Compacted layers are typically 12 to 22 inches deep. Ideally, the shank's tip should run 1 to 2 inches below the compacted soil layer. If the shank's tip is too deep, subsoiling may increase compaction because the compacted layer will not be fractured.

Shank spacing will vary depending on soil moisture, soil type, degree of compaction, and the depth of the compacted layer. Spacing should be adjustable so the worked area can be fractured most efficiently (figure 8). Horsepower requirements depend on soil moisture, the depth and thickness of the compacted layer, and to a lesser extent, the soil type. Each shank may require from 30 to 75 horsepower.

Equipment speed can affect subsoiling. Travel speed that is too high can cause excessive surface disturbance, bring subsoil materials to the surface, create furrows, and bury surface residues. Travel speed that is too slow may not lift and fracture the soil adequately. Contractors may prefer to travel more quickly to improve their profit per acre.

Figure 8—For most subsoiling operations, the shank tip should run 1 to 2 inches
below the compacted layer of soil. The spacing and depth of the shanks can
affect how completely and efficiently the soil is fractured.
Click here for a long description of this image.

Traffic Patterns

It is best to follow the ground contour whenever possible while subsoiling. This helps increase water capture, protect water quality, and reduce soil erosion, especially in burned areas or areas susceptible to erosion. Stay clear of waterways, ditches, and other areas where subsoiling could affect hydrology. Shanks should be lifted out of the ground frequently to clear stumps, rocks, and logs and to remove slash from the subsoiler.

It might be wise to consult your local silviculturist for advice on subsoiling next to trees and other established plants. Always be cautious of areas that might have buried utility lines, culverts, or diversion channels. Flag or mark such areas before subsoiling.

Tractor/Skidder Issues

The equipment used to pull subsoilers is heavy enough to create its own compaction problems. Make sure that the shanks on the subsoiler are spaced so that they run in the tracks of the tow vehicle. It may be best to specify smaller, lighter equipment to reduce ground pressure during subsoiling operations. Smaller, lighter equipment probably can't pull more than two to three shanks in most applications.

Excavator Subsoilers

Subsoiler attachments are available for excavators. They can be used in combination with buckets or rakes. A partial list of suppliers is included in the "Subsoiler Manufacturers" section.