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a Dep. of Crop Science, North Carolina State Univ., Raleigh, NC 27695-7620
b Dep of Soil Science North Carolina State Univ., Raleigh, NC 27695-7619
* Corresponding author (cbigelow{at}cropserv1.cropsci.ncsu.edu)
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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unamended sand. This response was probably due to improved water and nutrient retention of the amended rootzones. Although the forced air–vacuum technology provided little benefit in these newly constructed greens, it may be useful on mature putting greens that suffer from poor soil aeration or drainage.
Abbreviations: AI, air-injection • RMD, root mass density • TQ, turfgrass quality • WE, water evacuation
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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For the past 35 yr, the most common method of putting green construction has been the layered sand–gravel system (USGA, 1993). Sand is used as the rooting medium because it resists compaction, drains quickly, and maintains air-filled pore space even under heavy use. Normally, sand is combined with an organic amendment like peat moss to improve water and nutrient retention (Junker and Madison, 1967). Although peat moss is the most commonly used amendment, its gradual decomposition may adversely affect rootzone percolation rates. Over the years, inorganic amendments have been evaluated as alternatives to peat moss with mixed results (Davis et al., 1970; Waddington et al., 1974; Schmidt, 1980; Ferguson et al., 1986; Nus and Brauen, 1991; Kussow, 1996; Carlson et al., 1998; McCoy and Stehouwer, 1998). Once a suitable sand-based rootzone mixture is synthesized, it is installed directly above a specifically sized gravel layer (USGA, 1993). The gravel layer has two purposes: it facilitates rapid removal of excess water from the sand to the drainage lines and it conserves water and nutrients in the sand by creating a perched water table located above the sand–gravel interface (USGA, 1993).
Theoretically, the concept of a perched water table in the rootzone seems beneficial as a moisture reservoir, but it may actually create an undesirable condition that limits turfgrass rooting. A perched water table exists as a zone of near saturation and is characterized by a large volume of water-filled pores that could create an anoxic zone containing little or no O2. Saturated or anoxic soil conditions have long been observed to limit rooting depth (Morgan et al., 1964; Gliniski and Stepniewski, 1985; Huang, 1997). Some plants adapt to anoxic conditions by developing a dense, shallow root system near the soil surface where the O2 environment is more favorable (Gliniski and Stepniewski, 1985). While no specific minimum O2 threshold has been established for turfgrasses, it has been reported that roots of most crop plants cannot survive at O2 levels below 0.02 to 0.05 m3 m-3, with preferred levels between 0.05 and 0.15 m3 m-3, depending on species (Barden et al., 1987).
The composition of the soil atmosphere may also have an effect on turfgrass survival. Under well-drained conditions, soil gases approximate the ambient atmosphere, which contains about 0.20 m3 m-3 O2 and 0.004 m3 m-3 carbon dioxide (CO2) respectively (Hillel, 1999). Seasonal and daily fluctuations in the relative amounts of soil O2 and CO2 have been documented (Buyanovsky and Wagner, 1983; Jordan, 1998). As soil temperature increases and organic residues (e.g., senescing roots) decompose, CO2 concentrations generally increase because of microbial respiration (Wagner and Wolf, 1997). Normally, the evolution of CO2 is not a concern in coarse textured soils because adequate drainage and rapid O2 diffusion maintain a favorable gas composition. However, in putting greens, surface sealing of the rootzone may occur because of accumulation of excessive thatch or algal scum, which can restrict gas exchange. This may be particularly damaging at higher temperatures as microbial and root respiration rapidly depletes available O2 (Beard and Daniel, 1966; Ralston and Daniel, 1972; Kurtz and Kneebone, 1980; Huang et al., 1998).
During summer months, many golf course managers irrigate creeping bentgrass putting greens almost daily. This practice has been shown to affect soil gas composition, with irrigation events resulting in a diurnal periodicity of soil CO2, with CO2 concentrations ranging from 0.008 to 0.028 m3 m-3 on the day following irrigation (Jordan, 1998). The response was attributed to a restriction of gas exchange by the presence of water. While turfgrass response to elevated rootzone CO2 concentrations has not been established, the negative effects of elevated CO2 levels on crop plant roots and shoots have long been observed (Harris and Bavel, 1957; Stolwijk et al., 1957; Radin and Loomis, 1969; Geisler, 1963, 1967). Concentrations of CO2 as low as 0.01 m3 m-3 may be injurious to plant roots (Curl and Truelove, 1986). In barley (Hordeum vulgare L.) and corn (Zea mays L.), CO2 concentrations of 0.04 to 0.06 m3 m-3 decreased both shoot and root growth (Geisler, 1963; 1967). While it is possible that high concentrations of CO2 may be damaging to turfgrasses, the concentrations of CO2 and O2 in the soil atmosphere are inversely related. Therefore, it is equally possible that turfgrass damage may be due to insufficient O2 levels rather than excess CO2 effects.
