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Results and Discussion: densities of mobile electrical charges in soils

The geophysical methods do not measure individual charges in soils, but rather outline places with different densities of electrical charges. Thus, the measured with the geophysical methods electrical parameters provide information about volume density of mobile electrical charges in soils. Volume density of electrical charges is proportional to the number of electrically charged particles in an elementary volume of media. Volume density of mobile electrical charges designates the content of ions, which neutralize charges on a free surface (Schuffelen, 1972). As surface charge in soils is formed by sorbed (exchange) cations and anions (Sparks, 1997), the ion exchange capacity is equivalent to the density of exchange surface charges. The ion exchange capacity of the soil is the product of the soil specific surface and surface charge density (Uehara and Gillman, 1981).

Soil charge is determined by an ion exchange, which in turn depends on three factors: isomorphic substitutions in clay minerals, breakage of ionic bonds in organo­mineral complexes, and alteration of charge distribution in macromolecules of soil organic matter. Therefore, soil chemical properties, such as humus content, base saturation, cation exchange capacity (CEC), soil mineral composition, and the amount of soluble salts influence the ion exchange in soils. These soil properties are related with the volume density of mobile electrical charges in soils and, in turn, with the soil electrical parameters. Soil chemical properties, responsible for the formation of soil ion exchange capacity, are related with the total amount of available charges in soils.

Soil physical properties, such as water content and temperature, influence the mobility of electrical charges in soils. From our studies of the relationships between electrical resistivity and soil bulk density or soil water content (Figure 3) in laboratory conditions the mobility of electrical charges exponentially increases with the increase in those properties (Pozdnyakova, 1999). Other soil physical properties, such as soil structure, texture, and bulk density, alter the distribution of mobile electrical charges in soils. Thus, the volume density of mobile electrical changes is related to many soil physical and chemical properties.

Electrical parameters, such as resistivity and potential are exponentially related with the volume density of mobile electrical charges based on Boltzmann's distribution law (Bolt and Peech, 1953):

Boltzmann's distribution law         [3]

here is the ratio of the density of mobile electrical charges in the local volume vs. standard conditions, vi is the valence of the i-th ion, e is the electronic charge, k is the universal gas constant, and T is the absolute temperature. Therefore, from Eq. [3] the volume density of the mobile electrical charges is exponentially related to the electrical potential. According to Ohm's law the electrical potential is in direct proportion to the electrical resistivity. If the change of a soil property, such as water content, bulk density, or salt content causes a proportional change in the volume density of the mobile electrical charges, a relationship between electrical parameters and soil property (SP) can be expressed as

SP = al exp(– bl φ) = a2 exp(– b2ER)                              [4]

where al, a2, b1, and b2 are empirical parameters; φ is the electrical potential, and ER is the bulk electrical resistivity of the soil. Some relationships between soil properties and volume density of mobile electrical charges may not obey a single exponential equation on the whole range of property variation. For example, the relationship between soil water content and electrical resistivity was approximated with different exponents at different ranges of soil water content due to the influence of soil-water retention (Pozdnyakova, 1999).

                                                                                    relationship between electrical resistivity and peat soil water content (lab)

Figure 3 An example of experimental relationship between electrical resistivity and water content of a peat soil.

While measuring electrical parameters in situ, it is difficult to study separately the relationship between a soil property and electrical parameters. Therefore, the relationship of Eq. [4] may be less strong when measured under the simultaneous variations of many soil properties. Nevertheless, the general exponential relationships were obtained for many soil properties, such as total soluble salts, CEC, base saturation, humus content, etc. both in laboratory and field conditions (Pozdnyakov et al., 1996; Pozdnyakova, 1999; Pozdnyakova et al., 2001).

Considering the qualitative structure of mobile electrical charges soils can be broadly subdivided into two groups. The first group is soils with low soluble salts and CEC filled by Ca+2, Mg+2, A1+3, and Ir.  These soils are formed by the processes of podzolization, lessivage, eluviation-illuviation, humification, mineralization, and gleization in humid areas (Wilding et al., 1983). Spodosols, Alfisols, Gelisols, Histosols, Ultisols, and Mollisols can be considered as soils of the first group. The processes of calcification, salinization, alkanization, pedoturbation, humification, and mineralization in arid and semiarid areas form the second group of soils with CEC filled by Ca+2, Mg+2, and Na+ and, in some soils, high salinity. Soils of the second group represented by Aridosols, Vertisols, and some Mollisols. Inseptosols and Entisols can be assigned to either the first or second group depending on the primarily soil processes dominating in the soils.