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soil,surface layer of the earth, composed of fine rock material disintegrated by geological processes; and humushumus
, organic matter that has decayed to a relatively stable, amorphous state. It is an important biological constituent of fertile soil. Humus is formed by the decomposing action of soil microorganisms (e.g.
..... Click the link for more information. , the organic remains of decomposed vegetation. In agricultureagriculture,
science and practice of producing crops and livestock from the natural resources of the earth. The primary aim of agriculture is to cause the land to produce more abundantly and at the same time to protect it from deterioration and misuse.
..... Click the link for more information. , soil is the medium that supports crop plants, both physically and biologically. Soil may be from a few inches to several feet thick.
Components and Structure
The inorganic fraction of soil may include various sizes and shapes of rocks and minerals; in order of increasing size these are termed clayclay,
common name for a number of fine-grained, earthy materials that become plastic when wet. Chemically, clays are hydrous aluminum silicates, ordinarily containing impurities, e.g., potassium, sodium, calcium, magnesium, or iron, in small amounts.
..... Click the link for more information. , siltsilt,
predominantly quartz mineral particles that are between sand size and clay size, i.e., between 1-16 and 1-256 mm ( 1-406 – 1-6502 in.) in diameter. Silt, like clay and sand, is a product of the weathering and decomposition of preexisting rock.
..... Click the link for more information. , sandsand,
rock material occurring in the form of loose, rounded or angular grains, varying in size from .06 mm to 2 mm in diameter, the particles being smaller than those of gravel and larger than those of silt or clay.
..... Click the link for more information. , gravelgravel,
particles of rock, i.e., stones and pebbles, usually round in form and intermediate in size between sand grains and boulders. Gravel is composed of various kinds of rock, the most common constituent being the mineral quartz.
..... Click the link for more information. , and stone. Coarser soils have lower capacity to retain organic plant nutrients, gases, and water, which are essential for plants. Soils with higher clay content, which tend to retain these substances, are therefore usually better suited for agriculture. In most soils, clay and organic particles aggregate into plates, blocks, prisms, or granules. The arrangement of particles, known as soil structure, largely determines the soil's pore space and density, which translates into its capacity to hold air and water. Organic matter consists of decomposed plant and animal material and living plant roots. Microorganisms, living in the organic portion of soil, perform the essential function of decomposing plant and animal matter, releasing nutrients to be used by growing plants.
Besides organic matter, soil is largely composed of elements and compounds of silicon, aluminum, iron, oxygen, and, in smaller quantities, calcium, magnesium, sodium, and potassium. Factors determining the nature of soil are vegetation type, climate, and parent rock material; geographic relief and the geological age of the developing soil are also factors. Acidic soils occur in humid regions because alkaline minerals are leached downward: alkaline soils occur in dry regions because alkaline salts remain concentrated near the surface. Geologically young soils resemble their parent material more than older soils, which have been altered over time by climate and vegetation. For advice and information on soils, consult state agricultural experiment stations and their publications.
Undisturbed soils tend to form layers, called horizons, roughly parallel to the surface. The Russian system of soil classification, from which most others derive, is based on the distinctive horizons of the soil profile. The A horizon, the surface layer, contains most of the humus. The B horizon contains inorganic compounds formed by decomposition of organic material, a process known as mineralization; the material is brought to the B layer by the downward leaching action of water. The lowest soil layer, the C horizon, represents the weathered mineral parent substance.
Soil Fertility and Conservation
Soil fertility—the ability to support plant growth—depends on various factors, including the soil's structure or texture; its chemical composition, esp. its content of plant nutrients; its supply of water; and its temperature. Agriculture necessarily lowers soil fertility by removing soil nutrients incorporated in the harvested crops. Cultivation, especially with heavy machinery, can degrade soil structure. Agricultural soils are also vulnerable to mismanagement. Exposure of soils to wind and rain during cultivation encourages erosion of the fertile surface. Excessive cropping or grazing can depress soil-nutrient levels and degrade soil structure.
