The Pennsylvania State University
The Graduate School
College of Agricultural Sciences
EFFECTS OF PRE-HARVEST CULTURAL PRACTICES ON THE DIVOT
RESISTANCE OF THICK-CUT KENTUCKY BLUEGRASS SOD
A Thesis in
Agronomy
by
Evan C. Mascitti
2015 Evan C. Mascitti
Submitted in Partial Fulfillment
of the Requirements
for the Degree of
Master of Science
May 2015
ii
The thesis of Evan C. Mascitti was reviewed and approved* by the following:
Andrew S. McNitt
Professor of Soil Science
Thesis Advisor
Maxim J. Schlossberg
Associate Professor of Turfgrass Nutrition
Jack Watson
Professor of Soil Science
Peter Landschoot
Professor of Turfgrass Science
Agronomy Graduate Program Coordinator
*Signatures are on file in the Graduate School
iii
ABSTRACT
Athletic fields must provide stable and consistent footing to minimize injuries and
maximize player performance. Professional athletes in the National Football League
(NFL) impart extreme shearing forces to the surface during competition, producing
divots. Divots are defined as portions of the turf gouged from the surrounding area by
athletes’ cleated footwear. Divoting is the primary mechanism of damage on NFL fields,
as opposed to the abrasive wear and soil compaction common on more-frequently but
less-intensively trafficked surfaces.
During the latter stages of the NFL season, the cumulative removal of above- and below-
ground vegetation through divoting can destabilize the playing surface. At this time of
year the prevailing atmospheric and edaphic conditions in the northeastern United States
are unconducive to turfgrass growth and effectively eliminate turf recovery. To restore
stable footing the surface must be replaced with new sod.
Aside from ambient environmental conditions at sod farms, the quality of the sod
installed at NFL stadia is a direct consequence of management decisions by sod growers
prior to the sod harvest and installation. Since the surface will be used for competition
immediately after installation, there is little time for athletic field managers to alter the
surface characteristics through management practices. While some divoting is inevitable,
surface stability will be prolonged if divot sizes and numbers are minimized. It is thus
imperative that divot resistance is previously optimized through cultural practices at the
sod farm prior to harvest, transport, and installation.
iv
The objectives of this research were (1) Maximize divot resistance while maintaining
tensile strength of thick-cut Kentucky bluegrass sod through manipulation of pre-harvest
cultural practices, and (2) Determine the effects of mowing height, sand topdressing, and
nitrogen fertilization on shoot density, thatch accumulation, and below ground biomass
and their relationships to divot resistance.
Two field experiments were conducted to evaluate the effects of cutting height, sand
topdressing, and nitrogen fertilization on the divot resistance of thick-cut sod
immediately after harvest. Experiment 1 was spatially replicated at the Joseph Valentine
Turfgrass Research Center (University Park, PA) and Tuckahoe Turf Farms
(Hammonton, NJ). A four-way blend of Kentucky bluegrass (KBG) (Poa pratensis L.)
cultivars was seeded during early fall of 2012. Treatments were initiated in spring of
2013.
Cutting height treatments were established as discrete “experiments” maintained at 3.18
and 3.81 cm. Within each cutting height experiment a 2x6 topdressing x nitrogen
factorial experiment was conducted. The topdressing treatments were a control receiving
no sand and a treatment including three sand applications to total 8.5 kg sand m-2. The six
nitrogen treatments ranged from 96-244 kg total N ha-1 yr-1 and were further
differentiated by the application timing. Applications supplied 49 kg N ha-1 via granular
ammonium sulfate. Plots received either 2, 3, or 4 N applications from March-June and
either no additional N or a final N application in September of 2013.
v
Sod was harvested and tested in November 2013 at a standard “thick-cut” profile
thickness of 4.45 cm. Divots were produced with Pennswing, a weighted pendulum
device. The size of each divot was measured with smaller divots indicating high divot
resistance. Other parameters measured in the field included turfgrass color, surface shear
strength, and sod strength. To determine the associations among these parameters and
potentially related turfgrass morphological characteristics, core samples were collected
from each plot and evaluated for shoot density, thatch accumulation, and below-ground
biomass.
Experiment 2 evaluated the effects of four cutting heights (2.54, 3.18, 3.81, or 4.45 cm)
on divot resistance, shear strength, and sod strength. Plots were maintained under
identical fertilization and topdressing regimes. Treatment evaluation occurred in
November 2013 using the same methods as Experiment 1.
All plots in Experiments 1 and 2 were treated with trinexapac-ethyl (TE), a plant-growth
regulator which has been shown to increase divot resistance. The use of TE is common
among NFL field managers and by sod growers producing turf for the NFL, so TE
applications were included as part of the plot maintenance to better simulate a real-world
scenario.
Cutting height did not significantly affect divot resistance in Experiment 1 or 2. While it
is possible that cutting height in fact has no influence on divot resistance, the lack of a
cutting height effect in this project may be attributable to other factors in the experiments
masking such an effect. The blend of cultivars was uniform across all experimental units.
vi
KBG cultivars are known to respond differently to various cutting heights. The individual
cultivars therefore may have responded in opposing manners, masking the cutting height
effect. In addition, the use of TE regulates plant morphology in a fashion similar to close
defoliation (producing compact shoots and increased plant density). TE was regularly
applied to all plots in Experiments 1 and 2, and may have helped mask any cutting height
effect which would otherwise have occurred.
Lower cutting heights tended to produce higher shear strength at VRC (both Experiments
1` and 2), although TTF showed a weak trend in the opposite direction. Increased shear
strength under closer cutting may be partially attributed to higher shoot density. Shear
strength and density were positively, correlated, though the relationship was relatively
weak (r=0.26). The low correlation coefficient indicates other turfgrass properties also
influence shear strength.
Cutting height had no effect on sod strength in Experiment 1, but greater sod strength was
obtained in Experiment 2 with higher cutting heights. However the benefit was minor and
not realized until cutting height reached 4.45 cm, which is likely too high for sod
intended for the NFL. As acceptable sod strength was obtained under all cutting heights,
it is recommended that sod growers producing thick-cut sod for in-season replacements
maintain the production field close to 3.18 cm. This cutting height is comparable to those
used at NFL stadia and would not require the stadium manager to severely reduce the
cutting height prior to play.
vii
Divot resistance was negatively impacted by high N rates. Small divots indicated higher
divot resistance. At the VRC location divot lengths were 37% larger under the 4-1 N
treatment than the 3-0 treatment, which was most effective. Smaller yet analogous
increases in divot width and depth were also noted at each location under the 4-1 N
treatment.
The 4-1 N treatment (244 kg ha-1 total N) was the treatment most similar to the actual
fertilization schedule used by TTF in 2013 during production of thick-cut sod for NFL
stadia. This research project suggests N rates below the current standard may be
advantageous with regard to divot resistance. High shoot density and darker green color
were associated with greater N rates, but on NFL surfaces these traits are of minimal
importance compared to divot resistance.
Anecdotal concerns that close mowing and/or topdressing would render the turf
unharvestable at 4.45 cm sod thickness were not substantiated in this project. All
treatment combinations produced sod strength greater than 100 kg, which in these
experiments was estimated as the minimum acceptable strength for thick-cut, big-roll
sod.
Significant correlations among response variables occurred, but the correlation
coefficients were relatively weak. These relationships indicate divot resistance is affected
by multiple properties rather than a single turfgrass characteristic. Divot length was the
strongest measure of divot resistance and was negatively related to shear strength
viii
(r= -0.47) and sod strength (r= -0.38). Surprisingly, divot length was not significantly
correlated to below-ground biomass. Thatch thickness, however was significantly related
to divot length (r= -0.26). The relationship was negative, indicating plots with thicker
thatch layers also had shorter divots. Such a relationship is contrary to reports from
practitioners, most of whom opine divot resistance to be compromised by thatch buildup.
The relationship between thatch and divot resistance is complex and may depend on
factors not accounted for in this study such as depth of cleat penetration, impact angle,
and the physiochemical properties of the thatch layer (e.g. water status, lignin content,
state of decomposition). In the future, a research tool should be developed which more
accurately simulates the divot-producing mechanisms of NFL athletes.
Little research has investigated production of thick-cut sod with regard to in-season
replacement of American football fields. The goal of this project was to optimize the
cultural practices used by sod growers when producing thick-cut Kentucky bluegrass sod
for NFL football stadia. All treatments produced harvestable sod at 4.45 cm thickness
despite prior industry concern toward the imposition of close clipping as well as
applications of TE and sand topdressing. Under the conditions of this study, divot
resistance was improved by reducing nitrogen inputs from the current standard. Further
research is needed to understand the effects of more-refined nitrogen regimes as well as
the influence of thatch on divot resistance. The results of this project will aid sod growers
and turfgrass managers in producing divot-resistant natural grass playing fields.
ix
TABLE OF CONTENTS
LIST OF FIGURES ..................................................................................................... xi
LIST OF TABLES ....................................................................................................... xiv
DEFINITION OF TERMS ......................................................................................... xxi
ACKNOWLEDGEMENTS ......................................................................................... xxii
INTRODUCTION ................................................................................................................... 1
LITERATURE REVIEW ........................................................................................................ 4
Overview of turfgrass sod production .............................................................................. 4 Influence of clipping and nitrogen fertilization on turfgrass sod ..................................... 7
Clipping .................................................................................................................... 7 Nitrogen fertilization ................................................................................................ 10
Turfgrass thatch ................................................................................................................ 13 Consequences of thatch on athletic playing surface ................................................. 13 Effects of clipping on thatch development ............................................................... 15 Effects of nitrogen thatch development.................................................................... 15 Supplementary cultural practices to manage thatch ................................................. 17
Research methods used to evaluate surface stability and sod strength ............................ 19 Surface stability on athletic fields ............................................................................ 19 Sod strength .............................................................................................................. 24
OBJECTIVES .......................................................................................................................... 26
MATERIALS AND METHODS ............................................................................................. 27
Experiment 1: The effects of cutting height, topdressing, and nitrogen fertilization
on the divot resistance of thick-cut Kentucky bluegrass sod ................................... 28 Overview of Experiment .......................................................................................... 28 Plot Establishment .................................................................................................... 28 Treatment Application .............................................................................................. 32 Experimental Design ................................................................................................ 36 Additional Plot Maintenance .................................................................................... 36 Treatment evaluation ................................................................................................ 38 Statistical Analysis ................................................................................................... 51
Experiment 2: The effects of varying cutting height on the divot resistance of thick-
cut Kentucky bluegrass sod ...................................................................................... 54 Overview of Experiment .......................................................................................... 54 Plot Establishment .................................................................................................... 54 Treatment Application .............................................................................................. 54
x
Experimental Design ................................................................................................ 55 Additional Plot Maintenance .................................................................................... 55 Treatment evaluation ................................................................................................ 55 Statistical Analysis ................................................................................................... 55
RESULTS ................................................................................................................................ 56
Experiment 1: The effects of cutting height, topdressing, and nitrogen fertilization
on the divot resistance of thick-cut Kentucky bluegrass sod ................................... 57 Joseph Valentine Research Center (VRC) Location ................................................ 57 Tuckahoe Turf Farms (TTF) Location ..................................................................... 101 Variability across locations ...................................................................................... 146 Weather conditions ................................................................................................... 147 Correlations .............................................................................................................. 150
Experiment 2: The effects of varying cutting height on the divot resistance of thick-
cut Kentucky bluegrass sod ...................................................................................... 153 Divot length, width, and depth ................................................................................. 153 Sod strength .............................................................................................................. 155 Shear strength ........................................................................................................... 155 Shoot density ............................................................................................................ 158 Thatch accumulation ................................................................................................ 158 Below-ground biomass ............................................................................................. 158
DISCUSSION .......................................................................................................................... 161
Field Evaluations .............................................................................................................. 162 Divot Resistance ....................................................................................................... 162 Sod Strength ............................................................................................................. 169 Shear Strength .......................................................................................................... 175
Turfgrass morphological characteristics .......................................................................... 177 Shoot density ............................................................................................................ 177 Thatch ....................................................................................................................... 179 Below-ground biomass ............................................................................................. 181 Correlations among measured parameters ............................................................... 182 Potential for related future research ......................................................................... 185
SUMMARY AND CONCLUSIONS ...................................................................................... 190
REFERENCES ........................................................................................................................ 193
APPENDIX .............................................................................................................................. 202
Additional Materials ........................................................................................................ 202
xi
LIST OF FIGURES
Figure 1. Generalized Mohr-Coulomb failure envelope; soil fails when the shear
stress exceeds the normal stress (confining pressure) as indicated by red line.
Vegetative stabilization is responsible for most of the cohesion intercept in
sand-based turfgrass systems. ............................................................................... 23
Figure 2. Sod strips being removed for sod strength testing at TTF............................ 40
Figure 3. The Pennswing device about to be released. ................................................ 42
Figure 4. Examples of divots produced by Pennswing. Photo at left shows a plot
with low divot resistance while the photo at right shows a plot with high
divot resistance. 30 cm ruler included for scale. .................................................. 42
Figure 5. Point gauge used to measure divot depth. Device is placed across the
divot and the metal rod is lowered to the bottom of the depression. Height is
read and the reference height is subtracted to obtain the divot depth in cm. ........ 43
Figure 6. Operation of the shear strength measurement device; handle was turned
clockwise in the horizontal plane until the turf failed. ......................................... 45
Figure 7. Fins on the Turf Shear Tester. ...................................................................... 45
Figure 8. Operation of the sod strength device. ........................................................... 46
Figure 9. Sod strip following tensile strength test. ..................................................... 46
Figure 10. Measurement of thatch layer thickness. ..................................................... 49
Figure 11. Sample plug following removal of verdure for tiller count ........................ 49
Figure 12. Plug being sectioned into thatch portion (lower left) and soil/below-
ground biomass (upper right). ............................................................................... 50
Figure 13. Examples of thatch (left) and washed below-ground biomass samples
(right); top image shows samples before ashing and bottom image shows the
same samples following exposure to 440 °C for 16 hours. .................................. 50
Figure 14. Mean divot lengths for the nitrogen treatment main effect at VRC.
Treatments with overlapping error bars are not statistically different using
Fisher’s Protected LSD. ........................................................................................ 63
xii
Figure 15. Mean divot depths for the nitrogen treatment main effect at VRC.
Treatments with overlapping error bars are not statistically different using
Fisher’s Protected LSD. ........................................................................................ 69
Figure 16. Conceptual model depicting a possible explanation for the cutting
height by topdressing interaction in Experiment 1. Plugs on left have varying
BGB density with depth but contain essentially the same total amount of
BGB. Plugs on right have been topdressed, reducing the fraction of the plug
sampled from original soil. Less native soil is incorporated, resulting in the
lower CH plots having greater BGB per plug. Cartoon is for conceptual
purposes only and is not drawn to scale. .............................................................. 97
Figure 17. Scatter plot of divot lengths plotted against volumetric water content in
Experiment 1 at TTF. ............................................................................................ 106
Figure 18. Mean divot widths for the nitrogen treatment main effect at TTF.
Treatments with overlapping error bars are not statistically different using
Fisher’s Protected LSD. ........................................................................................ 109
Figure 19. Scatter plot of divot widths plotted against volumetric water content in
Experiment 1 at TTF. ............................................................................................ 112
Figure 20. Mean divot depths for the nitrogen treatment main effect at TTF.
Treatments with overlapping error bars are not statistically different using
Fisher’s Protected LSD. ........................................................................................ 116
Figure 21. Comparison of the 3.81 cm area (top) and 3.18 cm area (bottom) at
TTF. ...................................................................................................................... 124
Figure 22. Scatter plot of shear strength plotted against volumetric water content
in Experiment 1 at TTF. ........................................................................................ 129
Figure 23. Scatter plot of thatch thickness plotted against volumetric water
content in Experiment 1 at TTF. ........................................................................... 130
Figure 24. Mean air temperatures at both locations over the duration of
Experiment 1. Lines represent a 5-day moving average of the mean between
daily high and low temperatures. Black bold line at 20 °C represents the
temperature considered optimal for cool-season turfgrasses (Turgeon, 2012).
Green arrows represent treatment application dates. ............................................ 149
Figure 25. Mean sod strength values for the four cutting heights evaluated in
Experiment 2. Treatments with overlapping error bars are not significantly
different using Fisher’s Protected LSD. ............................................................... 156
xiii
Figure 26. Mean shoot density values for the four cutting heights evaluated in
Experiment 2. Treatments with overlapping error bars are not significantly
using Fisher’s Protected LSD. .............................................................................. 159
Figure 27. Divot lengths as affected by a cultivar by cutting height interaction; the
unpublished data were provided by McNitt (2014, personal communication). ... 164
xiv
LIST OF TABLES
Table 1. Particle size distribution for Ap horizon at the TTF location†. ..................... 31
Table 2. Particle size distribution of the top 15 cm of rootzone material at VRC†. .... 31
Table 3. Particle size analysis for the topdressing sand used at TTF. ......................... 34
Table 4. Particle size analysis for the topdressing sand used at VRC. ........................ 34
Table 5. Monthly schedule of topdressing and nitrogen treatments for Experiment
1. ........................................................................................................................... 35
Table 6. Notation for the linear contrasts performed to compare specific N
treatments and treatment combinations. ............................................................... 53
Table 7. Summary of treatment effects on field parameters in Experiment 1 at
VRC ...................................................................................................................... 58
Table 8. Summary of treatment effects on laboratory parameters in Experiment 1
at VRC. ................................................................................................................. 59
Table 9. Mean divot lengths for the cutting height main effect at VRC. ..................... 62
Table 10. Mean divot lengths for the topdressing main effect at VRC. ...................... 62
Table 11. Mean divot lengths for the nitrogen treatment main effect at VRC. ........... 62
Table 12. Selected contrasts comparing divot lengths at VRC as related to total
both total nitrogen applied and individual nitrogen treatments. ........................... 64
Table 13. Mean divot widths for the cutting height main effect at VRC. .................... 66
Table 14. Mean divot widths for the topdressing main effect at VRC. ....................... 66
Table 15. Mean divot widths for the nitrogen treatment main effect at VRC. ............ 66
Table 16. Mean divot depths for the cutting height main effect at VRC. .................... 68
Table 17. Mean divot depths for the topdressing main effect at VRC. ....................... 68
Table 18. Mean divot depths for the nitrogen treatment main effect at VRC. ............ 68
xv
Table 19. Selected contrasts comparing divot depths at VRC as related to total
both total nitrogen applied and individual nitrogen treatments. ........................... 70
Table 20. Mean sod strength values for the cutting height main effect at VRC. ......... 73
Table 21. Mean sod strength values for the topdressing main effect at VRC. ............ 73
Table 22. Mean sod strength values for the nitrogen treatment main effect at
VRC. ..................................................................................................................... 73
Table 23. Selected contrasts comparing sod strength at VRC as related to total
both total nitrogen applied and individual nitrogen treatments. ........................... 74
Table 24. Mean sod strength values for the cutting height by topdressing by
nitrogen treatment interaction at VRC. ................................................................. 75
Table 25. Mean shear strength values for the cutting height main effect at VRC. ...... 78
Table 26. Mean shear strength values for the topdressing main effect at VRC. .......... 78
Table 27. Mean shear strength values for the nitrogen treatment main effect at
VRC. ..................................................................................................................... 78
Table 28. Mean shoot density values for the cutting height main effect at VRC. ....... 81
Table 29. Mean shoot density values for the topdressing main effect at VRC. ........... 81
Table 30. Mean shoot density values for the nitrogen treatment main effect at
VRC. ..................................................................................................................... 81
Table 31. Selected contrasts comparing shoot density at VRC as related to total
both total nitrogen applied and individual nitrogen treatments. ........................... 82
Table 32. Mean shoot density values for the topdressing by nitrogen treatment
interaction at VRC. ............................................................................................... 83
Table 33. Mean thatch mass values for the cutting height main effect at VRC. ......... 87
Table 34. Mean thatch mass values for the topdressing main effect at VRC. ............. 87
Table 35. Mean thatch mass values for the nitrogen treatment main effect at VRC. .. 87
Table 36. Selected contrasts comparing thatch mass at VRC as related to total
both total nitrogen applied and individual nitrogen treatments. ........................... 88
Table 37. Mean thatch mass values for the cutting height by topdressing
interaction at VRC. ............................................................................................... 89
xvi
Table 38. Mean thatch mass values for the cutting height by nitrogen treatment
interaction at VRC. ............................................................................................... 90
Table 39. Mean below-ground biomass values for the cutting height main effect at
VRC. ..................................................................................................................... 93
Table 40. Mean below-ground biomass values for the topdressing main effect at
VRC. ..................................................................................................................... 93
Table 41. Mean below-ground biomass values for the nitrogen treatment main
effect at VRC. ....................................................................................................... 93
Table 42. Selected contrasts comparing below-ground biomass at VRC as related
to total both total nitrogen applied and individual nitrogen treatments. ............... 94
Table 43. Mean below-ground biomass values for the cutting height by
topdressing interaction at VRC. ............................................................................ 96
Table 44. Mean below-ground biomass values for the cutting height by nitrogen
treatment at VRC. ................................................................................................. 99
Table 45. Summary of treatment effects on field parameters in Experiment 1 at
TTF. ...................................................................................................................... 102
Table 46. Summary of treatment effects on laboratory parameters in Experiment 1
at TTF. .................................................................................................................. 103
Table 47. Mean divot lengths for the cutting height main effect at TTF. .................... 104
Table 48. Mean divot lengths for the topdressing main effect at TTF. ....................... 104
Table 49. Mean divot lengths for the nitrogen treatment main effect at TTF. ............ 104
Table 50. Mean divot widths for the cutting height main effect at TTF. ..................... 108
Table 51. Mean divot widths for the topdressing main effect at TTF. ........................ 108
Table 52. Mean divot widths for the nitrogen treatment main effect at TTF. ............. 109
Table 53. Selected contrasts comparing divot widths at TTF as related to total
both total nitrogen applied and individual nitrogen treatments. ........................... 110
Table 54. Mean divot depths for the cutting height main effect at TTF. ..................... 114
Table 55. Mean divot depths for the topdressing main effect at TTF.......................... 114
Table 56. Mean divot depths for the nitrogen treatment main effect at TTF. .............. 116
xvii
Table 57. Selected contrasts comparing divot depths at TTF as related to total
both total nitrogen applied and individual nitrogen treatments. ........................... 117
Table 58. Mean sod strength values for the cutting height main effect at TTF. .......... 120
Table 59. Mean sod strength values for the topdressing main effect at TTF. .............. 120
Table 60. Mean sod strength values for the nitrogen treatment main effect at TTF.... 120
Table 61. Selected contrasts comparing sod strength at TTF as related to total both
total nitrogen applied and individual nitrogen treatments. ................................... 121
Table 62. Mean sod strength values for the cutting height by nitrogen treatment
interaction at TTF. ................................................................................................ 123
Table 63. Mean shear strength values for the cutting height main effect at TTF. ....... 127
Table 64. Mean shear strength values for the topdressing main effect at TTF. ........... 127
Table 65. Mean shear strength values for the nitrogen treatment main effect at
TTF. ...................................................................................................................... 127
Table 66. Mean shoot density values for the cutting height main effect at TTF. ........ 132
Table 67. Mean shoot density values for the topdressing main effect at TTF. ............ 132
Table 68. Mean shoot density values for the nitrogen treatment main effect at
TTF. ...................................................................................................................... 132
Table 69. Mean thatch mass values for the cutting height main effect at TTF............ 136
Table 70. Mean thatch mass values for the topdressing main effect at TTF. .............. 136
Table 71. Mean thatch mass values for the topdressing main effect at TTF. .............. 136
Table 72. Selected contrasts comparing thatch mass at TTF as related to total both
total nitrogen applied and individual nitrogen treatments. ................................... 137
Table 73. Mean thatch mass values for the cutting height by topdressing
interaction at TTF. ................................................................................................ 138
Table 74. Mean thatch mass values for the cutting height by nitrogen treatment
interaction at TTF. ................................................................................................ 138
Table 75. Mean thatch mass values for the topdressing by nitrogen treatment
interaction at TTF. ................................................................................................ 140
xviii
Table 76. Mean thatch mass values for the cutting height by topdressing by
nitrogen treatment interaction at TTF. .................................................................. 140
Table 77. Mean below-ground biomass values for the cutting height main effect at
TTF. ...................................................................................................................... 144
Table 78. Mean below-ground biomass values for the topdressing main effect at
TTF. ...................................................................................................................... 144
Table 79. Mean below-ground biomass values for the nitrogen treatment main
effect at TTF. ........................................................................................................ 144
Table 80. Mean below-ground biomass values for the cutting height by
topdressing by nitrogen treatment interaction at TTF. ......................................... 145
Table 81. Mean values for field-measured parameters at each location when
averaged across all treatment levels. .................................................................... 148
Table 82. Mean values for laboratory-measured parameters at each location when
averaged across all treatment levels. .................................................................... 148
Table 83. Spearman correlation coefficients among parameters measured in
Experiment1 .......................................................................................................... 151
Table 84. Mean divot dimensions for the four cutting heights evaluated in
Experiment 2. ........................................................................................................ 154
Table 85. Mean sod strength values for the four cutting heights evaluated in
Experiment 2. ........................................................................................................ 156
Table 86. Mean shear strength values for the four cutting heights evaluated in
Experiment 2. ........................................................................................................ 157
Table 87. Mean shoot density values for the four cutting heights evaluated in
Experiment 2. ........................................................................................................ 159
Table 88. Mean thatch mass values for the four cutting heights evaluated in
Experiment 2. ........................................................................................................ 160
Table 89. Mean below-ground biomass values for the four cutting heights
evaluated in Experiment 2. ................................................................................... 160
Table 90. Comparison of data collected in Experiment 1 with selected published
values for KBG sod strength on a per-unit-area basis. ......................................... 172
xix
Table 91. Properties of sod from this research project compared with actual sod
produced by TTF and installed at NFL stadia in November 2013. All sod
tested was harvested at 4.45 cm profile thickness. ............................................... 202
Table 92. Mean visual color ratings for the cutting height main effect at VRC. ......... 203
Table 93. Mean visual color ratings for the topdressing main effect at VRC. ............ 203
Table 94. Mean visual color ratings for the nitrogen treatment main effect at
VRC. ..................................................................................................................... 203
Table 95. Selected contrasts comparing visual color at VRC as related to total
both total nitrogen applied and individual nitrogen treatments. ........................... 204
Table 96. Mean visual color ratings for the cutting height main effect at TTF. .......... 205
Table 97. Mean visual color ratings for the topdressing main effect at TTF. .............. 205
Table 98. Mean visual color ratings for the nitrogen treatment main effect at TTF.... 205
Table 99. Mean visual color ratings for the cutting height by nitrogen treatment
interaction at TTF. ................................................................................................ 206
Table 100. Selected contrasts comparing visual color at TTF as related to total
both total nitrogen applied and individual nitrogen treatments. ........................... 207
Table 101. Mean thatch thickness values for the cutting height main effect at
VRC. ..................................................................................................................... 208
Table 102. Mean thatch thickness values for the topdressing main effect at VRC. .... 208
Table 103. Mean thatch thickness values for the nitrogen treatment main effect at
VRC. ..................................................................................................................... 208
Table 104. Selected contrasts comparing thatch thickness at VRC as related to
total both total nitrogen applied and individual nitrogen treatments. ................... 209
Table 105. Mean thatch thicknesses for the cutting height by nitrogen interaction
at VRC. ................................................................................................................. 210
Table 106. Mean thatch thickness values for the cutting height main effect at TTF. .. 211
Table 107. Mean thatch thickness values for the topdressing main effect at TTF. ..... 211
Table 108. Mean thatch thickness values for the nitrogen treatment main effect at
TTF. ...................................................................................................................... 211
xx
Table 109. Selected contrasts comparing thatch thickness at TTF as related to
total both total nitrogen applied and individual nitrogen treatments. ................... 212
xxi
DEFINITION OF TERMS
Below-ground biomass- the structural components of turfgrass plants which reside
beneath the soil surface; namely roots and rhizomes, with a minor component of crowns
and leaf sheathes.
