Problems in CEC determination of calcareous clayey sediments using the ammonium acetate method

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 NH4+, especially during the washing steps. This leads to a more or less partial biionic exchange population resulting in an nderestimation of the CEC which is calculated based on NH4+ concentration of the clay by Kjeldahl analysis. The biionic exchange population was proven using the new silver thiourea technique with presaturation of calcite, AgTUcalcite. 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.