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From Grolier's Multimedia Encyclopedia
This Navajo house is made from red clay.
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George Burba/iStockphoto

The term soil generally refers to the loose surface of the Earth, as distinguished from solid rock. To the farmer, soil is the natural medium for growth of all land plants. Civil engineers consider soil an easily disaggregated earthen material that supports most constructed works and from which embankments and other earthen structures are built.


The term soil as used in agricultural contexts can be more specifically defined as a thin layer of loose earthen materials composed of weathered minerals and decaying organic matter. It provides physical support and nutrients as well as sufficient quantities of air and water for plant growth. Specialists who study and manage soils as a medium for plant growth are called agronomists, or soil scientists.

Soil Profile. All mature soils are made up of a sequence of distinct layers. These are called horizons and roughly parallel the Earth's surface. The horizons commonly range from a few centimeters to a few meters in thickness. Each horizon in the sequence differs markedly from the other horizons in chemical, physical, and biological properties. A section cut downward from the surface through the various soil horizons forms the soil profile. From the surface downward, the major soil horizons are designated A, B, and C, respectively. In some cases these layers are further divided into subhorizons designated A1, A2, B1, and so forth. The A horizon (topsoil) is characterized by high biotic activity and an accumulation of organic matter. Water percolating through the A horizon commonly carries fine-grained materials downward (primarily clay particles and colloidal organic matter). The water deposits the materials in a zone of accumulation called the B horizon. Thus a principal distinguishing characteristic of the B horizon is a high clay content. The A and B horizons together make up the solum, or true soil. The C horizon is composed of relatively unaltered materials underlying the B horizon.

The soil profile is dynamic. Four states of profile development are commonly recognized in the life cycle of a soil: parent material, immature soil, mature soil, and old-age soil. Soils begin their development with parent materials. These are loose earthen materials laid down by wind, water, or glacial ice, or materials weathered in place from rocks. Exposure of the parent material to the weather in most climates results in the establishment of plants. The plants die and leave organic residues on which animals, bacteria, and fungi feed. This activity breaks the residues down to more elementary chemical forms. The breakdown sets nutrients free, making the parent material more fertile for another cycle of plant growth. As more and more organic matter is worked into the parent material, the upper layer assumes a darker color, and an A horizon develops. Such a soil has only A and C horizons and is in the immature stage. The immature stage is commonly reached in less than 100 years.

Continued weathering releases additional nutrients into the soil. Weathering is usually accelerated by the actions of plant growth. This action leads to greater plant growth and more-demanding species. Weathering also breaks the soil particles down to finer sizes. The smallest particles may be carried downward by percolating water to an underlying soil layer. These finer particles accumulate there, forming the B horizon. Soils having all three (A, B, and C) layers are termed mature.

With continued weathering, nearly all of the mineral nutrients may be released and removed from the soil by plant growth and water percolation. This stage is in many cases accompanied by the development of deleterious acidic by-products. When soil nutrients have been removed or deleterious by-products have accumulated to an extent that retards plant growth, the soil is said to have reached old age. Fortunately, such processes as erosion, flooding, dust storms, and volcanic eruptions expose or provide new, unweathered parent material to begin a new soil life cycle.

Physical and Chemical Composition. The physical and chemical compositions of soil horizons also differ according to the various conditions under which soils develop. These differences form the basis for categorizing soils into different soil groups and types. The five major factors that control the kinds of soil that develop are climate, particularly temperature and precipitation; living organisms, especially the types of native vegetation; the nature of the parent material (chemical and mineralogical composition as well as particle size); topography (ground slope and elevation); and time.

Climate is the most influential of the factors controlling soil development. Part of this influence comes from the control it has on other factors, such as the types of plants that can grow in a given climate. Temperature and precipitation control the rates and types of chemical and physical processes that are active in weathering parent materials. Weathering in turn controls the rate of nutrient release and the profile development. For instance, in arid regions, the weathering process is slow. Soil profiles are much shallower and generally less well developed than in humid regions. Soils in cold regions tend to be shallower than soils in warm regions. The mineral and organic matter in cold-region soils also tends to be less decomposed.

Perhaps the second most important factor controlling soil development is living organisms. The rate of organic-matter accumulation and, to some extent, of weathering is dependent on plant growth. Chemical composition is in part dependent on the types of plants growing on the soil. For example, the soils that develop under grasslands are chemically different from the soils that develop under forests. Even within forested areas, soil profiles developed under coniferous trees differ from those developed under deciduous trees. In places where organic matter is produced faster than it can be decomposed and reused by new plant growth, an organic-rich horizon, or O horizon, develops on top of the soil profile. Such soils are called peats and normally form in marshy or boggy areas.

