Quantitative and qualitative responses of an established Kentucky ...

Quantitative and qualitative responses of an established Kentucky bluegrass (Poa pratensis L.) turf to N, P, and K additions Abdo Badra1, Léon-Etienne Parent2,5, Yves Desjardins3, Guy Allard3, and Nicolas Tremblay4 1Nutrite,

Brossard, Quebec, Canada J4Z 1A7; 2Department of Soil Science and Agri-Food Engineering, FSAA, Laval University, Quebec, Canada G1K 7P4; 3Department of Plant Science, FSAA, Laval University, Quebec, Canada G1K 7P4; 4Agriculture and Agri-Food Canada, Horticultural Research and Development Centre, St-Jean-sur-Richelieu, Quebec, Canada J3B 3E6. Received 16 July 2003, accepted 10 August 2004. Badra, A., Parent, L.-E., Desjardins, Y., Allard, G. and Tremblay N. 2005. Quantitative and qualitative responses of an established Kentucky bluegrass (Poa pratensis L.) turf to N, P, and K additions. Can. J. Plant Sci. 85: 193–204. Kentucky bluegrass is a common turf species used on golf courses, sports fields, municipal parks, sod farms, road banks, as well as residential and school yards. Our objective was to determine the effects of N, P, K rates on turfgrass quantitative response (clipping yield and underground turf biomass) and qualitative response (shoot density and foliage colour) under a continuous clipping removal. A 3-yr field study was conducted on two sites, a sand that met the specifications of the United States Golf Association (USGA) and a loam. The factorial experiment was arranged in a randomized complete block design with four replicates and different levels of three nutrients, N (0 or 50 to 300 kg ha–1 yr–1), P (0 or 21.8 to 87.3 kg P ha–1 yr–1), and K (0 or 41.7 to 250 kg K ha–1 yr–1). The maximum clipping yield was produced at the rate of 200 kg N ha–1 yr–1 in the loam and 300 kg N ha–1 yr–1 in the sand. Increasing N rates linearly reduced underground turf biomass. Added P and K had no effect on clipping yield and underground turf biomass. Nitrogen significantly improved shoot density and foliage colour. However, equivalent shoot density and colour ratings required 40 to 80 kg more N ha–1 yr–1 in the sand compared to the loam. Phosphorus and K had no significant effect on shoot density and colour in the loam. Colour response to P and K depended on N rates in the sand. Fertilizer units needed to increase soil test P averaged 6 kg added P ha–1 mg–1 PM-III kg–1 across soil types. To replenish soil K, 7 kg K ha–1 per mg KM-III kg–1 were required in the sand, and 3 kg K ha–1 per mg KM-III kg–1 in the loam. Phosphorus and K fertilizer programmes should account for P and K removals to maintain low to medium fertility levels for P, and medium for K when conditions are similar to those in this research. Key words: Turfgrass clipping yield, underground turf biomass, turfgrass shoot density, turfgrass foliage colour, Kentucky bluegrass fertilization Badra, A., Parent, L.-E., Desjardins, Y., Allard, G. et Tremblay N. 2005. Réponses quantitative et qualitative de gazon établi en pâturin des prés (Poa pratensis L.) aux ajouts de N, P et K. Can. J. Plant Sci. 85: 193–204. Le pâturin des prés est une espèce de gazon couramment utilisée dans les terrains de golf, les terrains sportifs, les parcs municipaux, les gazonnières, les bordures de route, et les aires résidentielles et scolaires. Notre objectif était d’évaluer les effets de la fertilisation en N, P et K sur le rendement de débris de tonte, la biomasse souterraine du gazon, la densité de tallage et la couleur du feuillage en cas de débris de tonte ramassés. Une étude sur trois années a été réalisée sur deux sites, l’un sableux conforme aux spécifications de la United States Golf Association et l’autre loameux. Le dispositif expérimental était en blocs complets aléatoires avec quatre répétitions et différents niveaux pour trois éléments fertilisants, soit N (0 ou 50 à 300 kg ha–1 an–1), P (0 ou 21.8 à 87.3 kg P ha–1 an–1), et K (0 ou 41.7 à 250 kg K ha–1 an–1). Le plus grand rendement de débris de tonte a été produit par la dose de 200 kg N ha–1 an–1 dans le loam et 300 kg N ha–1 an–1 dans le sable. La biomasse souterraine du gazon a été réduite linéairement en réponse aux doses d’azote. Les ajouts de P et de K n’ont pas affecté significativement le rendement de débris de tonte ni la biomasse souterraine du gazon. La réponse du gazon à l’azote était significative quant à la densité de tallage et à la couleur du feuillage. Il a fallu 40 à 80 kg N ha–1 an–1 de plus dans le sable pour produire une densité de tallage et une couleur comparables à celles observées dans le loam. Le P et le K n’ont eu aucun effet significatif sur la couleur du feuillage ni la densité de tallage dans le loam, par contre un effet de P et de K sur la couleur a été observé en fonction de la dose d’azote au site sableux. L’augmentation de P dans les deux sols a nécessité 6 kg P ajouté ha–1 par mg PM-III kg–1 alors que l’augmentation de K a nécessité 7 kg K ha–1 par mg KM-III kg–1 dans le sable et 3 kg K ha–1 par mg KM-III kg–1 dans le loam. Le programme de fertilisation en P et K devrait permettre de maintenir l’analyse de sol dans les classes de fertilité faible à moyenne pour le P et moyenne pour le K si les conditions sont semblables à celles de la présente recherche. Mots clés: Rendement de débris de tonte du gazon, biomasse souterraine du gazon, densité de tallage du gazon, couleur du feuillage du gazon, fertilisation du pâturin des prés

