Many athletic fields in the US are built with native soils in contrast to constructed sand rootzones such as those developed according to the United States Golf Association. Native soils high in silt and clay tend to have greater soil water contents and slower water infiltration rates compared to constructed sand root zones. The decreased water infiltration rates of cohesive soils (i.e., non-sand soils) are potentially problematic when precipitation occurs prior to athletic events. It has been reported that cohesive soil athletic fields with high soil water content tend to lose green turfgrass cover faster than those with lower soil water content.

Constructed sand rootzones are used on many US collegiate and professional football fields. Sand rootzones are preferred because of consistent air-filled porosity, rapid drainage, and compaction resistance, which help avoid rain delays or cancellations. While multiple types of constructed sand root zones exist, the USGA specification is the most common for high-end athletic fields because it provides acceptable stability and optimal drainage. However, sand rootzones may not be used on all athletic fields due to high construction costs.

The objective of this research was to determine the impact of soil water content on the performance of hybrid bermudagrass on cohesive soil (silt loam) and non-cohesive (USGA specification) rootzone when subjected to traffic. Two field studies were conducted from 2014-2015 at the University of Tennessee Center for Athletic Field Safety (Knoxville, TN) to determine soil water content impact on compaction and loss of green turfgrass cover on ‘Tifway’ hybrid bermudagrass. Study I was conducted using plots established on a Sequatchie silt loam soil (fine-loamy, siliceous, semiactive, thermic Humic Hapludult). This soil was selected due to its common use on high school athletic fields in Knoxville, TN. Study II used plots established on a sand meeting USGA specifications (0.7% very coarse, 14.3% coarse, 61.4% medium, 18.1% fine, 5.1% very fine, and 0.4% silt and clay by weight) mixed with 20% (volume) reed sedge peat moss.

Study I had four soil moisture ranges: low (6 – 13%), medium (14 – 21 %), medium-high (22 – 29%), and high (30 – 37%). Study II soil moisture ranges were: low (5 – 11%), medium (12 – 19%), and high (20 – 27%) throughout the study for both years. Differences in the amount of ranges between rootzones were due to plant available water of the soil texture. Water was applied to each experimental unit as needed based on the average of seven rootzone moisture measurements (3 in depth) collected daily using a handheld time domain reflectometer (TDR) probe. Traffic was applied to both studies using a self-propelled core aerifier similar to the Baldree Traffic Simulator (BTS) described by Kowalewski et al. (2013). Each plot received 50 traffic events each year.

Silt loam rootzone

This study’s findings indicate that increased soil water content on cohesive soils resulted in greater loss of green turfgrass cover when trafficked. High soil water content ranges lost green turfgrass cover approximately four times faster than the low or medium soil water content and three times faster than medium-high soil water content treatments. Surface hardness varied across traffic events due to soil water content. These findings indicate surface hardness of a field can be manipulated by adjusting soil water content, suggesting that high soil moisture and soil compaction have significant impacts on surface hardness values. Cohesive soils, due to the higher quantities of silt and clay, are more responsive to increases in water content.

Regardless of soil water content, soil bulk density increased as traffic events increased. The increase in soil bulk density was due to reduction of the air-filled pore space of soil. Shear strength declined most rapidly at the high soil water content treatment. The high soil water content had the greatest loss of green turfgrass cover, extremely low surface hardness values and unacceptable shear strength throughout a majority of this study. This study found plots maintained at 7 to 20% soil water content provided the optimal surface for athletic field performance for the silt loam athletic field.

USGA sand specification rootzone

Soil water content treatments had little impact on the non-cohesive root zone when trafficked. The high soil water content treatment resulted in less than ideal surface hardness values, but not unstable conditions. Soil bulk density increased six percent after 50 traffic events, which was accompanied by a six percent decrease in air-filled porosity. Results suggest that shear strength values were not affected by soil water content in the sand rootzone, but by the loss of green turfgrass cover due to traffic. No optimum soil water content range was identified of those tested in this study for the sand rootzones.

In this study 50% was selected as the worst case for low input athletic fields (i.e., parks, recreation, etc.). The authors are aware that higher green turfgrass cover levels could be the minimum acceptable limit for professional athletic fields. Also, the soil water content ranges determined as optimal are not for all root zones, these are only for the listed soils described above. Slight changes in the composition of sand, silt, and clay in addition to sand particle size could greatly change the optimum ranges for those soils.

Conclusions

Results from this research indicated that hybrid bermudagrass established on a silt loam soil performs best when soil water content ranges were in the low and medium range. These results of the optimal range for silt loam soils correspond to plant available water and potentially explain the superior results. The high soil water content treatment lost cover at a rate four times faster than the low and medium soil water content treatments. The high soil water content treatment decreased turfgrass stability and negatively impacted field performance because of the saturated soil conditions. Soil water content treatments minimal impact on hybrid bermudagrass traffic green turfgrass cover loss on sand root zones with few differences detected among field performance characteristics or soil physical properties. Our results indicate that low to medium soil water content provides optimum field performance for hybrid bermudagrass on silt loam rootzones, while no optimum range was identified in sand rootzones.

Kyley Dickson, PhD, is a turf researcher at the University of Tennessee; John Sorochan, PhD, is a professor turfgrass science at UT and director of the University’s Center for Athletic Field Safety.

The photo included here is being used for art purposes only, we do not think this field is hybrid bermudagrass.

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