Yellowish orange colored soil derived from granitic rocks

Abstract

The writer studied the yellowish orange colored soil developed on the granitic rocks of Oshima Island, Yamaguchi Prefecture. The locality belongs to the "Setouchi" climatic region, and more widely to the warm-temperate forest zone. Geologically it is situated in the "Ryoke" metamorphic zone (granodiorite). Primary minerals in sand fractions (coarse and fine sands) consist predominantly of acidic to neutral plagioclase, biotite and quartz. Except for the upper horizons, soil color is yellowish orange. This soil is, however, mainly at the stage of physical disintegration, accompanied by weak chemical decomposition (siallitization). Analytical data show that both the humus accumulation and clay formation are not so intensive and the soil is still at the younger stage of development. These characteristics of weathering and soil formation seem to reflect considerably the lithological nature of parent rock (granodiorite). Chemical properties of this soil are characterized by low values of pH, exchangeble acidity, C.E.C., and exchangable Ca and Mg, while the degree of base saturation are somewhat high. Leaching of base is fairly distinct through the profile. The contents of free oxides (Fe, Al) are relatively high, as compared with those of free silica. Since SiO_2/R_2O_3 mol-ratio in clay fraction (<2μ) is rather lower in surface horizon than in subsoil, slight enrichment of sesquioxides may have taken place in surface horizon. Clay minerals in this soil are mainly Al-vermiculite and metahalloysite with some amounts of mixed layer minerals and micaceous minerals. Absence of gibbsite in this soil is notable. The nature of weathering and soil formation of this soil is thought to be initial stage of siallitization. Judging from the properties above-mentioned, this soil corresponds neither to Brown forest soils and their affinities distributed in north-east Japan, nor to Red-Yellow soils developed on Pleistocene terraces and hills in south-west Japan. Instead, another taxonomical name should be devised to this soil at the level of genetic soil type.