The quality and longevity of an athletic field is directly related to the drainage capability of the soil. How fast water drains into and through the soil (infiltration rate, KSat) is the best indicator of how many games can be played and how the field will react during a rain game. Ideally, fields should have a minimum infiltration rate of 1 inch/hr., but it is not uncommon to see soils with infiltration rates of 0.01 inches/hr. or less.
The infiltration rate of a soil is influenced by the soil texture and the level of compaction the soil is subjected to. Fields with low infiltration rates create very poor playing conditions. When wet, they turn to mud, when dry they turn to concrete. Turf plants will not grow in these soil conditions and so the field gets taken over by weeds like prostrate knotweed, clover, dandelions and Poa annua. Turf growth is so poor that nutrients are not taken up, so the turf becomes chlorotic/yellow and does not recover from wear. Slow turf growth is more susceptible to diseases like red thread and rust. It is also impossible to get seed established on hard, compacted soils. Improving the infiltration rate of the soil therefore is the key to improving field conditions.
There are short term fixes to improve infiltration. These include using aeration equipment like a core aerator, spiker, deep-tiner or verti-drain. These machines punch holes in the soil, allowing water to enter and O2 and CO2 to enter and exit. After a period of a couple of weeks or less however, those holes seal over and the previous conditions return.
A long-term fix is to amend the soil with a material that improves the infiltration rate, namely sand. As well as improving infiltration rates, topdressing evens out the playing surface and fills holes that could cause athlete injury. Applying 50 tons of sand to a field per year via topdressing appears to be an effective rate. It is possible to apply higher rates of 60-100 tons, especially if the sand is applied in two increments (spring & fall). The sand is either applied alone or in combination with soil or compost. The soil &/or compost typically makes up 10-30% of the mix. Adding compost to the mix is a good way to get some organic material into the soil if it is lacking. Compost improves the soil health (increased oxygen, reduced bulk density, increased water holding capacity) and also adds nutrients. The soil component in a topdressing mix helps to maintain surface stability (sand alone does not have good shear strength) and also retains moisture, which is important for seed establishment.
The sand component of the mix should ideally be 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 or seed 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 contain far too much gravel. In one notable instance, a college soccer field was constructed with a material that contained 44% gravel. That particular field had also been graded and then leveled with a vibratory roller, making it as hard and impenetrable as a parking lot. The ultimate goal of topdressing with sand is to achieve at least 70% sand by weight in the rootzone. At this point, the sand particles bridge, creating macropores and reducing particle density. Without a doubt, initiating a sand topdressing program significantly improves native soil field quality and longevity.
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:
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
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
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.
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 what different soils feel like.
Perform a soil settlement test, sometimes called a “Jar test” (a good practice for baseball infield mixes too):
Take a small soil sample.
Quarter fill a water bottle or mason jar with the soil.
Add tap water until the bottle is three-quarters full.
Replace the lid and shake until the water and soil are thoroughly mixed.
Leave to settle for 2-3 days
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).
Measure the total thickness of the soil, then each individual layer, to determine what percentage sand, silt, and clay is present. For example, if the total mineral layer is 2 inches thick and the sand layer is 1-inch thick, there is 50% sand.
The sand, silt and clay components can be plotted against the soil textural triangle to determine soil texture
One of the main issues with starting a topdressing program is that a topdresser is needed to apply the material, unless an outside contractor is paid to make the application. Also, the program is not a “one time” occurrence, but must take place each year, ideally in conjunction with aeration, which will take time and money. However, in every instance where a sand topdressing program has been adopted, the results have been so dramatic and the fields have improved so significantly that school boards and administrators usually look for extra sources of funding to try to start topdressing programs on additional fields.
It may take a couple of years to see the benefits of dressing with sand. Obviously, the more sand applied, the faster the desired 70% by weight goal will be reached. It is not a good idea to apply more than 0.25 inch at any one time as the sand can be abrasive to both turf equipment and the grass, but two or three applications could be made each year, outside of the playing season. If there is money and manpower available, a new “fast-track” sand build-up system could be employed. This system was recently developed by Michigan State University and can be accessed through their website.
