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Effects of Reduction and pH Changes on Phosphate Sorption and Mobility in an Acid Soil¹

ABSTRACT

Suspensions of a silt loam in 0.01M CaCl₂ (1:6) were incubated at redox potentials of 500, 280, 65, and –150 mV for 26 ± 2 days in factorial combination with pH treatments of 5.0, 6.5, and 8.0. Samples were then removed and equilibrated with graded concentrations of KH₂PO₄ under O₂-free conditions. Isotopically-exchangeable P was measured on the zero treatment and P sorption on the other treatments after 24 hours equilibration. Sorption properties of the soil were determined by application of the Langmuir equation.

The largest changes occurred in the most reduced treatment (–150 mV) and at pH 8.0. At pH 5.0, an unnaturally low pH for an anaerobic soil, reduction to –150 mV decreased P sorption at solution P concentrations below 20 ppm because of the reduction and dissolution of ferric hydrous oxides. At higher concentrations sorption was increased by ferrous phosphate precipitation due to the very high concentrations of soluble Fe. At pH 6.5, reduction caused a very large increase in P sorption apparently because of increased adsorption on freshly precipitated amorphous ferrous hydroxides. At pH 8.0, P sorption was deceased at P concentrations below 2 ppm, probably because of increased surface negative charge, but increased at higher P concentrations due to precipitation of Ca phosphates. At all pH values, reduction increased the concentration of native P in solution. At pH 6.5 this was caused by an increase in labile P due to the release of P occluded and precipitated in ferric compounds. At pH 5.0 and 8.0 it was caused by a decrease in P bonding energy. Reduction to –150 mV caused the largest increase in soluble P and the smallest increase in P sorption capacity at pH 5.

¹ Contribution from the Laboratory for Wetland Soils and Sediments, Center for Wetland Resources, Louisiana State Univ., Baton Rouge, LA 70803. Supported in part by U.S. Environmental Protection Agency Grant No. R803612-01

² Research Associate (present address: Agricultural Research Centre, Tamworth, N.S.W. 2340, Australia); and Professor, respectively, Center for Wetland Resources, Louisiana State Univ., Baton Rouge.

Received for publication August 18, 1978. Accepted for publication October 13, 1978.

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