CHEMICAL EQUILIBRIUM OF SOIL SOLUTION IN STEPPE ZONE SOIL

Abstract

Dynamics of material composition, migration and accumulation of salts is determined by chemical equilibrium in soil solution. Soil solution contains associated electrically neutral ion pairs CaCO₃⁰; CaSO₄⁰, MgCO₃⁰, MgSO₄⁰, charged ion pairs CaHCO₃⁺, MgHCO₃⁺, NaCO₃^-, NaSO₄^-, CaOH⁺, MgOH⁺. Calculation method is proposed for quantitative assessment of real ion forms in the soil solution of chestnut solonetz soil complex. Were proposed equations to calculate free and associated forms of ions. To solve the equations were used an iteration, a linear interpolation of equilibrium constants, a Method of Ionic Pairs including a law of initial concentration preservation, a law of the operating masses of equilibrium system, the concentration constants of ion pair dissociation on the law of operating masses. Was determined the quantity of ion free form and a coefficient of ion association as ratio of ions free form to analytical content γ_e = C_ass/C_an. The association of ions varies in individual soils and soil layer. Increasing soil solution salinity amplifies the ions association. In form of ionic pairs in soil solution are: 11.8-53.8% of Ca²⁺; 9.4-57.3% of Mg²⁺; 0.7-11.9% of Na⁺; 2.2-22.3% of HCO₃^-, 11.8-62.7% of SO₄^2-. The ion CO₃^2- is high associated, the share of ions in associated form is up to 92.7%. The degree of soil solution saturation was obtained for three level of approximation accounting on analytical concentration, calculated association coefficient, calculated coefficient of association. Relating to thermodynamic solubility product S₀, the mathematical product of analytical ionic pairs indicated super saturation of soil solutions up to K₁ = 100, taking into account calculated coefficient of association ion activity super saturation of soil solutions is absent, K₃≈1. Only for solonetz chestnut meadow K₃≈2-5. The soil solution saturation degree in soil profile and laterally in landscape varies. The quantitative assessment of real ion forms in the soil solution allows explain evolution of landscape of salted soils, structure of soil cover. Calculations fulfilled show that a possibility of soil degradation scenario taking into account the laws of association of ions in soil solution is much more probable and dangerous than it was assessed before. New understanding of water-salt transfer, geochemical barriers and ecological functions of soil will help to improve rainfed and irrigational agriculture.