potassium fertilization and stress tolerance of …
TRANSCRIPT
The Pennsylvania State University
The Graduate School
POTASSIUM FERTILIZATION AND STRESS TOLERANCE OF INTENSELY
MANAGED CREEPING BENTGRASS PUTTING GREENS
A Thesis in
Agronomy
by
Benjamin E. Brace
Ó 2019 Benjamin E. Brace
Submitted in Partial Fulfillment
of the Requirements
for the Degree of
Master of Science
May 2019
The thesis of Benjamin E. Brace was reviewed and approved* by the following:
Max Schlossberg
• Associate Professor of Turfgrass Science Thesis Advisor
Benjamin McGraw
• Associate Professor of Turfgrass Science
Michael Fidanza
• Professor of Plant and Soil Science Charles White Assistant Professor and Extension Specialist, Soil Fertility and Nutrient Management
Peter Landschoot Professor of Turfgrass Science
• Director of Graduate Studies in Agronomy
*Signatures are on file in the Graduate School
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ABSTRACT
Potassium (K) requirement of creeping bentgrass putting greens is a highly-
debated topic. Recent studies evaluating K fertilization requirements contend its
importance, but golf course superintendents still apply it regularly, their justification
being that golf course putting greens established on sand-based rootzones have limited K
retention and that sufficiency is crucial during stress periods. A two-year study was
conducted to quantify Penn A- and G-series creeping bentgrass (Agrostis stolonifera L.)
putting green performance and stress-tolerance response to soluble K fertilizer rate and/or
frequency, to develop K fertilization guidelines and identify a critical K deficiency
thresholds. Foliar applications of KCl (0-0-60) were made on 7- or 14-day intervals to
supply 0, 15, 30, or 45 kg K2O ha-1 per growing month. Three putting greens were
maintained under an intense double-cutting, rolling, and limited soil moisture
management regime and in the second season height-of-cut was lowered and
management intensified to simulate tournament conditions for a three-week period.
Monthly clipping yields and associated leaf nutrient status indicated optimal vigor and
nutrient sufficiency for throughout most of the study. Mehlich-III soil analysis revealed
concentrations below recommended levels, but deficiency symptoms were never seen.
Canopy density and color, measured using multispectral radiometers, were not influenced
by the K fertilizer treatments. Leaf water content was influenced more by environmental
conditions than K fertilizer treatment. Under simulated duration of extreme drought and
wear stress, K fertilizer treatments did not benefit turfgrass canopy density or survival.
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TABLE OF CONTENTS
LIST OF FIGURES ............................................................................................................ vi
LIST OF TABLES .............................................................................................................. viii
ACKNOWLEDGEMENTS................................................................................................. ix
Chapter 1: LITERATURE REVIEW .................................................................................. 1
Introduction ................................................................................................................. 1 Golf course putting greens .......................................................................................... 1
Creeping bentgrass ............................................................................................... 2 Rootzone.............................................................................................................. 3 Maintenance ........................................................................................................ 5
Plant nutrition ............................................................................................................. 6 Potassium............................................................................................................. 7 Potassium deficiency ............................................................................................ 8 Potassium availability .......................................................................................... 8
Potassium fertilizers ................................................................................................... 10 Potassium fertilization ................................................................................................ 12
Potassium recomendations ................................................................................... 13 Potassium uptake ........................................................................................................ 15
Non-readily available soil potassium .................................................................... 16 Potassium and plant health ........................................................................................... 19
Drought ............................................................................................................... 19 Cold tolerance ...................................................................................................... 21 Wear tolerance ..................................................................................................... 23 Disease ................................................................................................................ 25 Turf performace ................................................................................................... 25
Purpose of research ..................................................................................................... 27
Chapter 2: POTASSIUM FERTILIZATION AND STRESS TOLERANCE OF INTENSELY MANAGED CREEPING BENTGRASS PUTTING GREENS .............. 28
Introduction ................................................................................................................. 28 Materials and Methods ................................................................................................ 31
Field trial ............................................................................................................. 31 Locations ...................................................................................................... 31 Experimental design ..................................................................................... 31 Potassium applications .................................................................................. 32 Experiment weather ...................................................................................... 32 Putting green maintenance ............................................................................ 35
Cultural managment ................................................................................... 35 Tournament simulation .............................................................................. 35 Chemical management ............................................................................... 36 Topdressing applications ............................................................................ 36
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Treatment evaluation .................................................................................... 39 Soil potassium analysis .............................................................................. 39 Clipping yield ............................................................................................ 39 Leaf potassium analysis.............................................................................. 39 Plant uptake ............................................................................................... 39 Turf quality ................................................................................................ 40 Ball roll ...................................................................................................... 40 Leaf water content ...................................................................................... 40
Wear tolerance field trial ...................................................................................... 41 Drought tolerance greenhouse trial ...................................................................... 42 Statistical analysis ................................................................................................ 43
Results ........................................................................................................................ 44 Soil potassium analysis ........................................................................................ 44 Clipping yield ...................................................................................................... 49 Leaf potassium analysis........................................................................................ 50 Plant uptake ......................................................................................................... 55 Turf quality .......................................................................................................... 56 Ball roll ................................................................................................................ 61 Leaf water content ................................................................................................ 64 Wear tolerance field trial ...................................................................................... 66 Drought tolerance greenhouse trial ...................................................................... 69
Discussion ................................................................................................................... 72 Conclusions ................................................................................................................. 83
LITERATURE CITED ....................................................................................................... 84
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LIST OF FIGURES
Figure 2-1: Daily high and low air temperatures (C) from University Park, PA airport from 25 April 2017 to 1 Oct. 2018 (PSU) ..................................................................... 34
Figure 2-2: Daily TDR volumetric water content % (7.62 cm depth) by putting green from 21 June to 1 Oct. 2018. ........................................................................................ 37
Figure 2-3: Tournament putting green performance. Ball roll distance in (m) by putting green during the final week of tournament simulation, higher number represents a faster surface (Stimpmeter readings(ft) on 8/8; Sand-1: 16.4, Sand-2: 15.8, Push-Up: 15.7).. ................................................................................................................... 37
Figure 2-4: Tournament putting green performance. Surface firmness (Tru-Firm) readings by putting green during the final week of tournament simulation, lower number represents a firmer surface.. ......................................................................................... 38
Figure 2-5: Tournament putting green performance. TDR volumetric water content % (7.62 cm depth) by putting green during the final week of tournament simulation ........ 38
Figure 2-6: Photograph illustrating water-filled push turfgrass roller with knobbed cover that was used to apply traffic treatments in the wear tolerance field trial. ...................... 41
Figure 2-7: Photograph illustrating the irrigation of putting green plugs by 0.5-cm tension mini-disk infiltrometer in the drought tolerance greenhouse trial. ................................. 42
Figure 2-8: Mean extractable soil K level, pooled over the three putting greens, by K fertilization treatment (month ha)–1 and time (DSI, days since initiation). Respective error bars denote the least significant difference at a 5% alpha level. ............................ 47
Figure 2-9: Mean extractable soil K level by putting green, K fertilization rate (month ha)–1, and days since initiation (DSI). ........................................................................... 48
Figure 2-10: Mean leaf K by K fertilization treatment (month ha)–1 and days since initiation. Respective error bars denote the least significant difference at a 5% alpha level. ........................................................................................................................... 53
Figure 2-11: Mean leaf K by putting green, fertilization rate (month ha)–1, and days since initiation (DSI). ........................................................................................................... 54
Figure 2-12: 2017 mean canopy density as normalized differential vegetation index (NDVI) by putting green, monthly fertilization rate (ha–1), and days since initiation (DSI). .......................................................................................................................... 59
Figure 2-13: 2018 mean canopy density as normalized differential vegetation index (NDVI) by putting green, monthly fertilization rate (ha–1), and days since initiation (DSI). .......................................................................................................................... 59
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Figure 2-14: 2017 mean canopy dark green color index (DGCI) by putting green, monthly fertilization rate (ha–1), and days since initiation (DSI). .................................. 60
Figure 2-15: 2018 mean canopy dark green color index (DGCI) by putting green, monthly fertilization rate (ha–1), and days since initiation (DSI). .................................. 60
Figure 2-16: Mean ball roll distance by K fertilization rate (month ha)–1, and days since initiation (DSI). ........................................................................................................... 63
Figure 2-17: Mean canopy density as normalized differential vegetation index (NDVI) by monthly K fertilization rate (ha–1), and days since initiation (DSI) during simulated traffic stress period. ..................................................................................................... 68
Figure 2-18: Mean canopy density as normalized differential vegetation index (NDVI) by K fertilization rate (month ha)–1 and days since watered (DSW) during simulated drought period. ............................................................................................................ 71
Figure 2-19: Mean fertilizer K uptake (kg ha–1) and K fertilizer use efficiency (%) pooled over the three creeping bentgrass putting greens, by K fertilization treatment ............... 79
Figure 2-20: Photograph illustrating putting greens on 3/18/2018, from left to right; Push-Up, Sand-1, Sand-2. Number code 0: unfertilized control, 1: weekly 15 kg K2O ha-1
month, 2: semi-monthly 15 kg K2O ha-1 month, 3: weekly 30 kg K2O ha-1 month, 4: semi-monthly 30 kg K2O ha-1 month, 5: weekly 45 kg K2O ha-1 month, 6: semi-monthly 45 kg K2O ha-1 month ..................................................................................... 80
Figure 2-21: Photograph illustrating putting greens on 5/29/2018, from left to right; Push-Up, Sand-1, Sand-2. Number code 0: unfertilized control, 1: weekly 15 kg K2O ha-1
month, 2: semi-monthly 15 kg K2O ha-1 month, 3: weekly 30 kg K2O ha-1 month, 4: semi-monthly 30 kg K2O ha-1 month, 5: weekly 45 kg K2O ha-1 month, 6: semi-monthly 45 kg K2O ha-1 month .................................................................................... 80
Figure 2-22: Photograph illustrating putting greens on 7/19/2018 (Day 2 of tournament simulation), from left to right; Push-Up, Sand-1, Sand-2. Number code 0: unfertilized control, 1: weekly 15 kg K2O ha-1 month, 2: semi-monthly 15 kg K2O ha-1 month, 3: weekly 30 kg K2O ha-1 month, 4: semi-monthly 30 kg K2O ha-1 month, 5: weekly 45 kg K2O ha-1 month, 6: semi-monthly 45 kg K2O ha-1 month. .................... 81
Figure 2-23: Photograph illustrating putting greens on 8/7/2018 (Day 20 of tournament simulation), from left to right; Push-Up, Sand-1, Sand-2. Number code 0: unfertilized control, 1: weekly 15 kg K2O ha-1 month, 2: semi-monthly 15 kg K2O ha-1 month, 3: weekly 30 kg K2O ha-1 month, 4: semi-monthly 30 kg K2O ha-1 month, 5: weekly 45 kg K2O ha-1 month, 6: semi-monthly 45 kg K2O ha-1 month ..................... 81
Figure 2-24: Photograph illustrating the Sand-1 green on 9/5/2018,(VWC: 4.5%),Number code 0: unfertilized control, 1: weekly 15 kg K2O ha-1 month, 2: semi-monthly 15 kg K2O ha-1 month, 3: weekly 30 kg K2O ha-1 month, 4: semi-monthly 30 kg K2O ha-1
month, 5: weekly 45 kg K2O ha-1 month, 6: semi-monthly 45 kg K2O ha-1 month .......... 82
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LIST OF TABLES
Table 2-1: Initial Mehlich-III soil analysis results by putting green on (20 April 2017). Samples were taken to a depth of 15 cm and a combination of four subsamples per green... ........................................................................................................................ 33
Table 2-2: Monthly rainfall totals from 2017 and 2018 at the Valentine Turfgrass Research Center University Park, PA. Average data for University Park, PA (1942-2018) from (Weatherbase, 2019). ................................................................................. 33
Table 2-3: Analysis of variance (ANOVA) of Mehlich-III extractable (M3) soil K (0-15 cm depth) or clipping yield by source, and least squares means by monthly K fertilization levels. ....................................................................................................... 46
Table 2-4: Analysis of variance (ANOVA) of putting green leaf K concentration or K uptake by source, and least squares means by monthly K fertilization levels. ................ 52
Table 2-5: Analysis of variance (ANOVA) of canopy density as normalized differential vegetative index (NDVI), or canopy dark green color index (DGCI) by source, and least squares means by monthly K fertilization levels. .................................................. 58
Table 2-6: Analysis of variance (ANOVA) of ball roll distance by source, and least squares means by monthly K fertilization levels. .......................................................... 62
Table 2-7: Analysis of variance (ANOVA) of leaf water content by source, and least squares means by monthly K fertilization levels. .......................................................... 65
Table 2-8: Analysis of variance (ANOVA) of canopy density collected during imposed 2018 intense traffic trial as normalized differential vegetative index (NDVI), by source, and least squares means by monthly K fertilization levels ................................ 67
Table 2-9: Analysis of variance (ANOVA) of canopy density collected during imposed greenhouse dry-down as normalized differential vegetative index (NDVI), by source, and least squares means by monthly K fertilization levels................................. 70
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ACKNOWLEDGEMENTS
This project would not have been possible without the support of the
Pennsylvania Turfgrass Council and the Penn State College of Agriculture Sciences.
Their support for turfgrass research and graduate students has propelled me through this
incredible program. I would also like to thank the Dr. George Hamilton Fellowship for
offering me a scholarship to help fund my tuition payment and the Stanley Zontek
Endowment for funding assistance.
I would like to thank my major advisor Dr. Max Schlossberg, for giving me the
opportunity to work under him and his guidance over the last two years. I learned so
much from him and together we were able to put together some really good research
projects. It was a true privilege to work under Dr. Schlossberg and I am looking forward
to continuing our relationship in the future. I am also thankful to the rest of my graduate
committee; Dr. Benjamin McGraw, Dr. Charles White, Dr. Michael Fidanza, for assisting
me over the last two years helping develop and execute this project. Their input was
crucial for the success of this project and the development of my graduate studies.
I would like to thank Tom Bettle and the crew at the Valentine Turfgrass
Research Center for their help over the last two years. Without them much of this project
would have not been possible. I would also like to thank colleagues and friends that
assisted me at times with this project; Nate Leiby, Seth Hildebrand, Job Stepanski and
Josh Dymond. Also, I would like to thank the Penn State turfgrass faculty and College of
Agriculture Science faculty for their support over the last two years.
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Finally, I would like to thank my parents, Bert and Holly Brace for continued
support and financial aid throughout my graduate and undergraduate education at Penn
State University. I could not have accomplished anything without them.
