Finding the balance

Core aeration affects turf health, soil physical properties and the playability of golf course greens.

Jeff Atkinson, M.S., and Bert McCarty, Ph.D.

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Finding the balance: photo 1

A recently core-aerated bermudagrass putting green. Core aeration is necessary to mitigate effects from compaction caused by human and mechanical traffic.
Photo by Bert McCarty

Each growing season, superintendents are conflicted between the need for core aeration and commitment to maintaining a consistent, playable putting surface. Over time, reducing or eliminating core aeration will result in deteriorating turf health, soil physical properties and soil chemical properties (6). The trick is finding the balance between providing sufficient soil cultivation to maintain long-term turf health while limiting disruption to surface playability.

Foot traffic and maintenance practices such as mowing and rolling are compressive forces that continually increase compaction. As compaction becomes more severe, the availability of nutrients, water and oxygen is reduced. Severe compaction leads to accumulation of toxic levels of carbon dioxide within the soil and increased incidence of localized dry spot, anaerobic soil conditions, disease and nutrient deficiency.

Using highly stoloniferous turfgrasses (like ultradwarf bermudagrasses) to increase wear tolerance of high-traffic areas, such as putting greens, increases the need for frequent cultivation to prevent excessive thatch accumulation. Thatch is a slowly decomposing layer of living and dead stems, leaves and roots that develops between turfgrass shoots and the soil surface (3). A limited amount of thatch is desirable to provide resilience to turf and act as a buffer for moderation of soil temperatures (1). Excessive thatch reduces infiltration rate, promotes mower scalping, localized dry spot and vulnerability to insect and disease damage (9).

Byproducts of microbial thatch degradation accumulate in the soil and increase total soil organic matter. It has been suggested when organic matter content of sand-based putting greens reaches 3%-4% by weight, soil macroporosity begins to decrease (2). Organic matter accumulation within the root zone increases microporosity, lowers permeability of the soil, further slows surface infiltration and subsurface drainage, decreases the amount of water available for plant uptake and impedes gas exchange.

Finding the balance: table 1
†Number of core aeration events/year to reach total surface area affected each year.
‡Topdressing amount was the mathematical equivalent of soil removed from core aeration, or half of this rate.
§In some treatments, two slightly offset passes with the core aerator were necessary to achieve the correct hole spacing.
Table 1. Treatment list showing percent of total surface area affected per year, core aeration events per year and amount of topdressing applied in Clemson, S.C., June–August 2008 and 2009.

Core aeration and topdressing are cultivation practices used to improve gas exchange, relieve compaction and slow thatch and organic matter accumulation. Previous recommendations have stated that removing 20% of the surface area on a yearly basis through core aeration is necessary to maintain highquality turf (4). Research has not been able to find the perfect balance among core-aeration programs, turf health and surface consistency.

The goal of this research was to provide superintendents with a decision-making framework for maximizing benefits to soil physical properties and turf health from core aeration while maintaining consistent playability.

Materials and methods

A field study was conducted at Clemson University, Clemson, S.C., during the summers of 2008 and 2009 to evaluate the effect of various core-aeration programs on turf quality and soil physical properties. All research was conducted on a 10-year-old TifEagle bermuda-grass research putting green built to USGA recommendations. The experimental design was developed to explore the effects of removing 15% or 25% surface area per year through one, two or three core aerations on turf quality and soil physical properties.

Finding the balance: photo 2

Two weeks after treatment removing 25% of the surface area with one core aeration. Increasing the amount of surface area removed through core aeration reduces bulk density, improves surface water infiltration and decreases surface hardness, while increasing healing time.
Photos by Jeff Atkinson

For each treatment, the combination of the percentage of surface area removed and the number of core aerations dictated the tine size and tine spacing. The actual amount of surface area affected per year varies slightly from the target values because of mechanical limitations. In some treatments, two slightly offset passes with the core aerator were necessary to achieve the correct hole spacing. Following each core aeration, treatments received one of two topdressing rates — either the mathematical equivalent of soil removed by aerification or half this rate (Table 1).

The first core aerations were performed on June 1, the second on July 4 (±3 days) (where necessary), and the third on Aug. 15 (±3 days) (where necessary) of each year, using a tractor-mounted core cultivator. Topdressing material similar to that used in putting green construction was measured by volume and applied by shaking it evenly over individual plots. Topdressing was incorporated by hand with a shop broom.

A 20N-8.8P-16.6K fertilizer was applied throughout the growing season to provide 1 pound nitrogen/1,000 square feet (48.42 kilograms/ hectare) each growing month. Plots were mowed five times per week and maintained at 0.125 inch (3.18 millimeters).

