Dinetofuran (DNT), imidacloprid (IMD) and thiamethoxam (THM) are among the neonicotinoid insecticides widely used for managing insect pests of agricultural and veterinary importance. Environmental occurrence of neonicotinoid in post-application scenario poses unknown issues to human health and ecology. A sorption kinetic study provides much needed information on physico-chemical interaction of neonicotinoid with soil material. In this research study, time-dependent sorption behavior of DNT, IMD and THM in vineyard soil was studied. Sorption kinetics studies were conducted over a period of 96 hours with sampling duration varying from 0, 2, 4, 8, 12, 24, 60 and 96 hours. All three neonicotinoids exhibited very low sorption potential for the soil investigated. Overall percent sorption for all three neonicotinoids was below 20.04 ± 2.03% with highest percent sorption being observed for IMD followed by DNT and THM. All three neonicotinoids are highly soluble with solubility increasing with IMD < THM < DNT. Although, DNT has the highest solubility among all three neonicotinoids investigated, it exhibited higher percent sorption compared to THM, indicating factors other than solubility influenced the sorption kinetics. Low sorption potential of neonicotinoids indicates greater leaching potential with regard to groundwater and surface water contamination.

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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.

Serpentinitic landscapes are characterized by unusual plant species composition and vegetation patchiness. We hypothesized that soil extractable Ca content is correlated with vegetation Ca concentration and may contribute to the unusual vegetation characteristics. Eleven pedons from California were sampled within soil survey mapping polygons containing the modal location for a classic “serpentine soil.” Soil NH4OAc‐extractable Ca/Mg ratio varied from 0.1 to 1.5 and total elemental Ca/Mg ratio varied from <0.01 to 1.9. Soil extractable Ca content was influenced by parent material mineralogy: soils with a Ca/Mg ratio of 0.2 or less were derived from serpentinite parent materials with only trace Ca‐bearing minerals; soils with a Ca/Mg ratio >1.0 were derived from non‐serpentinite parent materials, and soils with a Ca/Mg ratio from 0.2 to 1.0 were derived from serpentinite parent material that had minor amounts of accessory Ca‐bearing minerals. Based on greenhouse studies, the Ca concentration of the leaf of the grass Vulpia microstachys (Nutt.) Monro correlated better with soil‐extractable Ca (r2 = 0.69, P < 0.01) than with total elemental analysis Ca (r2 = 0.40, P > 0.05). The shallow soil depths of the pedons, together with the generally high coarse fragment content, limit the soil volume available to roots. Pedons with the highest extractable Ca/Mg (1.34 and 1.89) also had the greatest extractable Ca pool, 1169 and 394 g m−2, respectively. Among the 11 pedons, extractable Ca/Mg ratio, extractable‐Ca pool, and the distribution of the Ca pool within the pedon all varied, and these variables were important determinants of plant performance, as determined by leaf Ca content. Of these variables, soil extractable‐Ca content was the best determinant of plant performance as indicated by leaf Ca concentration.

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