The Benefits of Testing Materials for Sportsturf Construction
By Ann Murray
Year round play has led to increasing demands on golf courses and sports grounds. In the 1960's the United States Golf Association began research that is still ongoing today, into the best construction for golf greens to facilitate maximum play and satisfactory grass growth. The principles behind this research apply also to the construction of other sports grounds such as football, rugby etc.
Sports pitches and golf greens are expected to resist compaction, be free draining but retain sufficient water and nutrients to support grass growth.
In order to achieve these properties, the materials used to construct sports pitches must be selected carefully. Testing is the means of determining that the construction materials will perform as required.
Construction materials usually comprise of gravel for the drain lines and drainage carpet layer and a 'rootzone' layer placed above the gravel in which the grass is grown. The rootzone layer is predominantly made up of specially selected sand with an organic amendment added.
Physical Property Tests
There is a range of tests that can be carried out to assess physical properties.
A) Sand & Gravel - Particle Size Distribution
The most basic test for gravel and sand is the particle size distribution test (PSD). This test will determine the range of different sizes of grains present in sand and rock fragments present in gravel. The PSD of the sand for rootzone or the gravel for drains is critical to the performance of these construction materials in sports pitches.
In both gravel and sand, uniformity of particle size is very important in order that they are able to resist compaction due to foot traffic and remain free draining.
Sand that is predominantly comprised of 'medium sand sized grains' will produce the correct balance of large pore spaces through which water can drain while having enough capillary (small) pore spaces to retain water.
However, sand that is too single sized can lead to problems with stability at the surface particularly if the sand grain shape is rounded. With this in mind, the ideal PSD for rootzone sand is usually one that has around 20-30% of the particles in the coarse sand fraction, 40-65% of the particles in the medium sand fraction and 12-16% in the fine sand fraction with preferably low levels of very coarse particles >1mm or very fine particles <0.15mm. Spreading the particle distribution over 3 particle ranges helps the sand particles to pack to form a more stable surface while having the majority of the particles in the medium/coarse sand fractions ensures that free draining and water retention properties are maintained.
B) Sand - pH
Sand with a moderate pH level is preferred. Extreme pH levels, either very alkaline or very acidic, are best avoided as problems may be encountered with nutrient uptake and therefore grass growth as a result. Thatch problems on golf greens can also be experienced on golf greens when pH levels are very acidic due to very low microbial activity.
The pH of the rootzone mix will differ from the pH of the rootzone sand as the pH will be influenced by the addition of the organic or inorganic amendment to the sand to form the rootzone.
The ideal pH level for golf green grass growth is 5.5 - 6.5 so this range should be targeted for golf green rootzone. A wider range of pH levels is acceptable for other sports pitches, for example football pitches that are growing a more pH tolerant species of grass.
C) Sand & Gravel - Particle Shape
Moderate particle shapes are preferred for gravels, rootzone and topdressing sands. Sub-rounded to sub-angular particles with a medium degree of sphericity are ideal.
However, a more angular shape is better for bunker sand to facilitate stacking on the bunker face and to reduce the chance of buried balls.
D) Organic and Inorganic Amendments
The traditional organic amendments mixed with sand to form a rootzone are peat or soil.
In recent years new organic amendments have been introduced to rootzones, for example green waste products & processed sewage sludge. The use of these 'processed waste' products is now widely encouraged as it is seen to be environmentally more acceptable.
In the March 2004 revision of the USGA recommendations, inorganic amendments such as zeolites, diatomaceous earth and calcined clays are now considered acceptable for rootzone use.
Peat can be tested for organic matter content, pH and fibre content to determine if it is a suitable organic amendment.
The suitability of soil for rootzone use can be determined from a soil texture analysis.
E) Premixed Rootzone - Physical Properties - USGA Test
The use of suitable sand and a suitable amendment mixed together in the correct ratio will produce rootzone that is free draining while having the correct balance of air to water filled pore space to ensure that there is sufficient water retained for grass growth.
The USGA test includes the PSD, particle shape, pH, organic matter content, percolation rate, total porosity, air-filled porosity, water-filled porosity and water retention level at the relevant tension that corresponds to the depth of rootzone to be used in the construction.
F) USGA Design Test - 3 Recipe Set Test
Starting with the sand and amendment, the laboratory is able to determine the optimum mix ratio of sand to amendment in order to achieve the best balance of the physical properties in the rootzone mix.
G) Gravel Compatibility with Rootzone
Information derived from the particle size distribution analyses of rootzone and gravel materials is used to determine if the gravel is of a suitable size to 'bridge' with the rootzone and to ensure that 'permeability' will be maintained from the rootzone to the gravel layer below.
This information is vital in order to produce a profile where the gravel and rootzone layers sit cleanly on top of each other with no movement of rootzone into the gravel, whilst ensuring that adequate movement of water across the rootzone/gravel interface is maintained.
Guide to Test Sequence for New Constructions and Re-Constructions
- Source sand, rootzone, gravel and amendment materials that are available close to the project.
- Have these tested for suitability by the laboratory.
- Once suitable gravel is found it is advisable to check random deliveries to the site for quality control purposes.
- If suitable premixed rootzone is found then random deliveries to site should also be tested for quality control purposes.
- If the rootzone is to be mixed by the contractor when suitable sand and amendment materials have been located, have the laboratory determine the optimum mix ratio. The first bulk mix prepared should also be tested to determine that results correspond to the approved laboratory mix and then random subsequent mixes should be tested for quality control purposes.
Light and frequent topdressing of sports pitches helps to produce level, firm surfaces and to dilute organic material as it forms at the surface. Materials used for topdressing should, where possible, have a very similar particle size distribution, or preferably be identical to the material used in the construction in order to avoid layers of differing particle sizes forming at the surface.
Using light dressings of the selected material frequently rather than heavy topdressing applications infrequently will also help in the avoidance of layer formation.
Bunker sand should be tested for particle size distribution, pH, particle shape, percolation rate, crusting potential and penetrometer value. Particle size that is similar to that used in green construction is recommended as much bunker sand can be chipped onto adjacent greens.
Extreme pH levels should be avoided as these can be detrimental to grass. Angular particle shapes are preferred for bunker sand to reduce the chance of buried balls and to help the sand stack against the bunker face.
Bunker sand should drain at a rate in excess of 500 mm/hour.
The crusting potential in bunker sand is a check on the likelihood that the sand will form a crust at the surface when it dries out after wetting. Crusting is associated with high levels of silt and clay in the sand.
Penetrometer value indicates if the sand has a tendency to bury balls or if the bunkers are likely to be excessively firm.
Selection of Test House
An accredited laboratory that specializes in testing for the sportsturf industry should be used if possible. USGA approves only the use of laboratories accredited to ISO 17025 through A2LA for testing to the USGA recommendations.
Testing of the construction materials to ensure that the best available are used should be seen as a sound investment. When built and maintained properly golf greens and sports pitches can provide excellent playing surfaces over a period of many years.