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The basic laws of electricity relate the parameters of electrical fields with the density of mobile electrical charges in the media. The parameters of electrical fields, such as electrical resistivity, conductivity, and potential, are related with various soil properties exponentially, obeying Boltzmann’s distribution law. Chapter IV describes distributions of electrical charges in soil profiles under different soil-forming processes. The profile distributions of electrical parameters are provided for the major soil orders. The electrical parameters, easily measured in situ, are shown to be essential characteristics for evaluation of soil genesis and morphology.

Volume density of mobile electrical charges determines the values of the electrical parameters. Soil water content, cation exchange capacity; salt content, base cations, and humus content are soil properties responsible for the formation of mobile electrical charges in soils (Chapter III). In general, the density of mobile electrical charges in soils is determined by the exchangeable ions in the solid and liquid phases. Yet the influence of the ions on measured electrical parameters differs for different soils. Three basic cases of the influence can be considered. First is the formation of a high density of electrical charges primarily by the exchangeable ions of the electrical double layer around the soil matrix, which is the characteristic of Histosols, Mollisols, Vertisols, and some non-saline Aridisols. Second case is the establishment of medium and low densities of electrical charges by ions of the electrical double layer as in Spodosols and Alfisols. Third case is the formation of a high density of mobile electrical charges by soluble ions in the soil solution, which is the characteristic for saline soils, primarily for various Aridisols. The three cases of formation can occur in some soils simultaneously. Moreover, the influence of each case sometimes varies in different horizons of the same soil. However, such separation is convenient for study the distribution of mobile electrical charges and electrical parameters in different soil profiles.

Let us consider the first and third cases with a high density of mobile electrical charges, such as in Histosols, Mollisols, and saline soils. According to the Boltzmann’s distribution law, the variation of charge density does not influence much the variation of electrical potential or resistivity at the high density of electrical charges (Eq. [35]). Thus, the soils with high density of mobile electrical charges, e.g. Mollisols, saline Aridisols, and Histosols, possess low electrical resistivity and potential and appear in far right part of the exponent function (Figs. 20 and 22). These soils should not show high variations in density of mobile electrical charges and electrical parameters along the profiles (Fig 24a). On the contrary, even a slight variability in charge density due to eluvial-illuvial process should change the measured electrical parameters in profiles of Spodosols and Alfisols (Fig 24b). These soils have an overall low density of mobile electrical charges, which corresponds to the left part of the exponential function and represents a high change in electrical parameters (Fig. 20).

Besides the general ideas about the values of electrical charge density in various soils, the major soil-forming processes in different soils should be considered. The soil-forming processes continuously distribute electrical charges and form various charge densities in different soil structures, such as aggregates, horizons, profiles, and soil sequences (Glazovskaya, 1973; Targul’yan, 1978; Buol et al., 1989). Under the processes, various uniform or layered soil electrical profiles can be formed. Modern soil science recognizes a number of soil-forming processes, incorporated into four basic groups (Buol et al., 1989). The first group of the processes includes an addition to a soil body, such as the processes of enrichment, cumulization, melanization, and litter accumulation. The second group incorporates a loss from a soil body, such as the processes of leaching and surficial erosion. The third group includes processes of material translocation within a soil body: eluviation-illuviation, calcification-decalcification, salinization-desalinization, alkanization-dealkanization, lessivage, pedoturbation, podzolization, desilication, and gleization. The processes of fourth group involve those responsible for transformation of soil materials in place without translocation. These are the processes of decomposition, synthesis, humification, paludization (peat accumulation), ripening, mineralization, and loosening-hardening.

Different elementary soil-forming processes influence distributions of electrical charges in soil profile differently. Processes of the first and fourth groups usually lead to enrichment of soil horizons with electrical charges, whereas processes of the second group cause depletion in electrical charges. The coupled processes of third group redistribute electrical charges within soils forming pronounced layered soil profiles with electrically different horizons. For example, the humus accumulation in Mollisols leads to the enrichment of topsoil with mobile electrical charges (Fig. 25a). The result of all the processes (hydrolyses of primary and secondary minerals, leaching, lessivage) in Spodosols is depletion of albic horizon in mobile electrical charges and their translocation into spodic horizon (Fig. 25b).

Each type of the soil-forming processes or their combinations forms a distinct soil profile. The combination of horizons in a soil profile induces the distribution of electrical parameters in the profile. In a number of cases when one soil-forming process is primarily responsible for soil formation two horizons with contrast electrical properties are formed: the horizon of accumulation of electrical charges and horizon of depletion. The measured electrical parameters should reflect such soil profile organization. Electrical potential or resistivity should show low values in accumulation horizon and high values in depleted horizon (Fig 25). Such profile structure is common for some Spodosols, Mollisols, and saline Aridisols.

