THE CHOKER LAYER An examination of the water content of a USGA rootzone mixture in the presence and absence of a coarse intermediate layer. by RAYMOND H. SNYDER and JOHN L. CISAR, Ph.D.

E United States Golf Associ- greens are of similar USGA specifica- The trademark of the USGA green is atio~ has long .been re.garded as tions, a valuable characteristic when an enhancement of the water-holding TIan mnovator In puttmg green developing a turf management program capacity of the rootzone mix relative construction. Since the 1960s the because a turf manager is able to apply to what would occur in the surface USGA has suggested recommenda- similar management practices to all of horizon if the same-textured rootzone tions for putting green construction the greens, reducing confusion and mix extended to the subgrade of the based on research conducted by a headaches (in today's world, wasted green. This enhancement is created number of scientists at various insti- time equals wasted money). by placing the rootzone mix over a tutions. These recommendations have The components of a USGA green coarser-textured layer of sand, the been "the most widely used method as initially described (USGA Green intermediate layer or, as it is often for green construction throughout the Section Staff, 1960) are: 1) the sub- called, the choker layer. Water held in United States and in other parts of the grade, 2) drainage pipes, 3) gravel layer, the fine-textured rootzone layer does world" (Hummel, 1993). 4) intermediate layer, 5) rootzone mix. not pass into the coarse-textured layer The objectives of the USGA's recom- Each component is required to meet "until the pressure head at the inter- mendations for green construction are strict USGA recommended specifica- face builds up sufficiently for water to clear: Create a green that drains rapidly, tions to ensure a properly functioning penetrate into the coarse material" resists compaction, and provides a suit- green. (As of 1993, the need for the (Hillel, 1982). This essentially means able medium for plant growth. Addi- intermediate layer is determined by the that the pores in the rootzone mix tionally, a high level of uniformity exists properties of the gravel and rootzone must become nearly filled near the within a golf course when all of its mix.) interface with the coarser sand before

The research study investigated the moisture-holding capacity of a green constructed with and without the intermediate layer, using a rootzone mix with physical properties recommended by the USGA.

14 USGA GREEN SECTION RECORD water will move into the underlying coarse-textured layer. The enhanced Table 1 water-holding capacity is often called Particle Size Distribution of USGA Rootzone Mix a "perched water table." However, Used in This Experiment since the pores in the rootzone never Name Particle Diameter Specification Used % remain saturated for an extended Fine Gravel 2.0-3.4mm Not more than 10% of the total .3 period of time, a true water table does particles in this range, including not exist and the term "perched water a maximum of 3% fine gravel table" will not be used herein. The (preferably none) enhanced water-holding capacity is Very coarse sand 1.0-2.0mm 9.6 important, because without it, the Coarse sand 0.5-1.0mm At least 60% of the particles 48.0 rootzone mix would be less able to hold must fall in the coarse and sufficient water to maintain the turf- medium sand classes grass between reasonably scheduled Medium sand 0.25-0.50mm 34.1 irrigations during periods of heavy Fine sand 0.15-0.25mm Not more than 20% of the 7.9 play and tournaments, or when regu- particles may fall within this range latory agency irrigation restrictions are mandated. Very fine sand 0.05-0.15mm Not more than 5% 0.0 The intermediate layer component Silt 0.002-0.05mm Not more than 5% 0.0 (choker layer) of the USGA green has Clay Less than 0.002mm Not more than 3% 0.0 been the source of much debate. The VFS + S + C should not exceed 10% of total intermediate layer is comprised of sand particles between Imm and 4mm in diameter. Its original purpose was to the moisture content of the rootzone with a wooden rod after adding each prevent migration of sand particles mix overlying a coarse sand material is layer of Scm. When finished, there from the rootzone mix into the gravel less than that of a rootzone mix over- was Scm of unfilled column remaining layer (Hummel, 1993). However, many lying a gravel material. The soil material above the surface of the soil. The two golf course architects and superinten- used in their experiment was a sandy treatments were replicated five times dents have considered the intermedi- loam (54% sand, 36% coarse silt, 3% and the columns were arranged in a ate layer to be the reason for the fine silt, and 7% clay), which is not completely random fashion in the rack. enhanced water-holding capacity of a rootzone mix that meets USGA On the afternoon of the first day, the rootzone. In fact, enhanced water- specifications. water was added to the top of each holding capacity will occur with or An experiment was conducted to column until it dripped out of the holes without the Imm to 4mm particle-sized determine the moisture-holding capac- in the stoppers in the bottom of the intermediate layer, as long as the ity of a green constructed with and columns. Water was repeatedly added rootzone mix is placed over a coarser- without the intermediate layer, using a throughout the evening and the next textured layer, such as the gravel layer. rootzone mix with physical properties morning until 10:00 AM to ensure The high cost of the intermediate recommended by the USGA. that the soil was saturated with water. layer has also made it the subject of The columns were then covered with controversy. The cost of the inter- Methods and Materials aluminum foil to prevent evaporation mediate layer is high because guidelines The experiment was conducted using and allowed to drain for 24 hours. call for a narrow particle range, re- clear plastic columns having a diameter After draining, the columns were quiring careful sieving by suppliers. of 4.5cm and a length of 4Scm. The removed from the wooden rack. Using Furthermore, the additional time and columns were placed upright in a a hacksaw, the top, middle, and lower labor required to add this layer in green wooden rack. A rubber stopper having 10cm portions of the 30cm rootzone construction increases its cost. To re- two holes was pushed into the bottom mix sand were excised. The length of duce costs, many construction com- of each column and covered with a the cut portion of the column was panies eliminate this layer during the piece of wire screen. Gravel that met measured, since sometimes the top or construction of new greens; some such USGA guidelines was placed in the bottom portion was a little more or greens function correctly, but many fail. bottom of each column to a depth of less than 10cm, and the sand, still inside The USGA recognized that it could 10cm (if no choker layer was to be of the plastic column piece, was placed better serve the industry by providing used), or to a depth of Scm (if choker in an aluminum tray and weighed to specifications for construction when sand was to be added). The choker the nearest O.lgm on a top-loading bal- the intermediate layer is omitted, and sand was sieved to ensure that it would ance. After recording the wet weight, published revised specifications in all be less than 4mm and greater than the tray containing the column piece 1993. When the intermediate layer is Imm. In five columns, a layer of choker and column was placed in an oven set omitted, very strict gravel specifications sand Scm deep was added. The sand at approximately 70 degrees centigrade. must be met (Hummel, 1993). was packed gently with a wooden A higher temperature was not used to Many in the golf course construc- rod. The bulk density of each 10cm prevent melting of the plastic column. tion industry have discontinued the depth was determined (soil weight! soil Over the next two days, the samples practice of installing the intermediate volume) to verify the uniformity of were weighed several times to obtain a layer following the release of the new packing. Then rootzone mix sand that constant dry weight. After the first day, specifications; therefore, the effects of met USGA guidelines (Table 1) was the sand was shaken out of the plastic this practice should be examined. added to each column to a depth of column to speed up drying, but the Miller and Bunger (1963) showed that 30cm. It was gently packed in place empty column was placed back into the

