Cullen, Steve; Montagne, Cliff. 1981. Final report to the United States Forest Service (Region 1): Soil compaction study.
Eighteen forest soil pedons (boralfs and ochrepts) in the Bitterroot, Flathead and Kootenai National Forests were characterized and 54 pedons evaluated for the existence of soil compaction. The soils were examined over a range of parent materials: Tertiary Volcanics, limestone dominated glacial till and quartzite dominated glacial till. Characterization data included the amorphous character of surface horizons, texture, Proctor test, Atterberg limits, clay mineralogy, particle size distribution, bulk density, porosity, water holding characteristics, organic matter content, organic carbon content, pH, electrical conductivity, exchangeable cations, cation exchange capacity and calcium carbonate content.
Soil compaction was evaluated in timber harvest units ranging from 3 to 17 years old. Three treatment classes of soil compaction (control, moderate and severe) were established based on evidence of surficial disturbance and the presence of vegetation. Infiltration, bulk density and soil moisture retention data were collected and analyzed by two-way analysis of variance to determine if compaction had occurred as a result of treatment. Additionally, subsites were grouped by age as "young" (0-4 years since harvest), "medium" (5-9 years since harvest) and ''old" (10-17 years since harvest). The data were analyzed to determine possible amelioratory effects of time on compaction.
Amorphous character of the surface deposits was strongly expressed in the Flathead and Kootenai National Forests of northwest Montana, while the surface deposits of the Bitterroot National Forest soils in the southwest were not amorphous. Three independent measurements showed that significant changes in the physical properties of the study soils had taken place with treatment. Significant reductions in infiltration and significant increases in bulk density of the surface horizon occurred at all three study sites. Significant reductions in soil water retention at three water potentials occurred in the surface horizon of the limestone and quartzite glacial till sites. With three exceptions, no significant changes occurred in the physical properties of the study subsoils. No significant changes attributable to age grouping were detected in infiltration, soil water retention or bulk density.
Compaction occurred at all three study sites examined. Although the compaction was found only in the surface horizon, the author feels that the effects of the compaction process are expressed throughout the profile in the form of reduced water and gas movement.