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Physical Geography of Northern Eurasia
Soils of Northern Eurasia
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Heteronomous Humid Soils >>>
Soil-forming Processes
The concept of soil-forming processes (e.g., humification, formation of clay, formation
of soil, mixing of soil, and gleying) is of fundamental importance for genetic soil
science and soil geography. Ideas about soil-forming processes as an instrument for
studying soil origin have been developed and applied by many prominent soil scientists:
Kovda (1946, 1947); Gerasimov (1976); Rode (1976); Glazovskaya (1981); Duchaufour (1982);
Zonn (1987); and Sokolov (1993). The original Dokuchaev scheme 'soil
properties-soil-forming factors' has been modified by Gerasimov (1976) as 'soil
properties-soil-forming processes-soil-forming factors'. Many aspects of this concept are
contradictory and insufficiently researched and approaches reflect different levels of
complexity, hierarchy, and interaction with soil-forming factors (Elementarnye, 1992;
Chernyakhovsky, 1994; Zamotaev and Targulian, 1994).
Organic Matter Metamorphism
Processes of organic matter metamorphism play an important profile-forming role both in
humid and arid environments. These processes include transformation of organic remains,
humification, mineralization, formation of complexes, and migration of humification
products (Grichina and Orlov, 1978; Chichagova and Cherkinsky, 1979,1985;Dergacheva, 1984;
Trofimov and Dorofeyeva, 1994). The organogenous and/or organic-mineral zones are
distinguished according to the amount of organic matter, its distribution across the
profile and at the surface, and quality indices, such as degree of decomposition, type of
humus, and its enrichment by nitrogen. Seven main types of organic profiles, which are
distinguished in Northern Eurasia, are listed in Table 4.3.
Table 4.3 Types of organic profiles
Mineral Matter Metamorphism
The formation of clay minerals in  horizons in the soils of northern Eurasia is
controlled by two main processes of biochemical weathering: sialitization and
ferrallitization. The former occurs in humid cold and temperate regions, in warm arid and
semi-arid regions, and sub-humid subtropical environments; the latter develops in humid
subtropical conditions. The term 'subhumid subtropics', referring to the subtropical
regions which annually receive between 1300 and 1500 mm of precipitation, is used in
Russian soil science to distinguish between the areas where kaolinite-smectites develop
(humid subtropics with up to 2500 mm of precipitation per annum) or do not develop
(subhumid subtropics).
Across large areas, parent rocks accumulated clay products of different mineral
composition (smectite-hydromica, hydromica, kaolinite-smectite, and kaolinite) during
previous cycles of lithogenesis (Sokolova, 1985; Chernyakhovsky, 1991). Under the present
climatic and biotic conditions, these clay associations do not undergo significant
transformations by the processes of intrasoil weathering. Therefore, under humid cold and
temperate and to a certain extent under warm semi-arid conditions sialitization occurs due
to the selective partial or full biochemical dissolution of non-laminate minerals
(calcite, feldspars, pyroxenes, and hornblendes). Laminate silicates of the parent rock
(biotite, muscovite, chlorite, and smectite) are weathered or destroyed by mechanisms of
hydration and degradational transformation. For this reason, in cold and temperate humid
climates only vermiculite is formed as a result of biochemical weathering. Because of
selective decomposition of non-laminate minerals, leaching of carbonates and weak
transformation changes of laminar silicates, the most stable mineral associations are
accumulated in the soil profile. In the case of weathering of acid rocks and their
sedimentary derivatives, the stable minerals are quartz and dioctahedric micas. In the
case of weathering of middle and basic rocks and their sedimentary derivatives,
black-coloured minerals, hydromicas, and vermiculites are the predominant products of the
transformation of micas and chlorites of the parent rock. The relative stability of
laminate alumo-silicate minerals, inherited from parent rocks, is one of the principal
causes of the observed silitization in podzols, podzoluvisols of taiga, grey soils of
forest-steppe, and cambisols of deciduous forests, and to a lesser degree in chernozems
and chestnut soils of steppes.
In the subhumid subtropics, on the Russian part of the Black Sea coast (which is
considered 'subtropical' with respect to the soil-forming processes in contrast to
climatic classifications) and the foothills of the Talysh mountains, sialitization occurs
through the dissolution of minerals of non-laminar structure and degradational
transformation of trioctahedric micas, chlorites and mixed-laminar formations. It develops
further than in the soils of cold and temperate regions. Sialitization leads to the
accumulation of trioctahedric and sometimes dioctahedric smectite which binds most of the
iron in the soil. This is the final stage of degradational transformation of these
minerals. Due to the stability of smectites, insignificant amounts of iron, released in
the course of the destruction of pyroxenes, hornblendes, and chlorites, is insufficient to
produce dense pigmentation of the soil. Moreover, after iron is released from minerals
into the upper horizons of the soil profile, it is bound into mobile organomineral
complexes and is almost completely removed from the soil profile. Therefore, there are no
conditions for the formation of red and mottled colours and soils, termed yellow earths or
alisols, to develop.
Ferrallitization occurs only in a relatively small area of the humid subtropics in
western Georgia, where high rainfall provides continuous moistening and a deep washing
regime throughout the year. Observations have shown that in order to initiate
ferrallitization, the annual amount of rain should exceed 1400 mm (Chernyakhovsky, 1991).