Several technologies and rootzone amendments are marketed to improve soil aeration, optimize the soil moisture, and maximize the physical performance of sand-based rootzones. However, few published data are available on their performance. The objectives of this field study conducted on newly constructed sand-based rootzones were to (i) evaluate the effectiveness of a mechanical forced air blower–vacuum technology for its ability to manipulate sand-based rootzone water and gas contents, (ii) determine if this technology improves creeping bentgrass quality and rooting when used regularly throughout the summer months, and (iii) assess the effects of several commercially available inorganic soil amendments on turfgrass establishment, growth, rootzone physical properties, and potential as alternatives to peat moss.
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MATERIALS AND METHODS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Greens were constructed by excavating existing native soil (Cecil sandy-loam, fine, kaolinitic, thermic Typic Kanhapludult), and embedding 100-mm diam. perforated smooth walled interior drainage pipe (ADS N-12, Advanced Drainage Systems Inc., Columbus, OH), surrounded by 5 cm of gravel (particle distribution: >12.5 mm = 0, 9.5–12.5 mm = 14, 6.3–9.5 mm = 558, 4.8–6.3 mm = 315, 2.0–4.8 mm = 112, 1.0–2.0 mm = 1, <1.0 = 0; g kg-1, respectively), into the base of the soil cavity. The remaining drainage pipe was standard 100-mm diam. sewer and drain pipe. Sand-based rootzone mixtures and gravel components were installed according to USGA guidelines (USGA, 1993). Adjacent greens were separated by a naturally compacted zone of native soil at least 3 m wide to minimize green to green treatment effects. Within each green, five independently drained subplots (3.0 by 3.7 m) were constructed. The drainage pipe was located at the center axis of each subplot and drained perpendicular to the main axis of the larger green (Fig. 1). To prevent cross contamination of the treatment media, each subplot was physically separated from each adjacent subplot with a wooden board covered with a 15.2-µm (6-mil.) thick plastic liner.
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In May 1998, drainage treatments, mechanically induced or gravity (control) representing conventional golf greens, were assigned as main plot factors in a randomized complete block design. A randomized complete block design was employed to minimize the possibility of green to green vacuum treatment effects. On the day preceding mechanical drainage evaluation, the experimental area was irrigated with 12 mm of water via an overhead sprinkler system. Mechanical drainage and air-injection were applied with a (12 kW) centrifugal forced air blower–vacuum (Sub Air, Inc., Deep River, CT). Test greens were subjected to mechanical drainage–air-injection on a weekly schedule as suggested by the manufacturer. During mechanically induced drainage treatments, the blower device was operated at full capacity in the vacuum mode, conferring a -1.1 kPa soil water pressure on the rootzones as measured at the inlet pipe of each subplot, and operated for a 20 min period as selected by a preliminary study conducted on these particular rootzones. Air-injection (8.5 m3 min-1) treatments were conducted for 5 min immediately following water evacuation, equivalent to 2.5 total pore volume exchanges.
Three representative greens were characterized for within-plot vacuum and air-injection uniformity. A grid system for each subplot (Fig. 1) was established consisting of four equidistant transects perpendicular to the subsurface drainage pipe in each subplot, located 0.6, 1.2, 1.8, and 2.4 m from the edge of the green (denoted by point A, Fig. 1). Vacuum or air pressure was measured along each transect directly above the subsurface drainage pipe and at points 0.6, 1.2, and 1.8 m to either side of the drain line (denoted by points B, C, and D, Fig. 1). Individual cores (12-mm diam by 300 mm deep) were removed from each point along each of the transects and appropriately sized rubber stoppers were placed in each of the remaining holes to keep the rootzone sealed. Vacuum or air-injection was applied, and the corresponding pressures across the grid were recorded at each individual point using a Magnehelic pressure gauge (Dwyer Instruments Inc., Michigan City, IN) attached to a stainless steel probe designed to fit snugly into each sampling hole. Pressure and vacuum uniformity within each subplot was calculated by an irrigation uniformity equation (Emmons and Boufford, 1995). Lastly, the effect of sampling depth within the rootzone on vacuum pressure was also determined. Cores were removed incrementally to depths of 0 to 100, 0 to 200, and 0 to 300 mm while the system was operated in vacuum mode.