A combination of two elementary soil-forming processes, such as humification and leaching, may form a three-layered soil profile (Fig. 24b). The top accumulative horizon (A) has a relatively high humus content and cation exchange capacity hence, a high density of mobile electrical charges and low resistivity and potential. The underlying elluvial (albic) horizon (E) mostly consists of bleached sand grains. The density of mobile electric charges is much lower in E horizon than that in A horizon, therefore, the potential and resistivity of E horizon is higher. The electrical parameters of the illuvial (spodic or argillic) horizon (B) is the lowest in the profile attributable to enrichment with the fine clay material and Fe^+2/+3, Al⁺³, Mn⁺⁵, and other cations.

Our investigations show a similar three-layer distribution of electrical parameters within eluvial-illuvial soil profiles in a number of soils, such as Spodosols, Alfisols, Ultisols, and Oxisols (Pozdnyakov et al., 1996a). The combination of a large number of soil-forming processes can complicate profile organization of soils. In many soil profiles the variations of electrical parameters in the horizons are attributable to complex influence of three or more soil-forming processes, pollution, or differences in parent materials. The information about the soil morphology and soil-forming processes can be obtained by study of distributions of electrical parameters in soil profiles. The influence of soil-forming factors, such as climate, parent material, vegetation, and relief, on soil profiles also can be evaluated with measured electrical parameters. The research objective of this chapter was to establish relationships between soil-forming processes/factors and the measured electrical parameters. The electrical parameters, mostly electrical resistivity and potential, were measured in profiles of basic soil orders, such as Spodosols, Alfisols, Histosols, Mollisols, and Aridisols. The measurements were conducted in situ with the electrical geophysical methods of four-electrode profiling, vertical electrical sounding, and self-potential (Chapter II).

4. 2. Soil-forming processes and electrical parameters of soils in humid areas

Soils with eluvial-illuvial differentiation of profile, such as Spodosols and Alfisols are dominated in automorphous landscapes of humid areas (Ivanova and Rozov, 1970). The different processes are responsible for such differentiation. Soil genesis recognizes the processes of lessivage and podzolization, which form eluvial-illuvial soil profile. The lessivage and podzolization processes often occur in combinations with the processes of peat formation, gleyzation, and humus accumulation, forming a spectrum of soils in cool humid areas.

Eluvial processes (podzolization and lessivage) redistribute the mobile electrical charges in profiles of Spodosols and Alfisols. Podzolization prevails in soils of cool coniferous forests (Spodosols) under percolation of acid rainwater (Bridges, 1970). The water dissolves organic matter and causes breakdown and leaching of clay minerals from albic (E) horizon. The iron oxides, which are mobilized from the surfaces of the mineral soil particles, together with the aluminum and organic matter, are moved with the soil solution down through the profile. The material is then deposited in illuvial spodic (Bhs) horizon lower in the profile. The deposition of material in spodic horizon is explained by wetting-drying cycles, changes in pH, presence of flocculation ions, and the breaking down of the iron-humus bonds by aging or by bacterial attack (Bridges, 1970).

The lessivage process prevails in soils of temperate deciduous and coniferous-deciduous forests (Alfisols) with ustic water regime. The conditions are not so strongly acidic as in Spodosols. In most Alfisols a lighter-colored albic (E) horizon can be distinguished below accumulative A horizon. Clay is mechanically leached from the E horizon, practically without breakdown of alumosilicates (Bridges, 1970). The E horizon of Alfisols is also depleted of Ca, Mg, K, and Na. Below the albic horizon clay particles, some iron, and bases accumulate forming the distinct argillic (Bt) horizon.

Thus, the eluvial processes of podzolization and lessivage reduce the amount of dispersed minerals with high sorption in eluvial albic horizon and increase the amount of them in illuvial (spodic and argillic) horizons. As a result of the eluvial processes the relative amount of primary minerals, such as SiO₂ and field spars, increases in eluvial horizons. The molecular ratios of SiO₂/R₂O₃, SiO₂/Fe₂O₃, and SiO₂/Al₂O₃ increase in albic horizons of Spodosols in comparison with accumulative and illuvial horizons. The soils in humid areas are usually leached of soluble salts yet have low CEC and density of mobile electrical charges. The eluvial-illuvial variation of CEC in soil profiles highly alters the distributions of electrical parameters in the profiles. Therefore, the eluvial horizons, depleted of dispersed clay material, have the highest electrical resistivity and potential, whereas illuvial horizons have lowest electrical parameters in profile. The accumulative (A) horizon, if present, has the intermediate values of electrical potential and resistivity. Thus, the profile curves for Spodosol can be two-layered and for Alfisol mostly three-layered, as the accumulative horizon is usually present in profiles of Alfisols. Such distribution of electrical parameters is similar to SiO₂ distribution and inverse to clay distribution in Spodosols and Alfisols profiles.

Measurements of electrical potentials were conducted in soil pits at the central part of Klin-Dmitrov ridge, 107 km northwest from Moscow (Appendix). The ridge was formed by glaciers about 15,000 years ago. Parent material is sandy clay-loam under-laid on the depth of 2 m by clay loam glacial till. Soils are Alfisols formed under different vegetation types and varied by the intensities of eluvial processes.

