Root Penetration of Douglas-fir Seedlings into Compacted Soil

This experiment was designed to examine the relationship of soil compaction to root growth in one sandy loam and two loam soil materials. The experiment was conducted under controlled environment using test cores of experimentally compacted soils ranging in bulk density (BD) from 1.3 to 1.77 g/cm³. Root penetration of 35- to 45-day-old seedlings varied considerably among seedlings but generally declined linearly with increase in BD (r² for each soil varied from 0.60 to 0.71). Using the regression equations, the values for BD estimated to prevent root penetration by most seedlings, varied from 1.74 to 1.83 g/cm³. These values are higher than previously reported for Douglas-fir but generally compare to those reported for other plants in loam soils. The corresponding pore space at which rooting was prevented varied from 30 to 27 percent. When downward growth was restricted by high BD most roots grew laterally in the uncompacted surface soil to a greater total length than they grew vertically at the lowest BD level. Top growth of seedlings in this experiment was not significantly affected by BD. Root impedance in relation to effects of compaction and variability of root penetration among seedlings are discussed. Forest Sci. 27:660-666.