Rootzone Measurements
The mean volumetric water content at the 0- to 150-, 0- to 210-, and 0- to 270-mm depths were measured in each subplot using time domain reflectometry (Soil Moisture Equipment Corp., Santa Barbara, CA). Measurements were made at two locations near the center of each subplot, approximately 0.5 m on either side of the subsurface drain line. Volumetric soil water content at the 150- to 210- and 210- to 270-mm depths was calculated by difference according to the following formulae:
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Rootzone O2 and CO2 concentrations were measured across the 0- to 200-mm depth approximately every 2 wk throughout each summer with a portable infrared soil gas analyzer (Geotechnical Intruments, Leamington Spa., UK). The measurements were recorded before and after the mechanical drainage treatments were applied. Soil temperatures were continuously recorded at 100 and 200 mm below the rootzone surface in all sand-peat plots with temperature data logging probes (Onset Computer Corp., Pocasset, MA).
Plant Measurements
Visual percentage cover was rated on each subplot monthly from 30 d after seeding until full cover was achieved on all plots (June 1998) by a visual scale of 0 to 100%. Additionally, visual turfgrass quality in each subplot was rated every month by a scale of 0 to 9: 0 = brown, thin, nonuniform turf; 5 = minimum acceptable quality; and 9 = optimum color, density and uniformity.
To assess the effects of drainage and amendment treatments on bentgrass rooting, rootmass was measured in May and September in both 1998 and 1999 by means of a single continuous intact core (50-mm diam. by 300 mm deep) removed from the center of each subplot. The remaining hole was completely back-filled and compacted with the appropriate rootzone media. The surface 0 to 25 mm including thatch was removed and cores were sectioned into three segments: 25 to 100, 100 to 200, and 200 to 300 mm, and dried at 75°C for 24 h. Roots were separated from the rooting media by screening the dried rootzone mixture–rootmass over a 2-mm-mesh screen and weighed. Preliminary research confirmed no significant difference in root mass estimation using this method and ashing of the roots at 600°C for 24 h. Final root weights were converted to a mass volume basis, hereafter referred to as root mass density (RMD), and expressed as kilograms per cubic meter.
Statistical Design–Analysis
All data were subjected to analysis of variance (ANOVA) by the Statistical Analysis System (SAS Institute, Inc., 1996). The experimental design was a split-plot with main plot treatments (drainage) arranged in a randomized complete block design containing four replications and subplots (rootzone amendment) arranged in a random design. Data were analyzed for significant treatment x year interactions and treatment means separated by Fisher's least significant difference test (LSD) and preplanned orthogonal comparisons (Steel et al., 1997). Prior to analysis, visual percentage turfgrass cover data were transformed by the arc-sine transformation (Steel et al., 1997). Because of significant treatment x year effects, most data are presented for each year individually, soil moisture and turfgrass quality measurements were pooled across all dates for each individual year.
