Synthetically coated sand makes debut

Rootzone mixtures of putting greens constructed according to United States Golf Association (USGA) specification are composed of at least 60 percent medium- and coarse-sized sand fractions. The coarse nature of the rootzone mix promotes low compaction, good aeration and drainage, while at the same time offering little retention of moisture and nutrients.

In an attempt to limit migration of the rootzone mix into the subgrade pea gravel drainage system, a coarse sand layer (intermediate or choker layer) of approximately 2 inches in thickness was recommended in the original USGA specification, but due to the expense and difficulty incorporating this sand layer, many golf greens have recently been constructed without the choker layer. When you eliminate the choker layer, you must use a somewhat coarser rootzone mix.

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In Florida, commercially available sands essentially come as either coated or uncoated. Coatings are composed of a variety of substances separately or in combinations. They include mainly clay coatings (clay skins), Fe/Al compounds and calcite coatings. The nature and percentage of coatings greatly influence the properties of the rootzone mixture of putting greens, such as nutrient and moisture retention. In general, the available coated sands are of an overall finer texture with lower percentages of coarse sand particles. Thus, the coated sands are not as well-suited for greens construction when you eliminate the choker layer and need a coarser sand. Your only option in this case is a coarser uncoated sand. Unfortunately, the uncoated sands don't perform as well as coated sands in a number of important physical and chemical properties of the rootzone mix, such as moisture, P and K retention and turfgrass establishment and growth rate. What is needed is a coarser sand that offers the best of both worlds. Synthetically coated sands may be the solution, but they are only experimental and not yet commercially available.

TABLE 1. PHYSICAL AND CHEMICAL PROPERTIES OF VARIOUS SAND TYPES
Sand type Coatings CEC Water Retention P Al Fe Water Conductivity
% cmol/kg % mg/kg cm/hr
Uncoated 0 0 2.0 8 3 0 86
Lightly Coated 1.2 0.70 4.0 42 57 3 66
Heavily Coated 2.0 1.99 5.9 79 198 178 58
Synthetically Coated 7.2 10.4 7.4 615 166 23 80

PHYSICAL AND CHEMICAL CHARACTERIZATION

In most cases, you can detect the presence of a coating on sands simply through observation, as shown in the photo, left. The coated sands may be ranked from lightly to heavily coated based on the percentage of the sand grain covered with coating and the actual color of the sand. An uncoated sand, which is essentially inert quart sand, generally appears white in color. A lightly coated sand may appear beige to light red and a heavily coated sand usually is dark red or brown in color due to the presence of sesquioxides of iron and aluminum. These sesquioxides of Fe and Al aid in the retention of anions such as phosphorus. Naturally coated sands almost always also contain clay skins, which are composed of kaolinite-type clays. Cation exchange associated with the presence of the clay skins is responsible for the retention of nutrient cations, such as potassium, calcium and magnesium. In recognition of the need for a coarser sand rootzone mix and a coated sand product, experimental batches of synthetically coated sand products have been produced. These synthetically coated sand products are made up of the coarser uncoated sand, resins, clays and other proprietary products. Clays that are being used for coatings are generally smectite type (2:1), which have a higher cation exchange capacity (CEC). Some of the physical properties of the various sand types are shown in Table 1, page G26.

Studies indicate that water retention (percent of water retained in the soil at 1.0 bar) increases as the degree of coating increases. This represents the potential for more water being available to the turfgrass in response to the presence of coatings. The quantity of extractable P, Al and Fe also increases with the presence of coating, which suggests that more P can be retained. However, the large quantity of P associated with the synthetically coated sand is present because the clay used in the coating contains high levels of the rock phosphate (apatite). Even though the rates of water conductivity (water permeability) appear high according to USGA recommendations (sand columns were not packed according to USGA specifications prior to testing), the sands differ in conductivity relative to coating percentages. As indicated above, coated sands tend to contain higher percentages of fine and medium sands than do uncoated sands; therefore, the water conductivity of the coated sands is slower than the uncoated sand.

TURF ESTABLISHMENT

Rapid coverage during grow-in of a putting green is essential for economic and environmental reasons. Sand type and the presence of amendments such as peat have been shown to greatly influence turfgrass establishment rate. For economic reasons, some golf course putting greens are being built using uncoated sands alone without the inclusion of organic matter.

