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FOREST, RANGE & WILDLAND SOILS

Maximum Bulk Density of British Columbia Forest Soils from the Proctor Test: Relationships with Selected Physical and Chemical Properties

^a Faculty of Forestry, Univ. of British Columbia, Vancouver, BC V6T 1Z4, Canada
^b British Columbia Ministry of Forests and Range, Research Branch, Vernon, BC V1B 2C7, Canada
^c Dep. of Geography, Simon Fraser Univ., Burnaby, BC V5A 1S6, Canada

^* Corresponding author (krzic{at}interchange.ubc.ca).

The widespread use of heavy equipment during timber harvesting and site preparation can lead to reduced soil productivity and warrants development of new methods to assess compaction. We evaluated the effects of soil particle density, organic matter, particle size distribution, extractable oxides, and plastic and liquid limits on the maximum bulk density (MBD) of forest soils in British Columbia. Soil samples were collected from 33 sites throughout British Columbia, covering the major forest and soil types of the province. The standard Proctor test was used to determine MBD and related parameters, including the gravimetric water content (W_MBD) and porosity (f_MBD) at which MBD was achieved. The significance levels of single soil properties in predicting MBD were in the order plastic and liquid limits, organic matter, oxalate-extractable oxides, and particle size distribution. For all samples, liquid limit and clay were most closely related to MBD (R² = 0.83). Addition of organic matter to the model increased the regression coefficients, and oxidizable organic matter caused a greater increase than did total C. Stratification of the sample set into groups based on plasticity led to higher R² values in multiple regressions, and different soil properties were important for nonplastic soils than for those with high, moderate, and low plasticity. Prediction with multiple regression explained the most variation in MBD for nonplastic soils, while properties of highly plastic soils explained the least variation in MBD and moderately plastic soils were intermediate. Based on our findings, we propose an approach for using MBD to help better interpret bulk density data in forest soil compaction studies.

Abbreviations: BWBS, Boreal White and Black Spruce biogeoclimatic zone • CDF, Coastal Douglas-fir biogeoclimatic zone • CWH, Coastal Western Hemlock biogeoclimatic zone • f, porosity • f_MBD, porosity at MBD • ICH, Interior Cedar–Hemlock biogeoclimatic zone • IDF, Interior Douglas-fir biogeoclimatic zone • LTSP, Long-Term Soil Productivity Study • MBD, maximum bulk density • PCA, principal component analysis • SBS, Sub-Boreal Spruce biogeoclimatic zone • gravimetric water content at which MBD was achieved