Abstract: Potassium exchange-equilibrium were obtained from quantity-intensity (Q/I) isotherms, i.e., K equilibrium activity ratio (AR0k), K labile (Klab), equilibrium potential buffering capacity for k (PBCkequ), free energy of k replishment (-ΔGkequ), the Gapon selectively coefficient (kG), least soil exchangeable potassium (Emin) and initial equilibrium concentration solution potassium (C, k0). Characterization of these relations provides general information on the nature of K equilibrium and surve as a good index of K supplying power of soil. Plant availability of soil potassium is controlled by dynamic interactions among its different pools. Misunderstanding of these dynamics leads to mismanagement of soil fertility. These relationships were investigated in some selected soils of Fars province, Iran. K equilibrium activity ratio (AR0k) ranged between 1.74 to 19.90 (mmol dm-3)0.5, labile K values fluctuated within the range 1.28 to -9.78 meq 100 g-1 soil. And equilibrium potential buffering capacity (PBCkequ) fluctuated from 31.14 to 100.64 meq 100 g-1 (mmol dm-3)0.5. Potassium was significantly controlled by soil properties. Potassium activity was controlled more by silt (r = 0.80**), Mn-BCD (r = 0.67*), Mn-OX (r = 0.73*) and the -ΔGkequ values had significantly correlated with silt (r = 0.79*), Mn- BCD (r = 0.68*) Mn-OX (r = 0.71*). This result implies that studies of potassium dynamics of soils should additionally consider the level of Fe and especially Mn, as well. The Q/I parameters provide useful information for understanding K+ availability in calcareous soils and can be used for K+ fertilizer recommendations.