Plots were evaluated for turf quality, bulk density, surface hardness, thatch depth, soil organic matter content and surface-water infiltration rate. Turf quality was visually evaluated every two weeks on a 1-9 scale, where 1 was dead turf; 9 was dark green, dense turf; and a rating below 7 was unacceptable. Bulk density was measured at the end of each study year by removing an undisturbed soil core, drying it in an oven at 221 F (105 C) for 48 hours and then dividing dry soil core mass by total soil core volume.

Finding the balance: table 2
†% surface area impacted per year.
‡Averaged across all rating dates.
§Relative surface hardness value quantifies deceleration of 4.96-pound (2.25-kilogram) weight dropped from height of 17.7 inches (45 centimeters).
//Ashed organic weight of thatch layer per square centimeter of surface area.
††Values followed by different letters within the same year are significantly different.
Table 2. Response of turfgrass and soil physical properties to core aeration affecting 25% and 15% surface area per year averaged across all rating dates and number of core aeration events per year in Clemson, S.C., June−August 2008 and 2009.

Surface hardness was determined as the average of three Clegg impact values (CIV) per plot, a measurement of deceleration of a 5-pound (2.25-kilogram) weight dropped from a height of 18 inches (45 centimeters). Thatch depth was measured two weeks after each core aeration by removing four soil cores from each plot and measuring the distance between shoots above the thatch layer and roots below the thatch layer. Thatch samples were dried at 221 F (105 C) for 48 hours and weighed. Dry cores were then combusted in a muffle furnace to provide ashed organic weight and organic matter content determined by the difference between these two measurements.

Infiltration was measured 14 days (±2 days) after each aerification using a doublering infiltrometer. Infiltration (inches/hour) is reported as time for water in the center ring to empty from an initial height of 3 inches (8 centimeters) while maintaining a consistent hydraulic head in the outer ring.

Results and discussion

Topdressing rate did not affect any measured parameter in either year of the study, and interaction between percent surface area removed per year and the number of core aerations per year was inconsistent. The remainder of the article will focus on the effects that the amount of surface area removed per year and the number of core aerations per year had on turf quality, bulk density, surface hardness, thatch depth, soil organic matter content and infiltration rate.

 Turf quality

Any conversation about core aeration includes a discussion of its effect on turf quality. Understanding how turf quality is affected by the percent surface area removed and the number of core aerations per year is necessary to properly evaluate the trade-off between reduction in turf quality and improvement of soil physical properties.

When considering only the amount of surface area removed per year, turf quality was improved 4% in 2008 and 6% in 2009 by reducing the amount of surface area removed (Table 2). With less surface disruption, the turf required less time to fully heal from core-aeration injury, which contributed to the overall improvement in turf quality.

Finding the balance: table 3
†Initial aerification occurred on June 1 of each year with subsequent core aeration on July 4 and Aug. 15 (±3 days).
‡Turf quality values range from 9 (ideal turf) to 1 (dead turf).
§Values followed by different letters within the same year and weeks after initial core aeration event are significantly different.
Table 3. Turf quality response over time to 1, 2 or 3 core aerations per year averaged across all amounts of surface area affected per year in Clemson, S.C., June−August 2008 and 2009.

After initial core aeration on June 1, turf quality was unacceptable (<7) for approximately four weeks in 2008 and six weeks in 2009, regardless of amount of surface area removed. After these periods, in treatments involving two and three core aerations, turf quality generally improved but was reduced by subsequent core aerations on July 4 and Aug. 15 (Table 3). In 2008, removing 15% or 25% of the surface area in a single core aeration initially reduced turf quality more than treatments removing these amounts through two or three core aeration events.

Although the initial injury after removing 15% or 25% surface area in a single core aeration may be unacceptable to some turf managers, turf quality in these treatments was considered acceptable for more cumulative weeks throughout the study when compared to treatments with two or three core aerations. In 2009, initial turf-quality reductions were not as severe because less surface heaving occurred during core aeration.

Bulk density or soil compaction

 Soil bulk density is the mass of dry soil per given unit of soil volume. If interpreted correctly, bulk density can provide insight into the degree of soil compaction. An excessive increase in bulk density will result in reduced macroporosity, nutrient and water availability, oxygen concentration, and speed of surface water infiltration. Failure to adequately perform core aeration will cause soil to become excessively dense over time.