Divot- a portion of turf partially or completely gouged out of surrounding turf by a golf
club head, horse shoe, or studded footwear.
Newton-meter (Nm)- the SI unit for torque, calculated as the product of an applied
rotational force and the length of the lever arm.
Scuff- a portion of turfgrass shoots and thatch gouged from surrounding turf; less severe
than a divot in that most crowns, roots, and rhizomes remain intact.
Shear strength- the tendency of a material to resist strain in a direction coplanar with an
applied force. Generated in turfgrass systems mostly by vegetative stabilization but also
by friction, cementation, and molecular attraction among soil particles.
Shoot- the above-ground portion of an individual turfgrass plant; may be initially derived
from a seed, tiller, stolon, or rhizome.
Sod- a surface layer of turf harvested for transplanting.
Sod strength- the uniaxial tension force required to separate a strip of turfgrass sod into
multiple pieces.
Studded footwear- a type of shoe containing posts on the sole of the shoe that penetrate
into the turf at footstrike; commonly worn by athletes in order to improve traction during
athletic maneuvers.
Thatch- an accumulation of living and dead crown, root, rhizome, and leaf tissues
between the green verdure and the soil surface.
Turf- a community of plants, usually turfgrasses, in consort with the portion of soil in
which the plants grow.
Turfgrass- a plant species which forms a contiguous ground cover and persists under
regular defoliation and traffic.
Verdure- the layer of above-ground green living plant tissue remaining after mowing.
xxii
ACKNOWLEDGEMENTS
I am tremendously grateful to my major professor, Dr. Andrew McNitt. Thank you Andy, for
taking a chance on me as a new graduate student with no prior turf education. You always made
feel like I belonged here, and I feel privileged to have been trained by you.
Thanks are extended to the additional members of my graduate committee, Drs. Jack Watson and
Max Schlossberg. Jack, I admire your enthusiasm for soil science and your ability to engage
students in learning. Max, you’ve taught me more than you can realize; your unique blend of
cerebral thought and informal discourse have greatly heightened my appreciation for both
statistical analysis and turfgrass nutrition.
With regard to my mentors at Penn State, Tom Serensits was also among the most important.
Tom, thanks for answering all my questions, both astute and silly. I am also indebted for your
extensive aid with logistics and data collection. Thanks also to Dianne Petrunak for your practical
advice and camaraderie at various turf conferences and events.
Thanks to Tuckahoe Turf Farms for providing space, materials, and the impetus for this project,
as well as an enlightening glimpse into the practical side of sod production.
I would also like to acknowledge Tom Bettle and the Valentine Research Center staff. Your can-
do attitude and assistance with equipment and other tasks are greatly appreciated.
I am thankful to the numerous Penn State faculty, staff, and students who helped make my time
here both educational and enjoyable. In particular I’d like to recognize my office-mates, Nur
Suhada Abu Bakar and James Banfill. The countless Creamery breaks, late nights at the office,
and sessions of “unstructured learning” I shared with you were critical in my success during these
two years.
Finally, but most importantly, I am grateful to my parents Julie and Jason, and my brother Marco.
I attribute most of my success to the positive attitude and work ethic you have instilled in me, and
to your unwavering encouragement and love.
1
INTRODUCTION
An athletic field should provide a consistent and predictable level of footing for athletes
making maneuvers during competition. The surface should be stable and should not
excessively displace or move when contacted by the athlete. Expectations for quality
natural grass playing surfaces are at an all-time high. These demands are driven by the
availability of sophisticated equipment and scientific knowledge, increased scrutiny on
player safety and performance, and competition with infilled synthetic turf systems.
In an attempt to provide a consistent and predictable surface in all weather conditions,
professional sports stadia have installed very high-sand rootzones to provide excellent
drainage and good air exchange to the turfgrass root system (Canaway, 1983). Without
vegetative or synthetic reinforcement, sand is essentially a non-cohesive soil when
compared with a finer-textured soil containing moderate to low soil moisture (Brady and
Weil, 2007). Thus surface stability on high-sand athletic field rootzones is provided
primarily by the interaction of roots and rhizomes with the sand. When the roots and
rhizomes fail to stabilize the surface, a divot can form. On a natural turfgrass playing
surface, a divot is defined as the complete shearing of turfgrass shoots, crowns, and soil
adhering to roots and/or rhizomes from the soil below (McNitt, 2000). Divoting is of
particular concern on professional American football fields. The athletes possess
exceptional size and speed, and impart accordingly large forces to the surface. The
2
potentially low surface stability of high-sand rootzones can increase certain types of
athlete injuries if adequate turf cover is not maintained (McNitt, 2000); thus, turf
managers aim to maximize the turf’s above-ground cover as well as root and rhizome
production.
Athletes wearing studded footwear cause abrasive, compressive and shearing forces to
the surface during competition. These forces injure the turfgrass plants and reduce the
vegetative cover (Canaway, 1975). Heavy wear from field usage may damage the surface
at a rate faster than the plant community can recuperate. Such damage is commonplace in
American football as approximately 80% of the wear is concentrated over less than 10%
of the field (Cockerham and Brinkman, 1989). This thinning of the turfgrass stand
through either divoting or abrasion results in a decrease of surface stability. When
vegetative cover is reduced the field manager should attempt to remediate the damage by
limiting play, overseeding and allowing the field to recuperate. If the schedule of events
does not permit this remediation the damaged areas must be replaced with sod. Sod is a
layer of turfgrass plants and the adhering soil harvested for transport (Turgeon, 2012). On
high-profile fields the sod is expected to provide a safe and stable surface immediately
after planting. Most National Football League (NFL) stadia install new sod quite
frequently, as often as 2-3 times each year. Consequently, the playing surface stability of
the sod after it is installed at a stadium is largely dependent on the cultural practices
employed at the sod farm prior to harvest.
3
Newly installed sod is inherently less stable than a mature, healthy stand of turf. The
harvesting process is highly stressful to the turfgrass plants as it removes a significant
portion of their root system, hindering the plants’ ability to acquire water and nutrients.
Until the sod is sufficiently re-rooted, the interface between the sod layer and underlying
soil provides a plane of weakness along which portions of the turf can be sheared away
(Turgeon, 2012). In response to these challenges, most sports field managers use sod with
a relatively thick soil layer (up to ~5 cm) for in-season re-sodding. Thick-cut sod is a
relatively specialized product that is used only when there is not enough time for the sod
to root prior to play. It is also expensive as the added weight of soil necessitates
specialized machines to harvest and install the sod and may increase shipping costs up to
4 times that of conventional sod (Trulio, 1994). A thicker cut provides additional weight
to the sod strip to reduce the chance of the strip moving during play. Despite the popular
use of thick-cut sod for in-season repairs of athletic fields, little research-based literature
exists on production methods for thick-cut sod. Research is needed to improve these
methods in order to help growers produce sod with maximum surface stability.
4
LITERATURE REVIEW
The following literature review summarizes prior work concerning this research project’s
objectives. It is divided into four sections: (1) a synopsis of turfgrass sod production (2) a
summary of the influence of primary cultural practices on sod properties, (3) an overview
of thatch management in turfgrass systems, and (4) a review of research methods used to
evaluate the quality of sod and athletic fields.
Overview of turfgrass sod production
Before considering specific research goals pertaining to thick-cut sod, it is instructive to
briefly examine the overall practice of sod production. Production of sod as an
agricultural commodity in the United States began around the 1920s, when strips of
pasture grass were harvested and transplanted to provide rapid ground cover. Since then
the industry has advanced considerably through development of improved cultivars,
mechanized harvesting equipment, and heightened demand for “instant” turf (Beard et al.,
1969). Quality sod is defined by uniform appearance, high shoot density, acceptable
color, adequate carbohydrate reserves for rooting, adequate sod strength for handling, a
minimal thatch layer, and freedom from disease, weed, and insect pests (Beard, 1973).
Sod is produced on a variety of soil types including mineral soils, organic/muck soils, and
various types of organic waste (e.g. dairy manure, composted sewage sludge) (Beard et
al., 1969; Vietor et al., 2002; Tesfamariam et al., 2009). Mineral soils are the preferred
5
substrate for sod that will be planted on athletic fields, due to its greater weight and
stability in comparison to muck soils and organic by-products (Rieke and Beard, 1969).
Production time for cool-season turfgrass sod ranges from 6 to 24 months, depending on
edaphic, climatic, and management factors. In general, sod can be produced more rapidly
on organic soils than on mineral soils (Rieke and Beard, 1969). When possible, a very
sandy soil is chosen in order to match the high-sand rootzone over which the sod will be
installed. This reduces the chance of forming a perched water table at the textural
discontinuity between the sod layer and the rootzone. Such an interface could restrict
drainage and rooting, potentially creating a shear plane along the boundary.
Sod may be harvested as soon as roots and rhizomes have knitted together enough to
allow the sod to be handled without tearing (Beard et al., 1969). Once sufficiently knitted,
the sod is harvested using a machine with a horizontal blade 1-6 cm beneath the soil
surface. The blade severs roots and produces a layer containing soil and the turfgrass
plants. This layer can then be rolled up or stacked, transported, and re-laid on a prepared
site.
In northern U.S. climates, Kentucky bluegrass (Poa pratensis L.) is the predominant
species used for commercial sod production (Watson et al., 1992). Kentucky bluegrass
(KBG) is the only cool-season turfgrass species with a strong rhizomatous growth habit,
which permits the sod to knit together. Tall fescue (Festuca arundinacea) and fine
fescues (Festuca rubra spp. and Festuca ovina spp.) have also been used to produce sod.
Tall fescue provides improved heat and drought tolerance over Kentucky bluegrass, as
6
well as lower nitrogen requirements. However, because Festuca spp. have a
predominantly bunch-type or very weakly rhizomatous growth habits, sod produced
using these species has a low tensile strength and tends to tear apart during harvesting,
transport, and installation. Sod tensile strength is often referred to simply as sod strength.
To mitigate this lack of sod strength, some producers substitute a small percentage (10-
20%) of KBG seed for tall fescue. The KBG rhizomes provide sufficient sod strength for
harvesting, while tall fescue remains the dominant species in the sod. Alternatively,
reinforcing material such as plastic netting can be used to provide sufficient sod strength
for harvesting. The net method can produce harvestable tall fescue sod in as little as 7
weeks (Burns, 1980). The netting adds to production costs and is not preferred for use on
athletic fields as it poses a potential safety hazard if athletes’ studded footwear become
entangled in it. Tall fescue also has a coarse leaf texture, intolerance to close cutting,
slower recuperative capacity, and overall inferior turf quality when compared to KBG.
For these reasons use of tall fescue on sports fields is limited to lower-maintenance
scenarios such as those at community fields and parks, while KBG is preferred for
higher-profile athletic surfaces (Puhalla et al., 2010).
Among KBG cultivars, significant morphological differences exist including leaf texture,
optimum cutting height, shoot density, and lateral spread rate/aggressiveness (Murphy et
al., 2004). Specific attributes of interest in sod production include sod strength and lateral
spread rate. In an evaluation of 103 KBG varieties, Shearman et al. (2001) found that the
strongest cultivars had sod strength 3-5 times that of the weakest cultivars. In addition,
significant differences in lateral spread rate existed among cultivars. Interestingly, sod
7
strength was not correlated with lateral spread rate. This is contrary to the widely-held
assumption that rhizome growth is the main factor associated with both lateral spread rate
and sod strength. Other factors may also be important in these measured characteristics,
such as tiller production and root mass.
Influence of clipping and nitrogen fertilization on turfgrass sod
Factors of interest in this research project include two basic cultural practices- cutting and
fertilization. The following two sections summarize prior research on these practices as
they pertain to sod production and playing surface quality.
Clipping
The relationship between regular defoliation (i.e. clipping) and turfgrass plant
morphology is well documented in literature. A higher cutting height (CH) leads to
increased rooting depth, longer rhizomes, and decreased tiller/shoot density. Conversely,
lower cutting heights produce a greater number of aerial shoots with a shallower root
system and decreased rhizome production (Goss and Law, 1967; Adams et al., 1974;
Nyahoza et al., 1974). The turf compensates for removal of leaf tissue by producing
additional tillers (intravaginal branches) and a more compact growth habit with more
leaves per shoot (Eggens, 1981). This morphological change is attributed to hormonal
effects, primarily triggered by additional light penetration through the canopy. These
signals release axillary buds from apical dominance and permits them to differentiate as
tillers (Simon and Lemaire, 1987). On closely mown turf the leaf angles are more
8
prostrate, as opposed to the upright angles found on higher-cut turf. These adaptations
minimize the loss of photosynthetic area, although close cutting still results in a net
decrease of carbohydrate product (Eggens, 1981, 1982). The net energy loss is probably
the main reason for the decreases in root and rhizome growth associated with low cutting
heights (Juska and Hanson, 1961).
Cutting height may affect divot resistance and therefore is of interest to sports turf
managers and sod producers. Unpublished data from research at The Pennsylvania State
University suggest for certain cultivars, divot resistance is actually increased by closer
cutting (McNitt, 2014, personal communication). This phenomenon may be a function of
greater root density in the uppermost profile, higher shoot density, and/or other factors.
Divot length was negatively correlated to tiller density and below-ground biomass in a
further study of KBG cultivars (Serensits, 2008).
During sod production, height of cut should be within the optimal range for the turfgrass
species being grown. As root and rhizome growth are favored by higher cutting heights, it
might be expected that sod strength would also improve. Satari (1967) reported higher
sod strength under cutting heights of 5.72 cm and 3.81 compared to lower heights.
Other research suggests that within this optimal range, it makes little difference whether
the turf is maintained at the higher or lower ends of the range. In a 3-year study, Mitchell
& Dickens (1979) found varying results with respect to cutting height. In 1973, sod
strength of ‘Tifway’ bermudagrass (Cynodon dactylon x C. transvaalensis) was improved
with higher cutting heights (2.50 cm or 2.00 cm) compared to a lower height of 1.25 cm,
9
while in 1974, the opposite response occurred. In 1975, no significant difference was
observed. The authors suggested the differences may have occurred due to chance rather
than to a repeatable physiological response. Similarly, Li et al. (2011) reported no
difference in sod strength of KBG sod grown on clayey soil when cutting height was
raised from 4.5 cm to 7.5 cm.
Research by Li et al. (2011) and Mitchell and Dickens (1979) tested sod harvested at
standard thicknesses of 1.25 cm and 2 cm, respectively. When sod is harvested at greater
soil thickness, greater sod strength is needed to support the increased mass of adhering
soil. Cutting height therefore may be an important variable when sod is cut at greater
depths.
Given no difference in sod tensile strength due to cutting height, Li et al. (2011)
suggested sod producers maintain their turfgrasses near the high end of the range for
economic reasons; in other words, less-frequent cutting is required for grass maintained
higher heights of cut. It should be noted that this suggestion was geared towards growers
of standard sod, rather than thick-cut sod produced specifically for high-end sports fields.
Even the lower height tested by Li et al. (2011) (4.5 cm) would be considered too tall for
a professional football surface. If the sod were maintained at this height, lowering the
canopy to a more acceptable height would be required immediately before or after sod
installation. This practice would be discouraged due to its imposition of two simultaneous
stresses and aesthetics- a severe reduction in canopy height followed by root pruning (i.e.,
sod harvesting). The likelihood of damage from a MH reduction is compounded when
other stresses are present (Turgeon, 2012). Therefore it is preferable to maintain the sod
10
close to its eventually desired height of cut (~2.5 to 3 cm) during establishment. No
published data exist on the effect of these low cutting heights on KBG during sod
production.
Nitrogen fertilization
Nitrogen is the mineral nutrient required in greatest quantity by higher plants, and in
nearly all cases is the growth-limiting nutrient of a turfgrass system (Kussow et al., 2012;
Marschner, 2012). Maximum biomass production occurs in KBG under extraordinary N
rates of 600-800 kg N ha-1 yr-1 (Kussow et al., 2012). However maximum biomass
production is seldom the goal of turf management. N is instead applied to produce the
desired surface characteristics, which may be of a functional, recreational, and/or
aesthetic nature (Beard, 1973; Bowman, 2003). In addition to management goals,
recommended N rates on athletic fields and sod farms vary substantially according to soil
conditions, turfgrass genotype, and weather patterns.
Turfgrasses typically exhibit shoot priority, meaning that in general shoots are a preferred
sink for photosynthate compared to roots or stems (Bell, 2011). When carbohydrate
supply is limited, the available carbohydrates will be preferentially allocated to
production of leaf tissue rather than roots, crowns or rhizomes (Satari, 1967; Carrow et
al., 2001; Bell, 2011). Shoot priority is exacerbated by abundant N supply because the as
N becomes less limiting, the plants mobilize stored carbohydrates to produce leaf
biomass. Restricting N is hypothesized to favor root growth because of roots’ lower basal
11
N requirement relative to shoots. Under low N availability, the leaves are relatively N
deficient and thus are unable to utilize stored carbohydrates for growth. Roots, however,
are not as sensitive to low N supply and are able to mobilize photoassimilates for
structural growth (Adams et al., 1974).
When shoot growth is too rapid the plants consume carbohydrates at a greater rate than
they can be produced (Juska and Hanson, 1961). Numerous studies have examined the
effects of nitrogen fertilization on sod strength. Data from sod production research
generally support this phenomenon, showing weaker sod under high N, although data
documenting the opposite response have also been published (Kurtz, 1967).
An early study of Merion KBG concluded monthly applications of 17 kg N ha-1 improved
sod strength when compared with higher monthly rates of 34 or 68 kg N ha-1 (Satari,
1967). The sod was harvested 17 months after seeding. However this study was
conducted on an organic muck soil which likely contributed additional N through
microbial mineralization.
Similarly, bermudagrass sod strength was consistently higher when N was applied every
4 weeks at 25 or 50 kg ha-1, compared to 100 or 200 kg ha-1 (Mitchell and Dickens,
1979). Sod was harvested 4 months following a June sprigging. The authors attributed the
lower sod strength under high N to excessive aerial shoot growth, which depleted the
plant’s carbohydrate reserves, suppressing root and rhizome growth, although no data
were presented to support this hypothesis.
12
Li et al. (2011) tested sod strength of 14-month old KBG sod harvested in October. Sod
strength was higher under annual nitrogen applications of 120 kg ha-1 than under N rates
of 240 kg ha-1. N timing was also investigated by Li et al. (2011). Sod strength was
higher when N applications were restricted during the summer months, and the strongest
sod resulted from applying much of the N during fall. Direct measurements of roots and
rhizomes were not performed.
In contrast, Kurtz (1967) found KBG sod strength to increase with higher N rates, up to
175 kg N ha-1 yr-1 . These results were derived from a turf establishment study during
which the turf was seeded in spring and fertilized beginning in July before being
harvested in October. Highest sod strength was obtained with light applications in the
summer and a heavier dose of N at the final fertilizer application in September.
Although N fertilization is a practice fundamental to turfgrass culture, little published
research exists about the effects of N levels on divot resistance. Most research pertaining
to N and athletic playing surface quality has focused on tolerance to abrasive wear and
subsequent recovery. It is sometimes suggested that athletic fields receive higher than
normal fertilization to promote recovery from damage (Puhalla et al., 2010), although
excessive N levels will decrease wear tolerance via greater leaf moisture content and
decreased rooting (Hoffman et al., 2010). It is likely that wear tolerance shows a
quadratic response as N levels increase from zero, showing an initial improvement before
plateauing and eventually declining (Canaway, 1984). Sod transplanted to athletic
playing surfaces is not exposed to wear during production; consequently, a different
approach to N fertilization may be necessary.
13
Turfgrass thatch
Thatch is an intermingled accumulation of living and dead plant tissue which lies
between the green verdure and the soil surface in a turfgrass system. Any factor
increasing the rate of biomass production or decreasing the rate of biomass
decomposition will hasten the development of thatch, although interactions between
buildup and breakdown mechanisms are not fully understood (Beard, 1973; Waddington,
1992). The following subsections review the influence of thatch on athletic playing
surfaces.
Consequences of thatch on athletic playing surface
A small amount of biomass accumulation decreases surface hardness and increases wear
tolerance on athletic fields (Duncan and Beard, 1975; Puhalla et al., 2010). The increase
in wear tolerance is attributed to the thatch layer’s cushioning effect, which shields the
turfgrass crowns from physical injury. The cushioning effect is most beneficial on
frequently trafficked athletic field (e.g., multipurpose high school fields) because wear
injury on these fields is associated with soil compaction and repetitive abrasion of the
turfgrass plants (Canaway, 1975).
Conversely, a thatch cushion provides little benefit on sand-based American professional
football fields. On these fields, soil compaction is minimal due to the single-grained
structure of sand, and most wear injury is associated with formation of large divots rather
than repeated abrasion. Excessive accumulation of organic matter at the surface may
14
reduce surface stability (i.e. divot resistance) and playing quality (Sherratt et al., 2005;
Serensits, 2008).
Other detrimental effects of excessive thatch include slowed infiltration, reduced
fertilizer and pesticide efficacy, increased disease pressure, and proneness to scalping
(McCarty et al., 2007). These effects are detrimental to both playing quality and turfgrass
health.
For the reasons outlined above, NFL sports turf managers generally desire sod with
minimal thatch when resurfacing their fields. Sod would ideally be harvested and
installed before any thatch develops, but in reality this goal is difficult to achieve. The
sod may not be mature enough for use on high-end athletic fields before thatch begins to
form. Additionally, the aggressive cultivars used in athletic field sod production tend to
produce thatch quickly (Shearman, 1980).
Sod growers must take measures to minimize thatch development when producing turf
for NFL stadia. Turfgrass thatch involves interactions among many factors – thatch
reduction practices are widely conducted in the turfgrass industry, yet their effects in
controlled, replicated research trials are often murky. Waddington (1992) noted that
depending on the study, one could reasonably conclude cutting height, N fertilization,
clipping removal, core cultivation, verticutting, and topdressing all either do or do not
affect thatch levels.
15
Effects of clipping on thatch development
Cutting practices have been shown to influence thatch accumulation. Changes in cutting
height alter the plants’ morphology and partitioning of biomass (Shearman, 1989).
Shearman (1980) found that increasing cutting height of Kentucky bluegrass from 2.5 cm
to 5 cm resulted in greater thatch accumulation. A similar effect was observed by Dunn et
al. (1981), who found thatch was thicker in zoysiagrass (Zoysia japonica Steud.) at a
cutting height of 3.8 cm compared to 1.9 cm. The authors suggested the increased thatch
at higher cutting heights is a function of more stem tissue being produced by the plant.
This conclusion was based on stem tissue having higher sclerenchyma content and a
slower degradation rate.
Increased stem tissue production at higher cutting heights was also reported by (Juska
and Hanson, 1961). Their study showed greater rhizome weight for Kentucky bluegrass
mowed at 5.08 cm as opposed to 2.54 cm. Additional rhizome production can lead to
greater thatch accumulation since rhizome tissue degrades slowly (Dunn et al., 1981)
Effects of nitrogen thatch development
High N rates are often blamed for thatch buildup because nitrogen stimulates excessive
biomass production. However, research data indicate N rate plays a lesser role in thatch
buildup than is commonly assumed in the turf industry (Waddington, 1992). For
example, Carrow et al., (1987) determined that bermudagrass thatch accumulation was
not different among a wide range of N rates (96 to 296 kg ha-1 year1).
16
Similarly, Shearman (1980) found no difference in thatch thickness among plots of
Kentucky bluegrass fertilized at either 100 kg N ha-1 or 200 kg N ha-1. Much more
variation existed among cultivars (23 evaluated) and cutting heights (5 cm produced more
thatch than 2.5 cm), suggesting these factors play more important roles in regulating
thatch development compared to N rate.
Weston and Dunn (1985) investigated the effects of N rates on thatch accumulation in
‘Meyer’ zoysiagrass. The authors reported increased thatch development (both thickness
and % organic matter) when 96 kg ha-1 N was applied compared to no N, but no
additional thatch accumulation when the N rate was increased to 196 kg ha-1.
Taken together, these studies suggest when compared to no N fertilization at all, thatch
buildup is initially increased by N applications, but there is a plateau above which
additional N will not affect thatch development. This plateau varies according to species,
soil type, and non-N related management practices.
In addition to N rate, the N source applied to turfgrass can influence thatch accumulation
due to soil acidification. Thatch decomposition is governed by microbial populations and
earthworms which prefer soil with a neutral to slightly acid pH (Murray and Juska, 1970).
Sartain (1985) demonstrated that applying ammonium sulfate (the most acidifying of
common soluble N fertilizers used in turfgrass) significantly increased thatch
development when compared with isobutylidene diurea and a natural organic fertilizer.
The authors concluded that by decreasing the soil pH, ammonium sulfate inhibited
microbial activity and accelerated organic matter accretion. It is also worth noting that the
17
total N recovery from microbially-degraded products can sometimes be less than that of
soluble N salts. Thus the total quantity of plant-available N may have been greater for the
plots receiving ammonium sulfate.
Supplementary cultural practices to manage thatch
Even when correct cutting and fertilization practices are observed, a thatch layer is likely
to form due to the aggressive nature of modern Kentucky bluegrass cultivars. Additional
action must be taken to prevent excessive buildup. Two widely used practices of thatch
removal are core aeration and verticutting. When the cores are harvested following an
aeration event, the thatch within the cores is physically removed from the turfgrass
system. The holes are then filled with fresh sand or soil. Similarly, verticutting physically
removes thatch by invoking a series of vertically aligned blades to tear thatch from
beneath the canopy and deposit it at the surface, where it can be raked or blown into a
pile for removal (Turgeon, 2012). By puncturing the thatch layer, these processes also
expose additional surface area along which aerobic soil microbes can contact the thatch.
Turfgrass managers frequently use these core aeration and verticutting practices to
manage thatch. However, sod producers often wish to avoid core cultivation and
verticutting. These practices sever roots and rhizomes, temporarily reducing sod strength.
Even if the sod remains harvestable, its athletic playing quality may be compromised
once it is transplanted (Kowalewski et al., 2008).
18
A less disruptive means of thatch control known as topdressing involves the application
of a thin layer of soil (usually >90% sand) to the turfgrass surface. Topdressing is often
conducted concurrently with mechanical cultivation (coring and/or vertical cutting) on
established turf. However, some research suggests the mechanical removal can be
omitted so long as sufficient volume of topdressing is applied, and a significant thatch
layer does not already exist (Fermanian et al., 1985; McCarty et al., 2007). Light,
frequent topdressing progressively dilutes the thatch with fresh mineral matter as it
develops, preventing a layer of pure organic material from forming. It also helps keep soil
microorganisms in contact with the organic matter and maintains more constant humidity,
permitting more rapid bio-degradation (Parker, 2011; Beasley et al., 2013).
