Distribution and Properties of Podzolized Soils in the Northern Rocky Mountains

Core Ideas Podzolization is a widespread pedogenic process in the Northern Rocky Mountains. Podzolization is strongly influenced by terrain attributes and environmental gradients. Northern Rocky Mountain landscapes are dominated by Spodosols, spodic intergrades, and Andisols. Although not recognized on land system inventory maps, Spodosols have been documented in the volcanic ash–mantled landscapes of the Northern Rocky Mountains (Major Land Resource Area 43A). This study focuses on terrain attributes and environmental gradients as potential predictors of the distribution and properties of podzolized soils in the region. Seventy‐two forested sites were sampled using hierarchical randomization. Podzolization, as indicated by the presence of a spodic horizon, was observed at 43 sites. Soils meeting all or most taxonomic criteria for Spodosols were present at 26 sites. These Spodosols are concentrated above elevations of approximately 1100 m and on northerly aspects. In this mountainous region, more northerly aspects and increasing elevation serve as proxies for lower temperatures, increased effective precipitation, and greater snow cover, all of which promote podzolization. Spodosol E horizons are very strongly acidic, with pH values as low as 3.1 and Al saturation as high as 89% of the effective cation exchange capacity. Other soils exhibit evidence of podzolization as interpreted from subsurface increases in pyrophosphate‐ and oxalate‐extractable Fe and Al. However, most of these soils have darker and/or higher‐chroma colors in the surface mineral horizon and lack the albic–spodic morphology of Spodosols. Results indicate that podzolization is a widespread pedogenic process in the Northern Rocky Mountains and occurs under different climatic conditions, topographic settings, and parent materials than those of the upper Midwest and northeastern United States.