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Mineralogy and Classification of Andic Soils in Northeastern California

Dep. of Land, Air and Water Resources, Univ. of California, Davis, CA 95616

*Corresponding author.

ABSTRACT

Recently proposed changes to Soil Taxonomy will have a significant impact on the classification of soils with andic properties, most of which formed from volcanic materials. The vast majority of soils with andic properties occur in humid environments. A few have been identified in regions with semiarid to subhumid climates. Soils with andic properties and a xeric soil moisture regime are extensive in the Medicine Lake Highland and near the town of Burney in the Cascade physiographic province of northeastern California. Selective dissolution and x-ray diffraction were employed to determine the mineralogy of six soils being mapped in the region. Four soils in the Medicine Lake Highland, formed mainly from air-fall tephra and basalt, are mantled by rhyolitic pumice up to 81 cm thick. Soil pH in the 5 to 7 range favored initial allophane and smectite formation over formation of humus-metal complexes. Subsequent desilication in upper horizons produced imogolite and kaolins there, but favored allophane stability in resilicated lower horizons. More intensive desilication of two soils formed from mixed basalt and tephra in the more humid Burney area favors the stability of a noncrystalline Al hydrous oxide (kliachite?), gibbsite, kaolins and crystalline Fe oxides. Three of the pedons in the Medicine Lake Highland meet the criteria for Xerands. The fourth, formed in the thickest pumice deposit, is a Vitrandic Xerorthent. Organic C contents, Al/Si ratios, and total annual precipitation suggest that the soil moisture regime in the Burney area may be udic, not xeric. The pedons in the Burney area are tentatively classified as Udands.

Contribution from the Dep. of Land, Air and Water Resources, Univ. of California, Davis.

Received for publication January 17, 1989.

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