Soil conservation techniques have been developed to address the range of soil management issues. Various methods of cultivation conserve soil fertility (see cover cropcover crop,
green temporary crop grown to prevent or reduce erosion and to improve the soil by building up its organic content. Green-manure crops, which are specifically grown for their organic content and other feature that enable them to improve the soil, but which may be
..... Click the link for more information. ; rotation of cropsrotation of crops,
agricultural practice of varying the crops on a piece of land in a planned series, to save or increase the mineral or organic content of the soil, to increase crop yields, and to eradicate weeds, insects, and plant diseases.
..... Click the link for more information. ). Minimum-tillage systems, often entailing herbicideherbicide
, chemical compound that kills plants or inhibits their normal growth. A herbicide in a particular formulation and application can be described as selective or nonselective.
..... Click the link for more information. use, avoid erosion and maintain soil structure. Soil fertility and agricultural productivity can also be improved, restored, and maintained by the correct use of fertilizerfertilizer,
organic or inorganic material containing one or more of the nutrients—mainly nitrogen, phosphorus, and potassium, and other essential elements required for plant growth.
..... Click the link for more information. , either organic, such as manuremanure,
term used in the United States to refer to excreta of animals, with or without added bedding; also called barnyard manure. In other countries the term often refers to any material used to fertilize the soil.
..... Click the link for more information. , or inorganic, and other soil amendments. Organic matter can be added to improve soil structure. Soil acidity can be decreased by addition of calcium carbonate or increased by addition of sulfuric acid.
See F. R. Steiner, Soil Conservation in the United States (1990); M. Alexander, Introduction to Soil Microbiology (2d ed. 1991); E. J. Plaster, Soil Science and Management (2d ed. 1991); publications of the U.S. Soil Conservation Service.
a natural formation possessing properties inherent in animate and inanimate nature. It consists of genetically related horizons (the soil profile) created by the transformation of the surface layers of the lithosphere under the combined influence of water, air, and organisms. An important characteristic of soil is fertility.
In the last quarter of the 19th century, V. V. Dokuchaev, the founder of modern soil science, introduced the concept of soil as an independent natural body developing through the interaction of soil-formation factors and possessing properties that distinguish it from the parent (soil-forming) material. Until then soil was generally regarded as a geological formation. Its fertility, or ability to provide plants with water and food, enables soil to participate in the production of the biomass. Natural fertility varies with the composition and properties of soil and with the factors involved in soil formation. As a result of agrotechnical, agrochemical, and reclamation measures, soil, the principal means of production in agriculture, acquires effective, or economic, fertility, an indicator of which is crop yield.
Formation. The main factors involved in soil formation are climate, parent material, plants and animals, topography, the geological age of the land, and man’s economic activity.
Climate influences the weathering of rock, affects the soil’s thermal and water regimes, thereby regulating the processes that take place in soil and their intensity, and largely determines the type of vegetative cover and animal life.
Parent material is converted into soil during soil formation. The texture (granulometric composition) and structure of the parent material determine the physical properties of soil—including its permeability to water and air and its water-retention capacity—and, consequently, its water and thermal regimes, its aeration characteristics, and the rate at which substances move through it. The mineral composition of the parent material determines the mineral and chemical composition of the soil and its initial content of nutrients for plants.
Vegetation directly affects soil. The roots loosen and give structure to the soil mass and extract minerals from it. Under natural conditions, minerals and organic matter enter the soil and collect on its surface in the form of root and surface litter-fall. The annual amount of litter-fall varies from about 5–6 centners per hectare (ha) in deserts and 10 centners per ha in arctic tundras to 250 centners per ha in humid tropical forests. The qualitative composition of the litter-fall also varies: its ash content ranges from 1 percent to 15 percent. In the soil the litter-fall is exposed to microflora, which mineralizes up to 80–90 percent of its mass and participates in the synthesis of humic substances formed from decomposition products and microbial metabolites.
