Soil Compaction, managing it is one of a turf managers top tasks. Everyone knows that potential for soil compaction increases as the soil moisture increases. But have you ever seen numbers put to it? I hadn’t until recently. It turns out that the folks who build roads and walls think about this all the time. They have a test called the proctor density test to determine the optimum water content of a soil to maximize compatibility.
Soil Compaction Test
Obviously you don’t want to build a road on a foundation that is likely to settle. Therefore, they take soil samples and determine the moisture content where compaction will be maximized, then adjust the moisture content of the soil in the field to near optimum before compacting.
I thought that this information might be useful for helping to communicate to your stakeholders about the importance of preventing compaction on your turf.
[Tweet “Once the soil drops to 80% of potential saturation, rutting will become less evident but soil compaction will be maximized.”]
In a USDA research study, Aragon et al. (2000) took 30 different soils ranging in clay content from 7 to 27 and sand content from 25 to 76 and figured out the moisture content where compaction was maximized. While optimum water content for compaction depends on texture and organic matter, they found that the average soil (which had a sandy loam texture) had maximum compatibility around 24% volumetric moisture content. More universally, they reported that compaction was maximized for all soils when they were 80% saturated. For reference, field capacity – which is the moisture content after drainage ceases following a saturating rain – is around 50% saturation for most soils.
Using percent moisture saturation data to restrict cart traffic
Conflicts on when to allow cart traffic constantly arise between the pro and golf course superintendent. We now can explain that following a major rain event, when the soil is between 80 – 100% saturated that rutting will be common. Once the soil drops to 80% of potential saturation, rutting will become less evident but soil compaction will be maximized. As saturation drops from 80 to 50% (typically over a 48 hour period), compaction potential recede. We still don’t have an exact number for when to allow carts, but at least we can start to use these numbers to explain the basis for decisions related to compaction management.
Calculating percent moisture saturation
You can figure out the percent moisture saturation of your soil by measuring the soil moisture at saturation in a number of places on your property (you need to wait for a nice long rain), then divide any future moisture measurement by that number. For example, say we have a silt loam soil that has a 50% moisture content at saturation. The next day we measure the moisture in our fairways to be an average of 40% – 40 divided by 50 is 0.8 or 80%. You’d like to see a lower moisture level before allowing carts on areas with that level of moisture or above. If you have several different soil types, you may have several different moisture levels at saturation for example, a sandy loam soil may have a water content of 40% at saturation, so you’d be looking for moisture values below 32%.
There is still a lot to learn in this area, so the Wisconsin Golf Course Superintendents Association has recently funded a study specifically to examine the compaction potential for various soils under golf cart traffic. We hope that that data along with the information above will get us closer to using research-based metrics for making sound decisions on when to allow carts on the course – with obvious transferability to athletic field management.