After oxygen, silicon (Si) is the most abundant element in the Earth’s crust and a dependable constituent of the soil mineral fraction [1
]. Although not recognized as a plant-essential nutrient, Si naturally accumulates in tissue of many plant species [2
]. This plant uptake results in intra- and extracellular silica deposition in the epidermal and vascular tissue of monocotyledonous leaves and roots [3
]. Silica deposition in leaf/shoot cuticle and epidermis of rice (Oryza sativa
L.), sugarcane (Saccharum officinarum
L.), or bamboo (Phyllostachys heterocycla
Mitf.) has shown significant correlation to stomatal density, transpiration, and lodging resistance [5
Recent agricultural field assessments report soil Si depletion in shallow depths of intensively managed systems [7
]. Uptake of Si by roots of broadleaf and grass plants support recommendations for silicon fertilization [8
]. Silica uptake in rice has been shown to increase with SiO3
]. Topdressing or incorporation of Ca- and/or Mg-SiO3
conditioner(s) into rootzones underlying creeping bentgrass (Agrostis stolonifera
L.), tall fescue (Schedonorus arundinaceus
[Schreb.] Dumort., nom. cons.), perennial ryegrass (Lolium perenne
L.), bermudagrass (Cynodon dactylon
L.), and zoysiagrass (Zoysia japonica
Steud.) have resulted in statistically increased Si accumulation in leaf tissue [10
]; whereas, Si application to seashore paspalum (Paspalum vaginatum
L.) resulted in a significant leaf Si increase in one of two field evaluations [14
]. Similarly, spray application of liquid Si fertilizers to creeping bentgrass (Agrostis stolonifera
L.) has shown significant leaf accumulation in Kentucky [15
], but not in North Carolina [16
Considering the Ca-requirements of meristematic tissue in lengthening roots and aqueous activity of highly-soluble monovalent silicates [17
], calcium silicate appears better suited for sizable application to managed turfgrass than liquid potassium- or sodium-silicate products. Furthermore, while all liming agents neutralize exchangeable acidity, resulting products of these chemical reactions typically comprise only CO2
]. Acid neutralization by Ca/MgSiO3
liming agents uniquely produce silicic acid, H4
, which is readily assimilated by plant roots and re-distributed to vascular, epidermal, and cuticular tissue [3
Wear injury to turfgrass comprises abrasion, tearing, and compression of leaf/shoot tissue; and is an innate concern of those managing turfgrass on intensively used sites [20
]. Indirect effects of wear injury to turfgrass include degraded pest resistance, utility, and aesthetic appeal [20
]. Silicic acid deposition in the apoplast has been reported to elicit structural resilience and pathogen resistance by polymerizing cell walls at a significantly lesser metabolic cost than lignification [22
]. Varying Si concentration, supplanting lignin, cellulose, and/or hemicellulose content in rice stalks, demonstrated equal resistance to applied compressive force [27
Perennial ryegrass is a cool-season turfgrass species renowned for its dark green color, upright growth habit, rapid germination and establishment rate, tolerance of low mowing height, and minimal thatch contribution [21
]. In regions where cool-season turfgrasses are well-adapted and perennial athletic field vegetation is intended, perennial ryegrass pairs swimmingly with Kentucky bluegrass (Poa pratensis
L.) and/or turf-type tall fescue. Likewise, perennial ryegrass is commonly established and maintained as overseed where recreational warm-season turfgrass systems undergo prolonged dormancy. Given both its widespread use and reported tendency to accumulate soil Si, perennial ryegrass comprises an ideal candidate for soil conditioner and wear treatment evaluation. The primary objective of this field research is to comprehensively-assess perennial ryegrass response to intense wear/traffic and amendment by pelletized Ca/Mg-rich soil conditioners.
2. Materials and Methods
A field study of commercial soil conditioners was initiated July 2010 on a perennial ryegrass (1:1:1 ‘Manhattan,’ ‘Brightstar SLT,’ ‘Mach 1’) field maintained within the Joseph Valentine Turfgrass Research Center (University Park, PA, USA). The Hagerstown silt loam (fine, mixed, mesic, typic hapludalfs) comprising the athletic field rootzone was sampled to a 16-cm depth on 6 July 2010. Three (3) composite samples were submitted to the Pennsylvania State Univ. Agricultural Analytical Services Laboratory (PSU-AASL) for routine fertility analysis [18
]. The experiment was arranged as a split plot in randomized complete block design of six (6) replicates. One of two main plots in each replicate block was randomly assigned systematic wear treatment. Three (3) split plots (1.7 × 0.9 m) in each main plot were randomly assigned treatment applications of: granular Ca/Mg-silicate (SiO3
) ‘CrossOver’ (Harsco Minerals Intl., Sarver, PA, USA) totaling 1220 or 2440 kg ha−1
annually (manufacturer recommendation), or a 1:1 blend of pelletized dolomitic and calcitic limestones, Ca/MgCO3
(OldCastle Lawn and Garden, Thomasville, PA, USA) totaling 1220 kg ha−1
annually (Table 1
Granular applications were initiated 7 July 2010. Split applications of each treatment were reapplied over the 2010, 2011, and 2012 seasons (Figure 1
). Granular applications were followed by either irrigation or a rainfall event. Likewise, mowing (twice weekly at a 2 cm height of cut) was suspended for a 5 d period. On 3 Aug. 2010, wear treatments were initiated by three sets of two adjacent passes using an internally powered front-end Sweepster (0.91 m swath). This device caused significant defoliation and was retired in preference of a pull-behind wear simulator [30
]. On 9 Aug., main-plot wear treatments were applied by six sets of passes using the pull-behind wear simulator. In 2011, weekly wear treatments of five successive wear simulator passes were applied June through Aug. While two passes of the pull-behind wear simulator introduce the mean number of cleat dimples incurred between the mid-field hash marks of a regulation US football field during a 60-minute game [30
], this study employed five passes over the wear main plots each week. The above-described wear treatments commenced weekly in June 2012 but were concluded 13 Aug. (769 days after initial treatment, DAIT) due to saturated conditions arising from recent and further-forecasted precipitation events.
