Graduate Thesis Or Dissertation
 

Morphology, genesis, and classification of soils forming in recent age tephra deposits from Mt. St. Helens volcano

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  • The morphology, genesis, and classification of soils forming in multiple tephra deposits of recent age from Mt. St. Helens volcano in southwestern Washington Cascade Mountains was studied. Soils which occupied well drained and poorly drained positions on the landscape were characterized according to their morphology and the results of analyses of particle size, clay mineralogy, cation exchange capacity, exchangeable bases, organic carbon, total nitrogen, extractable iron and aluminum oxides, exchangeable acidity, pH, and bulk density. The results reveal that there are greater differences within the profiles than between soils themselves. The main difference was that the organic carbon contents were higher in the poorly drained soils than in the well drained ones, Cation exchange capacity tended to follow the pattern of organic matter content. Particle size results showed the dominance of sand size particles in these horizons. An interesting bimodal distribution of the sand size fractions is present in all soils examined. In soils dominated by amorphous gels the results obtained for the percent clay separation is of questionable value due to incomplete dispersion. Electron micrographs showed a higher degree of weathering in the buried A horizons of both paleosols. The x-ray diffraction patterns however do not reveal any significant difference between the clay mineralogy of each horizon. All horizons were dominated by amorphous constituents. The vegetation at each site is a better indicator of the internal moisture relations of these soils than are morphological properties. The well drained sites consisted of depauperate understories of Vaccinium membranceum and Xerophyllum tenax. The poorly drained soils typically had a much richer understory which consisted of species such as Vaccinium bValitolium, Menziesia ferruginea, Streptopus roseus, and Tiarella unifoliata to name a few. The classification of these soils was difficult due to inherited characteristics, buried soils, and the incompleteness of the soil classification system used in the United States on volcanic soils The dry sites were classified as-ashy over ashyskeletal, mixed Andeptic Cryorthents. The wet sites were tentatively classified as ashy over ashy-skeletal, mixed Andaqueptic Cryaquents. :Secondary classifications were also presented where these soils may have better fit the Inceptisol rather than the Entisol soil order. Deficiencies in Soil Taxonomy (Soil Survey Staff, 1975) occur in classifying these soils and brief discussion is included where these deficiencies occur.
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