by James C. Thomas C.P.Ag.
To many people, “sand is sand.” On a golf course, however, sands can be as different as night and day. While it may be tempting to save money by using bunker sand as a soil amendment or topdressing, the physical properties of a bunker sand may not be suitable in a root zone. Also, a root zone sand may be woefully unsuited for use in a bunker.
Superintendents must evaluate several characteristics of any sand before using it on the golf course. Properly selected sands will help ensure healthy greens. Happy golfers and playable bunker lies.
How big are the grains?
Sand grains in both bunkers and root zones should be predominately in the “medium” and “course” ranges, which means their particle sizes generally range from 0.25 millimeter to 1 millimeter.
You can use USGA’s sand-content recommendations for a finished root-zone mixture as a preliminary tool in selecting acceptable root-zone sands because sand comprises 80 percent or more of most mixes.
Root-zone sand should contain no more than 3 percent gravel (particles 2 to 4 millimeters in diameter) by volume. Sands free of gravel are ideal, while those containing more than 3 percent gravel are likely to damage mower blades and bed knives. Ideally, a root-zone sand should contain 60 percent or more medium- and coarse-sized particles and no more than 10 percent total of gravel and very coarse (1- to 2-millimeter) particles. The sand may also contain up to 20 percent of particles in the fine (0.1- to 0.25-millimeter) range with an additional 5 percent or less in the very fine (0.05- to 0.15-millimeter) range.
Since very-fine sand acts much like silt and clay particles, root-zone sand should contain no more than 10 percent very fine sand, silt and clay combined. Use of excessively fine sand results in root zones that are slow to drain and poorly aerated. Fine sand also requires careful management to prevent black layer.
Bunker sand needs to be cleaner than root-zone sand and should contain no more than 3 percent silt and clay. Bunker sand with more than 3 percent silt and clay may crust over or “setup” frequently, requiring repeated maintained to keep the sand in playable condition.
Because golfers often blast sand from greenside bunkers onto putting surfaces, ideal bunker sands are limited to less than 3 percent gravel and 7 percent very-coarse particles. Bunker sands with greater amounts of gravel and very-coarse particles will damage mowers, reduce putting quality and slow play on the course. An ideal bunker sand should also contain greater than 65 percent of its particles in the medium and coarse ranges (0.25 to 1.0 millimeter) and less than 25 percent in the 0.05- to 0.25-millimeter range.
Determine particle shape by looking at a sand sample under a microscope or hand lens. The predominate particle shape may be either angular, sub-angular, sub-rounded or rounded, depending upon the sharpness of the edges and corners of the particles. The particles are also rated for sphericity, with “ball-shaped” particles rating high in sphericity and very elongated or flat-shaped particles rating low.
Particles with either a sub-angular or sub-rounded shape and medium sphericity are preferred in root zones. Highly angular sands tend to pack tightly and may injure turf roots, while highly rounded sands may be loose and unstable during turf establishment, causing problems with footprinting and tracking. In contrast, the desired bunker sand shape is angular with a low degree of sphericity. The sharp angles and corners of an angular sand help it resist movement under impact from a golf ball, thus resulting in fewer buried balls, which reduces complaints from players.
Generally, sand color is of little importance for a root zone since it’s quickly covered by the turf and isn’t seen except for short periods immediately after topdressing or aerification. Bunker sands, on the other hand, are continuously exposed, and sand color adds an important aesthetic appeal to most golf courses. The contrast of a clean sand against a healthy green turf is important to break up the monotony of large turf areas, and well-designed bunkers add definition and challenge to each golf hole.
Generally, light-colored sands are preferred for bunkers use. The most common colors seem to be white, light brown, yellowish brown and light gray. While some clubs use a pure white sand to provide dramatic contrast in colors, tans or light browns are more natural, easier to maintain and are easier to play from because they reflect and glare less. Pure white sands can be hard to keep clean and often turn tan with age because of contamination from grass clipping, leaves and other foreign materials that are washed or blown into the bunkers.
Occasionally, other considerations may override color, and unusually colored bunker sands may be acceptable. Such was the case at the Jack Nicklaus designed Old Works course in a depleted Montana copper mine, where the bunkers contained black slag instead of sand. The on-site availability and the stability of the slag material combined with its very high resistance to partially burying golf balls in the “fried-egg” lies makes for a good bunker sand, even though the black color is unusual. Given the choice between a sand that plays well with few buried balls and one that looks good but plays poorly, our preference should be for the playability of the sand.
