.7, 22, 29.4, and 36.8 g K m-2 90 d-1) in conjunction with N applied monthly at 4.9 g m-2. Potassium chloride produced a more rapid shoot growth than did K2SO4, but this effect may be linked to the N source. Bermudagrass shoot growth rate and tissue K concentrations were increased by K fertilization up to 7.4 g K m-2 90 d-1. There was no additional increases in either of the aforementioned parameters, regardless of the K level applied. Turfgrass quality and root weight were not influenced by K application. Greater root weight was observed during May, June, and July than August, September, and October. Observed turfgrass growth responses to Mehlich-1 extractable K levels suggest that 30 mg K kg-1 soil may be adequate for optimum growth. For an N application rate of 4.9 g m-2 mo-1, K fertilization levels above 9.8 g K m-2 90 d-1 probably will not enhance Tifway bermudagrass shoot and root growth, quality, or tissue K concentration.

Abbreviations: K, potassium • N, nitrogen • KCl, potassium chloride • K2SO4, potassium sulfate • (NH4)2SO4, ammonium sulfate • NH4NO3, ammonium nitrate • TNC, total nonstructural carbohydrates • CEC, cation exchange capacity


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
NITROGEN (N) REQUIREMENTS for growth of ‘Tifway’ bermudagrass have been well established. To maintain good quality of Tifway bermudagrass, at least 20 to 30 g N m-2 yr-1 were required in Georgia (Carrow et al., 1987). Additionally, Callahan and Overton (1978) reported a significant increase in ‘Common’ bermudagrass density when N was increased from 10 to 20 g m-2 y-1 in Tennessee, but the turfgrass did not respond as readily to potassium (K) as to N. Previous studies have shown that Tifway bermudagrass either does not respond positively to K (Barrios and Jones, 1980) or only responded to low rates of less than 4.9 (Carrow et al., 1987) or 12.5 g K m-2 (Horn, 1969). When clippings were returned, Tifway bermudagrass growth rate did not respond to K (Sartain, 1993).

Typically, turfgrasses accumulate about twice as much N as K so these elements are recommended to be applied at a 2:1 (N:K) ratio (Turgeon, 1985). Augustin (1992) reported that turfgrass managers use relatively large amounts of K fertilizer, generally at levels equal to or exceeding the rate of N. Use of high K rates may have been prompted by reports that K improved disease resistance, and drought, heat and wear tolerance (Turner and Hummel, 1992) and enhanced root growth and cold-hardiness (Beard, 1982). Gilbert and Davis (1971) reported that a 4N:6K ratio provided excellent shoot growth of ‘Tifdwarf’ and ‘Tifgreen’ bermudagrasses following cold treatment, although shoot growth at the 4N:6K ratio was not statistically better than a 4N:3K or 2N:1K ratio fertilizers. Peacock et al., (1997) reported no improvement in turf quality, color, rooting, or cold tolerance of Tifgreen bermudagrass as a result of increasing rates of K. Snyder and Cisar (2000) found that increasing K beyond a N:K fertilization ratio of 2:1 had no effect on Tifgreen bermudagrass appearance, growth, root weight, or resistance to bermudagrass decline [Gaeumannomyces graminis (Sacc.) Arx. & D. Oliver var. graminis]. Increasing K relative to N did not result in commensurate increases in tissue K. Carrow et al. (1987) found somewhat less dollar spot (Sclerotinia homoeocarpa F.T. Bennett) in Tifway bermudagrass treated with 4.9 g K m-2, when compared with 9.8 and 19.6 g K m-2. Johnson et al. (1987) observed no benefit in reducing dollar spot in Tifway from 0.5 to 30 g K m-2. Decreases in winter injury or increases in bermudagrass winter hardiness due to increased K levels have been reported (Juska and Murray, 1974). Beard and Rieke (1966) stated that turfgrass winter survival was maximal when K rates were about one-half the N rate. Total nonstructural carbohydrates (TNC) decreased in roots and rhizomes of Tifdwarf and Tifway hybrid bermudagrasses with increasing rates of K, but had no effect on shoot and stolon TNC concentrations (Miller and Dickens, 1996).

