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A solubility diagram was developed that shows the activity of various ionic and molecular species of Si in equilibrium with amorphous silica as a function of pH. Below pH 8.0 only uncharged Si(OH) 4 contributes significantly to total soluble Si. The solubility of Si was measured on two soils in which equilibrium was approached from both undersaturation and supersaturation over a 50‐day period. In this study, 0.02 M CaCl 2 was included in all suspensions during their equilibration to flocculate colloidal Si and preclude its inclusion as soluble Si. The acid Paxton soil supported approximately 19 ppm of Si in solution while the calcareous Ulysses soil supported 25 ppm. These levels are intermediate between the solubility of quartz (2.8 ppm Si) and amorphous silica (51 ppm Si) and lie very near the level predicted for equilibrium between kaolinite and montmorillonite based on Kittrick's thermodynamic calculations.
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Elgawhary et al. (1972) studied this question.
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