5To

Abbreviations: CEC, cation exchange capacity; USGA, United States Golf Association

whom correspondence should be addressed ([email protected]). 193

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Kentucky bluegrass is a common turf species used on golf courses, sports fields, municipal parks, sod farms, road banks, as well as residential and school yards. Turfgrass productivity is largely influenced by fertilization, particularly nitrogen. A greater N use efficiency is observed at low N levels (Below 1995). A high N availability in soil-turfgrass systems stimulates shoot growth (Turner and Hummel 1992) and yields high amounts of clippings (Walker et al. 1963; Hull and Smith 1974; Hummel and Waddington 1981). There were significant increases in turfgrass shoot growth with ranges of 0 to 650 kg N ha–1 yr–1 (Walker et al. 1963; Juska and Hanson 1967; Hull and Smith 1974; Razmjoo and Kaneko 1993), and 12 to 98 kg N ha–1 mo–1 (Trenholm et al. 1998). Nitrogen also had a large effect on turfgrass shoot density and foliage colour ratings (Hummel and Waddington 1981). In Bermudagrass and over-seeded perennial ryegrass, low N rates (2.5 and 5 g N m–2 over 8 wk) were not adequate to maintain an acceptable turfgrass quality (Sartain and Dudeck 1982). Johnson et al. (1987) found that N increased bermudagrass shoot density with greatest responses using 100 and 200 kg N ha–1 yr–1 while 200 kg N ha–1 yr–1 was adequate to properly maintain lawns in the Piedmont region of Georgia. Foliage colour rating improved with N rates of 50 to 650 kg N ha–1 in perennial ryegrass (Razmjoo and Kaneko 1993). Close relationships were found between fresh clipping weight and foliage colour rating of Kentucky bluegrass (Hummel and Waddington 1981), hence, quantitative turfgrass attributes could corroborate aesthetic evaluations. Darker turfgrass colouration using N fertilization may be associated with shallow rooting (Turgeon 1991). High levels of N additions or above-optimum available N in the soil inhibit root and rhizome growth (Hylton et al. 1964; Watschke and Waddington 1974, 1975; Christians et al. 1981; Le Bot et al. 1994), allocating assimilates to aboveground shoot tissue (Le Bot et al. 1994). Turfgrass had smaller root systems with 200 kg N ha–1 yr–1 compared to controls (Bocker and Opitz von Boberfeld 1974). However, the turfgrass root system does not expand readily in response to N (Madison 1962). Responses of clipping yield and underground biomass of Kentucky bluegrass to P and K fertilizations are variable but not striking, depending on initial soil P and K levels (Hull and Smith 1974; Christians et al. 1979, 1981; Turner 1980; Turner and Hummel 1992). Additions of 50 to 350 kg P ha–1 yr–1 had no effect on turfgrass shoot density or foliage colour (Waddington et al. 1978; Razmjoo and Kaneko 1993). Carrow et al. (1987) observed no improvement on shoot density and foliage colour from using more than 49 compared to 98 and 196 kg K ha–1 yr–1. Clipping yields were not consistently influenced by K, yet K significantly increased clipping yields over time (Waddington et al. 1972). In soils with medium to high contents of P and K, P and K can reach detrimental levels for growth and quality of turfgrass (Christians et al. 1981). However, there is little information on the residual effect of N, P, and K fertilizations especially under continuous clipping removal. The purpose of this research was to determine the effect of N, P and K rates on clipping yield, underground turf bio-

mass, ratings of shoot density and foliage colour, and soil test P and K for Kentucky bluegrass stands grown on a loam or a USGA sand, under continuous clipping removal in southwestern Quebec. MATERIALS AND METHODS Experimental Sites This study was initiated in August 1991 at the Agriculture and Agri-Food Canada research station at L’Acadie, Quebec (45°17′45″N, 73°21′00″W). Soils were a Saint-BlaiseMacdonald loam (Humic Gleysol) and a reconstituted sandy soil meeting the specifications of the USGA for golf greens (Bengeyfield 1989). The reconstituted sandy soil was made of a layer of 30–35 cm of sand from Pine Hill near Lachute, Quebec, laid over the native loamy soil with the following specifications: infiltration rate of 18.8 cm h–1, water holding capacity of 0.191 m3 m–3 at –4 kPa, and carbon content of 39 g C kg–1. Peat humus from Alfred bog (Ontario) was added to the sand at an amount of approximately 2% by weight. Plot size was 3.0 × 1.2 m. The prior forage grass-legume stand was destroyed using glyphosate followed by a mixture of 2,4-D mecoprop and dicamba. The field was irrigated to soften the soil. The herbicide-treated forage sod was cut to a depth of 1 to 2 cm with a mechanical sod cutter. The bare soil was plowed to a depth of 20 cm and disked the same day. Based on soil chemical testing of both sites, the loam received 450 kg lime ha–1 (lime agricultural index of 75%; lime composition : 12% Mg and 21% Ca) and 92 kg of N ha–1 as urea (46% N); they were immediately incorporated by using a rototiller into the top 7 cm before seeding on 1991 Aug. 23. The sand received the following pre-seeding materials that were rototilled into the top 7 cm : 60 kg N ha–1 as urea, 10.9 kg P ha–1 as super-phosphate (8.7% P, 11.8% S and 20% Ca), 50 kg K ha–1 as potassium sulphate (41.7% K and 17% S), 235 kg S ha–1 as agricultural S (90% S), 26 kg Mg ha–1 as Epsom salt (9.8% Mg, 2% Ca and 14% S), and micronutrient Frit 358 GC (0.42 kg B ha–1, 1.04 kg Cu ha–1, 2.08 kg Fe ha–1, 1.04 kg Mn ha–1, 0.01 kg Mo ha–1, 1.04 kg Zn ha–1 and 1.04 kg Co ha–1). Establishment of Kentucky Bluegrass Turf An equal blend of four Kentucky bluegrass (Poa pratensis L.) cultivars (cv. Baron, Argyle, Gnome, and Regent) was seeded on 1991 Aug. 23 at both sites using a brillion seeder and a rate of 220 kg seed blend ha–1. Irrigation was set up immediately afterwards. Both sites received 30 kg N ha–1 as urea and 25 kg ha–1 as potassium sulphate on 1991 Sep. 26. Turfgrass grown in the sand was not mowed in 1991 due to lack of growth. The first mowing in the loam occurred on 1991 Sep. 26, while the sixth and last mowing of the year was done in mid-November 1991. Quintozene was applied in mid-November 1991 to prevent grey snow mould (Typhula spp.). Due to low shoot density after the 1991–1992 winter, plots were over-seeded with the same seed blend across sites at a rate of 350 kg seed blend ha–1 at the end of April 1992, and 25 kg N ha–1 as urea was applied. The same N rate was repeated in mid-May and in early June