However, the potential formation of minerals of the kaolinite group and iron and aluminium
oxides-hydroxides in western Georgia is often hampered by the occurrence of the older
oligomictous quartz-kaolinite or gibbsite-goethite-kaolinite rocks. These rocks cannot be
transformed by weathering to any significant extent even if other conditions are satisfied
(Zonn, 1987; Romashkevich, 1974, 1988). Complete ferrallitization occurs through
weathering in the mountains where young rocks are exposed by active tectonic processes and
erosion rates exceed weathering rates. Soils, termed red earths or nitisols, are formed
under these conditions. They are distinguished by a clay profile with a clear
differentiation of the genetic horizons, where minerals with kaolinite and
kaolinite-smectite mixed-laminar structures have a rock-forming importance, and have dense
red and mottled colours.
Aluminium-Iron-Humus Eluviation and Illuviation
Aluminium-iron (Al-Fe)-humus eluviation is one of the most common processes. It refers
to the combination of such processes as mobilization of low molecular humic matter by iron
and aluminium, the formation and removal of stable complex and chelate organomineral
compounds. Al-Fe-humus eluviation is typical of soils which have developed on substrata
with coarse grains, as these provide conditions for the free migration of solutions. In
humid boreal regions, this process leads to the formation of soils with eluvial horizons
(El) such as haplic and ferric podzols on acid rocks enriched by quartz and poor in bases
and sesquioxides. On the rocks, rich in bases, sesquioxides, and minerals which are easily
weathered, soils without El horizons develop. Podburs, otherwise known as cambic podzols,
belong to this type. The term 'podburs' is widely used in Russian-language literature but
it is not much used elsewhere.
Al-Fe-humus illuviation is genetically connected with Al-Fe-humus eluviation and
develops in the same profiles. It refers to the process of removal of Fe and Al oxides and
organic matter in organomineral forms and their subsequent deposition. One or several
illuvial horizons (illuvial-ferrogenous, illuvial-humic-ferrogenous and
illuvial-Al-Fe-humic), enriched by oxides and sometimes by humus in comparison to the
over- and underlying soil horizons, are formed.
Al-Fe-humus illuviation and Al-Fe-humus eluviation are often viewed as the two aspects
of the same process otherwise known as true podzolization (Duchaufour, 1982).
Lessivage
The eluvial-illuvial differentiation of profiles in the humid section of soil formation
may be caused by other processes such as lessivage and gley migration of iron and
manganese. Lessivage is the process of washing of clay particles without any change in
their chemical composition out of the eluvial horizon and their accumulation at depth as
local or complete drapes or cutans at the surface of peds, rock fragments, and walls of
pores. Intensive lessivage develops in humid and semi-humid regions, in soils forming on
loose Quaternary deposits enriched by fine particles. Lessivage is one of the
profile-forming processes in dystric and eutric podzoluvisols. It forms a series of
diagnostic indices in stagnic podzoluvisols, grey forest soils, alisols, and nitisols.
Gley Migration of Iron and Manganese
The process of gley migration of iron and manganese is as the movement of anaerobically
recovered forms of Fe2+ and Mn2+ in gels of organomineral complexes,
salts of mineral acids with subsequent oxidization and accumulation of Fe3+ and
Mn3+ hydroxides in the soil profile or outside. In soils, developing on loams
and clays, this process is connected with surface water, while in soils, developing on
sandy and silty rocks, it is affected by ground water. Although this process occurs in all
humid regions, it has a profile-forming function in gleysols. It is a direct result of
gleying.
Gleying of the Soil Mass
Gleying is a process of soil transformation under conditions of permanent or continuous
waterlogging. Gleying of soils is characterized by the formation of light blue or greenish
colours (Karavaeva, 1982). Gleying is typical of soils of all zones of Northern Eurasia.
However, it is most common in humid and subhumid environments with waterlogging. It takes
place both in autonomous soils on watersheds and in heteronomous soils in depressions
(e.g., umbric gleysols, gleyic chernozems, and mollic gleysols). In arid environments,
gleying is linked to ground and soil-ground waters and occurs in heteronomous soils of
meadows, wetlands, and various solonchaks (salinized soils).
Salinization and the Formation of Solonetz
Salinization is particularly widespread in arid environments while in humid regions
saline soils occur only locally. It may develop in soils or individual horizons of a soil
profile in response to seasonal and perennial variability in salt balance. Aridity and
widespread occurrence of salt-bearing rocks stimulate Salinization of soils in steppes,
semi-deserts, and deserts. Solonchaks are formed as a result of this process.
Solonetz formation is one of the profile-forming processes typical of the arid zone. It
refers to the accumulation of exchangeable sodium in the soil's absorbing complex with a
relatively low concentration of salts and an alkali reaction in the soil solution. The
most favourable conditions for the formation of solonetz soils are those of long-term salt
accumulation such as deltas, river and marine terraces in typical and southern steppes and
semi-deserts.
Solodization
Solodization is the replacement of absorbed sodium by hydrogen which leads to
hydrolithic splitting of minerals of the soil's absorbing complex and desilting of the
upper part of the soil profile. Solodization develops in response to the occurrence of
contrasting moisture regimes. For example, in steppes and forest-steppes, waterlogging
takes place in topographic depressions where intersoil drainage is limited, while in dry
periods this regime is replaced by a weak capillary regime.
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