Release of Al by hydroxy‐interlayered vermiculite and hydroxy‐interlayered smectite during determination of cation exchange capacity in fine earth and rock fragments fractions

The fine earth (<2 mm) and rock fragments (>2 mm) fractions of two soils derived from Oligocene sandstone have been examined to assess the origin of the discrepancies between cation exchange capacity (CEC) and effective CEC (ECEC). The soils differ in terms of acidity: soil A is more acid than soil B. When the A samples are treated with BaCl2, the solution became sufficiently acid (pH < 4·5) to dissolve and to maintain Al in solution. From these samples more Al is released than base cations. Aluminium was continuously replenished even after 192 h, so that the ECEC was always larger than the CEC. Samples from soil B contain less H and Al ions, and the BaCl2 solution could not lower the pH below 5·0. In these samples little Al is released, and the base cations dominate the exchangeable pool of ions. This Al can be considered to be exchangeable, and a good agreement exists between the ECEC and the CEC. The source of non‐exchangeable Al in the A samples is the OH‐Al polymers of the hydroxy‐interlayered vermiculite (HIV) and hydroxy‐interlayered smectite (HIS) that tend to dissolve during the BaCl2 treatments. In the less acid B samples the Al polymers are not affected by BaCl2 treatment. Different results were obtained when the clays, extracted from an Na‐dispersed suspension, were treated with BaCl2 solution. Because the clays are no longer acid, no H+ is released, and the OH‐Al polymers are not dissolved. Therefore, the saturating ions play an important role in the dissolution of the OH‐Al polymers and cause differences between the CEC and ECEC. We discount organic matter and specifically Al‐organo complexes as a source of non‐exchangeable Al. Both A and B soils contain very similar pyrophosphate‐extractable Al, but show substantial differences in the amount of exchangeable Al.