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Physical, Chemical, and Mineralogical Characteristics of Soils from Volcanic Ash in Northern Idaho: I. Morphology and Genesis¹

ABSTRACT

Mission soils, coarse-silty, mixed, frigid Andic Fragiochrepts, are representative of volcanic ash soils found extensively in northern Idaho, northeastern Washington, and northwestern Montana. They have developed under a western hemlock (Tusuga heterophylla) habitat type with 740 mm of precipitation, > 8°C mean summer soil temperature and 7.5°C mean annual soil temperature (weather data, Univ. of Idaho. Res. Stn., Sandpoint). The B2ir or cambic horizons have developed from volcanic ash and the underlying IIA'&B' and IIB'2x horizons have fragipans developed in glacial outwash sediments. Horizons from volcanic ash have higher values of estimated allophane, organic C, extractable Fe, Al, and Mn, exchangeable H, 1/3- and 15-bar moisture, and CEC than do underlying horizons. Bulk densities range from 0.80 to 0.90 in volcanic ash horizons compared with 1.41 to 1.65 for glacial sediments. X-ray patterns show little indication of crystalline minerals in the ash-related horizons. Concretions have greater contents of Mn and Fe whereas Al remains relatively constant in relation to the total soil. Development of a B2ir, without an Al horizon under the O horizon, is characteristic of soils associated with these climatic and vegetational zones. Diagnostic properties make the cambic gradational to a spodic horizon.

¹ Contribution from the Department of Plant and Soil Sciences, Univ. of Idaho, Moscow, ID 83843. Approved for publication by the Directors of the Idaho Agric. Exp. Stn., as Research Paper no. 7778.

² Professor of Soils, Scientific Aide, Professor of Soils, and Former Post-doctoral Research Assistant, respectively, Department of Plant & Soil Sciences, Univ. of Idaho, Moscow, ID 83843.

Received for publication August 8, 1977. Accepted for publication December 15, 1978.