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a type of soil that forms beneath steppe and forest-steppe vegetation in the sub-boreal belt. Chernozems form chiefly from carbonate-bearing parent rock—loesses, loess-like clays, and loams—and sometimes from more ancient limestones, sandstones, and marly clays in undrained or periodically drained circumstances. Chernozems are characterized by the accumulation of organic matter in the humus-accumulative horizon, by a high humus content, by a well-expressed cloddy-granular texture, and high potential fertility.
The first scientific observations about the origin of chernozems were made by M. V. Lomonosov (mid-18th century). Lomonosov believed that chernozems formed as a result of the decomposition of plant and animal matter. At the turn of the 19th century, P. Pallas and others proposed the hypothesis of a marine origin of chernozems, considering them to be marine oozes remaining after the retreat of the Caspian and Black seas; this hypothesis is only of historical importance, reflecting the idea prevalent at that time of soil as a geological formation. The hypothesis of the bog origin of chernozems also proved unfounded. Supporters of this theory, such as E. I. Eikhval’d and others in the mid-19th century, believed that the zone of chernozems had in the past been highly marshy tundra areas and that the decomposition of marsh vegetation caused by the onset of a warm climate led to the formation of chernozems. The theory of the terrestrial-plant origin of chernozems (F. I. Ruprekht, V. V. Dokuchaev, and others) links their formation to the establishment and development of meadow-steppe and steppe-grass vegetation. This theory was expounded in great detail by V. V. Dokuchaev in Russian Chernozem (1883), in which Dokuchaev proved that chernozems formed as a result of the close interaction of grassy vegetation, climate, topography, parent rock, and other soil-forming factors. The accumulation of humus is a consequence of this process.
Each year, grassy vegetation leaves extensive dead matter in the soil. This matter is plant remains, 70–80 percent of which are roots. The hydrothermal conditions of the steppe and forest-steppe zones are conducive to humification, as a result of which complex humus compounds are formed (primarily humic acids) and impart to the soil profile its dark coloring. The best conditions for humification occur in the spring and early summer, when the soil is supplied with adequate moisture as a result of autumn and winter precipitation and the snowmelt and when temperature conditions are favorable. During the period of summer drying out, the microbiological processes markedly weaken, which prevents humic matter from becoming rapidly mineralized. When plant remains, rich in ash elements and nitrogen, decompose, bases form (calcium is especially plentiful), which saturate the humic matter. This helps to fix the decomposed remains in the soil in the form of humates and to preserve a neutral or close-to-neutral reaction in the top horizons of the chernozems. In addition to the humus, the plant nutritional elements, such as nitrogen, phosphorus, sulfur, and iron, are fixed in the soil in the form of complex organomineral compounds, which form as a result of the interaction of humic acids with the ash elements of plant remains and the mineral part of the soil.
Chernozem soil formation is most intensive in the forest-steppe zone, where better moisture conditions promote more intensive development of grassy vegetation and active humification of its residue. In the steppe zone, inadequate moisture causes shallower root penetration, a decrease in the amount of dead matter entering the soil, and fuller decomposition of the matter.
The natural process of soil formation alters significantly when chernozems are brought into agricultural use. The cultivation of crops changes significantly the character of the biological cycle of matter and conditions of formation of the water and heat regimes. A large part of the biomass formed and consequently a significant quantity of nutrients are removed each year, and the period of vigorous interaction between the plant root systems and the soil is reduced. In addition, the soil remains for a long time without a plant cover, which reduces its absorption of winter precipitation. When virgin chernozems are first plowed, the structure of the soil is partially broken down and the content of humus and nitrogen in the plowed horizon drops, especially in the first years of cultivation. This process slows down in later years. At the same time, the activation of microbiological processes when chernozems are cultivated promotes the mobilization of nitrogen and phosphorus. Therefore, cultivated chernozems have a high content of nutrients that are available to plants. To maintain the high fertility of chernozems that have been brought into intensive cultivation, it is essential to introduce crop rotation, apply organic and mineral fertilizers, and use the proper soil tillage system.
