Fast drainage is a basic requirement for sport fields. Good drainage allows for the removal of large amounts of water in a short period of time and avoids cancellation or postponement of games due to accumulation of excessive water. For the sake of drainage, many sports fields are built with sand-based root zones. However, one limitation of a sand-based root zone is that the surface tends to be soft and unstable, especially when the grass cover is not fully established or deteriorated. Sports turf fields require a stable surface to support play and to prevent injuries. To stabilize sports turf root zones, many reinforcement materials, whole layer or pieces, have been evaluated (Baker, 1997). Advantages of using reinforcements generally include retaining more grass cover during play, reducing divots and soil compaction, and improving drainage and surface stability (Baker, 1997). However, rubber crumb, a popular reinforcement material, decreased capillary porosity, hardness, and shear strength when incorporated in sand-soil rootzones (Baker et al., 2001). Disadvantages in using reinforcement materials include poor turfgrass establishment, and extreme traction and surface hardness values (Adams, 1997; Baker 1997; McNitt and Landschoot, 2005). Furthermore, inclusion of reinforcement is not always cost effective (McNitt and Landschoot, 2001).
Research on the roles of the inherent properties of sand in playing surface stability has been limited to particle-size distribution (Ferguson, 1955; Bingaman and Kohnke, 1970; Adams et al., 1971; Baker et al., 1988; Whitmyer and Blake, 1989) and compressibility (Kezdi, 1979; Waddington, 1992). Dury and Craggs (1977) reported that the playing surface stability of a sports field is affected by the size, shape, and particle-size distribution of the medium based on ball bounce ratio. Sand shape and roundness have long been speculated to be important for surface stability of sand-based sport fields (Adams et al., 1971). However, their roles relative to other factors are not clearly understood due to a lack of quantitative evaluation of sand shape and roundness. Li et al. (2004) compared sand particles of different shape and roundness and found that angle at repose (AR), coefficient of uniformity (CU), and roughness index, are quantitative factors explaining 98.5% of the variance contributing to surface stability of sand-based media. Other research also noted that surface stability is not only affected by the sand particle size but also by grass roots, stolons/rhizomes, and moisture conditions (Adams, 2004). Information on the contribution of root mass and cellulose to the tensile strength of soils comes mainly from civil engineer research (Coutts, 1983; Operstein and Frydman, 2000; Dupuy et al., 2005). Minimal information is available on the relative importance of sand shape and roundness, and the interactions between sand and plant factors in surface stability and playability.
The objective of this study was to evaluate the contributions of sand particle size, particle-size distribution, angularity, plant roots, and root zone moisture conditions to surface playability and stability. The goal was to understand the effectiveness of using angular sand to increase root zone stability, which was compared with two alternatives, either adding native soil to the sand medium, or using sand sources with wider particle-size distribution ranges. A spherical sand and a locally available mason sand source with relatively high sphericity were included for comparison.
MATERIALS AND METHODS
This field study was initiated in September 2000 and lasted for 5 yr. Native soil was excavated from an existing turf area to form plots each measuring 1.5 m wide, 3 m long, and 0.15 m deep. Five sand-based root zone materials, that is, Hallet mason sand (Hallett Materials, Ames, IA), Hallet concrete sand (Hallett Materials, Ames, IA), Sidley Pro/Angle sand (R.W. Sidley, Inc. Painesville, OH), Bunker White sand (Specialty Minerals Group, Inc., Lucerne Valley, CA), and Hallet mason sand + 15% native soil (v/v) were back-filled to the plots and then compacted with a plate compactor for 5 min (Model: VP 1550V, Improved Construction Methods, Jacksonville, AR). The soil part of the mixture was Nicollet clay loam (fine-loamy, mixed, supractive, mesic Aquic Hapludoll). The plots were allowed to consolidate naturally for 8 mo. On 25 June 2001, half of each plot was planted with washed 2-yr-old sod and on 25 Aug. 2001, the other half was seeded at a rate of 150 kg ha−1 Grass used was ‘Unique’ Kentucky bluegrass (Poa pratensis L.). The turf received a total of 159 kg ha−1 N, 35 kg ha−1 P, and 96 kg ha−1 K 3 mo after sodding. When seeded, these areas received 50 kg ha−1 N using a 13–13–13 fertilizer contained all soluble N, 50% from urea and 50% from ammonium nitrate. All turf was fertilized with 200 kg ha−1 N from 12–0–22 in four applications in 2002 to 2005. Irrigation was applied uniformly to all plots as needed to prevent water stress. The turf was mowed once a week at a height of 5 cm. Plots were topdressed in November each year with the same material as in the root zones to a thickness of 0.5 cm.
