Grady Miller from the July issue of SportsTurf:
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?
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.
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.
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.
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).
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.
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).
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.
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.