McCarty et al. (2007) compared various methods of thatch control traditionally used by
turf managers. Topdressing, verticutting (6 or 19 mm depth), grooming (3 mm depth),
and core aerification all maintained baseline organic matter levels in a USGA sand
putting green. The only treatment to actually reduce OM was a combination of coring,
grooming, and verticutting. However, this research suggests that if a turf is topdressed
diligently before a substantial thatch layer develops, topdressing alone may be a viable
option of thatch control.
Similarly, Barton et al. (2009) found topdressing to be equally effective as verticutting at
reducing soil organic matter levels. This effect occurred regardless of whether the soil
had high or low OM at the beginning of the experiment.
19
Among others, these two studies suggest topdressing alone may be used to offset thatch
formation. However sand has little cohesion until roots and rhizomes can stabilize it, and
over-application on sports fields can result in an unstable surface (Kowalewski et al.,
2010). Little controlled research exists on how sand topdressing affects divot resistance.
Additionally, applying sand topdressing to a sod production field is a relatively new
concept and has not been evaluated in research trials.
Research methods used to evaluate surface stability and sod strength
Surface stability on athletic fields
The means by which a divot forms on a sports field is challenging to replicate within a
controlled research setting. Athletes contact the surface at various angles and speeds.
There are considerable differences among players’ body weights and shoe types, not to
mention the many types of motion involved in different sports. All are factors in the type
of force a given athlete applies to the surface. Several mechanisms in turfgrass research
have been used to simulate divot formation and related properties of surface stability.
Divot resistance
The Turf Shear Tester (TST) has been used in divot studies of turfgrass used for both golf
and sports turf (Sherratt et al., 2005; Kowalewski et al., 2011; Trappe et al., 2011;
20
Anderson, 2012). The TST has a 50-mm wide paddle that can be inserted to a depth
specified by the user. The paddle is connected to a lever, which when pulled rotates the
paddle upward until the turf fails. A force gauge measures the amount of rotational torque
necessary to rotate the paddle. A higher torque value is interpreted as more divot resistant
turf. Trappe et al. (2011) evaluated the divot resistance of various zoysiagrass and
bermudagrass cultivars. The researchers found the TST to have less variability in its
measurements as an indicator of divot resistance than divot volume or visual ratings of
divot severity. However, divot volume and visual ratings were measured on divots
produced by golfers actually hitting balls off the research plots. While there was no
statistical difference between the divot sizes produced by each golfer, variation among
golfers may also have contributed to the larger variability in these measurement
techniques.
A device termed Pennswing was used by McNitt and Landschoot (2001) and Serensits
(2008) to evaluate divot resistance of athletic fields. Pennswing creates divots by
impacting the turf surface with the head of a pitching wedge golf club. Shear strength
values from the shear vane described above correlated (r = -0.40) with values obtained by
Pennswing (Serensits, 2008). However, Pennswing was better able to detect differences
among treatments and may be a more precise indicator of divot resistance than the shear
vane.
21
Shear Strength
The Turf-Tec Shear Strength Tester (Turf-Tec International, Tallahassee, FL measures
shear strength in the upper portion of the turf-soil system. Shear strength is defined as the
maximum resistance of a soil or rock to shearing stresses (ASTM D653, 2014). A given
soil’s shear strength is the sum of: (1) electrostatic attraction and cementation between
mineral particles (2) friction between mineral particles during application of the shear
force, and (3) root reinforcement (Yokoi, 1968; Adams and Jones, 1979). In a high-sand
content soil, mineral cohesion and friction are low due to sand particles low surface area;
thus it can be assumed that the majority of shear strength is obtained from vegetative
stabilization (Ross et al., 1991). The Shear Strength Tester device consists of a 7.0 cm
diameter disk mounted to a shaft. The disk has 12 radially-mounted fins of alternating
lengths (2.0 and 1.0 cm) welded to one side. The disk is pressed into the soil surface and
the shaft is rotated until the surface fails under the shearing force. A torque wrench is
attached to the shaft during this process and used to record the maximum force in
Newton-meters (Nm). The Shear Strength Tester is essentially equivalent to the
Eijkelkamp Type 1B shear vane method (Kowalewski et al., 2008; Trappe et al., 2011),
which is no longer in production.
It should be noted that while the term “shear strength” is often used in soil science and
engineering to describe values obtained with the direct shear test (ASTM D 3080, 2011)
or triaxial tests (ASTM D7181, 2011), the Shear Strength Tester is more similar to the in-
situ system is tested in a “drained” condition (i.e. pore water pressure ~0) rather than an
“undrained” or saturated state, for which ASTM D2573 (2008) is intended. The shearing
22
mechanisms exerted by the Shear Strength Tester still follow the generalized Mohr-
Coloumb failure criterion:
Τ = σ' tan(φ') + c'
Where τ is the critical shear strength, σ' is the normal stress (perpendicular to the shear
plane), φ' is the angle of internal friction (a property inherent to a given soil), and c' is
the cohesion (Fig. 1) (Handin, 1969). Cohesion is defined in soil mechanics as a soil’s
shear strength when the compressive stresses are zero (Yokoi, 1968). This definition
differs somewhat from the conventional soil physics definition of cohesion as the
electrostatic forces which attract soil particles toward one another. Due to low inherent
cohesion of the engineered sand media commonly used in turfgrass culture, cohesion is
mostly a function of vegetative stabilization.
The measurement of shear strength with the Shear Strength Tester is simplified in that the
first term of the Mohr-Coloumb equation is essentially zero, since no confining pressure
(i.e. downward force) is used during the test. Thus the shear strength is essentially a
function of root and rhizome stabilization.
23
Figure 1. Generalized Mohr-Coulomb failure envelope; soil fails when the shear stress
exceeds the normal stress (confining pressure) as indicated by red line. Vegetative
stabilization is responsible for most of the cohesion intercept in sand-based turfgrass
systems.
24
Sod strength
Sod strength (SS), sometimes known as sod tensile strength, is defined as the longitudinal
force necessary to separate a piece of sod (Beard et al., 1969). A minimum SS is required
for successful harvest, handling, and installation of turfgrass sod. . Sod strength is
measured by applying a tensile force to a strip of sod until the strip tears. The applied
force at which the sod fails is considered the SS. This measurement is useful in
determining whether a given piece of sod is considered harvestable. Minimum SS values
for commercially harvestable sod range from 20-50 kg depending on the study (Li et al.,
2011). Such values are applicable to standard-size, palletized sod rather than “big rolls”
measuring up to 1.2 m wide and 19.8 m long. Such rolls would require additional sod
strength for harvesting, Researchers have investigated sod grown under climatic and
edaphic environments, as well as several turfgrass species. It is therefore instructive to
consider SS as a function of per unit cross-sectional area to standardize values across
studies. The amount of force per unit cross sectional area can be used to standardize
across sod strip dimensions. The minimum value for thick-cut sod is unknown, but is
likely higher than for standard-depth sod since during handling and installation the sod
must support additional force due to the greater soil weight.
The majority of devices used to test SS are variations of the same basic apparatus. This
type of machine consists of two horizontal platforms. One platform is fixed to the frame
and the other can be slid along a set of tracks. Both platforms have clamps that are
fastened to either end of a sod strip. The original design used by Rieke et al. (1968) used
a bucket hanging from a pulley to apply tensile force to the sod strip. Sand was added to
25
the bucket at a constant rate until the sod failed. The weight of sand in the bucket was a
measure of the sod’s strength. The device described by Jagschitz (1980) invoked an
electric motor to apply the tensile force rather than a sand bucket. A force gauge recorded
the maximum tensile force required to tear the sod strip, which was expressed as a mass
per unit area (i.e., kg dm-1). Other versions of this type of machine incorporated a lever or
wheel and a standard torque wrench. The device built by Parrish (1995) used a lever
mounted on the movable platform, rather than a pulley arrangement. The lever is rotated
up to 20 degrees, which was sufficient displacement to break a sod strip. The maximum
torque required to tear the sod strip was recorded.
A more sophisticated device incorporating an Instron Universal Testing Instrument has
been used in some sod studies (Burns and Futrap, 1979; Ross et al., 1991). The Instron
instrument was a force gauge initially developed for research and quality control of
engineering and textile materials. It can measure the elastic and plastic displacement of
an object as it is broken, with measurements taken at high frequencies throughout the
breaking process. However Burns and Futrap (1979 ) and Ross et al. (1991) adapted the
gauge to test SS. This device has the added advantage of measuring how much total
elongation the sod strip undergoes before tearing. The Instron device also records the
amount of elongation that occurs for each unit of added tensile force (i.e. breakage
pattern).
26
OBJECTIVES
Installation of thick-cut sod is an important strategy for managers of high-end athletic
facilities. By resurfacing a playing field with thick-cut sod, the surface can be used
almost immediately. In some cases, a field may be re-sodded so frequently that the turf
management practices at the sod farm have equal or greater influence on playing surface
quality than the management at the field itself. Despite its widespread use in sports such
as American football, little scientific research has investigated the production of thick-cut
sod. The goals of this research project are:
(1) Maximize divot resistance while maintaining tensile strength of thick-cut KBG
sod through manipulation of pre-harvest cultural practices
(2) Determine the effects of mowing height, sand topdressing, and nitrogen
fertilization on shoot density, thatch accumulation, and below ground biomass and
their relationship to divot resistance
27
MATERIALS AND METHODS
Although thick-cut Kentucky bluegrass sod is commonly installed on NFL playing fields,
little research-based knowledge exists about the production of this type of sod. The
Literature Review section of this thesis summarized the need for thick-cut sod and how
cutting, topdressing, and nitrogen fertilization practices influence the properties of
turfgrass plant communities.
Research was conducted to clarify how cutting, topdressing, and nitrogen fertilization
practices translate to thick-cut sod production, and how they ultimately impact surface
stability on athletic playing fields. This project consisted of two experiments: Experiment
1 – The effects of cutting height, topdressing, and nitrogen fertilizer regime on the divot
resistance of thick-cut Kentucky bluegrass sod, and Experiment 2 – The effects of
varying cutting height on the divot resistance and tensile strength of thick-cut Kentucky
bluegrass sod. The following sections detail how the experiments were conducted.
28
Experiment 1: The effects of cutting height, topdressing, and nitrogen fertilization
on the divot resistance of thick-cut Kentucky bluegrass sod
Overview of Experiment
Experiment 1 evaluated the effects of cutting height, sand topdressing, and nitrogen
fertilization on several response variables. Plots were subjected to one of two cutting
heights, one of two topdressing treatments, and one of six nitrogen regimes. Response
variables measured in the field included divot resistance, surface shear strength, and sod
tensile strength. Plugs were removed from each plot and analyzed in the laboratory for
thatch thickness and mass, tiller density, and below-ground biomass. The experiment was
replicated at two geographic locations.
Plot Establishment
Field plots for Experiment 1 were established at two locations: the Joseph Valentine
Turfgrass Research Center (“VRC location”- University Park, PA), and Tuckahoe Turf
Farms (“TTF location”- Hammonton, NJ). Tuckahoe Turf Farms holds over 1400 acres
within the New Jersey Pine Barrens, with roughly half the land in cool-season turfgrass
sod production at a given time. The native soils at the TTF location are sands and sandy
loams of fluvio-marine origin (National Resource Conservation Service, 2014). The
sandy texture of these soils makes TTF an ideal location for producing athletic field sod.
The native texture closely mimics that of the sand-based rootzones commonplace at
29
professional sporting venues, thus minimizing the textural discontinuity at the sod-soil
interface. A large discontinuity would likely impede drainage and inhibit root growth by
creating a perched water table (Li et al., 2013). A soil sample taken from the
experimental area contained 0.8% gravel, 85.2% sand, 12.3% silt, and 1.6% clay. A
detailed particle size analysis is given in Table 1. The treatments and experimental design
were identical at the two locations.
Plots were established to a Kentucky bluegrass blend containing the following cultivars:
30% ‘Everest,’ 30% ‘Boutique,’ 30% ‘P-105,’ and 10% ‘Bewitched.’ The experimental
area at the TTF location was seeded in October 2012. The turf that was utilized at the
VRC location was seeded with this blend in August 2012 at the TTF location and later
transplanted to University Park as described below. In both cases the turf was mowed as
necessary and fertilized twice in the fall of 2012, to supply a total of 84 kg N ha-1, 37 kg
P ha-1 139 kg K ha-1.
The sod used at VRC was harvested from the TTF location on April 12, 2013 as 1.21 m
wide x 9.14 m long rolls at a 3.8 cm soil depth. The sod was installed over a rootzone
profile conforming to United States Golf Association (USGA) specifications for greens
construction (USGA, 2004). The profile consisted of a 15 cm gravel drainage blanket
overlain by a 2.54 cm intermediate layer of fine gravel and coarse sand and 25 cm of an
80-20 sand-peat blend. The profile was originally constructed in 2001, and through years
of sand topdressing the rootzone thickness had increased to 35 cm by the time of this
project. The existing sod and top 3.8 cm of the profile were removed to allow the
30
experimental sod to be installed to a flat grade with the surrounding areas. Thus the
rootzone depth prior to sodding was approximately 30 cm. A particle size distribution of
the top 15 cm of rootzone material is shown in Table 2. Plot size at VRC was 1.2 m by
2.4 m. Plots at TTF measured 1.8 m by 1.8 m.
31
Table 1. Particle size distribution for Ap horizon at the TTF location†.
Size fraction Percent of sample
----- mm -----
--- % by mass ---
>2.0
0.8%
2.0-1.0
5.4%
1.0-0.5
23.1%
0.5-0.25
35.2%
0.25-0.15
14.8%
0.15-0.05
6.7%
0.05-0.002 12.3%
<0.002
1.6%
Total 100.0%
† As determined using ASTM F-1632-10 (2010)
Table 2. Particle size distribution of the top 15 cm of rootzone material at VRC†.
Size fraction Percent of sample
----- mm -----
--- % by mass ---
>2.0
0.4%
2.0-1.0
3.9%
1.0-0.5
19.6%
0.5-0.25
46.2%
0.25-0.15
17.6%
0.15-0.05
5.1%
0.05-0.002
4.8%
<0.002
2.4%
Total 100.0%
† As determined using ASTM F-1632-10 (2010)
32
Treatment Application
Plots were subjected to one of two cutting heights (3.18 cm and 3.81 cm). At TTF the
plots were mowed with a 5-reel Jacobsen® fairway mower. At the VRC location plots
were mowed with a walk-behind rotary mower. Plots at both locations were mown twice
weekly. Clippings were returned at both locations.
Sand topdressing treatments were applied to the plots using a drop spreader. The
appropriate mass of dry sand was weighed with a scale and applied evenly in two
directions. Rates were chosen on the basis of the observed thatch development rate. The
first two applications (May and June) were made at 3.4 kg sand m-2 and the September
application was made at 1.7 kg sand m-2 to total 8.5 kg sand m-2 for plots receiving
topdressing. Particle size analyses for the topdressing materials used at both locations
appear in Tables 3 and 4.
Two applications of soluble nitrogen were made to the entire experimental area in early
spring of 2013 (March 15 and April 1) prior to the start of experimental treatments. Each
application equated to approximately 49 kg N ha-1. N treatments included 0, 1, or 2
additional N applications (49 kg N ha-1) during the spring plus either 0 or 1 application
(also 49 kg N ha-1) in the fall. Thus the “negative control” plots received a total of 98 kg
N ha-1 during the entire experiment, while the most heavily fertilized plots received a
total of 244 kg N ha-1. The 244 kg N ha rate was similar to the actual N rate used by TTF
in thick-cut sod production, and thus served as a "positive control." The N source was
33
granular ammonium sulfate (21-0-0). Fertilizer was applied in two perpendicular
directions by hand using a shaker jar. The 12 combinations of topdressing treatment x
nitrogen regime are presented in Table 5.
Due to travel time between the sites, treatments were not applied on the same days at
both locations. However, the treatments were applied on as similar a schedule as possible
to maintain consistency between the sites. Fertilizer and topdressing were applied on the
following dates: 10 May, 6 June, and 6 September (TTF location); 13 May, 9 June, and
12 September (VRC location).
34
Table 3. Particle size analysis for the topdressing sand used at TTF.
Size fraction Percent of sample†
----- mm -----
--- % by mass ---
>2.0
0.0%
2.0-1.0
1.1%
1.0-0.5
15.9%
0.5-0.25
55.3%
0.25-0.15
23.8%
0.15-0.05
2.8%
0.05-0.002 0.8%
<0.002
0.4%
Total 100.0%
† As determined using ASTM F-1632-10 (2010)
Table 4. Particle size analysis for the topdressing sand used at VRC.
Size fraction Percent of sample†
----- mm -----
--- % by mass ---
>2.0
0.0%
2.0-1.0
0.3%
1.0-0.5
36.7%
0.5-0.25
52.5%
0.25-0.15
7.2%
0.15-0.05
0.9%
0.05-0.002 0.8%
<0.002
0.2%
Total 100.0%
† As determined using ASTM F-1632-10 (2010)
35
Table 5. Monthly schedule of topdressing and nitrogen treatments for Experiment 1.
Topdressing
rate
Number of N
applications Mar.† Apr.† May† June† July† Aug.† Sept.† Oct.† Nov.†
-- kg sand m-2 -- -- no. in spring-fall,
respectively --
0 2-0 N N
0 3-0 N N N
0 4-0 N N N N
0 2-1 N N
N
0 3-1 N N N
N
0 4-1 N N N N
N
8.5 2-0 N N T T
T
8.5 3-0 N N TN T
T
8.5 4-0 N N TN TN
T
8.5 2-1 N N T T
TN
8.5 3-1 N N TN T
TN
8.5 4-1 N N TN TN TN
† N indicates an application of 49 kg N ha-1.
T indicates an application of sand topdressing. May and June applications were made at 3.4 kg sand m-2 and September applications were made at 1.7 kg sand m-2.
36
Experimental Design
TTF had previously decided to maintain half of the sod production field utilized in this
experiment at 3.18 cm and the other half at 3.81 cm. While the effect of mowing height
was of interest in this research project (i.e. fixed effect), it was impractical to make
mowing a completely random factor, or even a split-plot or strip-split plot factor. The
time and effort required to mow individual, randomly located plots with the wide gang-
style reel mowers used at TTF would be prohibitive to such a design. Therefore a design
was chosen in which plot areas were established on both ends of the sod field, one under
each MH. The two areas were considered separate experiments and the main effect and
interactions associated with mowing height were tested using a combined analysis and
the error terms specified by (McIntosh, 1983). Within each mowing height “experiment,”
the two topdressing treatments and six nitrogen treatments comprised a 2x6 factorial
experiment arranged in randomized complete blocks with three replications.
Additional Plot Maintenance
Irrigation
At TTF, irrigation water was applied every 3-4 days with a pivot-type irrigation system.
At VRC, overhead irrigation was applied to prevent drought stress. Rainfall data for each
location are presented in the Results section of this thesis.
37
Fertilization
Soil tests revealed low levels of potassium at both locations. Potassium was supplied to
each location via three applications over the course of the season. Each application
consisted of 41 kg K ha-1 from potassium sulfate (0-0-50). No other pH or nutrient
deficiencies were detected.
Plant growth regulators
Beginning in May, all plots were treated with the gibberellin inhibitor trinexapac-ethyl.
At TTF, the TE was applied on 3-week intervals. At VRC, the TE was applied at the label
rate of 0.20 kg a.i. ha-1 every 28 days using a spray volume of 815 L ha-1.
Fungicides
At TTF, one application of propiconazole fungicide was made on 4 June to control rust
disease (Puccinia spp). At VRC, fungicide applications were made on 16 July, 24 July,
and 4 September to control dollar spot (Sclerotinia homoeocarpa) and other pathogenic
fungi. The three applications consisted (respectively) of chlorothalonil, a
vinclozolin/triticonazole tank mix, and a propiconazole/chlorothalonil tank mix.
Herbicides
One application of the pre-emergent herbicide prodiamine was made to the TTF plots on
6 May 2013 as a preventative control of crabgrass. No herbicides were applied at VRC.
38
Insecticides
No insecticides were applied at TTF. At VRC, one application of imidacloprid insecticide
was made on 1 July 2013 to preventatively control white grubs.
Treatment evaluation
Sod was cut and tested November 8-16 at VRC and November 18-22 at TTF. Late
November was chosen because American professional football fields are typically re-
sodded at that time of year. Sod was cut using a Ryan HD walk-behind sod cutter with a
46 cm cutting width. The sod profile was 4.45 cm thick, a standard thickness for sod
installed mid-season on football fields.
Half of each treatment plot was used to evaluate divot resistance and surface shear
strength. This portion of sod was severed from the underlying soil with the sod cutter, but
not removed from the surface. This produced a turfgrass surface that simulated a newly
sodded football field. The other half of each treatment plot was cut and removed to
evaluate sod tensile strength (Fig. 2).
After field measurements were complete, two 5.08-cm diameter cores were removed
from undamaged areas of each treatment plot with a tubular turf plugger (Turf-Tec
International, Tallahassee, Florida). Since the sod in each plot had already been severed
from the underlying soil, each plug measured 4.45 cm high. The cores were refrigerated
39
at 5 °C until they were analyzed. Measurements of thatch thickness, tiller density, thatch
mass, and below-ground biomass were collected from each plug. Both plugs from each
plot were analyzed and the average of the two values was used to represent that
experimental unit.
40
Figure 2. Sod strips being removed for sod strength testing at TTF.
41
Divot resistance
Divot resistance was evaluated with the “Pennswing” apparatus described by McNitt &
Landschoot (2001) and Serensits (2008). Pennswing consists of a pitching wedge golf
club head mounted on a pendulum. A 70 kg lead cylinder is fixed to the shaft to provide
additional force upon impact with the turf (Fig. 3-4). The device is mounted to the three-
point hitch of a tractor. Two adjustable metal pads support the device as it is lowered to
the turf surface. Six 11.3 kg cylindrical weights were secured to the device, to stabilize it
upon club head impact. Two fabric ratchet straps were fastened between the tractor axles
and Pennswing to further secure the device. The pendulum was released from a
horizontal position to ensure a consistent striking force for each divot. The club head
caused partial or complete shearing of the turf as it impacted the surface.
Divot length and width were measured with a standard ruler. Divot depth was measured
using a point gauge (Fig. 5). A “T” shaped rod was fitted into the gauge’s groove, and the
gauge was placed across the divot with its base resting evenly on the divot’s edges. The
rod was lowered to the deepest portion of the divot, the depth recorded and the “blank”
reading subtracted to yield the actual vertical displacement in centimeters.
Plots with small divots were considered to have high divot resistance. The dimensions of
three divots per plot were averaged to produce a representative divot length, width, and
depth for each experimental unit.
42
Figure 3. The Pennswing device about to be released.
Figure 4. Examples of divots produced by Pennswing. Photo at left shows a plot with
low divot resistance while the photo at right shows a plot with high divot resistance. 30
cm ruler included for scale.
43
Figure 5. Point gauge used to measure divot depth. Device is placed across the
divot and the metal rod is lowered to the bottom of the depression. Height is read
and the reference height is subtracted to obtain the divot depth in cm.
Actual divot depth
Reference height
Measured height
44
Shear strength
Shear strength was measured using the Shear Strength Tester (Turf-Tec International,
Tallahassee, Florida). The device was built to replicate the Eijkelkamp device described
in the literature review of this thesis. The vertical fins were inserted into the turf and the
device was rotated slowly with no down pressure until the turf gave way (Fig. 6-7). A
follower needle registered the maximum torque applied to the device in Newton-meters
(Nm). Three shear measurements were taken per plot with the average of the three used
to represent the shear strength of that experimental unit.
Sod strength
Sod strength (SS) was evaluated from the half of each plot not used to measure divot
resistance or shear strength. The apparatus used to measure sod strength was constructed
at Penn State, on the basis of previous published designs (Rieke et al., 1968; McCalla et
al., 2008). The device consisted of two platforms, one fixed to a large metal frame and
the other movable along a set of wheeled tracks (Fig. 8). A sod strip was secured to the
device via two hand-tightened clamps on each platform. Each clamp assembly had
several brass golf spikes on the inward-facing sides, preventing the sod from slipping as
the device was operated. One end of a braided steel cable was attached to the movable
platform, with the other end secured to a force gauge (Chatillon DFS II-1000, AMETEK
Test & Calibration Instruments, Largo, Florida) mounted on the fixed platform.
45
Figure 6. Operation of the shear strength measurement device; handle was
turned clockwise in the horizontal plane until the turf failed.
Figure 7. Fins on the Turf Shear Tester.
46
Figure 8. Operation of the sod strength device.
Figure 9. Sod strip following tensile strength test.
47
To test a piece of sod, a winch connected to the steel cable was activated by a 12-volt
marine battery until the sod ruptured (Fig. 9). The force gauge recorded the maximum
tensile force experienced during the test. STS was recorded for three sub-samples per
plot, which were averaged to produce a representative value for that experimental unit.
Soil volumetric water content
Soil volumetric water content in the top 3.8 cm was recorded for each plot using a Field
Scout TDR 300 probe (Spectrum Technologies, Inc., Aurora, Illinois).The average of
three measurements was used to represent the value for that plot.
Visual turfgrass quality (1-9 scale)
Visual ratings of turfgrass color were recorded in November using a protocol established
by the National Turfgrass Evaluation Program (NTEP) (Morris and Shearman, 1999).
Color was evaluated on a 1-9 scale, with 1 being poorest and 9 being best. A rating of 6 is
generally considered acceptable, though “acceptable” thresholds for turfgrass color vary
according to the turf’s function.
Thatch thickness
Thatch thickness was measured with a standard ruler under the compression of a 450 g
weight. The plug was first placed in a vertically-slit PVC cylinder to enable the plug to be
48
viewed (Fig. 10). The cylinder helped stabilize the weight atop the plug and provided a
straight edge along which to measure the thatch layer.
Shoot density
After thatch thickness was measured, verdure was removed using scissors. Two rubber
bands were placed around the vertical axis of the plug to divide the surface into four
quadrants and simplify the counting process (Fig. 11). The aerial shoots (tillers) on the
plug were counted. This value was divided by the plug’s cross-sectional area to produce a
density value (i.e. number of shoots per unit area).
Thatch mass and below-ground biomass
The remaining portion of the plug was sectioned to separate the thatch layer from the
portion of the plug containing soil (Fig. 12). The soil portion was enclosed between two
60-mesh sieves and immersed in a tub of water. The sieves were agitated by hand to
remove most of the soil from the roots and rhizomes. The total amount of roots and
rhizomes in this portion of the plug was considered below-ground biomass (BGB).
Thatch and BGB were dried in separate crucibles at 60° C. The crucibles were then
weighed, heated to 440°C for 16 hours, and re-weighed. The loss on ignition was used to
represent the mass of the thatch or BGB contained in the plug (Fig. 13).
49
Figure 10. Measurement of thatch layer thickness.
Figure 11. Sample plug following removal of verdure for tiller count
50
Figure 12. Plug being sectioned into thatch portion (lower left) and
soil/below-ground biomass (upper right).
Figure 13. Examples of thatch (left) and washed below-ground biomass
samples (right); top image shows samples before ashing and bottom image
shows the same samples following exposure to 440 °C for 16 hours.
51
Statistical Analysis
Data were subjected to analysis of variance using mixed models in the SAS system (SAS
Institute Inc., Cary, North Carolina). An initial analysis identified large statistical
differences among the two locations, so it was deemed more appropriate to analyze each
location separately, as opposed to pooling data from both locations for a combined
analysis. Within each location, data from both cutting heights were pooled after testing
for homogeneity of variance and the main effect of cutting height was tested as a fixed
effect. This analysis utilized the appropriate error term for combined experiments
(McIntosh, 1983). Topdressing and nitrogen fertilizer regime, as well as the associated
interactions among these factors and cutting height were treated as fixed effects. Block
was nested under cutting height and considered a random effect. Volumetric water
content was used as a covariate in the analyses of field data.
