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Effect of pH on Labile and Soluble Phosphate in Soils¹

ABSTRACT

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 — pCa 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.

¹ Agronomy Paper no. 795, New York State College of Agriculture at Cornell University, Ithaca. 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 US Army Cold Regions Research and Engineering Laboratory, Hanover, N.H.; and Professor of Soil Science, Cornell University, Ithaca, N.Y.

Received for publication May 20, 1968. Accepted for publication August 19, 1968.