ANDIC SOILS OF THE INLAND PACIFIC NORTHWEST, USA: PROPERTIES AND ECOLOGICAL SIGNIFICANCE

Abstract

Holocene tephra from the cataclysmic eruption of Mount Mazama in southwestern Oregon is an important component of many soils east of Cascade Mountains in the Pacific Northwest region. This article examines ecologically important properties of these tephra-influenced soils. Characterization data from 886 soil horizons (384 pedons) of the region meeting criteria for andic soil properties or subgroup classification as andic or vitrandic intergrades in Soil Taxonomy were used to compare morphological, chemical, and physical properties. Mazama tephra typically comprises a significant portion of a surface mantle with textures ranging from silt loam in areas distal to the tephra source to very gravelly loamy coarse sand in proximal locations. Tephra has been mixed to varying degrees with other parent materials across the region. On average, volcanic glass only comprises 31.1% of the 0.05 to 2-mm fraction and has a bimodal distribution, suggesting that some tephra was deposited with significant quantities of dust or, that since deposition, considerable reworking and mixing of glass has occurred. Oxalate-extractable Al ranges from 0.04 to 5.4% and provides the best indicator of colloidal surface activity as indicated by P retention. Cation exchange capacity (CEC) determined at field pH (ECEC) averages 7.8 cmol c kg -1 and is less than one-third the CEC determined at pH 8.2, indicating considerable variable charge and relatively low capacity of these soils to store and supply nutrient cations. Mineralogical data suggest that a reduced pool of secondary Al may be limiting allophane formation in soil horizons with less tephra influence. Volumetric water-holding capacity of ash-influenced horizons is as much as twice that of underlying horizons and underscores the importance of tephra in seasonally dry, forested ecosystems of the Inland Northwest region.