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Prediction of Phosphate Sorption in Soils from Regular Solid-solution Theory¹

ABSTRACT

Potassium dihydrogen phosphate (KP) solutions or monocalcium phosphate (MCP) spot placed solids were reacted with eight acidic Missouri soils. After reacting the solid MCP with moist soil for 30 d, a 1-g soil to 10-g water suspension was made. Monocalcium phosphate soil suspensions and similar suspensions made with KP solutions were shaken for 16 d. The pH, Eh, electrical conductivity, Al, Ca, Fe, and P contents of the solutions were measured. It was postulated that P sorption sites were not independent of each other, but were altered by P sorbed at adjacent sites and that P sorption could be considered to behave as predicted for a regular solid-solution. Observed values of P sorbed (PS) and P in solution were fitted to the following equation derived for a regular solid-solution system: In P = A + B(PS) + C(PS)². The data for the eight soils fit this equation with regression coefficients (r²) for KP which varied from 0.97 to 0.99 and for MCP from 0.88 to 0.97. The data were fitted to the linear form of the equation; In P = In P_max + B'(1-PS/PS_max)², and no appreciable curvature was observed. The addition of 200 to 800 mg P/kg to soil as KP resulted in higher concentrations of P in solution than when equal amounts of MCP were added. This effect was attributed to MCP dissolving additional reactants capable of precipitating P. When 100 mg P/kg or less was added the concentrations of soluble P from KP and MCP were similar, except in the Barco soil where the addition of MCP resulted in a higher concentration of soluble P than the addition KP.

¹ Contribution from the Missouri Ag. Exp. Stn., J. Ser. no. 9662. Approved by the Director.

² Professor of Agronomy and Former Research Assistant, Univ. of Missouri. Columbia, MO 65211, respectively.

Received for publication July 16, 1984. Accepted for publication December 28, 1984.