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Several factors examined herein which control opal dissolution are: specific surface area, Al content, hydration state, age, and rate of organic matter biodegradation of the encasing vegetative tissues. These factors are covariable with opal of different origin. Recent (6 months old) opal phytoliths of deciduous origin are most hydrated (11%), have lower A1 content (2%) and highest dissolution (9 mg Si/liter, cold water; 50 mg Si/liter, hot water; and 3 mg Si/liter under natural environments). In contrast, opal of coniferous origin is older (30 months), more rigid, has higher Al content (3 to 4%), is encased within litter that is more slowly biodegradable and yields lower dissolution (2 to 3 mg Si/liter, cold water; 20 mg Si/liter, hot water; and 0.5 mg Si/liter under natural environments). Gramineous phytoliths associated with understory forest vegetation generally are intermediate in the above properties and dissolution. Biogenic opal has solubilities that approximate geologic opal‐A. It is relatively stable and not sufficiently labile under most soil environments to support observed soluble Si levels.

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Approximately 75 grams of biogenetic opal were isolated from 45 kilograms of soil by employing gross particle-size and sink-float specific gravity fractionation procedures. After pretreatment of the sample to remove extraneous organic and inorganic carbon contaminants, the carbon occluded within opal phytoliths was dated at 13,300 +/- 450 years before the present. Therefore, biogenetic opal is stable for relatively long periods.