The reasons for the formation of light-colored acid eluvial horizons in the soil profile

Concepts concerning the origin of light-colored acid eluvial horizons within the soil profiles are considered. In Russia, the current concept is related to the polygenetic origin of these horizons indicating that their formation is affected by acidic hydrolysis, lessivage, and gleying. The acidic hydrolysis under aerobic conditions was shown not to provide the reduction of Fe (III) to Fe(II) and its transfer to soil solution. Lessivage is not an obligatory factor that controls the formation of light-colored acid eluvial horizons, since its signs are often absent in the profile of soils. The only process responsible for the eluviation of Fe, Mn, and Al; the removal of hydroxide and oxide iron coatings from mineral grains; increasing the relative Si content; and the appearance of the whitish color is gleying under conditions of a stagnant-percolative water regime. Precisely this factor is the single reason for the formation of light-colored acid eluvial (podzolic) horizons. Therefore, they are monogenetic in origin.     

Forms of acid hydrolysis and gley formation and their role in the development of light-colored acid eluvial (Podzolic) horizons

Nowadays, three processes, namely lessivage, acid hydrolysis, and gleying, are considered as responsible for the development of loamy and clayey podzolic soils. However, as was shown earlier, lessivage is not obligatory for their origin. In view of assessing the reasons for the formation of light-colored acid eluvial horizons, this article deals with the role of acid hydrolysis under aerobic conditions against the background of a percolative water regime and of two forms of gleying in the development of the horizons mentioned above. One form of gleying occurs under permanent anaerobic conditions against the background of a stagnant water regime; the other one is formed under pulsating anaerobic-aerobic conditions against the background of a stagnant-percolative water regime. As a result, three large genetically individual groups of soils are formed: nondifferentiated brown and gley, and differentiated podzolic soils on different parent rocks. The two latter forms of gleying are identical in their effects on the mineral substrates. They cause the iron removal from the soils. Among the three processes considered, the last one (gleying under a stagnant-percolative water regime) is the single reason for the leaching of most of the metals, the formation of light-colored acid eluvial horizons and their clay depletion, and for the differentiation of the soil profile.     

A concept of Gleyization and its role in the pedogenesis

Three simple factors ‐ excessive moistening, anaerobic microflora and organic matter, are indispensable and sufficient conditions for gley formation. This process is always characterised by a non‐silicate iron loss from the soil fine earth or soil plasma. Gley formation takes place under conditions of stagnant or stagnant‐percolative water regimes.

In the second case gley formation induces a drastic acidification of the mineral soil part, lessivage, removal of iron, aluminium, calcium, magnesium, bleaching of the soil fine earth, and it appears the features of soil with eluvial, acid, bleached horizons. Therefore soils with such horizons should be regarded as manifestations of gley formation in conditions stagnant ‐percolative water regime on acid, neutral or leached parent material. Under influence of stagnant water regimes Fe of mineral substrat is removed and unconsiderable eluviation of Ca and Mg takes place. pH of parent material does not change or has the trend to increase. In this case does not arise a soil with bleached horizons.     

Solods under conditions of excessive surface and ground moisture in Western Siberia: Properties,hydrology, and genesis

Depending on conditions of formation, solods should be differentiated into two groups: solods of ground overmoistening and solods of surface overmoistening. Criteria are offered to distinguish soils according to the ratio between the clay in the B2 horizon and that in the A2 horizon, as well as according to the changes in the soil pH. Formation of gley under conditions of stagnant to percolative water regime is a necessary and sufficient cause for light-colored acid eluvial horizons to form in their profile. In the main properties of the solid phase (acidity, total chemical composition, and distribution of silt), gley solods are identical to soddy-podzolic and chernozem-like podzolic gley soils.     