The electrical potentials of eluvial horizons are generally on 20-50 mV higher than the potentials of illuvial horizons. The more pronounced is the eluvial process the higher the difference between electrical potentials of eluvial and illuvial horizons (Fig.26) In Mollic Haplocryalf (Fig. 26e), for example, the difference is about 50 mV. The electrical potentials of the accumulative horizon (А1) have intermediate values between those of eluvial and illuvial horizons. Sharp changes of electrical potential are attributed to the border between accumulative and eluvial horizons if the border is morphologically distinguishable (Fig.26b and e). When the soil properties are gradually change from accumulative to eluvial horizon, the electrical potential also increases gradually (Fig. 26a, c, and d). The electrical potential increases from the border of A1 and A1E to the middle of E horizon. The maximum potential is at the center of eluvial horizon, and then the potential sharply decreases to the border between E and EВ1. The electrical potential also varies within the profiles. Variations of electrical potential in the transitional EB horizons are shown in Fig. 26c and d. The separate lines represent the measurements in albic “tongues” appeared in illuvial horizons of these Glossocryalfs. Starting from the lower boundary of EB1 horizon and further through B1, B2, and C horizons the electrical potential decreases steadily. Usually on the depth 100-120 cm the electrical potentials, is stabilized and almost constant to the depth of 300-320 cm (Fig. 27).

Fig. 27. Deep profile of electrical potential in Typic Haplocryalf.

Notably, the three-layered distributions of the electrical potential are opposite to the profile distribution of soil water content in Cryalfs (Fig. 26f). As was discussed in Chapter III, the water content of soils in humid areas is a relatively stable soil property and depends mostly on soil retention ability. Therefore, the horizons of fine texture have higher water content, resulting in a higher density of mobile electrical charges and a decrease in electrical potential of these horizons.

Electrical potentials were studied in a 20-m transect (Fig. 28). The transect was set on Typic Ferrudalf in south part of Klin-Dmitrov ridge where clay loam glacial till was closer to the surface (0.5-1.0 m). The electrical potentials of the till were very low and often did not exceed 5 mV (Fig. 28). The values of electrical potential for different depths are shown in Fig 28 a-e. The variations in potential are caused by variations in the depths of the horizons and boundaries between horizons. The separate intrusions of elluvial material in illuvial horizon are also common for this soil. Such intrusions and tongues of eluvial horizons had higher electrical potential than bearing them illuvial horizon, but lower than those for the eluvial horizon.

Thus, the electrical potential distribution in Alfisols is under major influence of lessivage and podzolization processes forming the eluvial-illuvial profile of the soils. The variations in electric potential are caused by soil morphology. The measurements show that a specific three-layered distribution of electrical potential in the profiles of Alfisols is relatively stable in space and time.

Electrical resistivity was measured in soil pits with the four-electrode probe (Chapter II) in Wenner configuration (a = 5 cm). Electrical profiling was also conducted from the soil surface with different electrode arrays to map the electrical resistivity of various soil layers. The profile curves were obtained for the soils with vertical electrical sounding from the soil surface. Measurements were conducted in the Moscow area on an experimental research station of Moscow State University "Chashnikovo" (Appendix). Soils formed on clay loam glacial till and on sandy glacial outwash were investigated to compare the influence of soil parent materials on the electrical parameters.

The distributions of electrical resistivity in profiles of Glossudalf reflect the same three-layered organization of soil profile as was revealed with electrical potential measurements. The electrical resistivity of surface accumulative horizons is always lower than that of eluvial horizons, but higher that resistivity of illuvial horizons and underlying parent material. Starting from the top of A horizon, electrical resistivity steadily increases to the middle of eluvial horizon. Then the resistivity decreases with the increase in illuvial material in soil profile (Fig. 29).

Parent materials influence the thickness of soil profiles. In general, if other soil-forming factors are similar, deeper soil profiles with thicker horizons are formed from sand parent material than clay and clay-loam materials. The soils formed from coarse sand material tend to have higher values of electrical resistivity in all horizons than soils of finer texture (Fig. 29).

The VES measurements show that minimum values of electrical resistivity for Bt horizons are 40-50 ohm m and found on AB/2>2.4 m for Haplic Glossudalf, formed on clay-loam parent material (glacial till). The electrical resistivity of accumulative (A) horizon (АВ/2=0.15-0.22 m) is higher than that of B horizon, but lower than that of E horizon (AB/2=0.6-2.4 m). The maximum electrical resistivity for Haplic Glossudalf formed on clay-loam does not exceed 800 ohm m. Figure 30 shows the VES profiles of electrical resistivity measured in the same locations as the resistivity profiles measured in the soil pits shown in Fig. 29.