Animals, mainly the invertebrates inhabiting the top layers of soil and the surface residues, considerably accelerate the decomposition of organic matter and promote the formation of organo-mineral soil aggregates, that is, the soil structure.
The main effect of topography consists in the redistribution of climatic (moisture, heat, and their correlations) and other factors involved in soil formation over the earth’s surface. The time required for the development of a mature soil profile varies from several hundred to several thousand years, depending on the conditions.
The age of the land in general and that of the soil in particular, as well as changes in the conditions of soil formation during their development, have a substantial effect on the structure, properties, and composition of soil. Under comparable geographic conditions of soil formation, soils differing in age and historical development may differ considerably and belong to different classification groups.
Man’s economic activity influences some factors of soil formation, for example, the Vegetation by the cutting down of forests and their replacement with grass phytocenoses. Man also affects the soil directly through cultivation, reclamation, and the addition of mineral and organic fertilizers. The soil-forming process and soil properties can be deliberately altered by an appropriate combination of these measures. Man’s influence on the soil processes is growing steadily because of the intensification of agriculture.
Composition and properties. Soil consists of solids, liquids, gases, and living organisms. The proportions vary not only from soil to soil but in different horizons of the same soil. The content of organic matter and living organisms steadily decreases with depth and the rate of transformation of the constituents of the parent material increases from the lower horizons to the upper ones.
Minerals dominate the soil’s solid matter. Primary minerals (quartz, feldspar, hornblende, mica), together with rock fragments, form coarse fractions; secondary minerals (hydromica, montmorillonite, kaolinite), created during weathering, form finer fractions. Friability is caused by the polydispersity of the solid matter, which includes particles of varying size, ranging from soil colloids measured in hundredths of a micron to fragments several dozen cm in diameter. The bulk of the soil is usually made up of fine earth, particles less than 1 mm in diameter. Soil texture (granulometric composition) is determined by the proportion of particles of different diameters, divided into groups, or granulometric fractions.
The classification of soil particles by diameter that has been adopted in the USSR is shown in Table 1.
|Table 1. Classification of soil particles in the USSR|
|Particle diameter (mm)||Name of fraction|
By granulometric composition, soils are divided into the following groups, or varieties, depending on the ratio of physical clay (particles smaller than 0.01 mm in diameter) to physical sand (particles larger than 0.01 mm in diameter): friable and consolidated sand, sandy loam, light and medium loam, and light, medium, and heavy clay. A more detailed division is made according to whether gravel, sand, coarse silt, silt, or clay particles predominate. N. A. Kachinskii’s classification of soils by granulometric composition is widely used in the USSR.
Naturally occurring solid particles do not fill the entire volume of the soil mass, only part of it. The other part consists of pores of different sizes and shapes between the particles and their aggregates. The total pore volume is called soil porosity. For most mineral soils, the porosity varies from 40 to 60 percent. It increases to 90 percent in organogenic (peat) soils but decreases to 27 percent in waterlogged, gleyed mineral soils. The soil’s water properties (permeability, capacity to raise water, moisture content) and its density depend on its porosity.
The pores contain soil solution and soil air, whose proportions are constantly changing because of the entry of atmospheric precipitation and, sometimes, irrigation water and groundwater and because of water loss through runoff, evaporation, and the drawing off of moisture by plant roots. Pore spaces freed from water fill up with air. These phenomena determine the air and water regimes of the soil. The more moisture there is in the pores, the more difficult the gas exchange (especially O2 and CO2) between the soil and the atmosphere and the slower the oxidation processes and the more rapid the reduction processes in the soil mass. The pores also contain soil microorganisms.
The soil’s bulk density in an intact state is determined by the porosity and average density of the solid phase. The bulk density of mineral soils varies from 1 to 1.6 g/cm3, sometimes reaching 1.8 g/cm3, that of waterlogged gleyed soils may be as much as 2 g/cm3, and that of peat soils is 0.1–0.2 g/cm3.
