CONTRIBUTION OF ORGANIC-MATTER AND CLAY-MINERALS TO THE CATION-EXCHANGE CAPACITY OF SOILS

The cation exchange capacity (CEC) at pH 7 was measured for samples of 347 A horizons and 696 B horizons of New Zealand soils. The mean CEC was 22.1 cmolc/kg for the A horizons and 15.2 cmolc/kg for the B horiz ons. Multiple regressions were carried out for CEC against organic car bon (C), clay content, and the content of seven groups of clay mineral s. The results, significant at p <0.001, were consistent with most of the CEC arising from soil organic matter. For the samples of A horizon , the calculated CEC was 221 cmolc/kg per unit C and for the B horizon s was 330 cmolc/kg C. There was also a contribution from sites on clay minerals. Multiple regression indicated that smectite had a higher CE C (70 cmolc/kg) than other minerals but it was not as high as that of type smectites; kaolin minerals had the lowest CEC. There was a signif icant effect of interaction between organic matter and some clay miner als on the CEC. Samples from B horizons containing allophane had lower CEC than those not containing allophane which is consistent with allo phane reacting with carboxyl groups on organic matter. For the samples from the A horizons, however the CEC was higher when allophane was pr esent.