| Abstract: | The effect of pH and calcium (Ca) on the dissolution of Gafsa (GPR) and Christmas Island A (CIPR) phosphate rocks (PR) was examined in closedincubation and open-leaching systems in six acid, Malaysian soils. The pH of the soils was increased to between 4.5 and 6.1 by incubating with calcium carbonate (CaCO3); soil was also incubated with calcium chloride (CaCl2) to provide equivalent amounts of Ca without causing any marked change in pH. In the closed-incubation system, dissolution of reactive GPR was overall higher (11–70%) than for the less reactive CIPR (12–43%) after 60 days of incubation. Dissolution of PR decreased with increasing levels of CaCO3 or CaCl2, but the decrease was more pronounced in CaCO3-treated than in CaCl2-treated soils. Increasing additions of CaCl2 decreased the size of the available sink for Ca from 84.0 to 0 mmol (+) kg–1 soil. Although the addition of CaCO3 increased the cation-exchange capacity (CEC) of these variable-charge soils (from 23.0 to 199.0 mmol (+) kg–1 soil), most of the newly-created exchange sites were occupied by Ca added through CaCO3. This was responsible for the decrease in size of the sink for Ca. Addition of CaCO3 also decreased the proton supply from 260.7 to 0 mmol (+) kg–1 soil, which in conjunction with the decrease in size of the Ca sink decreased the dissolution of PR. The effect of CaCO3 and CaCl2 on PR dissolution varied between soils and was related to pH-buffering and the Ca-sink size. In an open-leaching system, large amounts of Ca (8–40%) added as CaCO3 were removed in the leachate and hence the decrease in GPR dissolution with CaCO3 addition was less in the open-leaching than in the closed-incubation system. |