Numerous K sources are available and turfgrass response to different K sources have been evaluated. Horn (1965) reported that K2SO4 and K2CO3 induced superior Tifway bermudagrass quality relative to other K sources. However, when the plots were fertilized with ammonium sulfate, the quality differences attributed to the K sources subsided. Snyder and Cisar (1990) found no bermudagrass growth response to various K sources. Potassium sulfate leaches less rapidly than potassium chloride, so a greater utilization efficiency of potassium sulfate is anticipated (Chung et al., 1998).

Wide ranges in tissue K concentrations have been reported and reflect differences in turfgrass species (Mehall et al., 1983; Turner and Waddington, 1983), fertilization practices (Sartain and Dudeck, 1980; Snyder and Cisar, 2000), and time of sampling (Mehall et al., 1983). These wide differences make interpretation of tissue K levels difficult for purposes of diagnosing deficiencies. However, recent findings by Snyder and Cisar (2000) for Tifgreen bermudagrass suggest a relationship between tissue K and growth response. When the tissue K level was below 13 g K kg-1 dry matter, a growth response to K application was obtained. Once tissue K levels reached approximately 16 g K kg-1 dry matter, no increase in tissue K concentration or growth rate were observed in response to the application of additional K.

Arredondo fine sand is composed of 960g kg-1 sand and has a cation exchange capacity (CEC) of 7.7 cmol (+) kg-1 (Carlisle et al., 1989), thus the potential for K leaching is great. Sartain (1995) reported significant K leaching from this soil. Unfertilized Arredondo fine sand typically contains very low Mehlich-1 extractable K levels (Sartain, 1993).

Although the effects of K on turfgrasses has been studied for decades, there is no consensus among turfgrass managers as to the proper K fertilization program. This research was undertaken to evaluate the influence of K fertilization rate and source on Tifway bermudagrass shoot growth rate, root density, visual quality, and tissue K concentration.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
A 3-yr field study was initiated at the G.C. Horn Turfgrass Field Research Laboratory, Gainesville, FL, during May 1996 on an established stand of Tifway bermudagrass growing on a Arredondo fine sand (pH 6.6). Mehlich-1 extractable K prior to treatment application was 17 mg K kg-1. Treatments were applied to 2 by 3 m plots arranged in a randomized complete block split-plot design with three replications. Two K sources (KCl and K2SO4) were applied at 8 rates (0, 3.7, 7.4, 9.8, 14.7, 22, 29.4, and 36.8 g K m-2 90 d-1). Since only one 90 d cycle was evaluated in 1996, K and N were applied one and three times per year, respectively. In 1997 and 1998, two 90 d growth cycles were evaluated, thus K and N were applied two and six times per year, respectively. Nitrogen was applied at 4.9 g m-2 30 d-1 with either (NH4)2SO4 or NH4NO3. To balance the quantity of S applied, NH4NO3 was used on the K2SO4 plots and (NH4)2SO4 was used in the KCl plots. The Arredondo fine sand is a phosphatic soil which tests high in Mehlich-1 extractable P (204 mg kg-1): Therefore, no P was applied. Nitrogen and K sources were applied by hand. Approximately 1 cm of irrigation water were applied following each N and K fertilization, and irrigation was provided throughout the study to maintain adequate soil moisture based on the monthly average evapotranspiration. During the 90 d study period of 1996, and the 180 d periods of 1997 and 1998, 42.8, 83.0, and 83.7 cm of rainfall occurred, respectively. In addition, 55.5, 96.0, and 99.8 cm of water were applied by overhead irrigation in 1996, 1997, and 1998, respectively. The area was mowed to a height of 1.3-cm twice a week. The number of days elapsed since the last mowing was used to calculate the turfgrass growth rate. Clippings were returned, except when collections were made for growth and tissue analysis.

