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Transformation of Dibasic Calcium Phosphate Dihydrate and Octacalcium Phosphate in Slightly Acid and Alkaline Soils¹

ABSTRACT

Chemical, petrographic, and X-ray diffraction methods were used to investigate the transformation of CaHPO₄·2H₂O and Ca₈H₂(PO₄)₆·5H₂O incubated in slightly acid and alkaline soils for 4, 16, and 44 weeks at 25 and 35C. At 25C, hydrolysis of CaHPO₄·2H₂O to Ca₈H₂(PO₄)₆·5H₂O was confirmed after 4 weeks in soils having a pH value higher than about 7.9, and at 44 weeks the new phase was found in soils with pH values higher than 6.9. Transformation of CaHPO₄·2H₂O to Ca₈H₂(PO₄)₆·5H₂O increased with increasing pH, temperature, and time of incubation but decreased in the presence of much magnesium. There was no evidence of conversion of added Ca₈H₂(PO₄)₆·5H₂O to more basic phosphates.

¹ Journal Paper no. J-6457 of the Iowa Agr. & Home Econ. Exp. Sta., Ames, Iowa. Project no. 1183.

² Former Graduate Assistant and Professor. The senior author is now a Queen Elizabeth Fellow, Dept. of Soil Science & Plant Nutrition, Univ. of Western Australia, Nedlands, W. Aust.

Received for publication January 5, 1970. Accepted for publication March 2, 1970.