MINERALOGY AND CLASSIFICATION OF SOILS ON SERPENTINIZED PERIDOTITE OF THE TRINITY OPHIOLITE, CALIFORNIA

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Abstract
We sampled twenty-two soils in mixed conifer stands of ultramafic terrain in the Klamath Mountains. They are almost evenly split among Xerochrepts, Argixerolls, and Haploxeralfs. A close relationship between exchangeable Ca/Mg ratios and conifer productivity is evident from a previous investigation, even with a broad range of environmental conditions. The Ca/Mg ratios, in turn, are closely related to the ultramafic mineral content of the fine sand fraction in subsoil (30 to 60 cm) samples. Thus subsoil mineralogy is an indicator of fertility in these soils of ultramafic terrain. Exchangeable Ca and Mg, however, are more readily determined in most soil laboratories, and their ratio is a more direct index of fertility. To include all 16 of the less productive soils with low Ca/Mg ratios (Ca/Mg<0.5) in a serpentini-tic (or ultramafic) family, it would be necessary to exclude fine clay from the determinant particle-size fraction, add ortho-pyroxenes and olivine to the list of diagnostic minerals, and reduce the required quantity of these minerals. The surest way to separate less fertile from more—fertile soils of ultramafic terrain would be with an exchangeable Ca/Mg ratio criterion. It could be applied at either the subgroup or the family level of Soil Taxonomy. Any subsoil or control-section limit in the range 0.5 < Ca/Mg < 1.0 would make an effective separation of these soils of the Trinity ultramafic body. © Williams & Wilkins 1990. All Rights Reserved.