BADRA ET AL. — RESPONSES OF KENTUCKY BLUEGRASS TURF TO N, P, AND K ADDITIONS

1992. Some bare spots were over-seeded again with 250 kg seed blend ha–1 at the end of May 1992. The stand density was deemed uniform on 1992 Jul. 20, ready to start the fertilizer trials. Trials were conducted in 1992, 1993, and 1994. All clippings were removed, since returning clippings could reduce N fertilizer recommendations by 50% or more (Kopp and Guillard 2002). Fertilizer Treatments The yearly fertilizer rates in 1993 and 1994 were equally split across six dates to provide 0, 100, 200, and 300 kg N ha–1 yr–1, 0, 43.7, and 87.3 kg P ha–1 yr–1, and 0, 83.3, 166.7, 250 kg K ha–1 yr–1 in the loam. The N × P × K factorial trial consisted of 48 treatments arranged in a completely randomized block design with four replicates (192 experimental plots). Since nutrient diffusion was slower in the sandy soil materials (Olsen and Watanabe 1963), N, P and K control plots on the sand site were the ones receiving 50 kg N ha–1 yr–1, 21.8 kg P ha–1 yr–1, and 41.7 kg K ha–1 yr–1, while the control plots on the loam did not receive any fertilizer addition. In 1992, we applied on both sites 60% of the intended yearly rates because in early May 1992, turfgrass shoot density did not fully recover from winter injury. Fertilizer materials were urea, super-phosphate and potassium sulphate. All fertilizer materials were screened to 1.5–2.0 mm in diameter before weighing to remove dust and minimize pellet segregation during mixing and broadcasting. The fertilization dates for the loam site were: Jul. 20, Aug. 05, and Aug. 20 in 1992; May 14, Jun. 03, Jul. 02, Jul. 23, Aug. 12, and Sep. 03 in 1993; and May 18, Jun. 08, Jun. 30, Jul. 20, Aug. 11, and Aug. 30 in 1994. Fertilization dates for the sand site were: July 20, Aug. 05, and Aug. 20 in 1992; May 13, Jun. 03, Jul. 02, July 23, Aug. 12, and Sep. 02 in 1993; May 18, Jun. 08, Jun. 30, Jul. 20, Aug. 11, and Aug. 30 in 1994. This is not a standard fertilizer application schedule for the region (four application dates for intensive fertilization and two application dates for low maintenance turfrass), but highest N, P, and K rates necessitated splitting annual rates into six application dates to avoid burning from urea and salt damage. Fertilizers were hand-broadcast and uniformly spread on the plots that were then irrigated. A second light irrigation was done the following day. Properties of the irrigation water as mean ± standard deviation of six samples collected in 1993 and 1994 were as follows: pH of 9.23 ± 0.20, 7.3 ± 5.1 mg NO3–N L–1, 0.14 ± 0.17 mg P L–1, 13 ± 4 mg K L–1, 69 ± 14 mg Mg L–1, 73 ± 10 mg Ca L–1, and 259 ± 61 mg SO42–S L–1. Soil and Plant Analyses Soil samples were collected for analyses prior to fertilizer treatments in 1991. Soil pH was 6.17 ± 0.27 for the loam and 7.10 ± 0.16 for the sand, as measured using 1:1 soilwater paste. Buffer pH (Shoemaker et al. 1961) was 7.00 ± 0.21 for the loam before liming, but not in the sand (no lime requirement). The cation exchange capacity (CEC) was assessed from buffer pH and cation concentrations as follows (CRAAQ 2003): CEC = 9(7.5 – buffer pH) + ∑exchangeable cations (1)

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where exchangeable cations are Ca, Mg, and K (cmol+ kg–1). The CEC computed according to Eq. 1 was 19.0 cmol+ kg–1 in the loam and 10.9 cmol+ kg–1 in the sand. Bulk density using the cylinder method was 1.4 g cm–3 for the loam and 1.5 g cm–3 for the sand. We extracted available P, K, Mg, and Ca using standard methods (Mehlich 1984), S with a mono-basic calcium phosphate solution, Cu, Zn, Mn, Fe, and Mo with 0.1 N HCl, B with hot water, and organic matter using the Walkley-Black method (Nelson and Sommers 1982). The P (Laverty 1963) and B (Black 1965) were determined colourimetrically, and metals by atomic absorption spectrophotometry. Particle-size distribution was determined according to Day (1965) and sand sieved in a flow of tap water. Eight soil cores 10 cm deep by 2 cm in diameter were removed from each experimental unit after each clipping harvest and before broadcasting the next fertilizer treatments for chemical analyses. A total of 1536 soil samples were collected from each site during this 3-yr study. Fresh clipping mass was harvested at eight periods in the spring, summer and autumn during the 3-yr trial. Clippings were oven-dried at 70ºC for 24 to 48 h, ground in a Wiley mill, and wet-digested (Isaac and Johnson 1976). Phosphorus was determined colourimetrically (TRAACS 800), and K by plasma emission spectroscopy (ICAP 9000 from Jarrel Ash). The sand had a low clay content (10 g clay kg–1) (Table 1). The AlM-III (Mehlich-III) content of 333 mg AlM-III kg–1 was low in the sand compared to the range of 535 to 2424 mg AlM-III kg–1 reported by Khiari et al. (2000). Soil P saturation expressed as the (P/Al)M-III ratio averaged 6.01% in the loam and 17.42% in the sand (Table 1). Collection of Clippings Shoot density was maintained by controlling irrigation, mowing height, and weekly weeding following common practices on lawns. Turfgrass was mowed 2 d after fertilizer addition to avoid collecting fertilizer pellets in mower’s basket. The mowing height for clipping harvest was 38 mm and that for routine maintenance at other dates was 50 mm. Kentucky bluegrass height was measured using a ruler prior to each clipping harvest. Mowing frequency for routine maintenance was two to three times per week from early May till the end of September and less frequently in April and October. At each clipping harvest date, after brushing each experimental unit to set leaf blades upright immediately prior to mowing, fresh clippings were collected from an area of 2.38 m by 0.56 m per experimental unit. To avoid losing clippings from a blowing wind, plastic sheets were held on both sides of mower’s basket and a large piece of plywood was held perpendicularly to the wind direction. At each harvest, a crew of five people worked to minimize the length of time from the first to the last experimental plot in collecting fresh clippings, weighing, and then started drying them. Underground Biomass of Kentucky Bluegrass Turf Samples of sod plugs measuring 2 cm in diameter by 15 cm in length were taken in the experimental plots and rolled in

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Table 1. Initial physical and chemical characteristics of the experimental sites Characteristics

Loam

Sand g kg–1 (mean of four values, one per block)

Organic carbon (Walkley-Black) Clay (< 2 µm particles) Silt (2–5 µm) Sand (0.05–0.1 mm) Sand (0.1–0.25 mm) Sand (0.25–0.5 mm) Sand (0.5–1 mm) Sand (1–2 mm)

17.4 180 460 80 130 80 50 20

Mehlich-III extractsz kg–1)

P (26–71 mg K (loam, 126–175 mg kg–1; sand, 100–130 mg kg–1) Mg (loam, 151–200 mg kg–1; sand, 100–130 mg kg–1) Ca (loam, 1001–2000 mg kg–1; sand, 800–1100 mg kg–1) S (9–15 mg kg–1) Fe (12–24 mg kg–1) Zn (3–5 mg kg–1) Cu (0.9–1.5 mg kg–1) Mn (15–29 mg kg–1) B (1–1.5 mg kg–1) Al (1100–1600 mg kg–1) Phosphorus saturation index 100 × P/Al zSuggested

