小笠原諸島の土壌 : とくにその生成・分類について

抄録

The islands are situated at about 27° N and 142° E, in the Pacific ocean about 1,000 km south of Tokyo. The climate is subtropical with mean annual temperature, 22.6°C and annual rain fall, 1636mm. Four soil groups are etablished, Lithosol, Dark, Yellow-brown and Red groups. Soils of Dark group are high in pH, base saturation and montmorillonitic clay content and may be correlated to Chromuderts of U. S. Soil Taxonomy (S. T.) Soils of Yellow-brown group are slightly lower than those of Dark one in terms of the above-stated features, thus being probably Eutrochrepts of S. T. Red group contains three main subgroups, Dark red, Slightly dark red and Red. The former two are widely distributed. Soils of Dark red and partly of Slightly dark red subgroups are moderately high in pH and base saturation and may be Rhodudalfs if clay translocation be confirmed, otherwise Eutrohrepts of S. T. The clay mineralogy of the soils is characterized by dominant Al-vermiculite and metahalloysite. Other soils of the group are acid and low in base saturation, probably referred to Rhodudults or Dystrochrepts of S. T. Predominance of soils with high pH and base saturation is specific to the pedoshere of Bonin Islands and may be explained by their basic parent materials, igneous and tufaceous, and rapid release of bases by chemical weathering. Some of classification schemes well distributed in Japan need reexamination to accomodate such the soils. Degree of leaching increases in an order of Lithosol, Dark, Yellow-brown and Red groups. From this and their mode of occurrence in the field, an age sequence of the above order is also supported. This can be extended to a soil color series, a more widely applicable concept to soils of various climates and parent materials. For examples, the equivalent series in limestone soils may be Rendzinas, Terra Fusca-and Terra rosa-like soils in Okinawa Islands at about the same latitude as Bonin Islands, one and the latter half of the series in soils of acidic or sedimentary parent materials could be Yellow-brown forest soils and Red Yellow soils in warm temperate southwest Japan.