The widespread use of heavy machinery during harvesting and site preparation in timber plantations in British Columbia (BC) has led to concerns that compaction causes a reduction in long-term soil productivity. Compactability is the quantitative expression of the compactive response of a soil within specific boundary conditions to the application of a specified stress regime. An assessment of soil compactability is needed to establish effects of forestry operations, such as timber harvesting and site preparation, on soil compaction and consequently tree growth. Evaluation of effects of soil properties that are routinely measured in the laboratory (e.g., organic matter and texture) might lead to a better understanding of soil compactability. Establishment of a procedure that identifies soils most likely to be at risk for excessive compaction, and development of thresholds for soil compaction effects on tree growth, is needed for more reliable assessment of current forestry practices in BC and their impacts on site productivity.

The objective of this study was to determine relationships in representative forest soils in BC, among total carbon, water content, and particle size distribution with the compactability estimated using a standardized Proctor test. Two compactability indices were used: maximum bulk density (MBD) and susceptibility to compaction (SC) determined by the standard Proctor test.

Soil samples were collected from 16 sites within the Boreal White and Black Spruce (BWBS), Sub-Boreal Spruce (SBS), Interior Douglas-fir (IDF), and Interior Cedar-Hemlock (ICH) biogeoclimatic zones of BC. Typical soils included in this study were mainly developed on glacial till, with the exception of a Brunisol (Cambisol) in the ICH developed on colluvium and Luvisol in the SBS developed on lacustrine parent material. Soils varied in texture (12 to 87% sand, 9 to 76% silt, and 2 to 53% clay) and organic matter content (18 to 76 g kg^-1 total C).

A strong negative correlation (r²=0.90) was observed between MBD and gravimetric water content at which MBD was achieved (W_MBD) and between MBD and total C (r²=0.70). Similarly, WMBD and total C had strong effects on SC. The estimation of either MBD or SC values was not substantially improved by including texture parameters to the regression equations in addition to the total C. Organic matter, which has many profound impacts on soil physical, chemical, and biological properties, was found to have a strong impact on reducing compactability of forest soils in BC. This provides important information for soil sensitivity ranking, and also justifies efforts of achieving higher organic matter contents in forest soils, particularly those that have experienced severe mechanical disturbance.

The implication of the relationships observed in this study is that increases in soil organic matter reduce the risk of compactability, which is particularly important for forest soils where compaction is generally not corrected by implements after tree planting. The information is also useful for assessing the extent of compaction on soils affected by machine traffic.