10.4 10 10 20 140 470 270 80 mg kg–1 (mean of 192 values ± SD)

68 ± 12 63 ± 6 165 ± 17 2587 ± 443 21 ± 3 255 ± 42 0.35 ± 0.08 0.63 ± 0.22 43 ± 10 0.29 ± 0.23 1131 ± 67 6.0%

58 ± 10 39 ± 6 143 ± 1 1925 ± 186 12 ± 4 226 ± 14 0.30 ± 0.08 0.57 ± 0.18 28 ± 3 0.40 ± 0.08 333 ± 74 17.4%

range for the medium fertility level in parentheses (CRAAQ 2003).

plastic bags, then stored in a freezer at –20°C . A total of 2880 sod plugs per site (960 sampling units across 192 experimental plots for five periods × three sub-samples) were removed for assessing the underground turf biomass. Dates of sod plug removal were Sep. 08 and Oct. 14 in 1992; Jun. 18, Aug. 14, and Oct. 07 in 1993. No underground turf biomass was determined in 1994. Plugs were sprayed with water to defrost the soil, then subjected to a mild air pressure to clean the soil from the underground biomass of Kentucky bluegrass. Residual soil particles were cleaned from underground parts with a stream of water. The shoot was separated at the crown region from underground turfgrass parts, then dried at 70°C for 24 h.

and rate 0 for strip D, either on page 199 or page 161. The acceptable ratings for a high quality turf such as sports fields and golf fairways were 8.0 to 10.0 for shoot density, and 7.0 to 8.9 for foliage colour. Analyses of Variance The analyses of variance were performed using SAS version 6.10 for Windows (SAS Institute, Inc. 1990). Statistical analyses were conducted using the univariate repeated measures analysis. Polynomial contrasts across nutrient rates were used to detect response trends. We also presented the least significant difference test for comparison of the means. RESULTS AND DISCUSSION

Performance Criteria of Kentucky Bluegrass Turf Visual shoot density and foliage colour ratings of Kentucky bluegrass represent a visual perception of quality from an aesthetic viewpoint. Visual ratings were recorded weekly and summarized as the median value across weekly evaluations preceding and including each clipping date. Visual shoot density and colour ratings ranged from 0 to 10 and were expressed to the closest unit. Zero rating of shoot density was a bare soil or dead Kentucky bluegrass, and 10 was considered as 100% Kentucky bluegrass cover (i.e., no visible weed, no bare soil surface, and without leaf discolouration). The colour chart of the Royal Horticultural Society (London) was used to visually assess turfgrass foliage colour as follows: rate 10 for strip A, page 137; rate 9 for strip C, page 137; rate 8 for strip A, page 141; rate 7 for strip B, page 141; rate 6 for strip A, page 143; rate 5 for strip B, page 143; rate 4 for strip C, page 143; rate 3 for strip D, page 144; rate 2 for strip D, page 145; rate 1 for strip D, page 153;

Effect of Added N on Clipping Yield of Kentucky Bluegrass Turf Clipping yield was significantly increased by N addition (Table 2). The main effect of N on clipping yield was quadratic (Table 2). Clipping yield was larger at the high N addition in the sand than in the loam (Table 3) suggesting that the response of turfgrass to N addition was higher in the sand than that in the loam. The main effects of N addition on clipping yield were similar for the loam and sand (Table 2), but differed yearly among spring, summer and autumn (Table 3) probably due to differences in turfgrass growth rate in interaction with weather conditions (Christians 2004). The increase in clippping yield due to increased N addition levelled off at 200 kg N ha–1 yr–1 in the loam and 300 kg N ha–1 yr–1 in the sand (Table 3). This is in agreement with Johnson et al. (1987). While maximum clipping yield is not a desirable goal for Kentucky bluegrass man-


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Table 2. Statistical effects of N, P, and K addition on clipping yield of Kentucky bluegrass turf in 1992, 1993, 1994, and on underground turf biomass in 1992, and 1993 at both sites Dry clipping yield Source N N linear N quadratic P K Time × N Error A Error B

Loam df 3 1 1 2 3 21

Dry underground turf biomass Sand

Loam

F value 1020** 2962** 65** 2.19NS 0.80NS 89**

df 3 1 1 2 3 12

Error mean square 6.65994 7.16597 4.15584 3.54779

144 576

664** 1576** 415** 1.29NS 1.98NS 87**

144 1008

Sand F value

36** 107** 0.86NS 1.99NS 1.85NS 7.35**

21** 59** 0.32NS 0.61NS 0.80NS 4.23**

Error mean square 532542 939447 451348 566136

**Significant at the 0.01 level; NS, not significant.

Table 3. Temporal (spring, summer, and autumn) variations in clipping yield and underground turf biomass of Kentucky bluegrass turf in response to N addition in 1992, 1993, and 1994 at both sites (see Table 2 for polynomial contrasts) N rate (kg ha–1yr–1)

Period Summer 92

Autumn 92

Spring 93

0 100 200 300 SE

4.45a 5.85b 6.48c 6.86d 0.10

6.42a 7.70b 8.26c 8.36c 0.10

20.75a 29.61b 33.51d 31.70c 0.10

50 100 200 300 SE

5.72a 7.24b 9.27c 9.90d 0.10

4.53a 5.60b 7.02c 8.10d 0.10

12.36a 18.29b 27.07c 29.75d 0.10

Summer 93

Autumn 93

Dry clipping yield (g m–2) Loam 7.99a 7.32a 9.68c 9.20b 9.95d 10.08d 9.05b 9.84c 0.10 0.10 Sand 9.35a 12.27b 15.56c 17.25d 0.10

10.77a 12.77b 14.59c 14.63c 0.10

0 100 200 300 SE

1067.5a 989.6b 937.8b 1051.4ab 34.1

1073.5a 1126.6a 941.5b 906.8b 34.1

Dry underground turf biomass (g m–3) Loam 4062.3a 5012.2a 4569.2a 3955.3b 4627.6b 3811.3b 3788.8c 4168.8c 3535.2c 3471.5d 3701.6d 3404.5d 34.1 34.1 34.1