As supported by the studies of other Alfisols in Moscow and Tver’ areas, soils formed on sandy and gravel glacial outwash materials usually have more extended profiles and higher values of electrical resistivity for characteristic horizons. Electrical resistivity for B horizon (АВ/2 > 4.8 м) of sandy Alfisols is about 150-200 ohm m. The maximum values of electrical resistivity of E horizon in sandy Alfisols are higher than those of E horizons in Alfisols, formed on clay-loam parent material under the same vegetation and relief. The resistivity of sandy layered glacial deposits can be as high as 2000 ohm m and is about 900 ohm m in average, yet the electrical resistivity in eluvial horizons of Alfisols formed on coarse sandy parent material can reach 3000 ohm m.

Thus, the three-layered profiles of electrical resistivity were revealed for all Alfisols formed on different parent materials. Parent material always had lower electrical resistivity compared with that of soil horizons. The resistivity of clay and clay-loam parent material does not exceed 50 ohm m, the resistivity of sandy-loam is about 150 ohm m, and resistivity of sand can be 1000 ohm m or higher. For Alfisols formed on any parent material accumulative horizons have lower resistivity than that of eluvial horizons. The electrical resistivity of illuvial horizons is the lowest in soil profiles and close to the resistivity of parent material. Generally, the electrical resistivity measured with four-electrode probe in soil pits tends to be higher than resistivity measured on the same soils with VES method. The electrical resistivity measured directly in soil pits with four-electrode probe can reveal more details in soil profiles than the VES method applied from the soil surface. Nevertheless, the VES method outlines the elluvial-illuvial organization of Alfisols and allows quick estimation of thickness of soil horizons without any disturbance. By observation and interpretation of VES profiles we could obtain exact thicknesses and locations of soil horizons in Alfisol profiles. The values of the electrical resistivity in E and B horizons have shown to be good indicators of intensity of the elluviation-illuviation processes in soils.

Poor drainage induces a widely spread soil-forming process of gleyzation leading to formation of gley or hydromorphic soils (Bridges, 1970). Hydromorphic soils can be found in association with all zonal soils, where water can gather in sufficient volume and for sufficient time to produce the effect of gleyzation. The gley or hydromorphic soils occur in subordinated landscapes in arid areas, but most widely spread in humid areas.

Gleyzation develops when water saturates a soil, filling all the pore spaces and driving out the air. Any remaining oxygen is soon used by the microorganisms and anaerobic conditions are established. The soil water contains the decomposition products of organic matter. In the reducing conditions and in presence of organic matter, iron compounds are chemically reduced from the ferric to the ferrous state. In the ferrous form iron is much more soluble and removed from soil once the water logging disappears. The season water-logging in cycle with complete soil drainage is thought to be one reason of Spodosol formation (Zaidelman, 1991). If the water-logging persists for a considerable time, the mobile products of gleyzation can accumulate in soil profile. The Bt horizon of Alfisols under certain condition is a barrier of low water permeability and can cause gleyzation in E and A horizons of these soils. The enrichment of gley horizons with mobile ferrous iron, soluble organic matter, and reduced colloidal material decreases electrical parameters in the horizons.

The gleyzation decreases the electrical potentials in profiles of Haplocryalfs (Fig. 31). Without surface gleyzation the potential in the eluvial horizon is as high as 50 mV in Mollic Haplocryalf (Fig. 31a). Comparing the gleyed and non-gleyed horizons of similar texture, the electrical potentials of gleyed horizons are usually of 15 mV lower than potentials of non-gleyed horizons. The difference is most notable for the soils gleyed by the groundwater, when the whole soil profile is highly gleyed as shown on Fig. 32b.

Electrical resistivity of Oxyaquic Haplocryalf is also lower than that of its automorphous analogue, Mollic Haplocryalf, as shown in Fig. 32. The electrical resistivity measured with VES is lower by 100 ohm m for the eluvial gleyed horizon of Oxyaquic Haplocryalf than for the eluvial horizon of Mollic Haplocryalf (Fig. 32c). An even higher differences were obtained for electrical resistivity, measured in soil pits (Fig. 32b). As shown in Figs. 33a and b the electrical resistivity distribution changes from eluvial-illuvial three-layered profile for Mollic Haplocryalf and Oxyaquic Haplocryalf, to two-layered profile for Argic Cryaquoll. The decrease in electrical resistivity is more pronounced in surface alluvial and eluvial horizons than in illuvial horizons (Fig 32).

Conditions of very poor soil drainage encourage peat accumulation in association with the gley soils. Under centuries of water-logging, deep (3-6 m) peat soil profiles can develop, which have different physical properties from those of the under-laid parent mineral material and show practically no signs of gleyzation. Thus, the peat accumulation becomes a single soil-forming process in peat soils (Histosols). Peat accumulation is characterized by the accumulation of weak-decomposed organic matter and accompanied by changes in many soil properties (Puusijarvi and Robertson, 1975; Rahmatullan, et al., 1976). The most important change is increase in sorption ability of peat soil. The CEC of peat soils is extremely high (about 170 mg eq/100 g) (Fuchsman,1980). The organic compounds of peat, such as humic and fulvic acids and their salts, have relatively high dissociation rates and mobilities. These properties increase the density of mobile electrical charges in peat soils and horizons. Therefore, peat accumulation increases the density of mobile electrical charges in soils.

