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Calcium Carbonate Supersaturation in Soil Solutions of Calciaquolls¹

ABSTRACT

We found that soil solutions of Calciaquolls in western Minnesota were up to 40-fold oversaturated with respect to pure calcite. Selective dissolution of soil calcites and x-ray diffraction showed that the Mg content of these calcites ranged from 1 to 10 mole % MgCO₃. This amount of Mg substitution cannot explain the degree of oversaturation observed in the soil solutions. X-ray peak broadening and scanning electron microscopy (SEM) was used to determine the size and shape of soil calcites. The mean size of soil calcite crystallites was approximately 0.04 to 0.09 µm. Surface energy relationships predict that soil calcites of this size would have solubility products (–log K_sp) near 8.3, not low enough to explain the observed supersaturation. Energy dispersive analysis of x-rays (EDAX) and SEM observations of these samples showed that the larger soil calcites (0.5–20 µm) were irregularly shaped and did not exist as independent crystals, and were associated with other aluminosilicate solid phases. Soil suspensions of Calciaquolls prepared at 10-fold oversaturation remained oversaturated for 40 d. Adding pure calcite seeds to soil suspensions and soil solution supernatants induced some precipitation, but the solutions remained oversaturated, attaining CaCO₃ ion activity products (–long IAP) ranging from 7.6 to 7.9. These results suggest that the metastable condition of supersaturation is a result of inhibition of the calcite precipitation process by water-soluble organic components, which are adsorbed and block potential crystal growth sites.

¹ Contribution from the Minnesota Agric. Exp. Stn., Dep. of Soil Science, Univ. of Minnesota, St. Paul, MN 55108. Scientific Journal Series, Paper no. 14547.

² Graduate Research Assistant, and Associate Professor, respectively. Address for senior author now Dep. of Agronomy, Oklahoma State Univ., Stillwater, OK 74078.

Received for publication July 15, 1985.