Abstract

Cation speciation in soil extracts from a catenary sequence of salt-affected soils was studied to understand the chemical evolution of Solonetzic soils in a closed basin. Major changes in cation speciation were found to occur at each phase of soil development. The proportion of thermodynamically active cations was greater in the extracts from the more leached profiles. Saturation indices suggested that the extracts achieved equilibrium with calcite and gypsum, when present in the soil, within 24 h. Accumulations and/or losses of gypsum and calcite from saline layers indicate that there are ongoing changes in their stability. The ratio of Ca2+/Mg2+ suggested a possible equilibrium with Mg-bearing calcites; conditions existed in certain horizons for the formation of this group of minerals. Correlations between exchangeable sodium ratios and sodium adsorption ratios calculated using cation concentration and activity were similar; however, using activities to determine the exchange constants resulted in significant differences.Key words: Cation speciation, saturation indices, cation adsorption, exchange selectivity constants, Solonetzic soils

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