The calcium-magnesium ratio of serpentinitic soils in various topographic locations in Sekinomiya, Japan : a potential criterion for the classification of the Dark-red Magnesian soils

抄録

Serpentinitic soil has an exchangeable Ca/Mg ratio much lower than those shown by non-serpentinitic soils. The ratio of these cations is known to increase as serpentinitic soil is subject to natural weathering processes. We investigated the vertical trends in the exchangeable Ca/Mg ratio in four soil profiles developed on different topographic locations; a forested hill summit (Js-F1), a hill shoulder (Js-F2), a footslope (Js-F3) and a paddy field on an alluvial fan (Js-P) in the Sekinomiya serpentinitic terrain. These soils were classified as Epileptic Cambisols (Dystric), Haplic Cambisols (Dystric), Haplic Cambisols, and Gleyic Fluvisols (Eutric), according to the WRB soil classification, respectively. Chlorite and talc were dominant in the clay assemblages, and both transformation to vermiculite from chlorite and neoformation of smectite were not apparent in the clay assemblages of the hill soils, which indicated the secondary formation of clay minerals had not occurred to a significant extent. Additionally, illite, though it is not included originally in serpentinite, was evident in the clay assemblages of the soils in the alluvial fan (Js-P). This observation suggested that mineral contamination by felsic materials containing illite had occurred in the paddy field. In all of the profiles, the exchangeable Ca/Mg ratio at the surface horizon was higher than that of subsurface horizons. Weathering of Mg-silicates followed by leaching of Mg^<2+> in the development of the profiles, as well as Ca accumulation at surface horizon by nutrient cycling may have caused the vertical gradients. The exchangeable Ca/Mg ratio in the subsurface horizons increased as the elevation decreased from the hill summit to the alluvial fan. While the ratio in the Js-F1 and F2 profile was much less than 0.1, it was around 0.1 in the Js-F3 soil. The ratio in the Js-P profile had further increased to greater than 1.0, which can then be regarded as suitable for plant growth. As suggested by the soil type and clay mineralogy, the higher exchangeable Ca/Mg ratio in Js-P was not caused by strong weathering of the Mg-silicates but by contamination of non-serpentinitic minerals such as from fluvial deposits from up-stream. In summary, the hill soils had an extremely low exchangeable Ca/Mg ratio particularly at higher elevations due to both the direct inheritance of the elemental composition from serpentinite and the relatively weak degree of weathering there, whereas the paddy soils on the alluvial fan had a ratio higher than the threshold value unfavorable for most plant growth (i.e.<0.7), probably due to contamination by non-serpentinitic fine-grained materials. As an extremely low exchangeable Ca/Mg ratio has been determined as a qualifier for magnesic soils in the Australian soil classification system, such criteria could be applied to the classification of Dark-red Magnesian soils in the United Soil Classification System of Japan to better determine the agro-ecological functions of serpentinitic soils in Japan.