The large total surface of solid particles is related to their fineness: 3–5 m2/g in sandy soils, 30–150 m2/g in sandy loam and loamy soils, and about 300–400 m2/g in clay soils. As a result, the soil particles, especially the colloid and clay fractions, possess a surface energy that is reflected in the adsorbing capacity and buffering action of the soil.
The mineral composition of the solid matter is largely responsible for the fertility of a soil. Few organic particles (plant residues) are present, and only peat soils are made up of them almost entirely. The minerals contain Si, Al, Fe, K, N, Mg, Ca, P, and S, as well as small quantities of such trace elements as Cu, Mo, I, B, F, and Pb. Most of the elements occur in oxidized form. Many soils, especially those in regions receiving little rain, contain large quantities of CaCO3, particularly if the soils were formed on calcareous parent material. Soils in arid regions contain CaSO4 and other more readily soluble salts, and soils in the humid tropics are rich in Fe and Al.
However, whether these general patterns prevail depends on the composition of the parent material, age of the soil, topography, and climate. For example, soils with fairly large amounts of Al, Fe, alkaline-earth metals and alkali metals are formed on basic igneous rock, whereas Si abounds in soils developed on acidic rock. In the humid tropics, soils on young weathering crusts contain much less iron and aluminum oxides than those on older weathering crusts, and they are similar in content to soils in the temperate latitudes. On steep slopes, where erosion is pronounced, the composition of the solid matter scarcely differs from that of the parent material. Saline soils contain an abundance of calcium, magnesium, and sodium chlorides and sulfates (less commonly nitrates and bicarbonates) because of the original salinity of the parent material and the entry of these salts either from the groundwater or as a result of soil formation.
The solid part of soil includes organic matter, 80–90 percent of which consists of a complex of humic substances, or humus. The organic matter also consists of compounds of plant, animal, and microbial origin containing cellulose, lignin, proteins, sugars, resins, fats, and tannins, as well as the intermediate products of their decomposition. When organic matter decomposes, its nitrogen changes into forms available to plants. Under natural conditions, organic matter is the main source of nitrogen for plants. Many organic substances participate in the creation of organomineral structural separates (aggregates). The resulting structure largely determines the physical properties of the soil, as well as its water, air, and thermal regimes. The organomineral compounds include salts, clay-humus complexes, and complex and chelating compounds of humic acids with various elements (including Al and Fe). It is in these forms that the elements are transported through soil.
The liquid part of soil, or soil solution, is an active component, transporting substances within the soil or removing them from it. It also supplies plants with water and dissolved nutrients. The soil solution usually contains ions, molecules, colloids, and coarser particles and sometimes becomes a suspension.
The gaseous part of soil, or soil air, fills the pores not occupied by water. The quantity and composition of soil air (N2, O2, CO2, volatile organic compounds) are not constant but are determined by the many chemical, biochemical, and biological processes at work in the soil. For example, the concentration of CO2 in soil air varies considerably in the course of a year or 24-hour period owing to differences in the rate at which the gas is released by microorganisms and plant roots. Gas exchange between soil air and the atmosphere takes place chiefly through the diffusion of CO2 from the soil into the atmosphere and the diffusion of O2 in the opposite direction.
The living organisms in the soil include microorganisms (bacteria, fungi, actinomycetes, algae), various invertebrates (protozoans, worms, mollusks, insects and their larvae), and burrowing vertebrates. The active role of living organisms in soil formation makes soil a bioinert natural body, a major component of the biosphere.
Processes. During soil formation the parent material differentiates into soil horizons, which constitute the soil profile. Organic matter, nitrogen, phosphorus, and exchangeable compounds of aluminum, calcium, magnesium, potassium, and sodium accumulate in the surface horizons. The silicate compounds (with the exception of silica in the form of quartz) are often lost. The various processes that take place under the influence of the factors of soil formation may be divided into three main groups: (1) the exchange of matter and energy between the soil and other natural bodies, (2) the transformation of matter and energy in the soil body without the transport of substances, and (3) the movement of matter and energy in the soil.