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Chapter 1: LITERATURE REVIEW
Introduction
At the end of 2016, there were approximately 33,161 golf facilities worldwide,
equaling 576,111 total golf holes (i.e., ± 576,111 putting greens) in 208 countries with
45% of them located in the United States (Klien, 2017). In 2016 the game of golf drove
$84.1 billion dollars in economic activity across the United States while providing 1.9
million jobs (U.S. Golf Economy Report, 2017). Over 450 million rounds of golf were
played in the United States in 2017 (National Golf Foundation, 2018).
According to a survey funded by the Golf Course Superintendents Association of
America (GCSAA), golf courses in the United States applied roughly 46,906 metric tons
of potassium (K) fertilizer in 2014, which was down from 80,851 metric tons from a
survey conducted in 2006 (GCSAA, 2016). In 2014, the northeast region was responsible
for 14% of all applications in the United States with approximately 6,724 metric tons
applied (GCSAA, 2016). Of note, less than 1% of golf course superintendents in the
United States reported restrictions on K fertilizers within their area (GCSAA, 2016).
Therefore, the role of K fertilization practices in golf course putting green maintenance
and its effect on turf performance and stress tolerance warrants further examination.
Golf course putting greens
Putting greens are defined as the areas of a golf course located at the end of each
golf hole where the cup is located, that is specifically prepared for putting (USGA, 2019).
Putting green turf species and maintenance levels vary between and sometimes within
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golf facilities. They are typically mowed daily between 2.5 and 4.8 mm (Beard, 2005).
Throughout the day putting greens are subjected to intense traffic from golfers and
maintenance programs. On a typical par 72 golf course, 36 strokes are meant to be made
on the green and 18 strokes are meant to be played into the green (Turgeon, 2012). No
matter the level of the golf facility, putting greens usually require the highest level of
maintenance on a golf course. The turfgrass species often depends on geographic
location, age of golf course, expectations for putting surface quality and performance,
water and pesticide restrictions, and maintenance level (Vermeulen, 1992). Some
common species include bentgrass (Agrostis spp.), bermudagrass (Cynodon dactylon),
fine fescue (Festuca spp.) and annual bluegrass (Poa annua L.) (Turgeon, 2012).
Creeping bentgrass. Botanically, creeping bentgrass (Agrostis stolonifera L.) is
fine textured and has a stoloniferous growth habit, and it is a C3 tetraploid with 28
chromosomes (Turgeon, 2012). The United States Golf Association (USGA) Green
Section observed its success in putting greens established from an old seed mixture called
the ‘South German Mix’ which was a composed of primarily creeping, colonial (Agrostis
capillaris L.), dryland (Agrostis castellana) and velvet (Agrostis canina L.) bentgrasses
(Oakley, 1926; Turgeon, 2012). Turf samples were obtained from various well
performing greens and planted at the United States Department of Agriculture testing
facility in Arlington, Virginia were research eventually led to the release of vegetatively
propagated cultivars (Steiniger, 1968). In 1955, however, the seedable ‘Penncross’
cultivar was introduced from Pennsylvania State University by Dr. H.B. Musser and
quickly became the most popular creeping bentgrass cultivar for putting greens (Hein,
1958). ‘Penncross’ actually is a blend of three creeping bentgrass cultivars of ‘PennLu’
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and two numbered cultivars (Schery, 1970). Creeping bentgrass cultivars have improved
since the introduction of ‘Penncross’ and creeping bentgrass is regarded as the premier
turfgrass for putting greens in the northeast United States (Watson, 2001).
In the 1990s, new cultivars of creeping bentgrass were released specifically for
putting greens (Weidner et. al). ‘Penn A-4’ along with other popular creeping bentgrass
cultivars from the ‘Penn A and G’ series were developed by Pennsylvania State
University researcher Dr. Joseph Duich from ‘Penncross’ putting greens at the Augusta
National Golf Club in Augusta, Georgia (Moraghan, 2012). These ‘Penn A and G’ series
creeping bentgrasses are known for their summer stress tolerance, low mowing height
tolerance, upright growth, and high density (Stier and Hollman, 2003).
‘Penn A-4’ creeping bentgrass is categorized as a “high density” bentgrass and
has aggressive growth habits when mowed at heights between 2.5 and 3.3 mm (Sweeney
et al., 2001). It also has exceptional heat and wear tolerance (Landry and Schlossberg,
2001). In National Turfgrass Evaluation Program (NTEP) trials conducted nationwide,
‘Penn A-4’ creeping bentgrass ratings often exceed other bentgrasses particularly in the
density and color categories (NTEP, 2004). ‘Penn G-2’ has similar quality ratings to
‘Penn A-4’ when mowed at 3 mm (Stier and Hollman, 2003).
Rootzone. A putting green rootzone is the underlying soil and root growing
environment beneath the turf. For putting greens, an important quality of a rootzone is
drainage so the surface can be playable after heavy rains (Doak, 1992). The ideal soil
texture for golf turf is considered to be of sandy loam (Doak, 1992). Clay soils are not
suitable for rootzones because during times of drought, unirrigated areas can become dry
and crack while during wet periods these same areas will become waterlogged (Doak,
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1992). Both of these situations cause unplayable conditions for golfers. Consistent
putting surfaces across a golf course on a given day is directly related to consistent
rootzones and a requirement of maintaining putting greens (Doak, 1992; Waters, 2018)
The USGA updates their rootzone construction recommendations every few
decades for golf course putting greens. Over the last 50 years, most championship putting
greens built around the world have been constructed to USGA specifications. The report
gives recommendations for subgrade, drainage layer(s), and rootzone compatibility
(bridging) among other construction details (USGA Green Section, 2004). Construction
involves layers of imported soils to create a perched water table under the surface, which
supplies the turf with plentiful moisture while draining excess water and reducing
compaction (Doak, 1992). One of the main characteristics of a USGA rootzone is that it
is a minimum of 60% sand with particle sizes of 0.25 mm to 1.0 mm, because this
composition facilitates a rapid percolation rate and compaction resistance (USGA Green
Section, 2004). Of note, putting green rootzone construction recommendations are
continuously reviewed and updated as new research becomes available (USGA Green
Section, 2004; 2018).
A popular type of putting green built prior to USGA specifications was the “push-
up” green, in which existing soil on the site (i.e., native soil) is pushed up to form a
slightly elevated surface (Doak, 1992). It is essential that the green surface is constructed
without pockets that retain water and for drainage to be led away from the surface, with
the simplest way to do this is to elevate the green above the surrounding area (Doak,
1992). Some greens built prior to the USGA method where amended or capped with sand
or other coarse materials (Hurdzan, 2004).
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Maintenance. Putting greens require a high level of daily maintenance to provide
the best putting surface (Turgeon, 2012). These systems require daily mowing and ideally
routine rolling throughout the golf season. Pesticide applications are essential in most
regions and many superintendents in the northeast will apply plant protection and other
products on two-week intervals or based on weather conditions and environmental
monitoring. Plant growth regulators are routinely used to manage putting green turf
growth, annual bluegrass populations and seedhead management, and improve putting
green surface performance in times of abiotic stress (Kreuser, 2015; Bigelow, 2012).
Cultural practices such as core cultivation (i.e., aeration), sand topdressing, and brush
cutting can be performed multiple times throughout the year to maintain a desired playing
surface (Turgeon, 2012). Irrigation is very important because sand-based rootzones have
a limited water holding capacity (Bigelow et. al, 2000). Deep and infrequent watering
produces the highest quality creeping bentgrass (Jordan et. al, 2003) but many
superintendents will only water the parts of a putting green that are close to wilting to
keep surfaces dry for playability (Moeller, 2013). Wetting agents, surfactants that cause
liquids to penetrate into or spread easier across a surface, are often used to prevent or
rewet dry spots, improve irrigation efficiency, reduce water use, or as an adjuvant in
combination with pesticides or plant growth regulators (Zontek and Kostka, 2012).
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Plant nutrition
Fertilization is one of the three major turfgrass management cultural practices,
along with irrigation and mowing (Turgeon, 2012). Without sufficient plant and soil
nutrition levels, turfgrass will not respond to management practices and/or tolerate stress
(Carrow et. al, 2001). Turfgrass nutritional requirements for optimum growth are not
clearly understood and there is not one simple benchmark for quantifying turfgrass
response (Turgeon, 2012). Turfgrass quality is often dependent on fertilizer source, rate,
and timing (Turgeon, 2012).
There are seventeen essential nutrients that plants require to grow, perform crucial
functions, and complete their metabolic reactions and functions applications (Carrow et.
al, 2001). Carbon (C), hydrogen (H), and oxygen (O) are considered basic macronutrients
and required in the highest quantities by the plant but cannot be directly added through
fertilization (Carrow et. al, 2001). Nitrogen (N), phosphorus (P), and potassium (K) are
referred to as primary macronutrients and are supplied most frequently through fertilizer
applications (Carrow et. al, 2001). The three-number fertilizer grade that is found on a
fertilizer label is related to the three primary macronutrients of N, P, and K (Turgeon,
2012). Nitrogen is the nutrient required in the highest concentration by turfgrasses (Mills
and Jones, 1996). Color, shoot growth, and shoot density are all directly related to N
fertilizer applications (Carrow et. al, 2001). Phosphorus plays an important role in many
metabolic processes including photosynthesis, respiration, energy storage, and is crucial
to the vigor of turfgrass seedlings (Carrow et. al, 2001). Leaching and run-off potential of
N and P warrant environmental concerns in managed turfgrass sites (Carrow et. al, 2001).
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Potassium’s primary role is its influence on turfgrass tolerance to stress from drought,
cold, high temperature, wear and salinity (Carrow et. al, 2001).
Calcium (Ca), magnesium (Mg), and sulfur (S) are considered secondary
macronutrients (Carrow et. al, 2001). Calcium is a component of cell walls and is
important in cell production (Marschner, 2011). The Ca requirement for commelinoid
monocots (grasses) is low compared to other plant species (White and Broadley, 2003).
Magnesium is the central atom in chlorophyll and involved in protein synthesis
(Marschner, 2011). Sulfur is a component of amino acids required for protein synthesis
(Marschner, 2011). Iron (Fe), manganese (Mn), zinc (Zn), copper (Cu), molybdenum
(Mo), boron (B), chlorine (Cl), and nickel (Ni) are considered micronutrients and
required by the plant in very small amounts (Carrow et. al, 2001). Iron is the
micronutrient most likely to be deficient in turfgrass systems and plays an important role
in chlorophyll synthesis (Turgeon, 2012).
Potassium is found in relatively large quantities in most soils and makes up 1.9%
of the earth’s crust (Tisdale et. al., 1985). Potassium is the second most important mineral
nutrient and is essential for plant growth (Carrow et al., 2001). It is plant-available in its
monovalent form (i.e., K+). Potassium is highly mobile and used efficiently within the
plant, but it is also very mobile in the soil. In soil, K readily moves with water especially
in sand-based root zones (Carrow et al., 2001).
Potassium sufficiency in creeping bentgrass is generally assumed with tissue K
levels exceeding 22 g kg-1 (Mills and Jones, 1996). Many plant enzymes are dependent on
K for activation (Suelter, 1970). The number of known plant enzymes that require K to
activate conformational change in proteins exceeds 50 (Marschner, 2011). For example,
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K is required in high concentrations for protein synthesis (Marschner, 2011) including the
translation process of binding tRNA to ribosomes (Wyn Jones et. al., 1979). Cytoplasmic
K concentration directly governs starch synthase, pyruvate kinase, 6-phosphofructo-
kinase, and membrane-bound ATPase activity (Marschner, 2011). Photosynthesis is
reduced in K deficient plants due to the role K plays in stomatal regulation (Marschner,
2011). During stomata movement K is required to change turgor pressure of guard cells
which opens and closes the stomata (Marschner, 2011). Other plant functions that K has
been shown to play major roles in include carbohydrate formation, cell elongation and
root development (Carrow et. al, 2001).
Potassium deficiencies can be difficult to visually recognize, especially on golf
course putting greens because of their low mowing height (Christians, 1998). Tissue K
concentrations less than 10 g kg-1 may indicate a deficiency (Turgeon, 2012). Since K is a
mobile nutrient, deficiency symptoms will first appear in the oldest leaves as necrosis of
the leaf tip and/or margins, minor chlorosis, and sub-optimal turgor (Carrow et al., 2001;
Christians, 1998). Under highly evaporative conditions, K deficient turfgrasses may
demonstrate leaf firing and lack of turgor despite their underlying rootzones containing
adequate soil moisture levels (Carrow et al., 2001). Potassium deficient turfgrass is
commonly associated with high rainfall or irrigation, recent and sizable applications of
Ca, Mg or Na, clipping removal, and soils with low CEC and/or base saturation, or high
sand content, vermiculite, illite and/or smectite clay content (Carrow et al., 2001).
Potassium availability. Potassium in soil originates from the weathering of
rocks containing K bearing minerals (Tisdale et al., 1985). Soil concentrations normally
range from 0.5 to 2.5% and are usually lower in coarse soils and higher in fine textured
9
soils formed from K-bearing minerals (Tisdale et. al., 1985). Most of the K in soil is
retained within the lattice of feldspar minerals or fixed between layers of illite and
vermiculite clay minerals, and thus unavailable to the plant (Turgeon, 2012; Tisdale et
al., 1985). There are typically four ‘pools’ of soil potassium, in order of low to high plant
availability: mineral, non-exchangeable, exchangeable, and solution (Tisdale et al.,
1985). Transfer of K from the mineral and non-exchangeable to the exchangeable and
solution ‘pools’ is slow due to feldspars and micas resistance to weathering (Tisdale et
al., 1985). Transfer between the exchangeable and solution pools is relatively faster and
facilitated by cation exchange (Tisdale et al., 1985).
Cation exchange capacity (CEC) is the net sum of negative charge equivalents per
unit mass dry soil (Brady and Weil, 2010). Soil composed primarily of sand particles will
have a lower CEC than soils that are dominated by clay particles due to its charge density
of a specific surface area (Baird, 2007). Limited retention of K is observed in sand based
putting greens with low CEC and high infiltration rates, which increases the chance for K
to leach (Brady and Weil, 2010). Clay and silt particles have large surface areas, with
more exchange sites than sand particles with a smaller surface area (Brady and Weil,
2010). Potassium as K+ is a base cation in the exchangeable cation suite (Brady and Weil,
2010). Potassium is weakly attracted to the CEC and can be displaced by aluminum
(Al+3), calcium (Ca+2), and magnesium (Mg+2) on those exchange sites on soil colloids
(Brady and Weil, 2010). Therefore, when Ca and Mg fertilizers or liming agents are
applied, K occupancy of CEC can be reduced (Stanford et al., 1942). Since USGA
putting greens are dominated by sand they have low CEC and must be fertilized and
monitored regularly. Therefore, a “spoon-feeding” approach (i.e., the frequent application
10
of fertilizers at low rates) is a useful strategy in the nutrient management of putting
greens (Carrow et al., 2001; Baird, 2007).
