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Summary The amounts of aluminium liberated from a proto-imogolite (P-I) sol by 1–3 mM oxalate at pH 4·8–7·1 are found to be in excellent agreement with those calculated on the basis of the solubility product of the P-I sol, and the stability constants for aluminium oxalate complexes established earlier. Calculated solubility curves are presented for proto-imogolite sols in the presence of 10−3, 10−4 and 10−5 M total oxalate over the pH range 4–8, and also for the case where oxalate concentrations are controlled by solid calcium oxalate in the presence of 0·05, 0·5 and 5 mM calcium ion. We conclude that the presence of a proto-imogolite allophane in podzol B horizons of pH 5·0 requires oxalate concentrations less than 10−5 M. For fertile agricultural soils with typical calcium concentrations (approximately 5 mM) in the soil solution, aluminium oxalate concentrations would be limited by calcium oxalate solubility to less than 10−6 M in the presence of proto-imogolite allophane at pH values exceeding 5·5. In leached podzolic soils calcium concentrations are generally less than 0·05 mM, and would not limit the formation of aluminium oxalate complexes. |
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