Genesis,hydrology, and properties of soils in mesodepressions waterlogged by surface water in the northern Ryazan forest-steppe

On the interfluves and in small depressions of the Ryazan forest-steppe, under periodic stagnation of surface water, acid chernozem-like soils with a relatively thick humus horizon, podzolic horizons, and marble-colored gleyed B1 and B2 horizons are formed. The eluvial horizons of these soils contain Mn-Fe nodules, and dark humus coatings occur in the illuvial horizons. In the spring, the eluvial horizons of these soils are excessively moistened and gravitational water stagnates on the soil surface for 3–4 weeks. The formation of the acid light-colored eluvial horizons of the soils on leached rocks is related to gleying under the conditions of the stagnant-percolative regime. Their total thickness is 15–25 cm and more. According to the properties of their solid phase, these horizons are similar to the podzolic horizons of soddy-podzolic gleyed soils. These soils have not been represented in the classification systems of soils of the USSR and Russia. Based on the principles of the substantial-genetic classification, one of the authors of this article [9] referred this soil to gleyed podzolic chernozem-like soils, thus, considering it as an individual genetic soil type. The gleyed podzolic chernozem-like soils differ from the leached chernozems by their low productivity and difficulty of tillage. In humid and moderately moist years, the death of crops or a reduction in yield are probable because of the excess of moisture.     

Acid-base buffering of soils in transitional and transitional-accumulative positions of undisturbed southern-taiga landscapes

The method of continuous potentiometric titration (CPT) of soil water suspensions was used to evaluate the acid-base buffering of samples from the major genetic horizons of podzolic soils on a slope and soddy gley soils on the adjacent floodplain of a rivulet. In the soils of the slope, the buffering to acid upon titration from the pH of the initial titration point (ITP) to pH 3 in all the horizons was 1.5?C2.0 times lower than that in the podzolic soils of the leveled interfluve, which could be due to the active leaching of exchangeable bases and oxalate-soluble aluminum and iron compounds with the later soil flows. In the soddy gley soils, the buffering to acid in the mineral horizons was 2?C10 times higher than that in the podzolic soils. A direct dependence of the soil buffering to acid on the total content of exchangeable bases and on the content of oxalate-soluble aluminum compounds was found. A direct dependence of the buffering to basic upon titration from the ITP to pH 10 on the contents of the oxalate-soluble aluminum and organic matter was observed in the mineral horizons of all the studied soils. The soil treatment with Tamm??s reagent resulted in the decrease of the buffering to acid in the soddy gley soils of the floodplain, as well as in the decrease of the buffering to basic in the soils on the slopes and in the soddy gley soils. It was also found that the redistribution of the mobile aluminum compounds between the eluvial, transitional, and transitional-accumulative positions in the undisturbed southern taiga landscapes leads to significant spatial differentiation of the acid-base buffering of the mineral soil horizons with a considerable increase in the buffer capacity of the soils within the transitional-accumulative terrain positions.     

Solods of the Baraba Lowland and the Priobskoe Plateau: Their properties and genesis and the methods of their diagnostics

The properties, hydrological features, and genesis of the solods occurring in the Baraba Lowland and Priobskoe Plateau were studied. Methods for determining the hydromorphism degree are considered; the features of the similarity and differences between the solods and other soils with textural profile differentiation are shown. Depending on the reasons for the waterlogging, the solods should be divided into two groups: the solods of groundwater waterlogging and the solods of surface waterlogging. Criteria for their discrimination are suggested: the ratio between the contents of the clay fraction in the parent rock (or in the B2 horizon) and that in the A2 horizon, the changes in the pH values along the soil profiles, and the content of nonsilicate iron compounds. The solods studied are shown to be formed under the conditions of a stagnant-percolative regime and gleying. This circumstance is an obligatory and sufficient reason for the formation of the light-colored acid eluvial (A2) horizons. According to some basic properties of the soil solid phase (the acidity, the total chemical composition, and the clay pattern in the eluvial part), the gleyed solods are close or identical to the gleyed soddy-podzolic and gleyed chernozem-like podzolic soils. At the same time, the solods differ from the gleyed chernozem-like podzolic soils by their thicker A1 (or Ap) horizon and their higher humus content (5–7%).     