Potassium fertilizers
Potassium fertilizers were first produced commercially in Germany around 1861
(Engelstad, 1985). Due to their reserves and mining processes, Germany was the only
country able to produce agricultural grade K fertilizers until World War I. Today, many
countries produce K fertilizers for agricultural use including the United States where it is
mainly mined in New Mexico, Utah, and California (Engelstad, 1985).
The K content of fertilizer products is the third number of the fertilizer grade and
listed as K2O (i.e., potassium oxide, and referred to as “potash” by practioners) which is
83% K (Carrow et al., 2001). The majority of K sources used for turfgrass fertilization
come from sylvanite deposits that are mined and processed by hydrochloric acid into KCl
(0-0-60), a fairly concentrated source of K and chloride (Carrow et al., 2001). Potassium
chloride (KCl), also known as muriate of potash, dissolves in the soil when directly
applied. Potassium chloride is a widely used K fertilizer in agriculture. However, chloride
can lead to sodium issues if soil accumulations get too high (Tisdale et al., 1985).
Reacting sylvanite with sulfuric or nitric acid synthesizes potassium sulfate (0-0-
50) and potassium nitrate (13-0-44) (Carrow et al., 2001). Potassium sulfate (K2SO4)
contains 17% sulfur and behaves similar to KCl in the soil, but also applies sulfur which
is another plant essential nutrient (Tisdale et al., 1985). Potassium nitrate (KNO3) is a
great source of the two most important nutrients of N and K to turfgrasses (Tisdale et al.,
1985). Other K sources include mono-potassium phosphate (0-51-35), di-potassium
11
phosphate (0-41-54), potassium thiosulfate (0-0-25), potassium magnesium sulfate (0-0-
22), potassium carbonate (0-0-30) and potassium hydroxide (0-0-83) (Carrow et al.,
2001).
Controlled release K fertilizers, developed by coating soluble K salts with sulfur,
plastic, and/or resins, can help minimize loss in sand based rootzones (Snyder and Cisar,
1992). Applications of controlled-release resin-coated and sulfur-coated K2SO4 on
‘Tifgreen’ bermudagrass contained significantly more K in clippings three months after
fertilization than clippings from plots fertilized with conventional KCl and K2SO4
(Snyder and Cisar, 1992). Twelve months following application, clippings from plots
treated with varyingly-permeable resin-coated K2SO4 contained higher amounts of K than
any other treatments in the study (Snyder and Cisar, 1992). The least-permeable coated
granules resulted in highest K in clippings, while plots treated by the most-permeable
coating resulted in the least clipping K content (Snyder and Cisar, 1992).
In a comparison of different commonly used K fertilizers, there were few
differences in creeping bentgrass performance due to K source (Young, 2009). Potassium
was applied as KCl, KNO3, K2SO4, resin-coated K2SO4 or potassium thiosulfate at
quarterly rates of 56 kg K2O ha-1 to a ‘Penn G2’ creeping bentgrass putting green in
Auburn, AL. Initial Mehlich-1 extractable soil K levels in the 0 to 7.6-cm soil depth
averaged 18µg g-1. Downward movement of K (to 30-cm) was unaffected by K-fertilizer
source. There were no consistent differences in clipping yield or bentgrass color and
quality due to K source. Leaf tissue K concentrations in K-fertilized plot clippings
collected over the 17-month study ranged from 12 to 27 g kg–1. No differences in vigor
between K-fertilized and control plots were observed in the final seven clipping yields
12
despite the control plot clippings containing comparatively limited tissue K levels over
the last 14 months of the study (8 to 13 g kg–1). At experiment end, Mehlich-1 extractable
soil K levels in the 0 to 7.6-cm soil depth of control plots averaged 4 µg g–1, yet no
deficiency symptoms were observed (Young, 2009).
Potassium fertilization
Inherencies of putting green culture, such as frequent irrigation and systematic
removal of plant residues (i.e., clipping collection) complicate K nutrition and
management (Carrow et al., 2001). Potassium should be applied as light and frequently as
possible to reduce luxury consumption, leaching, and fixation (Brady and Weil, 2013).
Potassium fertilizer applications are recommended to improve wear tolerance, survival
during stress periods and cold, heat, and drought tolerances (Turgeon, 2012). Some
turfgrass managers will apply K in a ratio with N applications (Carrow et al., 2001; Park
et al., 2017). Potassium nutrition is very important on sites where irrigation water
contains high concentrations of Na because overaccumulation leads to decreases in K
uptake (Carrow et. al, 2001). High concentrations of Na can displace K from CEC sites
(Carrow et. al, 2001). Potassium fertilization does not enhance salinity tolerance but it is
required in some cases to keep a proper K nutritional status to carry out important plant
functions (Carrow et. al, 2001).
Soil properties such as soil textural class, CEC, and mineral source play a large
role in K management. Noer (1934) noted that applications of potash can last for several
years on silt and clay fairways. He concluded that the only fairway soils that K fertilizer
applications should be made are on poor sands, mucks, and peats (Noer, 1934).
13
Potassium recommendations. Turfgrass managers will often make K
applications based on results of soil or tissue testing or in response to stress events
(Carrow et al., 2001). Potassium fertilization is needed when soil K levels drop below a
desired crops critical level (Tisdale et al., 1985). In almost all soils, except those that
have a low CEC and are heavily leached, soil fertility analysis is the best method for
evaluating K requirements (Carrow et al., 2001). The most widely used extractant for
measuring K is 1M NH4O Ac (ammonium acetate). The Mehlich-III is a popular
universal extractant in the northeast United States (Carrow et al., 2001). Following soil
analysis, K levels are reported as parts per million (µg g–1) or pounds per acre in addition
to percent K saturation of CEC (Schlossberg, 2016).
It is recommended to follow the sufficiency level of available nutrients (SLAN)
approach when interpreting soil test results (Schlossberg, 2016). The SLAN method
attempts to quantify the amount of available nutrients in the soil and then ranks the levels
for each nutrient from low to high (Meentemeyer and Whitlark, 2016). These
recommendations were developed based on responses of forage, agronomic, and
horticulture crops but have been modified for turfgrass (Carrow et. al. 2004). Carrow et.
al. (2001) recommends using the high level of SLAN (116 µg g–1) as a target for K
fertilization on intensely managed recreational turf sites built on sand rootzones.
Minimum level of sustainable nutrients (MLSN) is a modified version of SLAN
and is designed to manage soil nutrients at or slightly above a minimum threshold
(Meentemeyer and Whitlark, 2016). It is based on numerous soil tests from well
performing turfgrass sites which resulted in the development of a guideline K level of 37
µg g–1 (Meentemeyer and Whitlark, 2016). The recommendation considers historical
14
weather averages and monthly N applications to estimate nutrient use for a specific
system along with current nutritional levels (Woods, 2016). The recommendation is
developed from a formula where K needed annually equals the estimated use of K per
year by the plant plus the MLSN guideline minus the most recent soil K level (Woods,
2017).
Base cation saturation ratio (BCSR) is an approach focused on creating a perfect
soil that maximizes crop yield by balancing the ratios of Ca, Mg, K, and Na (St. John and
Christians, 2013). Making recommendations using this method often leads to over
application of Ca and K (Meentemeyer and Whitlark, 2016). Most soil testing
laboratories that report BCSR use Graham (1959) interpretation where a perfect soil has
65-85% Ca, 6-12% Mg, 2-5% K. The BCSR approach is not recommended for
developing a fertility program for sand-based golf greens (St. John and Christians, 2013).
Another option turfgrass mangers have for evaluating K is by monitoring K leaf
status through tissue testing (Soldat, 2016). Tissue testing for K can provide information
on how a plant responds to fertilization and weather events (Carrow et al., 2001).
Interpretation of tissue analysis is still unclear as there is insufficient data relating tissue
nutrient concentrations and turf performance (Meentemeyer and Whitlark, 2016). Mills
and Jones (1996), however, declared 22 to 26 g kg-1 as sufficient ranges for K
concentration in creeping bentgrass clippings.
15
Potassium uptake
Plants differ in their ability to take up K and their critical K sufficiency level
(Tisdale et. al., 1985). Soil solution is the pool from which K is primarily assimilated by
turfgrass roots, and diurnally acquired by mass flow/convective acquisition processes.
Indicative of luxury consumption, K is often assimilated by turfgrass in concentrations
exceeding levels required to support normal growth (Bart and Jassen, 1929). Potassium
applications have failed to affect turfgrass clipping yields, even when soil K levels were
below locally-recommended levels (Dest and Guillard, 2001; Woods et al., 2006; Young,
2009).
Increases of annual K rates up to 243 kg ha-1 did not correspond to increases in
tissue K levels (Fitzpatrick and Guillard, 2004). Potassium was applied as K2SO4 to
Kentucky bluegrass (Poa pratensis L.) and tissue K levels were maximized equally
across plots receiving 81 to 162 kg K ha-1 yr (Fitzpatrick and Guillard, 2004). On annual
bluegrass, soil K levels below 50 µg g–1 and tissue levels below 20 g kg-1 are considered
deficient (Murphy et. al., 2015). Maximum tissue K content of 29 g kg-1 was achieved
when soil K concentrations were 100 µg g–1, so there was no benefit to fertilizing K
beyond this level (Murphy et. al., 2015). Applications of K2SO4 to a mixed stand of
Kentucky bluegrass, fine fescue and perennial ryegrass (Lolium perenne L.) did not lead
to increases of tissue K or clipping yield but soil K concentrations were increased
(Petrovic et. al, 2005).
Sandy soils with a low CEC are prone to leaching of K especially when high
inputs (or soil qualities) of Ca or Mg and when subjected to high irrigation or rainfall
(Carrow et. al., 2001). Leaching of K from high fertilization rates was observed from
16
deep soil sampling (Johnson et al., 2003). Applications were made on ‘Providence’
creeping bentgrass grown on a calcareous sand as KCl at yearly rates of 0, 101, 203, 304,
406 kg K ha-1. Soil K levels which ranged from 28 to 46 µg g–1 and were slightly
increased but not proportional to K application rates. Tissue K concentrations below 10 g
kg-1 were observed. Soil test results from multiple soil depths revealed that K was
moving through the rootzone and leaching out of the green (Johnson et al., 2003).
Non-readily available soil potassium. Much of the K in the soil occurs in
primary minerals like feldspars, micas and clays which is not directly available to plants
(Carrow et. al., 2001). Overtime through the weathering process this K will become plant
available but is not thought to contribute a great deal to plant needs during a year (Carrow
et. al., 2001). The release of K from non-exchangeable sources have been shown to
satisfy a portion of corn (Zea mays L.) K requirement on sand and loamy sand soils due
to lack of response from K fertilization (Liebhardt et. al, 1976; Woodruff and Parks,
1980; Rehm and Sorensen, 1985). Colby and Bredakis (1966) found that creeping
bentgrass utilized K from mineral sources in the soil, likely due to the large surface area
of its roots.
Recent work on K fertilization has shown that creeping bentgrass may be taking
up K from primary materials in sand based rootzones (Woods et al., 2006). When K2SO4
was applied on 14-day intervals to supply monthly K rates of 0, 10, 20, 40 or 60 kg ha-1,
K concentration in soil solution showed respective increases (Woods et al., 2006).
Research performed on a calcareous sand ‘L-93’ creeping bentgrass putting green found
that in spite of its highly conductive and inert nature, water extractions from the sand
rootzone sampled seven months after the last K fertilizer application revealed legacy
17
effects of those treatments (Woods et al., 2006). There was not a consistent relationship
between soil and tissue K concentrations found. Soil K levels were below recommended
levels, but tissue K concentrations of the unfertilized control remained within sufficiency
ranges throughout most of the study leading the authors to conclude that depletion of
solution and exchangeable K by the plant was replenished from non-exchangeable
sources (Woods et al., 2006).
No major benefits were observed from the addition of K fertilizer to eight
different sand rootzone mixes despite low CEC and initial K levels (Dest and Guillard,
2001). In the greenhouse, plastic pots were filled with 2 kg of each rootzone, grassed with
‘Penncross’ creeping bentgrass and treated with K as KCl at rates of 0 and 50 mg K kg-1.
Rootzone CEC values ranged from 0.55 to 3.96 cmol kg-1 and exchangeable K values
ranged from 2.4 to 15.7 µg g–1. Potassium fertilization significantly increased leaf tissue
K concentrations within each root zone on every harvest date. Potassium uptake in one
rootzone (CEC: 3.88 cmol kg-1, exchangeable K: 8.8 µg g–1) was significantly higher than
all the other root zones. The three root zones with the lowest CEC values removed
significantly less K than the other root zones. Potassium fertilization had little effect on
clipping yield and, significantly increased in one rootzone (CEC: 0.85 cmol kg-1,
exchangeable K: 4.0 µg g–1) and significantly decreased in one rootzone (CEC: 3.88 cmol
kg-1, exchangeable K: 8.8 µg g–1) (Dest and Guillard, 2001). Correlations between K
uptake and total clipping yield, root weight, and tissue K concentrations were highly
significant on the last three harvest dates. On the last harvest date, six of the root zones
had leaf tissue K concentrations under 10 g kg-1 with deficiency symptoms seen in only
four of the root zones. When leaching was measured, root zones were separated into
18
groups and those with more exchangeable K leached more in the beginning and then
leveled out. In some rootzones, K uptake was up to nine times higher than exchangeable
soil K concentrations. Laboratory studies indicated that release of K from non-
exchangeable ‘pools’ satisfied creeping bentgrass K requirements in some sand root
zones (Dest and Guillard, 2001).
Sand topdressing is a common practice on golf course putting greens to control
thatch and protect greens during the winter (Turgeon, 2012; Vavrek, 2013). Sand may be
derived from K feldspars which constitute the largest mineral reserve of K in the regolith
(Tisdale et. al, 1985). In a collection of topdressing sand from golf course superintendents
across the United States, Soldat (2018) recorded K content ranging from 0.1% (New
Jersey) to 2.3% (Arizona). In a long-term study assessing the effects of K fertilization,
Mehlich-III soil concentrations of unfertilized plots rose likely due to the release of K
from K feldspar in topdressing sand (Bier et. al., 2017). The K content of the sand used
was 0.7% by mass and by topdressing approximately 5 cm per year, 341 kg K ha-1 was
added to the soil annually as K feldspar. Potassium was applied as K2SO4 at rates up to
300 kg K2O ha-1 per year and did not improve creeping bentgrass appearance or
performance over the unfertilized plots (Bier et. al., 2017).
