Graduate Thesis Or Dissertation
 

Mineralogical and chemical characteristics of western Oregon Andic soils

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  • A study was conducted to determine the mineralogy and chemical properties of "Ando-like" western Oregon soils. The samples were collected from 13 sites which formed mainly from basaltic rocks, a contact metamorphic rock, sedimentary rocks, and alluvium derived from sedimentary rock and from volcanic rock. All samples contained small amounts of dacitic glass shards. The largest amount was 1.6% on a whole-soil basis. On the basis of refractive indices and the mineralogy of phenocrypts, it was concluded that most of the dacitic glass originated from Mt. Mazama. Large percentages of other mineral grains, especially in the light fractions of soils formed from basalt, were altered and unidentifiable with the petrographic microscope. The sand-sized fractions contained 2:1 layer type clay minerals. Amorphous materials in the separated clay fractions as determined with 0.5 N NaOH dissolution treatments ranged from 18 to 64%. Gibbsite contents in clay samples, indicated by differential thermal analysis peak heights, ranged from none to moderate (about 18%) amounts. The predominate crystalline phyllosilicate clay mineral in the samples of 10 sites was chloritic intergrade (aluminous chlorite). Montmorillinite was the major component in two soils and biotite plus vermiculite were major components in one soil. Smectite clay was present in all samples but the specific type could not be determined in the majority of the samples. Chlorite was also present in all samples. Vermiculite was present in all sites except two. Mica was absent in the clay of only one soil but it was only a minor constituent in all soils except one. A kaolin mineral was present in the clays of three sites. In all other sites the presence or absence of kaolin was not established. All soils were strongly to very strongly acid (pH 5.3-3.8) and base saturations were low (1-30%), with 10% or less for the majority of the soils. Organic matter contents of surface horizons of soils from five sites ran over 30%. The lowest organic matter content of any surface horizon was 14%. Considerable aluminum was complexed by the humic fraction of the majority of the soils. In general, the fulvic acid content of the crude humate fraction increased with depth. Low bulk densities of most of the surface horizons (about 0.6-0.8 g/cc) was probably due to the large amounts of organic matter. The soils in the study have many of the same properties attributed to soils formed from volcanic ash but the mineralogy of the soils is vastly dissimilar. The soils were placed into the Inceptisol order under the U.S.D.A. Comprehensive Soil Classification System. The soils were tentatively classified as Typic Dystrandepts, Andic Cumulic Haplumbrepts, and an Andic Entic Haplumbrept.
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