Clay soil is cohesive in nature and prone to compaction when wet.
Photos courtesy of Pamela Sherratt.
Soil compaction is defined as an increase in soil bulk density, and concomitantly a decrease in soil porosity, by the application of mechanical forces to the soil. Compaction occurs as an interaction between the level of applied stress and the strength of the soil in resisting deformation. In other words, if the applied stress exceeds the soil compressive strength, then compaction will occur.
Little compaction is observed from the use of turf maintenance equipment, but excessive and untimely foot traffic is a major cause of soil compaction on athletic fields. For example, a 165-pound person in street shoes will create a ground pressure (stress) of 0.01 ounces per square inch standing upright, while the same individual wearing cleated shoes and running shows an increased pressure of 2 ounces per square inch. A running athlete, planting his foot for rapid direction changes, will add significantly to these loadings.
Clearly, compressive stress from foot traffic can become an issue, but it’s important to remember that total load is small and the depth of stress penetration in the soil will be shallow. Additionally, a healthy turf with thatch or mat can absorb significant impact from foot traffic, but excessive foot traffic on bare soil can lead to devastating consequences.
The degree to which a soil will be compacted depends on the strength of the soil. Soils with a low compressive strength are classed as “cohesive” and contain an appreciable quantity of silt and clay. Sands are classed as “granular” and have high compressive strengths, which is why they are preferred as rootzones for soils exposed to high levels of foot traffic.
It is not foot traffic or soil texture alone that dictates the extent of soil compaction. Soil moisture plays a large role on the strength of a cohesive soil, with strength generally decreasing at higher water contents. Thus, play on a cohesive soil when at higher soil moisture leads to more severe compaction than when play occurs on the same soil when it’s drier.
The inevitable consequence of athletic sports is that they frequently occur regardless of rainfall or soil moisture conditions. Soil compaction results in a loss of total porosity, with the larger pore spaces (macropores) being lost and smaller pores (micropores) increasing in percent volume. The loss of macropores in compacted soils results in reduced infiltration rates, reduced drainage, poor soil aeration, and a platy or massive soil structure. These marginalized soil physical conditions result in less than favorable environmental conditions for turfgrass roots and for many beneficial soil microbes, earthworms and arthropods. For this reason, compaction is probably the most serious damage that can occur in athletic field soils.
In addition to the agronomic effects that soil compaction has on an athletic field, compaction also heavily influences the field’s playability, namely surface hardness, ball bounce, ball roll, surface evenness and traction. Generally speaking, as the clay content in the rootzone increases, surface hardness increases, and both ball bounce and roll increase.
Core cultivator/aerator. Having the tine unit run the full width of the machine gives uniform coverage without running tires over previously cored turf.
In cases where rainfall exceeds evapotranspiration during the playing season, and/or irrigation is improperly programmed, many clay-based cohesive soils become waterlogged, leading to a mud bath scenario. Hardness in these soils falls below the preferred lower limit of 20 gravities when the soil moisture content reaches 34 to 39 percent. In this instance ball response is slow and inadequate for play and surfaces are considered “softer.” In essence, clay soils are hard when dry and soft when wet.
Traction is also affected by wet and/or waterlogged soils, with lower traction readings recorded in saturated conditions. On the flip side of that, soils with high clay content maintain surface stability when devoid of grass compared to sand. Sand becomes very loose and provides poor traction unless it can be kept at high moisture content and rolled.
On a well-grassed sand rootzone, plant roots increase the shear resistance (surface stability) by a factor of two to three times. The effect of the roots is much greater than the increase in traction achieved by increasing the silt and clay content, although in soil with a fine fraction (< 50 millimeters) constituting 12 percent of the rootzone, there is an increase in shear resistance of about 50 percent over that of pure sand. An underlying rootzone that provides a dry, firm surface with good traction enhances the quality of the game and ensures better player safety.
Fields for sports such as football and soccer – played during periods of low temperature and high precipitation – demand a rootzone with a high sand content in order to perform at their best, ensure player safety and be usable throughout the season. Playing quality will improve with better surface construction, and level of use can be increased substantially from as many as 50 adult games per season on a marginally drained soil to 125 to 180 adult games per season for a sand-dominant construction.
Indicators of soil compaction: algae on surface, cannot push soil probe into soil, unhealthy looking turf and white clover thriving.
When firm soils are a good thing
In sports where ball bounce is paramount, such as lawn tennis and cricket, there is a higher percentage of clay in the underlying soil profile to give the surface binding strength and increase ball bounce height. Also, soil mixes used as a hard surface, such as the skinned areas of a baseball diamond, must provide uniform ball response, good traction, and an ability to remain playable at widely different moisture levels.