Main effects and interactions were considered statistically significant if an F-test yielded
a p-value less than 0.05. A sole exception was for below-ground biomass data; due to the
high degree of variability normally associated with root quantification, an alpha value of
0.1 was chosen prior to data analysis for below-ground biomass only.
When the p-value was smaller than the critical alpha value, means were separated with
Fisher’s least significant difference (LSD) test. The LSD value expressed the minimum
difference between two treatments necessary for them to be considered statistically
different. Per the stipulations of Fisher’s Protected LSD test, LSD values were not
calculated for analyses yielding non-significant p-values.
52
Selected linear contrasts were computed for the six nitrogen regimes, in order to compare
specific regimes and the individual influences of N rate and N timing. Null hypotheses
for these contrasts appear in Table 6.
Spearman correlation coefficients were calculated to determine whether measured
parameters were linearly related to one another. The measured parameters included divot
length, width, and depth, sod tensile strength, shear strength, thatch mass and thickness,
shoot density, and below-ground biomass.
53
Table 6. Notation for the linear contrasts performed to compare specific N treatments and
treatment combinations.
Contrast label Nitrogen applications†
A 0 fall vs. 1 fall
B 3 total vs. 4 total
C 4 total vs. 5 total
D 3 total vs. all other rates
E 5 total vs. all other rates
†
Each application supplied 49 kg N ha-1. Labels indicate which N treatment
combinations were pooled for the contrast. Nomenclature for each contrast's
null hypothesis is as follows:
A H0: μ 2-0, 3-0, 4-0 = μ 2-1, 3-1, 4-1
B H0: μ 2-1, 3-0 = μ 3-1, 4-0
C H0: μ 3-1, 4-0 = μ 4-1
D H0: μ 2-1, 3-0 = μ 2-0, 3-1, 4-0, 4-1
E H0: μ 4-1 = μ 2-0, 2-1, 3-0, 3-1, 4-0
54
Experiment 2: The effects of varying cutting height on the divot resistance of thick-
cut Kentucky bluegrass sod
Overview of Experiment
Space and labor limitations prevented experiment 1 from being conducted at more than
two cutting heights. Experiment 2 was designed to investigate the effects of additional
mowing heights and was conducted only at VRC.
Plot Establishment
Plot establishment for Experiment 2 was identical to that of Experiment 1 at the VRC
location.
Treatment Application
Plots were mown twice weekly at one of four cutting heights (2.54, 3.18, 3.81, or 4.45
cm). All plots were maintained at 3.18 cm for four weeks after sod installation.
Treatments were then "applied” at each mowing event throughout the growing season, up
until the simulated sod harvest.
55
Experimental Design
The experimental design was a randomized complete block. Each treatment was
replicated three times.
Additional Plot Maintenance
Fertilizer and sand applications followed the schedule of treatment 8 in Experiment 1 —
two N applications in the spring, no N applied in the fall, and topdressed three times to
total 8.5 kg sand m-2 (Table 1). Other plot maintenance (irrigation, K fertilization, and
applications of growth regulators/pesticides) was identical to that of Experiment 1.
Treatment evaluation
Data collection for Experiment 2 was identical to Experiment 1. Measurements of divot
resistance, shear strength, sod tensile strength, and turfgrass color were collected in the
field from 8 Nov-16 Nov, 2013. Following field evaluation, plugs were collected from
each plot and analyzed for thatch thickness, tiller density, thatch mass, and below-ground
biomass as in Experiment 1.
Statistical Analysis
Data were analyzed in the SAS system using the GLM procedure. Where statistically
significant differences occurred (p<0.05), means were separated with Fisher’s LSD.
56
RESULTS
The goal of this research project was to optimize cultural practices for thick-cut KBG sod
production, in the context of athlete safety and playability. Several dependent variables
were measured to evaluate the experimental treatments. Divot resistance, sod strength
and shear strength comprised the data collected from the experimental units in the field,
while shoot density, thatch accumulation, and below-ground biomass were evaluated in
the lab from samples extracted from the field plots. Values obtained from these
measurements are presented and compared in this section. Visual assessments of turfgrass
color were also made in November 2013. The visual quality of sod was not a focus of this
project, but these values are included for reference in the Appendix.
This section is divided into three main subsections: one for each experiment described in
the Methods section of this thesis, and a third to present correlations among the measured
parameters. In Experiment 1 notable disparities occurred between the VRC and TTF
locations; thus data are presented separately for the two. The trends across locations are
then compared. Within each location, the results are is further subdivided by response
variable. For consistency, all main effects are presented regardless of statistical
significance.
57
Experiment 1: The effects of cutting height, topdressing, and nitrogen fertilization
on the divot resistance of thick-cut Kentucky bluegrass sod
Joseph Valentine Research Center (VRC) Location
At the VRC location all dependent variables were significantly affected by at least one
treatment with the exception of divot width (Tables 7-8). Prior research with the
Pennswing apparatus has suggested divot length is the best indicator of divot resistance
while width and depth are mostly controlled by the swing plane and path of the device
(McNitt, 2000; Serensits, 2008). Divot lengths likely have the greatest practical value;
however, divot depths were affected in a similar fashion to lengths. For the sake of
consistency, data treating all three dimensions are reported below regardless of statistical
significance.
58
Table 7. Summary of treatment effects on field parameters in Experiment 1 at VRC
NS = not significant, * = significant at 0.05 level, ** = significant at 0.01 level
Source Degrees of freedom
Divot
Length
Divot
Width
Divot
Depth
Sod
Strength
Shear
strength
Cutting height ( C ) 1 NS NS NS NS *
Block (B) 2 N/A N/A N/A N/A N/A
CB (Error 1) 2 N/A N/A N/A N/A N/A
Topdressing (T) 1 NS NS NS ** **
Nitrogen treatment (N) 5 * NS * ** NS
CT 1 NS NS NS NS NS
CN 5 NS NS NS NS NS
TN 5 NS NS NS NS NS
CTN 5 NS NS NS * NS
Volumetric water content 1 NS NS NS NS NS
Residual error 43
Total 71
59
Table 8. Summary of treatment effects on laboratory parameters in Experiment 1 at VRC.
Source Degrees of freedom Shoot density Thatch Mass Thatch Thickness Below-ground Biomass
Cutting height ( C ) 1 * * * +
Block (B) 2 N/A N/A N/A N/A
CB (Error 1) 2 N/A N/A N/A N/A
Topdressing (T) 1 NS ** ** **
Nitrogen treatment (N) 5 ** ** ** +
CT 1 NS ** NS **
CN 5 NS ** ** **
TN 5 * NS NS NS
CTN 5 NS NS NS NS
Residual error 44
Total 71
NS = not significant, + = significant at 0.1 level * = Significant at 0.05 level,** = significant at 0.01 level
60
Divot length
Cutting height main effect-
Cutting height did not significantly affect divot length at VRC. The mean divot length
values for the two cutting heights were within 1 cm when averaged across all topdressing
and N treatments (Table 9).
Topdressing main effect-
Topdressing did not significantly affect divot length at VRC (Table 10). Topdressed plots
had slightly longer divots when compared to the control receiving no topdressing.
Nitrogen treatment main effect-
Significant differences were observed between nitrogen treatments at the VRC location
(Table 11). The longest divots at VRC were produced on plots receiving the 4-1 N
treatment. Fisher’s Protected LSD test indicated that these divots were larger than divots
from all other nitrogen treatments.
Experimental units treated with the 3-0 N treatment had the shortest mean divot length
(21.0 cm); however, this mean length was not significantly different from any treatment
other than the 4-1 N treatment using the protected LSD separation. (Table 11; Fig. 14).
Selected contrast statements were computed to further compare N treatments (Table 12).
Contrast statements permit comparison of an individual treatment of interest with the
61
mean of several other treatments. For example, the mean divot length of the 4-1 N
treatment was tested against the mean divot length of all other N treatments pooled
together. Additionally the difference between two pre-specified treatment groups can be
tested. For example, Table x also compares the mean divot length of all experimental
units receiving the fall N application (2-1, 3-1, and 4-1 treatments) with the mean divot
length of all those which did not (2-0, 3-0, and 4-0 treatments). In general, plots receiving
three total N applications had shorter divots than those receiving two, four, or five total
applications.
Three contrasts showed statistically significant differences in divot length as a function of
the N treatment. Plots receiving 3 total N applications (mean of 2-1 and 3-0 treatments)
had significantly shorter divots than the four other treatments averaged together. Plots
receiving 4 total applications had shorter divots than those receiving 5 total applications.
Similarly, the 4-1 application schedule had longer divots than the mean of all other N
treatments. The contrast between plots receiving fertilizer in fall and those not receiving a
fall application indicates divot lengths were shorter for the plots not fertilized in fall,
although the difference was small and not statistically significant.
Interactions-
No significant two-way or three-way interactions on divot length occurred at VRC
between cutting height, topdressing, and nitrogen treatment treatments.
62
Table 9. Mean divot lengths for the cutting height main effect at VRC.
Cutting height Divot length Letter grouping
---- cm ---- ---- cm ----
3.18 24.4 -
3.81 23.9 -
LSD (0.05) NS -
Table 10. Mean divot lengths for the topdressing main effect at VRC.
Topdressing applied Divot length Letter grouping
---- kg sand m-2 ---- ---- cm ----
0.0 23.3 -
8.5 25.1 -
LSD (0.05) NS -
Table 11. Mean divot lengths for the nitrogen treatment main effect at VRC.
Nitrogen treatment† Divot length Letter grouping
---- cm ----
2-0 24.7 B
2-1 22.0 B
3-0 21.0 B
3-1 24.3 B
4-0 23.6 B
4-1 29.4 A
LSD (0.05) 4.6 -
† indicates number of N applications (each at 49 kg N ha-1) made in the spring
and fall, respectively
63
Figure 14. Mean divot lengths for the nitrogen treatment main effect at VRC. Treatments
with overlapping error bars are not statistically different using Fisher’s Protected LSD.
10.0
15.0
20.0
25.0
30.0
35.0
2-0 2-1 3-0 3-1 4-0 4-1
Div
ot
len
gth
(cm
)
Nitrogen treatment
(no. of applications of 49 kg N ha-1 in spring-fall, respecitvly)
64
Table 12. Selected contrasts comparing divot lengths at VRC as related to total both total nitrogen applied and individual nitrogen
treatments.
Contrast label Nitrogen applications† Divot length
Difference relative to
second value Pr > F
Statistical
Significance ††
------ cm ------ -------- % --------
A 0 fall vs. 1 fall 23.1 vs. 25.2 -9% 0.115 NS
B 3 total vs. 4 total 21.5 vs. 24.0 -10% 0.131 NS
C 4 total vs. 5 total 24.0 vs. 29.4 -18% 0.009 **
D 3 total vs. all other rates 21.5 vs. 25.5 -16% 0.006 **
E 5 total vs. all other rates 29.4 vs. 23.1 +27% 0.001 **
† Each application supplied 49 kg N ha-1. Labels indicate which N treatment combinations were pooled for the contrast.
Nomenclature for each contrast's null hypothesis is as follows:
A H0: μ 2-0, 3-0, 4-0 = μ 2-1, 3-1, 4-1
B H0: μ 2-1, 3-0 = μ 3-1, 4-0
C H0: μ 3-1, 4-0 = μ 4-1
D H0: μ 2-1, 3-0 = μ 2-0, 3-1, 4-0, 4-1
E H0: μ 4-1 = μ 2-0, 2-1, 3-0, 3-1, 4-0
††
* = significant at 0.05 level, ** = significant at 0.01 level, *** = significant at 0.001 level
65
Divot width
Cutting height main effect -
Cutting height did not significantly affect divot width at VRC. Divot widths for each
cutting height are presented in Table 13.
Topdressing main effect-
Topdressing treatment did not significantly affect divot width at VRC. Mean divot widths
for each topdressing treatment are presented in Table 14.
Nitrogen treatment main effect-
Nitrogen treatment did not significantly affect divot width at VRC. Mean divot widths for
each topdressing treatment are presented in Table 15.
Interactions-
No significant two-way or three-way interactions on divot width occurred at VRC
between cutting height, topdressing, and nitrogen treatment treatments.
66
Table 13. Mean divot widths for the cutting height main effect at VRC.
Cutting height Divot width Letter grouping
---- cm ---- ---- cm ----
3.18 5.3 -
3.81 5.9 -
LSD (0.05) NS -
Table 14. Mean divot widths for the topdressing main effect at VRC.
Topdressing applied Divot width Letter grouping
---- kg sand m-2 ---- ---- cm ----
0.0 5.6 -
8.5 5.7 -
LSD (0.05) NS
Table 15. Mean divot widths for the nitrogen treatment main effect at VRC.
Nitrogen treatment† Divot width Letter grouping
---- cm ----
2-0 5.6 -
2-1 5.3 -
3-0 5.2 -
3-1 6.0 -
4-0 5.6 -
4-1 5.9 -
LSD (0.05) NS -
† indicates number of N applications (each at 49 kg N ha-1) made in the spring and fall,
respectively
67
Divot depth
Cutting height main effect-
Cutting height did not significantly affect divot depth at VRC. Divots were slightly
deeper under the 3.81 cm height of cut (Table 16)
Topdressing main effect-
Topdressing did not significantly affect divot depth at VRC. The mean divot depth was
equal for experimental units receiving topdressing and those which did not (Table 17).
Nitrogen treatment main effect-
The effect of nitrogen on divot depths was statistically significant and had a trend similar
to that for divot lengths, though less pronounced (Tables 18-19, Fig. 15). At the VRC
location divot depths ranged from 1.4-1.8 cm for the various N treatments. The highest N
treatments resulted in the deepest divots. The 4-1 N treatment produced divot depths 17%
greater than the mean of all other rates. The fall N application increased divot depth by
14% irrespective of the spring application.
Interactions-
No significant two-way or three-way interactions on divot depth occurred at VRC
between cutting height, topdressing, and nitrogen treatment treatments.
68
Table 16. Mean divot depths for the cutting height main effect at VRC.
Cutting height Divot depth Letter
grouping
---- cm ---- ---- cm ----
3.18 1.5 -
3.81 1.8 -
LSD (0.05) NS -
Table 17. Mean divot depths for the topdressing main effect at VRC.
Topdressing
applied Divot depth
Letter
grouping
---- kg sand m-2 ---- ---- cm ----
0.0 1.6 -
8.5 1.6 -
LSD (0.05) NS -
Table 18. Mean divot depths for the nitrogen treatment main effect at VRC.
Nitrogen treatment† Divot depth Letter grouping
---- cm ----
2-0 1.5 C
2-1 1.6 ABC
3-0 1.4 C
3-1 1.8 AB
4-0 1.6 BC
4-1 1.8 A
LSD (0.05) 0.2 -
† indicates number of N applications (each at 49 kg N ha-1) made in the spring and fall,
respectively
69
Figure 15. Mean divot depths for the nitrogen treatment main effect at VRC. Treatments
with overlapping error bars are not statistically different using Fisher’s Protected LSD.
0.0
0.5
1.0
1.5
2.0
2.5
2-0 2-1 3-0 3-1 4-0 4-1
Div
ot
dep
th (
cm)
Nitrogen treatment
(no. of applications of 49 kg N ha-1 in spring-fall, respectively)
70
Table 19. Selected contrasts comparing divot depths at VRC as related to total both total nitrogen applied and individual nitrogen
treatments.
Contrast label Nitrogen applications† Divot depth
Difference relative to
second value Pr > F
Statistical
Significance ††
------ cm ------ -------- % --------
A 0 fall vs. 1 fall 1.5 vs. 1.7 -14% 0.001 ***
B 3 total vs. 4 total 1.5 vs. 1.7 -9% 0.072 NS
C 4 total vs. 5 total 1.7 vs. 1.8 -9% 0.129 NS
D 3 total vs. all other rates 1.5 vs. 1.7 -9% 0.045 *
E 5 total vs. all other rates 1.8 vs. 1.6 +17% 0.008 **
† Each application supplied 49 kg N ha-1. Labels indicate which N treatment combinations were pooled for the contrast. Nomenclature
for each contrast's null hypothesis is as follows:
A H0: μ 2-0, 3-0, 4-0 = μ 2-1, 3-1, 4-1
B H0: μ 2-1, 3-0 = μ 3-1, 4-0
C H0: μ 3-1, 4-0 = μ 4-1
D H0: μ 2-1, 3-0 = μ 2-0, 3-1, 4-0, 4-1
E H0: μ 4-1 = μ 2-0, 2-1, 3-0, 3-1, 4-0
††
* = significant at 0.05 level, ** = significant at 0.01 level, *** = significant at 0.001 level
71
Sod strength
Sod strength (SS) was defined as the maximum tensile force recorded during extension of
the sod strip. During harvesting and installation of thick-cut, big-roll sod, the strip may
fall apart into clumps if not sufficiently anchored together by roots and rhizomes. Thus
SS must exceed a minimum threshold to be considered harvestable.
Cutting height main effect-
The main effect of cutting height was not significant in Experiment 1 at VRC. SS values
obtained from the two cutting heights were nearly identical (Table 20).
Topdressing main effect-
SS was significantly lower for plots receiving sand applications compared to the
untreated control plots at VRC. Mean SS values are presented in Table 21.
Nitrogen treatment main effect-
Nitrogen treatments significantly affected SS at VRC. Mean sod strength for the six N
treatments ranged from 203.1 kg to 219.3 kg (Table 22).
Lowest SS resulted from the 2-0 and 2-1 N treatments. Sod strength was increased by
additional N applications (i.e. the 3-0, 4-0 or 3-1 treatments), but tended to slightly
72
decline under the 4-1 program (highest N). Using Fisher’s protected LSD, there was no
significant difference among the 3 best-performing treatments.
Selected contrasts were computed to compare the effect of total N rate and make specific
comparisons among N treatments. These contrasts are presented in Table 23. The only
statistically significant contrast was between treatments totaling 3 N applications (2-1 and
3-0) with those including 4 total applications (3-1 and 4-0). While statistically significant,
the difference tested by this contrast was small (3%).
Cutting height x topdressing x N treatment interaction-
The CH x T x N interaction on sod strength at VRC was statistically significant (p=0.03).
Mean values for each three-way treatment combination are presented in Table 24. One
probable reason for the statistical significance of this interaction is that for two N
treatments (3-1 and 4-0) under the 3.18 cm cutting height, topdressed plots actually had
increased sod strength when compared to the non-topdressed control. This reversal of the
overall trend was not observed for any N treatments under the 3.81 cm cutting height.
Given that no significant two-way interactions were present at VRC, the practical value
of this interaction and the exact reasons for its occurrence remain unclear.
Other interactions-
The cutting height by topdressing, cutting height by nitrogen, and topdressing by nitrogen
interactions each were not significant at VRC.
73
Table 20. Mean sod strength values for the cutting height main effect at VRC.
Cutting height Sod strength Letter grouping
---- cm ---- ---- kg ----
3.18 211.7 -
3.81 211.3 -
LSD (0.05) NS -
Table 21. Mean sod strength values for the topdressing main effect at VRC.
Topdressing applied Sod strength Letter grouping
---- kg sand m-2 ---- ---- kg ----
0.0 216.1 A
8.5 206.8 B
LSD (0.05) 4.7 -
Table 22. Mean sod strength values for the nitrogen treatment main effect at VRC.
Nitrogen treatment† Sod strength Letter grouping
---- kg ----
2-0 205.7 BC
2-1 203.1 C
3-0 214.1 A
3-1 212.6 AB
4-0 219.3 A
4-1 214.2 A
LSD (0.05) 8.1 -
74
Table 23. Selected contrasts comparing sod strength at VRC as related to total both total nitrogen applied and individual nitrogen
treatments.
Contrast label Nitrogen applications† Sod strength
Difference relative
to second value Pr > F
Statistical
Significance ††
------ kg ------ -------- % --------
A 0 fall vs. 1 fall 213.0 vs. 210.0 1% 0.186 NS
B 3 total vs. 4 total 208.6 vs. 216.0 -3% 0.014 *
C 4 total vs. 5 total 216.0 vs. 214.2 1% 0.602 NS
D 3 total vs. all other rates 208.6 vs. 213.0 -2% 0.085 NS
E 5 total vs. all other rates 214.2 vs. 210.9 2% 0.327 NS
† Each application supplied 49 kg N ha-1. Labels indicate which N treatment combinations were pooled for the contrast.
Nomenclature for each contrast's null hypothesis is as follows:
A H0: μ 2-0, 3-0, 4-0 = μ 2-1, 3-1, 4-1
B H0: μ 2-1, 3-0 = μ 3-1, 4-0
C H0: μ 3-1, 4-0 = μ 4-1
D H0: μ 2-1, 3-0 = μ 2-0, 3-1, 4-0, 4-1
E H0: μ 4-1 = μ 2-0, 2-1, 3-0, 3-1, 4-0
†† * = significant at 0.05 level, ** = significant at 0.01 level, *** = significant at 0.001 level
75
Table 24. Mean sod strength values for the cutting height by topdressing by nitrogen treatment interaction at VRC.
N treatment
Cutting height Topdressing applied 2-0 2-1 3-0 3-1 4-0 4-1 LSD (0.05)
---- cm ---- ---- kg sand m-2 ---- --------------------------------- kg --------------------------------
3.18 0.0 202.3 210.6 226.2 206.3 213.2 228.2 16.2
8.5 201.7 200.5 215.6 217.4 217.2 201.6 16.2
3.81 0.0 219.5 210.3 209.3 219.9 232.9 215.2 16.2
8.5 199.2 190.9 205.0 207.0 213.9 211.9 16.2
LSD (0.05) 16.2 16.2 16.2 16.2 16.2 16.2 -
† indicates number of N applications (each at 49 kg N ha-1) made in the spring and fall, respectively
76
Shear strength
Shear strength at the soil surface is not a direct measurement of divot resistance. The
measurement is performed at a slower rate and interacts with the surface differently than
an athlete’s foot. Still, shear strength can be considered a related measurement because it
evaluates the surface’s resistance to mechanical deformation. It is more rapid and less
destructive than divots produced with Pennswing. Differences in shear strength occurred
due to treatments in this research project (Tables 25-27).
Cutting height main effect-
The main effect of cutting height on shear strength was statistically significant at VRC.
The 3.18 cm cutting height produced a mean shear strength 10% greater than that of the
3.81 cm height (Table 25).
Topdressing main effect-
Topdressing significantly lowered shear strength at VRC. Table 26 contains shear
strength values for topdressed and control plots at VRC. The mean difference due to
topdressing was 1.9 Nm. The reduced shear strength for topdressed plots was probably
related to the addition of non-cohesive sand particles to system.
77
Nitrogen treatment main effect-
Nitrogen fertilization did not significantly affect shear strength at VRC and there was
little variation among treatments. Data presented in Table 27 suggest that shear strength
is a function of factors other than N supply.
Interactions-
No significant two-way or three-way interactions related to shear strength occurred at
VRC among cutting height, topdressing, and nitrogen treatments.
78
Table 25. Mean shear strength values for the cutting height main effect at VRC.
Cutting height Shear strength Letter grouping
---- cm ---- ---- Nm ----
3.18 29.7 A
3.81 27.0 B
LSD (0.05) 2.3 -
Table 26. Mean shear strength values for the topdressing main effect at VRC.
Topdressing applied Shear strength Letter grouping
---- kg sand m-2 ---- ---- Nm ----
0.0 30.5 A
8.5 26.1 B
LSD (0.05) 0.6 -
Table 27. Mean shear strength values for the nitrogen treatment main effect at VRC.
Nitrogen treatment† Shear strength Letter grouping
---- Nm ----
2-0 28.4 NS
2-1 28.6 NS
3-0 28.4 NS
3-1 27.8 NS
4-0 28.8 NS
4-1 28.0 NS
LSD (0.05) NS -
† indicates number of N applications (each at 49 kg N ha-1) made in the spring and fall,
respectively
79
Shoot density
The number of turfgrass plants per unit area has been related to playing surface
characteristics including divot resistance, shear strength, surface hardness, and traction
(Shildrick and Peel, 1984; McNitt, 1994; Serensits, 2008). Significant differences and
related trends are described in the following paragraphs.
Cutting height main effect-
In Experiment 1, significantly higher shoot density was exhibited at the 3.18 cm cutting
height compared to 3.81 cm (Table 28). When averaged across all nitrogen and
topdressing treatments, the mean increase at was 5%.
Topdressing main effect-
Topdressing did not influence shoot density at either location. In fact, the mean values for
this main effect were exactly equal at VRC (Table 29).
Nitrogen treatment main effect-
Nitrogen fertilizer applications strongly affected shoot density (p<0.001). At VRC,
increased total N consistently produced greater density (Table 30). The fall N application
tended to overwhelm any influence from spring N applications. Treatments receiving N
in September had similar density regardless of the spring N rate; the 2-1, 3-1, and 4-1 N
treatments produced densities of 251, 252, and 256 shoots dm-2, respectively. This result
80
is reasonable since the turf had less time to metabolize the N from the fall application and
equilibrate to its prior shoot density.
However when N was not applied in the fall, higher rates of spring N still had a residual
influence on the stand density 5-6 months later. For example the 3-0 N treatment had
10% greater density than did the 2-0 program. Table 31 shows selected contrasts to
compare shoot density among various N treatments and treatment combinations.
Topdressing x nitrogen interaction-
A significant interaction occurred at VRC between topdressing treatments and N
treatments. The statistical significance of the interaction probably resulted mostly from
the behavior of the 3-1 N treatment when not topdressed (Table 32). When topdressing
was applied, shoot density increased by 44 shoots dm-2 with the 3-1 N treatment
compared to the 3-0 N treatment. In contrast, when not topdressed the density increased
by only 2 shoots dm-2 as N applications increased from 3-0 to 3-1.The reason for these
treatments’ behavior is uncertain. Though statistically significant, the interaction
probably has little practical meaning.
Other interactions-
The cutting height by topdressing, cutting height by nitrogen, and cutting height by
topdressing by nitrogen interactions each were not significant at VRC.
81
Table 28. Mean shoot density values for the cutting height main effect at VRC.
Cutting height Shoot density Letter grouping
---- cm ---- --- no dm-2 ---
3.18 244 A
3.81 232 B
LSD (0.05) 8 -
Table 29. Mean shoot density values for the topdressing main effect at VRC.
Topdressing applied Shoot density Letter grouping
---- kg sand m-2 ---- --- no dm-2 ---
0.0 238 -
8.5 238 -
LSD (0.05) NS -
Table 30. Mean shoot density values for the nitrogen treatment main effect at VRC.
Nitrogen treatment† Shoot density Letter grouping
--- no dm-2 ---
2-0 209 C
2-1 251 A
3-0 230 B
3-1 253 A
4-0 229 B
4-1 257 A
LSD (0.05) 15.9 -
† indicates number of N applications (each at 49 kg N ha-1) made in the spring and
fall, respectively
82
Table 31. Selected contrasts comparing shoot density at VRC as related to total both total nitrogen applied and individual nitrogen
treatments.
