2006 年 50 巻 1 号 p. 14-28
The fundamental soil characteristics of eight sites in five regions in Thailand-northeast (NE1, NE2), central highland (CH1), west (W1, W2), north (N1, N2) and south (S1)-were studied. Further, the grouping of the soils in the recently published classification system-the "World Reference Base for soil resources" (WRB) was examined. The values of the CEC/clay, which indicate the activities of clay, were highest (40 cmolc kg^<-1> clay) in the soils of the central highland followed by sites N2, W2, and NE2, which were comparatively high (20 cmolc kg^<-1> clay); the values were relatively low at sites NE1 and S1 (approximately 16 cmolc kg^<-1> clay) and the lowest at sites W1 and N1. The strength of the weathering and the chemical activity of the soils can be reflected in the results. The organic matter content of the surface horizons was highest at site W1, followed by sites W2, N1, and CH1. The soils at these sites, including their lower horizons, had considerable organic carbon storage. On the other hand, the soils at sites S1, N2, NE1, and NE2 had low organic matter contents not only in the subsurface horizons but also in the surface horizons. The exchangeable cations in the profile of sites CH1 and W2 did not diminish in the lower horizon, and the TRB value was high. On the other hand, a distinct decrease in the exchangeable bases in the subsoils was observed at the other sites. A very low TRB level at the northeastern and southern Thailand sites should indicate the low potential fertility of these soils. According to the WRB classification, the soils at sites NE1, NE2, and N2 were grouped as Acrisols, those at sites W1 and N1 were grouped as Ferralsols, those at sites CH1 and W2 were grouped as Luvisols, and the soil at site S1 was grouped as Plinthosols.
