Healthy root systems are essential to maintain turfgrass quality in the demanding environment of golf greens. Increased rooting of creeping bentgrass on golf greens can increase drought tolerance (DaCosta and Huang, 2006), nitrogen uptake (Bowman et al., 1998), and the general health of the turfgrass. When rooting is limited, like in compacted soils, more shallowly rooted species such as annual bluegrass (Poa annua L.) can become invasive (Klecka, 1937). Deeper-rooted creeping bentgrass would also reduce the need for supplemental irrigation because the plant would be able to extract water from deeper in the root zone. The ability to regulate root distribution in turfgrass systems would therefore be an important tool for turfgrass managers.
Phosphorus (P) availability regulates a variety of plant characteristics, including shoot development, leaf expansion, water use, reproductive phenology, and root development. Plants grown in low-P soils typically have increased root-to-shoot ratios (Lynch and Beebe, 1995), and a decrease in bentgrass root weight was observed when a complete fertilizer was applied as opposed to a fertilizer without P (Holt and Davis, 1948). Low P availability increases the intensity of leaf color in creeping bentgrass, which improves visual quality (Waddington et al., 1978). Pellet and Roberts (1963) noted that high rates of P led to the deterioration of Kentucky bluegrass during drought, whereas adequate P levels were needed for rapid recovery after drought. Evidence from these studies suggests that P availability can be optimized to improve several aspects of turfgrass stress tolerance and quality.
Conventional P fertilizers deliver high doses of P (mmol levels) for a limited time as a result of pellet dissolution. The ideal situation in turf would be to hold the P availability at a constant low level (μmol levels) as would be found in natural soils. A novel fertilizer, originally developed for growing horticultural crops in soilless media (Lin et al., 1996; Lynch et al., 1990), could provide a method for studying P placement in the root zone and eventually controlling P levels to manipulate plant growth. This fertilizer is a solid phase-buffered P fertilizer consisting of phosphate adsorbed to small particles of aluminum oxide (Al-P) and can be mixed into sand culture to maintain P levels at low concentrations (μmol levels) over an extended period of time (Coltman et al., 1982; Elliott, 1989; Lynch et al., 1990). The solid alumina releases micromolar concentrations of P, acting as a buffer, using equilibrium exchange between solution phosphate and solid phase-adsorbed phosphate. In this way, it mimics the P buffering that occurs in soil through complex chemical and biological mechanisms (Comerford, 1998) and the alumina can be used to provide optimal P nutrition over time. This is in contrast to slow-release fertilizers, which can maintain adequate P concentrations in the root zone over extended periods of time but release P as a function of pellet dissolution rather than plant requirements and do not prevent P leaching in sandy soils (Havis and Baker, 1985).
Research with soilless media indicates that addition of Al-P at 1% of the volume of the dry medium is sufficient for optimal plant performance of floriculture crops and for vegetable production in the field (Brown et al., 1999; Lin et al., 1996; Tanaka et al., 2006). Preliminary work with turf species demonstrated that creeping bentgrass grew satisfactorily in 80% sand:20% peat with 1% Al-P as the sole source of P (Lyons et al., 2000). Because soluble P concentrations remain very low with buffered P sources, negligible amounts of P are lost even if large quantities of water flow through the medium. In the systems that have been tested, leaching has been reduced to less than 1% of that in conventionally fertilized systems (Borch et al., 1998; Lin et al., 1996).
The placement of P deeper in the root zone may permit deeper rooting, thereby improving heat tolerance, recuperative ability, drought resistance, and overall turfgrass health. The objective of these experiments was to determine whether creeping bentgrass roots responded to spatial P supply and to determine whether deeper rooting could be encouraged by placing Al-P deeper in a sand-based root zone.
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