Method for the determination of CEC and exchangeable bases in calcareous soils.

The determination of CEC and exchangeable bases in calcareous soils has always been very difficult, because partial dissolution of the phases (e.g. CaCO3) is observed in these matrices as a result of interaction with the exchange solution rich in electrolytes. The proposed method foresees, on 10 g of soil, a saturation step with use of 1N NH4OAc, followed by washing with deionized water, contact with a solution of 0.4N NaCl and re-extraction of Na+ with the 1N NH4OAc solution. The exchangeable bases were determined in the supernatant after the 1st saturation step and CEC by measuring the concentration of Na+ removed by NH4+ and collected in the 2nd step. For both measurements optical emission spectrometry with inductively coupled plasma (ICP-OES) was used. For the trials two soil profiles of Vertisols were chosen, collected in the experimental farm 124 in Foggia (Italy), both with seven diagnostic horizons, with the amount of carbonates expressed as CaCO3 %, ranging from 10% (topsoil) to 30% (subsoil). The proposed method is well correlated with the official method for single and the sum of exchangeable bases; but not for CEC.