|Abstract or Summary
- 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
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.