Evaluating and quantifying the liming potential of phosphate rocks

The liming potential of phosphate rock was evaluated with theoretical calculations and quantified by laboratory titration and soil incubation. Three anions present in the carbonate apatite structure of phosphate rock that can consume protons and cause an increase in pH when dissolved from apatite are PO4(3−), CO3(2−), and F−. The pKa for HF is so low that F- has very little effect on increasing pH. The pKa for 2 protons on H2PO4 and H2CO3 are sufficiently high enough to cause an increase in pH with PO4(3-) and CO3(2-) released into solution if the pH range is between 4 and 6. Because of the greater molar quantity of PO4(3-) compared to CO3(2-), PO4(3-) exerts a greater affect on the liming potential of P rock. For a variety of phosphate rocks with a axes ranging from 9.322 to 9.374 A° in the carbonate apatite structure, the theoretical % calcium carbonate equivalence (CCE) ranges from 59.5 to 62%.With the presence of gangue carbonate minerals from 2.5 to 10% on a weight basis in the phosphate rocks, the theoretical %CCE ranges from 59.5 to 63.1%. Use of AOAC method 955.01 for quantifying the %CCE of North Carolina phosphate rock (NCPR) and Idaho phosphate rock (IDPR) resulted in %CCE ranging from 39.9 to 53.7% which were less than the theoretical values. The lower values measured in the AOAC method was presumed to be due to formation of CaHPO4 or CaHPO4 2H2O precipitates which would result in less than 2 protons neutralized per mole of PO4 (3-) released from carbonate apatite. The highly concentrated solution formed in the method was considered not indicative of a soil solution and thus determined %CCE values would be suspect. A soil incubation study was conducted to determine a more appropriate %CCE value in a soil environment using Copper Basin, Tennessee soil with a soil pH of 4.2. Agricultural limestone, NCPR, IDPR, and a granulated IDPR were added to 100 g of soil at rates of 0.1, 0.3, 1, 3, and 10 g/kg soil, incubated for 105 days at field moisture capacity, and analyzed for changes in soil pH and P. The %CCE of each phosphate rock addition was determined using limestone as a standard curve. The relationship between %CCE and % dissolved P followed a quadratic model 2 2 where %CCE=8.47 + 0.0078 (%dissolved P)2 (r2=0.84). At 0% dissolved P, the model predicted 8.47% CCE which was probably due to gangue carbonate minerals. The experimental model showed qualitative agreement with theory showing increased liming ability with increased dissolved P from the P rock. However, the model showed lower %CCE than theoretical calculations when %P dissolved ranged from 20 to 60%.