IMPORTANCE OF PROTON SUPPLY AND CALCIUM-SINK SIZE IN THE DISSOLUTION OF PHOSPHATE ROCK MATERIALS OF DIFFERENT REACTIVITY IN SOIL

The dissolution of a range of phosphate rock (PR) materials (Gafsa, GPR; Jordan, JPR; North Florida, NFPR; and Tundulu, TPR) was evaluated in three UK soils (Nercwys, Davidstow and Withnell) which differed in proton supply and P-buffer capacity. The Ca-sink size was adjusted by adding different amounts of a cation-exchange resin (CER) to the soils. In the presence of a large proton supply (pH(w) 3.8-4.8, pH-buffer capacity 52.5-36.5 mmol OH- kg-1 pH unit-1), the dissolution of GPR, JPR and NFPR in the Nercwys and Davidstow soil-CER mixtures was strongly influenced by the size of the Ca sink. A twofold increase in Ca-sink size in these two soils caused an increase in PR dissolution of 44-120%. Except for TPR, the increase in PR dissolution per unit increase in Ca-sink size was the same for the three PR materials. In the Withnell soil-CER mixtures, where the initial proton supply was relatively small (pH(w) 4.8-6.1, pH-buffer capacity 23.7-21.4 mmol OH- kg-1 pH unit-1), the increase in PR dissolution with increasing Ca-sink size was less (24-68%) than in the other two soils (44-120%). Also, for the Withnell soil, the increase in PR dissolution with increasing Ca-sink size decreased in the order GPR (68%) > JPR (49%)> NFPR (24%), a trend consistent with the decrease in PR reactivity. The maximum dissolution of TPR was only 8-22% under favourable conditions of proton supply and Ca sink, and was little affected by Ca-sink size.