19
Potassium and plant health
When K is deficient the plant is much more susceptible to biotic and abiotic
stresses (Marschner, 2011). Decreased stress tolerance of K deficient plants is due to the
enhanced production of reactive oxygen species and results in stress induced oxidative
stress (Cakmak, 2005). Plant stress situations in which K has been shown to help include
drought (Sen Gupta et. al., 1989), cold (Grewal and Singh 1980), high light intensity
(Marschner and Cakmak, 1989), iron toxicity (Li et. al, 2001), pest and disease pressure
(Amtmann et. al, 2008). An optimum K nutritional status is critical for plant survival
when these stresses occur (Marschner, 2011). Applications of K should be made prior to
stress occurrence and during the stress period. A spoon-feeding program is best in high
use sites where stress is constant (Carrow et al., 2001).
Drought stress is a common summer occurrence for turfgrasses managed in
temperate climates worldwide. Potassium plays a key role in plant water relations (Hsia
and Låuchli, 1986). Potassium nutrition influences plant water relations including
regulation of cell turgor pressure and stomata aperture, thus affecting drought tolerance
(Carrow et al., 2001). Potassium facilitates plant management of oxidative stress through
responsive osmoregulation. Influx of K into guard cells initiates and maximizes stomatal
conductance. Stomatal constriction or closure, induced by darkness, soil moisture
depletion, and/or abscisic acid (ABA) signaling, results from rapid efflux of K from
guard cells (Carrow et al., 2001). Delayed stomatal response adversely affects
carbohydrate production, enzyme synthesis, and water use efficiency (Marschner, 2011).
Accumulation of K solutes facilitates cellular osmotic adjustment, and has been observed
20
to account for 59-65% of the total ion concentration measured by Jiang and Huang (2001)
in Kentucky bluegrass cytoplasm.
Potassium can delay the onset of wilting due to its role in plant water relations
(Carrow et. al., 2001). Wilt symptoms were most pronounced in plots treated with high
rates of N and no K (Carrow, 1994). Potassium was applied as K2SO4 to ‘Penncross’
creeping bentgrass at yearly rates of 0, 147, 294 and 443 kg K2O ha–1 and N was applied
with each rate at 294 and 443 kg ha–1. During the summer, quality ratings were lowest on
high N and no K treated plots. Wilt symptoms decreased as K fertilization rate increased.
Root growth was maximized in the high N rate at 147 kg K2O ha–1 and in the low N rate
at 294 kg K2O ha–1 (Carrow, 1994).
Potassium applications resulted in reduced water use of Kentucky bluegrass when
applied as KCl (Schmidt and Breuninger, 1981). The plots that received greater K
fertilization treatments also recovered from drought stress more quickly than the
untreated plots (Schmidt and Breuninger, 1981). In a growth chamber study on Kentucky
bluegrass, Carroll and Petrovic (1991) suggested frequent applications of K fertilizer may
enhance leaf turgor pressure. Other studies conducted in that era report limited
enhancement of drought resistance in relation to K fertilization, fertility, and/or tissue
content (Shearman, 1982; Waddington et al., 1978).
Reports of negligible effects of K on drought resistance and/or water use have
become increasingly common. In the absence of N or P fertilization, KCl applied at
annual rates of 0, 87, 174, 261, or 348 kg K ha–1 to Kentucky bluegrass field plots had no
influence on evapotranspiration rate (Ebdon et al., 1999). Considering only replicated
plots receiving annual applications of 294 kg N and 43 kg P ha–1 in the second year of
21
study, the described array of K fertilization rates again did not influence water use
(Ebdon et al., 1999).
Potassium fertilization did not delay wilting as soil moisture levels decreased
when soil samples were examined once plants were at their permanent wilting point and
determining gravimetric moisture content (Dest and Guillard, 2001). When irrigation was
withheld K had no effect on the severity of localized dry spot on ‘Penncross’ when
fertilized as K2SO4 at rates of 0, 195 and 390 kg K2O ha–1 (Nikolai, 2002). Lawson (1999)
saw no response to drought stress from K applications on ‘Colonial’ bentgrass grown at
fairway height of cut (5 to 13 mm).
Relative to 1:1 N:K fertilizer treatments, plots fertilized by treatments comprising
N:K ratios exceeding one did not increase drought tolerance (Rowland et al., 2014).
Potassium was applied as KCl in conjunction with N at ratios (N:K) of 1:1, 1:2, 1:3 and
1:4 to bermudagrass, seashore paspalum (Paspalum vaginatum), and zoysiagrass hybrids
maintained as putting greens in Florida. Wilting was increased on two rating dates in the
highest K rate (1N:4K) compared to the 1:1 treatment. Potassium applications did not
increase canopy density on any of the cultivars studied (Rowland et al., 2014).
Cold tolerance. Winter kill is a major problem on putting greens in the northeast
United States, especially those comprised primarily of older bentgrass and annual
bluegrass (Schmid et al., 2016). Winter injury can be caused by several factors including;
ice suffocation, death by lack of oxygen or buildup of toxic gases, crown hydration
caused by rapid ice formation within crown tissue cells or loss of moisture from crowns,
low-temperature injury, or desiccation from cold dry winter winds (Vavrek, 2016). An
22
inadequate K level within plants is a factor that leads to an increase risk of frost damage
(Marschner, 2011).
Potassium applications greatly reduced winterkill damage from ice cover and/or
crown hydration on a New Jersey annual bluegrass putting green but no differences were
observed between sources and rates (Schmid et al., 2016). Potassium was applied to plots
for three years at as KCl or K2SO4 at rates of 63.5, 132, 264 kg K2O ha-1 and K2CO3 or
KNO3 at rates of 264 kg K2O ha-1 per year. Each green was covered with snow and ice
for 47 days in late winter. The first visual ratings, (five days after snow and ice melt)
showed that the control plots averaged 58% turf damage and the plots that received K
averaged less than 4% turf damage. The second visual ratings were conducted a month
later and showed the control plots averaged 32% turf damage and the plots that received
K had less than 1% turf damage (Schmid et al., 2016). Potassium fertilization resulted in
improved lethal temperature (LT50) in a controlled freezing test. Annual bluegrass not
fertilized with K had an LT50 of -13.8 o C when plants fertilized with K had a LT50 of
-16.6 o C (Schmid et al., 2016).
In Kentucky bluegrass, the greatest cold tolerance resulted from 2:1 and 3:1 N:K
fertilization ratio regimes (Beard, 1969). Cold hardiness of perennial ryegrass was
positively affected from K applications (Webster and Ebdon, 2005). Potassium was
applied as K2SO4 at yearly rates of 49, 245 and 441 kg K ha-1. Through its interaction
with N, K had a positive impact on LT50. The authors recommended applying N
fertilizer at moderate rates and keeping soil K levels high for optimum shoot growth and
cold tolerance (Webster and Ebdon, 2005).
23
Applications of K had no effect on the cold tolerance of ‘Toronto’ creeping
bentgrass (Beard and Rieke, 1966). Potassium was applied at rates of 98, 195, 293, 390.5
kg K ha-1 along with increasing rates of N. Plugs were subjected to -23o C temperatures
and all treatments survived. Authors noted that ‘Toronto’ creeping bentgrass has
excellent cold tolerance (Beard and Rieke, 1966). Likewise, Lawson (1999) reported no
benefit to response of winter stress from K applications on ‘Colonial’ bentgrass grown at
fairway height of cut.
Winter exposure of Tifton 44 coastal bermudagrass was improved from
applications of KCl at yearly rates up to 140 kg K ha–1 (Belesky and Wilkinson, 1983).
Roots and rhizome weights were significantly increased when K fertilization rate was
increased (Belesky and Wilkinson, 1983). Bermudagrass cold tolerance was not
improved with K fertility at levels that exceed sustainable growth (Miller and Dickens,
1996). Additionally, K fertilization had no effect on carbohydrate levels (Miller and
Dickens, 1996).
Wear tolerance. Wear stress is a major issue on turfgrass putting greens due to
the amount of foot and equipment traffic they endure. Turfgrasses are more durable when
subjected to wear than the majority of other plants. Different species of turfgrass will
vary in wear tolerance (Beard, 1973). Creeping bentgrass has average wear tolerance but
exceptional recuperative ability compared to other cool season turfgrass species
(Turgeon, 2012).
Potassium has been shown to increase wear tolerance of creeping bentgrass
(Shearman and Beard, 1975). The greatest increase in wear tolerance was seen at annual
rates between 270 and 360 kg K ha-1. As more K was applied tissue K levels rose, which
24
increased load-bearing capacity and leaf tensile strength of ‘Penncross’ creeping
bentgrass (Shearman and Beard, 1975). In a separate study, wear tolerance was increased
linearly with K fertilization rate (Shearman and Beard, 2002). Potassium was applied as
K2SO4 at annual rates of 0, 100, 200, 300 and 400 kg K ha-1 to ‘Penncross’ creeping
bentgrass. There was an increase greater than 40% in wear damage between the untreated
control and the 400 kg K ha-1 yearly treatment in the first year. In addition to wear
tolerance, K applications led to increases in tissue K concentrations, load-bearing
capacity and total cell wall content (Shearman and Beard, 2002).
Applications of K have also been shown to improve turfgrass quality under wear
stress from foot traffic (Kim and Kim, 2012). Potassium applications lead to a 10.25%
increase in visual quality of ‘Penncross’ creeping bentgrass. Where there was no traffic,
K applications had no effect on visual quality (Kim and Kim, 2012). Conversely, K
applications did not improve wear tolerance of four species, including ‘Penncross’
(Carroll and Petrovic, 1991). Potassium was applied as KCl at annual rates of 0, 48, 96,
192 kg K ha-1 in conjunction with rates of N (96 and 192 kg ha–1yr–1) over a four-year
period. Additionally, K did not affect the recovery from wear (Carroll and Petrovic,
1991). Similarly, there was no benefit seen from K applications to wear tolerance or
recovery of perennial ryegrass (Hoffman et. al., 2010).
Potassium fertilization did not improve wear tolerance from winter traffic in the
southeast United States (Mirmow, 2016). Potassium was applied at annual rates of 0,
36.6, and 73.3 kg K ha- 1 to a ‘Crenshaw’ creeping bentgrass putting green in Clemson,
South Carolina. Three fall applications were made to examine if there was an increase in
turfgrass performance when subjected to different morning and afternoon traffic levels
25
throughout the winter. Applications did not lead to increases in soil or tissue K levels.
Untreated plots averaged tissue K levels of 15.7 g kg-1 and soil K levels of 13.16 µg g–1
across both years. Potassium applications had no effect on creeping bentgrass
performance determined by visual ratings, canopy density, ball roll or surface firmness
(Mirmow, 2016).
Disease. Potassium can increase pathogen resistance because it changes enzyme
activities and metabolic concentrations leading to facilitated entry and development in
plant tissue (Marschner, 2011). Low tissue K concentrations have been shown to favor
spring dead spot, leaf blotch, take-all patch, red leaf spot, crown and root rot, dollar spot,
and red thread (Carrow et al., 2001). Potassium fertilization may be an important tool for
reducing anthracnose severity on annual bluegrass where moderate to low soil K
concentrations exist (Schmid et. al., 2018). Applications of KNO3 resulted in the greatest
reduction in anthracnose severity compared to other commonly used N sources (Schmid
et. al., 2018). Dollar spot infection was not affected by K fertilization (Nikolai, 2002;
Woods et. al., 2006). Potassium applications increased Microdochium patch (pink snow
mold) on creeping bentgrass but higher K rates did not lead to more disease (Soldat,
2008). It was found that stopping K applications after years of applying did not help
decrease Microdochium patch (Soldat, 2008). High K fertilizer applications lead to
increases in damage from Typhula incarnata (gray snow mold) in the spring of both 2003
and 2004 (Woods et. al, 2006).
Turf performance. Turfgrass quality is often determined by the visual rating
system based on the evaluator's judgement (Morris, 2009). As K nutritional
concentrations increased, less N was required to reach the best quality ratings on
26
Kentucky bluegrass and creeping bentgrass in a growth chamber study (Christians et al.,
1979). Potassium applications had no effect on the performance of ‘Penncross’ creeping
bentgrass when examined for 20 years (Fulton, 2002). No differences in visual quality
ratings were observed from KCl applications despite control K levels being low (20-30
µg g–1) over the last seven years of study (Fulton, 2002). High K rates had a negative
impact on turf performance of velvet bentgrass (Murphy and Schmid, 2014). Multiple
studies have failed to show effects from K fertilization on visual quality of creeping
bentgrass (Nus 1989; Nikolai, 2002; Johnson et. al, 2003; Woods et. al., 2006; Mirmow,
2016). Ball roll, a common metric of turfgrass performance, is the average distance a golf
ball rolls after release to a turf surface (Turgeon, 2012). Potassium applications had no
effect on ball roll of creeping ‘Penncross’ creeping bentgrass when subjected to different
putting green mowing heights (Nus, 1989). Ball roll of creeping bentgrass was not
improved by K fertilizer additions (Woods, 2006; Mirmow, 2016).
27
Purpose of Research
Recent research results do not support traditional K fertilization regimes but most
turfgrass managers (95%) are still applying K to putting greens (GCSAA, 2016). One
reason being the role K is thought to play in stress tolerance because putting greens are
always under stress and often perform best when highly stressed from management
practices and decreased irrigation. For golf course superintendents, a primary objective
for their turf management program is optimum visual turf appearance and also turf
function and playability (Oatis, 2010). If putting green performance is pushed too far,
from maintenance programs adding to the increasing stress, turf loss can occur (Zontek,
2012). Of all the current research published on K fertilization none focus on extreme
stress caused by maintenance programs. In response, this research seeks to examine the
effect K fertilizer applications on extreme stress situations imposed on greens height-of-
cut creeping bentgrass. Turfgrass performance as a reflection of canopy density is to be
measured during mechanical stress periods as provided from increased mowing events,
rolling and brushing, drought stress in a greenhouse scenario, and wear stress from
simulated traffic. The goal of this research is to provide turfgrass practitioners with
research-based information on the effects of K fertilization on creeping bentgrass putting
greens exposed to various stresses, and if K should be used as a tool to possibly increase
stress tolerance of creeping bentgrass putting greens.
28
Chapter 2: POTASSIUM FERTILIZATION AND STRESS TOLERANCEOF INTENSELY MANAGED CREEPING BENTGRASS
PUTTING GREENS
Introduction
Potassium (K) is an essential element for plant growth and is often considered the
second most important mineral nutrient to turfgrass (Carrow et al., 2001). It is plant-
available in its monovalent form (i.e., K+) and highly mobile in the plant, thus used
efficiently within the plant, but it is also very mobile in the soil and can readily leach
(Carrow et al., 2001). Putting green rootzones built to USGA specifications are
predominantly comprised of sand (USGA Green Section, 2004) which limits K
availability due to its low cation exchange capacity (CEC) and increases potential for
leaching (Carrow et. al, 2001).
