Although penetrometers are very useful for certainapplications, the data show trends, but not exact measures of soil density or compaction. The penetrometer provides very accurate information on soil strength, or resistance to penetration, to some depth. However, strength measurements are affected by moisture, porosity, and rock content.

When combined with other information, such as soil moisture, structure, and texture, data from penetrometers can contribute to a more accurate picture of soil properties. Typically, penetrometer readings taken in moist soils are easier to acquire and give more meaningful information than readings taken in dry, hard-packed soils. Penetrometers are best suited for studying soil strengths (resistance to penetration) in comparison studies between areas with similar soil composition and moisture, or in one area where impacts change over time, such as in areas that are farmed or logged.

The agricultural industry has been a leader in using electronic penetrometers. Forest tree nurseries could benefit from information provided by a penetrometer, such as identification of areas of high compaction, plow pans, and clay zones to help determine appropriate irrigation, fertilization, and cultivation practices.

Soil compaction begins to inhibit the root growth of most plants when the soil's strength is about 1,500 kPa. The roots of many plants quit growing when the soil's strength reaches about 2,500 kPa. Penetrometers can help identify these areas faster and easier than standard bulk density tests. More definitive soil testing may be required, but the penetrometer can identify the problem areas.

MTDC initially investigated the use of hand-held electronic cone penetrometers to evaluate how well contractors were compacting the soil around bareroot seedlings planted in holes drilled by augers at the Boise National Forest in Idaho (figure 5). Planting contracts usually specify that each planting hole be filled and compacted by thirds rather than simply being filled and compacted. Roots, rocks, sticks, gopher holes, voids, and the highly compactable granitic soils in the Boise National Forest made it difficult to identify the compacted layers. We concluded that penetrometers are not practical for inspecting proper compaction during tree-planting contracts.

Assessing the impact of logging operations on forest soil compaction is important. Penetrometers may help soil scientists and contracting officers verify that specifications on soil compaction limits are being followed. Indepth Forest Service studies using hand-held electronic cone penetrometers during logging operations have been conducted by the Forestry Sciences Laboratory in Olympia, WA, and the Pacific Southwest Research Station in Redding, CA.

Figure 5—Penetrometer testing in the Boise National Forest near
Idaho City, ID.