5.2 Electrical property of soil
            The knowledge of soil electrical property of soil system (solid, liquid and gaseous phase) is required for several applications in engineering and geosciences. Electrical properties of soil system have multiple phases due to the following reason (Fang and Daniels 2006): (a) Soil and water has inherent electrical characteristics, (b) electrical energy is related to thermal and magnetic properties and difficult to separate (c) electro-chemical interaction in soil-water system is sensitive to surrounding environment. The important factors influencing soil electrical properties are particle size distribution, compaction, water content, mineral structure, mineral surface condition, characteristics of pore fluid and ion exchange reaction. The direction of electric current is the direction of flow of ions. The zone of electric field depends on the magnitude of electric charge and soil-water system. The electrical property of soil is defined in terms of electrical resistivity, conductivity, capacitance and dielectric property. Resistivity and conductivity quantifies the flow of electric current through a medium. Electrical resistivity is the most common method for defining electrical property of soil-water system. There are a lot of literature that describe the use of resistivity or conductivity for indirectly assessing water content, extent of soil contamination or salinity, unit weight, porosity, frost depth, buried objects etc. (Fang and Daniels 2006). Capacitance is the charge storage capacity of a material. Dielectric property defined in terms of dielectric constant (κ) implies the ability of a material to perform as an insulator. This property is not measured but computed by Eq. 5.1.

κ = C x (d/A)

5.1

            C is the capacitance in Farad, d is the length of specimen and A is the cross sectional area of specimen. κ is an important property that has been used extensively for indirect correlation with different soil properties.
            When the soil is fully dry the electrical resistivity is very high because there is little interaction between the electrical charge (or energy) and ions present in the soil. When the soil is wet, resistivity decreases and electrical conductivity increases due to the formation of water film around soil surface. Such a film act as a bridge between electrical charge and ions present in the soil. Flow of electricity through soil can be due to direct current (DC) or due to alternating current (AC) of particular frequency. The effect produced by both on soil is different. To assess the effect of flow of alternating current in soils, it is necessary to determine κ and electrical conductivity (σ_ec) of the soil corresponding to the frequency of the current (Smith-Ross 1933). This is because these characteristics are dependent on the frequency of AC. The density, water content of soil and frequency of AC are the important parameters affecting electrical properties of soil under AC. The κ value for dry soil and minerals varies between 2.8 to 2.6 for a frequency variation from 100 to 10000 kHz. As moisture content increases, the κ variation with frequency increases considerably. For pure water, κ is close to 80. Such a wide variation in κ values is used for indirectly determining volumetric water content of soils.