What putting green firmness measurements actually tell us

Measurement devices used in this research. From left to right: TruFirm, Clegg and TDR350 with Turf Rod Spacers at 0.5-, 1.0- and 1.5-inch depths. (Photo: Daniel O’Brien)

Firmness is an important attribute of championship greens, affecting both playability and durability of the putting surface. However, firmness can be interpreted and managed in various ways. Underlying factors often associated with firmness include volumetric water content (VWC), organic matter (OM) and bulk density (BD).

Consequently, maintenance practices can include irrigation, aerification, verticutting, topdressing and rolling. By understanding the extent to which firmness measurements reflect each of these underlying factors, superintendents can improve the efficiency and precision with which they manage firmness. The objective of this research was to compare device measurements and ground-truth data for VWC, OM and BD.

This research was conducted in Fayetteville, Ark., on a USGA sand-based, Penn A1 creeping bentgrass putting green (Agrostis stolonifera L.). We used combinations of irrigation, rolling and cultural management practices to create individual plots. Surface firmness was measured using the Clegg Impact Soil Tester (Lafayette Instruments) and FieldScout TruFirm Turf Firmness Meter (Spectrum Technologies). We used the FieldScout TDR350 Moisture Meter to measure VWC at 0.5-, 1.0- and 1.5-inch depths (Spectrum Technologies). We extracted cup-cutter samples so that we could calculate ground-truth values for VWC, OM and BD by weight and thus compare to measurements from each device.

We found that while both Clegg and TruFirm detected differences in VWC, neither did so with the level of precision of the TDR350. For OM and BD, all devices demonstrated a much better ability to detect differences from rolling than differences from aerification, verticutting and topdressing. Based on these results, superintendents should not consider firmness data redundant of, nor a substitute for, TDR measurements. The next phase of this research will include ball-bounce and ball-mark-severity data, so firmness measurement can be better interpreted in terms of playability.

Daniel O’Brien is a research technician, and Doug Karcher, Ph.D., and Mike Richardson, Ph.D., are turfgrass scientists at the University of Arkansas. You may reach Daniel O’Brien at dpo001@uark.edu for more information.