From a golfer's perspective, a properly constructed golf course green will provide a firm, uniform surface on which to putt. Getting to this point is not as easy as it appears. From a superintendent's perspective, a properly constructed golf course green will provide the above and more: will resist compaction even under frequent close mowing and foot traffic; will allow adequate water and air infiltration and percolation; will retain nutrients; and will hold moisture and make it available when the turfgrass needs it. Proper soil amendments and the right construction methods are your keys to coming through with the goods.
LAYING THE GROUNDWORK
The primary ingredient for constructed root zones for turfgrass is sand. Sand is considered an ideal medium for bentgrass greens in terms of its physical characteristics. It is resistant to compaction, and it has a high infiltration rate and aeration porosity. However, sand does have a lower water-holding capacity and nutrient retention compared to other media. Due to these challenges, it is often a good idea to add amendments to the sand base to increase water and nutrient retention. Organic amendments such as peat increase water and nutrient retention, but peat decomposes over time, which results in lower air-filled porosity and water infiltration.
Other amendments for sand-based root zones include inorganic porous ceramics and zeolites that offer some benefits relative to organic amendments. They are less prone to compaction than organic materials and also have high cation exchange (nutrient retention) and water-holding capacities relative to sand. Additionally, because they are inorganic, they do not decompose with time.
Construction systems for root zones also influence the performance of turfgrass. An increasing number of golf courses are building greens in a California-style that reduces costs relative to United States Golf Association (USGA)-style greens by establishing a 10- to 12-inch sand layer over native soil without a gravel layer. Before construction, physical properties for the sand-based root zone must meet USGA recommendations that include saturated hydraulic conductivity, bulk density, porosity and water-holding capacity. Over time, these physical properties typically change due to an increased thatch layer and compaction as well as decomposition of the peat. Chemical properties of the root zone (organic matter content and nutrient retention) also may change.
PUTTING IT TO THE TEST
The University of Missouri-Columbia Turfgrass Research Center conducted a study comparing putting-green root zone amendments and greens construction systems. The researchers compared four treatments: a California profile, a USGA profile and two modified California profiles. The California profile, referred to as the California treatment, consisted of a 12-inch layer of 100 percent sand over a 5-inch layer of silt loam with a drain at the top of the silt loam layer. The USGA profile, referred to as the USGA treatment, consisted of 90 percent sand and 10 percent Dakota reed sedge peat by volume, with a 12-inch root zone mix over a 5-inch pea gravel layer (0.10 to 0.25 inch in diameter) over a drain. The sand/peat mix was blended at the supplier, Capitol Sand, Jefferson City, Mo. Researchers also studied two modified California profile green treatments, each consisting of a 10-inch root zone mix over a 7-inch layer of silt loam with a drain at the top of the silt loam layer. The first modified profile, referred to as the California-P treatment, consisted of 82 percent sand, 15 percent porous ceramic (Profile), and 3 percent humate; the second modified profile, referred to as the California-Z treatment, consisted of 85 percent sand and 15 percent zeolite (ZeoPro). These two mixes were blended at the Turf Research Center with a small cement mixer. Two sands were used for the different root zone mixes in this study: a coarse (25 percent) and medium (52 percent) sand was blended with Dakota reed sedge peat for the USGA treatment and a medium (53 percent) and fine (37 percent) sand was used in the three California-style treatments. The particle size distributions in the root zone mixes for the California and USGA systems met the criteria for these systems.
Researchers established field treatments in 4-foot × 4-foot wooden boxes equipped so that they could monitor drainage leachate. The amended and unamended root zones were installed in August and ‘Penncross’ creeping bentgrass was seeded Sept. 27, 1998, at 1 lb/M (M=1000 ft
The objective of the study was to compare these amendments and construction systems in terms of short- and long-term performance and resource efficiency by measuring responses such as: creeping bentgrass establishment, quality and color, root zone physical and chemical properties, and nitrogen and potassium leaching.
