Soil classification

Soil classification. The grouping of thousands of different soils on the basis of their common characteristics. Soil classification contributes to the organization and communication of information about soils and enhances the understanding of their genesis, the processes within them, their relationships with the physical environment, and their place in the landscape.

Despite more than a century of soil science research and international communication, there is no universal soil classification system accepted by all countries. Soil classification in Ukraine follows the Russian system. In that system the broadest soil groups (called ‘soil types’) are based on the properties of the soil profile (ie, horizons and other phenomena observed in a vertical cross-section of a soil body down to two meters). The old US soil classification system, which recognized three orders (‘zonal soils,’ ‘intrazonal soils,’ and ‘azonal soils’), employed nomenclature for the ‘great soil groups’ of the ‘zonal soils’ and conceptually differentiated them in a way that corresponded to the ‘soil types’ of the Russian system. Since that classification system is also familiar to many in North America, the terminology from the old US soil classification system is employed here to describe the soils of Ukraine and their distribution.

Location and environmental relationship. In Ukraine three broad belts of soils corresponding to the belts of natural vegetation dominate most of the territory. First is the podzols of forested Polisia, formed on outwash sandy plains or on clayey till plains, which contain pockets of bog soils and ribbons of meadow soils on floodplains. The southern boundary of this belt follows along the line LutskRivneZhytomyr–Kyiv–ChernihivNovhorod-Siverskyi. Second is the belt of soils associated with the forest-steppe, consisting of the gray forest soils (formed under the broad-leaved deciduous forest), the deep chernozems (formed under the prairie), and the transitional podzolized chernozems or degraded chernozems (so modified under conditions of encroaching forest), all of which evolved on calcium-rich loess deposits. This belt also contains some bog soils in depressions and meadow soils in river valleys. The southern boundary of the belt follows along the line Kishinev–Pervomaiske (Mykolaiv oblast)–KirovohradKremenchukKrasnohradKupianka–Valuiky. Third is a belt of soils associated with the steppe, consisting of common chernozems in the northern part and southern chernozems to the south. This belt arches around the Sea of Azov into the Kuban Lowland and part of the Stavropol Upland and contains chernozems particularly rich in available carbonates. Along the north coast of the Black Sea and along both sides of Syvash Lake are the chestnut soils. Interspersed among the southern chernozems and the chestnut soils, especially between the Inhul River in the west and the Molochna River in the east, are many shallow depressions consisting of leached meadow gley soils (known in Russian as solod and in German as Wiesenboden), and along the coast, solonetz soils (including the salt-laden solonchak soils).

In the Crimea there is an analogous, although inverse, sequence of soil belts associated with rising elevation from north to south: in the north, chestnut soils with associated solonetz soils and solonchak soils; in the middle, southern chernozems, followed by a carbonate-rich, shallow variant of the common chernozems and small areas of chernozems on heavy clays (Kerch Peninsula); and in the mountains, stony brown mountain forest soils interspersed with small pockets of mountain meadow soils at the highest elevations. On the warm south slopes the soil is transitional into the reddish brown soils typical in a Mediterranean climate.

A similar sequence occurs in Subcaucasia, with carbonate-rich variants of the common chernozems and the deep chernozems at low elevations in the north, carbonate-rich brown mountain forest soils on the mountain slopes, and mountain meadow soils at the highest elevations. On the southern slopes to the Black Sea coast are reddish brown soils. Only the broad alluvial plain of the Kuban River is dominated by ‘azonal’ alluvial soils; its delta contains ‘intrazonal’ bog soils.

In the Carpathian region one can observe both a vertical zonation and a transition to the Central European brown forest soils. With rising elevation the degraded chernozems and the gray forest soils of the Subcarpathian Basin give way to increasingly podzolized meadow gley soils. On higher slopes appear gray mountain forest podzols in areas covered by fir trees and brown mountain forest soils in areas of beech forests. The highest elevations have mountain meadow soils. On the Transcarpathian side of the Carpathian Mountains brown forest soils cover the foothills and meadow gley soils prevail in the lowland.

Characteristics. The major representatives of the great soils groups in Ukraine include, in order of importance, the chernozems and their related chestnut soils, the various podzolized chernozems of the forest-steppe, and the podzols of the forest.

Chernozems occupy 41 percent of Ukraine's surface area and even more of its agricultural land (54 percent) and plowland (58 percent).