50 100 200 300 SE

1456.4a 1422.4a 1244.9b 1189.6b 38.3

1689.3a 1459.7b 1329.3c 1382.8c 38.3

4177.4a 4242.3a 3924.8b 3801.1c 38.3

Sand 4833.8a 4864.4a 4384.3b 4196.5c 38.3

4784.0a 4592.2b 3817.2c 3418.8d 38.3

Average 92–94

Spring 94

Summer 94

Autumn 94

8.00a 16.58b 22.27c 24.04d 0.10

10.16a 18.56b 21.69c 21.71c 0.10

7.43a 16.51b 18.65d 17.56c 0.10

9.06a 14.21b 16.36c 16.14c 0.26

7.67a 11.43b 17.56c 21.38d 0.10

10.50a 17.01b 23.54c 25.61d 0.10

8.37a 14.47b 19.43c 19.64d 0.10

8.66a 12.38b 16.76c 18.28d 0.27

– – – –

– – – –

– – – –

3156.9a 2902.1b 2674.4c 2507.2d 74.5

– – – –

– – – –

– – – –

3388.2a 3316.2a 2940.1b 2797.8b 98.9

SE = standard error. a–d Means followed by the same letter within the same soil texture and at a given date are not significantly different at the 0.05 level (least significant difference test).

aged as turf, other criteria such as underground turf biomass, shoot density, and foliage colour should be considered in judging the effect of N rates on growth and aesthetical appearance. Effect of Added N on Underground Biomass of Kentucky Bluegrass Turf Increased N addition significantly reduced the underground turf biomass in a linear fashion in both sand and

loam (Table 2). Percent underground turf biomass reduction due to each N increment was higher in the loam than in the sand (Table 3). Compared to control treatments, the decline in underground turf biomass in response to N was 15.3% for 200 kg N and 20.6% for 300 kg N ha–1 yr–1 in the loam and 13.2% for 200 kg N and 17.4% for 300 kg N ha–1 yr–1 in the sand. Hull and Smith (1974) found that low N treatments stimulated more photosynthate translocation to the roots compared with high N treatments for Kentucky bluegrass.

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Bocker and Opitz von Boberfeld (1974) also found that turfgrasses fertilized with 200 kg N ha–1 yr–1 were lower in root mass than those without N addition. Beard (1973) wrote that as N addition increased from zero, there was an increase in the growth rate of both turfgrass roots and shoots. However, at higher N levels, carbohydrates became limiting for protein synthesis, which reduced root growth while shoot growth continued to respond to increased N levels. Effect of Added P on Clipping Yield and Underground Biomass of Kentucky Bluegrass Turf Added P had no significant effect on clipping yield and underground turf biomass in both sites (Table 2). Initial soil P analyses of 68 mg P kg–1 in the loam and 58 mg P kg–1 in the sand were considered as medium to high levels, which could explain the absence of turfgrass response. Christians (2004) explained that turfgrass has fibrous multi-branched root systems that make them relatively efficient at obtaining P from the soil. Even in soils that tested low to very low in P, little or no clipping yield response has been observed by Walker et al. (1963), Waddington et al. (1978), Turner (1980), Razmjoo and Kaneko (1993). Effect of Added K on Clipping Yield and Underground Biomass of Kentucky Bluegrass Turf Despite the low initial soil K levels (63 mg K kg–1 in the loam and 39 mg K kg–1 in the sand supplemented with 42 mg K kg–1 as exchangeable K), clipping yield and underground turf biomass showed no significant responses to K addition in both sites (Table 2). Similarly, clipping yield response to K addition has also been found to be non-significant in soils testing medium to high in K (Walker et al. 1963), in soils testing low to medium in K (Waddington et al. 1978), and even in soils testing very low to low in K (Waddington et al. 1972; Hull and Smith 1974; Barrios and Jones 1980; Turner 1980). Effect of P Addition on Soil Test Added P linearly increased soil test P at a rate of 1 mg PM-III kg–1 per fertilizer increment of 6.3 kg P ha–1 (14.4 kg P2O5 ha–1) in the loam and 5.8 kg P ha–1 (13.3 kg P2O5 ha–1) in the sand (an average of 6 kg added P ha–1 or 14 kg P2O5 ha–1 per mg PM-III kg–1) (Table 4). Those rates were within the range of 3.6 to 7.2 computed from Zhang et al. (1995) for corn. With more P required to enrich the soil, the loam presented a higher P buffering capacity than the sand, since AlM-III content in soils is closely related to their P sorption capacity (Khiari et al. 2000). Effect of K Addition on Soil Test Added K linearly increased soil test K at a rate of 7.1 kg K ha–1 (8.5 kg K2O ha–1) per mg KM-III kg–1 in the sand and 3.1 kg K ha–1 (3.7 kg K2O ha–1) per mg KM-III kg–1 in the loam (Table 4). Comparatively, soil test K increased at a rate of 8 kg K ha–1 (9.6 kg K2O ha–1) per mg KM-III kg–1 in excessively K-fertilized soils (Zhang et al. 1995). Although K leaching was not monitored in this study, results of K soil

Table 4. Effects of added P and K (x in kg ha–1 yr–1) on Mehlich-III soil test P and K (y in mg kg–1) contents of loams and sands seeded in Kentucky bluegrass turf averaged across eight clipping periods Soil texture

Equation

R2

1/(dy/dx)

P

Loam Sand

y = 0.1592x + 74.3 y = 0.1732x + 61.0

0.993 0.998

6.3 5.8

K

Loam Sand

y = 0.3272x + 119.9 y = 0.1403x + 49.8

0.992 0.997

3.1 7.1

Treatment

test indicated that considerably higher amounts of added K must have been leached out of the tested soil layer of the USGA sand compared to the loam due to higher clay and organic matter contents, hence higher cation exchange capacity, in the loam. Effect of Added N on Shoot Density of Kentucky Bluegrass Turf Nitrogen had by far the largest effect on turfgrass visual rating of shoot density (Table 5). The effect of P was significant only for shoot density in the sand and for foliage colour in the loam, and was thus inconsistent (Table 5). Visual shoot density was improved in a non-linear fashion by N at both sites (Table 5), although the amplitude of the response of shoot density and foliage colour varied among the eight clipping periods (Table 6). Johnson et al. (1987) found that N addition increased turfgrass shoot density with the greatest response from 100 to 200 kg N ha–1 yr–1. The N rates to achieve given visual shoot density ratings can be interpolated across sites using the equations of Fig. 1 that link average values of visual shoot density ratings (y, unitless) to N rates (kg N ha–1 yr–1) shown in Table 6. In order to meet the proposed visual shoot density ratings from 8 to 10 (on a visual scale of 0 to 10), Kentucky bluegrass turf required rates of 200 kg N ha–1 yr–1 in the loam and 235 kg N ha–1 yr–1 in the sand (Table 7). Nitrogen rates from 105 to 199 kg N ha–1 yr–1 in the loam and from 150 to 234 kg N ha–1 yr–1 in the sand were required to meet visual shoot density ratings from 7.0 to 7.9 (Table 7). Nitrogen rates from 45 to 104 kg N ha–1 yr–1 in the loam and from 100 to 149 kg N ha–1 yr–1 in the sand were required to meet visual shoot density ratings from 6.0 to 6.9 (Table 7). Visual shoot density ratings that are below 6.0 are not sought because it would mean that the lawn contains 60% turfgrass and 40% weeds, bare spots, dead turfgrass. Control treatments for N reached visual shoot density ratings of 5.1 in the loam and 4.8 in the sand (Table 6), which were insufficient. Effect of Added N on Foliage Colour of Kentucky Bluegrass Turf Nitrogen had a highly significant effect on visual rating of turfgrass foliage colour (Table 5). Many studies showed increased turfgrass foliage colour with increased N rates (Waddington et al. 1972; Hummel and Waddington 1981; Razmjoo and Kaneko 1993). Rates of N to achieve given foliage colour ratings can be interpolated using the equations of Fig. 1, which link average values of foliage colour ratings