The most important considerations with these soils are that they are graded correctly and they have adequate moisture retention. The soil surface needs to drain freely and support foot traffic after heavy precipitation, but not become too hard or dusty after heavy usage or prolonged dry periods. The surface should be favorable to frequent dragging and should not wash out. An example of an effective skinned area soil mix is 60 percent sand, 20 percent silt and 20 percent clay.
One specialized skinned surface is the baseball pitcher’s mound. To withstand the excess wear placed on a mound, clay-based soils are sometimes used to provide the necessary soil strength.
Proper irrigation management is critical on these types of athletic fields, and moisture management prior to games is generally handled by the use of turf covers.
Figure 1: The stress at the soil surface is proportional to tire pressure, or PSI. Therefore, a roller or mower with tires inflated to 15 PSI will apply 12 to 15 PSI pressure to the soil surface. The force applied dissipates as a function of depth.
Relieving soil compaction
The close connection between foot traffic and compaction would suggest that some degree of soil modification would substantially improve the soil and, consequently, turf growth and playing quality. This is particularly true for those areas exposed to the greatest traffic.
The main goal in soil modification is to replace the existing native soil that exhibits cohesive behavior with a rootzone having properties of a granular media. This is achieved by establishing sufficiently high sand contents in the rootzone, typically 75 percent by weight. The increase in sand results in 15 percent of the total pore volume composed of larger sized (> 0.05-millimeter) pores, which are responsible for rapid air and water movement. A soil should contain at least 10 to 20 percent of these macropores to allow adequate infiltration, drainage and gas exchange. Studies have also shown that soil containing this minimum sand content will yield saturated hydraulic conductivity values of about 2 inches per hour, which is a recommended minimum permeability for high-traffic athletic fields.
Installing sand slits.
Soil modification with sand is performed in a variety of ways: blending the entire rootzone on or off-site, installing sand slits/bands into the existing native soil, installing a sand cap, or topdressing with sand each year as part of the annual maintenance program. Adding organic material like bulky compost to soils that are devoid of organic matter is also an option. Composts reduce the bulk density of a soil and increase the nutrient status and water-holding capacity, but applications should be made thoughtfully and in conjunction with careful monitoring of infiltration rates.
Soil cultivation is one of the most important methods of relieving soil compaction. Compaction on athletic fields occurs most readily in the upper 2 to 3 inches of the soil profile. Therefore, cultivation strategies to address surface compaction need to be a top priority in the overall maintenance program. Ranking soil cultivation as a top priority is based on its benefits:
Release of toxic gases from the soil Decreased wilting of isolated dry spots on fields More rapid drying of wet or saturated soils Increased water penetration Improved root growth within coring holes Improved shoot growth in the vicinity of the coring holes Control of thatch where topdressing is incorporated Improved turfgrass response to fertilizers Preparation of a seedbed for overseeding
The frequency of cultivation is primarily dependent on the level of field use. Heavily used fields or fields with soil that is particularly prone to compaction may require cultivation a minimum of four to six times per year. The critical times to cultivate a field are: in the spring prior to fertilization and overseeding; in the early summer after the spring sports season; in the early fall prior to fall sports; and in the late fall after the fall sports season.
Table 1. Spacing and Tine Size Effect from Soil Cultivation
Where a field is in constant use, it may be necessary to cultivate during the season. A general rule of thumb is to cultivate whenever the turf begins to show the effects of soil compaction providing some time is available for recovery and it doesn’t reduce the quality of the playing surface.
Soil cultivation can be performed throughout the year as long as the soil is not frozen and post-cultivation conditions (i.e. surface disruption, presence of unbroken cores, soil debris, etc.) do not produce unsatisfactory playing conditions. Returning the cores (if using a hollow-tine aerator) can be accomplished by mat dragging the field or vertical mowing.
In most soils and conditions, full recovery from core cultivation will take two to three weeks. Fertilization and irrigation will help shorten the recovery time. Soil cultivation is also best accomplished when the soil is relatively moist but not wet. Moist soils facilitate deeper tine penetration. Cultivators will not effectively penetrate dry, compacted soils. Cultivation will cause additional compaction to wet soils.
The reincorporating of cores is usually based on time and cost. Core return is a good idea on most soil fields since it becomes a form of topdressing. However, cores can be removed by hand, brush or core harvester. Their removal provides the opportunity to ameliorate the rootzone with a more granular material. The logistical problems involved with core removal and the cost of an alternative material can be too much for a groundskeeper’s budget. The practical solution is to overseed, mix the cores with the seed, and drag or brush them back in.