Comparing the accumulative horizons of soils in humid areas, the umbric epipedon of Alfisols has much higher electrical resistivity and potential than fibric and hemic epipedons of Histosols. Virgin Histic Cryaquolls and Limnic Haplofibrists enriched with iron compounds and carbonates have very low electrical parameters, which only slightly vary in the profiles. The electrical potentials in these soils are not higher 3-7 mV (Shuch and Wanke, 1969). The electrical potentials of other Histosols do not exceed 12 mV, whereas in eluvial horizons of Alfisols the potentials can be as high as 40-60 mV.

The electrical resistivities of Histosols are about 20 ohm m, which is of two orders lower than resistivity of eluvial horizons in Alfisols. The profile distributions of electrical resistivity in various cultivated Histosols in Yachroma valley are shown in Fig. 33. The Limnic Haplosaprists with carbonate accumulation and Fluvaquentic Haplosaptists with ferrous nodules can be distinguished from Typic Luvihemist with VES measurements (Fig. 33a, b, c, and d). The electrical resistivity of carbonated Limnic Haplosaprists formed near terrace in Yachroma valley does not exceed 15-18 ohm m and usually lower in the whole profile than resistivity of ferric Fluvaquentic Haplosaptists formed in central part of the valley. The electrical resistivity of less decomposed Typic Luvihemist formed near the Yachroma river reaches 80 ohm m in the topsoil (Fig. 33d, 3). Measurements of electrical potential of the top horizons have also shown, that Saprists and, especially, those enriched with mobile iron and carbonate have smaller (3-5 mV) potentials than Hemists and Fibrists with low decomposition (8-12 mV).

The drainage, cultivation, and intensive agricultural usage with application of high rates of fertilizers decrease the electrical parameters of Histosols. The top part of cultivated horizon has the highest electrical resistivity (45-60 ohm m) due to leaching and consumption of nutrients by plants. The electrical resistivity decreases with the increase of soil depth. The difference in electrical resistivity between fertilized and non-fertilized Typic Haplohemist vanishes on about 0.5 m depth (Fig. 33e). Drainage and further usage of Sapric Haplohemrist under perennial grasses decreased electrical resistivity of topsoil from 58 to 28 ohm m (Fig. 33f).

Thus, the peat accumulation process and further cultivation of peat soils lead to increase in density of mobile electrical charges in these soils. The electrical resistivity and potential of peat soils are extremely low and not much differentiated in profile. Nevertheless, the analyses of the measured electrical parameters allow distinguishing between ferrous and carbonated version of Haplosaprists and provide control on fertilizer application on these soils. Agricultural management of Histosols and Alfisols based on electrical parameters measured with geophysical methods is discussed in Chapter V.

Catena is basically a "chain" of soils formed under variation of one soil-forming factor while other factors remain similar (Bushnell, 1942). The first recognized catenas were hydrological-topological sequences. Such catenas distinguish freely drained automorphous soils formed on the top positions in landscapes and water-logged hydromorphous soil of the low positions in landscapes. Catena concept embodies the basic laws of soil geography and geochemical relations of soils formed in toposequences (Glazovskaya, 1973).

The catena relations explain the mechanisms of soil formation and laws of electrical parameter distribution in soils. As shown in IV.2.1-IV.2.3, hydromorphic soils, such as gley and peat soils, have higher density of mobile electrical charges and lower electrical parameters than automorphous soils in humid areas. These soils are located in subordinated positions of humid landscapes and accumulate large amount of the mineral and organic substances leached from the automorphous soils of freely drained positions in landscapes. Thus, mobile electrical charges concentrate in subordinated soils of landscapes. Soil electrical parameters reflect the transport and redistribution of substances in landscapes, geochemical connection and formation of soil sequences.

In humid areas the typical catena consists of various Alfisols and Histosols as shown in Fig. 34a. The automorphic Alfisols have rather high values of electrical parameters especially in eluvial horizons. The geochemically subordinated Histosols are enriched with the mobile electrical charges and have the least values of electrical parameters.

Soils of eluvial part of catena have the highest electrical resistivity of all the studied catena soils, especially in the eluvial horizons (Fig. 34b, 1, 2, and 3). The electrical resistivity decreases in middle part of catena on Aquic Ferrudalf (4) and Oxyaquic Hapludalf (5), as these soils receive some material from the above soils. Gleyzation also decreases the electrical resistivity of Aquic Ferrudalf and Oxyaquic Hapludalf. The electrical resistivity of eluvial horizon of Aquic Ferrudalf does not exceed 500 ohm m and for Oxyaquic Hapludalf the electrical resistivity of the whole profile is less than 150 ohm m. The organic peat soils are located in the lowest part of catena accumulating all the minerals from the above located soils. Moreover, soluble organic material produces plenty of mobile electrical charges. These soils (6, 7, and 8) are characterized by high CEC and low electrical resistivity (30-50 ohm m). Electrical resistivity slightly increases in Aeric Humaquept formed on the near-river hill and showing some sights of eluviation in profile.