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Table 5. Statistical effects of N, P, and K additions on shoot density and foliage colour of Kentucky bluegrass turf stands in 1992, 1993, and 1994 Shoot density Source N N linear N quadratic N cubic P P linear P quadratic N×P K N×K Time × N Error A Error B

Loam df 3 1 1 1 2 1 1 6 3 9 21

Foliage colour Sand

Loam

Sand

F value 2310** 6255** 666** 8.09** 1.71NS 3.37NS 0.06NS 0.13NS 0.08NS 1.32NS 24**

144 1008

3353** 9729** 123** 208** 20** 33** 5.78* 2.16* 0.27NS 0.80NS 13**

3327** 9229** 751** 0.30NS 6.07** 8.51** 3.63* 1.14NS 1.09NS 2.57** 33**

Error mean square 0.291829 0.181090

0.354980 0.199351

3245** 9555** 113** 66** 5.22** 8.99** 1.45NS 0.46NS 1.09NS 1.17NS 12**

0.237684 0.159932

0.299371 0.226578

*, **Significant at the 0.05 and 0.01 levels, respectively; NS, not significant.

Table 6. Temporal (spring, summer, and autumn) variations in ratings of shoot density and foliage colour of Kentucky bluegrass turf in response to N addition in 1992, 1993, and 1994 (see Table 5 for polynomial contrasts) N rate (kg ha–1yr–1)

Period Summer 92

Autumn 92

Spring 93

Autumn 93

Spring 94

Summer 94

Autumn 94

Average 92–94

Shoot density rating Loam 4.85a 5.01a 6.70b 7.33b 7.67c 7.74c 7.74d 7.76c 0.02276 0.02276

5.19a 6.40b 8.38c 8.58d 0.02276

5.25a 7.33b 8.92c 8.98d 0.02276

4.98a 6.96b 8.10c 8.06c 0.02276

5.08a 6.86b 8.05c 8.27d 0.06081

Summer 93

0 100 200 300 SE

5.09a 6.98b 7.57c 8.44d 0.02276

5.25a 7.15b 8.04c 8.35d 0.02276

5.00a 6.04b 8.00c 8.21d 0.02276

50 100 200 300 SE

5.19a 6.02b 8.09c 8.55d 0.02167

5.17a 6.40b 8.08c 8.83d 0.02167

4.23a 5.52b 7.27c 7.67d 0.02167

5.02a 6.19b 7.79c 8.31d 0.02167

4.44a 5.83b 6.79c 7.35d 0.02167

4.98a 5.73b 7.89c 8.45d 0.02167

4.42a 6.04b 7.98c 8.56d 0.02167

4.79a 5.95b 7.70c 8.26d 0.05514

4.98a 6.00b 8.04c 8.38d 0.02035

4.23a 6.23b 7.65c 8.13d 0.02035

4.00a 6.44b 7.23c 7.58d 0.02035

4.42a 6.16b 7.24c 7.62d 0.04976

Sand 3.50a 5.07b 6.32c 7.27d 0.02421

3.00a 3.67b 5.21c 6.10d 0.02421

3.42 a 5.02b 6.94c 7.68d 0.02421

4.00a 5.65b 7.04c 7.44d 0.02421

3.69a 5.00b 6.53c 7.25d 0.05584

Sand 4.88a 5.91b 7.74c 8.34d 0.02167

0 100 200 300 SE

4.61a 5.95b 6.30c 7.26d 0.02035

4.31a 6.21b 7.15c 7.40d 0.02035

Foliage colour rating Loam 5.04a 4.04a 4.13a 6.27b 5.69b 6.47b 7.54c 6.97c 7.07c 7.75d 7.13d 7.33d 0.02035 0.02035 0.02035

50 100 200 300 SE

4.02a 5.10b 7.10c 7.51d 0.02421

4.38a 5.54b 7.23c 7.92d 0.02421

3.15a 4.69b 5.52c 6.60d 0.02421

4.04a 5.27b 6.85c 7.46d 0.02421

SE = standard error. a–d Means followed by the same letter within the same soil texture are not significantly different at the 0.05 level following the least significant difference test.

(y, unitless) to N rate (kg N ha–1 yr–1) shown in Table 6. Foliage colour ratings above 8.9 do not need to be attained because this would require too much N fertilization, which stimulates shoot growth at the expense of root mass. In order to meet the proposed visual foliage colour ratings from 7.0 to 8.9 (on a visual scale of 0 to 10), Kentucky bluegrass turf

required rates of 170 kg N ha–1 yr–1 in the loam and 245 kg N ha–1 yr–1 in the sand (Table 7). Nitrogen rates of 90 to 169 kg N ha–1 yr–1 in the loam and 160 to 244 kg N ha–1 yr–1 in the sand were required to meet the visual foliage colour ratings of 6.0 to 6.9 (Table 7). N rates of 30 to 89 kg N ha–1 yr–1 in the loam and 105 to 159 kg N ha–1 yr–1 in the sand were

200


Fig. 1. Relationship between N requirements and average turfgrass ratings of shoot density and foliage colour across eight periods (■ ■ sand site and ◆loam site).

