Comparison of TruFirm Turf Firmness Meter and Clegg impact soil tester measurements on athletic fields

Abstract The Clegg impact soil tester (CIST) and TruFirm Turf Firmness Meter (TruFirm) are devices that measure turf surface hardness. Correlations between the devices have been limited to putting greens with little attention paid to higher mowed athletic fields. The objectives of this study were to determine the ability to predict CIST values from TruFirm measurements, with and without considering soil volumetric water content (VWC), and to evaluate how traffic influences the predictive relationship between the devices. TruFirm, CIST, VWC, and Normalized Difference Vegetation Index (NDVI) measurements were obtained from bermudagrass [ Cynodon dactylon (L.) Pers × Cynodon transvaalensis Burtt Davy] research plots with and without the use of a Baldtree traffic simulator to apply traffic and from bermudagrass soccer fields receiving heavy play in 2 of the 3 years the study was conducted. Cultivation practices were imposed across all sites with the turfgrass overseeded with intermediate ryegrass ( Lolium × hybridum Hausskn.) in October each year. Poor fits were obtained from trafficked research plots ( r 2 = 0.26, p < 0.0001) and athletic fields ( r 2 = 0.19, p < 0.0001) when predicting CIST from TruFirm measurements. Adding VWC as a second variable raised the coefficient of determination in trafficked research plots ( R 2 = 0.46, p < 0.0001) and athletic fields ( R 2 = 0.41, p < 0.0001), with both relationships continuing to have a high degree of uncertainty in predicting CIST. NDVI was significantly higher for the trafficked research plots than for the non‐trafficked research plots and athletic fields. On the non‐trafficked research plots, the simple linear relationship between CIST and TruFirm was not significant ( p = 0.1782), which may be due to CIST and TruFirm having differential responses to changes in turf canopy properties. This may be due to the shape and mass of the objects used to measure hardness and firmness in the two devices.