During a recent trip to a school soccer field, I made a recommendation to start topdressing with a sand:organic mix. During field construction, the topsoil had been removed from the field and the clay-loam that was left was so compacted that grass cover was very poor and the surface was extremely hard. The sand component is added to increase soil oxygen levels for better drainage and grass growth. The organic component (in this case Comtil) improves soil bulk density, holds some moisture, and increases nutrient-holding capacity. The ratio of sand to organic is typically 70-90% sand, with the rest organic material. The sand component is ideally medium-coarse in size and uniform.
In many situations, the sand component of the mix does not meet this specification and it is not unusual to see sand mixes that contain large amounts of silt and clay or gravel. Silt and clay particles are very fine and they clog a soil system. Air spaces are blocked and the soil becomes prone to compaction. When dry, silt and clay soils are rock-hard, but they turn to a quagmire when wet. For these reasons, very fine sand, silt and clay are generally restricted in mixes to less than 15% of the total mix.
There are no set guidelines for the amount of gravel allowed on a sports field but there is a landscape recommendation. ASTM D 5268-92 “Standard Specification for Topsoil Used for Landscaping Purposes” suggests that no more than 5% deleterious material (rock, gravel etc.) be included in a topsoil mix. Gravel is not a suitable material to improve soil physical or chemical properties and on a playing surface it can disrupt play and possibly cause player injury. In addition, gravel on the surface could damage mower blades and be very difficult to grow grass in. For a whole multitude of reasons then, gravel should not exceed 3-10% of the total mix.
Unfortunately, this has not been the case in many situations. Site visits to sports fields over the years have shown that many topdressing mixes around the state contain far too much gravel. In one notable instance, a college soccer field was constructed with a material that contained 44% deleterious material. That particular field had also been graded and then leveled with a vibratory roller, making it as hard and impenetrable as a parking lot.
One way to ensure that topdressing material contains the right amount of sand, silt, clay and gravel is to carry out some DIY quality control. There are several easy ways to do this:
1. Firstly, avoid the temptation to accept any sand, just because it’s free or cheap. Dressing fields with high amounts of gravel or silt and clay will probably make the fields perform a lot worse than before
2. Get familiar with what the sizes look like. Being able to distinguish between gravel and coarse sand can be helpful when taking delivery of an order. Any particle greater in diameter than 2 mm is considered gravel)
3. Send a sample away to a soil testing lab to have a textural analysis done on the sand component. The lab will furnish results that state clearly the percent of fine, medium and coarse sand and gravel.
4. Carry out a hand-texture test. While not precise, it offers an idea of the type of soil in hand and it helps for the turf manager to become accustomed to different soils.
5. Perform a soil “settlement test” and plot on a textural triangle: (a)Take a small soil sample. (b) Quarter fill a water bottle with the soil. (c) Add tap water until the bottle is three-quarters full. (d) Replace the lid and shake until the water and soil are mixed. (e) Leave to settle for 2-3 days (f) After 2-3 days, the soil will have settled out into discreet layers, with the gravel and sand on the bottom, then the silt, and then the clay (being the smallest particle, clay settles last). (g) Measure the total thickness of the soil, then each individual layer, to determine what percentage gravel, sand, silt, and clay is present. (h) The sand, silt and clay components can be plotted against the soil textural triangle to determine soil texture.To plot sand, find the percent sand along the bottom and follow the line diagonally left. To plot clay, find the percent clay along the left edge and follow the line horizontally across from left to right. To plot silt, find the percent silt along the right edge and follow the line diagonally left. Example: Soil containing 30% sand, 30% clay and 40% silt would be a “Clay Loam”.
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6. Finally, find a topdressing supplier that is knowledgeable of the sports turf industry and can source good material from around the state. Once you’ve found that person, memorize their phone number.
Posted by Pam Sherratt, Ed McCoy & John Street, www.buckeyeturf.osu.edu