Crop Science 40:1719-1723 (2000)
© 2000 Crop Science Society of America
TURFGRASS SCIENCE
Nitrogen/Potassium Fertilization Ratios for Bermudagrass Turf
George H. Snydera and
John L. Cisarb
a Everglades Research and Education Center, Univ. of Florida, P. O. Box 8003, Belle Glade, FL 33430 USA
b Ft. Lauderdale Research and Education Center, Univ. Florida, 3205 College Ave., Ft. Lauderdale, FL 33314 USA
ghs{at}gnv.ifas.ufl.edu
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ABSTRACT
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Turfgrass fertilizers often contain approximately one-half as much potassium (K) as nitrogen (N), on a weight basis. Since K fertilization has been shown to be very beneficial for turfgrass appearance and growth, higher rates of K fertilization, relative to N, have been suggested. The effect of K/N fertilization ratios on `Tifgreen' bermudagrass [Cynodon dactylon (L.) Pers. x C. transvaalensis Burtt-Davy] growth and quality was studied over a 3-yr period in south Florida for three rates of N fertilization. Severe K deficiencies were observed in the absence of K fertilization. However, increasing K fertilization beyond a K/N fertilization ratio of 0.5 to 1 had virtually no effect on turfgrass appearance, growth, on resistance to bermudagrass decline, or on root weight. Increasing K fertilization relative to N fertilization did not provide commensurate increases in tissue K.
Abbreviations: FLREC, Ft. Lauderdale Research and Education Center • Q1, Q2, Q3, Q4, Quarter 1, Quarter 2, Quarter 3, Quarter 4
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INTRODUCTION
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TURFGRASS FERTILIZERS generally contain more N and K than other nutrients, since N and K are required by turfgrasses in greater amounts (mass basis) than those of other fertilizer elements. Because turfgrasses have been shown to take up approximately half as much K as N, it is sometimes recommended that these elements be applied in a 2/1 (N/K) ratio (Turgeon, 1985).
Various studies have focused on the importance of K for maintaining turfgrass quality. Improved disease resistance, drought, heat and wear tolerance (Turner and Hummel, 1992), and enhanced root growth and cold hardiness (Beard, 1973, p. 417) have been attributed to adequate K fertilization. Furthermore, K is subject to leaching in many soils (Beard, 1982, p. 141), or to fixation in others (Brady, 1990, p. 372–375). In a growth chamber study, Gilbert and Davis (1971) concluded that a 4N/6K ratio was superior among the ratios evaluated for providing top growth of bermudagrass (Cynodon spp.) following a cold treatment, although statistically (P < 0.05) the 4N/6K ratio was no better than a 4N/3K or 2N/1K ratio. They recommended applying "adequate" K in the late summer for improving cold resistance. However, even considering cold hardiness, Razmjoo and Kaneko (1993) reported that a 2N/1K ratio was sufficient to prevent winter dormancy and provide good winter quality of perennial ryegrass (Lolium perenne L.) in Japan. In a field study involving annual N fertilization rates ranging from 100 to 300 kg ha-1 and K rates from 50 to 300 kg ha-1, Johnson et al. (1987) concluded that bermudagrass turf quality and shoot density were as good at the 50 kg K ha-1 rate as at any higher rate. Nevertheless, Augustin (1992) reported that some turf managers are using relatively large amounts of K fertilizer; i.e., fertilizing with K in amounts equal to or even exceeding the rate of N, and the authors have observed an increasing trend for turfgrass managers to subscribe to this practice. For this reason, the current study was conducted to evaluate the effect of N/K fertilization ratios on bermudagrass turf growth and visual quality at three rates of N fertilization.