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RESULTS AND DISCUSSION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Examination of vacuum pressure with depth indicated little or no vacuum at the 0- to 100- or 0- to 200-mm depths. It was not until the entire soil core was removed down to the gravel blanket that vacuum was detected. These data indicate that the zone of increased wetness at the sand–gravel interface may function as a barrier or "gasket" that limits the potential for the vacuum effect to function near the rootzone surface. Also, it appears that in order for water near the rootzone surface to be extracted it would need to be interconnected with free water located at the bottom of the rootzone mixture receiving suction. Although untested, the vacuum uniformity data also imply the possibility for even greater reduced vacuum efficiency when the force is spread over a much larger area such as an average sized, 550 m2, putting green. In this situation, frictional losses due to increased drainage pipe length and distance from the vacuum unit would likely be further compounded. For example, if one assumes an average sized putting green rootzone that has been fully flushed and charged with water so that a zone of accumulated free water is only located at the sand rootzone–gravel blanket interface, then, hypothetically it is reasonable to assume that the zone of free water accumulation would be approximately one third the total depth of the sand rootzone mixture or 100 mm. To completely remove this zone of near saturation, a vacuum pressure of at least -1 kPa would be required. However, the overall depth of free water would also be influenced and confounded by additional principles of soil hydrology, most importantly slope effects. Most modern putting greens are not uniformly flat like the experimental rootzones tested and actually contain peaks and valleys, sometimes varying by differences of 1 m or more from the highest area to the lowest. These variations would subsequently influence the depth of the zone of wetness within the rootzone, a deeper zone in the valleys and less at the peaks. In this study we measured vacuum pressures within the experimental rootzones ranging from -0.2 to -0.6 kPa which would be insufficient to fully remove even one half of a 100-mm hypothetical zone of wetness. Furthermore, our data demonstrate that although the variations in pressures were relatively uniform, the pressures within the rootzone matrix may not be consistent or sufficient to remove water adequately from an average sized and shaped sand–gravel based rootzone.
Turfgrass Establishment
Creeping bentgrass establishment on all rootzone mixtures was relatively slow, requiring >250 d to reach full coverage on all rootzones, which may have been due to the droughty sand and the selected, relatively moderate, fertility program. Bentgrass established faster on the amended sands (Table 2). Rootzone mixtures ranked in order of decreasing effectiveness were: sphagnum peat > Ecolite = Profile > Greenschoice = unamended sand, with Greenschoice being similar to unamended sand on two rating dates. The faster establishment on the amended sands probably is related directly to greater water retention and increased cation exchange compared with unamended sand.
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Water Contents
One day prior to imposing drainage treatments, the entire experimental area was irrigated to restore field capacity. This resulted in all plots being at or near gravity drainage prior to mechanically induced drainage treatments. The water evacuation (WE) and WE plus air-injection (AI) treatments significantly reduced water contents compared with gravity drained plots both years and at all depths except for the 150- to 210-mm depth in 1998 (Table 3). The reductions in soil moisture ranged from 0.01 to 0.05 m3 m-3 relative to the gravity drained plots, depending on the sampling depth. With reductions in soil moisture, corresponding increases in air-filled porosity occurred. It is unclear whether these relatively small changes in water content and air-filled porosity would impact turfgrass growth and quality. The biggest change occurred at the 210- to 270-mm depth, with water content decreasing 0.03 to 0.05 m3 m-3. However, even the gravity drained plots retained 0.20 m3 m-3 water or less at this depth. Critical levels for optimum soil aeration porosities have been suggested, ranging from 0.10 to 0.20 m3 m-3 (Baver, 1956; Flocker et al., 1959; Wesseling and van Wijk, 1957). Current guidelines for putting green rootzones specify an aeration porosity of 0.15 to 0.30 m3 m-3 (USGA, 1993). On the basis of total porosity, complete saturation for the sand mixtures is approximately 0.40 to 0.45 m3 m-3. This information suggests that with this medium-coarse sand, even the plots not subjected to mechanical drainage had adequate aeration porosity near the bottom of the rootzone.
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Rootzone amendments had significant effects on water retention in both years, particularly in the 0- to 150-mm depth. Water retention generally increased in the order: Ecolite = unamended sand 
Greenschoice < Profile = peat moss. Though differences were small, averaging 0.02 to 0.04 m3 m-3, higher water retention was associated with higher turfgrass quality in both years of the study (Table 3).
Turfgrass Quality
Visual turfgrass quality (TQ) in 1998 was generally low with mean values <6 (Table 3). Turfgrass quality ratings followed a seasonal pattern, with higher TQ values in the spring and lower in summer (data not shown). The main reason for the low overall TQ in 1998 was the incomplete turfgrass coverage during its slow establishment. Turfgrass quality was significantly higher the second year (1999), but was still rather low compared with accepted industry standards for creeping bentgrass putting greens. Drainage treatment had no significant effect on TQ in either year. Most rootzone amendments significantly improved TQ compared with unamended sand in both years. There was no significant difference in TQ of sands amended with sphagnum peat moss compared with the inorganic amendments. As with establishment, the improved TQ of amended sand rootzones may be due to the slightly higher water and nutrient-holding capacities.
