A study was made of the effect of varying soil pH, by the addition of either HCl or Ca(OH)2, on the concentration of phosphate in 0.01M CaCl2 extract as well as the amount of labile phosphate, as determined by isotopic exchange, in both acid and limed soils to which liberal amounts of superphosphate had been applied over a long period of time but to which no phosphate had been applied for at least five years prior to sampling.For any given soil, the amount of labile phosphate in the soil and the concentration of phosphate in soil solution both reached a minimum value at about pH 5.5 and increased rapidly as the pH was increased or decreased from this value. When the value of pH + pH2PO4 was plotted against pH — ½pCa on the conventional calcium phosphate solubility diagram, the experimental points did not fall on the solubility isotherm of any known crystalline phosphate compound except when the pH was increased sufficiently to cause supersaturation with respect to octocalcium phosphate at which point octocalcium phosphate began to precipitate and the values of pH + pH2PO4 fell on the octocalcium phosphate solubility isotherm.The observed variation in the amount of soluble and labile phosphate with pH was found to be reversible. The addition of soluble phosphate to a soil also resulted in increases in the amounts of soluble and labile phosphate at any given pH. These findings would support the conclusion that the phosphate concentration in soil solution was determined by the amount of labile phosphate rather than by the solubility of some crystalline phosphate as the pH of the soil varied.

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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.