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Reaction Products of Applied Phosphate in Limed Soils¹

ABSTRACT

A study was made of the solubility of phosphate in limed soils to which liberal amounts of superphosphate had been added over a long period of time but to which no phosphate had been applied for at least 5 years prior to sampling. The control limed soils to which no phosphate had been applied were also included for comparison. A 20-g sample of soil was extracted with 200 ml of 0.01M CaCl₂ for 108 hours and the concentration of phosphate and pH were determined in the clear supernatant solution after centrifugation. Although the extracts of all soils were found to be undersaturated with respect to hydroxyapatite, most of the extracts were found to be supersaturated with respect to fluorapatite in the presence of solid phase CaF₂. By assuming the formation of an ideal solid solution with fluorapatite and hydroxyapatite as end-members, it was shown that fluorapatite should be the stable phase even in the absence of solid phase CaF₂ provided that the F^- concentration in the soil solution is > 10^-8 M. Consistent with this deducation, the solubility of phosphate in many of the limed soils examined in this study agreed well with that expected for fluorapatite in the presence of 10^-5M F^- solution, which was the concentration of F^- found in several of the extracts that were examined. It may be concluded, therefore, that fluorapatite was the ultimate reaction product of the applied superphosphate in these limed soils.

¹ Agronomy Paper no. 788, New York State College of Agriculture at Cornell University, Ithaca, N.Y. This work was carried out at Cornell University in cooperation with the Tennessee Valley Authority, Wilson Dam, Ala.

² Formerly Graduate Research Assistant at Cornell University, now located at U.S. Army Cold Regions Research and Engineering Lab., Hanover, New Hampshire; and Professor of Soil Science, Cornell University, Ithaca, N.Y.

Received for publication November 10, 1967. Accepted for publication January 15, 1968.