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Evaluation of Criteria for Separating Soils with Xeric and Udic Moisture Regimes¹

ABSTRACT

This study was designed to evaluate criteria for classifying soil series in the Coast Range Mountains of western Oregon into categories with xeric and udic moisture regimes. Relationships among soil-water suction, plant-water suction, and cambial activity of Douglas-fir (Pseudotsuga menziesii [Mirb.] Franco) were also evaluated. Soil-water content was measured with gypsum blocks. Plant-water suction of Douglas-fir branch tips was measured with a pressure bomb. These measurements were taken early in the morning at least once every 10 days. Experimentals sites were selected on three soil series and one north slope soil phase tentatively classified as xeric and on four soil series tentatively classified as udic. The four study sites in the xeric zone had soil-water suctions equal to or greater than 15 bars within the soil-water control section for at least 60 consecutive days. None of the four study sites in the udic zone were dry that long. Cumulative water deficit in the xeric zone was about 75% of the seasonal total when the entire soil-water control section attained 15 bars. Almost all available water was removed from the upper third of the profile in the xeric zone when the trees stopped producing springwood. Much less water was removed from the upper third of the profile in the udic zone. The same relationship held true for the available water in the entire profile at time of dormancy. Trees in the xeric zone produced springwood an average of 23 more days and have 26 more days total growing time than trees in the udic zone. Changes of soil and plant-water suction with time showed similar patterns but different magnitudes. It was concluded that plant-water suction cannot be used in lieu of soil-water suction for purposes of soil classification.

¹ Contribution from the Oregon Agr. Exp. Sta., Oregon State Univ., Corvallis, Oregon 97331 as Technical Paper no. 3535.

² Formerly NDEA Fellow in Soils, currently Soil Scientist, Bureau of Land Management, Portland, Oregon; and Professors of Soil Science, Oregon State University, respectively.

Received for publication March 5, 1973. Accepted for publication June 15, 1973.