A series of air permeability tests were conducted on four hand‐packed samples of alluvial sands and glass beads using a newly developed air permeameter. The permeameter was tested and found capable of precisely controlling soil‐water matric potential (in the range 0 to 1 bar) while simultaneously facilitating the direct measurement of air permeability in porous media. Permeameter results indicate that air permeability increases with a corresponding decrease in water content over a monotonic drainage cycle. It was observed that the rate of change in air permeability with respect to changes in water content is highest at high water content and lowest at low water content. In several soil samples, the air permeability approached a constant value at low water content. Air permeability variations with water content were observed to differ among soils of different texture. For example, the intrinsic permeability of water was 11 to 86% of the maximum air permeability. The new permeameter allowed rapid and accurate measurements of air permeability in fine‐textured materials over a wide range of matric potentials and water content.

Little evidence is documented regarding the effects of timber‐harvest traffic on volcanic‐ash‐mantled glacial till soils and clay‐rich Tertiary volcanic‐derived soils of northwestern Montana. We identified and characterized differences in the soil physical properties between trafficked and nontrafficked areas. Soil‐clod bulk density, infiltration, and soil‐clod water retention were measured on 54 pedons from nontrafficked and moderately and severely trafficked sites. Soil mechanical properties were characterized using 18 nontrafficked pedons. Compared with nontrafficked areas, bulk densities in severely trafficked areas at the 15‐cm depth were 76, 21, and 21% greater in ash over limestone till, ash over quartzite till, and Tertiary volcanic soils, respectively. Water retention at 0.002, 0.010, and 0.033 MPa was significantly lower at the 15‐cm depth in trafficked than in nontrafficked areas of ash over limestone till. Similar differences were observed in ash over quartzite till. Compared with nontrafficked areas, cumulative 1‐h infiltration in severely trafficked areas was 81% less in ash over limestone till, 79% less in ash over quartzite till, and 87% less in Tertiary volcanic soils. Similar but smaller differences were detected in moderately trafficked areas. Three independent measurements demonstrated significant differences in surface horizon physical properties between trafficked and nontrafficked areas. We detected significant physical manifestations of traffic‐induced soil compaction below 30 cm.

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