The AR of the five root zone materials was measured from three subsamples before the study after drying at 105°C in an oven for 24 h following the methods of Li et al. (2004) Particle-size distribution of the materials was analyzed using wet sieving for the sand fraction and pipette method for the silt and clay fractions (Gee and Bauder, 1986). From the cumulative particle-size distribution curve, CU (D60/D10) and the gradation index (D90/D10) were derived, where D value is particle size at which the percentage of the subscription value was smaller. The sand particle density was measured with a pycnometer (Blake and Hartge, 1986) instead of using the standard particle density of 2.65 g cm−3 for soil minerals. This was done because most of the sand materials used in this study had values that exceeded 2.65 g cm−3 The volume of the particles was calculated from the mass and density of water displaced by the sample. Three subsamples were used for each material. Particle shape/roundness was assessed following the method by Li et al. (2004) and expressed as roughness index, a ratio of measured surface area of the sand particle to the surface area of a true sphere of equivalent volume. Three subsamples each weighing 2 g were used for this measurement. Descriptions of sand shape and roundness based on a chart specified by the United States Golf Association (USGA Green Section Staff, 1993) were also provided (Tables 1 and 2 ).
|Weight of sand retained on sieves with nominal openings 0.053–4 mm|
|Hallet mason sand||0||0.1||0.3||1.1||7.3||32.8||45.9||10.6||1.9||0|
|Hallet concrete sand||0||0.1||0.2||0.8||6.6||22.3||35.8||16.8||11.9||5.6|
|Hallet mason sand/15% soil v/v||1||5.5||1.4||4.1||32.2||46.0||8.5||1.1||0.3||0|
|g cm−3||degree||g cm−3|
|Hallet mason sand||mined sand||subround, medium sphericity||2.72||36.4||2.56||3.89||1.61||1.56||1.61|
|Hallet concrete sand||mined sand||subround, medium sphericity||2.72||38.9||3.14||43.75||1.58||1.73||1.82|
|Sidley Pro/Angle||crushed stone||angular, low sphericity||2.70||37.0||3.33||17.14||2.38||1.55||1.57|
|Bunker white||crushed stone||round, medium sphericity||2.77||34.0||1.54||2.56||1.53||1.74||1.75|
|Hallet mason sand/15% soil v/v||–||–||2.67||36.4||2.18||6.43||–||1.55||1.62|
Surface properties were evaluated in three phases over a 5-yr period. Phase one (September 2000–June 2001) measurements were conducted on root zone media with no grass cover. This phase is called the consolidation stage throughout this manuscript. In soil mechanics, soil consolidation is a process by which soils decrease in volume. Several processes may be involved in the consolidation. In this study, it primarily involved elastic deformation of dry, moist, or saturated soil (Das, 1998). Phase two focused on the turf establishment. The sodded areas were evaluated from July to October 2001, and the seeded plots were evaluated from August 2001 to June 2002. Phase three measurements were conducted on established turf, from 2002 and 2005 for the sodded area and from July to October in 2002 and again in 2005 for the seeded area. Water content was measured with time domain reflectometry (TDR100, Campbell Scientific, Logan, UT) when surface stability was evaluated. Soil cone penetration resistance was evaluated using an American Corps Engineering cone penetrometer (Standard angle 30 degree, surface area 6.5 cm2). Surface hardness was measured with a B&K 2500 vibration equipment (Brüel & Kjær North America Inc. Norcross, GA) and a Clegg Soil Tester (Lafayette Instrument, Lafayette, ID) using a 2.25 kg impact hammer with one drop from 0.3-m height. The readings from B&K 2500 vibration equipment were equivalent to 3.5 times the Clegg impact value (CIV) in this study. Therefore, data from the B&K 2500 vibration equipment expressed in acceleration due to gravity (g) was converted to m s−2 by multiplying by 9.81. Surface traction was determined by the maximum torque required to rotate a traction test device on the turf surface, using a device of the type developed by Canaway (1983) The traction device had a 45 kg weight load on a round disk with six football cleats (1.25 cm) installed at the bottom 60 degree apart. All measurements had three subsamples within each plot. Within each phase, the effects of soil water content on the surface stability were assessed by repeated measurements 24 h following water saturation.
