The thermodynamic model is proposed of heavy metals state in nosaline chernozem common carbonate of steppe zone of South Russia. The climate is continental semiarid, annual precipitation of 500-550 mm. The parent rocks are Carbonate and Carbonate-sulfate loess-like loam and clay. The chernozem common is thick, not solonized, humus 4.2%, physical clay 49.3%, clay 31.3%, CaCO3 0.14% (up to 3-6% at the depth of 1,3-1,6 m), pH = 7.8, exchangeable cations: Ca 2+-342 mmol kg-1 , Mg 2+-27 mmol kg-1 , Na +-6 mmol kg-1. The soil section in autoromorphic landscape. The soil solution water extract analyzed by standard methods. The state of ions in soil solution is influenced by ion association. The method of ion pairs in water solution is used. At high ionic force in soil solution are formed electrically neutral ion pairs СаСО3°; CaSO4°, MgCO3°, MgSO4°, charged ion pairs CaHCO3 + , MgHCO3 + , NaCO3-, NaSO4-, CaOH + , MgOH +. On the basis of of ion pair's method the algorithms and computer programs were developed to calculate the real equilibrium forms of ions in the soil solution. The concentration of free and associated macro-ion's forms were calculated by iteration procedure according to analytical ion concentration considering ion material balance, linear interpolation of equilibrium constants, Method of ionic pairs, laws of: initial concentration preservation, operating masses of equilibrium system. Concentration constants of ion pair dissociation were calculated following the law of operating masses. Were determined the quantity of macro-ion free form and coefficient of ion association γe as a ratio of ions free form to its analytical content e ass an C / C . To interpret a behavior of heavy metal in soil solution two appraches are proposed: an additional equation for microelement to the mathematical model of macro-ions; an individual