Contrast label Nitrogen applications† Shoot density
Difference relative
to second value Pr > F
Statistical
Significance ††
--- no. dm-2 --- -------- % --------
A 0 fall vs. 1 fall 223 vs. 253 -12% <.0001 ***
B 3 total vs. 4 total 240 vs. 241 0% 0.939 NS
C 4 total vs. 5 total 241 vs. 257 -6% 0.076 NS
D 3 total vs. all other rates 240 vs. 237 1% 0.558 NS
E 5 total vs. all other rates 257 vs. 234 10% 0.006 **
† Each application supplied 49 kg N ha-1. Labels indicate which N treatment combinations were pooled for the contrast.
Nomenclature for each contrast's null hypothesis is as follows:
A H0: μ 2-0, 3-0, 4-0 = μ 2-1, 3-1, 4-1
B H0: μ 2-1, 3-0 = μ 3-1, 4-0
C H0: μ 3-1, 4-0 = μ 4-1
D H0: μ 2-1, 3-0 = μ 2-0, 3-1, 4-0, 4-1
E H0: μ 4-1 = μ 2-0, 2-1, 3-0, 3-1, 4-0
†† * = significant at 0.05 level, ** = significant at 0.01 level, *** = significant at 0.001 level
83
Table 32. Mean shoot density values for the topdressing by nitrogen treatment interaction at VRC.
N treatment
Topdressing applied 2-0 2-1 3-0 3-1 4-0 4-1 LSD (0.05)
---- kg sand m-2 ---- ------------------------------------- no. dm-2 -------------------------------------
0.0 220 261 230 232 235 250 23
8.5 197 241 230 274 223 263 23
LSD (0.05) 23 23 23 23 23 23 -
† indicates number of N applications (each at 49 kg N ha-1) made in the spring and fall, respectively
84
Thatch accumulation
Thatch is of interest to sod growers and athletic field managers for its influence on
playing surface quality and turfgrass health. In this research project, thatch accumulation
was measured from plugs in terms of both mass (via loss on ignition, LOI) and
compressed thickness (with a ruler). Masses and thicknesses were respectively expressed
as grams of oven-dry organic matter per plug and millimeters of thatch thickness. Cutting
height, topdressing treatment, and N treatment all significantly affected thatch levels.
Two significant interactions, between cutting height and nitrogen and cutting height
topdressing, also occurred.
The relative orders of treatment means were similar between the two measurement
techniques; however, the LOI method detected additional main effects and interactions
compared to the thickness method. The greater inference power of the LOI method was
likely due to its greater measurement precision. Since the two methods were generally in
agreement and highly correlated (r=0.86; p<0.0001), only thatch mass data are presented
in this section. Thatch thickness values for all main effects and statistically significant
interactions are presented in the Appendix (Tables 101-105
Cutting height main effect-
Cutting height significantly affected thatch mass at VRC. Table 33 presents thatch mass
values for the two cutting heights averaged across all other treatment levels
85
Thatch mass was greater for the higher cutting height. Plots maintained at 3.81 cm
averaged 12% greater thatch mass than those maintained at the 3.18 cm height. Such an
effect has also been observed in other studies (Murray and Juska, 1970; Shearman, 1980)
Topdressing main effect-
Topdressing had a highly significant effect on thatch mass at VRC (p<0.0001). Control
plots produced more than double the thatch of those receiving sand topdressing (Table
34). This treatment effect is logical because the primary goal of topdressing is to
dilute/reduce thatch. A very small amount of thatch still accumulated at the surface of
plots receiving topdressing. However in most cases the layer was so thin that it could
have been eliminated simply by making an additional application of topdressing sand.
Nitrogen treatment main effect-
Thatch levels at VRC were significantly affected by N treatment. The greatest thatch
levels occurred under the highest N treatments (Table 35). This effect was consistent
across other main effects.
Contrast statements revealed other differences based on N rate and timing. By far the
largest single influence among N treatments was the application of 49 kg N ha-1 in
September. This application increased thatch mass by 15% at VRC. This difference was
also reflected in contrasts between the 4-1 N treatment and the mean of all other
treatments. The 4-1 application schedule resulted in significantly greater thatch than N
86
treatments receiving just one fewer application (i.e., average of the 3-1 and 4-0
programs), and also when compared against all other rates averaged together (Table 36).
A large fraction of these differences probably can be attributed to the additional fall
application.
Cutting height x topdressing interaction-
The cutting height by topdressing interaction was statistically significant at VRC (Table
37). A very small increase in thatch was observed in topdressed plots when moving from
the 3.18 to 3.18 cm cutting height. The effect of cutting height was minimal for un-
topdressed plots but greater for plots receiving topdressing.
Cutting height x nitrogen treatment interaction-
A statistically significant interaction occurred between cutting height and nitrogen
treatment at VRC. Experimental units not fertilized in the fall tended to have more thatch
at the higher cutting height compared to the lower height (Table 38). This may indicate
that under very low or very high N the cutting height has less influence on thatch
accumulation than does the N application schedule.
Other interactions-
The topdressing by nitrogen and cutting height by topdressing by nitrogen interactions
were not significant at VRC.
87
Table 33. Mean thatch mass values for the cutting height main effect at VRC.
Cutting height Thatch mass Letter grouping
---- cm ---- --- g sample-1 ---
3.18 1.49 B
3.81 1.67 A
LSD (0.05) 0.17 -
Table 34. Mean thatch mass values for the topdressing main effect at VRC.
Topdressing applied Thatch mass Letter grouping
---- kg sand m-2 ---- --- g sample-1 ---
0.0 2.26 A
8.5 0.90 B
LSD (0.05) 0.08 -
Table 35. Mean thatch mass values for the nitrogen treatment main effect at VRC.
Nitrogen treatment† Thatch mass Letter grouping
--- g sample-1 ---
2-0 1.29 D
2-1 1.58 BC
3-0 1.49 C
3-1 1.70 AB
4-0 1.57 BC
4-1 1.84 A
LSD (0.05) 0.15 -
† indicates number of N applications (each at 49 kg N ha-1) made in the spring and fall,
respectively
88
Table 36. Selected contrasts comparing thatch mass at VRC as related to total both total nitrogen applied and individual nitrogen
treatments.
Contrast label Nitrogen applications† Thatch mass
Difference relative to
second value Pr > F
Statistical
Significance ††
----- no. dm-2 ----- -------- % --------
A 0 fall vs. 1 fall 1.45 vs. 1.71 -15% <0.001 ***
B 3 total vs. 4 total 1.53 vs. 1.63 -6% 0.061 NS
C 4 total vs. 5 total 1.63 vs. 1.84 -11% 0.002 **
D 3 total vs. all other rates 1.53 vs. 1.60 -4% 0.145 NS
E 5 total vs. all other rates 1.84 vs. 1.53 21% <0.001 ***
† Each application supplied 49 kg N ha-1. Labels indicate which N treatment combinations were pooled for the contrast. Nomenclature for
each contrast's null hypothesis is as follows:
A H0: μ 2-0, 3-0, 4-0 = μ 2-1, 3-1, 4-1
B H0: μ 2-1, 3-0 = μ 3-1, 4-0
C H0: μ 3-1, 4-0 = μ 4-1
D H0: μ 2-1, 3-0 = μ 2-0, 3-1, 4-0, 4-1
E H0: μ 4-1 = μ 2-0, 2-1, 3-0, 3-1, 4-0
†† * = significant at 0.05 level, ** = significant at 0.01 level, *** = significant at 0.001 level
89
Table 37. Mean thatch mass values for the cutting height by topdressing interaction at
VRC.
Cutting height
Topdressing
---- kg sand m-2 ----
0.0 8.5 LSD
(0.05)
---- cm ---- ------ g sample-1 ------
3.18 2.24 0.73 1.14
3.81 2.27 1.07 0.14
LSD (0.05) 0.14 0.14 -
90
Table 38. Mean thatch mass values for the cutting height by nitrogen treatment interaction at VRC.
N treatment †
Cutting height 2-0 2-1 3-0 3-1 4-0 4-1 LSD (0.05)
---- cm ---- --------------------------- g sample-1 ---------------------------
3.18 1.10 1.57 1.36 1.76 1.55 1.58 0.22
3.81 1.48 1.59 1.62 1.63 1.59 2.11 0.22
LSD (0.05) 0.22 0.22 0.22 0.22 0.22 0.22 -
† indicates number of N applications (each at 49 kg N ha-1) made in the spring and fall, respectively
91
Below-ground biomass
Roots and rhizome are of interest in sod production because they anchor the soil together
and permit the sod to be harvested. They also stabilize the surface and prevent divots
from forming during athletic competition. Thus a larger quantity of BGB is desirable. In
this research project, below-ground biomass was considered any plant material growing
beneath the soil surface. The values are expressed in grams to represent the total mass of
BGB obtained from the cylindrical plugs with 5.08 cm diameter and 4.45 cm height.
Roots were not differentiated from rhizomes. Thatch was removed prior to measurement
of BGB and was discussed in the previous section. Due to the large degree of variability
in sub-surface growth and root recovery during analysis, an α-value of 0.1 was used
during the statistical analyses of BGB measurements.
Cutting height main effect-
Cutting height significantly affected below-ground biomass at VRC. BGB increased by
11% under the 3.18 cm cutting height compared to 3.81 cm (Table 39).
Topdressing main effect-
Topdressing had a large, statistically significant effect on BGB at VRC. Mean BGB
values were 2.628 g for topdressed plots and 1.442 g for control plots (Table 40).
It is critical to note, however, that despite the magnitude of this effect the addition of
topdressing sand did not necessarily cause the turfgrass plants to produce more roots and
92
rhizomes. A much more likely explanation for this effect relates to partitioning of plant
material in the sample plugs. When sand was applied, the new mineral matter
encapsulated any plant material that had begun to accumulate around the crowns,
effectively raising the soil surface. This caused what would have otherwise been thatch to
become below-ground biomass; this effect is essentially the goal of topdressing. Plots
receiving no topdressing continued to accumulate plant debris at the soil surface (thatch).
During plug analysis this material was removed from the plug and processed separately
from the BGB fraction. Thus the thatch: BGB ratio was altered such that a greater portion
of the plug was considered to be below the soil surface in topdressed plots, compared to
those that did not receive sand applications.
Nitrogen treatment main effect-
Nitrogen fertilization significantly affected below-ground biomass at VRC. The lower N
rates tended to produce more BGB than did the high N rates (Table 41). The greatest
mean BGB (2.187 g) was observed under the 2-1 N treatment while the lowest mean
value (1.914 g) occurred under the 4-1 N treatment.
Contrast statements revealed additional differences considering the effect of N
fertilization on below-ground biomass at VRC (Table 42). Plots receiving 3 total N
applications (either 2-1 or 3-0) had 8% more BGB than those receiving 4 total
applications and 7% more than all other treatments averaged together. The 4-1 N
treatment (5 total applications) had 7% lower BGB than the average of all other N
treatments.
93
Table 39. Mean below-ground biomass values for the cutting height main effect at VRC.
Cutting height Below-ground biomass Letter grouping
---- cm ---- --- g sample-1 ---
3.18 2.14 A
3.81 1.93 B
LSD (0.1) 0.18 -
Table 40. Mean below-ground biomass values for the topdressing main effect at VRC.
Topdressing applied Below-ground biomass Letter grouping
---- kg sand m-2 ---- --- g sample-1 ---
0.0 1.44 B
8.5 2.63 A
LSD (0.1) 0.10
Table 41. Mean below-ground biomass values for the nitrogen treatment main effect at
VRC.
Nitrogen treatment† Below-ground biomass Letter grouping
--- g sample-1 ---
2-0 2.11 AB
2-1 2.19 A
3-0 2.06 ABC
3-1 1.95 BC
4-0 1.98 BC
4-1 1.91 C
LSD (0.1) 0.17 -
94
Table 42. Selected contrasts comparing below-ground biomass at VRC as related to total both total nitrogen applied and
individual nitrogen treatments.
Contrast label Nitrogen applications†
Below-ground
biomass Difference relative to
second value Pr > F
Statistical
Significance ††
--- g sample -1 --- -------- % --------
A 0 fall vs. 1 fall 2.05 vs. 2.02 2% 0.572 NS
B 3 total vs. 4 total 2.12 vs. 1.97 8% 0.037 *
C 4 total vs. 5 total 1.97 vs. 1.91 3% 0.531 NS
D 3 total vs. all other rates 2.12 vs. 1.99 7% 0.039 *
E 5 total vs. all other rates 1.91 vs. 2.06 -7% 0.070 +
† Each application supplied 49 kg N ha-1. Labels indicate which N treatment combinations were pooled for the contrast. Nomenclature
for each contrast's null hypothesis is as follows:
A H0: μ 2-0, 3-0, 4-0 = μ 2-1, 3-1, 4-1
B H0: μ 2-1, 3-0 = μ 3-1, 4-0
C H0: μ 3-1, 4-0 = μ 4-1
D H0: μ 2-1, 3-0 = μ 2-0, 3-1, 4-0, 4-1
E H0: μ 4-1 = μ 2-0, 2-1, 3-0, 3-1, 4-0
†† + significant at 0.1 level, *significant at 0.05 level, **significant at 0.01 level, ***significant at 0.001 level
95
Cutting height x topdressing interaction-
A significant interaction occurred between cutting heights and topdressing treatments at
VRC. When plots were not topdressed, the two cutting heights produced similar BGB.
However when sand was applied, the 3.18 cm height produced more BGB per core than
did the 3.81 cm height. Table 43 presents values for each combination in this interaction.
This interaction is difficult to explain but conceptually could be a function of root density
with depth. It is possible that plots mowed at 3.18 cm produced most of their roots in the
very top portion of the soil profile. When topdressing was applied, the effective rise of
the soil surface moved the bottom of the sod layer closer to the zone of high root density
for the 3.18 cm plots. The 3.81 cm plots would have had no such zone if their roots were
distributed more evenly with depth. Thus the amount of soil sampled beneath the
topdressing layer would contain fewer roots and rhizomes for the 3.18 cm plugs (see
conceptual model in Fig. 16).
96
Table 43. Mean below-ground biomass values for the cutting height by topdressing
interaction at VRC.
Cutting height
Topdressing
---- kg sand m-2 ----
0.0 8.5 LSD (0.1)
---- cm ---- -------- g sample-1 --------
3.18 1.44 2.83 0.16
3.81 1.44 2.42 0.16
LSD (0.1) 0.16 0.16 -
97
Figure 16. Conceptual model depicting a possible explanation for the cutting height by
topdressing interaction in Experiment 1. Plugs on left have varying BGB density with
depth but contain essentially the same total amount of BGB. Plugs on right have been
topdressed, reducing the fraction of the plug sampled from original soil. Less native soil
is incorporated, resulting in the lower CH plots having greater BGB per plug. Cartoon is
for conceptual purposes only and is not drawn to scale.
98
Cutting height x N treatment interaction-
A significant interaction occurred between cutting heights and N treatments at VRC.
Table 44 contains below-ground biomass for each combination treatment present in this
interaction.
At moderate N levels, the two cutting heights performed similarly. BGB for the cutting
heights was not significantly different under the 3-0, 3-1, and 4-0 N application
schedules. At the very low or very high N levels, the two cutting heights behaved
differently. At two of the lowest N levels (2-0 and 2-1 programs), and the highest N
treatment (4-1) the 3.18 cm cutting height produced more BGB than did the 3.81 cm
height. BGB was reduced by the highest N treatment regardless of the cutting height,
consistent with the N main effect described earlier in this section.
The lower cutting height produced slightly greater BGB irrespective of N rate (CH main
effect). This may have occurred due to the roots being concentrated in the top portion of
the profile for the shorter cutting height. The lower N rates produced greater BGB than
higher N (N main effect). It is possible that a synergistic effect occurred between these
factors- by mowing lower and also applying less N, the benefits of both practices
combined to produce greater BGB than the combined effects of higher cutting height and
also higher N.
99
Table 44. Mean below-ground biomass values for the cutting height by nitrogen treatment at VRC.
N treatment †
Cutting height 2-0 2-1 3-0 3-1 4-0 4-1 LSD (0.1)
---- cm ---- ------------------------------------- g sample-1 -------------------------------------
3.18 2.38 2.36 2.10 2.00 1.88 2.10 0.30
3.81 1.84 2.02 2.01 1.91 2.09 1.73 0.30
LSD (0.1) 0.30 0.30 0.30 0.30 0.30 0.30 -
† indicates number of N applications (each at 49 kg N ha-1) made in the spring and fall, respectively
100
Other interactions-
The topdressing by nitrogen treatment and cutting height by topdressing by nitrogen
treatment interactions both were non-significant at VRC.
101
Tuckahoe Turf Farms (TTF) Location
At the TTF location all dependent variables were significantly affected by at least one
treatment, with the exceptions of divot length and shoot density (Tables 45-46). The
following subsections present data from Experiment 1 at the TTF location.
Divot length
Cutting height main effect-
At the TTF location divot length was not significantly affected by cutting height (Table
47). The trend was slightly larger divots at the lower height of cut.
Topdressing main effect-
Topdressed plots had a very slight decrease in divot length at TTF, but the effect was not
significant. Mean divot lengths for both topdressing treatments are presented in Table 48.
Nitrogen treatment main effect-
At TTF, the main effect of nitrogen treatments on divot length was non-significant. Mean
divot lengths for each N treatment appear in Table 49.
Interactions-
No significant two-way or three-way interactions on divot length occurred at TTF among
cutting height, topdressing, and nitrogen treatment treatments.
102
Table 45. Summary of treatment effects on field parameters in Experiment 1 at TTF.
NS = not significant, * = Significant at 0.05 level,** = significant at 0.01 level
Source
Degrees of
freedom
Divot
Length
Divot
Width
Divot
Depth
Sod
Strength
Shear
strength
Cutting height ( C ) 1 NS * * NS *
Block (B) 2 N/A N/A N/A N/A N/A
CB (Error 1) 2 NS N/A N/A N/A N/A
Topdressing (T) 1 NS NS * * **
Nitrogen treatment (N) 5 NS * * * NS
CT 1 NS NS NS NS NS
CN 5 NS NS NS * NS
TN 5 NS NS NS NS NS
CTN 5 NS NS NS NS NS
Volumetric water content 1 ** ** NS NS **
Residual error 43
Total 71
103
Table 46. Summary of treatment effects on laboratory parameters in Experiment 1 at TTF.
Source
Degrees of
freedom
Shoot
density
Thatch
Mass
Thatch
Thickness
Below-ground
Biomass
Cutting height ( C ) 1 NS NS NS +
Block (B) 2 N/A N/A N/A N/A
CB (Error 1) 2 N/A N/A N/A N/A
Topdressing (T) 1 NS ** ** **
Nitrogen treatment (N) 5 NS ** ** NS
CT 1 NS * NS NS
CN 5 NS ** NS NS
TN 5 NS * NS NS
CTN 5 NS * NS *
Residual error 44
Total 71
NS = not significant, + = significant at 0.1 level * = Significant at 0.05 level,** = significant at 0.01 level
104
Table 47. Mean divot lengths for the cutting height main effect at TTF.
Cutting height Divot length Letter grouping
---- cm ---- ---- cm ----
3.18 33.6 -
3.81 29.6 -
LSD (0.05) NS -
Table 48. Mean divot lengths for the topdressing main effect at TTF.
Topdressing applied Divot length Letter grouping
---- kg sand m-2 ---- ---- cm ----
0.0 32.5 -
8.5 30.8 -
LSD (0.05) NS -
Table 49. Mean divot lengths for the nitrogen treatment main effect at TTF.
Nitrogen treatment† Divot length Letter grouping
---- cm ----
2-0 31.2 -
2-1 32.5 -
3-0 33.7 -
3-1 31.1 -
4-0 30.7 -
4-1 30.6 -
LSD (0.05) NS -
† indicates number of N applications (each at 49 kg N ha-1) made in the spring and fall,
respectively
105
Volumetric water content as a covariate with divot length-
Volumetric water content (VWC) was measured with a 3.81 cm time domain
reflectometry probe concurrently with divot production. VWC was used as a covariate
during the analysis of variance for field data. Smaller divots were produced with greater
VWC, though the coefficient of determination was relatively small (r2=0.17) (Fig. 17).
The maximum VWC value was less than 25%. It is unclear why the turf was more divot
resistant under higher moisture contents, as soil cohesion tends to decrease with
increasing moisture content (Dafalla, 2013). However as described in the literature
review section of this thesis, in vegetated soils electrostatic cohesion and friction among
soil particles is considered to be of minimal influence on shear strength compared to root
stabilization. Perhaps this covariance is actually related to another property of the turf
which separately affected both moisture retention and divot resistance. One example is
thatch. The influence of thatch on divot resistance is discussed is discussed in subsequent
portions of this thesis.
106
Figure 17. Scatter plot of divot lengths plotted against volumetric water content in Experiment 1 at TTF.
y = -1.5227x + 63.877
R² = 0.1727
10.0
15.0
20.0
25.0
30.0
35.0
40.0
45.0
50.0
15.0 17.0 19.0 21.0 23.0 25.0 27.0
Div
ot
len
gth
(cm
)
Volumetric water content (%)
107
Divot width
Cutting height main effect-
Cutting height significantly affected divot width. Wider divots were produced under the
3.81 cutting height than the 3.18 cm height (Table 50)
Topdressing main effect-
Topdressing did not significantly affect divot width. Mean divot widths for each
topdressing treatment are shown in Table 51.
Nitrogen treatment main effect-
The effect of nitrogen on divot widths was statistically significant at TTF (Table 52; Fig.
18). These differences were relatively small (less than 10%). Experimental units
receiving less nitrogen tended to have smaller divots. Table 53 presents contrast
statements comparing the divot widths for individual treatments of interest and selected
treatment groups. These contrasts revealed that treatments including a fall N application
produced wider divots. In addition the divots produced under the 4-1 N treatment were
the widest of any treatment.
Interactions-
No significant two-way or three-way interactions on divot width occurred at TTF among
cutting height, topdressing, and nitrogen treatment treatments.
108
Table 50. Mean divot widths for the cutting height main effect at TTF.
Cutting height Divot width Letter grouping
---- cm ---- ---- cm ----
3.18 7.4 A
3.81 6.8 B
LSD (0.05) 0.4 -
Table 51. Mean divot widths for the topdressing main effect at TTF.
Topdressing applied Divot width Letter grouping
---- kg sand m-2 ---- ---- cm ----
0.0 7.0 -
8.5 7.2 -
LSD (0.05) NS
109
Table 52. Mean divot widths for the nitrogen treatment main effect at TTF.
Nitrogen treatment† Divot width Letter grouping
---- cm ----
2-0 6.8 B
2-1 7.2 AB
3-0 6.9 B
3-1 7.1 B
4-0 7.2 AB
4-1 7.7 A
LSD (0.05) 0.5 -
† indicates number of N applications (each at 49 kg N ha-1) made in the spring and fall,
respectively
Figure 18. Mean divot widths for the nitrogen treatment main effect at TTF. Treatments
with overlapping error bars are not statistically different using Fisher’s Protected LSD.
5.0
5.5
6.0
6.5
7.0
7.5
8.0
2-0 2-1 3-0 3-1 4-0 4-1
Div
ot
wid
th (
cm)
Nitrogen treatment
(no. of applications of 49 kg N ha-1 in spring-fall, respectively)
110
Table 53. Selected contrasts comparing divot widths at TTF as related to total both total nitrogen applied and individual nitrogen
treatments.
Contrast label Nitrogen applications† Divot width
Difference relative to
second value Pr > F
Statistical
Significance ††
------ cm ------ -------- % --------
A 0 fall vs. 1 fall 6.94 vs. 7.29 -5% 0.023 *
B 3 total vs. 4 total 7.01 vs. 7.11 -1% 0.579 NS
C 4 total vs. 5 total 7.11 vs. 7.65 -7% 0.015 *
D 3 total vs. all other rates 7.01 vs. 7.17 -2% 0.299 NS
E 5 total vs. all other rates 7.65 vs. 7.01 9% 0.002 **
† Each application supplied 49 kg N ha-1. Labels indicate which N treatment combinations were pooled for the contrast. Nomenclature for
each contrast's null hypothesis is as follows:
A H0: μ 2-0, 3-0, 4-0 = μ 2-1, 3-1, 4-1
B H0: μ 2-1, 3-0 = μ 3-1, 4-0
C H0: μ 3-1, 4-0 = μ 4-1
D H0: μ 2-1, 3-0 = μ 2-0, 3-1, 4-0, 4-1
E H0: μ 4-1 = μ 2-0, 2-1, 3-0, 3-1, 4-0
†† *significant at 0.05 level, **significant at 0.01 level, ***significant at 0.001 level
111
Volumetric water content as a covariate with divot width-
Smaller divots were produced with greater VWC, though the coefficient of determination
was very small (r2=0.04) (Fig. 19). The maximum VWC value was less than 25%. It is
unclear why the turf was slightly more divot resistant under higher moisture contents;
however this degree of association is minimal even though it was statistically significant.
.
112
Figure 19. Scatter plot of divot widths plotted against volumetric water content in Experiment 1 at TTF.
y = -0.0768x + 8.7401
R² = 0.038
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
10.0
15.0 17.0 19.0 21.0 23.0 25.0 27.0
Div
ot
wid
th (
cm)
Volumetric water content (%)
113
Divot depth
Cutting height main effect-
Cutting height significantly affected divot depth. Divots were deeper under the 3.18 cm
cutting height than the 3.81 cm cutting height (Table 54).
Topdressing main effect-
Topdressing significantly affected divot depth. Experimental units receiving topdressing
produced slightly deeper divots compared to the control (Table 55).
114
Table 54. Mean divot depths for the cutting height main effect at TTF.
Cutting height Divot depth Letter grouping
---- cm ---- ---- cm ----
3.18 1.8 A
3.81 1.6 B
LSD (0.05) 0.2 -
Table 55. Mean divot depths for the topdressing main effect at TTF.
Topdressing applied Divot depth Letter grouping
---- kg sand m-2 ---- ---- cm ----
0.0 1.6 B
8.5 1.8 A
LSD (0.05) 0.1 -
115
Nitrogen treatment main effect-
The effect of nitrogen on divot depths was statistically significant and had an influence
similar to that for divot widths, though less pronounced (Table 56; Fig. 20). Mean divot
depths ranged from 1.5-1.9 cm for the various N treatments. Divots were deeper under
higher N.
Contrast statements revealed additional differences among N treatments (Table 57). The
higher N treatments resulted in the deepest divots at TTF. The 4-1 N treatment produced
divot depths 11% greater than the mean of all other rates. The fall N application increased
divot depth by 12% regardless of the spring application schedule.
Interactions-
No significant two-way or three-way interactions on divot depth occurred at TTF among
cutting height, topdressing, and nitrogen treatment treatments.
116
Table 56. Mean divot depths for the nitrogen treatment main effect at TTF.
Nitrogen treatment† Divot depth Letter grouping
---- cm ----
2-0 1.5 B
2-1 1.7 AB
3-0 1.6 B
3-1 1.9 A
4-0 1.7 AB
4-1 1.9 A
LSD (0.05) 0.2 -
† indicates number of N applications (each at 49 kg N ha-1) made in the spring and fall,
respectively
Figure 20. Mean divot depths for the nitrogen treatment main effect at TTF. Treatments
with overlapping error bars are not statistically different using Fisher’s Protected LSD.
0.0
0.5
1.0
1.5
2.0
2.5
2-0 2-1 3-0 3-1 4-0 4-1
Div
ot
dep
th (
cm)
Nitrogen treatment
(no. of applications of 49 kg N ha-1 in spring-fall, respectively)
117
Table 57. Selected contrasts comparing divot depths at TTF as related to total both total nitrogen applied and individual nitrogen
treatments.