The profile curves of electrical resistivity also change in soils of the catena. The profile curves are presented in Fig. 34c for the different soils of the catena. Some profiles are shown twice in different scales for the comparison with adjacent soils. The cultivated Typic Ferrudalf on the top of catena has two-layered distribution of electrical resistivity (Fig. 34c, 1). The soil is heavily manured and the A horizon is mixed by plowing with the most of the AE horizon. Slight increase of electrical resistivity is noted at AB/2=2 m corresponding to the EB horizon. The non-cultivated Typic Ferrudalf has typical three-layered distribution of electrical resistivity with the maximum of 800 ohm m in the E horizon (Fig. 34c, 2). The increase of electrical resistivity is notable for Typic Fraglossudalf on AB/2=2.4 m (Fig. 34c, 3). This increase is corresponded to the placic horizon of cemented iron with high resistivity. The distribution of electrical resistivity is still three-layered for Aquic Ferrudalf and Oxyaquic Hapludalf, but with smaller values in all horizons (Fig. 34c, 4, 5). The electrical resistivity of gleyed eluvial horizons of these soils is about 300-400 ohm m. The peat soils at the bottom of catena are formed under the major process of peat accumulation. They have the lowest electrical resistivity in the catena, the resistivity varies around 30-50 ohm m (Fig. 34c, 6, 7, 8). The three-layered resistivity profile is observed for Aeric Humaquept formed on mineral layered river deposits (Fig. 34c, 9). The soil is young and the variation of resistivity indicates the variability in parent materials rather than the differentiation of soil profile caused by soil-forming process.

The catena example described above shows the change in intensities of different soil-forming processes with the change of water availability conditions and parent material in the typical toposequence of humid landscapes. When water percolation changes to water logging, the intensities of podzolization and leaching processes decrease, but gleyzation and peat accumulation increases, increasing the density of mobile electrical charges. The same situation appears under the change of climatic conditions from north to south of European part of Russia. Soils of this extended area are distributed according to climatic zones and soils of automorphous landscapes change in a row of Spodosol-Alfisol-Mollisol-Aridisol from approximately 70⁰ to 40⁰ latitude (Appendix) with the increase of annual temperature and decrease of precipitation. The intensities of eluviation processes decrease in the row of Spodosol-Alfisol-Mollisol, whereas the intensity of humus accumulation increases. The humus accumulation than decreases from Mollisols to Aridosols, but calcification, alkanization, and salinization of soils increase.

The change in intensities of major soil-forming processes with the climatic conditions leads to increase in densities of mobile electrical charges in soil profiles from north to south of Russia. The distributions of electrical parameters in soils should change accordingly. The distributions of electrical resistivity shown in Fig. 35 were measured in the soil pits in different areas of Russia. With the decrease in intensity of the eluviation-illuviation process in a row of Ferrudalf-Kandiudalf-Hapludalf-Argialboll-Calciudoll, the three-layered differentiation of electrical resistivity in the soil profile vanishes (Fig. 35). Soils of hot climate and ustic water regime, such as Typic Calciudoll and Xeric Calcigypsid, have much lower electrical resistivities than soils of cool humid areas (Typic Ferrudalf and Typic Kandiudalf).

The soil sequence in Fig. 34 are the classic example of extended soil zonal organization, widely reported by Russian soil scientists (Gerasimov, 1968; Kovda et al., 1969; Fridland, 1976). However, on other continents, soil distributions usually lack of pronounced zonal organization with the change of longitude. Nevertheless, the change in mobile electrical charges should appear under the change of water regimes in soils of different climatic zones. Munn et al. (1978) reported that the depth of carbonate layer and thickness of mollic epipedon in Mollisols decrease with the average annual precipitation from east to west of the USA indicating decrease in intensity of humus accumulation and increase in calcification and salinization of soils in this direction. Consequent increase in accumulation of soluble salts in soil profiles of this extended soil sequence would decrease the electrical resistivity and potential of the soils.

The soils in arid and semiarid areas are formed under less amount of water than soils in humid areas. Therefore, the eluvial-illuvial processes practically do not occur in these zones. The major soil-forming process in Mollisols under ustic water regime is humus accumulation. In Aridisols under xeric and aridic water regimes the processes of calcinization-decalcinization, salinization-desalinization, and alkanization-dealkanization take place.

Humus accumulation is a process of darkening of light colored mineral initial unconsolidated materials by admixture of organic and formation of surface mollic and umbric horizons. The humus accumulation actually consists of a several processes including littering (the accumulation of plant debris on soil surface), humification and mineralization of this debris, and melanization (relocation) of organic matter in soil profiles (Buol et al., 1989). During the humus accumulation the complex organic compounds break into simpler, more soluble organic and mineral compounds. Therefore, the humus accumulation leads to the increase in density of mobile electrical charges in surface accumulative horizons. The humus formation in Mollisols create abundance of complex humic acids with high amount of aromatic cyclic compounds (Duchaufour, 1965). Intensive humus accumulation as in profiles of Mollisols usually occurs on calcium enriched parent materials. Therefore, the complex organic acids can form insoluble salts with calcium, which preserve these organic compounds from leaching. These organo-mineral calcium compounds form the largest part of soil CEC. Therefore, the humus accumulation process in soils of semi-arid areas usually occurs together with calcification.