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Methods and materials
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The study was established on a mature sward of Tifgreen bermudagrass growing on Hallandale fine sand (Siliceous, hyperthermic Lithic Psammaquent) at the Ft. Lauderdale (Florida) Research and Education Center (FLREC). The field design was a split plot, with six replications. Main plots, 3 m x 8 m, were fertilized at 2.5, 5.0, or 10.0 g N m-2 from (NH4)2SO4 beginning 13 Feb. 1990, and were similarly fertilized near the middle of each month through February 1993. Subplots, 2 by 3 m, were fertilized with K (KCl) on the same day as the N fertilizations to provide K/N ratios of 0, 0.5, 1.0, or 2.0 (K/N fertilization ratios are stipulated instead of N/K ratios, since the "no K" treatment would produce an undefined N/K ratio, i.e., infinity). A Turf-Tender drop-type spreader (Gandy Co., Owatonna, MN) was used for the N fertilization and weighed amounts of KCl were applied to subplots by hand. Approximately 10 mm of irrigation was applied following each N and K fertilization, and irrigation was provided throughout the study to maintain adequate soil moisture. The area was mowed at 1.3-cm height, and mowing frequency was adjusted to minimize scalping. Clippings were removed. During April 1991, the cutting height was lowered gradually over several weeks to 0.6 cm to increase stress on the turf. This mowing height was maintained throughout the remainder of the study.
On 30 Jan. 1991, a material containing 850 g elemental S and 50 g Mn kg-1 (STM-5; Traylor Chemical Co., Orlando, FL) was applied at the rate of 50, 25, and 0 g m-2 to plots receiving N fertilizer at the rate of 2.5, 5.0, and 10.0 g m-2, respectively. On 14 July 1992, the entire plot area received Mg, Fe, Mn, Zn, and Cu at the rate of 100, 10, 10, 3, and 1 kg ha-1, respectively. Herbicides and insecticides were applied according to label specifications when required.
Two weeks after each N and K fertilization, plots were rated visually for quality on a 1-to-10 scale with 10 = best possible turf, 6 = minimally acceptable turf, and 1 = dead or brown turf. In conjunction with visual ratings, clippings from a 1.36-m2 area were taken from each plot with a greens mower. The clippings were oven dried at 60°C, weighed, ground in a stainless steel mill (Arthur Thomas Co., Philadelphia, PA), wet digested (Lowther, 1986), and analyzed for N by automated colorimetry (Technicon AutoAnalyzer II; Technicon Instr. Corp., Tarrytown, NY) and for K by atomic absorption spectrometry (Varian Techtron Pty. Ltd, Springvale, Australia). Periodically, soil samples 0 to 10 cm deep were taken from each plot, or were composited across selected treatments, and analyzed for pH (1 soil:2 water), and Mehlich I (0.05 M HCl in 0.0125 M H2SO4) extractable P, K, Ca, and Mg by the Extension Soil Test Laboratory, University of Florida, Gainesville. On 4 Oct. 1990, 25 Mar. 1992, and 24 Feb. 1993, root weights were determined in cores 0 to 10, 10 to 20, and 20 to 30 cm deep (measured below the thatch layer) and 7.5-cm diam taken from each plot (two cores per plot on the first date, one core on the latter two dates) with a tractor-mounted sampler (Giddings, Ft. Collins, CO). Thatch depth was recorded. The roots were washed fairly free of soil, oven dried (60°C), weighed, ashed in a muffle furnace (550°C), and reweighed. Root data are presented as "ash-free" weight, i.e., oven dry weight minus ash weight.
All observations that were collected on a monthly basis (e.g., ratings, clipping weights, tissue analyses) were averaged on a plot by plot basis over 3-mo intervals beginning April, 1990, to condense and simplify data presentation. Statistical analyses were performed on these 3-mo averages. Data were analyzed as a split-plot design (N = Main plot, K/N = sub-plot) by SAS (1988) PROC ANOVA.