The first group includes the exchange of gases, moisture, and solid particles between the atmosphere, soil, and vegetation (aboveground organs); the exchange of gases and moisture (with dissolved substances) between the soil and the rocks beneath the soil, including parent material and bedrock; the exchange of short-wave and long-wave radiation between the sun, vegetation, soil, atmosphere, and outer space; the exchange of thermal energy between the atmosphere, vegetation, soil, and underlying rock; the exchange of ash substances, nitrogen compounds, CO2, and O2 between the soil and higher vegetation; the mostly one-way entry of moisture from the soil into plants through their roots; and the one-way entry into the soil of organic matter synthesized by the higher plants and containing accumulated energy.
The second group encompasses a large number of very different processes: the decomposition of organic compounds and synthesis of humus; the synthesis and breakdown of microbial plasma; the formation and breakdown of organomineral compounds, that is, processes related to the carbon cycle (decomposition of carbohydrates, tannins, lignin); processes related to the nitrogen cycle (ammonification, nitrification, denitrification, and fixation of atmospheric nitrogen); the decomposition and conversion of primary and secondary minerals and the synthesis of secondary ones; oxidation and reduction, especially of iron and manganese; and the freezing and thawing of soil moisture, as well as the evaporation and condensation of moisture within the soil.
The third group includes the movement of soil air under the influence of changing pressure and temperature; the diffuse movement of gases and water vapor and the movement of the soil solution in response to gravity and to capillary, sorption, and osmotic forces; and the movement of the soil mass by burrowing animals and as a result of root pressure.
The soil processes, closely related and interdependent, either take place in the entire soil mass or are concentrated in certain parts. They occur within the earth’s gravitational field and have a cyclical character related to the cyclical flow of radiant energy (diurnal, annual, and multiyear cycles) to the soil surface, as well as to the biological cycles of living organisms. The fact that the processes are cyclical does not mean that the soil returns to its original state. The results of the cyclical processes occurring in the soil mass from the very beginning of its formation also determine the development and evolution of the soil. The nature of the processes and their intensity vary depending on the amount of soil and especially on the distance from the surface. The soil as an open system is also related to other natural systems (atmosphere, underlying rock, living organisms) by a reciprocal and many-sided exchange of matter.
The processes by which soil horizons are formed are called elementary soil processes. They include the formation of steppe matting, forest litter, and peat (accumulation of organic residues on the soil surface); the humus-accumulation process (the accumulation of organomineral compounds and ash elements in the upper horizons); salinization (the movement of dissolved salts followed by their precipitation from the solution); desalini-zation (the leaching of dissolved salts into the lower horizons or out of the soil); argillization, or the conversion of primary minerals into secondary clay minerals (the decomposition of primary minerals and synthesis of secondary ones); ¡lluvial processes (the dissolving of various substances in the upper horizons and the migration of solutions to the deeper horizons, where some substances are precipitated and accumulate); clay migration, or the gravity-induced movement of tiny solid particles in suspension; and gleization (the reduction of elements with a variable valence, primarily iron and manganese, and the related loss of structure of the soil mass). Among other processes are solonetsization, solodization, podzolization, iron accumulation, ferralitization, and pedocryogenesis.
Main soil groups and their distribution. The changes over space and time in soil-forming factors and, consequently, in past and present soil processes account for the great diversity of soils. Before Dokuchaev, soils were classified by individual properties —chemical composition, granulometric composition, and so on. The modern genetic classification of soils is based on the structure of the soil profile, which reflects all the processes involved in the origin, development, and evolution of soils and their regimes.
The main classification unit is the genetic group. Dokuchaev identified ten soil groups, and modern classifications comprise more than 100 groups. The groups are subdivided into subgroups, genera, species, varieties, and categories, and they are combined into classes, series, formations, generations, families, and associations. The principle by which soil groups are combined into larger units varies from one classification to another. Soils have been grouped according to the conditions of soil formation (ecological principle), the relations between soil groups (evolutionary-genetic principle), and the structure and genesis of the soil profiles (profile-genetic principle).