The genesis,classification, and melioration of gleyed podzolic chernozem-like soils in the north of the forest-steppe zone of European Russia

Chernozem-like soils with light-colored acid eluvial horizons are widespread in the forest-steppe zone of European Russia. Their formation is related to gleying under the conditions of a stagnant-percolative water regime on leached rocks. It is closely associated with the evolution of salinized soils (Gedroits’s scheme). However, these soils have not been included in the soil classifications of the Soviet Union and Russia. Based on the principles of substantial-genetic classification, one of the authors of this article [3–5, 10] referred them to gleyed podzolic chernozem-like soils, which are considered as an individual genetic soil type. With respect to agroecological aspects, they are different from the leached chernozems in their low productivity and difficulty of tillage. This article covers the problems of genesis, classification, and melioration of gleyed podzolic chernozem-like soils in the north of the forest-steppe zone of European Russia and their possible association with dark-colored podbels.     

Al,Fe, and Si compounds in Tamm and Mehra-Jackson extracts from mucky-peaty-podzolic gley soil: Contents,reserves, and profile and particle-size distributions

Equal or comparable contents of Fe and Al extractable by Tamm and Mehra-Jackson solutions have been revealed in all the horizons of a loamy mucky-peaty-podzolic gley soil on binary deposits. The content of Si extractable by the Mehra-Jackson solution has exceeded that of oxalate-soluble Si by an order of magnitude. The distributions of Al in the Tamm solutions from the entire soil and its fractions of 1–5 and >5 μm are of accumulative type with a maximum in the mucky H horizon and a gradual decrease of the content with depth in relation with the analogous distribution of Al-organic complexes. The maximum content of oxalate-soluble Al in the clay fraction has been found in the eluvial ELg horizon, which can be due to the partial dissolution of Al hydroxide interlayers in soil chlorites. The distribution of Fe in the entire soil has two maximums, in the H horizon due to the accumulation of Fe-organic complexes and in the concretion-rich ELnn,g horizon due to the accumulation of Fe hydroxides. Depletion of oxalate-soluble Fe in the eluvial ELg horizon has been observed in all the fractions, which can be related to its mobilization and removal under strongly acidic conditions and the development of reductive processes, as well as the enrichment of the concretion-rich horizon with these compounds because of an increase in pH and the development of conditions favorable for water stagnation and Fe segregation.     

Iron minerals in soils on red-earth deposits in the Cis-Ural region

In soils developed from the red-earth deposits in the Cis-Ural region (Perm oblast), hematite does not ensure the theoretically possible redness due to the concealing effect of rivaling pigments, i.e., humus in the upper horizons and Fe(II) in the gleyed horizons. The soil color depends on the minimal (spring) values of the hydrogen partial pressure index rH_min rather than on the average value of this index rHav. The hematite content decreases in the gleyed and humus horizons (despite the absence of the morphological features of gley in the latter due to the concealing effect of humus). The gley horizons are heterogeneous with respect to the state of iron. Upon the maximum wetting in the gley horizons of the mucky-humus gley soil, hematite is being reduced to Fe(II), which is proved by the low values of rH_min (<19). In a less humified dark humus gley soil, the values of rH_min exceed 19, which points to the inherited gley features in this soil. In the mucky-humus gley soil, an inverse dependence between the magnetic susceptibility χ and E_Hmin is observed upon E_Hmin <320 mV. In this case, the degree of reduction of the highly magnetic iron oxides rises from 0.3 to 1.0 due to a decreasing portion of maghemite γFe₂O₃ and an increasing portion of magnetite Fe₃O₄.     

Lessivage and its relation to the hydrological regime of soils

By the examples of four typical catenas in the East European Plain, the role of lessivage in the development of automorphic and hydromorphic loamy and clayey soils with light-colored acid eluvial horizons and with different degrees of gleyzation has been studied. It is found that characteristic features of lessivage are often observed in the soils without hydrological barriers hampering or preventing the vertical migration of soil water and mass transfer processes. The hydrological barriers may be represented by the shallow horizons of temporarily perched water, or by the ascending capillary fringe of the ground water, or by the water-saturated horizons, in which the volume of free pores does not exceed 2–4%. It is shown that light-colored acid eluvial horizons may be formed in the profiles of loamy and clayey soils without any signs of lessivage. The development of strongly gleyed soils (gleyed soddy-podzolic soils and pseudogley soils (Stagnosols)) is not related to colmatage (silting of their illuvial horizons through lessivage); it is conditioned by the actual hydrological regime of these soils. The role of lessivage, podzolization, and gleyzation in the development of clay-differentiated soils is discussed.     