Soil dry bulk density (2.5–7.5 cm depth) was measured at the end of each phase by taking undisturbed soil core samples and drying them at 105°C for 24 h. Three samples were taken in each plot by pushing a brass ring (137.5 cm3) with a soil sampler (A-145, ELE International Inc., Lake Bluff, IL) into the media. Grass shoots, thatch, and roots were sampled in the phase two and three using 5-cm diam. tubes at the 0 to 15 cm depth. Shoot dry weight was recorded after drying at 68°C for 48 h (Mills and Jones, 1996). Grass roots were washed from the media using tap water over a 2-mm sieve and dry weight recorded after drying at 68°C for 48 h (Mills and Jones, 1996). The organic content of thatch or mat layer was determined by the loss of ignition method at 490°C for 8 h (Rowell, 1994). At the end of the study, turf quality was evaluated based on a 9 to 1 scale, with 9 as the best, 1 as dead grass, and 6 as the minimum acceptable level.
The experiment design was a split plot with the main plots in a randomized block arrangement with three replications. The root zone material was the main plot and the turf establishment method was the subplot. Year was treated as repeated measurement not a treatment factor. Data were subjected to ANOVA using SAS GLM procedure (SAS Institute, 2008). Root zone material and turf establishing method were treated as fixed variables and block was treated as a random variable. Treatment means were separated with Fisher protected least significant difference (LSD). Regression analysis was conducted using the SAS REG procedure to fit data to stepwise models (SAS Institute, 2008).
RESULTS AND DISCUSSION
During the consolidation stage of the root zone, the most noticeable change in the measured parameters was an increase in cone penetration resistance (Fig. 1 ). Cone resistance varied among sand materials on all measuring dates at this stage. At the end of consolidation, the materials separated into four groups (Fig. 1). Sidley Pro/Angle had the highest cone resistance of 1245 kPa, followed by Hallet mason sand and Hallet mason sand/15% soil at 1087 kPa. Hallet concrete sand, which had the highest coefficient of gradation index, had cone resistance of 967 kPa (Table 2). The lowest cone resistance was found in the Bunker White sand at 763 kPa (Fig. 1). Bunker White sand had also the lowest AR, CU, gradation index, and roughness index (Table 2). Data taken within a 24 h period after water saturation, demonstrated that the cone resistance peaked at a soil water content that varied with soil types. The connection between soil moisture conditions and cone resistance is well established from soil engineering research (Das, 1998). Combining the data of different sand materials showed no significant correlation between soil moisture and cone resistance, indicating that the cone resistance-moisture relationship is material specific. Combined data from the whole consolidation stage showed no significant correlations between soil moisture and cone resistance, which may be explained by the changes in bulk density of sand materials during the consolidation (Table 2).