The optimal value for bulk density varies depending on soil texture. For sand-based putting greens, bulk density should fall between 1.25 to 1.55 grams/cubic centimeter to provide a balance of soil aeration, water retention, nutrient availability, and oxygen concentration (5). Bulk density should be a major consideration during the planning stage of a cultivation program and should be measured every two years.

In this study, bulk density decreased 5% in 2008 and 4% in 2009 by increasing surface area removed per year from 15% to 25% (Table 2). Increasing the number of core aerations per year from one to three reduced bulk density 8% in 2008 (Table 4). In 2009, increasing the number of core aerations from one to two reduced bulk density 4%. Bulk density was similar between treatments with one and two core aerations in 2008 and two and three core aerations in 2009. Although the effect of the number of core aerations per year on bulk density was somewhat inconsistent between years, bulk density generally decreased as the percent of surface area removed and the number of core aerations per year increased.

Surface hardness

Surface hardness, or firmness, is a measure of soil compaction and surface cushioning due to thatch accumulation and soil strength. Since measurements made with the Clegg impact hammer are a relative barometer of surface firmness, the effects discussed here should be used during the planning stages of a core-aeration program to identify the program’s potential effects on playability.

Finding the balance: table 4
†Relative surface hardness value quantifies deceleration of 4.96-pound (2.25-kilogram) weight dropped from height of 17.7 inches (45 centimeters).
‡Ashed organic weight of thatch layer per square inch of surface area §Values followed by different letters within the same year are significantly different.
Table 4. Soil physical properties response to one, two or three core aerations per year, averaged across all rating dates and percent of surface area affected each year in Clemson, S.C., June−August 2008 and 2009.

Overall, two years of core aeration were needed before surface hardness was significantly affected by the percent of surface area removed per year. During 2009, removing 25% of surface area reduced surface hardness 4% compared to removing 15% of surface area (Table 2). The effect of the number of core aerations per year on surface hardness was more consistent between years. Increasing the number of yearly core aerations from one to three reduced surface hardness 5% in 2008 and 19% in 2009 (Table 4).

Although surface hardness is a topical indicator of soil compaction, superintendents must consider how reducing putting surface firmness affects playability. A firm putting surface may be desirable to encourage fast ball roll speeds and allow predictable ball action on approach shots for skillful players.

Conversely, a firm putting surface slows overall play, as fewer balls will hold the green on an approach shot, causing more strokes to be played around the greens complex.

Because surface firmness affects playability, it must be considered during the planning stages of a core-aeration program. Allowing adequate time for firming of the surface is necessary to provide conditions appropriate for championship play. Although this study quantified the overall effect on surface hardness of various core-aeration programs across the growing season, additional research is needed to determine the amount of time necessary to restore firmness to a desired level after soil cultivation.

Thatch depth

Due to the waxy, hydrophobic nature of thatch, soil moisture management can become challenging when thatch depth is excessive. Putting green surfaces typically require relatively high nutrient input to maintain adequate growth. This, in combination with the use of highly stoloniferous turfgrasses, leads to rapid thatch accumulation in the absence of proactive cultivation. In this study, neither increasing the percent of surface area removed per year nor the number of core aerations per year reduced thatch depth. However, thatch depth did not increase throughout the study (Tables 2, 4). Further research is needed to determine how other thatch-cultivation techniques, such as vertical mowing, complement core-aeration programs and affect surface playability.

Organic matter content

Organic matter is the byproduct of microbial degradation of shoots, thatch and roots. Collecting these byproducts in the upper soil profile is the driving force behind accumulation of soil organic matter. Although soil microbe activity should be encouraged, failure to reduce accumulation of organic byproducts slows soil drainage, increases microporosity and decreases soil oxygen concentration.

Finding the balance: photo 3

Two weeks after removing 5% of the surface area with one core aeration. Decreasing the amount of surface area removed during core aeration limits improvements in bulk density, surface water infiltration and surface hardness, while decreasing healing time.

In this study, reduction of soil organic matter accumulation was not consistent between years. In 2008, increasing the percent of surface area removed per year or the number of core aerations per year did not reduce organic matter accumulation. In 2009, treatments with three core aerations slowed organic matter accumulation 10% more than treatments with only one core aeration (Table 4); however, overall organic matter accumulation was not reduced below pre-study levels.

Although research has not consistently quantified significant reductions in soil organic matter following core aeration, numerous studies have shown that core aeration prevents organic matter accumulation above pre-aerification values (6,7,8). Long-term observation of soil organic matter accumulation is necessary to determine the cumulative effect of core aeration over several growing seasons.