JULY/AUGUST1997 15 cause the matrix suction at which pores Table 2 fillwith water decreases as the pore size Bulk Density, Moisture Content, Pore Space, and Air-Filled Pore Space increases. Thus, the larger pore-sized for Different Soil Depths With or Without the Intermediate Layer gravel layer draws. less water from the overlying rootzone mix than does the Bulk Soil Moisture Total smaller pore-sized choker sand. The Factor Density By Weight By Volume Pore Space Air-Filled Pore Space results of this study are consistent glee ------% ------0/0 % with the results found by Miller and INTERMEDIATE LAYER Bunger (1963). However, their data None 1.53 16.52 22.95 42.2 45.1 could not be used to calculate the With 1.55 15.03 21.11 41.5 49.5 magnitude of the difference in water Significance NS ... NS ... content in a USGA green with and without the choker layer. The water- DEPTH (cm) holding capacity of a soil can be re- 71.1 0-10 1.56 8.74 12.10 41.2 lated to rainfall, irrigation, or evapo- 10-20 1.53 16.17 22.55 42.3 46.1 transpiration by considering a column 20-30 1.54 22.42 31.46 42.0. 24.7 of soil with unit surface area. As shown ...... Significance NS NS in Table 2, the 0-10cm soil column with CHOKER DEPTH the choker held 11.67% moisture by 3 3 - 0-10 1.55 9.07 12.53 41.3 70.0 volume (1l.67cm moisture/100 cm soil). Therefore, a 0-10cm column of 10-20 1.51 17.38 23.98 43.0 42.6 soil with a 1cm2 surface area would 20-30 1.53 23.11 32.35 42.2 22.6 hold 1.17cm of moisture (lcm x 1cm x + 0-10 1.56 8.41 11.67 41.0 72.1 10cm x 11.67g water/100cm3 soil = 10-20 1.55 14.96 21.11 41.5 49.5 1.67cm of precipitation, irrigation, or 20-30 1.54 21.73 30.57 41.8 26.9 evapotranspiration per cm2 soil sur- Significance NS NS NS NS NS face). Using this reasoning and the of the data in Table 2, it appears that with a Choker x Depth interaction choker layer, the 30cm of rootzone mix held a total of 6.34cm of water NS means there is no statistically significant difference for the treatment per 1cm2 of surface area (1.17 + 2.11 + * and ** refer to statistical significance at the 0.05 and 0.01 probability levels, respectively 3.06), and without a choker layer the total was 6.89cm (1.25 + 2.40 + 3.24), for a difference of 0.55cm. So what is tray with the sand. Finally, the sand and the other factor being the depth of the importance of 0.55cm of water? itself was weighed separately from the the rootzone mix. During cool weather in Florida, pre- column. dicted evapotranspiration (pET) for The moisture in each column piece Results and Discussion turf may be only 0.127cm per day was calculated by subtracting the The presence or absence of the (McCloud, 1971). By having an extra weight of the dry sand, including the choker layer and the depth (rootzone 0.55cm of water in the soil profile, a column piece and tray, from the weight mix section) significantly affected the turf manager may be able to delay irri- of the wet sand, including the column moisture content (by weight or by gations by approximately four days, if piece and tray. The moisture content volume) and the air-filled pore space of it is assumed that the turf can extract based on weight was calculated by the rootzone mix, but did not affect water from the entire 30cm of the soil dividing the moisture by the dry weight the total pore space or bulk density profile. Probably the actual delay time of the sand. The bulk density of each (Table 2). The factors of choker and will be less, since most of the turf column segment was calculated by depth should not have affected the roots will be in the upper part of the dividing the dry weight of the soil by bulk density or the pore space (which profile. On warm, sunny, windy days the column segment volume. The is calculated from the bulk density) in Florida, the turf may have a pET of moisture content of the soil on a since the rootzone mix was packed over 0.864cm per day. Under these volume basis was directly calculated by above the choker layer and an effort conditions, the extra water held in the dividing the moisture weight in grams was made to pack it uniformly through- absence of the choker layer probably (which also is the moisture volume in out the soil profile. There were no sig- would not permit the turf manager to cubic centimeters) by the volume of the nificant interactions among the choker delay irrigating even one day. Thus, column piece. The pore space was cal- and depth factors. although the presence or absence of culated as 100 - (bulk density/particle It was expected that the lower por- the intermediate layer (choker) was density), assuming a particle density tions of the soil profile would contain found to have a statistically significant of 2.65g/cc. Finally, the percent of more water than the upper portions, effect on the moisture-holding capacity air-filled pore space was calculated and that is what was found. When the of the overlying rootzone mix, the as 100 - {(volumetric moisture/pore choker layer was absent, the rootzone effect is probably not of major agro- space)100}, where a mean value of 41.8 mix contained more water at all three nomic importance. was used for the pore space term. rootzone mix sections (Figure 1) and It also is interesting to note that the The data were analyzed as a factorial correspondingly less air-filled pore 30cm profile averaged approximately experiment with one factor being the space (Figure 2), than when the choker 6.6cm of water per square centimeter presence or absence of the choker layer, layer was present. This occurred be- of surface area across the two choker