During the first year of the study, one 90 d growth cycle was investigated and clippings were collected three times at 30 d intervals for growth rate and K uptake estimates. Two, 90 d growth cycles were studied during the second and third years. Clippings were collected twice monthly for a total of 12 times during the second yr and four times 45 d apart during the third year. Clippings were oven dried at 65°C for 48 h, weighed, ground in a stainless steel mill and ashed for 8 h in a 450°C muffle furnace for K analysis. Total tissue K was determined by atomic absorption spectrometry (Varian Spectr AA-20 Plus, Mulgrave, Victoria, Australia). Three soil samples were collected from each plot with a 1.25 (length) by 10 (width) by 15 (depth) cm flat root sampler for root growth estimates. In 1996 and 1997, root samples were collected in June. Root samples were collected every 14 d over a 180 d period, for a total of 13 times in 1998. Roots were washed free of soil, oven dried at 65°C for 24 h and weighed. At the end of each growth year, soil samples were taken (0 to 10 cm deep) from each plot and analyzed for pH (1 soil:2 water), and Mehlich-1 (0.05 M HCl in 0.0125 M H2SO4) extractable P, K, Ca, and Mg. Visual quality ratings, based on a 1 to 9 scale, were taken twice monthly each year. A quality rating of 9 represented superior quality turf, 5.5 = the minimally acceptable turf, and 1 = dead or brown turf.

An analysis of variance was preformed on individual clipping collections and on yearly composites. Data were analyzed as a split-plot design with Statistical Analysis System (SAS) software, procedure PROC ANOVA (SAS Institute, Inc., 1985). Separation of means was accomplished with the general linear model procedure (PROC GLM) and single degree of freedom contrasts at P <= 0.05. Since this study involved the use of K rates and one of the objectives was to determine the rate at which no additional response to applied K was obtained, single degree of freedom contrasts were generated with the general linear model procedure. In each contrast, the response variable at a K rate was compared with the average effect of response at K rates greater in value, which was referred to as ‘rest’ in the figures. The K rate being compared with the remaining larger rates was assigned a positive value and the higher K rates were assigned a proportional negative value such that the sum of the weighted contrast values was zero.


   RESULTS AND DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Shoot Growth Rate
Shoot growth rate varied with K source and year (Table 1). Yearly differences in growth rate most likely were influenced by proximity of the clipping collection date to the N application date. Shoot growth rates were greater in 1998 than in 1996 because only 15 d elapsed following N application in 1998, whereas 30 d elapsed in 1996. In each of the 3 yr, KCl treated bermudagrass produced greater growth rates than turf fertilized with K2SO4. The K source effect may have been an artifact of the experimental design in that (NH4)2SO4 was applied to the KCl plots in order to balance the quantity of S applied to all plots. In previous studies, (NH4)2SO4 produced a more rapidly growing Tifdwarf bermudagrass than NH4NO3 (Horn, 1965; Volk, 1972).


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  		Table 1. Effect of potassium source on yearly mean Tifway bermudagrass shoot growth rate.

 
Quantity of applied K influenced Tifway bermudagrass shoot growth rate in all 3 yr, but only data for 1997 and 1998 are presented (Fig. 1) . Data for 1996 were omitted because they represent limited observations and because turfgrass shoot growth rate was low due to the sampling sequence used. Single degree of freedom contrasts of shoot growth rate versus rate of K applied revealed differences in growth rate up to approximately 9.8 g K m-2 90 d-1. Application of additional K beyond 9.8 g K m-2 90 d-1 did not increase shoot growth rate. A total of 14.7 g N m-2 90 d-1 was applied, and this equates to an N:K application ratio of 1.5:1, which may be different from the N:K ratio being used by some turfgrass managers. Snyder and Cisar (2000) reported a similar response to N:K fertilization ratio on Tifgreen bermudagrass growing in a Hallandale fine sand (siliceous, hyperthermic Lithic Psammaquent) in South Florida.