Soil formation and weathering on ultramafic rocks in the mountainous tundra of the Rai-Iz massif,Polar Urals

Gravelly clay loamy and clayey soils developed from the derivatives of ultramafic rocks of the dunite-harzburgite complex of the Rai-Iz massif in the Polar Urals have been studied. They are represented by raw-humus pelozems (weakly developed clayey soils) under conditions of perfect drainage on steep slopes and by the gleyzems (Gleysols) with vivid gley color patterns in the eluvial positions on leveled elements of the relief. The magnesium released from the silicates with the high content of this element (mainly from olivine) specifies the neutral-alkaline reaction in these soils. Cryoturbation, the accumulation of raw humus, the impregnation of the soil mass with humic substances, gleyzation, and the ferrugination of the gleyed horizons are also clearly pronounced in the studied soils. Despite the high pH values, the destruction of supergene smectites in the upper horizons and ferrugination (the accumulation of iron hydroxides) in the microfissures dissecting the grains of olivine, pyroxene, and serpentine, and in decomposing plant tissues take place. The development of these processes may be related to the local acidification (neutralization) of the soil medium under the impact of biota and carbonic acids. The specificity of gleyzation in the soils developing from ultramafic rocks is shown in the absence of iron depletion from the fine earth material against the background of the greenish blue gley color pattern.     

Mobilization of dissolved organic matter, aluminium and iron in podzol eluvial horizons as affected by formation of metal-organic complexes and interactions with solid soil material

Interactions with dissolved organic matter (DOM) are generally believed to play a crucial role in the translocation of Al and Fe in acid sandy soils. Binding of Al and Fe to DOM affects their mobility in soils by altering sorption equilibria of charged sites on solid soil material, inducing precipitation of organo‐metallic complexes and preventing the formation of inorganic Al and Fe phases. The relative importance of the different processes, especially with respect to the translocation of Al, Fe and organic matter in podzols, remains unresolved. We determined the effect of the presence of solid soil material from the eluvial (AhE and AE, respectively) horizons of a Fimic Anthrosol and a Haplic Podzol on the metal‐to‐organic carbon (M/C) ratio in solution and the formation of dissolved organic Al and Fe complexes. Furthermore, we assessed the resulting influence on the mobilization of Al, Fe and DOM. Even under considerable metal loading, the M/C ratios and ‘free’ metal fractions in solution remained low and relatively constant, due to an apparent buffering by the solid phase and the formation of organo‐metal complexes in solution. The M/C ratios remained so low that significant precipitation of organo‐metal complexes due to saturation with metals was not found. The apparent buffering by the solid phase can be explained by a strong release of organic matter from solid soil material and adsorption of non‐complexed Al and Fe on solid organic matter upon metal addition. Adsorption of organo‐metal complexes most likely played only a minor role. The observations confirm the expected mobilization of Al, Fe and DOM in eluvial horizons and seem to indicate that even under fluctuating input of Al, Fe and DOM the soil solution will have a constant composition with respect to M/C ratios and percentage of Al and Fe present in dissolved organo‐metal complexes.     

Clay minerals in chernozem-like soils of mesodepressions in the northern forest-steppe of European Russia

In the northern forest-steppe of European Russia, under the conditions of surface waterlogging (freshwater) and a stagnant-percolative regime, gleyic podzolic chernozem-like soils with thick light-colored eluvial horizons are formed. These horizons are close or similar to the podzolic horizons of bog-podzolic soils in many properties of their solid phase. They are bleached in color and characterized by the removal of Ca, Mg, Fe, Al, and Mn and the relative accumulation of quartz SiO₂. These soils differ from leached chernozems in their acid reaction and very low CEC, the presence of Fe-Mn concretions and coatings, and the significant decrease in the clay content in the A2 horizon as compared to the parent rock. The soils studied differ significantly from loamy podzolic and bog-podzolic soils by the composition of the clay minerals in the A2 horizons: (1) no essential loss of smectite minerals from this horizon was found as compared to the rest of the solum, (2) pedogenic chlorites (HIV and HIS) are absent, and (3) the distinct accumulation of illites is observed as compared to the subsoil and parent material, probably, due to the process of illitization.     

