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Using Subsoiling To Reduce Soil Compaction

Testing for Compacted Areas

The best time to check for compacted layers is when the soil is saturated. Several areas should be tested, especially areas such as logging decks or skid trails known to have been compacted by heavy equipment.

To test for compacted layers:

  • Dig a test hole or ditch 24 to 30 inches deep in ground that is uniformly saturated.

  • Probe down a face of the ditch's vertical walls with a nail or knife blade (figure 9). A small hand-held soil penetrometer can also be used to probe the face of the wall. If a backhoe or excavator is used to dig the ditch, probe the sides of the ditch, not the ends. Resistance to penetration indicates a compacted layer. If soil is compacted, you should be able to dig away the looser soils and leave a well-defined compacted layer.

  • Note the depth and thickness of the compacted layer so the information can be included in contract specifications defining the subsoiling depth.

Soil penetrometers, designed to test soil strengths when a rod is pushed into the ground, can be used in some applications to identify compacted layers and how well subsoiling operations fracture the soil (figure 10).

Soil penetrometers are most accurate when soils are at their full moisture capacity. Penetrometers are difficult to use accurately in rocky soils or soils with a lot of roots, because it can be difficult to tell the difference when the penetrometer hits rocks or roots or a compacted layer.

The Missoula Technology and Development Center prepared a report on three models of penetrometers and their use. It is available at http://www.fs.fed.us/t-d//pubs/htmlpubs/htm05242837/ (Username: t-d, Password: t-d).

Drawing illustrating how to test for compacted soil. In the illustration there is a knife that has dug a hole in the ground to the compacted layer and there are three nails in the soil each in the three layers of soil. Text included in image reads: Compacted layer.
Figure 9—To test for compacted soil, dig a test hole or ditch 24 to 30 inches
deep. Push a nail or knife blade into the soil along a face of the ditch walls.
Resistance to penetration indicates a compacted layer.

Photo of a man using a soil penetrometer to test the soil.
Figure 10—Soil penetrometers can be useful when determining whether soil
layers have been compacted or when checking to see how well areas have
been fractured by subsoiling. Penetrometers don't work well in areas with
lots of rocks or roots.

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