Contrast label Nitrogen applications† Divot depth
Difference relative
to second value Pr > F
Statistical
Significance ††
------ cm ------ -------- % --------
A 0 fall vs. 1 fall 1.6 vs. 1.8 -12% 0.001 ***
B 3 total vs. 4 total 1.7 vs. 1.8 -7% 0.081 NS
C 4 total vs. 5 total 1.8 vs. 1.9 -4% 0.393 NS
D 3 total vs. all other rates 1.7 vs. 1.8 -5% 0.163 NS
E 5 total vs. all other rates 1.9 vs. 1.7 11% 0.032 *
† Each application supplied 49 kg N ha-1. Labels indicate which N treatment combinations were pooled for the contrast.
Nomenclature for each contrast's null hypothesis is as follows:
A H0: μ 2-0, 3-0, 4-0 = μ 2-1, 3-1, 4-1
B H0: μ 2-1, 3-0 = μ 3-1, 4-0
C H0: μ 3-1, 4-0 = μ 4-1
D H0: μ 2-1, 3-0 = μ 2-0, 3-1, 4-0, 4-1
E H0: μ 4-1 = μ 2-0, 2-1, 3-0, 3-1, 4-0
†† *significant at 0.05 level, **significant at 0.01 level, ***significant at 0.001 level
118
Sod strength
Cutting height main effect-
A slight decrease in SS was observed at TTF under the higher cutting height compared to
the lower height, but the effect was not significant (Table 58).
Topdressing main effect-
SS was significantly lower for plots receiving sand applications compared to the
untreated control plots at TTF. Mean SS values are presented in Table 59.
Nitrogen treatment main effect-
Nitrogen treatments significantly affected SS at TTF. Sod strength values by N treatment
ranged from 144.1 kg to 170.2 kg. Mean sod strength values for each N treatment are
presented in Table 60.
The lowest sod strength was produced with the 2-0 and 2-1 N treatments. Sod strength
was increased by additional N applications (i.e. the 3-0, 4-0 or 3-1 treatments), but tended
to decline again under the 4-1 program (highest N). The differences between the 3 best-
performing treatments were not significant.
119
Selected contrasts were computed to compare the effect of total N rate on SS and make
specific comparisons among specific N treatments and treatment combinations. These
contrasts appear in Table 61. The only significant contrast was between treatments
totaling three N applications (2-1 and 3-0) with those including four total applications (3-
1 and 4-0). Making a fourth application increased SS on average by 9% compared to
experimental units receiving only three total N applications.
120
Table 58. Mean sod strength values for the cutting height main effect at TTF.
Cutting height Sod strength Letter grouping
---- cm ---- ---- kg ----
3.18 160.9 -
3.81 150.8 -
LSD (0.05) NS -
Table 59. Mean sod strength values for the topdressing main effect at TTF.
Topdressing applied Sod strength Letter grouping
---- kg sand m-2 ---- ---- kg ----
0.0 162.3 A
8.5 149.5 B
LSD (0.05) 8.9 -
Table 60. Mean sod strength values for the nitrogen treatment main effect at TTF.
Nitrogen treatment† Sod strength Letter grouping
---- kg ----
2-0 144.1 C
2-1 147.8 BC
3-0 152.2 BC
3-1 170.2 A
4-0 158.1 ABC
4-1 162.8 AB
LSD (0.05) 15.5
† indicates number of N applications (each at 49 kg N ha-1) made in the spring and fall,
respectively
121
Table 61. Selected contrasts comparing sod strength at TTF as related to total both total nitrogen applied and individual nitrogen
treatments.
Contrast
label Nitrogen applications† Sod strength
Difference relative to
second value Pr > F
Statistical
Significance ††
------ kg ------ -------- % --------
A 0 fall vs. 1 fall 151.5 vs. 160.3 -5% 0.133 NS
B 3 total vs. 4 total 150.0 vs. 164.1 -9% 0.022 *
C 4 total vs. 5 total 164.1 vs. 162.8 1% 0.817 NS
D 3 total vs. all other rates 150.0 vs. 158.8 -6% 0.091 NS
E 5 total vs. all other rates 162.8 vs. 154.5 5% 0.234 NS
† Each application supplied 49 kg N ha-1. Labels indicate which N treatment combinations were pooled for the contrast.
Nomenclature for each contrast's null hypothesis is as follows:
A H0: μ 2-0, 3-0, 4-0 = μ 2-1, 3-1, 4-1
B H0: μ 2-1, 3-0 = μ 3-1, 4-0
C H0: μ 3-1, 4-0 = μ 4-1
D H0: μ 2-1, 3-0 = μ 2-0, 3-1, 4-0, 4-1
E H0: μ 4-1 = μ 2-0, 2-1, 3-0, 3-1, 4-0
†† * = significant at 0.05 level, ** = significant at 0.01 level, *** = significant at 0.001 level
122
Cutting height x N treatment interaction-
Thus the N treatments did not behave the same way under each cutting height. SS values
for each CH-N combination are presented in Table 62. The statistical significance
resulted from the different trends across nitrogen combinations within each cutting
height. A fairly consistent SS increase was attributed with fall fertilization under 3.81 cm
height of cut. All three levels of spring fertilization (2, 3, or 4 applications), exhibited
greater SS when a fall application was also made. For example, the 2-1 treatment had
26.8 kg stronger SS than the 2-0 treatment. However at the 3.18 cm height this trend was
not evident.
One possible explanation for this phenomenon is that the high CH area responded less
favorably to N applications throughout the year than did the low CH area. As a whole the
area had reduced color and vigor compared to the 3.18 cm area despite receiving an
identical treatment array (Fig. 21; visual quality data presented in Tables 96 and 99 in
Appendix). Thus these plots may have been more deficient throughout the season and
strongly benefited from additional N applied in the fall, whereas the lower CH area was
generally less deficient and failed to show as dramatic a response to additional N in the
fall. The reason for the striking difference in turf quality among CH areas is unclear, and
is not likely to have occurred solely due to cutting height. The reduced quality may have
resulted from irregular plot maintenance, misapplication of a chemical, or other unknown
factors.
123
Table 62. Mean sod strength values for the cutting height by nitrogen treatment interaction at TTF.
N treatment †
Cutting height 2-0 2-1 3-0 3-1 4-0 4-1 LSD (0.05)
---- cm ---- ---------------------------------------------- kg ---------------------------------------------
3.18 160.4 145.4 164.2 173.4 166.5 155.7 30.1
3.81 127.8 150.2 140.2 167.0 149.7 169.9 30.1
LSD (0.05) 30.1 30.1 30.1 30.1 30.1 30.1 -
† indicates number of N applications (each at 49 kg N ha-1) made in the spring and fall, respectively
124
Figure 21. Comparison of the 3.81 cm area (top) and 3.18 cm area (bottom) at TTF.
125
Other interactions-
The cutting height by topdressing, topdressing by nitrogen, and cutting height by
topdressing by nitrogen interactions on sod strength each were not significant at TTF.
126
Shear strength
Cutting height main effect-
The main effect of cutting height was statistically significant at TTF. The higher CH
(3.81 cm) produced greater shear strength (Table 63).
Topdressing main effect-
Topdressing significantly lowered shear strength at TTF. Table 64 contains shear strength
values for topdressed and control plots at TTF. The mean difference due to topdressing
was 1.9 Nm. However the decrease in shear strength for experimental units receiving
topdressing was relatively small and of questionable practical value.
Nitrogen treatment main effect-
Nitrogen fertilization did not significantly affect shear strength at VRC and there was
little variation among treatments. Data presented in Table 65 suggest that shear strength
is a function of factors other than N rate.
127
Table 63. Mean shear strength values for the cutting height main effect at TTF.
Cutting height Shear strength Letter grouping
---- cm ---- ---- Nm ----
3.18 23.1 B
3.81 25.6 A
LSD (0.05) 2.4 -
Table 64. Mean shear strength values for the topdressing main effect at TTF.
Topdressing applied Shear strength Letter grouping
---- kg sand m-2 ---- ---- Nm ----
0.0 25.2 A
8.5 23.5 B
LSD (0.05) 1.0 -
Table 65. Mean shear strength values for the nitrogen treatment main effect at TTF.
Nitrogen treatment† Shear strength Letter grouping
---- Nm ----
2-0 24.3 NS
2-1 23.5 NS
3-0 24.5 NS
3-1 24.6 NS
4-0 25.3 NS
4-1 24.0 NS
LSD (0.05) NS -
† indicates number of N applications (each at 49 kg N ha-1 made in the spring and fall,
respectively
128
Volumetric water content as a covariate with shear strength-
Shear strength tended to increase with moisture content. The coefficient of determination
for this relationship was 0.30 (Fig. 22). As inherent soil cohesion and shear strength of
un-vegetated soil tend to decrease with increasing water content, the relationship
observed here is somewhat puzzling. It is possible that this relationship is actually
capturing a separate property of the experimental units – one which separately increased
both moisture retention and shear strength. One potential property could be thatch
accumulation, as turf with a thicker thatch layer may retain more moisture and was also
positively related to shear strength (see Correlations section below). Thatch also
positively correlated with water content (Fig. 23).
Interactions-
No significant two-way or three-way interactions related to shear strength were detected
at TTF among cutting height, topdressing, and nitrogen treatments.
129
Figure 22. Scatter plot of shear strength plotted against volumetric water content in Experiment 1 at TTF.
.
y = 0.6017x + 12.424
R² = 0.2983
0.0
5.0
10.0
15.0
20.0
25.0
30.0
35.0
15.0 17.0 19.0 21.0 23.0 25.0 27.0
Sh
ear
stre
ngth
(N
m)
Volumetric water content (%)
130
Figure 23. Scatter plot of thatch thickness plotted against volumetric water content in Experiment 1 at TTF.
y = 0.5594x - 5.9717
R² = 0.1586
0.0
2.0
4.0
6.0
8.0
10.0
12.0
14.0
16.0
18.0
20.0
15.0 17.0 19.0 21.0 23.0 25.0 27.0
Th
atc
h t
hic
kn
ess
(mm
)
Volumetric water content (%)
131
Shoot density
Cutting height main effect-
Shoot density was not significantly affected by cutting height at TTF, although a trend of
higher density at the lower cutting height was observed (Table 66). Experimental units at
the lower height of cut were 9% denser on average.
Topdressing main effect-
Topdressing did not affect shoot density at TTF. Mean density values were within 1 tiller
dm-1 (Table 67).
Nitrogen treatment main effect-
Nitrogen treatment did not significantly affect shoot density at TTF. However a trend was
evident toward greater density values for plots fertilized in the fall (Table 68).
Interactions-
No significant two-way or three-way interactions related to shoot density were detected at
TTF among cutting height, topdressing, and nitrogen treatments.
132
Table 66. Mean shoot density values for the cutting height main effect at TTF.
Cutting height Shoot density Letter grouping
---- cm ---- --- no dm-2 ---
3.18 231 -
3.81 212 -
LSD (0.05) NS -
Table 67. Mean shoot density values for the topdressing main effect at TTF.
Topdressing applied Shoot density Letter grouping
---- kg sand m-2 ---- --- no dm-2 ---
0.0 221 -
8.5 222 -
LSD (0.05) NS -
Table 68. Mean shoot density values for the nitrogen treatment main effect at TTF.
Nitrogen treatment† Shoot density Letter grouping
--- no dm-2 ---
2-0 216 -
2-1 232 -
3-0 220 -
3-1 222 -
4-0 212 -
4-1 227 -
LSD (0.05) NS -
133
Thatch accumulation
The two measurement techniques used to quantify thatch in this project (mass and
thickness) produced similar results. Since the two methods were generally in agreement
and highly correlated (r=0.86; p<0.0001), only thatch mass data are presented in this
section. Thatch thickness values for all main effects and statistically significant
interactions are presented in the Appendix (Tables 106-109).
Cutting height main effect-
Cutting height did not significantly affect thatch mass at TTF. Table 69 presents thatch
mass values for the two cutting heights averaged across all topdressing and N treatments.
Topdressing main effect-
Topdressing had a strongly significant effect on thatch mass at TTF (p<0.0001). Control
plots produced more than double the thatch of those receiving sand topdressing (Table
70). This treatment effect is logical because the primary goal of topdressing is to
dilute/reduce thatch. A very small amount of thatch still accumulated at the surface of
plots receiving topdressing. However in most cases the layer was so thin that it could
have been eliminated simply by making an additional application of topdressing sand.
134
Nitrogen treatment main effect-
Thatch levels at TTF were significantly affected by N treatment. The greatest thatch
levels occurred under the highest N treatments (Table 71). This effect was consistent
across other main effects.
Contrast statements revealed other differences based on N rate and timing (Table 72). By
far the largest single influence among N treatments was the application of 49 kg N ha-1 in
September. This application increased thatch mass by 18% at TTF. This difference was
also reflected in contrasts between the 4-1 N treatment and the mean of all other
treatments. The 4-1 application schedule resulted in significantly greater thatch than N
treatments receiving just one fewer application (i.e., average of the 3-1 and 4-0
treatments), and also when compared against all other rates averaged together. A large
fraction of these differences probably can be attributed to the additional fall application.
Cutting height x topdressing interaction-
The cutting height by topdressing interaction was statistically significant at TTF. A very
small increase in thatch was observed in topdressed plots when moving from the 3.18 to
3.18 cm cutting height. A decrease in thatch was observed at the higher cutting height.
This decrease was likely due to reduced plant vigor rather than a physiological change
associated with the higher cutting height. Mean thatch mass values for all combinations
of the cutting height by topdressing interaction are presented in Table 73.
135
Cutting height x nitrogen treatment interaction-
A statistically significant interaction occurred between cutting height and nitrogen
treatment at TTF. Plots not fertilized in the fall tended to have greater thatch at the 3.18
cm cutting height compared to 3.81 cm. However this effect may have been related to the
reduced vigor of the 3.81 cm area rather than an effect due to cutting height. Table 74
contains thatch mass values for all combinations of the cutting height by N treatment
interaction.
136
Table 69. Mean thatch mass values for the cutting height main effect at TTF.
Cutting height Thatch mass Letter grouping
---- cm ---- --- g sample-1 ---
3.18 1.66 -
3.81 1.49 -
LSD (0.05) NS -
Table 70. Mean thatch mass values for the topdressing main effect at TTF.
Topdressing applied Thatch mass Letter grouping
---- kg sand m-2 ---- --- g sample-1 ---
0.0 2.18 A
8.5 0.97 B
LSD (0.05) 0.15 -
Table 71. Mean thatch mass values for the topdressing main effect at TTF.
Nitrogen treatment† Thatch mass Letter grouping
--- g sample-1 ---
2-0 1.29 D
2-1 1.58 BC
3-0 1.49 C
3-1 1.70 AB
4-0 1.57 BC
4-1 1.84 A
LSD (0.05) 0.146 -
† indicates number of N applications (each at 49 kg N ha-1) made in the spring and fall,
respectively
137
Table 72. Selected contrasts comparing thatch mass at TTF as related to total both total nitrogen applied and individual nitrogen
treatments.
Contrast label Nitrogen applications† Thatch mass
Difference relative to
second value Pr > F
Statistical
Significance ††
--- g sample-1 --- -------- % --------
A 0 fall vs. 1 fall 1.43 vs. 1.73 -18% <0.001 ***
B 3 total vs. 4 total 1.51 vs. 1.58 -5% 0.422 NS
C 4 total vs. 5 total 1.58 vs. 1.90 -17% 0.005 **
D 3 total vs. all other rates 1.51 vs. 1.61 -7% 0.164 NS
E 5 total vs. all other rates 1.90 vs. 1.51 26% <0.001 ***
† Each application supplied 49 kg N ha-1. Labels indicate which N treatment combinations were pooled for the contrast.
Nomenclature for each contrast's null hypothesis is as follows:
A H0: μ 2-0, 3-0, 4-0 = μ 2-1, 3-1, 4-1
B H0: μ 2-1, 3-0 = μ 3-1, 4-0
C H0: μ 3-1, 4-0 = μ 4-1
D H0: μ 2-1, 3-0 = μ 2-0, 3-1, 4-0, 4-1
E H0: μ 4-1 = μ 2-0, 2-1, 3-0, 3-1, 4-0
†† * = significant at 0.05 level, ** = significant at 0.01 level, *** = significant at 0.001 level
138
Table 73. Mean thatch mass values for the cutting height by topdressing interaction at TTF.
Cutting height
Topdressing
---- kg sand m-2 ----
0.0 8.5 LSD (0.05)
---- cm ---- ------ g sample-1 ------
3.18 2.40 0.92 0.21
3.81 1.97 1.02 0.21
LSD (0.05) 0.21 0.21 -
Table 74. Mean thatch mass values for the cutting height by nitrogen treatment interaction at TTF.
N treatment †
Cutting height 2-0 2-1 3-0 3-1 4-0 4-1 LSD (0.05)
---- cm ---- ------------------------------------------- g sample-1 -------------------------------------------
3.18 1.72 1.55 1.65 1.69 1.47 1.90 0.36
3.81 1.08 1.59 1.24 1.76 1.40 1.90 0.36
LSD (0.05) 0.36 0.36 0.36 0.36 0.36 0.36 -
† indicates number of N applications (each at 49 kg N ha-1) made in the spring and fall, respectively
139
Topdressing x nitrogen treatment interaction-
A statistically significant interaction occurred at TTF between topdressing treatment and
N treatment (Table 75). The relative performance of all N treatments was similar across
topdressing treatments, with the exception of the 4-1 treatment. When topdressed, this N
treatment produced more thatch than other N treatments receiving topdressing. This may
indicate that topdressing was less effective at controlling thatch buildup when plots were
subjected to the highest N level, although it is unclear why this N treatment did not also
produce more thatch than all other N treatments when no sand was applied.
Cutting height x topdressing x N treatment interaction-
A significant three-way interaction on thatch mass occurred at TTF among cutting height,
topdressing treatment, and N treatment. Table 76 presents thatch mass values for each
three-way treatment combination. Several treatment combinations produced nearly
identical amounts of thatch. All of the highest-thatching combinations did not receive
topdressing (as expected), and the least thatch tended to occur under low N regardless of
cutting height. The statistical significance appears to have resulted from the lesser
separation of N treatments at the 3.18 cm cutting height compared to the 3.81 cm height.
A considerable increase in thatch production was also observed under the 4-1 N treatment
at 3.18 cm cutting height. This trend was not observed at the higher cutting height,
although the 4-1 N treatment still produced the most thatch. While statistically
significant, there was no synergistic trend among cutting height, topdressing, and
nitrogen, so the practical importance of the interaction probably does not supersede the
main effects of each main effect taken individually.
140
Table 75. Mean thatch mass values for the topdressing by nitrogen treatment interaction at TTF.
N treatment
Topdressing applied 2-0 2-1 3-0 3-1 4-0 4-1 LSD (0.05)
---- kg sand m-2 ---- --------------------------- g sample-1 ---------------------------
0.0 2.07 2.29 2.11 2.42 2.00 2.22 0.36
8.5 0.73 0.85 0.78 1.03 0.87 1.58 0.36
LSD (0.05) 0.36 0.36 0.36 0.36 0.36 0.36 -
† indicates number of N applications (each at 49 kg N ha-1) made in the spring and fall, respectively
Table 76. Mean thatch mass values for the cutting height by topdressing by nitrogen treatment interaction at TTF.
N treatment
Cutting height Topdressing applied 2-0 2-1 3-0 3-1 4-0 4-1 LSD (0.05)
---- cm ---- ---- kg sand m-2 ---- --------------------------- g sample-1 ---------------------------
3.18 0.0 2.61 2.22 2.42 2.52 2.37 2.60 0.50
8.5 0.82 0.89 0.88 0.85 0.57 1.54 0.50
3.81 0.0 1.53 2.36 1.79 2.31 1.62 2.19 0.50
8.5 0.64 0.81 0.68 1.20 1.18 1.62 0.50
LSD (0.05) 0.50 0.50 0.50 0.50 0.50 0.50 -
† indicates number of N applications (each at 49 kg N ha-1) made in the spring and fall, respectively
141
Below-ground biomass
Cutting height main effect-
Cutting height significantly affected below-ground biomass at TTF (Table 77). BGB
increased under the 3.18 cm cutting height compared to 3.81 cm. At TTF, plots
maintained at the lower cutting height had 11% greater BGB than those maintained at the
higher height It is possible that this effect occurred solely due to cutting height, although
the 3.81 cm area had reduced vigor which may have also contributed to the difference.
Topdressing main effect-
Topdressing had a statistically significant effect on BGB at TTF. Experimental units
receiving topdressing averaged 2.069 g compared to 1.118 g for the non-topdressed
control (Table 78).
It is critical to note, however, that despite the magnitude of this effect the addition of
topdressing sand did not necessarily cause the turfgrass plants to produce more roots and
rhizomes. A much more likely explanation for this effect relates to partitioning of plant
material in the sample plugs (c.f. p. 93-95 and Table 40).
Nitrogen treatment main effect-
Nitrogen fertilization did not significantly affect below-ground biomass at TTF. The 4-1
N treatment (most applied nitrogen) produced the lowest BGB, although the magnitude
142
of the difference between this and other treatments was small. Greatest BGB resulted
from the 4-0 N treatment. The second most effective treatment in terms of BGB was the
2-1 schedule. Other N treatments performed similarly (Table 79).
Cutting height x topdressing x N treatment interaction-
A significant three-way interaction occurred among cutting height, topdressing, and N
treatment (p=0.03). The statistical significance probably resulted mostly from the unusual
pattern exhibited by topdressed plots maintained at 3.18 cm (Table 80). Under lower N
treatments, these plots performed similarly to other CH/T combinations. However their
BGB increased appreciably under higher N, which was contrary to other treatment
schemes. At the highest N level (4-1 treatment) these plots did show a decline in BGB.
The three-way combination of cutting height, topdressing, and N fertilization producing
the most BGB was a 3.18 cm height of cut, with topdressing and the 4-0 N treatment
applied. The combination producing the least BGB was a 3.81 cm cutting height with no
topdressing and the 3-0 N treatment.
As this interaction involves three variables, the mechanisms behind its occurrence are
probably complex. Perhaps the increased vigor of the 3.18 cm area (described earlier in
the SS portion of the Results section) was further stimulated by additional N applications.
The practical implications of the interaction are somewhat unclear, although it does
permit certain treatment combinations to stand out as particularly effective or ineffective
at the TTF location.
143
Other interactions-
The cutting height by topdressing, cutting height by nitrogen, and topdressing by nitrogen
interactions each were not significant at TTF.
144
Table 77. Mean below-ground biomass values for the cutting height main effect at TTF.
Cutting height Below-ground biomass Letter grouping
---- cm ---- --- g sample-1 ---
3.18 1.67 A
3.81 1.51 B
LSD (0.1) 0.15 -
Table 78. Mean below-ground biomass values for the topdressing main effect at TTF.
Topdressing applied Below-ground biomass Letter grouping
---- kg sand m-2 ---- --- g sample-1 ---
0.0 1.12 B
8.5 2.07 A
LSD (0.1) 0.12 -
Table 79. Mean below-ground biomass values for the nitrogen treatment main effect at
TTF.
Nitrogen treatment† Below-ground biomass Letter grouping
--- g sample-1 ---
2-0 1.51 -
2-1 1.67 -
3-0 1.53 -
3-1 1.58 -
4-0 1.78 -
4-1 1.50 -
LSD (0.1) NS -
† indicates number of N applications (each at 49 kg N ha-1) made in the spring and
fall, respectively
145
Table 80. Mean below-ground biomass values for the cutting height by topdressing by nitrogen treatment interaction at TTF.
N treatment
Cutting height Topdressing applied 2-0 2-1 3-0 3-1 4-0 4-1 LSD (0.1)
---- cm ---- ---- kg sand m-2 ---- --------------------------- g sample-1 ---------------------------
3.18 0.0 0.94 1.24 1.07 0.95 1.32 1.52 0.40
8.5 2.15 2.32 1.93 2.44 2.55 1.66 0.40
3.81 0.0 1.14 1.12 0.94 1.09 1.13 0.97 0.40
8.5 1.80 2.02 2.17 1.86 2.10 1.84 0.40
LSD (0.1) 0.40 0.40 0.40 0.40 0.40 0.40 -
† indicates number of N applications (each at 49 kg N ha-1) made in the spring and fall, respectively
146
Variability across locations
Sod performed better at the VRC location than the TTF location with respect to nearly all
measured parameters (Tables 81-82). In the cases of divot length, width, and depth, the
values were larger at TTF than at VRC. Sod strength, shear strength, shoot density, and
below-ground biomass were all greater at VRC than at TTF. An exception to this trend
was thatch development, which was more extensive at VRC than at TTF. This can
probably be attributed to greater total biomass production at VRC due to the more
favorable weather conditions at this location.
Treatment effects were not always equivalent between the locations. For example, the
main effect of cutting height on shear strength was statistically significant at both TTF
and VRC. However at VRC the trend was greater shear strength under the 3.18 cm
cutting height, while the opposite was true at TTF (greater shear strength at the 3.81 cm
cutting height). The cause of this reversal is unclear; in any case, the small differences
due to mowing treatment are probably of little practical value.
In many cases the relative difference between the locations was larger than differences
ascribed to the experimental treatments. This observation highlights the importance of
weather conditions and other turf management practices in addition to the experimental
treatments. In November 2013, measurements were also recorded on sod harvested from
the actual 2013 thick-cut sod production field at TTF. The values obtained from the
147
production field were similar to mean values (averaged across all treatment levels) from
the TTF research plots. These data are presented in the Appendix (Table 91).
Weather conditions
Air temperatures were generally higher at TTF, especially during the months of July and
August (Fig. 24). These conditions were expected due to the more southern latitude of the
TTF location (39.68 deg. N) compared with the VRC location (40.81 deg. N).
During the summer months, high temperatures and humidity are stressful to cool-season
turfgrasses. The higher air temperatures may have contributed to the diminished sod
quality at the TTF location.
148
Table 81. Mean values for field-measured parameters at each location when averaged across all treatment levels.
Divot Sizes
Location Length Width Depth Sod strength Shear strength
-----------------cm----------------- -----kg----- -----Nm-----
VRC 24.2† 5.6† 1.6† 211.5† 28.3†
TTF 31.6 7.1 1.7 155.9 24.4
% change from VRC to TTF +30.6% +26.8% +6.2% -26.3% -13.8%
† denotes the more “preferred” value between the two locations
Table 82. Mean values for laboratory-measured parameters at each location when averaged across all treatment levels.
Location Shoot density Thatch mass Thatch thickness Below-ground biomass
-----no. dm -1----- ----g sample -1---- ------mm------ ----g sample -1----
VRC 238† 1.578 7.3 2.035†
TTF 221 1.578 5.2† 1.594
% change from VRC to TTF -7.1% 0.0% -28.8% -21.7%
† denotes the more “preferred” value between the two locations
149
Figure 24. Mean air temperatures at both locations over the duration of Experiment 1. Lines represent a 5-day moving average of
the mean between daily high and low temperatures. Black bold line at 20 °C represents the temperature considered optimal for
cool-season turfgrasses (Turgeon, 2012). Green arrows represent treatment application dates.
0
5
10
15
20
25
30
35M
ean
dail
y t
emp
era
ture
(d
eg. C
)
Calendar Date
daily mean temperature- TTF
daily mean temperature-VRC
150
Correlations
By measuring the degree of association among dependent variables, ideas can be formed
about which traits are most important in producing sod with excellent athletic playing
quality. Table 83 presents correlation coefficients between each combination of
dependent variables measured in this project.
