Abstract/Summary

A carbonate-substituted phosphate rock (Gafsa, GPR) was allowed to dissolve in the presence of one or more of the following: an acid pH buffer (a source of protons), hydrous ferric oxide gel (Fe gel, a sink for P), and a cation exchange membrane (CEM, a sink for calcium). This allowed a quantitative evaluation of the individual and combined influences of three of the most important soil-related factors (pH, P sink, and Ca sink) which control phosphate rock (PR) dissolution in soils. At 44 d, pH alone (pH 6.1 compared to 4.5) had little effect on the extent of dissolution (1.5 and 6.0% dissolution of P, respectively), probably because of the Ca common-ion effect resulting from the preferential dissolution of the more-soluble CaCO, accessory phase. The inclusion of a P sink at pH 6.2 resulted in only a 3.2% dissolution of P after 44 d, whereas at pH 4.5 it increased dissolution from 6.0% to 9.8%. This suggests that pH and P sorption have an interactive effect in influencing GPR dissolution. Inclusion of a Ca sink had the largest influence on GPR dissolution. This was due to the ability of the Ca sink to remove accessory CaCO2. At pH 6.2 the dissolution of P was 10 times larger in the presence of a Ca sink than it was in the presence of a P sink. Despite the smaller ratio of Ca sink size to GPR-Ca content relative to that of the P sink size to GPR-P content, the Ca sink was more effective in promoting GPR dissolution. The presence of the Ca sink resulted in twice as much dissolution of P at pH 4.5 than at pH 6.5. At pH 4.5, and in the presence of both the Ca sink and the P sink, 95% dissolution of GPR occurred over the 44-d experimental period.