Root zone amendments are typically organic materials or inorganic materials of diverse mineralogy that when added to sand at modest rates of 5% to 20% by volume are capable of substantially and permanently altering the physical and chemical properties of the root zone. There are a variety of reasons why specific amendments are selected and the choice of amendment should be associated with a specific reason.
Amendments are included in a sand-based root zone to increase water and nutrient retention. By increasing the water retention of unamended sand the intent is to avoid the early onset of drought between routine irrigation applications; and by increasing the nutrient retention the intent is to avoid the onset of nutrient stress between routine fertilizer applications. Amendments can also mitigate environmental concerns of excess water drainage and nutrient leaching from sand-based root zones. Guidelines have been proposed for the needed magnitude of water and nutrient retention within root zones. Thus, it has been suggested that the CP measured at 30-cm suction, using American Society for Testing and Materials (ASTM) F 1815 protocols should exceed 0.2 m3·m−3 (Murphy et al., 2001), or alternatively that the surface 3-inch water content at 24 h of drainage should exceed 0.1 m3·m−3 (Carlson et al., 1998). In both cases, these minimums were offered as requirements to achieve adequate turfgrass establishment from seed. Also it has been suggested that the CEC of a root zone need not exceed ≈4 cmolc·kg−1 to provide adequate nutrient retention (Carrow et al., 2001).
Root zone response to amendments is generally influenced by texture, uniformity, and the amount of silt and clay of the amended sand (Davis et al., 1970; Waddington et al., 1974). Thus, fine- to medium-textured sands exhibit a lesser response, particularly with regard to water retention, to added amendments than do medium- to coarse-textured sands. This is also generally true when comparing less uniform sand (e.g., having a wide range of particle sizes) with more uniform sand. A lesser response to amendments is also observed with sands containing appreciable silt and clay contents. Further, an amendment rate effect is observed with most sands where increasing amendment amounts lead to greater water and nutrient retention. But there is a subtle difference in the way nutrient and water retention increase at greater rates of incorporation. In the case of nutrient retention expressed as the CEC of the root zone, amendments that possess CEC yield a linear increase in CEC with incrementally increasing rates of incorporation. However, increasing water retention at successively greater incorporation rates shows diminishing returns. That is, incremental amendment additions have progressively smaller effects. Consequently, there is a practical upper limit in the ability of a particular amendment to increase the water retention of sand (McCoy, 1998).
Carlson et al. (1998) state that increasing root zone water content is the most important reason for using an amendment. Yet there has been a recent trend to consider other goals for incorporating amendments in sand-based root zones, with a focus not so much on establishment from seed, but on longer-term, agronomic and playability performance. This recent trend has led to formulating root zones that yield drier surface-layer conditions, motivated by improved playability (where drier soil conditions generally result in firmer and smoother surfaces) and by the potential to reduced disease problems associated with a drier surface. An obvious trade-off occurs because drier root zones need more frequent irrigation, contributing to greater water use, and more precise irrigation application. However, this is not considered to be much of a concern because the total putting green area of a golf course is rather small and greater water use on greens should not substantially increase overall irrigation costs. Further, the need for more precise irrigation has been met by newer irrigation technologies and the use of root zone moisture sensors for irrigation scheduling.
Sand texture, uniformity, and the amount of silt and clay have a profound effect on the hydraulic properties of sand-based root zones (Bigelow et al., 2004; Davis et al., 1970; Waddington et al., 1974; Zhang and Baker, 1999). More subtle but also important are the effects of amendments that may be added to a sand to create a root zone. Thus, the purpose of this article is to summarize recent reports on the water and nutrient retention properties of sand-based root zones containing the most commonly used and commercially available amendments. The focus of much of the studies is on new construction of a USGA putting green root zones. Thus, the sand into which amendments are incorporated generally conformed to USGA particle size guidelines and hereafter will be referred to as USGA-guideline sand. However, it is important to note that USGA-guideline sands are not all identical but rather span a reasonably narrow range of particle size distributions.
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