Rootzone Atmosphere and Temperature
In general, there was little or no effect of WE or WE plus AI on O2 or CO2 concentrations in either year (data not shown). Only on one single date in 1999 (29 June), mechanical drainage significantly decreased CO2 from 0.003 m3 m-3 to near ambient levels. While these effects were highly significant, the CO2 concentrations in the gravity drained plots were still very low, <0.003 m3 m-3, and all levels were well below the 0.04 to 0.06 m3 m-3 levels considered to be damaging to grasses (Geisler, 1963, 1967). Over the 2 yr of the study, gas exchange was apparently adequate to maintain soil gases near atmospheric conditions. This is probably due to the excellent diffusive properties of the newly constructed, highly permeable rootzones. Another mechanism that may have contributed to maintaining soil aeration is mass flow during natural drainage following irrigation. Since the plots were irrigated almost daily during hot weather, the rootzones were subject to frequent partial gas exchanges by liquid displacement.
Rootzone temperatures were not affected by WE or WE plus AI on any date at either sampling depth (data not shown). The absence of an effect may be due to the short, 5 min., time period for air-injection treatment. Little opportunity existed for heat exchange by the mechanically forced air. Naturally, changes in soil temperature occur mainly by radiation, convection, or conduction. Heat exchange by mechanically forced air is by convection, which is relatively inefficient for temperature modification of mineral materials or water because of the low thermal capacity of air (Hillel, 1999). Our observations regarding soil temperatures and gas contents in newly constructed sand rootzones are consistent with data from a similar experiment conducted on a similar aged (<2 yr old), sand-based green located in South Carolina (Dodd et al., 1999). In that case, only small reductions (<2°C) in rootzone temperatures were observed but these occurred after applying AI for hours rather than minutes. Further, they also measured low CO2 concentrations, <0.005 m3 m-3, and attributed this to the relatively young age of the putting green. In the present study, although no influence of AI on temperature was observed, there were important changes in rootzone temperatures during the summer months (see below).
Root Mass Density
Root mass density showed a consistent seasonal decline in both years of the study. Root mass density declined significantly from May to September in both years, decreasing 31% in 1998 and 14% in 1999 (Table 4). There was, however, a significant increase in RMD from 1998 to 1999, possibly because of an increase in plant density over time.
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A number of factors can adversely affect root growth, including low O2 levels, compaction, fertility status, and soil temperature. In compacted and/or wet soils like some putting greens, the main limitation to proper root growth is poor O2 diffusion (Bertrand and Kohnke, 1957). Recent studies have emphasized the importance of adequate soil O2 for maintaining viable creeping bentgrass roots during high temperature stress (Huang et al., 1998). In this study, on the basis of water and soil gas content measurements, the rootzones were well-aerated and constructed to resist compaction. Fertility, especially N was maintained slightly below optimum, but this often increases root growth.
Thus, the primary factor that seems responsible for bentgrass root system losses from spring to late-summer was supraoptimal soil temperatures. During the study, a seasonal increase in rootzone temperatures was apparent at both the 100- (Fig. 2) and 200-mm depths (data not shown). Rootzone temperatures fluctuated diurnally from 17 to 26°C in late-May and 25 to 30°C in middle and late-summer at the 100-mm depth, where most of the bentgrass roots occupied (Table 4). These middle and late-summer temperatures are above the generally accepted optimum for cool-season turfgrass root development (Beard, 1973). The data demonstrate that rootzone temperatures fluctuate diurnally and a period of night-time cooling occurs. Thus, during summer months the rootzones were subjected to extended periods of supraoptimal heat, and the diurnal cooling period was not sufficient for recovery of the cool-season turfgrass root system. Our observations are consistent with those from previous studies that indicated damage to bentgrass root systems when exposed to elevated soil temperatures (Beard and Daniel, 1966; Schmidt and Blaser, 1967; Ralston and Daniel, 1972).
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CONCLUSIONS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Although mechanically induced drainage and air-injection were without pronounced effect in these newly constructed sand rootzones, the technology may have merit and should receive attention for use under other situations. Further research should address its effects where putting greens are severely compacted, possess low percolation rates, or are maintained in unfavorable environments (e.g., shade, excessive wetness). In these situations mechanically induced subsurface drainage or supplemental aeration may have the capacity to improve the edaphic growing environment and enhance the summer survival of creeping bentgrass.
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ACKNOWLEDGMENTS |
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NOTES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Received for publication March 10, 2000.
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REFERENCES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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