Regional trends in the chemical and mineralogical properties of upland soils in humid Asia: With special reference to the WRB classification scheme

Soils in humid Asia exhibit relatively incipient mineralogical characteristics because of the dominant steep slopes, crust movement and volcanic activity compared with many tropical soils on stable plains. In the present study, the relationship between the mineralogical and chemical properties of these soils was investigated with special reference to the World Reference Base (WRB) classification scheme. A total of 186 upland soil profiles were collected and the chemical and mineralogical properties of the B-horizon soils were analyzed for clay mineral composition, pH(H2O), exchangeable cations, cation exchange capacity (CEC), total C content, particle size distribution and sodium-dithionite-extractable oxides (Fed and Ald). The majority of the soils were acidic. The CEC/clay of the soils derived from sedimentary rocks (excluding limestone) or felsic parent materials showed a clear regional trend, which was usually higher than 24 cmolsub(c)/kg (corresponding to Alisols if the argic horizon is recognized) under the udic and perudic soil moisture regimes in Indonesia and Japan, whereas it was predominantly lower than 24 cmolsub(c)/kg (corresponding to Acrisols) under the ustic soil moisture regime in Thailand. In contrast, soils derived from mafic volcanic rocks or limestone were more variable in clay mineral composition, CEC/clay and pH, and were often high in Fed. This trend is in accordance with the clay mineral composition, in that mica and kaolin minerals dominated under the ustic soil moisture regime in Thailand, whereas significant amounts of 1.4 nm minerals formed under the udic and perudic soil moisture regimes in Japan and Indonesia. In conclusion, of the 186 soils studied, only nine and eight soils are classified into Luvisols (or Lixisols) and Cambisols (eutric), respectively, whereas the majority (169 samples) are classified as acid soils, such as Andosols, Podzols, Alisols, Acrisols and Cambisols (dystric). The WRB classification is generally consistent with regional trends in the chemical and mineralogical properties of the soils and successfully describes the distribution patterns of the acid soils in humid Asia using the criteria of CEC/clay=24 cmolsub(c)/kg.