Tectonic Inclusions in Serpentinite Landscapes Contribute Plant Nutrient Calcium
2008
McGahan, Donald G. | Southard, Randal J. | Claassen, Victor P.
Serpentinite-derived soils give rise to botanically distinct systems primarily because of inadequate parent material Ca content. We hypothesized that Ca content varies widely in what have been mapped as soils derived from serpentinite. An exchangeable Ca/Mg ratio <0.7 is often used to relate the imbalance of these nutrient elements in serpentinite-derived soils. We sampled six parent materials and soils from the Coast Ranges of California in Henneke soil series (clayey-skeletal, magnesic, thermic Lithic Argixerolls) modal location map unit polygons. Parent material total CaO content varied from 1.0 to 230 mg kg⁻¹, and CaO/MgO varied from <0.1 to 4. A combination of x-ray diffraction (XRD), polarized light microscopy (PLM), and electron microscopy was used to identify the Ca-bearing accessory minerals diopside, grossularite, andradite, and tremolite. Accessory mineral content was often too low to be detected by XRD or minerals were too finely disseminated and difficult to detect in thin section by PLM. Electron microscopy, in concert with XRD and PLM, was needed to fully characterize the mineral assemblage. Two sites, Napa and Tehama, contained no serpentine minerals, were not serpentinites, and were tectonic inclusions in the serpentinite landscape. Napa rocks contained almost no Ca-bearing minerals and would be identified as a serpentinite if relying on elemental analysis CaO/MgO ratio alone. Tectonic inclusions and Ca-bearing accessory minerals affect Ca distribution and presumably its availability for plants. Careful mineralogical analysis may be required to identify Ca-bearing accessory minerals.
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