Multiple passes in several directions are usually required with each cultivation event to affect a meaningful amount of surface area. Table 1 provides the amount of surface area impacted by tine size and spacing. A general rule is to produce no less than 12 to 15 core holes per square foot of turf. Where intensive cultivation is required due to heavy compaction, or where coring will be used as the main cultivation method for overseeding, 45 to 50 holes per square foot is a more desirable target.
There are many types of cultivation equipment available, with varying modes of action (see Table 2). Shallow spiking or solid tining doesn’t reduce soil bulk density, but it does have a contributory effect on surface water infiltration and aeration. Also, it does not adversely affect soil strength as much as hollow coring does. Solid-tine aeration is used regularly during the playing season because it’s less disruptive than coring and doesn’t affect surface strength or playability.
Table 2: Various cultivation equipment available for modification of established sports turf surfaces and rootzones
In essence, to reduce soil bulk density the soil must be physically displaced to create fracturing so the same mass of soil occupies a greater volume (e.g. verti-draining), or the soil must be removed so that a smaller mass of soil occupies the same field volume (e.g. hollow coring). The conventional method of hollow-core aeration at variable depths still appears to be the groundskeeper’s favorite technique. However, there has been an influx of equipment such as the Verti-Drain, pressure injection, deep drillers and vibrating subsoilers on the market in recent years. These are used to break up hardpans and relieve compaction deeper in the soil profile.
Compaction and rolling athletic fields
Rollers are available in walk-behind, ride-on and pull-behind models. They should always have rounded edges to prevent damage to the turf. Rollers come in all sizes and weights, typically ranging from 300 to 2,000 pounds. This equates to a load of approximately 3 to 15 pounds per square inch (PSI) applied to the soil surface, which is similar in weight to ride-on mowers.
While there is no set weight for athletic field rollers, the maximum recommended weight for native soil fields is 1 ton (2,000 pounds). The weight of a roller can be increased by filling the roller with a material like water, sand or cement.
The action of a roller is similar to that of vehicular traffic (e.g. ride-on mowers). The stress at the soil surface is proportional to tire pressure, or PSI. Therefore, a roller with tires inflated to 15 PSI will apply 12 to 15 PSI pressure to the soil surface. What’s also interesting to note is that the force applied dissipates as a function of depth (Figure 1). For higher loads, the stress penetrates more deeply into the soil. How much stress a certain soil can withstand depends on many factors. In particular, a soil’s ability to resist compaction depends on soil texture (sand or native soil) and moisture content.
Benefits of rolling:
To smooth out uneven surfaces after winter heave or heavy traffic. Rolling cannot rectify poor grades, but is used to address minor surface undulations. To produce a firm surface that would be considered “faster.” Rolling is a common practice in golf green and soccer field management to increase speed short term (hours). To produce a firm surface critical for those sports that require ball bounce, such as tennis, cricket and baseball. Rolling is often used on newly seeded or sodded turf areas to aid turf-to-soil contact and speed up establishment. Mowing patterns, typically created by the rear roller on a cylinder mower, can also be achieved with a roller. There have been some research reports of rolling reducing the incidence of disease, such as dollar spot, on golf greens. This is directly related to the use of lightweight rollers first thing in the morning, whereby the roller is helping to remove dew/guttation water from the leaf tissue.
Problems associated with rolling:
Overuse results in thinning turf and a significant reduction in quality. Wet and/or frozen soils are susceptible to surface compaction. The overuse of rollers also results in surface compaction. It is critical that rolled fields are regularly aerated. Soils that are too dry will not benefit from rolling. Furthermore, if the turf is wilted or dormant it will be severely stressed and may die. Rolling should only be performed when grass is actively growing. Fields with 100 percent grass cover and a moderate thatch layer are less likely to be affected by rolling as a method to increase field speed. Fields with disease issues, particularly infectious diseases like gray leaf spot, Pythium or brown patch, should never be rolled.
The field manager and coach should decide whether or not to roll a sports field on a field-by-field basis. Factors such as athlete safety and playability, soil moisture, recovery time and turf quality all come into play. As a general rule of thumb, rolling should only be carried out as needed, not routinely. This may be once a year in the spring or several times during the playing season to keep the field safe and playable if grass cover is lost. Lastly, keep in mind that soil athletic fields are already prone to compaction, so any rolling that is carried out should always be counter-balanced by a strong aeration program.
Pam Sherratt is a sports turf specialist at Ohio State University and served on the STMA board of directors from 2010-2011. Dr. Karl Danneberger has been a turfgrass professor at Ohio State University since 1983. Dr. John Street has been a professor in turfgrass science at Ohio State University for the last 30 years.