Numerous significant correlations were detected among plot characteristics. Divot length,
width, and depth were all positively related to one another. This relationship is not
surprising because it is logical that less divot-resistant turf impacted by Pennswing would
be more severely damaged in all dimensions.
All other significant correlations related to divot length were negative. This indicates
plots with smaller divots also tended to have higher sod strength (r= -0.38) and shear
strength (r= -0.47). Divot length also had a significant, negative correlation with thatch
thickness (r= -0.26). In other words, plots with more thatch had larger divots.
The low magnitude and non-significant p-value of the correlation between divot length
and below-ground biomass is somewhat surprising. It might be expected that more
below-ground biomass would be associated with shorter divots. In this experiment thatch
appears to have played a larger role in divot size than did below-ground biomass.
151
Table 83. Spearman correlation coefficients among parameters measured in Experiment1
* = Significant at 0.05 level,** = significant at 0.01 level, *** = significant at 0.001 level
Divot
length
Divot
width
Divot
depth
Sod
strength
Shear
strength
Shoot
density
Thatch
thickness
Thatch
mass
Below-
ground
biomass
Divot
length -- 0.66 *** 0.33 *** -0.38 *** -0.39 *** - 0.09 -0.26 ** -0.11 -0.08
Divot
width -- 0.49 *** -0.52 *** -0.47 *** -0.10 -0.23 -0.05 -0.14
Divot
depth -- -0.04 -0.12 0.07 0.02 0.06 * -0.14 **
Sod
strength -- 0.67 *** 0.30 *** 0.40 *** 0.21 ** 0.19 **
Shear
strength -- 0.26 *** 0.62 *** 0.51 *** -0.24 ***
Shoot
density -- 0.07 0.16 * 0.10
Thatch
thickness -- 0.86 *** -0.64 ***
Thatch
mass -- -0.75 ***
Below-
ground
biomass
--
152
Shear strength and sod strength were highly correlated (r= 0.67) and also had similar
relationships with other properties. High shoot density and greater thatch thickness were
associated with higher shear strength (r=0.26 and r=0.62). Similarly, shoot density and
thatch thickness were positively correlated with greater sod strength (r=0.30 and r=0.40).
Thatch thickness was highly correlated with thatch mass (r= 0.86). This is logical because
the two techniques measure the same property.
Thatch measurements were negatively correlated with below-ground biomass (r=-0.64 for
thatch thickness and r= -0.75 for thatch mass). This could be a function of the plants
partitioning biomass above or below the surface based on cultural and environmental
factors. This strong relationship also could be related to the method in which the sod was
harvested. For all sod strips, the profile was 4.45 cm thick. Thus plots with thicker thatch
layers also had less soil in the sod profile and would tend to also have less below-ground
biomass.
153
Experiment 2: The effects of varying cutting height on the divot resistance of thick-
cut Kentucky bluegrass sod
Research and anecdotal reports indicate cutting height may influence divot resistance, sod
strength, and related characteristics. The goal of Experiment 2 was to further elucidate
the influence of cutting height on the same parameters measured in Experiment 1. In
Experiment 2, four cutting heights were evaluated under identical fertilization and
topdressing regimes. All plots in Experiment 2 were identically fertilized with the 3-0 N
treatment and were topdressed three times during the season, on the same dates as
Experiment1 plots.
Divot length, width, and depth
Divot sizes were not significantly affected by cutting height in Experiment 2 (Table 84).
The smallest divots were produced under the lowest cutting height of 2.54 cm while the
largest divots were produced under the 3.18 cm cutting height. It is not clear why this
treatment performed so poorly; the same treatment combination in Experiment 1
produced a mean divot length of 24.2 cm and width of 5.0 cm. Perhaps the small number
of experimental units in the experiment was insufficient to obtain a representative divot
size for each cutting height.
154
Table 84. Mean divot dimensions for the four cutting heights evaluated in Experiment 2.
Divot Dimensions
Cutting height Length Width Depth
---- cm ---- --------------------- cm ---------------------
2.54 27.0 5.8 1.5
3.18 34.3 7.4 1.7
3.81 29.2 6.5 2.1
4.45 30.7 7.0 2.2
LSD (0.05) NS NS NS
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Sod strength
The cutting height treatment significantly affected SS in Experiment 2, with the 4.45 cm
cutting height producing higher SS than all other cutting heights (Table 85). The overall
trend was greater SS under higher heights of cut, but the greatest jump in SS occurred
between the 3.81 cm and 4.45 cm treatments (Fig. 25).
Shear strength
Shear strength was not affected by the cutting height treatments in this experiment (Table
86) although the trend was greater shear strength for lower heights of cut. It should be
noted that the shear strength values obtained in Experiment 2 were similar to those from
plots receiving the corresponding T - N maintenance regime in Experiment 1 (26.7 Nm).
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Table 85. Mean sod strength values for the four cutting heights evaluated in Experiment
2.
Cutting height Sod strength Letter grouping
---- cm ---- ---- kg ----
2.54 193.5 B
3.18 203.6 B
3.81 207.8 B
4.45 234.5 A
LSD (0.05) 21.6 -
100.0
120.0
140.0
160.0
180.0
200.0
220.0
240.0
260.0
2.54 3.18 3.81 4.45
Sod
str
ength
(k
g p
eak
forc
e)
Cutting height (cm)
Figure 25. Mean sod strength values for the four cutting heights evaluated in
Experiment 2. Treatments with overlapping error bars are not significantly
different using Fisher’s Protected LSD.
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Table 86. Mean shear strength values for the four cutting heights evaluated in Experiment
2.
Cutting height Shear strength Letter grouping
---- cm ---- ---- Nm ----
2.54 25.3 -
3.18 24.8 -
3.81 24.5 -
4.45 23.0 -
LSD (0.05) NS -
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Shoot density
Shoot density was not significantly affected by cutting height in Experiment 2. However
shoot density increased with each incremental decrease in canopy height, showing a 22%
increase from the highest to lowest heights of cut (Table 87; Fig. 26). An analogous trend
was observed in Experiment 1. Increased density under closer mowing is a well-
substantiated phenomenon in turfgrass culture. The lack of a statistical difference among
cutting heights in Experiment 2 probably can be attributed to the small number of
experimental units in this study.
Thatch accumulation
Thatch accumulation was not significantly affected by cutting height in Experiment 2
(Table 88). It is likely that the applied topdressing was sufficient to dilute most of the
thatch, overwhelming any cutting height effect that may have occurred.
Below-ground biomass
Below-ground biomass was not significantly affected by cutting height in Experiment 2
(Table 89). The lack of a statistical difference could be attributed to the small number of
experimental units in this study, or to the application of sand topdressing to all plots
muting the cutting height effect.
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Table 87. Mean shoot density values for the four cutting heights evaluated in Experiment
2.
Cutting height Shoot density Letter grouping
---- cm ---- --- no. dm-2 ---
2.54 243 -
3.18 223 -
3.81 214 -
4.45 200 -
LSD (0.05) NS -
Figure 26. Mean shoot density values for the four cutting heights evaluated in Experiment
2. Treatments with overlapping error bars are not significantly using Fisher’s Protected
LSD.
100
120
140
160
180
200
220
240
260
280
2.54 3.18 3.81 4.45
Sh
oot
den
sity
(n
o. d
m-2
)
Cutting height (cm)
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Table 88. Mean thatch mass values for the four cutting heights evaluated in Experiment
2.
Cutting height Thatch mass Letter grouping
---- cm ---- --- g sample-1 ---
2.54 0.94
3.18 1.13
3.81 0.92
4.45 1.08
LSD (0.05) NS -
Table 89. Mean below-ground biomass values for the four cutting heights evaluated in
Experiment 2.
Cutting height Below-ground biomass Letter grouping
---- cm ---- --- g sample-1 ---
2.54 2.65 -
3.18 2.61 -
3.81 2.62 -
4.45 2.53 -
LSD (0.1) NS -
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DISCUSSION
The primary objective of this research project was to maximize divot resistance of thick-
cut KBG sod through manipulation of pre-harvest cultural practices, while maintaining
adequate sod strength for harvesting and installation. Data from each measured parameter
were presented in the Results section. In addition, a number of morphological
characteristics of the Kentucky bluegrass were measured and correlated to the divot
resistance and sod strength resulting from the varying treatments. This section
synthesizes these data and interprets their practical meaning within the context of sod
production and surface performance.
The Discussion section first addresses how the experimental treatments affected divot
resistance, shear strength, and sod strength which are the foci of this research project. A
discussion of the changes in morphological characteristics of the Kentucky bluegrass
resulting from the various treatments and their relationship with divot resistance and sod
strength follows. The section ends with a discussion of future research opportunities and
suggestions.
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Field Evaluations
Divot Resistance
Cutting height effect on divot resistance
It was somewhat surprising that cutting height had no significant effect on divot
resistance as measured by the length of divots produced with Pennswing. As regular
cutting is the most fundamental practice to turfgrass culture, cutting height has a central
influence on the growth habit and morphology of a sward (Turgeon, 2012). In
Experiment 1, two relatively similar cutting heights were evaluated. The small difference
in cutting heights may have contributed to the lack of a difference in divot length
measurements. Experiment 2; however, contained a wider range of cutting treatments
from 2.54 to 4.45 cm which also had no significant effect on divot length. A slight trend
of smaller divots under lower cutting heights was evident. The shortest cutting height of
2.54 cm produced a mean divot length of 27.0 cm compared with 30.7 cm for the tallest
height of cut (4.45 cm). Experiment 2 contained only 12 total experimental units. Due to
the large degree of variability in the size of divots produced by Pennswing, it is possible
that the number of experimental units in Experiment 2 was too small to accurately
capture the differences in divot resistance.
Aside from the experimental design, cultivar influences and TE-induced growth
regulation may have also masked cutting height effects on divot length. Unpublished data
from research at The Pennsylvania State University suggested very low cutting heights
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(<2.5 cm), improved divot resistance in Kentucky bluegrass (McNitt, 2014, personal
communication). However this effect occurred only for certain cultivars, while other
cultivars were either not influenced or performed better at higher heights of cut (Fig. 27).
Marked genetic diversity exists among KBG cultivars and other studies have also shown
the cultivar influence to play an integral role in divot resistance (Murphy et al., 2004;
Serensits, 2008; Trappe et al., 2011).
The cultivars used in Experiments 1 and 2 are considered to have compact (‘Everest ‘and
‘Boutique’) and laterally aggressive (‘P-105’) growth habits (Murphy et al., 2004).
Because all plots in Experiments 1 and 2 contained the same cultivar blend, the cutting
height effect could have been muted.
Growth regulation is an additional factor which may have masked potential cutting height
effects on divot resistance. All the turf in both Experiments 1 and 2 was maintained under
growth regulation with trinexapac-ethyl (TE). TE has been shown to produce
morphological effects similar to those observed under close defoliation. For example,
TE-treated plants exhibit a more rapid tillering rate and greater shoot density (Ervin and
Koski, 1998, 2001a). Additionally, TE reduces elongation of leaf blades and sheaths,
producing a compact or “miniature” growth habit (Ervin and Koski, 2001b; McCarty,
2014). It is possible that the growth-regulating effects of TE masked any effects on divot
size which would have otherwise occurred due to cutting height treatments.
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Figure 27. Divot lengths as affected by a cultivar by cutting height interaction; the unpublished data were provided by McNitt
(2014, personal communication).
0.0
5.0
10.0
15.0
20.0
25.0D
ivot
len
gth
(cm
)
Cultivar
2.22 cm CH
3.49 cm CH
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Topdressing effect on divot resistance
A potential concern in applying sand topdressing to young sod is the possible decrease in
divot resistance and sod strength. Sand particles possess little inherent cohesion, so if
more sand is applied than can be encapsulated by turfgrass stems and roots, the surface
can become unstable and prone to scuffs and divots.
In this experiment the topdressing treatments did not cause significantly larger or smaller
divots. This result was consistent across all cutting and N treatments, suggesting that 8.5
kg sand m-2 yr-1 is an acceptable topdressing rate under the other cultural circumstances
of this project. In other words it is apparent that when 14-month old KBG sod is
maintained under this range of cutting heights and N rates, applying 8.5 kg sand m-2 will
not influence the sod’s divot resistance when harvested at 4.45 cm depth. The total sand
rate was split over three applications to progressively dilute the organic matter
accumulation and avoid creating a layered profile. Since the application of 8.5 kg sand
m-2 had no effect on divot resistance compared to no topdressing, other rates should be
investigated. Perhaps a lower rate would improve divot resistance or at a minimum
produce similar results to the rate used in this project. The cost of sod production may be
lowered with a lower topdressing rate.
While the results of this project did not indicate a strong relationship between topdressing
and divot resistance, sand topdressing is still recommended to sod growers who produce
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thick-cut KBG for in-season football field replacements. The current market for thick-cut
in-season sod replacement demands little to no thatch and topdressing is currently the
only effective, non-mechanical method to mitigate thatch accretion. Topdressing is
performed ubiquitously by golf and sports turf managers, especially following core
cultivation, but topdressing during sod production is a relatively new practice. In most
cases the material costs are prohibitive, and benefits of topdressing are not evident when
the sod is produced for applications other than high-end athletic fields. On such fields, the
presence of thatch has been observed to cause more prevalent ‘scuffs’ and sometime
more prevalent divots. Scuffs are defined as when the surface portion of the plant (and
often the thatch) is torn out by athletes maneuvering on the surface wearing cleated
footwear. Typically, scuffs are primarily only an aesthetic concern as grass crowns
remain and regrowth is relatively rapid. However, some observations suggest that thatch
leads to an increase in divots where crown material is also removed. Since a slight
relationship between thatch thickness and divot size was indicated in this research, and
topdressing had no negative effect on divot size, it is suggested that topdressing practices
are continued and more research be conducted to further investigate the effect of various
topdressing types, rates, and timings on divot resistance.
Nitrogen treatment effect on divot resistance
Nitrogen fertilization was the only treatment with a significant influence on divot sizes at
both locations. The highest N treatment (4 applications in spring and 1 in fall, totaling
244 kg N ha-1) produced the largest divots by a wide margin; the mean divot length for
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this program was 21% greater than the average of all other N treatments and 37% greater
than the most effective treatment (3 spring applications; no fall N). Divot depths were
affected similarly, with the 4-1 treatment producing depths 19% greater than the average
of all other treatments and 29% greater than the most effective treatment (again the 3-0
schedule). At the TTF location no effect occurred on divot lengths, but divot widths and
depths were significantly larger under high N. The 4-1 N treatment produced 9% wider
divots than the mean of all other treatments, and a 12 % reduction in divot depth was
produced by withholding the September N application. High-N plots exhibited darker
green color (data presented in Appendix; Tables 92-100) and had excellent shoot density.
However these traits may come at the cost of reduced divot resistance, which is of greater
consideration than aesthetic appeal on American football fields.
The 4-1 N treatment was the treatment most similar to the fertilization schedule actually
used by TTF in current production of thick-cut sod for NFL stadia. This research project
suggests N rates below the current standard may be advantageous with regard to divot
resistance. During seedling establishment, N was applied on two dates to total 84 kg N
ha-1. After the precursory 98 kg N ha-1 was applied in spring of 2013, the turf had
established 100% ground cover and N treatments began. In this project the smallest
divots were produced with just one additional N application, for a total of 146 kg N ha-1
over the 2013 growing season. This rate is lower than those commonly suggested for
maintenance of Kentucky bluegrass, which are 196-294 kg N ha-1 yr-1 (c.f. Carrow et al.,
2001; Puhalla et al., 2010). Kentucky bluegrass is widely considered by turf managers to
have an N requirement among the highest of all cool-season turfgrass species. A
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perception also exists among some sod growers that a high N supply can “push” the turf
to accelerate sod development. This perception likely stems from the positive color,
density, and vigor responses to additional N. In the past these sod growers have applied
as much as 400 kg N ha-1 yr-1 to young Kentucky bluegrass sod (McNitt, 2014, personal
communication). No research data are available on the effects of increasing N rates in an
attempt to accelerate sod harvest. However reports from sod industry representatives
indicate such attempts usually fail (Charbonneau, 2000; Cisar, 2000).
Once a new turf reaches a maximum leaf area index, shoot density does not respond to
additional N (Simon and Lemaire, 1987). Furthermore, below-ground responses are
critical for a successful sod harvest and are likely to be hampered by over-fertilization
with N (Badra et al., 2005). Thus a successful post-seeding strategy may be to apply
sufficient N to quickly reach a maximum leaf area index, before “hardening off” the turf
during the summer months. The greater number of individual plants produced during the
spring can then produce more roots and rhizomes as the N supply is subsequently
depleted.
Larger divots under higher N may be attributable to factors including reduced below-
ground biomass, greater thatch, and/or plant succulence. In this project, below-ground
biomass was not significantly correlated to divot length (see Correlations section below),
although smaller divots and greater BGB were both observed under low N treatments.
This effect can likely be attributed to shoot priority, which dictates that as N becomes less
limiting, turfgrasses will preferentially allocate energy to leaf and tiller production rather
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than growth of subterranean stems and roots (Carrow et al., 2001; Badra et al., 2005;
Bell, 2011).
It must be acknowledged that other benefits to the sod grower are associated with rapid
establishment of contiguous ground cover through high N rates. Perhaps the most notable
are reductions in soil erosion and weed encroachment. In the case of thick-cut sod
production for NFL stadia, the production interval is relatively fixed, so rapid harvest
potential is not a concern. The seeding date is constrained by favorable growing
conditions in late summer the year before the sod harvest, and the harvest date is dictated
by the NFL playing season.
Sod Strength
Estimation of minimum acceptable sod strength
Harvesting and installation of sod requires a minimum level of sod strength. If not
sufficiently knitted, the strip cannot be removed from the sod field and transplanted to a
site without falling apart. Thus SS must exceed a minimum threshold to be considered
harvestable. The minimum acceptable SS for thick-cut sod is higher than for standard-
depth sod due to the greater soil weight; however this value has not been estimated in
prior studies. Published values for minimum acceptable sod strength range from 20-50
kg. These values do not account for varying thicknesses or widths of the sod strips tested
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in such studies. Additionally the values are applicable to standard-size palletized sod
rather than thick-cut big rolls.
SS can instead be expressed as a function of cross-sectional area to standardize values
across studies and predict SS values for various sod strip dimensions. A minimum
acceptable SS value for thick-cut sod could then be estimated by calculating the cross-
sectional area of a thick-cut, big roll strip and multiplying by the sod strength per unit
area obtained in other studies (Table 90). However this approach would assume SS to be
equal at all vertical positions of the sod strip. As root and rhizome mass nearly always
declines with depth, the bottom portion of the sod strip might be expected to have
relatively lower SS than the upper portions, where more roots and rhizomes reside. Since
all published studies have tested sod with appreciably thinner sod, this method could
underestimate the actual minimum value needed for the harvest.
As opposed to the calculation described above, an estimate for the minimum acceptable
SS for thick-cut, big-roll sod was obtained through direct SS measurements taken at the
TTF location (see Table 91 in Appendix). Three-month old Kentucky bluegrass sod was
harvested at 4.45 cm soil depth, as for Experiments 1 and 2. At the date of testing, TTF
personnel considered this sod to be slightly below the minimum strength needed to
harvest it in big rolls at 4.45 cm depth. The mean SS for this sod field was 39.0 kg. Thus
a very conservative estimate of 100 kg was chosen as the minimum SS needed to
successfully harvest, transport, and install thick-cut, big-roll sod in this research project.
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Sod strength was significantly affected by all three treatments: cutting height,
topdressing, and nitrogen fertilization. The following subsections relate these effects to
related literature and practical considerations.
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Table 90. Comparison of data collected in Experiment 1 with selected published values for KBG sod strength on a per-unit-area
basis.
Reference Sod age Sod strength† Sod strength
per unit area Comments
mo. after
seeding ---- kg ---- ---- kg dm-2 ----
Experiment 1- VRC location 14 232.9 114.6 -
Experiment 1- TTF location 14 173.4 85.3 -
Ross et al., 1991 7 30.1 154.4 greenhouse experiment
Heckman et al., 2001 27 65.8 101.8 sod heating study
Shearman et al., 2001 12 62.0 172.2 cultivar evaluation
Kowalewski et al., 2008 11 84.1 96.6 field experiment
Li et al., 2011 14 27.1 71.0 conducted on high clay soil
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Cutting height effect on sod strength
Cutting height had only a small influence on sod strength. No significant effect was
detected in Experiment 1, which tested heights of 3.18 and 3.81 cm. In Experiment 2, sod
tensile strength increased with cutting height, but the highest cutting height of 4.45 cm
was the only treatment statistically different from the others. While sod strength was
slightly reduced for the lower cutting heights, 2.54-3.81 cm heights still produced
adequate tensile strength for a thick-cut, big-roll harvest (well above 100 kg). These data
dispute prior anecdotal concern that maintaining the sod under cutting heights less than
3.81 cm could weaken the root system and result in unharvestable sod. Cutting height
should instead be chosen to match the eventual maintenance cutting height chosen by the
football field manager. This CH is typically between 2.54 and 3.81 cm. Cutting height
should therefore fall within this range, which falls on the low end of cutting heights to
which KBG is adapted (Turgeon, 2012). Producing sod for in-season replacements at a
CH greater than 3.81 cm is not recommended since the canopy height will be likely be
lowered at the stadium to about 3.18 cm. This severe reduction could weaken the turf by
compounding the stressful conditions imposed during the sod harvest and transport
process (Crider, 1955). The lower cutting height would also aid in more rapid
establishment from seed (Brede and Duich, 1984).
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Topdressing effect on divot resistance
Topdressing reduced sod strength by 6% on average. The mean sod strength for
topdressed experimental units was 149.5 kg at the TTF location and 206.8 kg at the VRC
location. These values would still be considered more than adequate for a thick-cut big-
roll harvest sod strength (>100 kg). Care should still be taken to avoid over-application of
sand, as this could reduce SS below critical levels.
Nitrogen treatment effect on divot resistance
Highest sod strength was obtained with 196 total kg N ha-1. At the TTF location the
greatest sod strength was 375 kg under the 3-1 N treatment. At the VRC location the
highest SS was 483 kg under the 4-0 N treatment. At both locations these “optimum”
treatments were not statistically different from any other N treatments receiving 144 or
196 kg total N (3-1, 4-0, or 4-1 treatments). No N treatments produced unharvestable sod;
consequently, growers can apply N at the rates which optimize divot resistance without
fear of deleterious effects on sod strength.
Other recent work with KBG suggested slightly lower N (120 kg N ha-1) produces
optimum sod strength (Li et al., 2011). However the study by Li et al. (2011) was
specifically conducted to evaluate sod production on clayey soil. The soil used in their
experiment contained 52% clay and 46% silt, and also contained 4.2% organic matter by
mass. The soil in this research project was a loamy sand with nearly 90% sand and just
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1.2% organic matter. It might be hypothesized that the clayey soil used by Li et al. (2011)
would have had less N leached from the profile, as well as more microbial mineralization
of organic N during the season. These differences in soil properties may help explain why
higher N rates produced greater sod strength in the current research project.
Shear Strength
Shear strength was measured with the Turf-Tec Shear Strength Tester, the same device
currently utilized by the NFL in their Game Day Certification program. The device does
not measure divot resistance, but is a related measurement and is more portable and less
destructive than Pennswing. It was used in Experiments 1 and 2 to compare shear
strength values with the divot sizes produced using Pennswing.
Cutting height effect on shear strength
Cutting height had a varied effect on shear strength. In Experiment 1 at VRC, shear
strength was significantly greater under higher cutting heights, while at TTF the opposite
occurred. No significant difference occurred in Experiment 2 across the four cutting
heights, although the trend was greater shear strength for higher heights of cut.
Regardless of the statistical outcome, all these differences were small and of little
consequence. Nearly all shear strength values were above 20 Nm, considered
“exceptional” by the shear strength device’s manufacturer (Mascaro, 2013). Typically,
the shear strength of all newly installed sod in NFL stadia measure high in shear strength.
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The shear strength is diminished as the above-ground verdure and below-ground biomass
are reduced by traffic (Serensits, 2008; Kowalewski et al., 2011). Most research on shear
strength therefore includes a simulated wear component to better elucidate the treatment
responses. As no wear was applied in this project, it is likely that any cutting height
effects on shear strength were less important than the fact that all plots had 100%
vegetative cover.
Topdressing effect on shear strength
Topdressing significantly reduced shear strength at both locations. The engineered sands
used for topdressing have little cohesion and this property is likely responsible for the
reduction in shear strength observed for topdressed sod. This is probably the same
phenomenon responsible for slightly lowered sod strength due to topdressing. The
magnitude of the shear strength differences were small, and the benefits of topdressing in
order to offset thatch buildup probably outweigh any marginal decreases in shear
strength.
Nitrogen treatment effect on shear strength
When averaged across all other treatment levels, nitrogen treatments had no statistical
effect on shear strength, nor did they result in any discernable trend. All mean shear
strength values were very similar among nitrogen programs. This result was somewhat
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177
surprising given that divot sizes were larger with the highest N inputs. While shear
strength and divot resistance may be considered related properties, the direction and
magnitude of the force created by Pennswing is quite different from that of the shear
vane. In addition, the shear vane was designed as a simple device for rapid field
measurements, rather than a precise research tool. Serensits (2008) found that the shear
vane was less sensitive than Pennswing in detecting differences among treatments.
Turfgrass morphological characteristics
Various morphological characteristics of the turf that have been previously described
were measured for each plot in these experiments. These characteristics were evaluated
for their known or purported relationships to divot resistance. This section discusses the
effect of the cutting height, topdressing, and nitrogen fertilization regime on these
morphological characteristics in November 2013, when divot size and sod strength was
measured.
Shoot density
Cutting height effect on shoot density
Shoot density was significantly affected by cutting height. As commonly reported, the
lower height of cut produced greater density at both locations. This phenomenon occurs
as the turfgrass plants attempt to maintain a constant total leaf area under more severe
defoliation (Eggens, 1981). However the small increase in density may be solely of
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aesthetic value, as the increased density did not correlate to divot size (see Correlations
sub-section below).
Topdressing effect on shoot density
Topdressing had no significant effect on shoot density at either location. Shoot density
was essentially equal for topdressed and untreated plots.
Nitrogen treatment effect on shoot density
Nitrogen treatment significantly affected shoot density. Higher spring N rates resulted in
higher November shoot density if no N was applied beyond June. This difference was
observed in November - several months after the last spring N application. However,
when a September N application was made, any influence from spring fertilization rate
was masked. Each of the three treatments receiving a fall N application had statistically
equal density regardless of differences in spring application rate. A practical implication
of this finding is that even under low total N (desirable for greater divot-resistance), high
November shoot density could be achieved by using little N in the spring and making a
single N application in the fall. This could produce denser, more aesthetically pleasing
turf without compromising divot resistance.
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Thatch
Cutting height effect on thatch
At the VRC location, a small but statistically significant difference in thatch
accumulation was detected among cutting height treatments. Thatch mass was 12%
greater at the higher CH of 3.81 cm. Such an effect was not observed at TTF. A positive
relationship between height of cut and thatch accumulation is consistent with prior
research (Murray and Juska, 1970; Shearman, 1980). The practical value of this
difference is unresolved, as the integration of other cultural practices (specifically
nitrogen management and topdressing) and turfgrass genotype can complicate the cutting
height influence (see Results section; c.f. Shearman, 1980).
