## Basic Principles And Theory Of Ec

Electrical conduction through soil is due to the presence of free salts in the soil solution and exchangeable ions at the surfaces of solid particles. The soil equivalent resistance model (Sauer et al., 1955) provides the basis for the formulation of a mechanistic soil ECa model applicable to the entire range of soil solution concentrations (Rhoades et al., 1989; Shainberg et al., 1980). Three pathways of current flow contribute to the ECa of a soil: (1) a conductance pathway traveling through alternating layers of soil particles and soil solution, (2) a conductance pathway traveling through the continuous soil solution, and (3) a conductance pathway traveling through or along the surface of soil particles in direct and continuous contact (Rhoades et al., 1989, 1999a). These three pathways of current flow are illustrated in Figure 4.3. Conceptually, the first pathway can be thought of as a solid-liquid, series-coupled element, and the second and third pathways represent continuous liquid and solid elements, respectively.

Rhoades et al. (1989) introduced a model for ECa describing the three separate current-flow pathways acting in parallel:

ECa = (e; +E9rws)2+EflCwsE:CsS + (0sc • ECsc) + (0wc • ECwc) (4.5)

9ss • ECws + 9ws • ECss where 8ws and 8wc are the volumetric soil water contents in the series-coupled soil-water pathway (cm3 cm-3) and in the continuous liquid pathway (cm3 cm-3), respectively; 8SS and 8sc are the volumetric contents of the surface-conductance (cm3 cm-3) and indurated solid phases of the soil (cm3 cm-3), respectively; ECws and ECwc are the specific electrical conductivities of the series-coupled soil-water pathway (dS m-1) and continuous-liquid pathway (dS m-1); and ECss and ECsc are the electrical conductivities of the surface-conductance (dS m-1) and indurated solid phases (dS m-1), respectively. Equation (4.5) was reformulated by Rhoades et al. (1989) into Equation (4.6):

(ess + ews)2 • ECW • ECSS + (0w _ 0 J, ecw (4.6)

where 8w = 8ws + 8wc = total volumetric water content (cm3 cm-3), 8sc • ECsc was assumed to be negligible, and solution conductivity equilibrium was assumed (i.e., ECw = ECws = ECwc, where ECw is average electrical conductivity of the soil water assuming equilibrium). According to Equation (4.6), ECa is determined by the following five parameters: 8w, 8ws, 8SS, ECss, and ECw. Using the following empirical approximations, Rhoades et al. (1989) showed that these five parameters are related to four measurable soil properties, which include soil salinity (ECe; dS m-1), saturation percentage (SP; SP is the gravimetric soil water content at saturation), bulk density (pfc; Mg m-3), and gravimetric water content (8g; kg kg-1):

Pathways of Electrical Conductance

Soil Cross Section

Pathways of Electrical Conductance

Soil Cross Section

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