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Ion Products and Solid-Phase Activity to Describe Phosphate Sorption by Soils¹

ABSTRACT

Solution equilibria and regular solid-solution concepts were combined to relate phosphate solubility to amounts of added phosphate in eight acidic Missouri soils. Phosphate sorption studies were done by shaking KH₂PO₄ solutions with soil for 16 d and measuring pH, Eh, electrical conductivity, Ca, Mn, Al, Fe, and P contents of the solution. These data were used to compute ion products and to estimate the activities of solid-phase phosphate minerals. It was shown that solubility relationships for the reaction of H₂PO₄ with Al(OH)₃ could be interfaced with estimates of solid-phase activities and used to predict phosphate in solution as a function of a soil constant (C), soil pH, and the amount of phosphate added. Activities for the equilibrium equation,

p(H₂PO₄) = 10.50 – pH – p[Al(OH)₃] + p(AlPO₄),

were estimated. The value of p[Al(OH)₃] was determined from the data to be a linear function of pH, 2.41 – 0.42 pH, for soils used in this study. The value of p(AlPO₄) was determined by statistically fitting observed p(AlPO₄) and P_added to a polynomial equation similar in form to the one for a regular solid solution:

p(AlPO₄) = C – 9.4 x 10^–3 (P_added) + 6.9 x 10^–6 (P_added)²,

with r² = 0.96. Values of p(H₂PO₄) were calculated from pH and P_added by these combined equations, and compared with measured p(H₂PO₄) with an r² of 0.96. The system was shown to have solubility features similar to those described for a regular solid solution. Combining solution equilibria and regular solid-solution concepts was useful in providing a chemical basis for interpreting phosphate sorption by soils.

¹ Contribution from the Missouri Agric. Exp. Stn., J. Ser. no. 9147. Approved by the Director.

² Professor of Agronomy and Former Research Assistant, Univ. of Missouri, Columbia MO 65201, respectively.

Received for publication August 13, 1982. Accepted for publication June 11, 1984.