The following genetic horizons are identified in the chernozem profile: A (humus-accumulative)—the most intensively colored, top part of the humus layer; B1 (humus-transitional)—the lower, less colored part of the humus layer; B2 (the horizon of humus streaks)—not observed in all types of chernozems; Bc (illuvial-carbonate horizon)—often combined with horizon B2; and C—parent rock. Horizons A and B1 both measure 40–120 cm in thickness and have a humus content of 4–12 percent.
Chernozems are broken down into the following subtypes: podzolized, leached, typical, ordinary, and southern. Each subtype has its own characteristic features; each differs in the thickness of the humus horizons, the humus content, the depth of carbonate occurrence, and other features determined by the conditions of development (see Table 1).
|Table 1. Characteristics of chernozem subtypes|
|Subtype||Thickness of horizons A and B1 (cm)||Depth of occurrence of carbonates (cm)||Humus content (percent)||Humus reserves in 1-m layer (ton/ha)|
A clear pattern can be observed in the geographic distribution of chernozem subtypes. The zone of chernozems is subdivided from north to south into the subzones of podzolized, leached, typical, common, and southern chernozems. These subzones are most clearly expressed in the European USSR.
Also identified are micellar-carbonate chernozems (Azov and Ciscaucasian), which form in regions with warm winters, where the soil does not freeze, and chernozems that develop under conditions of winter freezing. Chernozems are classified as ordinary, carbonate, solonets, solonets-solonchak, and the like according to the degree of salinity. According to the thickness of the humus layer (A and B,), they are classified as thin (less than 40 cm), medium thick (40–80 cm), thick (80–120 cm), and super thick (more than 120 cm). They are grouped according to humus content into low-humus (less than 6 percent), medium-humus (6–9 percent), and high-humus, or fertile (more than 9 percent). The clay and loam varieties of chernozems predominate. They possess favorable physical and physicochemical properties, such as good water capacity and high capacity for absorption of cations (30–70 mg per 100 g of soil).
There are about 240 million hectares of chernozems in the world, confined to Eurasia and North and South America. In Eurasia the chernozem zone, the largest in the world, encompasses Western and Southeastern Europe (Hungary, Bulgaria, Austria, Czechoslovakia, Yugoslavia, and Rumania), stretches in a broad band across the USSR, and continues on into Mongolia and China. In North America chernozems cover several western states of the USA and the southern provinces of Canada. In South America they are located in southern Argentina and the southern foothill regions of Chile.
In the USSR, chernozems are found in Moldavia, the Ukraine, the central part of the RSFSR, the Northern Caucasus, the Volga Region, and Western Siberia. They are very fertile and nearly all of them are farmed. The chernozem zone is the most important agricultural region of the country, containing more than 50 percent (130 million hectares) of the country’s plowed land. Winter and spring wheat, sugar beets, sunflowers, hemp, flax, buckwheat, and beans are grown on chernozems; animal husbandry, fruit and vegetable growing, and viticulture are highly developed in the chernozem zone.
The accumulation of moisture and its rational use are of paramount importance for increasing the fertility of chernozems, especially in the steppe regions of the zone, where a greater deficit of atmospheric moisture is observed. To improve the water regime of chernozems, irrigation is practiced and forest windbreaks are planted. Measures to prevent soil erosion, contamination by pesticides, and the salinization that can occur during irrigation play a special role.
REFERENCESDokuchaev, V. V. Nashi slepi prezhde i leper’. St. Petersburg, 1892.
Kostychev, P. A. Pochvy chernozemnoi oblasti Rossii: Ikh proiskhozhdenie, sostav i svoistva. Moscow, 1949.
Gavriliuk, F. Ia. Chernozemy Zapadnogo Predkavkaz’ia. Kharkov, 1955.
Afanas’eva, E. A. Chernozemy Sredne-Russkoi vozvyshennosti. Moscow, 1966.
Krupenikov, I. A. Chernozemy Moldavii. Kishinev, 1967.
Glazovskaia, M. A. Pochvy mira, parts 1–2. Moscow, 1972–73.
Chernozemy SSSR, vol. 1. Moscow, 1974.
Pochvovedenie, 2nd ed. Edited by I. S. Kaurichev. Moscow, 1975.
I. S. KAURICHEV