Adam Van Dyke, P. Johnson, P. Grossl
2008
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Abstract
Humic substances are often used as an amendment in putting greens to improve turf health, but little is known regarding their effects on soil moisture retention. Commercial humic substance products and pure organic acids were applied to three golf course putting greens in Utah in 2006 and the Utah State University research putting green in 2006 and 2007. These treatments were evaluated for effects on soil volumetric water content, phosphorus uptake, and chlorophyll content of creeping bentgrass (Agrostis stolonifera L.). Three irrigation levels — 80%, 70%, and 60% — of reference evapotranspiration (ETo) were imposed on the turf at the research putting green. Humic substances did not increase moisture retention in putting green soils as pure humic acid significantly decreased soil volumetric water content compared to the control. Both humic acid and fulvic acid-treated plots had lower soil moisture content readings than the control at a depth of 10 to 15 cm during the growing season. Uptake of P by creeping bentgrass was significantly decreased with the application of humic acid, and no differences were observed for chlorophyll content of the turf with any humic substance treatment. While they may provide other benefits, humic substances may not provide superintendents with benefits of reducing water or P fertilizer on putting greens. Turf Management and Humic Substances Creeping bentgrass (Agrostis stolonifera L.) is the predominant cool-season grass grown and managed on putting greens in the Intermountain West region of the United States. While adapted to golf course conditions, both the climate and calcareous soils of the region can impose difficult growing conditions for this and other turfgrass species. The large transpiration gradient created by warm temperatures and low humidity during the summer can create stressful conditions for bentgrass growth. Plus, sand root zones have low water holding capacity that requires frequent irrigation. The calcareous sand commonly used in the Intermountain West has a relatively high pH (~ 7.5 to 8.5), making phosphorus and some micronutrients less available to the turf. In addition to these challenges, many golf course superintendents are expected to reduce water use, especially during droughts, and minimize fertilizer use while still maintaining high quality turf. Thus, they are always seeking ways to be more efficient with their management practices while improving turf health. In order to meet these challenging demands, one management practice that is often implemented is the use of natural organic products, such as those containing humic substances. However, many questions exist regarding their effectiveness and what exactly these products can do for putting green turf (9). Humic substances are a component of soil humus, which can be divided into fractions of fulvic acid, humic acid, and humin depending on their solubility as a function of pH (17). These fractions represent an operationally defined heterogeneous mixture of organic materials (12) that are characterized as being yellow or black in color, of high molecular weight, and refractory (1). Humic substances have been studied and used on a variety of agricultural crops for 6 October 2008 Applied Turfgrass Science years, but only in the last twenty years have they been studied on turfgrass systems. Of the humic substances that have been studied, humic acid is the most common, but results with creeping bentgrass have been highly variable (5). Humic substances have caused hormone-like effects on plant growth and metabolism (3). Growth responses may also result from increased nutrient availability (15), including bioavailability of phosphorus (6), increased tissue levels of iron (4), zinc (4), and manganese (11). However, less growth effects from humic substances have been reported on creeping bentgrass when adequately supplied with nutrients (5). Humic substances increased photosynthesis in creeping bentgrass (11,20) and root mass (11) and length (5) in controlled studies. However, similar responses have not been observed in the field (8). The lack of responses on turf when using humic substances in the field may be attributed to the difficulty in isolating the effects of nutrients and other ingredients often included in humic substance products, and the confounding effects of the variability and uncontrolled nature of field conditions. Regardless of the inconsistencies that have been reported, products containing humic substances are common in the turf industry, with claimed benefits including the ability to increase soil moisture and nutrient availability. While positive growth effects of humic substances on creeping bentgrass have been well documented, scientific literature on improved moisture retention in putting greens has not. This study tested organic acids, including a pure humic acid and commercial humic substance products, on established putting greens to test their effects on (i) increased water retention, and (ii) uptake of phosphorus by creeping bentgrass in sand putting greens. Testing Effects of Humic Substances on Putting Greens Two experiments were conducted. One involved three golf courses in Utah, and the other at a research putting green at Utah State University. Organic acids, including a pure humic acid and commercial humic substance, products were applied to established creeping bentgrass putting greens. Evaluations were done during the summer growing season (June, July, and August) of 2006 and 2007 at the research putting green at Utah State University, and in 2006 at the three golf courses in Utah. The research sites for this experiment were the Utah State University Greenville Research Farm in North Logan, Birch Creek Golf Course in Smithfield, The Country Club in Salt Lake City, and Talons Cove Golf Course in Saratoga Springs. At the golf courses, plots were laid out on practice putting greens. The root zones consisted of primarily calcareous sands. None of the putting greens were built to USGA recommendations, with the research putting green being the closest of all the sites. At the research putting green, the sand mix contained higher percentages of fine (14%) and very fine (9%) sand particles. The Talons Cove putting green was built to California style specifications. The Country Club and Birch Creek greens were native soil pushup green with sand topdressing applied. In all locations, the putting green turf was predominantly creeping bentgrass (Agrostis stolonifera L.) with varying percentages of annual bluegrass (Poa annua L.) (Table 1). Cultural practices at all of the locations were considered typical for the Intermountain West region of the United States. Details of the management are outlined in (Table 1). At the three golf courses, the putting greens were used extensively by golfers, but no traffic was applied on the research putting green at Utah State University. 6 October 2008 Applied Turfgrass Science Table 1. Site characteristics and turf management practices at the golf course and research putting green locations. v Ratio of bentgrass to annual bluegrass. w Ratio of sand to organic matter in the root zone mix. x Mowing height was gradually lowered during the summer evaluation period at all locations. y Weekly foliar fertilization at the USU site was not done in July 2007. z Number of irrigations that occurred during the 91 day evaluation period (June 1 to August 31). Experimental design. Both experiments were laid out as a split-split-plot design. The experiment with the golf courses had location as the whole-plot factor, organic treatment the sub-plot factor, and observation date the sub-subplot factor. The experiment at the research putting green had irrigation level as the whole-plot factor, organic treatment the sub-plot factor, and observation date the sub-sub-plot factor. Individual organic treatment plots measured 1.5 × 1.5 ft with three replications. At the research putting green only, each block of organic treatments was centered in a 10.7 × 10.7 m plot irrigation block where different irrigation levels were applied. Irrigation treatments were randomized in a Latin square consisting of 80%, 70% and 60% of reference Birch Creek 2006 The Country Club 2006 Talons Cove 2006 Research putting green 2006 Research putting green 2007 Turf Type Penncross Many old bentgrasses L-93 and Southshore Dominant blend Dominant blend Bent: P. annuav 50:50 60:40 99:1 80:20 80:20 Soil Type Loamy sand Loamy sand Loamy sand Loamy sand Loamy sand Root zone mixw 100:0 80:20 100:0 90:10 90:10 pH 7.65 7.50 7.81 7.52 7.52 ECe (dS/m) 0.4 0.8 0.8 0.3 0.3 Manage -ment Mowing heightx (mm) 3 to 3.5 2 to 3 3 to 3.3 3 to 3.5 3 to 3.5 Fertilizery 6/10/06: 10-2-4, N at 183 kg/ha, &