東南アジア研究

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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.

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A case study survey on yield and its components in the Central Plain of Thailand was conducted in the Saraburi-Ayutthaya area during the harvesting season of 1967. The average values of yield components of the thirty plots of this study were compared with those of other surveys done in Thailand and Malaya. Basing on these data an attempt to figure out the yield components of rice plants actually grown by farmers in southeast Asian countries was made. Simple correlation coefficients among yield components were also calculated and compared for the results of several surveys including this one. However, no general rule of correlation coefficients common for southeast Asia was found. All the yield components of the samples were presented graphically. According to the shapes of the yield component patterns of each sample on this graph, the thirty samples were grouped into six types. Their characteristics were discussed.

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