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Results and discussion
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Soil Analysis
Soil pH decreased both with time and with the rate of N fertilization (Table 1)
, as might be expected since (NH4)2SO4 was the N source. Nevertheless, in Jan. 1991, soil pH was sufficiently high in plots receiving 2.5 or 5.0 g N m-2 monthly that a S and Mn mixture was applied to these plots, as was described in the Methods section, in order to prevent Mn deficiency due to high pH. Soil pH declined after the S and Mn application (Table 1). In a previous study at the FLREC (Snyder et al., 1979), Mn deficiency was observed when soil pH exceeded 7.0, and was prevented when soil pH was reduced by (NH4)2SO4 as the N source to counter the liming effect caused by high-pH irrigation water containing appreciable amounts of carbonates and bicarbonates. It was for this reason that (NH4)2SO4 was chosen as the N source in the current study. In the previous study, excellent bermudagrass turf quality and growth were obtained even when soil pH was below 5.0, since the soil contains little Al or other elements that reach toxic levels of availability in acid soils. Therefore, we do not believe that the low soil pHs observed in the latter part of the current study were deleterious to bermudagrass growth and quality.
Potassium was applied monthly to maintain a range of soil K levels across the K treatments, since K is subject to leaching in irrigated sand soils with little organic matter and cation exchange capacity. Apparently this goal was achieved, judging by the wide range of soil K values observed in samples taken just prior to K fertilizations (Table 2)
, which extend below and above the `critical' soil-test value of 60 mg kg-1 provided by Sartain et al. (1999).
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Table 2 Effect of potassium fertilization on soil potassium in samples taken just prior to potassium fertilizations
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Soil-test values for P appeared adequate for turfgrass growth throughout the study (data not presented).
Quality Ratings
Nitrogen fertilization always had a significant (P < 0.05) influence on visual quality ratings (Table 3)
. In all quarters except one, N increased visual rating scores. However, in the third quarter (Q3) of 1991, there was a significant decrease in visual rating with increased N. A severe outbreak of bermudagrass decline disease (a.k.a. Take all, caused by Gaeumannomyces graminis (Sacc.) von Arxd D. Olivier; Elliott, 1991) was observed during this quarter. Ratings for disease severity on 18 Sept. 1991 revealed that the disease was aggravated by high N (personal communication, M. Elliott, Turf Pathologist, FLREC). When rated on a scale of 1 to 5, where 5 indicated no disease and 1 indicated 80 to 100% thinning or chlorosis of turf due to disease, the 2.5, 5.0, and 10.0 g N m-2 rates produced ratings of 4.7, 2.8, and 2.3, respectively (LSD 0.05 = 0.4). Outbreak of this late-summer disease was not observed in the first or last year of the study. However, plot ratings at higher N rates were somewhat lower after Q3 1991, than they were before this period. Lower ratings may have resulted from incomplete recovery from bermudagrass decline, although they also may have been the result of the lowered cutting height imposed in the first month of Q2 1991, and maintained thereafter.
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Table 3 Effect of N and K/N Fertilization Ratio on visual quality ratings of bermudagrass turf, averaged by quarters (Q) of a year for 3 yr
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Potassium to N fertilization ratio (K/N Ratio) significantly affected visual ratings in all quarters of the study. This effect occurred mainly because of the poor performance of plots receiving no K (K/N Ratio = 0). There were only minor differences in visual ratings among plots receiving widely differing K/N ratios, as long as some K was applied. There were some significant interactions between N and K/N, occurring mainly in the final year of the study (Table 3). This interaction occurred because, with increasing N, there were greater differences in plot ratings between the K/N = 0 ratio and the ratios that were greater than 0. In other words, at low rates of N fertilization, the absence of K fertilization created less loss of visual quality than occurred when higher N rates were used. However, the effect of K/N remained the same at all three N rates, i.e., there was little difference in visual quality among the three K/N ratios that provided some K. A similar observation was made for the relationship between bermudagrass decline ratings and K/N. There was little difference in rating among any of the three treatments providing K, although greater disease severity was observed when no K was provided. For example, for K/N ratios of 0, 0.5, 1.0, and 2.0, the disease ratings (1-to-5 scale, 5 = no disease) were 2.6, 3.5, 3.6, and 3.4 (LSD 0.05 = 0.4).
