Pedogenesis on Outwash and Glacial Marine Drift, Northwestern Washington

Abstract

Spodosol, Alfisol, and Andisol classification in parts of the northwest USA is difficult because many soils have properties characteristic of more than one soil order. Four soils in the Puget Lowland of western Washington, with properties overlapping two or more orders, were studied to suggest guidelines by which similar soils may be classified and mapped. Edmonds (coarse-loamy over sandy or sandy-skeletal, mixed, mesic, ortstein Typic Sideraquod) and Tromp (sandy, mixed, mesic Aquic Haplorthod) soils are on glacial outwash, and Labounty (fine-loamy, mixed, mesic Typic Umbraqualf) and Whatcom (fine-loamy, mixed, mesic Aqualfic Haplorthod) are on glacial marine drift. All are capped by a tephra-containing loamy eolian mantle. Methods were selected to identify properties of Andisols, Spodosols, and Alfisols. The B horizons in the eolian mantle in Edmonds, Tromp, and Whatcom soils qualify morphologically as spodic horizons. These three soils then are Spodosols even though they do not qualify chemically. Spodosol morphology in the Labounty B horizon is weaker and it does not qualify chemically. In the Whatcom and Labounty soils, illuviation argillans and papules are in paleosols buried by the eolian mantle. These palesols are believed to have formed during the warm-dry period after isostatic rebound of the area from the sea and prior to deposition of the eolian mantle. Presence of an argillic horizon within 50 cm, and absence of the other horizons meeting andic or spodic criteria, places the Labounty soil as an Alfisol. In the Puget Sound area we find (i) sandy and coarse-loamy soils are more likely to have spodic horizons than are fine-loamy soils, (ii) soils in depressional areas with fine-textured underlying strata, or with buried paleosols, are less likely to have spodic horizons than soils on micro rises in similar landscapes, and (iii) more soils meet the morphological criteria for Spodosols than meet chemical criteria.