Putting greens are subjected to many forms of stress from maintenance practices,
weather events and traffic from golfers (Dowling and Meentemeyer, 2017). The main
role of K in the plant is stress tolerance and when K is deficient the plant is much more
susceptible to biotic and abiotic stresses (Marschner, 2011). Decreased stress tolerance of
K deficient plants is due to the enhanced production of reactive oxygen species and
results in stress induced oxidative stress (Cakmak, 2005). Potassium fertilization is
recommended to improve wear tolerance, survival during stress periods and cold, heat,
and drought tolerances (Turgeon, 2012) and applications should be made prior to stress
occurrence and during the stress period (Carrow et al., 2001). It is recommended to
follow the sufficiency level of available nutrients (SLAN) approach when interpreting
soil test results (Schlossberg, 2016) which attempts to quantify the amount of available
29
nutrients in the soil and then ranks the levels for each nutrient from low to high
(Meentemeyer and Whitlark, 2016). Carrow et. al. (2001) recommends using the high
level of SLAN (116 µg g–1) as a target for K fertilization on intensely managed
recreational turf sites built on sand rootzones.
Conflicting results have been seen in the literature on the effect of K fertilization
on stress tolerance of cool season grasses. Drought stress, a common summer occurrence
for turfgrasses, has been shown to be positively influenced by K fertilization (Schmidt
and Breuninger, 1981; Carrow, 1994). Potassium fertilization has also been shown to
have no effect on increases in drought stress tolerance (Dest and Guillard, 2001; Nikolai,
2002). Wear stress is another issue on turfgrass putting greens due to the amount of foot
and equipment traffic they endure. Potassium fertilization has been shown to improve
wear stress tolerance (Shearman and Beard, 1975, 2002; Kim and Kim, 2012) but other
studies have resulted in no benefits (Carroll and Petrovic, 1991; Hoffman et. al., 2010;
Mirmow, 2016). Recent studies have failed to show benefits to turfgrass quality from K
fertilization despite K nutritional concentrations below recommended levels (Fulton,
2002; Johnson et al., 2003; Woods et. al, 2006; Young, 2009). Creeping bentgrass has
been shown to utilize K in the soil from non-exchangeable sources that are not accounted
for through soil testing (Dest and Gulliard, 2001; Woods et. al, 2006; Bier et. al, 2017).
Recent research results contend traditional potassium (K) fertilization benefits to
creeping bentgrass putting greens. Likewise, the origins and accuracy of long-standing
soil K recommendations for creeping bentgrass, as well as its critical K deficiency
threshold in leaf clippings, has been questioned. There is a lack of current research
focused on the relationship between K fertilization and extreme stress of creeping
30
bentgrass putting greens. Thus, additional research is needed to examine the roles K
fertilization plays in stress tolerance.
We examined the effects from K fertilization on creeping bentgrass performance
along with its role in stress tolerance from simulated mechanical, drought, and wear. Our
objectives were to (i) quantify creeping bentgrass putting green canopy color, density,
vigor, water relations, nutrient content, and spring vigor/survival to soluble potassium
fertilizer application rate and/or frequency under an intense management regime; and (ii)
develop evidence-based K fertilization guidelines to manage creeping bentgrass under
stress for golf course superintendents.
31
Materials and methods
Field trial
Locations. A multi-site experiment was initiated in April 2017 to examine the
effects of K fertilization on intensely-managed creeping bentgrass putting greens. One
subset of three (3) blocks was established on a ‘Penn A-4’ creeping bentgrass putting
green (Sand-1), a second on a separate putting green established by a blend of ‘Penn A-
1/A-4’ creeping bentgrass (Sand-2), and the third on a separate ‘Penn G-2’ putting green
(Push-Up), all maintained at the Valentine Turfgrass Research Center (University Park,
PA). The former two putting greens are comprised of United States Golf Association
(USGA) specified rootzones 30-cm in depth and situated atop a 10-cm gravel drainage
layer. The ‘Penn G-2’ creeping bentgrass putting green rootzone is comprised of 8-cm
sand above a native Hagerstown silt loam (fine, mixed, semiactive, mesic, Typic
Hapludalf).
Initial soil analysis (Table: 2-1) resulted in both the Sand-1 and Push-Up green
with Mehlich-III K levels above 50 µg g–1, which is considered moderate concentrations
(51 to 116 µg g–1) (Carrow et al., 2001). The Sand-2 green had a K level of 27 µg g–1
which is considered low (<50 µg g–1) (Carrow et al., 2001). The last K fertilizer
applications made on each green were as followed; Push-Up: 30 Aug. 2016, Sand-1: 14
April 2017, Sand-2: 15 Oct. 2016. No K deficiency symptoms were observed before the
start of this study
Experimental design. Treatments were arranged in an augmented (3 x 2)
factorial of monthly K2O application rate and frequency in repeated randomized complete
block design (RCBD). Monthly K2O application rate of 15, 30, or 45 kg ha–1 comprised
32
the first factor, and weekly (7-day) or semi-monthly (14-day) delivery (± 1 day)
comprised the second factor. The augmented treatment was a zero-K ‘negative’ control.
Thus, the total experiment contained nine replications of seven treatments across three
identically-managed putting greens.
Potassium applications. Soluble spray applications were prepared using reagent-
grade potassium chloride (KCl, 0-0-60) and applied by a single nozzle (Teejet 11008E)
CO2 backpack sprayer in 600 L ha–1 carrier volume. Total K2O applied over both years
were 180, 360 or 540 for the respective 15, 30 or 45 kg ha-1 monthly treatments.
Treatments were applied 25 April to 17 Oct. 2017 totaling 22 weeks of applications in
2017. Total kg K2O ha-1 applied in 2017 from each rate; 15: 97.5, 30:195.0, 45: 292.5.
From 25 May 2017 to 5 September 2017, liquid potassium phosphite fertilizer (SilStar, 0-
0-26) supplanted 40% of the KCl in support of pathogen control (Cook et al., 2009).
Semi-monthly treatment applications, on dates when all plots were treated, were
supplemented with MgSO4●7H2O and MnSO4●H2O to provide Mg, S, and Mn at 1.1,
1.6, and 0.3 kg ha–1 rate, respectively. In 2018, K treatments were applied 25 April to 19
Sept. 2018 totaling twenty-two weeks of applications in 2018. Total kg K2O ha-1 applied
in 2018 from each rate; 15: 82.5, 30: 165.0, 45: 247.5. Phosphite or MgSO4●7H2O and
MnSO4●H2O additions were not included in any applications in 2018.
Experiment weather. Rainfall totals were above average in both years (Table: 2-
2) with 2018 setting a new record for total rainfall in a year in University Park, PA. There
were some dry periods in both seasons, most notably August and September 2017 and
late August and early September 2018 (Table: 2-2). There were some extreme
temperature periods over the two years (Figure: 2-1).
33
Table 2-1: Initial Mehlich-III soil analysis results by putting green on (20 April 2017). Samples were taken to a depth of 15 cm and a combination of four subsamples per green.
Initial soil analysis Field ID Push-Up Sand-1 Sand-2
pH 7.09 7.34 7.43 P µg g–1 48.5 36 14.5 K µg g–1 58 57 27
Mg µg g–1 195.5 86 65 Ca µg g–1 1374.9 1073.8 1127.7
CEC meq(100 g soil)-1 8.65 6.25 6.25 K % Saturation 1.7 2.35 1.1
Mg % Saturation 18.85 11.5 8.7 Ca % Saturation 79.45 86.15 90.2
Zn µg g–1 6.45 2.25 0.95 Cu µg g–1 6.8 2.35 1.15 S µg g–1 17.7 13.45 5.2
Organic Matter 2.18 0.99 0.51
Table 2-2: Monthly rainfall totals from 2017 and 2018 at the Valentine Turfgrass Research Center University Park, PA. Average data for University Park, PA (1942-2018) from (Weatherbase)
Rainfall (cm)
Month 2017 2018 Average Jan 7.37 7.06 7.37 Feb 3.94 15.06 6.35 Mar 10.95 4.78 8.64 April 8.08 8.84 8.64 May 16.33 12.45 10.41 June 10.06 13.46 10.16 July 12.73 20.47 9.65 Aug 6.20 18.80 8.89 Sept 4.34 22.94 7.37 Oct 15.80 11.86 7.37 Nov 6.40 10.92 6.86 Dec 2.29 14.05 6.86
34
Figure 2-1: Daily high and low air temperatures (C) from University Park, PA airport from 25 April 2017 to 1 Oct. 2018 (PSU)
-20.0
-10.0
0.0
10.0
20.0
30.0
40.0
4/25/17 7/6/17 9/16/17 11/27/17 2/7/18 4/20/18 7/1/18 9/11/18
Tem
pera
ture
s (C
)
Max Temp (C ) Min Temp (C )
35
Putting green maintenance
Cultural management. Putting greens were mechanically cored (Toro Procore
648, Bloomington, MN) by 12.3-mm diameter hollow tines on 51-mm centers in early-
and mid-Sept. 2016, and cores were collected and removed then verticut in 2 directions
(Ryan Mattaway, Johnson Creek, WI) on 18 April and 4 Oct. 2017. Greens were mowed
by reel mower (Toro Greensmaster 1000, Bloomington, MN) at a 2.6-mm height of cut,
seven times weekly in 2017 and ten times weekly in 2018. Management intensified in
2018 to enhance stress conditions, greens were double mowed 52 days and triple mowed
24 days between 12 April to 1 Oct. 2018. For enhanced canopy defoliation, greens were
manually brushed prior to mowing events on eight occasions in 2017 and six in 2018. For
more added stress, plots were rolled (Salsco HP 11, Cheshire, CT) twice per week in
2017 and three times per week in 2018. Moisture content was monitored daily by TDR
(Spectrum FieldScout TDR 350, Aurora, IL) between 21 June and 1 Oct. 2018 (Figure: 2-
2). The irrigation program was designed to limit the amount of water being added and
irrigation was added when moisture levels dropped below 10%.
Tournament simulation. During the weeks of 21 July to 8 Aug. 2018
management intensified to simulate tournament conditions. Height of cut was dropped to
2.4- mm on 21 July 2018 and then to 2.3-mm on 31 July 2018. Over the 3-week period
all greens were cut 46 times with 21 cuts during the final week. Greens were rolled daily
during the final week. During this period the Valentine Turfgrass Research Center
received 24.2 cm of rain with 9.5 cm on 3 Aug. 2018 alone. Putting green performance
data collected during the final week includes green speeds (Figure: 2-3), surface firmness
36
(Spectrum Tru-Firm, Aurora, IL) (Figure: 2-4), and soil moisture (Spectrum FieldScout
TDR 350, Aurora, IL) (Figure: 2-5).
Chemical management. Gypsum (CaSO4 •2H2O) was applied at a rate of 48.8 kg
ha–1 to each green prior to taking the soil samples in 2017 and 2018. Semi-monthly
soluble N applications, as methylol urea (Coron, 30-0-0), were made foliarly at a 12 kg N
ha–1 rate from 1 June to 13 Oct. 2017 and 5 June to 26 Aug. 2018. Nitrogen applications
totaled 120 kg N ha–1 in 2017 and 111 kg N ha–1 in 2018. Paclobutrazol (Trimmit,
Syngenta) plant growth regulator was applied semi-monthly per label directions between
May and August of both years. Putting greens were treated with fungicides and/or
insecticides as needed throughout both seasons, often combined with N applications.
Commercially available wetting agents were applied in accordance with label directions
and micronutrient fertilizers applied to deliver 2 kg Fe ha–1 each month from May to
September in both years.
Topdressing applications. Sand topdressing was performed in both years with a
sand that contained 0.53% K as K feldspar. In 2017 greens were sand top-dressed every
four weeks at an average rate of 500 kg ha–1. After discovering the K content of the sand
source in the fall of 2017, only three topdressing events were performed in 2018 at an
average rate of 325 kg ha–1. Estimated K additions from topdressing sand were 19 kg
K2O ha-1 in 2017 and 5 kg K2O ha-1 in 2018.
37
Figure 2-2: Daily TDR volumetric water content % (7.62 cm depth) by putting green from 21 June to 1 Oct. 2018
Figure 2-3: Tournament putting green performance. Ball roll distance in (m) by putting green during the final week of tournament simulation, higher number represents a faster surface (Stimpmeter readings(ft) on 8/8; Sand-1: 16.4, Sand-2: 15.8, Push-Up: 15.7).
0%
5%
10%
15%
20%
25%
30%
35%
40%
6/21 7/1 7/11 7/21 7/31 8/10 8/20 8/30 9/9 9/19 9/29
Push-Up Sand-1 Sand-2
3.25
3.50
3.75
4.00
4.25
4.50
4.75
5.00
5.25
7/31 8/1 8/2 8/3 8/4 8/5 8/6 8/7 8/8
Push-Up Sand-1 Sand-2
38
Figure 2-4: Tournament putting green performance. Surface firmness (Tru-Firm) readings by putting green during the final week of tournament simulation, lower number represents a firmer surface.
Figure 2-5: Tournament putting green performance. TDR volumetric water content % (7.62 cm depth) by putting green during the final week of tournament simulation.
0.37
0.38
0.39
0.40
0.41
0.42
0.43
0.44
0.45
0.46
0.47
0.48
7/31 8/1 8/2 8/3 8/4 8/5 8/6 8/7 8/8
Push-Up Sand-1 Sand-2
10%12%14%16%18%20%22%24%26%28%30%
7/31 8/1 8/2 8/3 8/4 8/5 8/6 8/7 8/8
Push-Up Sand-1 Sand-2
39
Treatment evaluation
Soil potassium analysis. Soil sampling employed a 2.1-cm id punch to remove
three (3) 15-cm-deep cores per plot. The top centimeter (thatch) of each core was
removed, cores combined, and the composite sample submitted to the Pennsylvania State
University Agricultural Analytical Services Laboratory (University Park, PA) for routine
analysis of soil pH (1:1 DI H2O); Mehlich-III extractable P, S, Cu, and Zn; Mehlich-III
exchangeable Ca, Mg, and K, and soil organic matter by loss on ignition (Ben-Dor and
Banin, 1989).
Clipping yield. Clipping biomass, a canopy vigor measure, was assessed in June,
July, and August of 2017 and 2018; and in September of 2017. Samples were collected
from a single mower (Toro Greensmaster 1000, Bloomington, MN) pass across the 3-m
length of each plot following a 3- to 4-day break in daily mowing. Samples from clipping
yield were oven-dried (60 C) and weighed to 0.1-mg resolution (Zhu et al., 2012).