required to meet the visual foliage colour ratings of 5.0 to 5.9 (Table 7). Control treatments for N reached a visual foliage colour ratings of 4.4 in the loam and 3.7 in the sand (Table 6), which were visually unacceptable as turfgrass. Using N Rates for Kentucky Bluegrass Turf The practical interpretations of the above-mentioned N rates for turfgrass shoot density and foliage colour are proposed for use as follows: (a) A golf course superintendent or a manager of sports fields trying to achieve a shoot density rating ≥ 8.0 and a foliage colour rating ≥ 7.0 for Kentucky bluegrass in a USGA sand for a golf tee, soccer, baseball, or football fields can use N rates of 235 kg N ha–1 yr–1 for shoot density and 245 kg N ha–1 yr–1 for foliage colour (Table 7) to apply N rates ranging from 235 to 245 kg N ha–1 yr–1 assuming the mowed clippings are collected as per the present research. Since the clipping yield per unit area per year and its N concentration can vary from one golf course or sports field to another and from one year to another, it was not

possible from this study to provide an estimate of the amount of kg N ha–1 yr–1 to be deducted from the abovementioned N rates in case the mowed clippings were not collected. Furthermore, part of the clipping N can be absorbed by turfgrass roots and another part may be leached. (b) A sod farm manager trying to achieve a shoot density rating ≥ 8.0 (on a scale of 0 to 10) and a foliage colour rating ≥ 7.0 (on a scale of 0 to 10) for Kentucky bluegrass in loam can use N rates of 200 kg N ha–1 yr–1 for shoot density and 170 kg N ha–1 yr–1 for foliage colour (Table 7) to apply N rates ranging from 170 to 200 kg N ha–1 yr–1 assuming the mowed clippings are collected. Nitrogen rates in this experiment were applied six times per growing season. Therefore, applying a total of 200 kg N from urea ha–1 yr–1 in three applications instead of six on a sod farm is not recommended because it may not yield the expected shoot density and foliage colour ratings, and may even result in adverse effects on sod rooting that delay the harvest date.

BADRA ET AL. — RESPONSES OF KENTUCKY BLUEGRASS TURF TO N, P, AND K ADDITIONS Table 7. The N requirements (computed from equations of Fig. 1) of Kentucky bluegrass turf to meet specified ratings of shoot density or foliage colour as a season average Target ratingz

Loam

Sand (kg N ha–1 yr–1)

Shoot density ≥ 8.0 unitless 7.0–7.9 6.0–6.9

≥ 200 105–199 45–104

≥ 235 150–234 100–149

Foliage colour ≥ 7.0 unitless 6.0–6.9 5.0–5.9

≥ 170 90–169 30–89

≥ 245 160–244 105–159

zRatings

are on a scale from 0 to 10.

(c) A golf course superintendent trying to achieve a shoot density rating ≥ 8.0 and a foliage colour rating ≥ 7.0 for Kentucky bluegrass in loam for golf fairways can use N rates of 200 kg N ha–1 yr–1 for shoot density and 170 kg N ha–1 yr–1 for foliage colour (Table 7) to apply N rate ranging from 170 to 200 kg N ha–1 yr–1 assuming the mowed clippings are collected. (d) A golf course superintendent, a home owner, or a manager of medium maintenance municipal lawns trying to achieve a shoot density rating ≥ 8.0 and a foliage colour rating of 6.0 to 6.9 for Kentucky bluegrass in loam for golf rough, residential lawn, or municipal landscape can use N rates of 200 kg N ha–1 yr–1 for shoot density and 130 kg N ha–1 yr–1 for foliage colour (Table 7) to apply N rates ranging from 130 to 200 kg N ha–1 yr–1 assuming the mowed clippings are collected. (e) A manager of municipal parks, school landscape, or cemetery turfgrass trying to achieve a shoot density rating of 7.0 to 7.9 and a foliage colour rating of 6.0 to 6.9 for Kentucky bluegrass in loam can use N rates of 152 kg N ha–1 yr–1 for shoot density and 130 kg N ha–1 yr–1 for foliage colour (Table 7) to apply N rates ranging from 130 to 152 kg N ha–1 yr–1 assuming the mowed clippings are collected. (f) Finally, a low maintenance turfgrass of low shoot density rating of 6.0 to 6.9 and a low foliage colour rating of 5.0 to 5.9 for Kentucky bluegrass in loam can be fertilized with N rates of 74 kg N ha–1 yr–1 found for shoot density and 60 kg N ha–1 yr–1 for foliage colour (Table 7) to apply N rates ranging from 60 to 74 kg N ha–1 yr–1 assuming the mowed clippings are collected. The above-mentioned N rates were based on shoot density and foliage colour of Kentucky bluegrass because these qualitative ratings were correlated with the quantitative responses, in particular the dry clipping yield. In addition, the qualitative ratings on a scale of 0 to 10 can easily be used by the turfgrass manager compared to the quantitative responses that need a precise and continuous collections of clipping yield and underground turf biomass for weighing and drying.

201

Urea was the nitrogenous material in the present research applied six times per growing year. Should the turfgrass manager use a nitrogenous material other than urea or in combination with urea, its date and rate per application will depend on the release characteristics in time of the material to be applied. Effect of Added P on Shoot Density and Foliage Colour of Kentucky Bluegrass Turf There was a significant linear P effect on foliage colour in the loam and the sand. Phosphorus demonstrated a quadratic effect on visual shoot density in the sand and on foliage colour in the loam. (Table 5). There was also a significant N × P interaction on shoot density in the sand (F = 2.16, P < 0.05) (Table 5). Dest and Allinson (1981) found a significant N × P interaction on the shoot density of a golf fairway made of annual bluegrass. However, P addition had no influence on shoot density and foliage colour of a golf fairway made of annual bluegrass and creeping bentgrass, nor was N × P interaction significant (Dest and Guillard 1987). In a greenhouse study in silica sand, Monroe et al. (1969) found a N × P interaction on clipping yield, underground parts, tops, and vigour of Kentucky bluegrass. Razmjoo and Kaneko (1993) found that P addition did not have a significant effect on turfgrass shoot density or foliage colour under field conditions. In our study, the effects of P treatments were relatively small, yet significant, on foliage colour across sites and on visual shoot density in the sand (Table 5) indicating a medium to high fertility class. The response of visual shoot density to P addition in the sand depended on N rate (Fig. 2). The P requirement increased with N rate, although such an effect was small (Fig. 2). On average, added P showed no effect on visual shoot density ratings at 50 kg N ha–1 yr–1, and linear and quadratic effects (P < 0.05) on visual shoot density ratings at higher N rates in the sand (Fig. 2). At P rates of 21.8, 43.7 and 87.3 kg P ha–1 yr–1, visual shoot density ratings increased from 5.9, 6.0, and 7.8, respectively (Fig. 1). With the 300 kg N ha–1 yr–1 treatment, the highest visual shoot density was obtained with 87.3 kg P ha–1 yr–1, with ratings of 8.1 to 8.4 (Fig. 2). This indicated little benefit from P addition other than maintaining soil test P. The relationship between P rates and visual shoot density ratings was environmentally important, especially in the sandy soil material [17.4% as (P/Al)M-III saturation index in the sand versus 13.4% as a proposed critical environmental value for runoff (Khiari et al. 2000) and of 8% for leaching (Beauchemin et al. 1998). Although target soil test for maintenance cannot be determined from this study, soil testing of approximately 60 mg PM-III kg–1 in both loam and sand appeared sufficient for Kentucky bluegrass turf. Effect of Added K on Shoot Density and Foliage Colour of Kentucky Bluegrass Turf There was a significant N × K interaction (F = 2.57, P < 0.01) on foliage colour in the loam (Table 5). Potassium showed no significant main effect on shoot density or foliage colour. Despite low soil K level, visual shoot density and foliage colour did not respond to K in the loam