- Root Zone Properties
Researchers removed samples from the treatments and evaluated them in terms of saturated hydraulic conductivity (Ksat), air-filled porosity, capillary porosity, total porosity and bulk density. Results of the physical property evaluation indicated that the construction treatments met the criteria set for California and USGA construction systems. Plots with amendments increased the capillary porosity in relation to the California treatment (100 percent sand), resulting in greater water retention. The plots with peat amendments (USGA) had the lowest saturated hydraulic conductivity values measured in laboratory tests for the root zone mixes. They found higher hydraulic conductivity values in the profile- and zeolite-amended California treatments compared to the other treatments; they attributed this to the higher total porosity of these treatments. Field water infiltration measurements were taken two to three times each year. Results of field infiltration measurements for the study indicated that all treatments experienced a decrease in the infiltration rate over time. Infiltration rates decreased over time, probably due to the increased accumulation of a thatch layer. The zeolite-amended California treatment had the highest field infiltration rates throughout the study. At the end of the experiment, infiltration rates were found to be higher in the Profile- and zeolite-amended California treatments compared to the USGA and un-amended California treatments.
The USGA and the Profile- and zeolite-amended California treatments had increased values of cation exchange capacity (nutrient retention) in the root zone compared to the un-amended California treatment. Researchers found the highest values were for the Profile-amended California treatment. The zeolite-amended California treatment contained more plant-available phosphorus and potassium compared to the other treatments, while the Profile-amended California treatment contained high amounts of available phosphorus, calcium and magnesium relative to the un-amended California and USGA treatments. Greater nutrient retention due to the additions of zeolite and Profile seemed to correlate with higher average bentgrass quality and color ratings over the three-year study. These results indicate that amending root zones with Profile or zeolite may allow for maintaining high quality bentgrass with less frequent fertilizer inputs.
- Bentgrass Performance
Conditions for the bentgrass establishment during the fall of 1998 were optimum and bentgrass germination and fill-in were excellent. The USGA and un-amended California treatments had better early establishment compared to the Profile- and zeolite-amended California treatments. No differences among treatments occurred three months after seeding.
The zeolite-amended California treatment had the highest overall quality ratings during most of the experiment. The only decrease occurred during July 2000 when irrigation water was not applied for 30 days. During this sampling period, the zeolite-amended treatment experienced a significant decrease in bentgrass quality. Researchers also measured bentgrass color throughout the experiment. The zeolite-amended California treatment had the highest color ratings during 1999 and early 2000. However, during the rainfed period of July 2000, this treatment experienced a significant decline in color. After this period, the Profile-amended California treatment was found to have the best bentgrass color for the treatments.
- Nutrient Leaching
Researchers connected drains for each plot to outlet collectors and performed periodic collection of the drainage water to evaluate the quality of the water for nitrate and potassium concentrations. They found that the USGA treatment had the highest nitrate leachate concentration for the November 1998 sampling. However, at three years after planting, the results showed that there were no significant nitrate-leaching differences among the treatments. Results from the study also demonstrated that nitrate-leaching concentrations typically increased at the first late-fall sampling following seeding, and then drastically decreased during the next year's growing season. The zeolite-amended California treatment consistently had the lowest potassium leachate concentration, probably due to its higher cation exchange capacity (nutrient retention). The USGA treatment had the highest measured losses for potassium in June and September 1999.
THE COST OF PERFORMANCE
Overall, the zeolite-amended California treatment had the highest infiltration rates throughout the experiment. The Profile- and zeolite-amended California treatments had higher cation exchange capacities (nutrient retention) and plant nutrient levels. The Profile- and zeolite-amended California treatments produced the highest bentgrass quality and color throughout the experiment. It is speculated that inorganic amendments may be superior replacements for peat as amendments in terms of some criteria for sand-based putting greens. In comparing construction costs of the four treatments, the USGA green was considered as the standard: $14,000 per 10,000 square feet. The un-amended California treatment construction costs were $5,000 per 10,000 square feet, while the Profile- and zeolite-amended California treatment costs were $13,000 per 10,000 square feet and $18,000 per 10,000 square feet, respectively. Thus, it appears that the Profile-amended California treatment achieves good quality bentgrass for a cheaper cost compared to the zeolite-amended California treatment.
Chang-Ho Ok is a graduate student in horticulture and Steve Anderson is professor of soil science, both at the University of Missouri (Columbia, Mo.)
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