Testing Basics

Consider How Testing Basics Can Provide Improved Quality, Better Player Experiences and Safer Athletic Fields

By Dr. Kyley Dickson, Associate Director of the Center for Athletic Field Safety and Dr. John Sorochan, Distinguished Professor, Plant Sciences Department The University of Tennessee

This article appeared previously in the February 2020 issue of Sports Field Management online. It is reprinted here with permission of STMA and the authors.

Athletic fields require regular maintenance whether the surfaces are natural or synthetic. One under-appreciated and challenging aspect for managing athletic fields is that systems change as a season progresses. One of the best ways to reduce player injuries and increase player performance is to have a consistent playing surface that is kept within acceptable ranges for athlete safety. Without regularly testing fields, it is hard to determine variances in playing surface consistency as use and wear increases. Knowing how a field is changing throughout the year can help field managers make data-driven decisions to optimize the performance of the playing surface and help to keep athletes safe. Keeping records of different field conditions across years and within season can help a field manager to visualize what is going on below the field surface. Although testing takes time and can be expensive, the knowledge gained by field managers can help extend the quality and longevity of a field, can improve player performance while protecting player health, and can mitigate risk.

One of the main benefits of testing a field is that testing results reveal consistency and characteristics of a field that have direct impacts on athletes. Tests that are conducted give clues to the health of a field and help identify maintenance actions that are needed. While there are different testing criteria for natural and synthetic surfaces, some shared tests are beneficial for both systems. In determining what tests are needed for a given surface, a few questions need to be answered. First, is the field natural or synthetic? Second, what sport or sports are played on the field? Lastly, what is the budget and time available for testing? Answers to these questions will help determine what tests will provide the most beneficial information for each surface. To start, some basic tests need to be established for field managers as a baseline.

Some of the biggest challenges facing turfgrass managers about testing fields come with decisions about which tests to perform, and then budgeting time, interrupted field scheduling, and dollars to complete the tests. The University of Tennessee Center for Athletic Field Safety (UTCAFS) has outlined its suggestions for a basic kit for natural and synthetic fields. For natural fields, the basic test kit should include a soil moisture probe, a side soil profiler, and a rotational traction, testing device. If buying these test components new, estimated costs will range between $2,500 and $5,000 depending on which products are selected. There are a variety of suitable products available. On a synthetic surface, the recommended basic kit includes an infill depth gauge, a measuring device for surface temperatures, and some type of rotational traction device. Estimated costs for synthetic turf kit components range from $850 to $1,000. There are also companies that will perform these tests on both natural and synthetic athletic fields. Companies can provide a wide range of tests and provide a summary of their findings and recommendations for any actions needed.

Remember that testing results provide a snapshot of the condition of that field at that particular time. Depending upon weather, level of play, and other factors, the same test conducted the following week can yield very different results. For this reason, taking multiple readings across a year will give a more detailed picture of what is happening. The other key requirement for getting a good snapshot is testing for the variables that have the greatest impact. A few variables have an influence on many parts of the field. In natural grass fields, for example, the soil moisture content of the field effects surface hardness, traffic tolerance of grass, rotational traction/resistance, increase in soil bulk density when trafficked, head injury criterion, and translational traction. There are several different kinds of devices that measure soil moisture, and most provide relatively rapid test results. Soil type in the field is also important and interacts with soil moisture content. In other words, soil moisture content will have a greater negative influence on the playability of a soil that is higher in silt plus clay than a sand-based field. While there are a multitude of tests for additional field performance parameters, irrigating (or withholding irrigation) to achieve appropriate soil moisture content will improve safety, longevity, and performance of a field, in addition to improving the overall quality of the grass.

Fig. 1. A side-soil profiler cross-section of turfgrass field reveals roots and layering structure of the sod/soil profile.

Another tool for natural grass is a side soil profiler. This device lets you remove a side-cut slice of turfgrass and sand or soil from your field to see what is really going on below the surface (Figure 1). From the slice, a turfgrass manager can quickly determine depth of roots, visualize layering issues, and identify buried objects. For example, a sample from a thinning turfgrass stand in a sand-based root zone may reveal a pocket of clay that is preventing consistent grass growth (Figure 2). In this case, the grass above the clay appeared more stressed than the surrounding areas, and a soil profiler revealed the problem. After a soil profile is taken, it can be reinserted back into the area tested with minimal surface disruption.

Fig. 2. This side-soil cross-section shows a clay inclusion within the cut turf profile.

Rotational traction is an additional tool that is very useful for both natural and synthetic surfaces and provides more of a performance and safety standpoint for the athletes on the field. Rotational devices normally have a cleat form on the bottom that is inserted into the surface and turned with a torque wrench around a rotational axis seeing the amount of force is required to break traction (Figure 3). The turfgrass manager’s goal is to keep a field consistent for rotational traction, which provides better footing for players and a potentially safer playing surface. Failure to maintain uniform rotational traction conditions have been associated not only with both lower extremity injuries to players, but also to reduced grass health. The smaller, portable devices for measuring rotational traction (Figure 3) are relatively easy and quick to use. There are several kinds available. The one shown in figure 3 is one of several that can be purchased. Use of these devices will cause a slight disruption to the playing surface where tested, but this minimal surface disruption can be tamped down and will recover.