Infiltration

The resistance water encounters as it travels through hydrophobic thatch and a compacted soil surface typically slows water infiltration. Properly constructed putting greens should balance sufficient drainage with adequate soil water-holding capacity to promote healthy turf growth. For newly constructed sand-based putting greens, water should infiltrate the turf surface 10 to 15 inches (25-38 centimeters)/hour (5).

Thatch accumulation and byproducts of thatch decomposition combine to slow infiltration over time. As infiltration is reduced, the proportion of water lost through runoff increases, the amount of plant-available water is reduced, and saturation of the thatch layer is encouraged. Removing thatch through core aeration has long been relied on to reduce or slow thatch accumulation, thereby improving infiltration rate.

In this study, increasing the percent of surface area removed per year did not increase infiltration speed in 2008 or 2009. Increasing the number of yearly core aerations did not affect infiltration speed in 2008; however, in 2009 increasing the number of core aerations from one to two decreased the speed of surface water infiltration 32%, and increasing the number of core aerations from one to three decreased surface water infiltration speed 20% (Table 4).

The season-long effect of increased infiltration in treatments with only one core aeration in 2009 may be explained by the initial removal of a large percent of surface area early in the growing season. Increasing the number of yearly core aerations to remove the same percent of surface area reduces the size and number of channels opened through the turf surface to facilitate infiltration. When single core-aeration treatments were repeated in consecutive years (2008 and 2009), numerous channels to facilitate water movement through the turf surface were opened early in the growing season, improving season-long infiltration speed.

Conclusions

The research shows that superintendents should develop core-aeration programs that fit their needs while keeping in mind agronomic considerations and playability. Generally, as the number of core aerations per year and the percent of surface area removed per year increase, soil physical properties improve. As the number of core aerations per year and the amount of surface area removed per year decrease, average turf quality across the entire growing season improves, but soil physical properties show less improvement.

Regular monitoring of bulk density, surface- water infiltration, surface hardness, thatch depth and organic matter accumulation is necessary to identify soil physical properties in need of amelioration through core aeration. These properties — as well as consideration of the effects of core aeration on turf quality — should be used to determine amount and frequency of core aeration necessary to provide a healthy growing environment for turf.

Developing a framework for superintendents to balance agronomic practices with maintaining a consistent playing surface is an ongoing process. Continuing research is needed to refine timing, spacing and tine size selection to minimize putting surface disruption while maximizing the benefits gained through core aeration. Additional research is also needed to gain an understanding of the long-term effects of various cultivation programs on turf health.

Funding

Funding was provided by the Clemson Agricultural Research Station.

Acknowledgments

The authors would like to acknowledge Alan Estes, Ray McCauley and Jeff Marvin for their assistance conducting this research.

Literature cited

  1. Butler, J.D. 1965. Thatch: A problem in turf management. Pages 1-3. In: Illinois Turf Conference Proceedings, Lemont, Ill. 1965. University of Illinois Cooperative Extension Service, College of Agriculture and the Illinois Turfgrass Foundation, University of Illinois, Urbana, Ill.
  2. Carrow, R. 1998. Organic matter dynamics in the surface zone of a USGA green: Practice to alleviate problems. USGA Turfgrass and Environmental Research Summary. Online (http://archive.lib.msu.edu/tic/ressum/1998/15.pdf). Verified Dec. 10, 2013.
  3. Engel, R.E. 1954. Thatch on turf and its control. Golf Course Reporter 22:12-14.
  4. Hartwiger, C., and P. O’Brien. 2001. Core aeration by the numbers. USGA Green Section Record 39:8-9.
  5. McCarty, L.B. 2011. Best Golf Course Management Practices. Pearson Education, Upper Saddle River, N.J.
  6. McCarty, L.B., M.F. Gregg and J.E. Toler. 2007. Thatch and mat management in an established creeping bentgrass golf green. Agronomy Journal 99:1530- 1537.
  7. McWhirter, E.L., and C.Y. Ward. 1976. Effect of vertical mowing and cultivation on golf green quality. Report 2. Mississippi Agriculture and Forestry Experiment Station, Starkville, Miss.
  8. Smith, G.S. 1979. Nitrogen and aerification influence on putting green thatch and soil. Agronomy Journal 71:680-684.
  9. White, R.H., and R. Dickens. 1984. Thatch accumulation in bermudagrass as influenced by cultural practices. Agronomy Journal 76:19-22.

Jeff Atkinson is a graduate student and Bert McCarty is a professor in the school of agriculture, forestry and environmental sciences, Clemson University, Clemson, S.C.