Soil testing by a physical testing laboratory can provide a wealth of information. Testing materials prior to their use in construction can help to ensure that quality materials are used and design specifications are met.  Quality control testing during construction can provide assurance that consistent materials are being used, and provide reasonable expectations for consistent performance.

Unfortunately the testing reports are not always the easiest documents to read and understand.  This newsletter is intended to provide information on one of the most misunderstood testing procedures, the Physical Performance Evaluation.

Physical Performance Evaluation
The physical performance evaluation (PE) is tailored to design agronomically acceptable rootzones for sand based systems, or to evaluate an existing rootzone material.  This evaluation provides information concerning the saturated hydraulic conductivity (which is often referred to as K-SAT, perc rate, or infiltration rate), bulk density, particle density, and porosity characteristics of a proposed rootzone material.  The methods used for determining the parameters are based on standard agronomic test methods and the USGA (United States Golf Association) protocol.  Testing involves compaction of a sample at field capacity, followed by determination of the aforementioned parameters.  The K-SAT and capillary porosity values are very important parameters that aid in this assessment.

Infiltration Rates are determined using a constant head saturated hydraulic conductivity.  The desired infiltration rate on any given project is dependent on a variety of factors including, usage, typical rainfall and evaporation conditions, and water quality.  The USGA recommends that infiltration rates greater than 6 inches/hour are acceptable, while other construction methods suggest higher or lower rates are desirable.  Regardless of the desired rate, care should be utilized when evaluating this result.  Reproducibility of results within a single laboratory is typically around +/- 3 in/hr.  Results from different labs can show a high amount of variability.

The Porosity data is broken up in to three sub-categories:  total porosity (the amount of space between the soil particles), capillary porosity (the pore space that is water-filled) and non-capillary porosity (the pore space that is air-filled).  It is desirable for a rootzone to have approximately half of its volume solid and half pore space.  The amounts of air-filled and water-filled pore spaces ought to be present in roughly equal amounts.  This should provide advantageous conditions for root growth, proper oxygen levels, and good mineral/water holding.

Bulk Density is a measurement of the mass of the bulk rootzone material per unit volume.  Most turf systems will have a bulk density between 1.4 and 1.6 grams/cubic centimeter.  Higher values indicate the turf system may be too hard, and lower values indicate the turf system may be too soft.

Particle Density is a measurement of the mass of the individual rootzone particles per unit volume.  The primary use of the particle density is in calculation of the porosity values.  Caution should be used to avoid confusing the particle density with the bulk density.  Bulk density is a measurement of the particles plus the air space between them, while the particle density excludes the air.  Particle density for sand is around 2.6 g/cc.

Organic Matter (OM) is reported on a dry weight basis in order to provide the most accurate assessment of the rootzone.  Soils and rootzone mixes used in sand-based golf and sports turf usually contain less than 2% OM by dry weight.
Sand-based rootzone mixes that are prepared in the field are often referred to by volume of organic matter.  A direct conversion from weight to volume depends on the materials being used and the amount of moisture present.  For example, a 90/10 sand/sphagnum peat blend is approximately 0.3 to 0.6% OM by dry weight, while a 90/10 sand/reed sedge peat mix is usually between 0.5 to 0.8% OM.

A Couple Thoughts
The next time you look at laboratory data, try not to concentrate on only one test parameter or test value.  It is always better to have a range of acceptable results rather than a single value.  Laboratory data should be thought of as a risk assessment tool.  Be sure to look at the entire report, as well as any comments the lab may have made, when evaluating and selecting turf and rootzone products.

To read this article, our other recent publications in national magazines, and our past newsletters, check out the turf library section of our web site www.turfdiag.com.

Turf Diagnostics and Design serves the golf course, sports field, and landscape industries.  We have performed testing and/or consulting on over 1,000 golf courses and hundreds of sports field and landscape projects.  Our services have been instrumental in ensuring that our clients utilize quality building materials and maintain healthy turf systems.

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