During the turf establishment stage, cone resistances in Sidley Pro/Angle sand were higher than that during the consolidation stage, 1350 kPa (Fig. 2a ) and 1500 kPa (Fig. 2b) for seeded and sodded areas, respectively. The relative levels of cone resistance of the five materials maintained the same trend as shown at the end of the consolidation stage for sodded areas (Fig. 2b). In the seeded areas, Hallet mason sand/15% soil had cone resistance close to Bunker White sand, which is different from the ranking at the end of the consolidation stage (Fig. 1 and 2a). Decreases in a cone resistance for Hallet mason sand/15% soil were also noticed in the sodded areas but did not change the ranking compared with the end of consolidation stage. The decrease of cone resistance in Hallet mason sand/15% soil may have been caused by freezing and thawing cycles of the clay and by the higher water content of the media (Table 3 ). The impact of soil moisture on cone penetration during the turf establishment stage was similar to the consolidation stage (Table 4 ).
|Water content†||Bulk density||Cone resistance||MD‡||Traction||Turf quality§|
|Root zone materials||Seed||Sod||Seed||Sod||Seed||Sod||Seed||Sod||Seed||Sod||Seed||Sod|
|% (v/v)||g cm−3||k Pa||ms−2||Nm|
|Hallet mason sand||12.32||12.05||1.58||1.60||758.4||804.4||250.3||234.6||45.7||47.0||7.0||6.7|
|Hallet concrete sand||11.64||9.48||1.75||1.80||735.4||804.4||289.1||235.9||41.0||49.0||7.3||7.0|
|Hallet mason sand/15% soil (v/v)||17.30||18.44||1.54||1.58||758.4||873.3||335.7||279.9||53.7||61.0||7.7||7.0|
|df||Water content||Bulk density||Maximum deceleration†||Cone resistance‡|
|Root zone materials||4||130.08**||0.03**||7,301.10**||99,238.48**|
As the turf matured, both the seeded and sodded areas demonstrated the same ranking of cone resistances, that is, Sidley Pro/Angle > Hallet mason sand > Hallet concrete sand > Hallet mason sand/15% soil > Bunker White sand (Fig. 3a and 3b ). The cone penetration resistance started decreasing slightly toward the end of those measuring cycles. By July 2005, the cone resistances decreased for all materials except Bunker White sand (Table 3). The effects of soil water content, root biomass, and organic matter in the mat layer were analyzed separately by root zone materials because of the differences among them (Table 5 ). Multiple linear regression equations showed that cone resistance was negatively correlated to root biomass whereas water content and thatch did not show determined direction when correlated (Table 6 ). As root system developed, cone resistance decreased. This may explain the decrease of cone resistance in the fourth year when soil bulk density did not change significantly from the end of consolidation stage (Table 2 and 3).
|df||Water content||Cone resistance†||MD‡||Traction||Biomass§||OM¶||Quality#||BK††|
|Root zone materials (M)||4||0.01**||22639.82**||11297.63**||367.20**||0.27**||0.14||3.87**||0.06**|
|B × M||8||0.01**||1137||1455.55**||45.45**||0.03*||0.14||0.34||0.00|
|M × E||4||0.00||19074.49**||444.03||14.33||0.01||0.09||0.63||0.00|
|Equation†||R 2||Equation||R 2|
|Cone penetration resistance|
|Hallet mason sand||Y = 1186.3 + 13.7X 1 – 578.6X 2 + 118.3X 3||0.93||Y = 1221.9 – 228.0X 2||0.68|
|Hallet concrete sand||Y = 1265.8 – 21.4X 1 – 94.8X 2||0.88||–||ns‡|
|Sidley Pro/Angle||Y = 1717.0 – 411.8X 2 – 164.9X 3||0.81||Y = 1794.5 – 9.0X 1 – 410.4X 2 – 43.5X 3||0.77|
|Hallet mason sand/15% soil v/v||Y = 942.4 – 131.9X 2||0.68||Y = 919.7 – 168.0X 2 + 77.9X 3||0.86|
