We studied clay mineralogy and differences in oxidic ratio and CEC of selected low-activity clay soils in temperate and tropical regions. Samples were collected from six Ultisols in Georgia (Bradson, Cecil, Davidson, Decatur, Greenville, and Madison soils). Two Oxisols (Cibinong and Sitiung) and four Ultisols (Ciomas, Jasinga, Jonggol, and Kedunghalang soils) were sampled in Indonesia. The samples from A, Bt, and B horizons were analyzed for CEC by the NH4OAc and NH4Cl methods, effective CEC (ECEC), and oxidic ratio ((extractable Fe2O3% + gibbsite%)/ clay%). Clay mineralogy was studied by x-ray diffraction (XRD), and gibbsite concentrations were measured by differential thermal analysis (DTA). Iron oxide was extracted from the soils by the dithionitecitrate-bicarbonate (DCB) procedure, and the concentration was determined in the extract by atomic absorption spectroscopy. The results indicated that the Bt horizons of the soils in Georgia had values for CEC(NH4OAc) and ECEC ranging from 14.7 to 28.9 and 8.1 to 11.9 cmol(+)kg-1 clay, respectively. With the exception of the Decatur soil, the oxidic ratio of the Bt horizons of the Georgia soils was >0.20, which could qualify them for placement in the oxidic mineralogy class, provided their quartz content would support it. The Davidson soil, especially, exhibited CEC and ECEC values within the requirement limits for both kandic and oxic horizons. These chemical properties were related closely to the clay mineralogy, which was composed of large amounts of kaolinite and especially gibbsite. Some hydroxy-A1-interlayered clays were also detected. In comparison, the Bt and B horizons of the soils in Indonesia, with oxidic ratios <0.20, were less oxidic in nature. On a soil basis, the Indonesia soils were generally higher in CEC values than the six Georgia soils. These differences in chemical characteristics were attributed to a variation in soil mineralogy, indicating the absence of quartz and considerably lower amounts of sesquioxides, but larger kaolinite contents, in the Indonesian than in the Georgian soils.
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