Leaf potassium analysis. A one-gram subsample of each clipping yield was
ground to pass a 0.15-mm sieve. Ground tissue samples were submitted to the
Pennsylvania State University Agricultural Analytical Services Laboratory (University
Park, PA) for acid- digest determination of leaf P, K, Ca, Mg, S, Fe, Mn, Cu, B, and Zn
concentration (Miller, 1998) and total leaf N by medium temperature furnace combustion
(Horneck and Miller, 1998).
Plant uptake was calculated as the product of oven-dry clipping yield (kg ha-1)
and nutrient content in leaf tissue (g kg-1) on a per plot basis and analyzed as a dependent
variable (Schlossberg and Schmidt, 2007).
40
Turf quality. Putting green canopy density and color were calculated to analyze
turfgrass quality. Beginning in early-May of each year, triplicate readings of 460-, 510-,
560-, 610-, 660-, 710-, 760-, and 810-nm canopy reflectance were recorded using a
passive multi-spectral radiometer (CropScan MSR87, Rochester, MN). This data was
used to calculate normalized differential vegetative (NDVI) and dark green color indices
(DGCI) of the putting green canopies, quantifying canopy density and color respectively
(Zhu et al., 2012). Collection involved taking two readings per plot and averaging them
for statistical analysis.
Ball roll was collected at least 11 hours after mowing (Pelz Meter, Spicewood,
TX). Six balls were rolled on each plot on Sand-2 Green and length of roll was measured
and averaged for statistical analysis. Data was only collected on Sand-2 Green because of
the slight undulations on the Push-Up and Sand-1 green that influenced results.
Leaf water content was determined in clippings collected following high
temperature and/or limited rainfall periods. Fresh clippings collected when taking yield
were sealed in tared Ziploc bags immediately following collection and stored in a cooler
for transport. Mass of sealed bags was then determined to 0.1-mg resolution in the
laboratory. Fresh weight was recorded as the difference in mass of sealed bag and its tare
weight. Following oven drying to constant mass, dry clipping mass was recorded to 0.1-
mg resolution. Leaf water content (g g–1) was calculated as (fresh mass - dry mass)/fresh
mass.
41
Wear tolerance field trial
Between 5 June 2018 and 6 Aug. 2018 traffic treatments were performed on a
separate plot area on Sand-2 green. Initial soil K levels were 14 µg g–1 and applications of
the weekly three K2O rates were applied between 30 May and 5 Aug. 2018. Traffic
treatments involved 6 passes with dimpled roller (Figure: 2-6) three to four times per
week totaling 204 passes per plot. This green was managed with the same intensity as all
other greens including during the tournament simulation. Turfgrass quality of the wear
treated plots were analyzed by canopy density and color. Triplicate readings of 460-, 510-
, 560-, 610-, 660-, 710-, 760-, and 810-nm canopy reflectance were recorded using a
passive multi-spectral radiometer as described. This data was used to calculate NDVI and
DGCI of the putting green canopies, quantifying canopy density and color respectively
(Zhu et al., 2012).
Figure 2-6: Photograph illustrating water-filled push turfgrass roller with knobbed cover that was used to apply traffic treatments in the wear tolerance field trial.
42
Drought tolerance greenhouse trial
In August and/or September of each growing season, a cup-cutter was used to
extract two (2) 8-cm deep × 10.8-cm id plugs from each plot of the field trial. Putting
green plugs were transferred into gravel-filled HDPE cylindrical containers (11 cm id) for
dry-down in greenhouse. Water content of the plugs was initially standardized by
applying 90 mL deionized H2O using a 0.5-cm tension mini-disk infiltrometer (Decagon
Devices, Pullman, WA) (Figure: 2-7). Irrigation was then withheld. Photosynthetically-
active canopy density, as a proxy for drought resistance, was measured every day over a
25-d period using an ambient light-excluding FieldScout TCM-500 turfgrass chlorophyll
meter (Spectrum Technologies Inc., Plainfield, IL).
Figure 2-7: Photograph illustrating the irrigation of putting green plugs by 0.5-cm tension mini-disk infiltrometer in the drought tolerance greenhouse trial.
43
Statistical analysis
All clipping yield, canopy reflectance, tissue nutrient concentration, soil
physicochemical, and other response data were combined for global analysis in
SAS/STAT (Ver. 8.2, SAS Institute, Cary, NC) using PROC MIXED (Zhu et al., 2012).
Model fixed effects were fertilizer treatment (Fert) and days since initiation (DSI).
Significance of fertilizer treatment (and associated contrasts) were tested by the expected
mean squares of its respective putting green (PG) interaction term (df = 12), as
determined by Model I of Hocking (1973) and later described by McIntosh (1983).
Means within significant main effects were separated by Fisher’s least significant
difference at an α level of 0.05.
The significance of main effect(s)-by-time (repeated measures) interactions were
analyzed using the residual error term and time-series covariate structures as appropriate.
Canopy density and color data collected over successive days, inherent to optimal passive
sensor operation and requisite weather conditions/patterns, were pooled in support of
figure readability and computational resources. Means within significant interactive
effects were separated by Fisher’s least significant difference at an α-level of 0.05.
44
Results
Soil potassium analysis
Global ANOVA of soil K data sampled over four dates showed a significant
effect (P £ 0.01) of K fertilization (Table: 2-3). Frequency of K application, weekly v.
semi-monthly, did not dependably influence soil K levels. Thus, each response to a non-
zero K fertilization rate is presented as the average of the two frequencies (Table: 2-3).
Mean Mehlich-III extractable soil K increased linearly with K fertilization rate.
Significant differences in mean soil K were observed between monthly K fertilization
rate increments of 30 kg K2O ha–1, but not 15 kg K2O ha–1 (Table: 2-3).
Mehlich-III extractable soil K levels were further influenced (P £ 0.01) by time
(days since initiation, DSI) and an interaction (P £ 0.01) of K fertilization and DSI
(Table: 2-3). Mean Mehlich-III extractable soil K levels in response to each K
fertilization frequency-rate combination and sampling event are shown (Figure: 2-8).
Frequency of K application did not influence soil K levels on any sample date. Plots
receiving K fertilizer treatments showed experiment-high extractable soil K levels in
October 2017 (DSI: 200).
Relative to data observed in October 2017, the rootzone of all plots sampled April
2018 (DSI: 353) revealed reduced Mehlich-III extractable soil K levels (Figure: 2-8). Soil
K levels in plots fertilized at the 30 or 45 kg K2O ha–1 monthly rate varied little over the
2018 season. Conversely, extractable soil K in plots receiving 0 or 15 kg K2O (month
ha)–1 steadily declined over the 2018 season. Final soil K levels (DSI: 525) in unfertilized
plots, as well those fertilized at the low K rate, show lesser mean soil K levels than at
experiment initiation (Figure: 2-8).
45
While typically not subjects of statistical inference via mixed model, random
variables and/or their interactions may describe meaningful influence of treatment on the
sampled population, and merit presentation (Zhu et al., 2012). Least square means of soil
K levels, as observed by K fertilization rate, time, and putting green are shown (Figure:
2-9). This data indicates lesser retention of soil K by the Sand-2 putting green, despite
having received equal fertilization rates, and being subjected to identical maintenance and
environmental conditions over the experimental period.
46
Table 2-3: Analysis of variance (ANOVA) of Mehlich-III extractable (M3) soil K (0-15 cm depth) or clipping yield by source, and least squares means by monthly K fertilization levels.
Source †type M3 Soil K Clipping yieldnum den ǂP r>F num den ǂPr>F
Putting green (PG) R 2 6 ** 2 6 *K fertilization (Fert) F 6 12 ** 6 12 0.25Days since initiation (DSI) F 3 144 ** 6 322 **Fert x DSI F 18 144 ** 36 322 0.99
M3 Soil K Clipping yieldµg g─1 kg ha─1
0 34.9 12.715 44.8 12.930 55.5 12.745 72.3 12.5Least significant difference, a = 0.05 17.9 ns
K fertilization contrasts P r>F P r>F§Frequency (Freq) 1 12 0.82 1 12 0.13Quadratic rate 1 12 0.46 1 12 0.33Cubic rate 1 12 0.79 1 12 0.61Freq x linear rate 1 12 0.72 1 12 0.12Freq x quadratic rate 1 12 0.92 1 12 0.58Freq x cubic rate 1 12 0.87 1 12 0.10†; Type of variable; R, random; F, fixed.ǂ; *, ** Significant at respective P values ≤ 0.05, 0.01.§; Weekly v. semi-monthly applications.
deg. freedom
Monthly K fertilization, kg ha─1
deg. freedom
47
Figure 2-8: Mean extractable soil K level, pooled over the three putting greens, by K fertilization treatment (month ha)–1 and time (DSI, days since initiation). Respective error bars denote the least significant difference at a 5% alpha level.
Days since initiation (DSI)0 60 120 180 240 300 360 420 480 540
Mea
n M
ehlic
h-III
ext
ract
able
soi
l K (µ
g g-1
)
20
30
40
50
60
70
80
90
100
110
120 0 kg K2O15 kg K2O weekly30 kg K2O weekly45 kg K2O weekly15 kg K2O semi-monthly30 kg K2O semi-monthly45 kg K2O semi-monthly
48
Figure 2-9: Mean extractable soil K level by putting green, K fertilization rate (month ha)–1, and days since initiation (DSI).
30405060708090
100110120130
Days since initiation (DSI)0 60 120 180 240 300 360 420 480 540
10152025303540455055
Sand 2
Push-UpM
ean
Meh
lich-
III e
xtra
ctab
le s
oil K
(µg
g-1)
2030405060708090
100110120
0 kg K2O15 kg K2O30 kg K2O45 kg K2O
Sand 1
49
Clipping yield
Potassium fertilization did not dependably influence creeping bentgrass clipping
yield. A global ANOVA of seven collection events showed clipping yield to be
influenced (P £ 0.05) only by days since initiation (DSI) (Table: 2-3). Given the
recognized influence temperature, light, soil moisture, and available N have on turfgrass
shoot growth, variation in clipping yield over seven sampling dates spanning two seasons
is expected (data not shown).
50
Leaf potassium analysis
Global ANOVA of leaf K data sampled over six dates showed a significant effect
(P £ 0.01) of K fertilization (Table: 2-4). Frequency of K application, weekly v. semi-
monthly, neither dependably influenced leaf K levels nor interacted with K rate to
influence leaf K levels. Thus, each response to a non-zero K fertilization rate is presented
as the average of the two frequencies (Table: 2-4). The quadratic response of mean leaf K
to K fertilization rate proved significant at a 0.01 P-level (Table: 2-4). Mean leaf K
response to rate of K fertilization significantly differed (P £ 0.01) by each 15 kg K2O
(month ha)–1 increment of the employed array (Table: 2-4).
Leaf K levels were further influenced (P £ 0.01) by time (days since initiation,
DSI) and an interaction (P £ 0.05) of K fertilization and DSI (Table: 2-4). Mean leaf K
levels in response to each K fertilization frequency-rate combination and sampling event
are shown (Figure: 2-10). Frequency of K application did not influence leaf K level on
any sample date. Excepting the 12 June 2017 sampling event (DSI: 48), K fertilization
supported leaf K levels significantly exceeding those observed in unfertilized plots.
Regardless of K fertilization, the lowest leaf K levels recorded over the experimental
period were collected 4 June 2018 (DSI: 405). Plots receiving K fertilizer treatments
showed experiment-high leaf K levels 495 DSI (Figure: 2-10).
Least square means of leaf K levels, as observed by K fertilization rate, time, and
putting green are shown (Figure: 2-11). In the second experimental year (2018), data
indicate less leaf K accumulation by unfertilized plots of the Sand-1 and Sand-2 putting
greens than of the Push-Up putting green, despite having endured identical maintenance
51
and environmental conditions over the experimental period (Figure: 2-11). Regarding the
4 June 2018 sampling date (DSI: 405), the Push-up putting green leaf K levels mirrored
their respective 2017 means, whereas simultaneously sampled leaf K levels from all plots
of the Sand-1 and Sand-2 putting greens constituted experiment-low leaf K levels
(Figure: 2-11).
52
Table 2-4: Analysis of variance (ANOVA) of putting green leaf K concentration or K uptake by source, and least squares means by monthly K fertilization levels.
Source †type Leaf K K uptakenum den ǂPr>F P r>F
Putting green (PG) R 2 6 ** **K fertilization (Fert) F 6 12 ** **Days since initiation (DSI) F 5 280 ** **Fert x DSI F 30 280 * 0.99
Leaf K K uptakemg kg─1 g ha─1
0 21.19 240.815 23.72 268.030 25.49 291.445 26.36 297.8Least significant difference, a = 0.05 0.76 12.2
K fertilization contrasts P r>F P r>F§Frequency (Freq) 1 12 0.93 0.79Quadratic rate 1 12 ** **Cubic rate 1 12 0.87 0.32Freq x linear rate 1 12 0.86 0.79Freq x quadratic rate 1 12 0.85 0.91Freq x cubic rate 1 12 0.88 0.36†; Type of variable; R, random; F, fixed.ǂ; *, ** Significant at respective P values ≤ 0.05, 0.01.§; Weekly v. semi-monthly applications.
deg. freedom
Monthly K fertilization, kg ha─1
53
Figure 2-10: Mean leaf K by K fertilization treatment (month ha)–1 and days since initiation. Respective error bars denote the least significant difference at a 5% alpha level.
Days since initiation (DSI)0 60 120 180 240 300 360 420 480
Mea
n le
af K
(g k
g-1)
18
19
20
21
22
23
24
25
26
27
28
29
300 kg K2O15 kg K2O weekly30 kg K2O weekly45 kg K2O weekly15 kg K2O semi-monthly30 kg K2O semi-monthly45 kg K2O semi-monthly
54
Figure 2-11: Mean leaf K by putting green, fertilization rate (month ha)–1, and days since initiation (DSI).
30 70 110 400 440 480
Mea
n le
af K
(g k
g-1)
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
30 70 110 400 440 480
Mean leaf K (g kg
-1)
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
Days since initiation (DSI)30 70 110 400 440 480
0 kg K2O15 kg K2O30 kg K2O45 kg K2O
Sand 1 Sand 2Push-up
55
Plant uptake
Global ANOVA of putting green K uptake, sampled over six dates, showed a
significant effect (P £ 0.01) of K fertilization (Table: 2-4). Frequency of K application,
weekly v. semi-monthly, did not dependably influence mean K uptake. Thus, each
response to a non-zero K fertilization rate is presented as the average of the two
frequencies (Table: 2-4). The quadratic response of mean K uptake to K fertilization rate
proved significant at a 0.01 P-level (Table: 2-4). As with mean leaf K concentration,
significant differences (P £ 0.01) in mean K uptake were observed between all monthly
K fertilization rate increments (Table: 2-4). Rate of K uptake was further influenced (P £
0.01) by time (days since initiation, DSI) but not an interaction of K fertilization and DSI
(Table: 2-4).
