INFLUENCE OF CALCIUM-CARBONATE ON THE DISSOLUTION OF SECHURA PHOSPHATE ROCK IN SOILS

Different amounts of CaCO3 (5.3 to 20% w/w) (180-75-mu-m) were mixed with Sechura phosphate rock (SPR) (180-75-mu-m) and incubated with Davidstow and Withnell soils. These soils differ in their proton supply and Ca-buffering capacity. The Ca-buffering capacity of Davidstow soil was also changed by adding different amounts of cation-exchange resin (CER). The consumption of protons and the release of Ca during the preferential dissolution of CaCO3 decreased the dissolution of the SPR (measured by P release). However, the negative effect of CaCO3 on SPR dissolution at near equilibrium (60 d) depended on both the proton supply and Ca-sink size of the soil. The Davidstow soil had an adequate proton supply (43.6 mmol H kg-1 pH unit-1) but a small Ca sink (32.0 mmol kg-1), and the dissolution of SPR at 60 d decreased linearly from 27.5 to 19.5% with increasing CaCO3 content. The Withnell soil had an adequate Ca sink (75.5 mmol kg-1) but a small proton supply (21.4 mmol H kg-1 pH unit-1), and the amount of P dissolved at 60 d also decreased (from 49 to 35%) with increasing CaCO3 content. Adding CER to the Davidstow soil increased the Ca-sink size from 32.0 to 39.0 mmol kg-1 and almost prevented the decrease in SPR dissolution with increasing CaCO3 content. This suggests that, in soils with an adequate proton supply and Ca-sink size, CaCO3 present as an accessory mineral in PR materials has a negligible effect on the dissolution of the PR.