Recent research suggests that this might be an ill-advised decision in the long term. Comparison studies in the field using uncoated sands, coated sands and synthetically coated sands without the inclusion of peat suggest that 100 percent cover can be delayed as much as four weeks when you use uncoated sand alone as a putting greens rootzone (see Figure 1, page G26).

At 35 days after sprigging, the coated sand and synthetically coated sand plots had 25 percent more coverage than the uncoated sand plots. The primary reason of the more rapid coverage rate on the coated sand plots appeared to be related to available moisture. Even though the plots were irrigated every two hours during the day, the uncoated sand plots still appeared to dry out on the surface between irrigations, thus resulting in a slower establishment of the turfgrass. Full coverage was attained on the synthetically coated sand plots at 51 days after sprigging (DAS). At 51 DAS the uncoated and coated sand plots were 65 and 75 percent covered, respectively.

TURF GROWTH

Sand type has also been shown to influence overall growth of the turfgrass after establishment. As shown in Figure 2, page G28, the mean growth rate of the turfgrass was influenced by sand type for the next nine weeks after complete coverage was attained. These differences in growth rate relative to sand type were noted throughout the first year of the study. In the latter part of the second year, differences relative to sand type were no longer observed. Apparently, the maturation of the turfgrass and the production of organic matter over time balanced out the sand effect, but those first two years of poor establishment and growth could have been avoided by selecting the proper sand type in the beginning.

NUTRIENT LEACHING

Reactive surfaces on sands enhance nutrient retention (P and K) and reduce nutrient loss due to leaching. As stated above, uncoated sands have very little, if any, coating and their natural ability to retain anything against leaching losses is exceedingly small. Iron and aluminum sesquioxide coatings on sand particles aid in the retention of P. When the coatings are not present, as in uncoated sands, it is very difficult if not impossible to establish and maintain adequate P levels in the rootzone media. The P will tend to leach out of the rootzone. Phosphorus has become an element of environmental concern in that, at very low concentrations, it can cause eutrophication of surface waters. The drainage system of a golf course may be connected to a surface body of water into which the P, applied to an uncoated sand green, may leach, thus resulting in P contamination of the water body. Not only is P leaching a possibility in uncoated sand mixes, but establishment of the turfgrass during grow-in may be hindered due to the inability to maintain adequate levels of P in the rootzone. More P was leached from the synthetically coated sand in this study because of the high levels of P in the coating material. You can reduce these excessive levels of P leached from the synthetically coated sand by properly choosing the coating material.

Clay skins associated with sand coatings impart cation exchange to the sands and serve to retain cations such as K, Ca and Mg against leaching losses. These elements generally are not considered as elements of environmental impairment; but maintenance of adequate levels in the rootzone enhance growth and nutrient uptake. Uncoated sands possess very little, if any, cation exchange capacity and thus tend to leach cations freely.

Naturally coated sands reduced K leaching by 50 percent during grow-in; whereas the synthetically coated sand, which had a much higher CEC, reduced K leaching losses to approximately 4 percent of that applied (see Figure 4, left). Even though P and K were retained against leaching losses by the coated and synthetically coated sands, P and K were still available to the turfgrass for uptake and the maintenance of optimum levels of these two nutrients in the turfgrass tissue.

PUT YOUR COAT ON

Sand coating is important relative to the maintenance of adequate levels of moisture and nutrients during establishment and subsequent growth of turfgrasses. Using uncoated sands in putting green rootzone mixes can result in delays in complete coverage during grow-in and the contamination of surface waters during grow-in. In addition, in some cases, it may be difficult if not impossible to maintain adequate levels of P and K in the rootzone profile if you use uncoated sands. Recently produced synthetically coated sands, which are not yet commercially available, have shown potential for promoting establishment during grow-in and subsequent growth after establishment and for the retention of P and K. Economics may play a role in the acceptance of this new technology, but studies are continuing to determine if researchers can find a mixture of uncoated and synthetically coated sand that has the same beneficial attributes without being prohibitively expensive.

Dr. Jerry B. Sartain is professor of turfgrass science in the Soil and Water Science Department at the University of Florida (Gainesville, Fla.).

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