56
Turf quality
Canopy density. Global ANOVA of putting green canopy densities sampled over
63 dates showed a significant effect (P £ 0.05) of K fertilization (Table: 2-5). Frequency
of K application, weekly v. semi-monthly, did not dependably influence mean canopy
density. Thus, each response to a non-zero K fertilization rate is presented as the average
of the two frequencies (Table: 2-5). Mean canopy density decreased linearly with K
fertilization rate yet plateaued from the 15 to 30 kg ha–1 monthly K2O level. While
statistically significant (P £ 0.05), differences in canopy density means were not visibly
detectable (Table: 2-5).
Canopy density was further influenced (P £ 0.01) by time (days since initiation,
DSI) but not an interaction of K fertilization and DSI (Table: 2-5). Given the recognized
influence temperature, light, soil moisture, and available N have on turfgrass shoot
growth, variation in canopy density measures spanning 63 dates over two seasons is
expected. However, presentation of this time effect and its interaction with K fertilizer
rate and putting green provides interested readers a graphical representation of canopy
density response to heightened maintenance intensity (Figure: 2-12 and 2-13).
Canopy color. Global ANOVA of putting green canopy color, or dark green
color index (DGCI), sampled over 63 dates showed a significant effect (P £ 0.05) of K
fertilization (Table: 2-5). Frequency of K application, weekly v. semi-monthly, did not
dependably influence mean canopy DGCI. Thus, each response to a non-zero K
fertilization rate is presented as the average of the two frequencies (Table: 2-5). Mean
canopy color decreased linearly with K fertilization rate yet plateaued from the 15 to 30
57
kg ha–1 monthly K2O level. While statistically significant (P £ 0.05), the reported
differences in mean canopy DGCI were not visibly detectable (Table: 2-5).
Canopy color was further influenced (P £ 0.01) by time (Table: 2-5). For reasons
already described, the significant influence of time on canopy color was expected (data
not shown). Sampling date (DSI) did not interact with K fertilization (Table: 2-5). As
calculation of both DGCI and NDVI employ canopy reflectance of 660-nm light
measured by the CropScan multi-spectral radiometer, they do not constitute fully-
independent indices. Correlation analysis of the three 929 pairs of DGCI and NDVI data
proved direct and significant (r=0.893, P £ 0.001), and precludes unnecessarily-
redundant narration of DGCI levels presented for the benefit of interested readers
(Figures 2-14 and 2-15).
58
Table 2-5: Analysis of variance (ANOVA) of canopy density as normalized differential vegetative index (NDVI), or canopy dark green color index (DGCI) by source, and least squares means by monthly K fertilization levels.
Source †type density colornum den
Putting green (PG) R 2 6 ** **K fertilization (Fert) F 6 12 * *Days since initiation (DSI) F 62 3448 ** **Fert x DSI F 372 3448 0.99 0.99
density colorNDVI DGCI
0 0.790 0.64915 0.788 0.64730 0.788 0.64745 0.785 0.645Least significant difference, a = 0.05 0.003 0.002
K fertilization contrasts§Frequency (Freq) 1 12 0.92 0.37Quadratic rate 1 12 0.60 0.50Cubic rate 1 12 0.27 0.23Freq x linear rate 1 12 0.64 0.16Freq x quadratic rate 1 12 0.54 0.70Freq x cubic rate 1 12 0.96 0.42†; Type of variable; R, random; F, fixed.ǂ; *, ** Significant at respective P values ≤ 0.05, 0.01.§; Weekly v. semi-monthly applications.
P r>F
Monthly K fertilization, kg ha─1
Canopy
Canopy
deg. freedomǂP r>F
59
Figure 2-12: 2017 mean canopy density as normalized differential vegetation index (NDVI) by putting green, monthly fertilization rate (ha–1), and days since initiation (DSI).
Figure 2-13: 2018 mean canopy density as normalized differential vegetation index (NDVI) by putting green, monthly fertilization rate (ha–1), and days since initiation (DSI).
20 40 60 80 100 120 140 160
Canopy density (N
DVI units)
0.74
0.75
0.76
0.77
0.78
0.79
0.80
0.81
0.82
0.83
0.84
0.85
0.86
0.87
Sand 2
Days since initiation (DSI)20 40 60 80 100 120 140 160
Can
opy
dens
ity (N
DVI
uni
ts)
0.74
0.75
0.76
0.77
0.78
0.79
0.80
0.81
0.82
0.83
0.84
0.85
0.86
0.87
Push-Up
20 40 60 80 100 120 140 160
0 kg K2O15 kg K2O30 kg K2O45 kg K2O
Sand 1
360 390 420 450 480 510
Canopy density (N
DVI units)
0.48
0.51
0.54
0.57
0.60
0.63
0.66
0.69
0.72
0.75
0.78
0.81
0.84
0.87
Sand 2
Days since initiation (DSI)360 390 420 450 480 510
Can
opy
dens
ity (N
DVI
uni
ts)
0.48
0.51
0.54
0.57
0.60
0.63
0.66
0.69
0.72
0.75
0.78
0.81
0.84
0.87
Push-Up
360 390 420 450 480 510
0 kg K2O15 kg K2O30 kg K2O45 kg K2O
Sand 1
60
Figure 2-14: 2017 mean canopy dark green color index (DGCI) by putting green, monthly fertilization rate (ha–1), and days since initiation (DSI).
Figure 2-15: 2018 mean canopy dark green color index (DGCI) by putting green, monthly fertilization rate (ha–1), and days since initiation (DSI).
20 40 60 80 100 120 140 160
Canopy dark green color (D
GC
I units)
0.59
0.60
0.61
0.62
0.63
0.64
0.65
0.66
0.67
0.68
0.69
Sand 2
Days since initiation (DSI)20 40 60 80 100 120 140 160
Can
opy
dark
gre
en c
olor
(DG
CI u
nits
)
0.59
0.60
0.61
0.62
0.63
0.64
0.65
0.66
0.67
0.68
0.69
Push-Up
20 40 60 80 100 120 140 160
0 kg K2O15 kg K2O30 kg K2O45 kg K2O
Sand 1
360 390 420 450 480 510
Canopy dark green color index (D
GC
I units)
0.45
0.48
0.51
0.54
0.57
0.60
0.63
0.66
0.69
0.72
Sand 2
Days since initiation (DSI)360 390 420 450 480 510
Can
opy
dark
gre
en c
olor
inde
x (D
GC
I uni
ts)
0.45
0.48
0.51
0.54
0.57
0.60
0.63
0.66
0.69
0.72
Push-Up
360 390 420 450 480 510
0 kg K2O15 kg K2O30 kg K2O45 kg K2O
Sand 1
61
Ball roll
Potassium fertilization did not dependably influence ball roll distance measured
on the Sand-2 green. A global ANOVA of nine collection events showed ball roll
distance to be influenced (P £ 0.01) only by days since initiation (DSI) (Table: 2-6).
Given the recognized influence temperature, shoot growth, and/or climatic conditions
have on ball roll distance (green speed), significant variation over nine sampling dates
spanning two seasons is expected, but presented for the benefit of interested readers
(Figure: 2-16).
62
Table 2-6: Analysis of variance (ANOVA) of ball roll distance by source, and least squares means by monthly K fertilization levels.
Source †type Ball roll distancenum den ǂP r>F
K fertilization (Fert) F 6 12 0.51Days since initiation (DSI) F 8 112 **Fert x DSI F 48 112 0.55
Ball roll distancem
0 2.1615 2.1630 2.1845 2.16Least significant difference, a = 0.05 ns
K fertilization contrasts P r>F§Frequency (Freq) 1 12 0.72Quadratic rate 1 12 0.23Cubic rate 1 12 0.11Freq x linear rate 1 12 0.62Freq x quadratic rate 1 12 0.77Freq x cubic rate 1 12 0.86†; Type of variable; R, random; F, fixed.ǂ; *, ** Significant at respective P values ≤ 0.05, 0.01.§; Weekly v. semi-monthly applications.
deg. freedom
Monthly K fertilization, kg ha─1
63
Figure 2-16: Mean ball roll distance by K fertilization rate (month ha)–1, and days since initiation (DSI).
Days since initiation (DSI)40 80 120 360 400 440 480 520
Ball r
oll d
istan
ce (m
)
1.8
1.9
2.0
2.1
2.2
2.3
2.4
0 kg K2O15 kg K2O30 kg K2O45 kg K2O
64
Leaf water content
Potassium fertilization did not dependably influence creeping bentgrass leaf water
content. Global ANOVA of five collection events showed leaf water content, like
clipping yield and ball roll distance, to be influenced (P £ 0.01) only by days since
initiation (DSI) (Table: 2-7). Given the recognized influence temperature, soil moisture,
and available N have on turfgrass shoot growth and water status, significant variation in
leaf water content over five sampling dates spanning two seasons is expected (data not
shown).
65
Table 2-7: Analysis of variance (ANOVA) of leaf water content by source, and least squares means by monthly K fertilization levels.
Source †type Leaf water contentnum den ǂP r>F
Putting green (PG) R 2 6 0.28K fertilization (Fert) F 6 12 0.87Days since initiation (DSI) F 4 216 **Fert x DSI F 24 216 0.83
Leaf water contentg kg─1
0 749.415 744.030 746.445 743.3Least significant difference, a = 0.05 ns
K fertilization contrasts P r>F§Frequency (Freq) 1 12 0.96Quadratic rate 1 12 0.79Cubic rate 1 12 0.47Freq x linear rate 1 12 0.39Freq x quadratic rate 1 12 0.60Freq x cubic rate 1 12 0.41†; Type of variable; R, random; F, fixed.ǂ; *, ** Significant at respective P values ≤ 0.05, 0.01.§; Weekly v. semi-monthly applications.
deg. freedom
Monthly K fertilization, kg ha─1
66
Wear tolerance field trial
Potassium fertilization did not dependably influence canopy density (NDVI) of a
highly-trafficked Penn A-1/A-4 creeping bentgrass putting green. Global ANOVA of 23
collection events showed canopy density to be influenced (P £ 0.01) only by days since
initiation (DSI) (Table: 2-8). Given the recognized detriment frequent mechanical wear
has on turfgrass being mowed daily at a less than 3.0-mm height of cut, variation in
canopy density over the 23 sampling dates in 2018 is expected, and presented for the
benefit of interested readers (Figure: 2-17).
67
Table 2-8: Analysis of variance (ANOVA) of canopy density collected during imposed 2018 intense traffic trial as normalized differential vegetative index (NDVI), by source, and least squares means by monthly K fertilization levels.
CanopySource †type density
num den ǂP r>FK fertilization (Fert) F 3 6 0.92Days since initiation (DSI) F 22 176 **Fert x DSI F 66 176 0.99
CanopydensityNDVI
0 0.77215 0.77230 0.77045 0.771Least significant difference, a = 0.05 ns
K fertilization contrasts P r>FLinear rate 1 6 0.75Quadratic rate 1 6 0.82Cubic rate 1 6 0.61†; Type of variable; R, random; F, fixed.ǂ; *, ** Significant at respective P values ≤ 0.05, 0.01.
deg. freedom
Monthly K fertilization, kg ha─1
68
Figure 2-17: Mean canopy density as normalized differential vegetation index (NDVI) by monthly K fertilization rate (ha–1), and days since initiation (DSI) during simulated traffic stress period.
Days since initiation (DSI)420 430 440 450 460 470 480 490 500
Can
opy
dens
ity (N
DVI
uni
ts)
0.62
0.64
0.66
0.68
0.70
0.72
0.74
0.76
0.78
0.80
0.82
0.84
0.86
0.88
0 kg K2O15 kg K2O30 kg K2O45 kg K2O
69
Drought tolerance greenhouse trial
Canopy density data collected from 189 unique plot plugs over three drought
studies were combined for analysis by mixed model. A global ANOVA of 15 data
collection events showed canopy density during an imposed 20-day dry-down to be
influenced (P £ 0.01) only by days since watering (DSW) (Table: 2-9). Given the
recognized requirement of soil water by turfgrass under otherwise-optimal growing
conditions, gradual deterioration of canopy density over 15 sampling dates spanning three
weeks is expected, and presented by monthly K fertilization rate for the benefit of
interested readers (Figure: 2-18).
70
Table 2-9: Analysis of variance (ANOVA) of canopy density collected during imposed greenhouse dry-down as normalized differential vegetative index (NDVI), by source, and least squares means by monthly K fertilization levels.
Source †type Canopy densitynum den ǂP r>F
Putting green (PG) R 2 6 **K fertilization (Fert) F 6 12 0.25Days since watering (DSW) F 14 2217 **Fert x DSW F 84 2217 0.99
Canopy densityNDVI
0 0.62915 0.62530 0.62645 0.620Least significant difference, a = 0.05 ns
K fertilization contrasts P r>F§Frequency (Freq) 1 12 0.45Quadratic rate 1 12 0.81Cubic rate 1 12 0.34Freq x linear rate 1 12 0.63Freq x quadratic rate 1 12 0.74Freq x cubic rate 1 12 0.07†; Type of variable; R, random; F, fixed.ǂ; *, ** Significant at respective P values ≤ 0.05, 0.01.§; Weekly v. semi-monthly applications.
deg. freedom
Monthly K fertilization, kg ha─1
71
Figure 2-18: Mean canopy density as normalized differential vegetation index (NDVI) by K fertilization rate (month ha)–1 and days since watered (DSW) during simulated drought period.
Days since watered4 6 8 10 12 14 16 18 20
Mea
n ca
nopy
den
sity
(ND
VI u
nits
)
0.45
0.50
0.55
0.60
0.65
0.70
0.75
0 kg K2O15 kg K2O30 kg K2O45 kg K2O
72
Discussion
Potassium fertilization is often recommended in maintenance of turfgrass
nutritional sufficiency and quality during periods of stress (Carrow et. al, 2001;
Christians, 1993). Golf course superintendents follow soil test or programmatic K
fertilizer recommendations to mitigate the effects of stress inherently imposed upon
putting greens through necessary cultural practice (Woelfel, 2017). The results of this
study question the traditional roles K fertilization has long been believed to impart upon
creeping bentgrass.