Clipping Weights
Results for clipping weights were similar to those for visual ratings. Clipping weights generally were increased by increasing N fertilization (Table 4)
. However, a significant decrease in clipping weight was observed at the highest N rate in Q3, 1991, when bermudagrass decline was observed. The consistently observed significant effect of K/N ratio on clipping weights is mainly attributable to the reduction in growth that occurred in the absence of any K fertilization (i.e., K/N Ratio = 0). There were few differences in clipping weights among plots receiving some K, regardless of whether K was applied at half or at twice the rate of N. When interactions occurred between N and K/N, the effect was mainly attributable to the increased response to any level of K, as opposed to no K, that occurred as N increased. In this regard, the interaction was similar for visual ratings and clipping rates, and was due to the magnitude, but not the direction, of the response.
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Table 4 Effect of N and K/N Fertilization Ratio on clipping weights of bermudagrass turf, averaged by quarters (Q) of a year for 3 yr
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Tissue N and K
The N content of clippings generally increased with increasing N fertilization, but the K/N fertilization ratio had no effect on tissue N, and there generally was no interaction between these treatments (Table 5)
. Tissue K was also increased by N fertilization in all quarters except for Q3, 1991 (Table 6)
. This probably occurred because as N fertilization increased, the absolute level of K fertilization was increased to achieve the target K/N fertilization ratios. The K/N fertilization ratio significantly increased tissue K throughout the study. However, most of the increase occurred between K/N = 0 and the three treatments providing some K fertilization. There was, however, a small but consistent trend for increasing tissue K as the K/N fertilization ratio increased among the three ratios that provided some K. Nevertheless, averaged across all 11 quarters, increasing the K/N fertilization ratio four-fold (from 0.5–2.0) increased tissue K by only 8%. A significant interaction between the treatment factors was observed in all quarters. As with the ratings and clipping weights, the effect was one of magnitude, and not of direction. When no K was applied, there was little effect of N rate on tissue K. But when some K was applied, tissue K increased with increasing N fertilization, probably for the reason previously presented.
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Table 5 Effect of N and K/N Fertilization Ratio on the N content of bermudagrass turf clippings, averaged by quarters (Q) of a year for 3 yr
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Table 6 Effect of N and K/N Fertilization Ratio on the K content of bermudagrass turf clippings, averaged by quarters (Q) of a year for 3 yr
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Tissue N/K
Nitrogen fertilization had little effect on the N/K ratio in plant tissue, but this observation should be viewed in the light of the N x K/N interaction described below (Table 7)
. The ratio of K/N fertilization significantly affected the tissue N/K ratio, but most of the effect was between the ratio providing no K, and the three ratios providing some K. In the absence of K fertilization, the tissue N/K ratio increased from approximately 3.5 to over 7.0 during the course of the study. But in the presence of K fertilization, the N/K ratio varied only slightly around 2.0. Although there was a trend for decreased tissue N/K ratio with increasing K/N fertilization ratios that provided some K, averaged across all 11 quarters, a four-fold increase in K/N fertilization ratio resulted in only a 7% decrease in the tissue N/K ratio. Significant interactions between N fertilization and K/N fertilization ratio occurred because in the absence of K fertilization, N fertilization increased the tissue N/K ratio, whereas in the presence of K fertilization, increasing N fertilization, with subsequent increases in K fertilization to maintain K/N fertilization ratios, decreased the tissue N/K ratio.