16 USGA GREEN SECTION RECORD At the start of the study, each soil column was saturated with water before calculating the moisture content. conditions. A native deep sand soil tion specifications. USGA Green Section U.S. Golf Association Green Section Staff. probably would hold only 2cm to 3cm Record. March/April, p. 7-21. 1960. Specifications for a method of putting of water in the upper 30cm of profile McCloud, D. E. 1971. Water Requirements green construction. USGA Green Section (Smajstrla, 1996). Thus, the value of for Turf. Proc. FL. Turfgrass Management Record 11(3):1-8. having a coarse-textured layer below Con£. 19:88-90. a finer-textured layer for holding Miller, D. E., and W C. Bunger. 1963. additional water was illustrated. Moisture retention by soil with coarse RAYMOND H. SNYDER is the 1997 layers in the profile. Soil. Sci. Soc. Amer. References Cited Granville C. Horn Graduate Student Proc.27:586-589. Scholarship Award recipient, and JOHN Hillel, Danile. 1982. Introduction to Soil Smajstrla, A. G. 1996. Water Requirements L. CISAR, Ph.D., is Associate Professor, Physics. Academic Press, San Diego. for Florida Agricultural Crops. University Turf Water Management, at the University Hummel, N. W, Jr. 1993. Rationale for the of Florida, IFAS, Extension Report 96 - 14. of Florida. This report is based on a class revisions of the USGA green construc- pp.21. project.

Figure 1 Figure 2 Effect of USGA Profile Construction Effect of USGA Profile Construction on Air-Filled on Water Content by Volume Pore Space (based on 0/0 of total pore space) 35 80 30 cf2. 25 0;; 60 W 20 o -a..::J« 40 15 u,(J) 9;w 10 «0: 0 20 5 - No Intermediate Layer a.. - No Intermediate Layer - - - Intermediate Layer - - - Intermediate Layer o 0 0-10 10-20 20-30 0-10 10-20 20-30 DEPTH (em) DEPTH (em)

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