The process of calcification is characteristic for soils of low-rainfall areas in continental interior situations. Although downward movement of materials does take place in these soils leaching is slight and soluble salts are not removed completely from the soil profile. These soils are usually wetted to a depth of 1-1.5 m when the water begins to move upward due to transpiration and evaporation. A calcic horizon of calcium carbonate accumulates where the impetus of the downward percolating rainwater (or snow melt) is lost. As these soils are relatively unleached, the exchange capacity is dominated with calcium ions, and to a less extent with magnesium ions (Bridges, 1970).

The calcium and magnesium cations of soil CEC influence the density of mobile electrical charges in Mollisols and electrical parameters as shown in Chapter III. High CEC of Mollisols determines the low electrical resistivity and potential of these soils. Electrical resistivity of an accumulative horizon about 15 ohm m, and resistivity of an illuvial horizon about 20-30 ohm m as were obtained for Argiudolls of Central Chernozem Reserve of Russia (Chan and Kirichenko, 1976), Calciaquolls of Tambov area (Chan, 1975), and Haploxerolls of Moldova (Karapet’yan, 1977). Since the humus accumulation decreases with the soil depth, the two-layer distribution of electrical parameters should be characteristic for Mollisols. The electrical resistivity and potential are lower in A horizon than in B horizon, due to enrichment of A horizon with organic and mineral materials, but the difference between these two horizons is less pronounced than in Alfisols (Fig. 35). Such distribution reflects the profile organization of Mollisols, which have basically two soil horizons: accumulative epipedon (A) and subsoil (B). The two-layer profile of electrical parameters in Mollisols is determined by the humus accumulation in A horizon, biological fixation of mineral elements in humus compounds, uniform distributions of SiO₂ and R₂O₃ in soil profile, illuviation of carbonates in B horizon, and absence of soluble salts (Kononova, 1961; Ivanova and Rozov, 1970). Summarily, profiles of Mollisols are uniform in texture and basic chemical properties; A and B horizons differ only by humus content.

Mollisols have relatively low electrical resistivity (20-30 ohm m) in accumulative (A) horizons, compared with that of Alfisols (200-600 ohm m) (Fig. 36). In carbonated illuvial (B) horizons electrical resistivity is slightly higher than in accumulative horizon, but does not exceed 40-50 ohm m. The VES profile shows a two-layered distribution of electrical resistivity with a gradual increase of resistivity in transition zone between A and B horizons.

The electrical resistivity decreases in soils located in subordinated positions of catena on Mollisols (Fig. 36). The decrease reflects the relocation of the material from the top soils in catena to the bottom soils as described for the landscapes of humid areas. However, the decrease in electrical resistivity for the Mollisol catena is less pronounced than that for Alfisol-Histosol catena (Figs. 34 and 36). This can be explained by the less developed leaching in soils of the area and the conditions of the catena formation. Mollisols on Central European Plain in Russia are formed on the uniform loess deposits, which started to deposit after the last glaciation (Valderan) at about 10,000 years ago. The soils are formed on extensive plain areas. Therefore, the soil catena in the area mostly consists of differently eroded Hapludolls (Fig. 36). The catena is located on the slope of less than 15 degrees. Nevertheless, the leaching of organic and mineral materials from the top of catena and the surficial erosion slightly decreased the electrical resistivity in soils of the bottom part of the catena.

The intensity of humus accumulation decreases with the decrease of the average annual precipitation and increase in average temperature. Thus, Aridisols have less humus content in A horizon than typical Mollisols, which should generally increase the electrical potential and resistivity in top horizons of Aridisols compared with those of Mollisols.

As climate became more arid the processes of salinization, alkanization, and dealkanization occur more widely in Mollisols and Aridisols overlaying with the humus accumulation and calcification processes. When the water evaporation considerably prevails the water infiltration in case of a high groundwater table, soluble salts tend to accumulate in the top soil horizons. Salts in soil solution are drawn upwards by capillary action and are then deposited as the water is evaporated. Such process is commonly called salinization. It was recognized that if periodically larger amount of water is applied to saline soils (Haplosalids), the sodium cations are involved in exchange sites of soil clays, replacing the calcium and magnesium ions (Van Beek and Van Breemen, 1973). The enrichment of soil CEC with sodium is called alkanization and natric horizons of distinct columnar structure are formed in such soils (Natrargids). Dealkanization (solodization) may occur on these soils under persistent water logging conditions. The soluble salts and exchange cations are hydrated and finally leached from the top soil horizons (Aquic Natrargid). Variations in water regimes can occur in arid areas within the different forms of microrelief, forming the high complexity of soil cover in arid areas (Fridland, 1976).