|Hallet mason sand||Y = 112.5 + 61.2X 2 + 26.4X 3||0.91||–||ns|
|Hallet concrete sand||Y = 83.0 + 11.9X 1 – 57.1X 2 + 23.8X 3||0.98||Y = 105.7 + 6.5X 1 – 25.1X 2 + 10.7X 3||0.73|
|Sidley Pro/Angle||Y = 49.6 + 170.6X 2 + 46.6X 3||0.89||Y = 102.9 + 94.6X 2 + 23.1X 3||0.47|
|Bunker white||Y = 93.8 + 3.8X 1 – 86.8X 2 + 92.9X 3||0.94||Y = 141.3 + 45.6X 2||0.59|
|Hallet mason sand/15% soil v/v||Y = 34.6 + 4.7X 1 + 119.3X 3||0.94||Y = 7.4 + 3.4X 1 + 62.7X 2 + 36.7X 3||0.47|
|Hallet mason sand||–||ns||–||ns|
|Hallet concrete sand||–||ns||Y = 37.0 + 0.3X 1 + 5.3X 2||0.55|
|Sidley Pro/Angle||Y = 36.9 + 10.4X 2 + 3.7X 3||0.72||Y = 35.5 + 13.8X 2 + 1.4X 3||0.62|
|Hallet mason sand/15% soil v/v||Y = 37.9 + 5.9X 2 + 3.8X 3||0.69||Y = 20.2 + 0.75X 1 + 12.3X 2 + 2.2X 3||0.66|
The maximum deceleration showed similar trends to the cone resistance during the consolidation stage, ranking as Sidley Pro/Angle > Hallet mason sand, Hallet mason sand/15% soil > Hallet concrete sand > Bunker White sand (Fig. 4 ). The peak deceleration in seeded areas decreased at the turf establishing stage (Fig. 5a ) and remained relatively stable for the sodded areas (Fig. 5b). During establishment, Sidley Pro/Angle showed significant higher peak deceleration than other materials except for Hallet concrete sand in seeded areas. In the sodded areas, only Sidley Pro/Angle showed consistent higher peak deceleration than Bunker White during turf establishment. Toward a more mature stage, both seeded and sodded areas had the same ranking of peak deceleration among the five materials with Sidley Pro/Angle and Hallet concrete sand higher than the others (Fig. 6a and 6b ). Also, the peak deceleration of sodded areas decreased to levels similar to that of seeded areas ranging from 100 to 150 m s−2 The peak deceleration of Bunker White sand maintained the value at the end of consolidation stage in the sodded area but decreased over time in the seeded areas (Fig. 4, 5a, and 6a) until the fourth year of establishment (Table 3). Unlike the cone resistance, peak deceleration reached highest levels by the fourth year in July 2005 (Table 3).
Established turf showed that peak deceleration was positively correlated with organic matter content in the thatch/mat layer and soil moisture whenever the model was significant (Table 6). Rogers (1988) reported that thatch decreased impact absorption values on fine fescue (Festuca rubra L.), Kentucky bluegrass, and zoysiagrass (Zoysia japonica Steud.). Results from this study did not contradict Rogers (1988) because no clearly defined thatch layers were observed in this study. Since soil bulk density did not change significantly (Tables 2 and 3), it is not likely that bulk density was the reason for the change of peak deceleration. Therefore, the thatch intermingled with topdressing sand may have resulted in a mat with lower impact absorption, that is, higher peak deceleration values. McCarty et al. (2005, 2007) also reported increased organic matter content in the mat layer was accompanied by higher surface hardness values on establishing and established creeping bentgrass putting greens. The effect of verdure and thatch on peak deceleration depends on the grass species and the weight of the impact hammer (Rogers, 1988). Rogers (1988) also reported that under certain conditions, ground cover did not affect impact absorption. The relationship between peak deceleration and root biomass did not show a determined trend in this study.