Characteristics of Clay Minerals in Podzols and Podzolic Soils

The clay minerals in Podzols and podzolic soils developed under coniferous forests in the Subarctic and Cool-temperate zones are characterized by the predominance of smectite and/or mica-smectite interstratified minerals in the eluvial horizons and chlorite-vermiculite intergrade in the illuvial horizons. A large amount of vermiculite is present in the eluvial horizons of some podzolic soils in the Cool-temperate zone. The illuvial horizons of these soils also contain free iron oxides such as goethite. Imogolite and allophane are present in the illuvial horizons of several soils derived from volcanic ashes. It is suggested that the critical bioclimate for the release of interlayered aluminum from the 2:1-type minerals lies between the Cool- and Warm-temperate zone. In the eluvial horizons of Podzols and podzolic soils, mica minerals and chlorite, as primary minerals, have been transformed to smectite through the pedogenic process. Based on previous studies on the structure and degradation of the dioctahedral mica minerals, it is considered that smectite is transformed from 1M-type mica minerals directly, and from 2M-type mica minerals via mica-smectite interstratifled minerals. The formation of a smectite lattice in the eluvial horizon should be a clay-mineralogical indicator of podzolization.     

Particle-size distribution patterns in Vertisols and vertic soils of Russia

Five variants of the distribution of clay (<0.001 mm) and physical clay (<0.01 mm) fractions along the vertical profiles of Vertisols (slitozems) and vertic soils (slitic subtypes of different soil types) from the European part of Russia are distinguished: (1) accumulative, (2) even, (3) regressive, (4) with a maximum in the middle-profile horizon and with their approximately equal contents in the upper and the lower horizons, and (5) eluvial–illuvial. These distribution patterns are related to the lithological specificity of sedimentation and formation of parent materials composed of swelling clays of different geneses and ages. Solonetzic, eluvial- gley, and solodic processes contribute to the development of the eluvial–illuvial and, partly, regressive variants of clay distribution. All the five variants with a predominance of the even distribution pattern can be found in Vertisols. Most of Vertisols in the European part of Russia have a medium clayey or a heavy clayey texture in the entire profile. The regressive distribution pattern is typical of the group of vertic soils. In the upper horizons of Vertisols, where slickensides do not form, the texture is usually heavier than that in the analogous horizons of vertic soils. The middle-profile and lower horizons with slickensides have similar statistical distributions of particle-size fractions in Vertisols proper and in vertic soils. However, in Vertisols, a tendency for a more frequent occurrence of the soils with a higher content of the clay fraction and with a higher portion of this fraction in the physical clay fraction is observed (as compared with the vertic soils).     

Iron oxyhydroxides in soils developed from Lower Palaeozoic sedimentary rocks in mid-Wales and implications for some pedogenetic processes

Lepidocrocite and goethite are pedogenetic crystalline Fe oxyhydroxides in the soils of mid-Wales. Lepidocrocite is more abundant and widespread, occurring in brown earths (Denbigh series) as well as morphological gleys. It is commonly dominant in horizons intermediate in depth. Goethite occurs in subsoils and was found to be dominant in the subsoil of surface water gleys (Cegin series). Apart from the Cegin series, a large proportion of the total free Fe occurs in non-crystalline forms. This amounted to 50–80% of the free Fe in organic and eluvial horizons of podzols. Despite lower proportions of non-crystalline Fe in Bs horizons, amounts were similar to those in eluvial horizons. Data on the distribution of crystalline and non-crystalline free Fe together with correlations observed between Fe and C extracted by pyrophosphate suggest that transformation and translocation of Fe in the podzols examined are governed by its association with organic fractions.     