Chestnut soils, related to the chernozems, occupy only 3.3 percent of the area of Ukraine and 3.4 percent of its agricultural land but account for 3.9 percent of its plowland. Whereas in the chernozems the zone of calcium carbonate accumulation occurs about a meter below the surface, that mineral concentration is characterized in chestnut soils by the presence of sodium cations. Their presence causes the chestnut soils to change gradually into salinized chestnut soils and solonetz soils.

The solonetz is low both in humus (1–3 percent) and in available plant nutrients. In the solonetz the salts are leached to a certain depth, which allows for a broader range of plants to grow. The solonchak, by contrast, is salinized right to the surface, and only salt-loving species of plants (halophytes) can survive. The process of salinization can be reversed only with expensive meliorative measures, such as subsurface drainage, deep plowing, the application of gypsum, or other agronomic techniques.

The podzolized soils of the forest-steppe developed with the encroachment of deciduous forest into the domain of the steppe. One-third of the soils in the forest-steppe are podzolized; their area represents 12 percent of the area of Ukraine but 18 percent of its agricultural land and 21 percent of its plowland.

Degraded chernozems still retain the chernozem habitus. Chemical analyses of degraded chernozems, compared with analyses of chernozems, reveal some decline in the proportion of absorbed calcium and magnesium and the presence of hydrogen in the absorbing complex of the soil. Inherent soil fertility is diminished as a result but can be enhanced with the application of chemical fertilizers and manure. Podzolized chernozem is more severely leached than degraded chernozem, as is apparent by the presence of a narrow leached zone at the bottom of the topsoil layer. Gray forest soils, by contrast, have a thinner (20 cm) dark gray layer of topsoil with a lower (1.5 to 3.5 percent) concentration of humus.

Podzols of the forest and associated soils are found in northern and northwestern Ukraine. The prevalent soils are the podzols, formed on the fluvioglacial deposits under conditions of humid continental climate and under the cover of coniferous and mixed forests. The podzols occupy about 13.5 percent of the area of Ukraine; another 1 percent, the remaining part of the region, consists of alluvial and organic soils. Known for their infertility, the podzols account for only 7.8 percent of the agricultural land and 6.8 percent of the plowland of Ukraine.

Bog soils, formed under poor drainage conditions, are representative of the hydromorphic suborder of the intrazonal soils. In Ukraine bog soils are abundant in the wet regions of Polisia (the Prypiat River Basin), in Chernihiv Polisia, and in the shallow troughs of small river valleys of the forest-steppe. Because of their high organic content, bog soils can be agriculturally productive, but first they must be drained and then treated with fertilizer. Bog soils occupy about 5.5 percent of the area of Ukraine and 4.5 percent of its agricultural land but only 0.24 percent of the plowland.

Meadow soils are formed on the floodplains of streams and rivers, where occasional floods and imperfect drainage provide for increased moisture. Excluding the solonetz, meadow soils occupy 4.3 percent of the area of Ukraine and 4.4 percent of its agricultural land but only 2.1 percent of the plowland.

Rendzina soils are thin, stony, dark-colored soils developed from soft limestone, chalk, or marl. Small areas of rendzina soil are found in the Volhynia-Kholm Upland. Soviet classification referred to them as carbonate-rich turfy soils formed on marl, chalk, or limestone, and considered them a subcategory of the chernozems.

Azonal soils are determined neither by climate nor by any particular soil-forming process associated with a topographic feature, but by the nature of the parent material. Commonly found on steep slopes, azonal soils show scant soil development and are generally of little significance to agriculture.

Mountain soils, developed on weathered solid rocks, are shallow and full of rock fragments. In the Carpathian Mountains, for example, are found gray mountain forest podzols, brown mountain forest soils, and mountain meadow soils. The mountain forest soils scarcely resemble the podzols. The mountain meadow soils developed under alpine meadows (sometimes overgrown with peat moss) are shallow. They usually undergo podzolization and develop a gley horizon.

In the Caucasus Mountains the main soils at higher elevations where coniferous forests grow are gray mountain forest podzols. At lower elevations, under deciduous forests, are the brown mountain forest podzolized soils. The latter soils, also found in the Crimean Mountains, are darker than those of the Carpathian Mountains because of the warmer climate and higher concentrations of calcium carbonate; they are also less acidic (pH 5–6).

(See also Soil conservation, Soil erosion.)

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