In addition to improving the rootzone soil, there are several drainage options for fields:
Surface Drainage refers to the ability of water to shed or run off the surface due to the installation of a slope or crown. The severity of the slope or crown depends upon the amount of sand in the rootzone. Fine textured soils with <45% sand should have crowns/slopes of 1.5 – 2 % while rootzones with >70% sand can have smaller crowns/slopes of 0.75 – 1.0 %. For a more detailed guideline on this, refer to ASTM WK37583 – New Guide for Construction or Renovation of Native-soil Athletic Fields.
Internal Pipe Drain: The origin of this type of installation is in agriculture and it is relatively cheap to install. It consists of a grid of piping below the surface of the subsoil. The benefits are (1) the gradual lowering of the water table and (2) shorter drying time. However, this system is not accustomed to dealing with high volumes of water in short periods of time, such as during a game. Also the area affected by the drainage pipes is usually restricted to within a few feet or so of the pipes. The main objectives of pipe drainage is to lower water tables, control or cut off flow of extraneous water, and drain any surface water directed to them.. The disadvantages to this system are that the water must flow to the drains in order for them to be effective.
Slit Drains: Slit-drained fields are designed so that surface water bypasses the native soil, and the local soil has less of an influence on drainage rate. A common specification is to install sand slits 1-inches wide, 10-inches deep, on 3 ft. spacing. Most importantly, the slits must transmit surface water through the native soil surface to a more permeable material underneath – such as a gravel layer or permeable fill over pipe drains. The slits run perpendicular to the pipe drains. Two problems can occur with slit drained fields: (1) when the permeable material does not come into contact with the sand slit (i.e. there is a soil layer between the sand slit and the underlying permeable material), or (2) when the slit is not kept directly at the field surface and the slit is sealed off by adjacent native soil. This can occur rapidly, even during one game if field conditions are very wet. To prevent the latter, a heavy annual sand topdressing program has to be initiated to make sure that the slits are not “capped off” over time. Research by the STRI has indicated that these types of field can accommodate 6 hours adult play per week (95-125 events per season). In addition, they have suggested that, if managed correctly, a slit-drained field should last about 7 years before needing to be slit again.
Suspended Water Table (USGA, PAT system or similar): By far the most expensive of the options to install and maintain, the suspended water table (SWT) construction consists of internal drain pipe, a gravel blanket and a sand rootzone. The biggest benefit is that it resists compaction and has very high infiltration rates. The challenges are that they require greater input (water, fertilizer) and they can lose grass cover quickly from over-use if the sand is not stable, or if regular over-seeding and topdressing is not performed. Organic matter accumulation is also a challenge. Switching from a native soil field to a SWT field is not economically viable in many cases and can only really be justified from a financial point of view if play has to be guaranteed irrespective of the weather (except snow and frost).
Other Native Soil Field Improvements
Moving on from discussing the intricacies of removing water quickly from soil fields, adding water to soil fields is also an important part in native soil field improvement. Irrigation is undoubtedly the most underused management practices on native soil fields, especially during renovations or overseeding/sodding operations. It is highly unlikely that turf will recuperate from wear or that new seedlings will survive if supplemental water is not added to the field during drought. Even in Ohio, with 40-inches of precipitation per year, it is not uncommon to have drought conditions June, July and August.
Adding water to a field does more than grow healthy grass, but can significantly reduce surface hardness on fine textured, dry and compacted soils. Dry and compacted native soil fields can have Gmax (hardness) readings in excess of 400, which is four times harder than recommended for athlete safety. And with the CDC reporting that 135,000 children between the ages of 5 and 18 are treated each year for concussion and other head injuries, it is best to make sure that the playing surface is not one of the contributing factors.
Guidelines for best management practices on native soil fields, like mowing, regular and timely applications of fertilizer, over-seeding etc. can be found through the STMA website or University turfgrass program website. For example, Ohio State has a free factsheet entitled “Standard Guide for Maintaining Sports Fields and Recreational Turf in Ohio.”
Pamela J. Sherratt is the sports turf extension specialist for the turf program at The Ohio State University in Columbus.