Fig. 3. A rotational traction sampler comes with cleat attachments to assess break force on natural and synthetic turfgrass surfaces.

For synthetic turf, infill depth can be just as important as soil moisture is to a natural grass system. The infill depth is often taken for granted on many synthetic fields. As seasons progress across the lifespan of a synthetic turf, infill will be moved around and spots will form on the field, which are lower or higher than adjacent areas. Research at the Center for Athletic Field Safety has demonstrated that variances in infill depth, surface temperature, and rotational traction each can impact surface hardness. Measuring infill depth uses a metal rods (or rod) that are inserted until the backing is contacted by the rods, the top of the infill is determined and the distance the rods that are inserted into the surface is the infill depth (Figure 4). Results will let the field manager know if additional infill is needed, or if the infill simply needs to be redistributed from areas that are too high to areas that are too low. The goal is keeping the infill depth as close to manufacturers recommendation.

Fig. 4. An infill depth gauge uses metal rods that easily insert into infill materials.

Surface temperature is another important variable to turf health and field and player performance. Synthetic turfs have temperatures that can be much higher than natural grass fields during full sun when air temperatures are hot. Heat increases to temperatures as high as 175ºF (as was recorded in Knoxville, TN at 3 pm on 13 August 2019) on synthetic turf. This has a detrimental impact on athletes, decreasing performance and increasing the need for breaks and rehydration. Surface temperatures can be taken with a variety of tools, but a hand-held temperature gun (available at most automotive and do-it-yourself supply stores) is an inexpensive, fast and easy device for gauging the surface temperatures of a field. While little can be done to reduce synthetic turf temperature after a system is installed, educating field stakeholders of potential heat concerns is one potential plan of action. The tests described above are just the basics. There are many more tests available if budget and time permits.

Another consideration is what sport/sports are played on the field. In football, knowing surface hardness and rotational traction are of greater importance than ball to surface interaction questions. In soccer, FIFA has requirements about ball roll and ball rebound that take place on a field. Key sections of a field are also important for choosing where to test (Figure 5). Recommendations are to test the same 8–12 spots on a football field or soccer pitch each time while testing additional areas that may also be of concern. The more locations that can be tested on a field the better. Testing the same spot across time, and recording the results in a spreadsheet or other mapping platform, will help the manager interpret the findings in a meaningful way. These records will tell you how it is changing each time testing is completed. Comparing multiple fields in a sports complex can show how different fields may vary from each other (and where) due to soil type, construction, grass, infill, and other factors.

Recommendations from professional sports governing bodies (i.e., FIFA Handbook) can also help direct what types of tests are important for a given sport. Currently, most field-testing is only required at the professional level and some sports do not have sports-specific tests. However, there are universal tests such as surface hardness and rotational traction on most surfaces that can be completed to increase the performance of an athletic field.

Tests for Playing Field Standards (FIFA)

Tests for both natural and synthetic turf:

• Ball roll

• Ball rebound (“bounce”)

• Shock absorption

• Vertical deformation

• Energy restitution

• Surface planarity (“levelness”)

• Skin/surface friction

Additional tests for synthetic turf:

• Infill depth

• Free pile height

• Surface temperature

References and Additional Resources

Baker, S.W. 1991. Temporal variation of selected mechanical properties of natural turf football pitches. Journal of the Sports Turf Research Institute 67: 83–92.

Baker, S.W., and R.J. Gibbs. 1989. Making the most of natural turf pitches. Case studies: II. Playing quality. Natural Turf Pitches Prototypes Advisory Panel Report 4. Sports Council, London.

Charalambous, L., H.C.V.L. Wilkau, W. Potthast, and G. Irwin. 2016. The effects of artificial surface temperature on mechanical properties and player kinematics during landing and acceleration. Journal of Sport and Health Science, 5: 355-360.

Dickson, K.H., J.C. Sorochan, J.T. Brosnan, J.C. Stier, J. Lee, and W.D. Strunk. 2018a. Impact of soil water content on hybrid bermudagrass athletic fields. Crop Sci. 58:1416-1425.

Dickson, K.H., W. Strunk, and J. Sorochan. 2018b. Head impact criteria of natural grass athletic fields is affected by soil type and volumetric water content. Proceedings 2: 270. doi:10.3390/proceedings2060270

Lim, L., and R. Walker. 2009. An assessment of chemical leaching, released to the air and temperature at crumb-rubber infilled synthetic turf fields. New York State Dept. of Health. pp. 1–140

Orchard, J., H. Seward, J. McGivern, and S. Hood. 1999. Rainfall evaporation and the risk of non-contact anterior cruciate ligament injury in the Australian Football League. Medical Journal of Australia 170: 304–306.

Stier, J. C., J. N. Rogers, J. R. Crum, and P. E. Rieke. 1999. Flurprimidol effects on Kentucky bluegrass under reduced irradiance. Crop Sci. 39:1423-1430.

Thoms, A.W.; Brosnan, J.T.; Zidek, J.M.; Sorochan, J.C. 2014. Models for predicting surface temperatures on synthetic turf playing surfaces. Procedia Eng. 72: 895–900, doi:10.1016/j.proeng.2014.06.153.