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  		Fig. 1. Effect of K application on growth rate of Tifway bermudagrass during 1997 and 1998. Single degree of freedom contrasts were generated with the SAS GLM Proc. *, *** at P <= 0.05 and P = 0.001 and nonsignficant (NS) at P > 0.05. Rest refers to the average of treatment means larger in magnitude than the one in the contrast.

 
Shoot Tissue K
Tifway tissue K concentrations varied with K rate and year (Fig. 2) . Greater tissue K concentrations were observed during 1997 than 1998, possibly because the greater growth rate in 1998 caused a dilution effect. In both years, a maximum tissue K level was achieved (16 and 14 g kg-1 in 1997 and 1998, respectively) in response to K applied at 9.8 g m-2 90 d-1. Greater applied K rates did not further increase tissue K. Maximum tissue K concentration was achieved in response to the application of 9.8 g K m-2 90 d-1 in both 1997 and 1998. Maximum shoot growth rate was achieved at 9.8 g K m-2 90 d-1, suggesting that the application of additional K does not enhance shoot growth or K uptake. Apparently, Tifway bermudagrass does not exhibit ‘luxury consumption’ of K and high K:N ratios were not beneficial. Snyder and Cisar (2000) reported maximum tissue K levels in Tifgreen bermudagrass in response to K application of 1.25 g m-2 30 d-1, with no additional tissue K increases with increasing rates of K.



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  		Fig. 2. Effect of K application on Tifway bermudagrass tissue K concentration during 1997 and 1998. Single degree of freedom contrasts were generated with SAS GLM Proc. *** and nonsignificant (NS) at P <= 0.01 and P > 0.05, respectively. Rest refers to the average of treatment means larger in magnitude than the one in the contrast.

 
Root Growth
A total of 1872 samples were taken during 1998 for root growth estimates. Potassium rate did not significantly influence root dry weight (Fig. 3) . A June sampling in 1996 and 1997 revealed no K effect on root growth (data not shown). One reason typically given for application of elevated levels of K relative to N is enhanced root growth. Beard (1973) suggested that turfgrass root growth was enhanced by K fertilization. Snyder and Cisar (2000) and Peacock et al. (1997) did not show increased root growth of Tifgreen bermudagrass by increasing the K:N ratio. Trenholm et. al. (1998) reported a slight decrease in root growth of ‘FloraDwarf’ bermudagrass in response to K under long day (>13 h) conditions and increased Tifdwarf root growth under short day (<13 h) conditions. The findings presented in this study suggest that there is limited influence of applied K on Tifway bermudagrass root growth, even when the turfgrass is growing in a sandy soil with limited CEC and an inability to retain K against leaching losses.



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  		Fig. 3. Effect of K application on Tifway bermudagrass root dry weight during 1998. Single degree of freedom contrasts were generated with SAS GLM Proc. Nonsignifant (NS) at P > 0.05. Rest refers to the average of treatment means larger in magnitude than the one in the contrast.

 
Time of year influenced root growth (Fig. 4) . Maximum root weight was observed during May and was lowest during September. In general, the greatest root weight was observed during the months of May, June, and July, and the lowest root weights occurred during August, September, and October. This reduction in root growth could have been related to a day length phenomenon. Trenholm et al. (1998) reported a 13% reduction in Tifdwarf bermudagrass root growth when the day length was reduced to <13 h d-1. A reduction of approximately 16% in Tifway bermudagrass root growth was observed during the months of August, September, and October relative to that of longer day length periods in May, June and July.



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  		Fig. 4. Effects of sampling month on Tifway bermudagrass root dry weight in 1998. Single degree of freedom contrasts were generated with SAS GLM Proc. *** at P <= 0.01 and nonsignificant (NS) at P > 0.05.

