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Change of Phosphate Potentials of Calcareous Soils on Adding Phosphorus¹

ABSTRACT

In two calcareous soils studied the phosphate potentials (0.5 pCa + pH₂PO₄) and lime potentials (pH – 0.5 pCa) indicated that the forms of mineral phosphates that determine the amount of phosphorus in solution were basic calcium phosphates similar in solubility to octocalcium phosphate, Ca₄H(PO₄)₃·3H₂O. Phosphorus as H₃PO₄ added to the soils at the rate of 200 ppm P saturated them with respect to octocalcium phosphate, and 500 ppm P saturated them with respect to dicalcium phosphate. Wetting and drying the soils for 5, 10, or 20 cycles after adding phosphorus increased the phosphate potentials. No tendency for calcium phosphates to convert to hydroxyapatite was observed after 20 wetting and drying cycles. Repeated extraction of soil samples—some in their original condition and some to which phosphorus was added—with 0.01M CaCl₂ solution increased their phosphate potentials. The dominant form of calcium phosphate in the soil to which phosphorus had been added changed with extraction from dicalcium phosphate to a form or mixture of forms intermediate between dicalcium and octocalcium phosphate.

¹ Contribution No. 900, Department of Agronomy, Kansas Agr. Exp. Sta., Project 269, Kansas State University, Manhattan. Paper presented before Div. S-2, Soil Science Society of America, Nov. 18, 1963, at Denver, Colo. Part of the senior author's Ph.D. thesis, 1963, Kansas State Univ.

² Associate Professor of Soils and Professor of Soils, respectively, Department of Agronomy, Kansas State University.

Received for publication November 12, 1964. Accepted for publication July 7, 1965.