202


Fig. 2. Influence of N (kg N ha–1 yr–1) by P and N by K interactions on average turfgrass ratings of shoot density and foliage colour of Kentucky bluegrass turf in the sand across eight periods.

(Table 5). Only in the sand did foliage colour respond to K addition, depending on N rates (Fig. 2). At N rates of 50 to 200 kg N ha–1 yr–1, there was no significant response of foliage colour to K addition. At 300 kg N ha–1 yr–1, K addition slightly decreased the foliage colour rating from 7.8 in control (41.7 kg K ha–1 yr–1) to ratings between 7.5 and 7.7 in other K treatments. Similar to P addition, K addition showed little visual benefit other than maintaining soil test K. Turner (1980) also found that visual quality of Kentucky bluegrass was not improved by K addition even in soils that tested low to very low in K. Holben (1952) stated that turfgrasses may not always show a visual growth to K addition. CONCLUSIONS Nitrogen addition had by far the largest effect on clipping yield, underground turf biomass, shoot density, and foliage colour of Kentucky bluegrass compared to P and/or K addition under field conditions and continuous clipping removal. Less N fertilization was required to attain the

highest turfgrass shoot density and foliage colour ratings reached at 185 kg N ha–1 yr–1 than the highest clipping yield at 300 kg N ha–1 yr–1. Since N was the most important nutrient in turfgrass growth and quality, the management levels (intensive, medium, or low maintenance) of Kentucky bluegrass turf should be considered in deciding the yearly N need and its rate per application as well. Depending on soil texture (loam or sand), added N should be adjusted according to Kentucky bluegrass growth, weather and soil conditions to minimize the decline in underground turf biomass and improve nutrient uptake capacity for increased shoot density. Over-fertilization of N for any given period of time on sod farms to speed up sod harvest date is likely to be at the expense of underground turf biomass. Any turfgrass management system accepting excessive root mass decline due to high N rates will likely require additional inputs of seed, irrigation water, fertilizers, and pesticides (to repress weeds, destroy pest insects, and control diseases).


Control treatments of P and/or K were found to be acceptable for the four parameters under study. Addition of P and/or K resulted in similar responses to the addition of N alone for clipping yield, underground turf biomass, shoot density, and foliage colour, meaning that Kentucky bluegrass demonstrated its ability to provide acceptable quantitative and qualitative responses under initial soil P and K fertility levels that appeared sufficient in the loam and in the sand over this 3-yr study. Hence, fertilization of Kentucky bluegrass would require maintenance rates of K and even a reduction in P rates for an already established turfgrass. The reconstituted USGA sandy soil, low in Mehlich-III extractable Al, appeared at high risk for P contamination [P saturation of 17.4% as (P/Al)M-III]. Mehlich-III P decreased with P removal, depending on soil P buffering capacity. Soil K decreased with both K removal and possibly leaching. Soil test P and K were more likely to fluctuate in the USGA sand than in the more buffered loam. ACKNOWLEDGEMENTS The statistical advice of Hélène Crépeau (Department of Mathematics and Statistics, Laval University) is gratefully acknowledged. We thank Gilles Routhier and the personnel of the L’Acadie Research Station for their kind assistance. The financial support from Nutrite, Pelouses Richer Boulet Inc., Groupe Richer, Comptoir Richelieu, Bigelow Sand, and Institut québécois du développement de l’horticulture ornementale is highly appreciated. Barrios, E. P. and Jones, L. G. 1980. Some influence of potassium nutrition on growth and quality of Tifgreen bermudagrass. J. Am. Soc. Hortic. Sci. 105: 151–153 . Beard, J. B. 1973. Turfgrass: Science and culture. Prentice-Hall Inc., Englewood Cliffs, NJ. 658 pp. Beauchemin, S., Simard, R. R. and Cluis, D. 1998. Forms and concentration of phosphorus in drainage water of twenty-seven tile-drained soils. J. Environ. Qual. 27: 721–728. Bengeyfield, W. H. (ed.). 1989. Specifications for a method of putting green construction. United States Golf Association, Green Section Staff, Golf House, Far Hills. NJ. 24 pp. Black, C. A. 1965. Methods of soil analysis. Pages 949–951, Part II. Agronomy no. 9. ASA, Madison, WI. Below, F. E. 1995. Nitrogen metabolism and crop productivity. Pages 275–301 in M. Pessarakli, ed. Handbook of plant and crop physiology. Marcel Dekker Inc., New York, NY. Bocker, P. and Opitz von Boberfeld, W. 1974. Influence of various fertilizers on root development in a turfgrass mixture. Pages 99–103 in E. C. Roberts, ed. Proc. 2nd Int. Turfgrass Res. Conf., Blacksburg, VA . ASA-CSSA, Madison, WI. Carrow, R. N., Johnson, B. J. and Burns, R. E. 1987. Thatch and quality of Tifway bermudagrass turf in relation to fertility and cultivation. Agron. J. 79: 524–530. Christians, N. E. 2004. Fundamentals of turfgrass management. 2nd ed. John Wiley and Sons Inc., Hoboken, NJ. 359 pp. Christians, N. E., Martin, D. P. and Karnok, K. J. 1981. The interactions among nitrogen, phosphorus, and potassium on the establishment, quality, and growth of Kentucky bluegrass (Poa pratensis L. “Merion”). Pages 341–348 in R. W. Sheard, ed. Proc. 4th Int. Turfgrass Res. Conf., Guelph, ON. Christians, N. E., Martin, D. P. and Wilkinson, J. F. 1979. Nitrogen, phosphorus, and potassium effects on quality and growth of Kentucky bluegrass and creeping bentgrass. Agron. J. 71: 564–567.

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