Abstract
In view of the fact that the extremely low fertility and poor tilth of plateau soil in Northeast Thailand, as confirmed in the preceding paper, are conditioned by their materials, further characterization of the nature of soil materials and material classification were carried out.
Fractionation of sand by sieving into 5 fractions and Fine/Coarse ratio derived were found to be an effective method for characterizing the sandy materials. The composition of the sand fraction was peculiar to each transect and the range of Fine/Coarse ratios for soils on one transect did not show any overlap with that for soils on other transects. This finding appears to support the hypothesis that the surface materials of the Korat plateau were formed by local alluvio-colluvial processes from the weathered basement rocks in the relative vicinity.
Soil material classification was attempted as the basis for understanding the pedogenesis of the area. A cluster analysis method was applied to a set of characters, including the sand composition, which are relevant to the nature of soil materials. The results confirmed the above finding, revealing that pedons on one transect were more similar among themselves in their materials as compared with those on other transects. Moreover, occurrence of the same surface material in common to all the costituent pedons of a transect could be and evidence of local transport/deposition prosess.
