Phosphorus is a limiting factor in the Pine Rockland slash pine forest (Pinus elliottii var. densa) because of high soil pH (average 7.5) and abundance of Ca, Mg, Fe, and Mn. Increase in available content of P after fire can result in alterations of P-mineral solubility in soils. The objective of this research was to assess impacts of fire on solubility of phosphate minerals by constructing activity diagrams between HPO₄^2- versus with different pH values. Stability diagrams of phosphate within a presence of monocalcium-P, monetite, dicalcium-P, beta-tricalcium-P, octacalcium-P, hydroxyapatite, strengite, vivianite, Mg₃(PO₄)₂, MgHPO₄:3H₂O, Mn₃(PO₄)₂, and MnHPO₄ were constructed basing on ionic activities simulated from the Minteq’s geochemical models at the atmospheric CO₂ partial pressure of 0.00038 atm. The input data for the Minteq’s equilibrium speciation models included soil pH, ionic strength, and concentrations of PO₄-P, Ca, Mg, K, Na, Fe, Mn, Zn, Cu, NH₄⁺, and NO₃^- extracted from soil solution. Ionic strength was calculated from EC (electrical conductivity) by Marion-Babcock equation. Our result showed that monocalcium-P and strengite were not present in the diagram, vivianite mineral was at equilibrium in pre-fire while other selected phosphate minerals were under-saturated, the equilibrium shifted to MnHPO₄ in 14-day postfire, both vivianite and MnHPO₄ reached the equilibrium in 30-day postfire, and the equilibrium went back the pre-burn status in 180 days after fire. With these findings, we concluded that solubility of phosphate in the Pine Rockland forest ecosystem was primarily controlled by vivianite and MnHPO₄.