 
Quality Ratings
Quality ratings were not influenced by K in any year. Turfgrass quality remained high through out the study period. Mean quality ratings ranged from 6.7 to 6.9 with a coefficient of variability (CV) of 15.7%, 6.9 to 7.2 with a CV of 11.9%, and 7.0 to 7.4 with a CV of 12.9% in 1996, 1997, and 1998, respectively. Somewhat lower ratings were recorded early in the growing seasons, but at no time was there an improvement in visual quality related to K application. Growth rate was increased by K application, but the response did not translate into a visual improvement in quality. Peacock et al. (1997) also did not observe an improvement in Tifgreen quality in response to K application. Snyder and Cisar (2000), however, reported an improvement in turfgrass quality in response to K, which mainly was due to the poor performance of Tifgreen bermudagrass on plots receiving no K. The lack of a response to K on Tifway quality may be related to the return of clippings. Clippings were returned in this study, but were removed by Snyder and Cisar (2000). In a previous study on Tifway, a growth and quality response to K was observed on plots where clippings were removed (Sartain, 1993). The influence of K on bermudagrass quality has most often been reported as an indirect effect on disease. Horn (1970) reported reduced dollar spot incidence in response to K application. However, Carrow et al. (1987) reported somewhat less dollar spot in Tifway fertilized with 4.9 g K m-2, but not at greater K rates. Johnson et al. (1987) found no benefit in reducing dollar spot on Tifway from using 5.0 to 30 g K m-2. Incidence of disease in the current study was minimal; therefore, the influence of increasing K rates on disease could not be effectively determined.

Soil Analysis
Soil pH and Mehlich-1 extractable nutrient levels prior to treatment application were within the optimum range for bermudagrass growth (Hanlon et al. 1990), except for K (Table 2). Based on current soil test interpretations, Mehlich-1 extractable K levels were "very low" and a response to K fertilization would be expected at least 75% of the time (Hanlon et al. 1990). Mehlich-1 extractable P and Mg were "very high", and were not applied during the study. Adequate levels of micronutrients were present and also were not applied.


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  		Table 2. Soil chemical properties prior to treatment application.

 
Mehlich-1 extractable K increased during the 1998 growth season and in response to K application (Table 3). By the end of the growing season, soil samples from plots receiving 9.8 to 22.0 g K m-2 tested medium in Mehlich-1 extractable K, and a response to K addition would be expected 25% of the time (Hanlon et al. 1990) No growth or tissue K response was observed when the application of K was 9.8 g K m-2 or greater (Fig. 1 and 2) and the Mehlich-1 extractable K level were 31 to 47 mg kg-1 soil. These findings suggest that the current interpretation of Mehlich-1 soil test results for bermudagrass may not be correct, and that the sufficient Mehlich-1 extractable K level should be adjusted downward to possibly as low as 30 mg K kg-1 soil.


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  		Table 3. Mehlich-1 soil extractable levels of selected nutrients as influenced by potassium applications during the 1998 bermudagrass shoot growth period.

 

   CONCLUSIONS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
When a critical minimum K tissue concentration was achieved, approximately 1.5 mg kg-1 tissue, additional K does not result in additional K uptake, shoot growth, or improved visual quality or root growth in Tifway bermudagrass. Monthly variations in root dry weight were observed, but application of K did not result in additional root growth. The critical Mehlich-1 extractable level of soil K appeared to be near 30 mg K kg-1 soil. Information collected through this 3 yr study suggested that K rates in excess of 0.50 to 0.67 times that of N application rates (4.9 g N m-2 30 d-1) do not result in additional tissue K uptake, shoot and root growth or enhanced visual quality. Additional research with intensive soil sampling and measurement of corresponding turfgrass tissue K levels should be performed prior to establishing new interpretations relative to the critical Mehlich-1 extractable level of soil K.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
This research was supported by the Florida Agricultural Experiment Station, and approved for publication as Journal Series No. R-08092.

Received for publication April 24, 2001.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 




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