How to Test Soil Compaction

Compacted garden soils are usually the result of too much foot traffic.

Compacted soils impact the ability of water and air to move through the earth's particles. Compaction rates also affect the stability of the soil, and are therefore important to know when you are building on a site. Overly compacted soils can be difficult to grow plants in while soils with low compaction rates may cause undermining and in some cases the failure of a building's foundation. Compacted soils can be fixed with compost and organic amendment as well as tillage and aeration. A simple soil test for compaction can be done by squeezing a handful of soil from a foot or two down in your fist. Extremely compacted soils will allow you to mold the earth, while soils with good tilth will not conform tightly. However, a more accurate reading requires more sophisticated tools.

Things You'll Need

• Compaction testing rod
• Digital reader rod
• Steel railroad spike or pin
• Hammer

Instructions

1. Push a manual compaction rod into the soil in early spring. This is the best time of year because soil moisture content is high and you will get a better reading. Insert the tester 36 to 48 inches deep with even, firm pressure. The rod should penetrate evenly in several test locations with minimal effort if soils are not compacted. Compacted soils will resist penetration.

2. Use a digital reader rod for a professional level of assessment. This tool records the depth and pressure used to insert the rod at each test location. The recorded depth in high pressure areas indicates the level where soil is compacted so you will know how deep to dig to begin amending the soil.

3. Hammer a steel railroad spike or pin into the soil for a less scientific but still useful assessment. In heavy clay soils that are near the surface or in hard pan, the spike will be difficult to drive in. Soils with good compaction will allow you to push the spike in manually. This is the very simplest test but useful for the home gardener who is planting shallow-rooted plants.

Tips & Warnings

• Rent a compaction rod rather than purchasing it unless you are going to be using it frequently. You can get a manual rod or a digital reader rod. Both give you an assessment of the soil compaction but in different levels of reading.

• Amend your soil as needed with compost, organic matter such as leaf litter and small amounts of grit such as sand. Use a tiller to work 5 to 7 inches of compost and 2 to 4 inches of grit into at least 2 feet of soil.

References

• Gemplers: Testing Soil Compaction
• Giles Engineering Associates; Soil Compaction and Stability; Charles S. Gresser P.E.
• Colorado State University Extension; Soil Compaction; David Whiting, Adrian Card, Carl Wilson, Jean Reeder Ph. D.