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Ionic Activity Product of Calcium Carbonate Precipitated from Soil Solutions of Different Degrees of Supersaturation¹

ABSTRACT

Calcium carbonate (CaCO₃) was precipitated by equilibrating samples of calcareous soil and noncalcareous soil with a dilute sodium bicarbonate solution at two soil-to-solution ratios. Different degrees of supersaturation of the soil solutions prior to precipitation were thus achieved by releasing variable amounts of exchangeable calcium (Ca). Six sequential equilibrations were performed on each soil at each soil-to-solution ratio. Both Ca and magnesium (Mg) were released from exchange sites during equilibration, but mainly calcium carbonate precipitated. Precipitation in the soil extracts actually occurred if the degree of saturation after precipitation was lower than the degree of saturation prior to precipitation. The logs of the lonic activity product of calcium carbonate, pIAP_c, of the noncalcareous soil were 7.57 and 7.83, and of the calcareous soil, 7.78 and 7.86, at the 1:1 and 1:10 soil-to-solution ratios, respectively. These pIAP_c values were within experimental error of those obtained from undersaturation at a 1:1 soil-to-water ratio (Levy, 1981). The same pIAP_c values were obtained regardless of the path by which equilibrium was approached, apparently because the same inhibitors were present in each case.

¹ Paper presented before a joint meeting of Div. S-2 and Div. S-6, Soil Science Society of America, 4 Dec. 1980, in Detroit, Mich. Contribution from the Agric. Res. Organ. Volcani Center, Bet-Dagan, Israel.

² Soil Scientist, Institute of Soils and Water, Agric. Res. Organ., Volcani Center, Bet-Dagan, Israel. On sabbatical leave with the LAWR Dep., UCD, Davis, Calif.

Received for publication April 13, 1981. Accepted for publication July 8, 1981.