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Some Soil Characteristics Which Affect Root Penetration and Timber Site Quality of Douglas Fir in Western Washington¹

ABSTRACT

This study was initiated to determine the relationships between soil properties and site quality for Douglas fir in Western Washington. The ultimate objective is to predict timber site quality and desirable management practices directly from soil survey maps.

Forest production measurements and soil profile descriptions were obtained concurrently on 203 sites representing 14 soil types. Profile properties of five soil types are discussed in detail to show by comparison their relationship to rate of tree growth. The highest timber site quality was produced on sites with deep, friable soils of moderate to high fertility level. Progressively lower timber site quality was reflected on sites with increasing strong profile development or degree of horizonation. There is a close relationship between timber site quality and soil depth over claypan, fragipan, and hardpan horizons. Soils having similar profiles, though developed from different parent materials, have similar site qualities.

The data collected provide a sound basis for assigning average site indices to the different soil types included in this study.

Factors of slope, aspect, position and land form are more important in woodland regeneration, management practices, and logging operations. Most important to productive site quality is the combination of properties which determine soil type.

¹ Presented before Div. V-A, Soil Science Society of America, St. Paul, Minn., Nov. 11, 1954.

² Soil Scientist, Woodland Conservationist, and Soil Scientist, respectively, Soil Conservation Service, U.S.D.A. The authors express appreciation to Dr. R. W. Simonson and A. O. Ness for suggestions on manuscript preparation, and to Soil Scientists O. P. Mueller, W. A. Call, D. A. McGee, and C. J. McMurphy for help in collecting field data.

Received for publication October 14, 1954.