Topdressing effect on thatch
Topdressing significantly reduced thatch levels; an expected result, as thatch dilution is a
primary goal of topdressing (Turgeon, 2012). The total sand rate of 8.5 kg m-2 reduced
the mean thatch mass to 0.94 g per plug, compared 2.23 g for the control plots. It is
important to apply the sand in light, frequent doses to incorporate the material evenly and
avoid creating layers in the soil profile. Such layers could become shear planes along
which divots could form due to restricted drainage and rooting.
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Nitrogen treatment effect on thatch
Greater N rates significantly increased thatch in Experiment 1. This result is consistent
with some prior thatch research (Duncan and Beard, 1975; Weston and Dunn, 1985)
although such a result is not always obtained under higher N (Shearman, 1980; Carrow et
al., 1987). The discordance among research trials can probably be associated to the
complexity of the thatch system, which contains many interacting components including
species/cultivar, weather, irrigation frequency, and soil conditions (Waddington, 1992).
The simplest explanation for increased thatch under high N is greater total biomass
production. The turfgrass plants grow more vigorously, thus fixing carbon at a greater
rate than microbial organisms can decompose the biomass. However, a complication in
ascribing thatch levels solely to N supply is that most N fertilizers are weak acidulants.
Soluble ammonium N sources (such as the ammonium sulfate used in this study) tend to
decrease soil pH and slow microbial breakdown of thatch with greater application rates.
This process may occur over longer time scales than the 14 months spanned by this
project, although the coarse texture and low buffering capacity of the soil would indeed
be conducive to rapid pH shifts. Prior to the initiation of treatments, a composite sample
from each location indicated pH values of 6.4 and 6.5 at TTF and VRC, respectively. Soil
pH was not measured in Experiments 1 or 2 following the initiation of treatments.
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Below-ground biomass
Cutting height effect on below-ground biomass
In Experiment 1, the lower cutting height (3.18 cm) produced significantly higher below-
ground biomass in the 4.45 cm-thick sod profile than did the higher cutting height (3.81
cm). This result was observed at both TTF and VRC. This effect was not observed in
Experiment 2, although the small number of experimental units may have contributed to
the lack of a trend.
It is well-understood that gross root mass is diminished by closer cutting (Juska and
Hanson, 1961; Eggens, 1981; Shearman, 1989). However, divot resistance and sod
strength are not influenced by roots deep in the profile. It is thus desirable to maximize
BGB within the 4.45 cm sod layer. Unpublished research has suggested that while close
cutting reduces overall BGB production, root and rhizome density in the uppermost
portion of the profile is actually increased. (McNitt, 2014, personal communication).
Data from this project would support such a phenomenon, as the 4.45 cm sod profile
contained greater BGB under the lower of the two heights. The reduced vigor of the 3.81
cm area at TTF could also have contributed to the significant difference at TTF.
Topdressing effect on below-ground biomass
Topdressing significantly increased below-ground biomass. However this effect was most
likely due to the sampling method rather than the grass actually producing more BGB as
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a result of sand applications. Topdressed plots had sand incorporated among the majority
of their rhizome and crown tissues, while this same material became thatch for control
plots. Thus similar plant material was classified differently among topdressing
treatments, and the literal result of this effect does not have a practical meaning.
Nitrogen treatment effect on below-ground biomass
Nitrogen treatment significantly affected below-ground biomass at the VRC location,
though this trend was not observed at the TTF location. Higher N rates decreased below-
ground biomass. This result is consistent with the volume of research showing higher N
decreases root: shoot ratios and favors partitioning of axillary buds to tillers rather than
rhizomes (McIntyre, 1964; Goss and Law, 1967; Adams et al., 1974).
Correlations among measured parameters
While statistically significant correlation among the various parameters were found, few
very strong and meaningful relationships were detected. Divot length was perhaps the
most important characteristic measured in this experiment. Divot length significantly
correlated with divot width and depth (r=0.66 and 0.33 respectively). It is logical that for
a less divot-resistant plot, divots would be larger in all three dimensions. Divot length is
considered the best indicator of divot resistance and had the strongest relationships with
other measured parameters.
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Curiously, divot length was negatively correlated with thatch thickness (r=-0.26). This
finding is contrary to the prevailing opinion among turf managers that thatch increases
divoting. In this project experimental units with greater thatch levels tended to produce
smaller divots, although such an association does not necessarily reflect a causal
relationship. It is possible that the perceived association between thatch and more
divoting is related to the scuffing effect previously described in this Discussion section.
During competition, athletes’ cleated footwear tends to tear the turfgrass shoots and small
portions thatch from the surface, colloquially termed “scuffs.” Scuffs differ from divots
in that after a scuff forms the turfgrass crowns remain intact and little soil is removed.
Scuffs therefore pose little threat to player safety and performance, yet the prevalence of
scuff debris across the surface is unsightly and can contribute to perception that the
surface is unstable. While Pennswing simulates the severe impact energy of a large
athlete contacting the surface at high speed, it does not capture the surface disruption
caused by frequent, less intense athletic maneuvers also performed during competition.
Informal observations during data collection indicated that upon impact the club head
tended to “bounce” on plots with greater thatch while on plots with less thatch the club
was more likely to penetrate through the thatch and into the underlying soil to produce a
divot. The relationship between thatch and divot resistance is not fully resolved and
merits further study. A device which more accurately simulates athlete-to-surface contact
should be developed.
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Thatch mass and thickness were each positively correlated with sod strength (r=0.21 and
0.40 respectively). The experiences of TTF growers corroborate the notion that a thatch
layer increases sod strength (J. Betts, 2013, personal communication). Increased N levels
tended to produce more thatch, less below-ground biomass, and also greater sod strength.
Perhaps the thicker thatch layer helped prevent a decrease in SS, which would have
otherwise been expected with reduced below-ground biomass. A similar relationship was
observed for shear strength (r=0.62 with thatch mass; r= 0.51 with thatch thickness).
These correlations probably resulted from the looser surface produced by topdressing
treatments, which simultaneously reduced thatch levels. Greater thatch levels have been
previously related to increased surface shear strength (Shildrick and Peel, 1984; Chivers
et al., 2005)
Shoot density was positively correlated with sod strength (r=0.30) but not significantly
related to divot resistance. Other research has related shoot density to divot resistance on
a trafficked football field (Serensits, 2008). However, the current study did not include a
simulated wear treatment to thin plots. The absence of traffic may account for the lack of
a significant correlation between density and divot resistance. The fact that all
experimental units had 100% turfgrass cover may have diminished the importance of
density.
Shoot density was significantly correlated with shear resistance, although the correlation
was not strong (r=0.26). Contrary to Pennswing, this relationship indicates the Turf Shear
Tester still detected density differences under conditions of 100% ground cover. Shoot
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density was also positively correlated with shear strength in other studies (Shildrick and
Peel, 1984; Serensits, 2008).
Potential for related future research
Fraise cutting
Minimizing thatch buildup is a common objective of turf managers. In this project light,
frequent topdressing applications mitigated thatch. Other means of thatch control are
available. A novel method of thatch removal is colloquially known by practitioners as
fraise cutting or “KORO-ing,” in reference to inventor Ko Rodenburg (KORO by Imants,
2014). KORO machines utilize an array of toothed, helical blades to grind away the green
verdure and thatch, leaving only rhizomes and crowns intact. The waste debris is
deposited into a hopper via conveyor belt and removed from the system. With proper
management the turf will regenerate from the remaining meristems.
Mechanical thatch control methods are not usually practiced by sod growers due to their
expense and negative impact on sod strength. If used to control thatch in sod fields, the
turf would need time to regenerate before being harvested- extending the production
period and raising the production cost. However if fraise cutting or related practices were
able to produce a marked increase in divot resistance, it is likely sod growers would adopt
these practices for high-end thick-cut sod. Customers would be willing to bear the
increased cost in exchange for the increase in quality.
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Anecdotal evidence suggests that following fraise cutting, regenerated turf has intact
rhizomes and excellent stability (Minnick and Reed, 2013). Perhaps these observations
are related to an apical dominance phenomenon, which is described in the following
section on verticutting. The observations may also result from abundant generation of
young, very photosynthetically active leaf tissue (Dunn and Engel, 1971), or simply to a
reduction in thatch. Research is needed to determine the value of fraise cutting during
production of thick-cut KBG sod.
Verticutting
Practitioners have reported greater divot resistance following mechanical cultivation and
attribute it to increased rhizome production. Divot resistance was in fact improved by
cultivation in the work by (Serensits, 2008) despite no measured change in root or
rhizome mass due to aerification and vertical mowing. Greater rhizome production
following verticutting has not been substantiated in field research trials, but the
phenomenon would agree with fundamental experiments on rhizomatous grass plants
(McIntyre, 1970; Mcintyre and Cessna, 1998). Several experiments with quackgrass
(Elytrigia reptans) showed that by severing rhizomes, undeveloped rhizome buds were
released from apical dominance and developed as new rhizomes and aerial shoots.
Dominance of rhizome apices and parent shoots over rhizome lateral buds by is a
complex phenomenon also governed by temperature, photoperiod, and N availability.
These environmental signals moderate various hormonal feedback loops involving
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auxins, abscisic acid, gibberellins, and ethylene (Gang, 2013). More research is needed to
better understand these processes and whether they occur with KBG in a field setting,
where significant interplant competition exists for light, water, nutrients, and physical
space.
TTF has in fact attempted to verticut KBG for thatch control prior to a thick-cut sod
harvest. The subsequent harvest failed due to insufficient sod strength. A post-verticut
addition of sand topdressing buried the remaining thatch layer, which coupled with
unfavorable weather conditions may also have contributed to the failed harvest.
Verticutting warrants a more controlled evaluation with regard to timing and intensity of
cultivation events before definitive claims can be made about its influence on sod
strength and divot resistance.
Nitrogen and divot resistance on established turf
This study tested divot resistance of immediately after sod installation. Nitrogen had an
influence on divot resistance in this project and has previously been shown to impact
wear resistance and subsequent recovery (Canaway, 1984; Carroll and Petrovic, 1991;
Hoffman et al., 2010). Thus a study could test N treatments during sod production, with
the addition of a simulated wear component following installation. Such an experiment
would help evaluate the pre-conditioning N programs’ efficacy over the duration of the
sod’s lifetime (typically 4-6 NFL games). However, it should be noted that during the
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time of year when thick-cut sod is typically installed, turf recovery is minimal due to low
light and temperature conditions. Thus wear tolerance is mainly a function of divot
resistance immediately after harvest (as tested in this project) and N supply would be
expected to have a lesser effect due to reduced growth potential.
Nitrogen effects on divot resistance for established turf
Voluminous research data exist with regard to the effects of nitrogen fertilization on
athletic field wear tolerance. However these studies have used percent ground cover,
turfgrass color, or other visual ratings to evaluate wear tolerance (e.g. Canaway, 1984;
Carroll and Petrovic, 1991; Sorochan et al., 2001). In the case of NFL or facilities with
infrequent yet intense usage, divot resistance may be a more important response variable.
Nutrient requirements can vary considerably between newly established and mature turfs.
Since the current experiments geared toward sod production showed N to be important in
optimizing divot resistance, the effects of N on the divot resistance of established turf
warrant study.
Improved research tools for evaluating divot resistance
In this project, both Pennswing and the Shear Strength Tester detected differences among
treatments, although different treatments affected the two measurements. Nitrogen
treatment was the only factor to affect divot sizes produced by Pennswing, but the two
other treatments (cutting height and topdressing) affected shear strength. The shear vane
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probably does not simulate a divot-forming impact as well as the pendulum device, but it
was still able to detect treatment effects of cutting height and topdressing in this project.
In the work by Serensits (2008) the shear vane was less sensitive in detecting differences
among treatments than Pennswing.
While significant differences in the size of divots produced by Pennswing were detected
in this study, turfgrass researchers would benefit from an improved divot production
device. Most devices designed to test divot resistance are inserted into the turf at a
specified depth before being activated in order to produce a divot. An example is the
Clegg Turf Shear Tester. This device utilizes a paddle inserted into the soil, which is then
rotated about a horizontal axis to produce surface disruption. Pennswing forcefully
impacts the surface from above without prior insertion, as is the case during actual
athletic competition. However Pennswing may not accurately simulate the actual
magnitude or direction of forces imparted to the surface by athletes with studded
footwear. A divoting device which better simulates these forces should be developed for
future studies and compared to the current methodology. Ideally such a device would use
the actual studded footwear used by professional athletes, a feature of the traction testing
device described by McNitt et al. (1997). An improved divoting device should also utilize
various loading weights and permit significant flexure upon impact with the surface in
order to allow divot size to be more sensitive to the turf properties than the device’s
dimensions. Finally it would be advantageous if the device were more portable, in order
to permit measurements to be taken at various sites.
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SUMMARY AND CONCLUSIONS
The goal of this project was to optimize the divot resistance of newly installed thick-cut
Kentucky bluegrass sod. Data from the experiments can inform cultural management
strategies during the sod production period, prior to harvest and installation.
Cutting height did not significantly affect divot resistance. The uniform blend of cultivars
and application of trinexapac-ethyl to all experimental units may have overshadowed the
influence of cutting height on plant morphology and divot resistance. Of the two heights
evaluated in this study, the shorter cutting height of 3.18 cm is recommended on the basis
of its similarity to actual cutting heights used on professional American football fields.
By keeping the canopy at a consistent height, the stress from a reduction in cutting height
following the sod harvest can be eliminated. Lower cutting heights also tend to produce
greater linear traction, a surface characteristic desired by most athletes (McNitt, 1994).
The 3.18 cm cutting height also produced better shoot density, although this density
increase may be only of nominal aesthetic value with little value towards playability.
Closer clipping did not prevent a successful sod harvest, despite prior concern by
practitioners.
A topdressing rate of 8.5 kg sand m-2 was sufficient to essentially eliminate thatch
accumulation. Applying sand at this rate did not increase or decrease divot resistance. In
the future other rates of topdressing should be evaluated to further elucidate the effects of
sand topdressing on divot resistance. A very small reduction in sod strength was observed
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with topdressing, but the sod was still harvestable, as all sod strength values were well
above the 100 kg threshold.
Nitrogen had the greatest influence on divot resistance of any experimental treatment.
Divot resistance was negatively impacted by greater N supply. At the VRC location, the
highest N rate (4-1 treatment; 244 total kg N ha-1) produced 37% larger divots than the
most effective N rate (3-0 treatment; 144 total kg N ha-1) and 27% larger than the average
divot length of all other N treatments. The 4-1 N treatment also produced the least below-
ground biomass and the most thatch. This treatment was most similar to actual
fertilization programs in place at TTF during the experiments. These data suggest a
reduction in the N input will improve divot resistance.
Divot length was not affected by N treatment at TTF, but divot lengths and depths were
larger under higher N treatments. N treatment effects may have been partially muted at
TTF by the overall deterioration of the plots. Supervisors of the TTF facility were unable
to pinpoint the cause of decline. The reduced turf quality may have resulted from
inadequate irrigation, a faulty pesticide application, insufficient N prior to the
experiment, or other unknown factors.
The lack of simulated traffic in this project may account for the small number of
significant treatment effects on divot resistance. In prior studies of divot resistance, plots
receiving different treatments but not exposed to traffic showed only small differences in
divot resistance (Serensits et al., 2011). Although nitrogen did significantly affect divot
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resistance, all experimental units had 100% turfgrass cover which may have diminished
the magnitude of nitrogen and other treatment effects.
The sod produced at the VRC location measured considerably higher in divot resistance
and sod strength compared to sod at the TTF location (31% and 26%, respectively). As in
all agricultural endeavors, climatic and edaphic conditions play a central role in the sod
production process. Higher air temperatures at TTF were less conducive to healthy cool-
season grass than the temperatures at VRC. Greater heat and humidity could have
stressed the turf and prevented maximum root and rhizome growth. While treatments
such as reduced nitrogen inputs were beneficial in this project, perhaps the greatest
improvement to the quality of the thick-cut sod produced at TTF would be attention to
detail. The overall level of cultural intensity was probably lower at TTF due to the plots’
location in a production field, rather than a more controlled research facility. Diligent
cutting and ample yet judicious irrigation are two such examples of basic practices
essential to successful turfgrass culture.
Data from this research project can provide sod growers with a deeper understanding of
their product. These data also will help refine the cultural practices used during
production of thick-cut sod. Such knowledge can ultimately improve the safety and
playability of professional football surfaces.
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APPENDIX
Additional Materials
Table 91. Properties of sod from this research project compared with actual sod produced
by TTF and installed at NFL stadia in November 2013. All sod tested was harvested at
4.45 cm profile thickness.
Sod type
Divot dimensions
Sod strength Shear strength length width depth
-------- cm -------- -- kg peak force -- --- Nm ---
VRC research plots 24.2 5.6 1.6 211.5 28.3
TTF research plots 31.6 7.1 1.7 155.9 24.4
3.81 cm† NFL sod at TTF 31.0 7.5 2.0 135.6 23.9
3.18 cm† NFL sod at TTF 30.0 7.7 1.5 138.7 22.0
Standard KBG sod at TTF* - - - 177.7 26.9
3-month old KBG sod* - - - 35.0 -
† indicates cutting height at which tested sod was maintained throughout the 2013 growing season
* indicates sod was tested from a standard production field, rather than one selected for thick-cut
NFL sod.
203
Table 92. Mean visual color ratings for the cutting height main effect at VRC.
Cutting height Color rating Letter grouping
---- cm ---- ------------
3.18 6.0 B
3.81 7.1 A
LSD (0.05) 0.7 -
Table 93. Mean visual color ratings for the topdressing main effect at VRC.
Topdressing applied Color rating Letter grouping
---- kg sand m-2 ---- ------------
0.0 6.4 -
8.5 6.6 -
LSD (0.05) NS -
Table 94. Mean visual color ratings for the nitrogen treatment main effect at VRC.
Nitrogen treatment† Color rating Letter grouping
--------
2-0 4.9 D
2-1 7.6 A
3-0 5.5 C
3-1 7.6 A
4-0 6.0 B
4-1 7.6 A
LSD (0.05) 0.5 -
204
Table 95. Selected contrasts comparing visual color at VRC as related to total both total nitrogen applied and individual nitrogen
treatments.
Contrast label Nitrogen applications† Color rating
Difference
relative to
second value
Pr > F Statistical
Significance ††
--------------
-------- % -------
-
A 0 fall vs. 1 fall 5.5 vs. 7.6 -28% <.0001 ***
B 3 total vs. 4 total 6.5 vs. 6.8 -4% 0.156 NS
C 4 total vs. 5 total 6.8 vs. 7.6 -10% 0.001 ***
D 3 total vs. all other rates 6.5 vs. 6.5 0% 0.890 NS
E 5 total vs. all other rates 7.6 vs. 6.5 +16% <.0001 ***
† Each application supplied 49 kg N ha-1. Labels indicate which N treatment combinations were pooled for the contrast. Nomenclature
for each contrast's null hypothesis is as follows:
A H0: μ 2-0, 3-0, 4-0 = μ 2-1, 3-1, 4-1
B H0: μ 2-1, 3-0 = μ 3-1, 4-0
C H0: μ 3-1, 4-0 = μ 4-1
D H0: μ 2-1, 3-0 = μ 2-0, 3-1, 4-0, 4-1
E H0: μ 4-1 = μ 2-0, 2-1, 3-0, 3-1, 4-0
†† * = significant at 0.05 level, ** = significant at 0.01 level, *** = significant at 0.001 level
205
Table 96. Mean visual color ratings for the cutting height main effect at TTF.
Cutting height Color rating Letter grouping
---- cm ---- ------------
3.18 5.8 B
3.81 4.4 A
LSD (0.05) 1.3 -
Table 97. Mean visual color ratings for the topdressing main effect at TTF.
Topdressing applied Color rating Letter grouping
---- kg sand m-2 ---- ------------
0.0 5.1 -
8.5 5.1 -
LSD (0.05) NS -
Table 98. Mean visual color ratings for the nitrogen treatment main effect at TTF.
Nitrogen treatment† Color rating Letter grouping
------------
2-0 4.2 D
2-1 5.4 A
3-0 4.5 C
3-1 5.9 A
4-0 4.6 B
4-1 5.9 A
LSD (0.05) 0.6 -
† indicates number of N applications (each at 49 kg N ha-1) made in the spring and fall, respectively
206
Table 99. Mean visual color ratings for the cutting height by nitrogen treatment interaction at TTF.
N treatment †
Cutting height 2-0 2-1 3-0 3-1 4-0 4-1 LSD (0.05)
---- cm ---- ---------------------------------------------------------------------
3.18 5.0 5.8 5.7 6.2 5.5 6.3 1.0
3.81 3.3 5.0 3.3 5.7 3.7 5.5 1.0
LSD (0.05) 1.0 1.0 1.0 1.0 1.0 1.0 -
† indicates number of N applications (each at 49 kg N ha-1) made in the spring and fall, respectively
207
207
Table 100. Selected contrasts comparing visual color at TTF as related to total both total nitrogen applied and individual nitrogen
treatments.
Contrast label Nitrogen applications† Color rating
Difference
relative to
second value
Pr > F Statistical
Significance ††
-------------- ------- % -------
A 0 fall vs. 1 fall 4.4 vs. 5.8 -23% <.0001 ***
B 3 total vs. 4 total 5.0 vs. 5.3 -6% 0.150 NS
C 4 total vs. 5 total 5.3 vs. 5.9 -11% 0.009 **
D 3 total vs. all other rates 5.0 vs. 5.1 -4% 0.282 NS
E 5 total vs. all other rates 5.9 vs. 5.1 +16% <.0001 ***
† Each application supplied 49 kg N ha-1. Labels indicate which N treatment combinations were pooled for the contrast. Nomenclature
for each contrast's null hypothesis is as follows:
A H0: μ 2-0, 3-0, 4-0 = μ 2-1, 3-1, 4-1
B H0: μ 2-1, 3-0 = μ 3-1, 4-0
C H0: μ 3-1, 4-0 = μ 4-1
D H0: μ 2-1, 3-0 = μ 2-0, 3-1, 4-0, 4-1
E H0: μ 4-1 = μ 2-0, 2-1, 3-0, 3-1, 4-0
†† * = significant at 0.05 level, ** = significant at 0.01 level, *** = significant at 0.001 level
208
Table 101. Mean thatch thickness values for the cutting height main effect at VRC.
Cutting height Thatch thickness Letter grouping
---- cm ---- ---- mm ----
3.18 6.8 B
3.81 7.7 A
LSD (0.05) 0.8 -
Table 102. Mean thatch thickness values for the topdressing main effect at VRC.
Cutting height Thatch thickness Letter grouping
---- cm ---- ---- mm ----
3.18 6.8 B
3.81 7.7 A
LSD (0.05) 0.8 -
Table 103. Mean thatch thickness values for the nitrogen treatment main effect at VRC.
Nitrogen treatment† Thatch thickness Letter grouping
---- mm ----
2-0 6.3 B
2-1 6.0 B
3-0 6.6 B
3-1 7.0 B
4-0 8.6 A
4-1 9.0 A
LSD (0.05) 1.0 -
† indicates number of N applications (each at 49 kg N ha-1) made in the spring and fall,
respectively
209
Table 104. Selected contrasts comparing thatch thickness at VRC as related to total both total nitrogen applied and individual
nitrogen treatments.
Contrast label Nitrogen applications† Thatch thickness
Difference relative
to second value Pr > F
Statistical
Significance ††
------ mm ------ -------- % --------
A 0 fall vs. 1 fall 7.2 vs. 7.3 -2% 0.579 NS
B 3 total vs. 4 total 6.3 vs. 7.8 -19% <.0001 ***
C 4 total vs. 5 total 7.8 vs. 9.0 -13% 0.010 **
D 3 total vs. all other rates 6.3 vs. 7.7 -19% <.0001 ***
E 5 total vs. all other rates 9.0 vs. 6.9 30% <.0001 ***
† Each application supplied 49 kg N ha-1. Labels indicate which N treatment combinations were pooled for the contrast. Nomenclature for
each contrast's null hypothesis is as follows:
A H0: μ 2-0, 3-0, 4-0 = μ 2-1, 3-1, 4-1
B H0: μ 2-1, 3-0 = μ 3-1, 4-0
C H0: μ 3-1, 4-0 = μ 4-1
D H0: μ 2-1, 3-0 = μ 2-0, 3-1, 4-0, 4-1
E H0: μ 4-1 = μ 2-0, 2-1, 3-0, 3-1, 4-0
††
* = significant at 0.05 level, ** = significant at 0.01 level, *** = significant at 0.001 level
210
210
Table 105. Mean thatch thicknesses for the cutting height by nitrogen interaction at VRC.
N treatment †
Cutting
height 2-0 2-1 3-0 3-1 4-0 4-1 LSD (0.05)
---- cm ---- --------------------------------- mm --------------------------------
3.18 5.5 6.2 5.3 7.2 8.8 7.7 1.5
3.81 7.2 5.8 7.8 6.8 8.3 10.3 1.5
LSD (0.05) 1.5 1.5 1.5 1.5 1.5 1.5 -
† indicates number of N applications (each at 49 kg N ha-1) made in the spring and fall, respectively
211
Table 106. Mean thatch thickness values for the cutting height main effect at TTF.
Cutting height Thatch thickness Letter grouping
---- cm ---- ---- mm ----
3.18 5.2 -
3.81 5.3 -
LSD (0.05) NS -
Table 107. Mean thatch thickness values for the topdressing main effect at TTF.
Topdressing applied Thatch thickness Letter grouping
---- kg sand m-2 ---- ---- mm ----
0.0 8.2 A
8.5 2.3 B
LSD (0.05) 0.9 -
Table 108. Mean thatch thickness values for the nitrogen treatment main effect at TTF.
Nitrogen treatment† Thatch thickness Letter grouping
---- mm ----
2-0 3.8 C
2-1 4.7 BC
3-0 4.5 BC
3-1 5.8 B
4-0 4.7 BC
4-1 8.0 A
LSD (0.05) 1.6 -
† indicates number of N applications (each at 49 kg N ha-1) made in the spring and fall,
respectively
212
Table 109. Selected contrasts comparing thatch thickness at TTF as related to total both total nitrogen applied and individual
nitrogen treatments.
Contrast label Nitrogen applications† Thatch thickness
Difference relative
to second value Pr > F
Statistical
Significance ††
------ mm ------ -------- % --------
A 0 fall vs. 1 fall 4.4 vs. 6.1 -29% <0.001 ***
B 3 total vs. 4 total 4.6 vs. 5.2 -13% 0.234 NS
C 4 total vs. 5 total 5.2 vs. 8.0 -34% <0.001 ***
D 3 total vs. all other rates 4.6 vs. 5.6 -18% 0.040 *
E 5 total vs. all other rates 8.0 vs. 4.7 70% <0.001 ***
† Each application supplied 49 kg N ha-1. Labels indicate which N treatment combinations were pooled for the contrast. Nomenclature for
each contrast's null hypothesis is as follows:
A H0: μ 2-0, 3-0, 4-0 = μ 2-1, 3-1, 4-1
B H0: μ 2-1, 3-0 = μ 3-1, 4-0
C H0: μ 3-1, 4-0 = μ 4-1
D H0: μ 2-1, 3-0 = μ 2-0, 3-1, 4-0, 4-1
E H0: μ 4-1 = μ 2-0, 2-1, 3-0, 3-1, 4-0
††
* = significant at 0.05 level, ** = significant at 0.01 level, *** = significant at 0.001 level
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