We investigated the electrical parameters in soils of Haplocalcid-Haplosalid-Natrargid-Aquic Natrargid complex. For all the soils, the higher potentials are attributable to eluvial horizons and lower to illuvial horizons. The greatest difference of potentials between these horizons is observed for Typic Haplosalid (>40 mV), slightly smaller for a Typic Natrargid (35-40 mV), and 20-25 mV for Typic Haplocalcid. The gradients of potentials at the transition from A to B horizons in Natrargid is about 20 mV/cm and in Haplocalcid is less than 10 mV/cm (Fig. 37a). Regardless the difference in electrical potential gradients of Haplocalcid and Natriargid, the water content distributions are almost the same for these two soil profiles (Fig. 37b). Therefore, the measured differences in electrical potential gradients in these soils are related with some other soil properties, such as humus content and CEC.

The electrical potential was measured in a 20-m transect with Sodic Haplocalcid and Typic Natrargid. Figure 38 shows that the electrical potentials for the upper horizons of Sodic Haplocalcid and Typic Natrargid do not differ to the depth of 15 cm. However, the electrical potentials of eluvial and illuvial horizons in Natrargid are much lower than the potentials in Sodic Haplocalcid (Fig. 38d and e). The data analysis has shown, that the differences between electrical potential of illuvial horizons of Typic Natrargids and Sodic Haplocalcid are statistically authentic. Iso-potential areas of < 0 mV outline the illuvial horizon of Typic Natrargid (Fig. 38e). The electrical potentials of natric horizons with some soluble salts are less than 5 mV. The electrical potential distributions in profiles of Sodic Haplocalcid and Typic Natrargids are stable in time, especially for the depths lower than 20 cm.

Many researchers reported the low electrical resistivity and high conductivity of saline Aridisols (Chan and Kirichenko, 1976; Halvorson and Rhoades, 1976; Karapet’yan, 1977; Lesch et al., 1992). Although the surface layers of these soils can be very dry and posses relatively high resistivity, the sub-surface diagnostic horizons (Bn) of sodic soils have resistivity about 10 ohm m and saline soil layers have resistivity as low as 3 ohm m. Thus, the saline and sodic soils can be easily distinguished from other Aridisols in a soil complex by the low values of electrical resistivity. The non-saline Aquic Calciudolls (Tambov area) has the greatest electrical resistivity (40-60 ohm m), the resistivity of saline Calciudolls is lower (20-30 ohm m), and the resistivity of saline Natrudolls is lowest (5 ohm m) (Chan, 1975). Karapet’yan (1977) reported the similar values of electrical resistivity, measured with VES for complexes of saline, sodic, and non-saline Mollisols and Aridisols of Moldova.

Our VES measurements were conducted on Natrargids in Dzanibek Research Center (Crimea) and on saline Mollisols in Don valley (Fig. 39). Notably lower electrical resistivity was measured in saline soils than in their less saline analogues in both areas. The profiles of electrical resistivity suggest two-layered electrical organization of the soil profiles. The upper accumulative-eluvial layer has lower salt and water content and outlined by higher resistivity. Soluble salts and/or exchangeable sodium are accumulated in illuvial horizons having considerably lower resistivity.

The soil-forming processes alter the distributions of mobile electrical charges in soil profiles. Three major soil-forming processes, such as eluviation, gleyzation, and peat accumulation, influence the profile distributions of electrical resistivity and potential in soils of humid areas. The podzolization process redistributes of mobile electrical charges and forms eluvial-illuvial differentiation in profiles of Spodosols and Alfisols. Typically, the electrical resistivity and potential are highest in the eluvial horizons, lowest in the illuvial horizons, and moderate in alluvial horizons of Alfisols (Table 10), forming typical three-layered electrical profile organization. The peat accumulation and gleyzation lead to enrichment of soil horizons or whole soil profiles with mobile electrical charges. Intensive soil cultivation with high impact of tillage and fertilizers also increases the densities of mobile electrical charges in topsoil and decreases the electrical parameters. The cultivation, humus/peat accumulation, and gleyzation can considerably decrease the differentiation in electrical parameters in eluvial-illuvial profiles of soils in humid areas (Table 11). The different intensities of the soil-forming processes form various soil profiles and horizons with distinct distributions of electrical parameters. The typical values of electrical resistivity and potential for the different horizons of soils in humid areas are summarized in Tables 10 and 11.

The intensities of humus accumulation, calcification, alkanization, and salinization processes increase in that order with the aridity of climate. All the processes basically increase the density of mobile electrical charges and decrease the electrical parameters. The water regime of Mollisols allows leaching of soluble salts and calcium from the topsoil. The soluble salts and calcium can accumulate in subsoil, decreasing the electrical parameters. The subsoil of Mollisols and, especially, Aridisols are enriched with soluble salts, carbonates, and gypsum. Thus, the two-layered electrical profiles with electrical resistivity and potential higher in A horizon than in B horizon are characteristic for the soils in arid areas (Table 12).

Table 12. Electrical parameters for characteristic horizons of soils in arid and semi-arid areas.