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Reaction of Orthophosphate with a Sandy Soil at Constant Supersaturation¹

ABSTRACT

Phosphate (P) sorption experiments with an acid sandy soil at constant supersaturation with respect to metal phosphates are described. This constant supersaturation was obtained primarily by the phosphato-stat (c_P-stat) technique, which enables accurate measurement of a reaction rate at constant supersaturation. The reaction rate is found to decrease with the amount sorbed, S, according to dS/dt = k₁ exp (–k₂S). This is ascribed to the formation of coatings of metal phosphates on the metal oxides present in the soil. The reaction rate is also dependent on the accompanying cation (Na, K). The above mentioned rate equation is also applicable to systems that already contain P at the start of the measurements, implying that it can be used for the prediction of the long-term sorption capacity of similar agricultural soils.

¹ Contribution from the Laboratory of Soils & Fert., Agriculture State Univ., 3 De Dreyen, 6703 BC Wageningen, The Netherlands.

² Chemist and Soil Scientist, respectively.

Received for publication March 21, 1980. Accepted for publication September 29, 1980.