In recent years, an uprising has emerged concerning United States Golf Associations recommendations as they related to the physical properties of a root zone mix. The situation has also brought accredited physical soil testing laboratories into the fray as well. It appears that all of the commotion is based around the recommendations as they relate to infiltration rates. What we are now beginning to hear is that one should not be concerned with infiltration rates so much; we should concentrate on pore space balance and total porosity of the root zone material. I agree, but after reviewing hundreds of physical analysis of root zone mixes I can with all certainty state that the two issues are directly related. When analyzing laboratory data, one cannot ignore infiltration rates and rely on porosity percentages. The fact is that as the infiltration rate fluctuates so does non-capillary (Air) porosity and capillary (Water) porosity. Let’s look at Table 1 for an actual example of how the differences in laboratory reporting, due to variation in procedures, can produced variations of infiltration rates and how these variations affect porosity.

It is not surprising that the two labs see this variance of gradation from the same sand sample. However, based on the gradation of the sand, (particle size distribution) and shape, it should not be difficult for an experienced practioner to determine which of the following reports is accurate. Let’s take a look at the differences in the physical performance reporting in Table 2.

There is a huge variance in the reporting of the sand when comparing laboratory #1 to laboratory #2. According to laboratory #1, the infiltration rate is reported at 29.7 inches/hour. The non-capillary porosity is reported at 28.6%, the capillary porosity is reported at 14.1%, total porosity is reported at 42.7% and water retention is reported at 9.3%.

The sand is of a medium-fine gradation and it is sub-angular in shape. It is difficult to imagine that this material would have a saturated hyudraulic conductivitiy rate of 50.5 inches per hour as reported by laboratory number #2. This is the reason that most everyone in the industry has lost faith in infiltration rates as reported by physical laboratories. The non-capillary porosity is reported at 25%, the capillary porosity is reported at 9.0%, total porosity is reported at 34% and water retention at field capacity is reported at 7.9%.
It is apparent that the variation in the reporting of saturated hydraulic conductivity has an affect on the reporting of pore space balance. The degree of variation is very significant. Generally speaking, the higher the infiltration rate, the higher the percentage of non-capillary (Air) porosity and the lower the percentage of capillary (Water) porosity is displayed. The reverse is true when infiltration rates are lower.
This article is not intended to rehash problems of testing procedures. It is written to illustrate a point. In some situations this disparity in reporting does not occur to this illustrated degree. In some instances, however, It does occur. The fact of the matter is that experienced people can deduce what laboratory results are accurate and those that are not. It is up to each and everyone one of us in the industry to have educated people in the field of physical soil science to make decisions based on the interpretation of laboratory data.