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Table 7 Effect of N and K/N Fertilization Ratio on the ratio of N to K in bermudagrass turf clippings, averaged by quarters (Q) of a year for 3 yr
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Thatch and Roots
Nitrogen fertilization significantly increased both thatch accumulation and the ash-free root weight in the 0- to 10-cm layer in the fall of the first year of the study, but had no effect on these or on other root weights during the remainder of the study (Table 8) . The K/N fertilization ratio had no effect on thatch accumulation, and only affected root weights in the final sampling. In the Feb. 1993 sampling, there were less roots in the top (0–10 cm), bottom (20–30 cm), and total (0–30 cm) portions of the sampled soil profile when no K was applied (Table 8). However, when K was applied in any of the three K/N ratios studied, there were no differences in root weights among the ratios.
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Table 8 Effect of N and K/N Fertilization Ratio on thatch accumulation in bermudagrass turf, and on root mass in the 0- to 10- and 20- to 30-cm depths, and on total root mass in the 0- to 30-cm depth for three sampling dates
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A range of soil pH, soil K, clipping height, and disease pressure occurred during the 3 yr of the study. Nevertheless, even across this range of conditions, increasing K fertilization beyond a K/N ratio of 0.5 to 1 had virtually no effect on turfgrass appearance, growth, resistance to bermudagrass decline, thatch depth, or root weight. Perhaps this occurred because increasing K fertilization relative to N fertilization did not provide commensurate increased in tissue K.Elliott 1991; SAS 1988
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ACKNOWLEDGMENTS
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The authors express their appreciation to Dr. Bruce Augustin (presently The Scotts Co.), and LESCO, Inc., for agrochemicals used in this study, and to Karen Williams for technical management of the experiment. The technical assistance provided by Eva Green, Theresa Sanford, Esther Figueiras, and Norman Harrison has also been very much appreciated.
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NOTES
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Contribution from the Florida Agric. Exp. Stn. Journal Series no. R-06865.
Received for publication March 22, 1999.
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REFERENCES
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- Augustin B.J. Surviving drought. Grounds Maintenance 1992;27(5):65,94.
- Beard J.B. Turfgrass: Science and culture. Englewood Cliffs, NJ: Prentice-Hall, Inc., 1973.
- Beard J.B. Turf management for golf courses. New York: Macmillan Publishing Co, 1982.
- Brady N.C. The nature and properties of soils, 10th Ed New York: Macmillan Publishing Co, 1990.
- Elliott M.L. Determination of an etiological agent of bermudagrass decline. Phytopathology 1991;81:1380-1384.
- Gilbert W.B., Davis D.L. Influence of fertility ratios on winter hardiness of bermudagrass. Agron. J. 1971;63:591-593.[Abstract/Free Full Text]
- Johnson B.J., Carrow R.N., Burns R.E. Bermudagrass turf response to mowing practices and fertilizer. Agron. J. 1987;79:677-680.[Abstract/Free Full Text]
- Lowther J.R. Use of a single H2SO4-H2O2 digest for the analysis of Pinus radiata needles. Commun. Soil Sci. Plant Anal. 1986;11:175-188.
- Razmjoo K., Kaneko S. J. Plant Nutr. 1993;16(8):1531-1538.
- Sartain J.B., Miller G.L., Snyder G.H., Cisar J.L., Unruh J.B. Fertilizer program. In: Unruh J.B., Elliott M.C., eds. Best management practices for Florida golf courses, 2nd ed Gainesville, FL: SP-141. University of Florida, 1999:65-72.
- SAS. SAS/STAT Guide for personal computers, 6th ed Cary, NC: SAS Inst, 1988.
- Snyder G.H., Burt E.O., Gascho G.J. Correcting pH-induced manganese deficiency in bermudagrass turf. Agron. J. 1979;71:603-608.[Abstract/Free Full Text]
- Turgeon A.J. Turfgrass management. Reston, VA: Reston Publishing Company, Inc., 1985.
- Turner T.R., Hummel N.W., Jr. Nutritional requirements and fertilization. In: Waddington D.V., et al. , ed. Turfgrass. Madison, WI: Agron. Monogr. 32. ASA, 1992:408-410.
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ABSTRACT
INTRODUCTION
