| Abstract: | Cation‐exchange–capacity (CEC) results of calcareous soils and clays can be erroneous if the ammonium acetate method is used. In this study, a model is proposed to explain the process for systematic underestimation of the CEC. Seven clayey sediments from Germany with varying calcite and low organic‐C content were studied. After several exchange treatments with concentrated ammonium acetate (NH4Ac) solutions, the exchange population is assumed to be in homoionic ammonium form. Throughout the cation‐exchange experiment, calcite reacts with the NH4Ac exchange solution generating Ca2+ cations. During the necessary washing steps to remove excess salt, calcite dissolution is lower but still occurs. The permanently added Ca2+ ions compete successfully with NH , especially during the washing steps. This leads to a more or less partial biionic exchange population resulting in an underestimation of the CEC which is calculated based on NH concentration of the clay by Kjeldahl analysis. The biionic exchange population was proven using the new silver thiourea technique with presaturation of calcite, AgTU calcite . The clay with 148 g kg–1 calcite had a fraction of 16.4 cmol+ kg–1 exchangeable Ca2+. This is ca. 50% of the CEC of this clay being 31.8 cmol+ kg–1. For clays with similar mineralogical composition, this trend is proportional to the calcite content. |
