A SIMPLE CONCEPTUAL-MODEL FOR PREDICTING THE DISSOLUTION OF PHOSPHATEROCK IN SOILS

Laboratory experimental and literature data were integrated to develop a simple, conceptual model to describe and predict the dissolution of a reactive phosphate rock (Gafsa phosphate rock, GPR) in soils. The m odel showed that initial solution Ca concentration strongly influences the time required for a single application of GPR (at 30 kgP ha(-1)) to dissolve. The model predicts that all of the GPR will dissolve with in a year of application in an unlimed, acid (pH(w) 4.5) loam. However , if the soil has previously been limed to pH(w) 5.8, and contains per manent charge only, the model predicts that only about 50% of the GPR would have dissolved by the end of the second year after application. On the other hand, if a previously limed soil (pH(w) 5.8) contains var iable-charge components, the model predicts that virtually all of the GPR would have dissolved in this soil by the end of the second year af ter application. These results emphasise that, even in the presence of a high proton supply, solution Ca has an overriding influence on the dissolution of GPR. The faster rate of GPR dissolution in the limed so il with variable charge, compared to that in the limed soil with perma nent charge only, demonstrates the ability of the variable-charge comp onent of soil to act as a sink for some of the lime-derived Ca. Accord ing to the solubility product principle, this allows more GPR to disso lve. Because of the generally stronger buffering of soil for P than fo r Ca, a relatively large proportion of any P removed by leaching and p lant uptake is buffered by the sorbed phase. Consequently, the influen ce of leaching and plant uptake on GPR dissolution is attributed prima rily to the removal of the relatively less-strongly buffered Ca.