Across the three experimental putting greens, monthly K fertilization increments
of 30 kg K2O ha–1 resulted in significantly different experiment-wide mean extractable K
concentrations in the 0 to 15 cm soil depth. Similar results were reported of ‘Penncross’
putting greens response to annual rates of 0 to 390 kg K2O ha–1 (Nikolai, 2002). Annual
increasing K2O fertilizer rates from 0 to 243 kg ha–1 positively influenced extractable soil
K concentrations in mineral soil (Fitzpatrick and Guillard, 2004). Similarly, extractable K
concentrations were increased by K application rates to creeping bentgrass (Waddington
et. al. 1972; Woods et. al, 2006). Johnson et al. (2003) report extractable K concentration
in soil depth segments from 0- to 30-cm increased as a result of K fertilization, but not in
proportion to the rates applied. Potassium concentrations were highest in the surface layer
(0-7.5 cm) and decreased with depth in the profile (Johnson et. al. 2003). Similar reports
from Young (2009) showed quarterly-application of 56 kg K2O ha–1 increased mean soil
extractable K relative to that measured in unfertilized control plots. However, in a recent
‘Crenshaw’ creeping bentgrass putting green study, extractable soil K levels sampled in
73
April were unaffected by either 0, 36.6, or 73.2 kg K ha–1 fertilizer treatment sprayed
over three split-applications the previous November (Mirmow, 2016).
Concurrent with findings from numerous cool-season turfgrass studies (Dest and
Guillard, 2001; Fitzpatrick and Guillard, 2004; Nikolai, 2002; Woods et al., 2006;
Young, 2009), K fertilization rate did not dependably influence mean shoot growth
(clipping yield). However, 15 kg K2O increments applied weekly or semi-monthly over
the range of 0 to 45 kg K2O (ha month)–1 incited statistically-different, yet
disproportionate, leaf K concentration of Penn A- and G-series creeping bentgrass putting
greens. In agreement with other published data (Woods, 2006; Young, 2009), these
results confirm luxury consumption of K by creeping bentgrass putting greens. Reports of
tissue K concentrations not statistically-responding to increasing K fertilizer rate are less
common, and perhaps the result of more sporadic and/or conservative K fertilization
protocols (Mirmow, 2016; Petrovic et. al, 2005; Waddington et al., 1972).
Despite tissue K concentrations of unfertilized plots falling below time-honored
tissue K thresholds of 22 g kg–1 (Mills and Jones, 1996), no deficiency symptoms were
observed in 2018. Absence of visual K deficiency symptoms in creeping bentgrass
clippings containing less than 18 g K kg–1 has been reported (Mirmow, 2016; Woods et.
al, 2006; Young, 2009). Likewise, upon conclusion of a creeping bentgrass greenhouse
study, leaf K in clippings collected from six different sand rootzones all registered less
than 10 g K kg-1, yet visual K deficiency symptomology was observed in the canopy
vegetation of only four rootzones (Dest and Gulliard, 2001).
While consistently increasing leaf K response to all treatments draws attention to
the second season (2018) data, it further calls into question the source of K to the
74
unfertilized plots. Numerous researchers have implicated the non-exchangeable soil K
pool as a source of available K to creeping bentgrass roots, but Dest and Guillard (2001)
verified a stronger correlation of bentgrass K uptake and nitric acid extraction than K
uptake and ammonium acetate extraction. Tissue K concentrations collected from
unfertilized plots of the Sand-2 green increased from 17 to 21 g kg-1 over the 2018 season
despite a simultaneous decrease in soil extractable K from 24 µg g–1 (DSI: 357) to 14 µg
g–1 (DSI: 525). These results suggest that creeping bentgrass may have been utilizing K
from non-exchangeable sources in the rootzone and/or from K-feldspars additions (24 kg
K2O ha–1 yr–1) in topdressing sand (Bier et. al, 2017; Dest and Gulliard, 2001; Woods et.
al, 2006).
Creeping bentgrass K uptake was positively influenced by K fertilization rate,
which was not unexpected given the observed sensitivity of leaf K to K fertilization rate.
While not analyzed as a dependent variable, K fertilizer use efficiency is readily
calculated from the significantly different K uptake means observed. Corrected for
control plot mean uptake (background K availability) and averaged over the 90-day
periods when monthly K uptake was measured in both years; 6.6, 6.1, or 4.1% of the
respective 15, 30, or 45 kg ha–1 K2O fertilizer rates were recovered in bentgrass leaf
clippings (Figure 2-19). Given the season-long protocol of foliar fertilization on weekly
or semi-monthly intervals, the observed mean fertilizer K recoveries were surprisingly
limited.
Regarding extractable soil K levels measured over the 2018 season, temporal
increases were only observed in the upper 15 cm of the Push-up putting green rootzone.
Thus, that associated cation exchange suite comprises the sole candidate to have retained
75
unrecovered K fertilizer. While speculative, additional unrecovered fertilizer K may have
been retained by the cation exchange suite within the 15 to 30 cm depth of all putting
green rootzones. Given the record level of precipitation endured in 2018, leachate
contributions to the sub-surface drainage may account for a further portion of
unrecovered K fertilizer.
Potassium applications did not increase NDVI, a highly resolute and dependable
measure of turfgrass canopy density (DaCosta and Huang, 2006; Zhu et al., 2012).
During stress periods, K additions had either an immeasurable or adverse influence on
canopy density. Similarly, K applications exceeding a 1:1 ratio of N fertilizer rate did not
affect canopy density of bermudagrass, seashore paspalum, and zoysiagrass hybrids
maintained as putting greens in Florida (Rowland et. al, 2014). Mirmow (2016) saw no
differences in canopy density of ‘Crenshaw’ creeping bentgrass following K applications.
There was no benefit to creeping bentgrass canopy color (DGCI) from K
applications observed over this two-year study. Furthermore, there were no visible
differences in canopy color by treatment observed between plots at any point over the
two-year trial (Figure: 2-20, 2-21, 2-22 and 2-23). No differences in visual quality/color
ratings were seen from K applications when measured over 20 years on a ‘Penncross’
creeping bentgrass putting green despite extractable K concentrations considered low
over the last seven years (Fulton et. al., 2002). The described work confirms the
predominate absence of canopy density/color response to K fertilization by creeping
bentgrass putting greens in recent published reports (Johnson et al., 2003; Kim and Kim,
2012; Mirmow, 2016; Nikolai, 2002; Woods et al., 2006).
76
While research conducted over the last 30 years indicates K fertilization rarely
influences creeping bentgrass canopy quality, proponents of traditional K fertilizer
regimes typically argue increased stress tolerance as its foremost attribute. Putting green
canopy density and color indices deteriorated rapidly in response to increased
maintenance and mechanical wear during the second-year tournament simulation, yet no
benefit to turfgrass quality or recovery rate was observed as a result of K fertilizer
additions (Figure: 2-13 and 2-15).
Potassium applications did not affect end of day ball roll distance (green speed)
on any sampling dates performed on the Sand-2 green. Green speed increased with
greater mowing and rolling event frequency. Reduction in mowing height increased ball
roll distance of ‘Penncross’ creeping bentgrass, whereas K fertilization did not (Nus,
1989). More recent creeping bentgrass putting green research report K fertilization had
no effect on ball roll distance (Nikolai, 2002; Woods et. al, 2006). This research confirms
K fertilization does not dependably influence putting green speed and should not be used
a tool to do so.
Potassium fertilization did not increase leaf tissue water content in the field.
Likewise, the described K applications did not influence leaf/shoot turgidity or tolerance
to wilting during multiple occurrences of drought stress in the field (Figure: 2-24).
Damage from localized dry spot on ‘Penncross’ creeping bentgrass was unaffected by K
fertilizer additions (Nikolai, 2002). Similarly, K applications to ‘Colonial’ bentgrass did
not significantly affect dieback from drought stress (Lawson, 1999). In Florida, K applied
in K:N ratios exceeding 1:1 did not improve drought tolerance of five species of warm
season turfgrass maintained as putting greens (Rowland et. al, 2014).
77
Likewise, K fertilization did not benefit creeping bentgrass drought resistance in a
terminal dry-down study. On no occasion did K fertilization support improved canopy
density relative to unfertilized creeping bentgrass as plot plugs dried down to dormancy
over a 20-day period. Drought stress tolerance of ‘Penncross’ creeping bentgrass
established to eight different sand rootzones was not influenced by K fertilizer additions
(Dest and Guillard, 2001). Despite the extreme measures taken to elucidate the benefit of
K nutrition to creeping bentgrass drought tolerance in the described experiments, no
benefit was observed. On this basis, the role K fertilization plays in creeping bentgrass
water relations and drought tolerance needs to be de-emphasized.
Potassium fertilizer did not improve wear tolerance when examined as canopy
density under severe stress from simulated traffic and increased mowing and rolling
events. Recovery from wear stress was similarly independent of pre-emptive and ongoing
K fertilization rate. While increasing K fertilization rates have been reported to improve
wear tolerance of ‘Penncross’ creeping bentgrass (Shearman and Beard, 2002; Kim and
Kim, 2012), said improvement required an annual K application rate in excess of 270 kg
ha–1 (Shearman and Beard, 1975). Conversely, K fertilization did not improve wear
tolerance or recovery of ‘Penncross’ or ‘Penneagle’ creeping bentgrass (Carroll and
Petrovic, 1991). Upon subjecting ‘L-93’ creeping bentgrass to traffic six days per week
representing 30,000 golfers in a year, multi-year K fertilizer treatments imparted no
influence on visual quality ratings (Woods et. al, 2006). Neither canopy density nor
visual ratings of ‘Crenshaw’ creeping bentgrass under simulated morning or afternoon
traffic were positively affected by K applications (Mirmow, 2016). While wear/traffic
injury remains a major issue for managers of highly-used turfgrass systems, results of this
78
and other recent studies safely preclude the role of K fertilization as a curative or
preventative management practice.
The results of this study show little influence of potassium fertilization on
creeping bentgrass putting green quality or performance parameters. Although K
applications increased both soil and tissue K concentrations, no benefits to turfgrass
quality measures were observed as a result. These results agree with much of the recent K
fertilization results conducted on creeping bentgrass. In response to imposed mechanical,
drought, and wear stresses, K fertilization did not positively influence creeping bentgrass
tolerance. Golf course superintendents should consider these results and reconsider their
K fertilizer programs, particularly as they relate to stress tolerance support.
Future research should be conducted on K deficient creeping bentgrass systems,
along with annual bluegrass systems. The authors encourage interested parties to begin
now by withholding all K additions from candidate systems. Deficiency symptoms were
never seen on the described putting greens, despite tissue levels less than 20 g K kg–1, and
Mehlich-III soil extractable K levels less than 15 µg g–1. Additional research is needed to
determine a minimum soil K threshold concentration, for which deionized water may
comprise the most appropriate extractant. A tissue K deficiency threshold for creeping
bentgrass remains a meaningful goal of future research, as this study and others have not
unequivocally defined it. Research suggests that creeping bentgrass readily assimilates K
from non-exchangeable soil minerals, but long-term studies are needed to evaluate how
long K-bearing sand rootzones and/or topdressing can support bentgrass requirements.
79
Figure 2-19: Mean fertilizer K uptake (kg ha–1) and K fertilizer use efficiency (%) pooled over the three creeping bentgrass putting greens, by K fertilization treatment.
80
Figure 2-20: Photograph illustrating putting greens on 3/18/2018, from left to right; Push-Up, Sand-1, Sand-2. Number code 0: unfertilized control, 1: weekly 15 kg K2O ha-1 month, 2: semi-monthly 15 kg K2O ha-1 month, 3: weekly 30 kg K2O ha-1 month, 4: semi-monthly 30 kg K2O ha-1 month, 5: weekly 45 kg K2O ha-1 month, 6: semi-monthly 45 kg K2O ha-1 month.
Figure 2-21: Photograph illustrating putting greens on 5/29/2018, from left to right; Push-Up, Sand-1, Sand-2. Number code 0: unfertilized control, 1: weekly 15 kg K2O ha-1 month, 2: semi-monthly 15 kg K2O ha-1 month, 3: weekly 30 kg K2O ha-1 month, 4: semi-monthly 30 kg K2O ha-1 month, 5: weekly 45 kg K2O ha-1 month, 6: semi-monthly 45 kg K2O ha-1 month.
81
Figure 2-22: Photograph illustrating putting greens on 7/19/2018 (Day 2 of tournament simulation), from left to right; Push-Up, Sand-1, Sand-2. Number code 0: unfertilized control, 1: weekly 15 kg K2O ha-1 month, 2: semi-monthly 15 kg K2O ha-1 month, 3: weekly 30 kg K2O ha-1
month, 4: semi-monthly 30 kg K2O ha-1 month, 5: weekly 45 kg K2O ha-1 month, 6: semi-monthly 45 kg K2O ha-1 month.
Figure 2-23: Photograph illustrating putting greens on 8/7/2018 (Day 20 of tournament simulation), from left to right; Push-Up, Sand-1, Sand-2. Number code 0: unfertilized control, 1: weekly 15 kg K2O ha-1 month, 2: semi-monthly 15 kg K2O ha-1 month, 3: weekly 30 kg K2O ha-1
month, 4: semi-monthly 30 kg K2O ha-1 month, 5: weekly 45 kg K2O ha-1 month, 6: semi-monthly 45 kg K2O ha-1 month.
82
Figure 2-24: Photograph illustrating the Sand-1 green on 9/5/2018,(VWC: 4.5%),Number code 0: unfertilized control, 1: weekly 15 kg K2O ha-1 month, 2: semi-monthly 15 kg K2O ha-1 month, 3: weekly 30 kg K2O ha-1 month, 4: semi-monthly 30 kg K2O ha-1 month, 5: weekly 45 kg K2O ha-1 month, 6: semi-monthly 45 kg K2O ha-1 month
83
Conclusions
Over the two years of study there was no benefit from K fertilizer applications
seen on the creeping bentgrass putting greens tested, although K applications
significantly increased soil and tissue K concentrations. There were no positive effects
seen on canopy density (NDVI), canopy color (DGCI), clipping yield, leaf tissue water
content, or ball roll. Further, K applications did not improve tolerance to drought,
mechanical (tournament simulation), or traffic stress. Fertilizer recovery was very poor at
all rates and the unutilized K was not effectively stored as exchangeable K in the sand
based greens. In this study there was no benefit to applying small amounts of K more
frequently (spoon-feeding). Visual differences were not seen at any time from K
applications. This research agrees with recently completed studies that show little to no
benefit from K fertilization.
These results and others indicate creeping bentgrass requires less K than once
thought and recommendations for golf course superintendents need to change. From the
results of this project, monthly applications should not exceed 15 kg K2O ha-1 per
growing month. This includes not applying K at all. When interpreting soil K levels,
superintendents should apply K to the low level of SLAN (Mehlich-III: 40-50 µg g–1) as a
target for creeping bentgrass putting greens. For tissue K analysis, an applicable critical
threshold for leaf K sufficiency in creeping bentgrass is 15 g kg-1. These
recommendations may still be excessive but more research is needed where K is deficient
to determine critical soil and tissue levels for creeping bentgrass.
84
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