Editor’s note: This article was originally published in the September 2014 issue of SportsField Management.
The quality and longevity of an athletic field is directly related to physical soil characteristics in relation to two divergent factors: (1) the drainage capability of the soil and (2) water retention for use by plant roots. In an ideal scenario, an athletic field should be capable of coping with the greatest local rainfall intensity that is likely to occur during the sporting event.
For example, it is not uncommon in the Midwest to have rainfall intensity as high as 1 inch of water over a couple of hours. Ideally then, fields in the Midwest should have a minimum infiltration rate of 1 inch per hour, but it’s not uncommon to see soils with infiltration rates of 0.01 inch per hour or less. Not surprisingly, fields with low infiltration rates may take several hours (or days) to drain, resulting in standing water and possibly canceled games.
Fine-textured soils that contain appreciable amounts of silt and clay generally have low infiltration rates because they contain a lot of small air spaces called micropores (\< 0.08 millimeters). Micropores hold onto water after gravity has drained all the free water, and they can impede drainage and plant root growth. For maximum infiltration of water, soil drainage, aeration within the soil profile, and unimpeded root extension, there needs to be a sufficient quantity of large-diameter air spaces, called macro- pores (\> 0.08 millimeters).
The extent to which a field can function then depends upon the climate, the amount of use and also the soil type. Sandy soils (\>70 percent sand) contain enough macropores for adequate drainage and aeration, but finer-textured soils are much more prone to soil saturation and compaction. Turf plants will not grow in saturated and compacted soil conditions, 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 makes it 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 poor soils is therefore the key to improving field conditions.
There are short-term fixes to improve water infiltration and percolation rates on fields. These include using aeration equipment like a core aerator, spiker, deep-tiner or verti-drain. These machines punch holes in the soil or create channels in the soil, allowing water to enter and oxygen and carbon dioxide to enter, and exit. However, after a couple of weeks or less, those holes may seal over and the previous conditions will return.
One long-term fix is to amend the soil with a material that increases macroporosity, namely sand. Along with improving infiltration rates, topdressing with sand also evens out the playing surface and fills holes that could cause athlete injury. Applying 50 tons of sand per year to a field via topdressing appears to be an effective rate. It’s possible to apply higher rates of 60 to 100 tons, especially if the sand is divided up and applied at two different times (spring and fall).
Topdressing sand should ideally be medium-coarse in size and uniform. In many situations, the sand 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. Macropores 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 percent 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 percent 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 can be difficult to grow grass or seed in. For a whole multitude of reasons then, gravel should not exceed 3 to 10 percent 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 percent 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 percent 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. It may take a couple of years to see the benefits of topdressing with sand. Obviously, the more sand applied, the faster the desired 70 percent by weight goal will be reached. It’s not a good idea to apply more than 0.25 inch at any one time, as the sand can be abrasive to turf equipment and the grass, but two or three applications could be made each year outside of the playing season.
If there’s 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 turfgrass science website.
In addition to improving the infiltration and percolation rate of rootzone soil, there are several drainage design options for sports fields. As might be expected, there are large geographical variations in the amount of rainfall a location receives, so drainage designs vary from location to location. Another important consideration when designing a drainage system is the time of year the fields are used, since fields used during winter months, when evapotranspiration rates are low, are less likely to have good infiltration rates and will require a more rigorous drainage setup.
Surface drainage refers to the ability of water to shed off the surface due to the creation of a slope or crown. A crown on a native-soil field or a sloped grade on a baseball field influence how quickly water and soil move from the surface during a rain event. Excessive/high grades result in quick water movement, but can also result in soil erosion, particularly on skinned infields. Conversely, a flat or undulated grade results in slow water movement and puddling.
The severity of the slope or crown depends upon the amount of sand in the soil profile. Fine-textured soils with less than 45 percent sand should have crowns/slopes of 1.5 to 2 percent. Sand-based field surface drainage rates are not as reliant on runoff, since infiltration rates are high, and grades may range from virtually flat to 1 percent slope. For a more detailed guideline on this, refer to ASTM WK37583 – New Guide for Construction or Renovation of Native-soil Athletic Fields.
On baseball infields the grade is critical, since infiltration rates are extremely slow but there is no turf cover to hold the infield soil mix in place. Even the smallest failure in grade can lead to standing water. For that reason, it’s important that the majority of any renovation budget be invested in good grading. A grade of 0.25 to 0.5 percent slope is common on skinned infields, with the pitcher’s mound the highest point and all slopes running from it to the back and sides. Outfields have a grade slope of 1 percent. Higher grades on the infield would facilitate faster water movement, but would also result in much greater movement of the infield mix, with it migrating into the surrounding turf and creating lip problems.
One final point on the subject of grades is that water doesn’t flow “upwards,” so any kind of hole or undulation – outside a turtle-back crown on a football field, a depressed soccer goalmouth, a lip on a baseball field, etc. – will trap water and create puddles. The slope and grade are critical during the building phase, but the annual upkeep of surface evenness through filling in holes, topdressing and divoting is also important. Having an infield laser-graded every couple of years is also a good idea. One last point to mention with crowns and slopes is that there should be a trench or grid system in place on the sidelines to catch the water and direct it towards an outlet. If there is no sideline trench, the water collects on the sideline and creates a muddy mess.
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 perforated plastic piping below the rootzone. The benefits include the gradual lowering of the water table and 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. Thus, 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.
Pipes are usually installed perpendicular to the crown or slope to maximize points of water interception. For example, on football fields the crown would run from the center to the sideline and the internal pipe would run from end zone to end zone or diagonally, from corner-to-corner. The spacing of internal drainpipe ranges from 10 to 20 feet apart, depending on soil type and budget, with pipes placed closer together in poor soils. Pipes are laid in trenches 1.5 to 3 feet deep and backfilled with gravel. In most cases, internal drainage pipe is not effective on baseball infields because the infiltration rate of the infield mix is too slow. In that instance, surface drainage is the solution.
Slit drains: Slit-drained fields are designed so 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 inch wide and 10 inches deep on 3-foot 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 internal 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 ensure that the slits are not “capped off” over time. Research by the Sports Turf Research Institute (STRI) has indicated that these types of fields can accommodate six hours of adult play per week (95 to 125 events per season). In addition, they have suggested that when managed correctly a slit-drained field should last about seven 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, which in turn is conducive to healthy root growth.
The challenges are that they require greater agronomic input, namely water and fertilizer, and they can lose grass cover quickly from overuse if the sand is not stable, or if regular overseeding and topdressing is not performed. Establishment of new seedlings can also be a challenge, as the sand creates quite a harsh environment. Organic matter accumulation is also a problem, though there have been technological advances in recent years, such as fraize mowing (removing the organic material and leaf tissue while leaving desirable crowns intact).
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.
Pam Sherratt is a sports turf specialist at Ohio State University and served on the STMA board of directors from 2010-2011.