Origin of the pH-dependent cation exchange capacities of Irish soil clays

Values of cation exchange capacity determined at pH 4.8 and at pH 8.2 are reported for peroxidized clay separates from a group of Irish soils derived from a variety of parent materials. The correlation between the two measurements was poor. The differences between the values (pH-dependent CEC's) ranged from 0.1 to 58.6 mequiv./100 g, with a mean of 12.9 mequiv./100 g. The highest values were associated with clays from spodic horizons and clays from basalt-derived soils. CEC at pH 8.2 and pH-dependent CEC were highly correlated with citrate-dithionite-bicarbonate (CDB) extractable Al. In contrast, CEC at pH 4.8 was negatively correlated with this parameter, suggesting that hydrous oxides reduced CEC at low pH values.Values of CEC of pH 4.8 and pH 8.2 are also reported for a selection of 16 clays subjected to sequential extraction with CDB and NaOH. CDB treatment reduced mean pH-dependent CEC from 20.4 to 9.0 mequiv./100 g, which was reduced to 3.9 mequiv./100 g by subsequent extraction with NaCH. The material removed by CDB was apparently positively charged at pH 4.8 and negatively charged at pH 8.2. Most of the pH-dependent CEC remaining after CDB treatment was associated with the clays from spodic horizons. The material extracted by NaOH was apparently negatively charged at both pH 4.8 and pH 8.2. No direct evidence of the physical nature of this material was obtained but it was present in sufficient amounts to suggest that it existed in particular forms in some of the clays from the spodic horizons. Following the dissolution treatments, the correlation between CEC values at pH 4.8 and pH 8.2 was very good (r = 0.99).