Diagnostic parameters of soil formation in gray forest soils of abandoned fields overgrowing with pine forests in the middle reaches of the Angara River

The content, differentiation in the profile, and dynamics of the mobile iron compounds can serve as adequate diagnostic parameters of the direction of the pedogenesis upon the overgrowing of abandoned fields with pine forests in the middle reaches of the Angara River Region in the area affected by the Bratsk water reservoir. The bulk chemical composition of the soil remains relatively stable in the entire profile against the background of the eluvial-illuvial redistribution of the finest particle-size fractions and mobile iron compounds under the impact of the long-term seasonal freezing, the percolative soil water regime, and the alternating redox conditions. The development of accumulative processes in the soil is accompanied by the weak manifestation of eluvial and pulsating gley processes as the initial stages of podzolization under the influence of the growing pine forests.     

Specific features of iron behavior in soddy-podzolic and alluvial gleyed soils of the Middle Cis-Urals region

The regime of observations revealed that the Eh dynamics in soddy-podzolic and alluvial soils in the Middle Cis-Urals region depends not only on the rate of iron (hydr)oxides reduction but also on the rate of opposite reactions in the gleyed horizons. Both processes depend on the temperature. The Eh value decreases on heating in automorphic soils, when the reduction of Fe(III)-(hydr)oxide particles accelerates. On the contrary, in gley soils, the Eh decreases on cooling, probably, because of the reactions opposing the reduction of Fe(III)-(hydr)oxide particles, including Fe(II) fixation on the surface of mineral particles. Fe(III)-(hydr)oxides are, for the most part, preserved in gleyed soils of the Cis-Urals; the content of (Fe₂O₃)_dit reaches 3.3% with iron minerals being usually represented by goethite. The increase in moistening influences the soil parameters (i.e., the redoxpotential rH and the content of conventional red pigment Hem_conv) in an intricate manner. Both direct and reverse branches on the curve of the Hem_conv-rH dependence point to the equilibrium and nonequilibrium conditions in the soil. The reverse branch probably stands for the initial phase of gleying in strongly humified soils, where, despite extra electrons in the solution, the brown pigment in the form of Fe(III)-(hydr)oxides is preserved.     

Zu Eigenschaften,Horizontaufbau und Gliederung der „Haftnässepseudogleye”︁

Properties, horizons and classification of the “Haftnässepseudogleye” (Stagnosols periodically waterlogged with capillary water) The term “Haftnässe” (soil wetness due to capillary moisture) can be used in describing soils with Sg-horizons in which long-term waterlogging and anaerobic conditions occur in the absence of gravitational water. “Haftnässe” is caused by water held in pores with an equivalent diameter of 0.2–50 μm by soil-water tension (pF) between 1.8 and 4.2, when the air capacity of the horizons is very low. “Haftnässe” moves primarily by capillary forces and is available to plants (available water). In some soils, the horizon below the Sg-horizon contains large pores, is well aerated and tends to impede the movement of capillary water. This type of horizon is often wetter than the overlying and underlying horizons, due to the presence of capillary water in the immediately overlying Sg-horizon. The symbol “So” is proposed for such horizons. In these soils, in the Sg-horizon reduced iron compounds are oxidized and precipitated, forming rusty mottles. The sequence of horizons developed in the “Haft(nässepseudo)-gleye” (Stagnosols periodically waterlogged with capillary water) typically affects the continuity of the pathways along which capillary water normally moves. The “Haft(nässepseudo)gleye” are divided into two subtypes on the basis of the sequence of horizons in the soil profile:

• Typical “Haft(nässepseudo)gley” (Shn) exhibits a sequence Ah/Sg/(II)So and shows transitions to Luvisol and Glossisol,
• Thick “Haft(nässepseudo)gley” (Shm) exhibits a sequence Ah/Sg and shows transitions to “Stau(wasserpseudo)gley” (Gleysol periodically waterlogged due to perched water), Gleysol, Fluvisol and tidal marsh soil.