Preliminary soil test results indicated neutral pH and optimal saturation of a 9.3 meq (100 g soil)−1
cation exchange capacity. Likewise, Mehlich-3 extractable soil P (94 mg kg−1
) resided within the optimal range for a mature turfgrass athletic field. Therefore, supplemental fertilization practices were limited to monthly applications of various urea fertilizers to ensure plant-availability of 10 to 20 kg N ha–1
per growing month of the two-year study [31
]. Monthly scouting of disease and/or nutrient deficiency symptoms were assessed and recorded throughout the 2011 and 2012 growing seasons.
Triplicate soil samples of 0 to 16 cm depth were collected from all non-trafficked split plots in July and Sept. 2011, and April and Aug. 2012. Soil cores were divided into 0 to 8 cm, and 8 to 16 cm depth segments, dried, and ground to pass a 1 mm sieve. Sieved depth segments were split for 0.5 M acetic acid extraction of plant-available soil Si [9
] and 1:1 soil pH measurement [32
]. Stock acetic acid was prepared and stored in high density polyethylene (HDPE) bottles, and soil extractions were conducted and stored in HDPE centrifuge tubes.
To avoid potentially confounding effects of wear treatment applications, clipping yields were collected from only non-trafficked split plots in June, July, and Sept. 2011; and May, July and Aug. 2012. The justification for this practice was to protect the shoot tissue from contamination by silica-rich soil particles during multiple mechanized traffic simulator transits each week. Clippings were thoroughly dried in a forced-air oven (70 °C), weighed to 0.1-mg resolution, and split for Si-extraction [33
] or acid digestion and plant-essential nutrient concentration analysis by PSU-AASL. Stock solution of 4,5-dihydroxy-1,3-benzenedisulfonic acid disodium salt monohydrate, C6
(Sigma-Aldrich, Merck KGaA, Darmstadt, Germany), was prepared in HDPE bottles, and leaf tissue Si-extractions conducted and stored in HDPE centrifuge tubes.
On 10 August 2010, and every 8 ± 3 days from late June through Aug. (2011 and 2012), simultaneous measures of 660 and 850 nm canopy reflectance from every split plot (four unique readings per) were recorded using an ambient light-excluding FieldScout TCM-500 turfgrass chlorophyll meter (Spectrum Technologies Inc., Plainfield, IL, USA). Reflectance data was used to calculate the normalized differential vegetative index (NDVI). On an identical frequency, a FieldScout TCM-500-RGB color meter (Spectrum Technologies Inc., Plainfield, IL, USA) was employed to collect quadruplicate measures of green, red, and blue canopy reflectance from every split plot. Percent color reflectance was converted to hue, saturation, and brightness levels for calculation of the dark green color index (DGCI) [34
]. These NDVI and DGCI indices provide resolute and dependably reproducible measures of turfgrass canopy density and color, respectively [35
Soil pH and plant-available soil Si levels were sorted by soil depth then modeled by conditioner treatment using PROC MIXED (SAS Institute, v. 8.2, Cary, NC, USA). The main effect of soil conditioner was F-tested using its block interaction term (df = 10). Repeated soil measures by depth were analyzed as split plots in time from initial treatment. Time-series covariate structures, selected using best fit criteria, facilitated F-tests of time and time interactions by the residual error term (df = 45).
The main effect of soil conditioner on clipping yield, leaf Si content, Si uptake, and tissue nutrient concentration was F-tested by its block interaction term (df = 10), while F-tests of time and its interaction were facilitated by previously described time-series covariate structures and a 75 df residual error term.
The main effect of wear (main plot) treatment on canopy density or canopy color was F-tested using its block interaction term (df = 5). The remaining soil conditioner effect and its interaction with wear was F-tested using the block × wear × amendment term (df = 20). Repeated canopy quality measures were treated as split-split-plots in time from initial treatment. Fit-selected covariate structures facilitated F-tests of time and time interactions by the residual error term (df = 690).
All main and split-plot effect hypothesis tests employed two-tailed separation of treatment means by Fisher’s protected least significant difference (LSD) at the 0.05 alpha level. For presentation clarity, treatment means within interacting levels of wear, soil conditioner, and/or time sources were separated using a one-tailed hypothesis test against the calcitic-/dolomitic blended-limestone treatment, and statistically significant treatment means separated using Fisher’s protected LSD at 0.05 alpha level.