Changes in solid‐ and solution‐phase phosphorus in soil on acidification

Summary There is confusion about the effects of acidification on soil P because its impact on sorption and on P dissolution have not been separated. We investigated these effects with two soils treated with H 2 SO 4 by both percolation and batch titration methods. Repeated percolation with dilute acid removed less P than repeated percolation with water. In contrast, the more acid was added in the batch method the more P was released. The conflicting outcome was attributable to the differences in the closed and open systems; the solutions used to remove P differed in their ionic strength and composition. The Q/I plots and the fractionation analyses revealed that the P release and the changes in the dynamic relation between solid‐and solution‐phase P are a net effect of two coincident, but opposing reactions on oxide surfaces. As a result of acidification the sorption sites available on the Fe and Al oxide surfaces will diminish through the resorption of P dissolved by acid from the Ca‐bound fraction. This increase in the P saturation enhances the desorption tendency, which is counteracted by the protonation of the oxide surfaces enhancing the sorption affinity.