The long-term effect of organic anions on phosphate sorption by soils has not been studied extensively. In this work, we investigated the time course of P sorption in 12 Cerrado (mostly Oxisols) soils by measuring the P concentrations in soil suspensions supplied with various rates of phosphate over a 256-day period. Phosphate sorption fitted extended Freundlich or Langmuir equations that included a potential time term. Equation parameters reflecting the significance of slow relative to fast P sorption were positively correlated with the organic matter (OM)/specific surface area (SSA) ratio. This suggests that the more extensive the coverage of soil-sorbing surfaces by organic compounds, the more significant the long-term P sorption as a result of phosphate slowly replacing organic anions at sorption sites. The fact that the relative affinity for phosphate at a low equilibrium concentration decreases as the OM/SSA ratio increases supports the hypothesis that phosphate and organic anions compete strongly for the same sorption sites. Oxalate and malate were the only two low-molecular-weight organic anions that occurred in measurable amounts in the soil suspensions; the amount of (oxalate + malate) released from the solid phase was related roughly, in a 2:1 ratio, to that of P sorbed after the initial (1-day) P sorption. The results suggest that the blocking action of organic compounds on P sorption is only transient. In practice, this fact should be considered when timing application of P fertilizers.
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