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One sample of H2O2‐treated, iron oxide‐extracted soil is treated with 2% Na2CO3 solution for 60 minutes and another with NaOAc of pH 3.5 for 15 minutes, at boiling. Both samples are washed with neutral NaOAc and the cation‐exchange capacity is determined with Ca replaced with NaOAc. The difference obtained between the two samples is referred to as the “cation‐exchange capacity delta value,” and averaged 100 me. per 100 g. for allophane from Ando soils of Japan developed on rapidly weathered volcanic ash and in some allophanes of Ross and Kerr. For example, the allophane content of fine clay of an Ando soil containing predominantly halloysite determined by delta value was 23.8% compared to 21.4% by water contents of constituent minerals.

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