Dependence of the Osmotic Pressure and Electrical Conductivity of Soil Solutions on the Soil Water Content

Abstract

The osmotic pressure and electrical conductivity were determined with the use of instrumental methods (centrifuging, conductometry, and cryoscopy) in equilibrium soil solutions as the functions of moisture (the mass ratio between solid and liquid phases) for the soils of different geneses and textures. These functions proved to have a nonlinear character with one extremum. A theoretical substantiation to this phenomenon is suggested. It is based on the concept of competing interphase interactions in soil and develops the classical ideas by A.A. Rode. Upon the low soil water content, surface (molecular and ion-electrostatic) forces of the solid phase bind water molecules and prevent them from hydrating ions and dissolving electrolyte salts in the soil solution; as a result, electrical conductivity and osmotic pressure values approach zero in this case. With an increase in the water content, water molecules escape gradually from the energy field of surface forces, and their activity (chemical potential) and dissolving capacity also increase. This results in the expected growth of the electrical conductivity and osmotic pressure of the equilibrium solution extracted from the soil. The maximum values of the studied parameters are observed at the water content approximately equal to the maximum molecular water capacity (or to the boundary zone, within which capillary gravitational forces influencing the soil liquid phase become prevailing over the surface interphase interaction forces). The further increase in the soil water content lowers the electrical conductivity and osmotic pressure of equilibrium solution because of dilution of the fixed mass of electrolyte salts. For quantitative description of the revealed functional dependence, we suggest an empirical mathematical model in the form of modified equation of lognormal distribution.