An important aspect of soil classification is the diagnosis of soils to establish the set of objective criteria for distinguishing soils at all the taxonomic levels of classification. Of special significance are the diagnostic criteria for identifying groups and lower taxonomic units because these subdivisions appear on most soil maps. Soil groupings according to agricultural production, land reclamation, and forest management are of great practical value.
No standard international classification of soils has yet been worked out, but a large number of national soil classifications are available. The classifications adopted by the USA, the USSR, and France include all the soils of the world. The first attempt to create a universal system was made by the Food and Agricultural Organization and UNESCO (1968–74) in conjunction with the compilation of the International Soil Map of the World.
Most of the earth’s land area is occupied by the comparatively small number of soil groups that Dokuchaev and N. M. Sibirtsev classified as zonal soils. These soils develop through soil-forming processes typical of each natural zone. The occurrence of zonal soils in wide bands, or zones, stretching along bands with similar precipitation (in regions with inadequate moisture) or along bands with identical annual temperatures (in regions with adequate or excessive moisture) accounts for the main pattern of soil distribution on plains—horizontal soil zonation, either latitudinal or meridional. On the East European Plain, for example, there are distinct zones of tundra, podzolic, gray-forest, chernozem, and brown desert-steppe soils. The subgroups of the zonal soils also occur in parallel bands, thus helping to identify soil subzones. The chernozem zone, for example, is subdivided into subzones of leached, typical, ordinary, and southern chernozems, and the chestnut zone comprises dark chestnut, chestnut, and light chestnut subzones.
I. P. Gerasimov and other scientists established the patterns of change in the properties of soils within zones and subzones resulting from changes in the climate and bioclimatic conditions. This phenomenon, called provinciality and faciation, was used to distinguish provinces within zones and subzones and to combine similar provinces in several zones and subzones into facies. Differences were found in the series of soil zones on different continents and in major areas of the largest continents. For example, zones of tundra, cryogenic-taiga, and podzolic soils, podburs, brown forest soils, cinnamonic soils of dry forests and scrub, yellow and red earths, and red-yellow ferralitic soils succeed one another from north to south in eastern Asia; in Central Asia (Western Siberia, Kazakhstan, Middle Asia) the series consists of tundra, surface-gley, and podzolic soils, chernozems, chestnut soils, brown desert-steppe soils, gray-brown desert soils, and sierozems. These differences are useful in distinguishing soil regions, each of which is characterized by a series of horizontal soil zones.
Altitude zonality is pronounced in mountainous countries. In mountains with inadequate moisture, the succession of vertical zones is determined by changes in the amount of moisture and by the exposure of the slopes (the soils here vary according to the exposure), whereas in mountains with adequate or excessive moisture, the zonation is determined by changes in thermal conditions.
The geographic soil patterns discussed above, determined mainly by bioclimatic factors, are responsible for the zonal-provincial structure of the soil cover. However, within the zones, subzones, and provinces, the soils are not homogeneous. The frequent alternation of soil in the soil cover is caused by changes in the topography, parent material, and depth of the groundwater, that is, it is dependent mainly on lithological and geomor-phological factors. The variations in more or less genetically related soil bodies create the pattern of the soil cover. They also account for the structure of the soil cover, all of whose components can be shown only on large-scale or detailed soil maps. The various structures of the soil cover are associated with distinct lithological-geomorphological and neotectonic structures, thereby clearly proving that they are closely related genetically.
Soil is one of the natural elements of man’s habitat. Disruption of the soil processes by improper use of the soil cover leads to intensified erosion, salinization, or waterlogging. The Basic Principles of Land Legislation of the USSR and Union Republics (1968) provides for measures to increase soil fertility and control erosion.
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A. A. RODE and V. M. FRIDLAND