Crusting and setup
Golfers tend to frown on bunkers with crusted setup sand. Crusting is the formation of a thin, hard shell on the sand surface, much like the upper crust of a pie shell. Setup is the formation of a thick crust as deep as water penetrates. Calcareous sand and sands with excess silt and clay particles have the greatest tendency to crust and setup. A severe tendency to crust or setup requires more frequent raking to keep the sand in playable condition. Thus, ideal bunker sands are free of both crusting and setup.
Since 1970, researchers have reported that quartz sand (commonly called silica sand) is preferred for use in both greens construction and bunkers because of its hardness, which makes it resistant to further weathering and helps it retain its original shape. Softer sands, particularly those derived from limestone, create special problems that require special management by the superintendent.
As soft, calcareous sands weather calcium carbonates dissolve and migrate in the green. The carbonates act as weak cementing agents and have been known to precipitate, forming restrictive layers above the drain systems of putting greens. In addition, the calcium carbonates will increase the amounts of crusting and setup in the bunker sands and thus require more frequent raking to keep bunkers in playable condition.
Furthermore, as the carbonates dissolve from the sand, the particle size of the sand may gradually degrease, causing a reduction in both water flow and the amount of air-filled porosity. Calcareous sands tend to have alkaline pH values which, depending on the severity of the pH, may limit the availability of micronutrients. Thus, adjustments in the fertility plan must compensate for this. Because of these various problems, quartz is the preferred mineral for all golf course sands.
Another problem unique to bunker sands is the potential for golf balls to partially bury in a fried-egg lie. Golfers tend to complain about “soft sand” when their scorecards record extra strokes from digging out of such lies.
A Test can reveal the potential for sands to form fried-egg. A standard quantity of air-dried bunker sand is placed in the test vessel and stirred. The force required to press a golf ball halfway into the sand is measured and compared with a table of values allowing the sand to be rated as having a very low, slight, medium or high potential for burying golf balls. Ideal bunker sands give a reading of 2.4 kilograms per square centimeter or above on a pocket penetrometer, indicating a very low tendency to bury golf balls landing in bunkers.
Saturated hydraulic conductivity is a measure of the rate at which water will pass through sand. Good root-zone mixtures have conductivities of 6 to 24 inches an hour as specified by the USGA.
This allows playing surfaces to drain rapidly and have adequate aeration to promote grass rooting. In addition, sand based root-zone mixtures need to meet other criteria, including pore size distribution, as set forth by the USGA.
No standards have been established for the conductivity of a bunker sand, although inadequate drainage is an oft-discussed problem of bunkers. And because bunker sand is frequently blasted onto nearby putting surfaces, it seems reasonable that a bunker sand should have a minimum conductivity of 20 inches an hour, with rates preferred.
Unlike root-zone mixtures, there is no upper limit to an acceptable hydraulic conductivity of a bunker sand.
Over time, bunker sands inevitably are contaminated with silt and clay particles from the base and edges of the bunkers, as well as soil, dust and organic debris washed or blown into the bunkers. These contaminants not only discolor the sand, but also clog pore spaces and bind sand particles together. This reduces water conductivity and increases water retention and the potential for crusting and setup. So it’s best to start with sand having high conductivity to reduce the effects of future contamination.
Care is essential in choosing the proper sands for a golf course. Evaluate sands used in root-zone mixtures or topdressing for their particle-sized distribution, shape and physical measurements (saturated hydraulic conductivity, density and pore size distribution). Evaluate bunker sands for particle-size distribution, shape, color, crusting and setup, hydraulic conductivity and their potential for burying golf balls. If the choice comes down to two or three bunker sands with similar test results, it’s often a good idea to establish some test areas and get your members opinions about which sand they prefer.
While the ideal particle sizes of root-zone and bunker sands are similar, bunker sands should be more angular, have less silt and clay, should not crust or setup and should be resistant to the formation of fried-egg lies. Consider the investment of time and lab testing as “doing your homework” before spending thousands of dollars and countless work hours to purchase and install large volumes of sand on your golf course.
This can pay big dividends in the form of happy golfers, fewer complaints to the superintendent and less maintenance to keep the course in top condition.