東南アジアにおける赤色土壌の類別, 分布および生成について

Abstract

Based on descriptions from soil survey literature, red-colored soils occurring in Southeast Asia were grouped into the following five great soil groups : 1. Dark Red and Reddish Brown Latosols (RBL), 2. Red Yellow Latosols (RYL), 3. Reddish Brown Lateritic Soils (RBLS), 4. Red Yellow Podzolic Soils (RYP), 5. Red Brown Earths (RBE). (Names and definitions are after Dudal and Moormann.) Their genetic conditions can be summarized as follows : [table] The following conclusions were tentatively drawn from considerations of genetic conditions : (1) Genesis of upland soils in the humid tropics is characterized by a greater intensity and a greater speed of weathering as compared to that of upland soils in the humid temperate zone. On the other hand, fundamental soil-forming (horizon differentiating) processes are common to both climatic zones, i. e., podzolization and lessivage. (2) Weathering on basic parent rocks produces sesquioxide-rich weathering crusts so rapidly that soil-forming processes are almost totally inhibited due to coating and cementation of kaolinic clay with sesquioxides. Thus, the resulting soil is of the latosol type. (3) Weathering products of acidic rocks contain kaolin minerals, quartz and relatively small amounts of sesquioxides. Thus, lessivage and podzolization can proceed along with weathering. Soils formed in this condition morphologically resemble the red yellow podzolic soils described for the warm temperate zone. (4) Morphology is not a sufficient criterion for classifying tropical soils. The degree of weathering in soil materials should also be taken into consideration. Thus, the red yellow podzolic soils, which have been described for the tropics, should be distinguished from those described for the warm temperate zone on the basis of soil-imprinted weathering features.