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Kinetics of non-exchangeable potassium release as a function of clay mineralogy and soil taxonomy in calcareous soils of southern Iran

The release of non-exchangeable potassium from 16 calcareous soils collected from southern Iran was examined. For this study, a method was used involving a total of 14 successive 15-min extractions with 0.01 M CaCl2 as an extractant. Cumulative potassium released from soils was in the range of 144–509 mg kg−1 for surface and 115–445 mg kg−1 for subsurface soils. The studied soils were categorized into two groups based on the cumulative K released, clay mineralogy and soil taxonomy. (I): Smectitic soils including Vertisols, Vertic subgroups and some Alfisols and Inceptisols; (II) other soils including Mollisols, Entisols, and other subgroups of Inceptisols and Alfisols. Cumulative K release was significantly (p < 0.01) higher in Group I than in group II. The release kinetics of non-exchangeable K showed the best fit to Elovich, parabolic and power function kinetic models. The Elovich and parabolic release rate constants were significantly (p < 0.05) higher in Group I than in Group II. Greater K release rate constants in Group I soils could be due to the heavy soil texture and domination of expansible clay minerals.