Dr. Grady Miller from the July issue of SportsTurf:
Q: I see reference to field hardness in articles and was wondering how that is measured. Would it be practical for me to measure our fields to see if our aerification program is working?
A: The concept of measuring field hardness has been around for a number of years now. There are a number of reasons that measuring a field’s hardness is a good idea. It can give some indications of safety, field performance, and potential for growing healthy turfgrass. I will write my response considering natural grass fields since you mentioned aerification. But synthetic fields are also evaluated for hardness, just using some different equipment and techniques.
There is at least one concept that may be helpful to understand before answering your question. That is the difference in soil compaction and surface hardness. Soil compaction influences surface hardness but can also influence root health of the plant. Compaction removes air space in the soil profile and pore volume for available for water. So a compacted soil holds less water, less space for root growth, and can increase surface hardness. Surface hardness is related to the point of contact of the athlete but may not directly relate to soil conditions that influence plant health.
The traditional soil science method for measuring compaction was to measure the soil’s bulk density. This is nothing more than a measure of dry soil weight per unit volume. There are some standard methods of accomplishing this measurement and data exist to compare bulk density and reduction of turfgrass root growth. This measurement method is slow and destructive and may not give you relative data on surface hardness.
A second method that is sometimes used is to measure resistance of pushing a metal rod with a cone shaped tip into a soil profile. The device, called a penetrometer, measures pressure (PSI) as the device is pushed into the profile. The greater the pressure, the more compacted the soil. One can measure compaction at depth intervals so it can be very useful in evaluating rooting and moisture potential throughout the soil profile. The device comes in several different versions, but the best ones are pretty pricey (approximately $1,000).
Anyone can use the “pentrometer concept” to get a relative indication of their field’s compaction. All one had to do is take a full-size Philips screwdriver and push it into the soil profile while noting the resistance. If the shank is easily buried to the handle, your field is not excessively hard. If you cannot get it into the profile below the thatch layer, then your field is compacted and the surface is probably too hard. It is important to note that the soil moisture can influence compaction and hardness.
The most common technology-based device used to measure field hardness is a Clegg Impact Tester. Unlike a penetrometer, a Clegg Soil tester is only measuring a surface parameter. A 2.25-kg (about 5 pounds) missile is dropped from a specific height above the turfgrass surface. An accelerometer measures how fast the missile stops after it comes in contact with the surface. This value in gravities has been related to surface hardness—the higher the numerical value, the harder the surface. Years ago I did research with college and professional soccer athletes and found they could not tell the difference between 90 and 120 gravities using the older four-drop Clegg measurement. The NFL has a cut-off of 100 gravities (one-drop Clegg measurement).
The Clegg is non-destructive so one can take as multiple readings without damaging the field. It is super easy to use and provides instantaneous readings. There are really only two negatives. Soil moisture has been shown to dramatically influence measured values. It is best to measure your fields near field capacity (well-watered), unless you really want to know how hard your dry fields have become since the last rainfall or irrigation. Researchers often take a soil moisture reading (using a TDR probe or similar instrument) when they take surface hardness values so they can reasonably compare field hardness over time. The other negative is cost of the Clegg instrument. A new 2.25-kg Clegg Impact Soil Tester will set you back about $4500. That is substantially more than a screwdriver. University turfgrass scientists, field builders, and turfgrass consultants often have Clegg testers.
Getting a feel for your field’s compaction/hardness under various conditions and in different areas (when wet or dry, beginning of season, before and after an aerification, goal mouths, between hash marks, etc.) is a great management tool. Over time you will come to better understand if your field just needs a bit more water (to soften it) or requires corrective aerification to reach a desired hardness.