Physical and Chemical Characteristics of Ash-influenced Soils of Inland Northwest Forests

Abstract

Holocene ash from the cataclysmic eruption of Mount Mazama (now Crater Lake) in southeastern Oregon is a major component of many forest soils that lie to the east of Cascade Mountains in the Pacific Northwest region. The relatively high productivity of the region’s ecosystems is closely linked to this volcanic ash component. This paper reports on the ecologically important properties of these ash-influenced soils by examining soil characterization data from over 500 soil horizons of the region contained in the National Soil Survey database. Volcanic ash-cap textures are generally silty in areas distal to the eruption. Volumetric water-holding capacity of ash-cap horizons is as much as twice that of underlying horizons and underscores the importance of ash caps in seasonally dry, forested ecosystems of the Inland Northwest region. Cation exchange capacity (CEC) determined at field pH (ECEC) averages 8.0 cmolc kg–1 and is less than one-third the CEC determined at pH 8.2, indicating considerable variable charge. These data suggest that ash-influenced soils have limited ability to store and exchange nutrient cations such as Ca, Mg, and K. In addition, strong sorption of SO4 and PO4 significantly limits the bioavailability of these nutrients. Erosion and compaction of ash caps are major management concerns, but relationships between these processes and ash-cap performance remain unclear.

Parent Publication

• Volcanic-ash-derived forest soils of the inland Northwest: Properties and implications for management and restoration

Citation

McDaniel, P. A.; Wilson, M. A. 2007. Physical and Chemical Characteristics of Ash-influenced Soils of Inland Northwest Forests. In: Page-Dumroese, Deborah; Miller, Richard; Mital, Jim; McDaniel, Paul; Miller, Dan, tech. eds. 2007. Volcanic-Ash-Derived Forest Soils of the Inland Northwest: Properties and Implications for Management and Restoration. 9-10 November 2005; Coeur d’Alene, ID. Proceedings RMRS-P-44; Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station. p. 31-45