Surface traction was not measured at the turf establishment stage because it is a destructive measurement. Surface tractions of the established turf from sod were not different among root zone materials except for the first measurement when Hallet mason sand/15% soil showed higher traction than other materials (Fig. 7b ). For the seeded areas, Bunker White sand had lower traction at all measurements (Fig. 7a). In the fourth year, surface traction was the highest for Sidley Pro/Angle and Hallet mason sand/15% soil for both seeded and sodded areas (Table 3). Where multi linear regression models were significant, traction was positively correlated to root biomass, organic matter in the thatch/mat layer, and soil moisture with R 2 values ranging from 0.55 to 0.72 (Table 6). The presence of roots was reported to increase traction values regardless of turfgrass species (Rogers, 1988).
Soil water content of the five root zone materials varied at approximate field capacity during the study (Table 3). The water holding ability may have contributed to the turf quality differences (Table 3) as well as surface stability data.
When constructing a sand-based root zone for a sports field, many sand sources may be available that follow the USGA specification. The Bunker White sand used in this study conformed to the USGA specifications (USGA Green Section Staff, 1993) and the Hallet mason sand came very close to the USGA specification with only 2.5% too much in the very coarse and gravel combination. Visual analysis of the roundness and sphericity would have put all materials used for this study as acceptable, except Bunker White. Angle at repose is often measured when surface stability is concerned. Li et al. (2004) reported that AR has a higher correlation coefficient with the roundness factors and roughness index than with the coefficient of uniformity. However, field measurement ranked the stability of root zone materials differently from AR values. Based on AR, Hallet concrete sand would have made the most stable root zone. Cone resistance and maximum deceleration ranked Sidley Pro/Angle more stable than the other materials. Also, cone resistance and maximum deceleration were affected by the turf establishment stage and establishment methods.
Using angular sand to improve the surface stability is recommended. The advantages of angular sand over subround sand were still evident 4 yr after turfgrass establishment. There has been no reported acceptable range of cone resistance, but the maximum deceleration results in the same period showed lower values than heavily used fields (60–90 Gmax) reported by Rogers (1988) Cultural practices such as core aeration and proper topdressing could help to incorporate topdressing materials into undecomposed plant materials and increase the stability. Subround sand with particle-size distribution within the specified ranges of USGA specification should be avoided for construction or topdressing. Using sand with a wider range of particle-size distribution (higher CU) as contributed from more coarse fractions did not result in higher surface stability.
Adding soil to the regular mason sand did not always result in increased cone penetration resistance due to freeze-thaw effects. Increased root growth led to decreased cone penetration resistance. Therefore, using the cone penetration method alone to assess the established turf may not provide sufficient information on root zone stability.
The cone penetration method was able to differentiate sands of different angularity but was not sensitive to different particle size distributions as reflected in the CU and gradation index, such as the Hallet mason sand and Hallet concrete sand. Using cone penetration resistance at soil moisture conditions close to field capacity, sand materials can be consistently ranked as would be predicted from the roughness index.
Without vegetative cover, maximum deceleration separated the sand materials similarly as the cone penetration method. With grass cover, hardness measurement revealed that angular sand provided consistently higher maximum deceleration. Increasing the gradation index (as in Hallet concrete sand) and/or adding soil to the mason sand (as in Hallet mason sand/15% soil) occasionally resulted in harder surfaces as reflected in the higher maximum deceleration values. Converting thatch layer into intermingled mat by topdressing could increase surface hardness as peak deceleration was positively correlated with organic matter content in the mat layer. Since soil moisture content was positively correlated with the peak deceleration, it is necessary to measure both soil water content and peak deceleration when assessing surface hardness.
The cleated-disk device was not a sensitive method to assess turf surface playability. When significant differences were detected, angular sand (Sidley Pro/Angle) or adding soil to mason sand (Hallet mason sand/15% soil) resulted in larger shear strength. Both root biomass and organic matter content in mat layer were positively correlated to the shear strength. Therefore, measuring traction may complement the cone penetration and hardness measurement in the assessment of surface stability.