The type of parent material has a great influence on the texture (particle sizes and shapes) and chemistry of the soil matrix. These factors in turn directly influence soil-profile development. For example, the rate of downward movement of water is controlled in part by the texture of the soil. Also, chemical and mineralogical composition affects rates of weathering. For instance, olivine minerals weather much faster than do quartz minerals. Chemical composition has some influence on the species of plants that grow in the soil. For example, only salt-resistant plants will grow in saline soils. The influence of parent material on soil characteristics is much more pronounced in younger than in older soils.

Topography affects soil development largely by influencing drainage and erosion. Excess water is removed much more slowly over smooth, flat terrain than over rolling terrain. Water may accumulate in some lowlands and depressions to form swamps, marshes, and bogs. Erosion is more active on sloping ground than on flat ground. Each of these factors has an important influence on the depth and character of the soil profile.

Finally, soils evolve with time. Soils on relatively new surfaces, such as an eroded area or an area covered by glacial debris from the most recent ice age, may be very different from nearby soils that appear to be in similar settings but have developed on an older landscape surface.

The physical properties of a soil have much to do with its suitability as a medium for plant growth. This fact has been recognized by farmers from the beginning of modern civilization. Thus an understanding of basic soil properties, combined with the use of procedures developed by modern technology for managing and improving soils, has led to revolutionary advances in crop production. These advances are necessary to provide food and fiber for a growing world population.


Many thousands of different soil types exist. These range from mature soils that have attained equilibrium with their environment to young soils still undergoing development. The factors involved in producing any soil type include the following: the parent rock materials; the landform of a given region; its climate; the local ecology; and the time period over which these other factors have been operating. Wherever the combination of these factors is similar, the resulting soils will also be similar.

Classification Systems. Many classification systems have been developed for grouping soil types. The oldest were based on use—that is, on the suitability of soils for various forms of agriculture. Scientists in the United States in the 19th century attempted a geological approach. This approach used the underlying and presumably parent rock as the basis for classification. In the later 19th century, however, Russian agriculturists began to develop grouping methods that have influenced all subsequent approaches. These methods were based on field and laboratory studies of distinctive soil properties as well as on broad climatic differences. The development of such systems remains an ongoing field of activity.

Many modern systems assemble soils into a hierarchy in which the lowest unit is the soil type, characterized by its distinctive soil profile. (One influential European system is based simply on these profiles, in terms of the ABC horizons described above.) Soil types alike in profile and parent material but differing in texture are then grouped in soil series. Usually the series are named for the site where the soils were first found. Similar series are placed in families, which in turn are gathered into groups. The groups typically coincide with broad climate and vegetation groupings. Finally, soil groups are classified as zonal, intrazonal (falling in two or more zones), or azonal (very young soils that may occur within any of the zones).

Group Characteristics. The soil groups can be characterized as follows.

Tundra soils (whose name dervies from the Finnish for "barren land") have dark brown surfaces and darker subsoils. In arctic regions, they are underlain by permafrost. The soils can be farmed if they are well drained and permafrost is absent or deep-lying.

Podzol soils (a term taken from the Russian for "ashlike") are moderately to strongly leached soils typical of forested, humid regions. They are not naturally very productive for agriculture but can be made so through good land management.

Chernozem soils (from the Russian for "black earth") have a dark surface layer underlain by more lightly colored soil. They typically develop under grasses in temperate to cool, subhumid climates. Chernozem soils are highly productive, although they require fertilizers after longer use.

Tropical and subtropical soils generally are characterized as lateritic. These reddish, leached soils form under conditions of good drainage, high temperatures, and extensive rainfall. When exposed, however, such soils tend to harden into an agriculturally useless material.

True desert soils are light-colored, shallow, and low in organic matter. In some locations they have lateritic characteristics. Under irrigation other types of desert soils can be made highly productive.

The remaining soil groups are intrazonal or azonal. These include the alluvial soils deposited by major rivers and the saline and alkali soils found in desert regions. Rendzina soils resemble chernozems but are underlain by chalk or soft limestone at shallow depths.

Seventh Approximation System. In 1965 the U.S. Department of Agriculture adopted a new classification system, now called the Seventh Approximation. This system can be applied to all soils, whatever their state of development. Using a terminology that describes soils in terms of surface and subsurface diagnostic horizons, the system incorporates ten major soil orders: alfisols, aridisols, entisols, histosols, inceptisols, mollisols, oxisols, spodisols, ultisols, and vertisols. These orders are subdivided into a descending hierarchy of suborders, groups, subgroups, families, and series. The names of the subdivisions further typify the soil types according to increasingly specific characteristics of texture, mineralogy, temperature, and so forth.


To the engineer, the materials making up the Earth's crust are divided into the categories of soil and rock. Soil is a natural aggregate of mineral grains often containing some organic particles. The natural aggregate can be separated into individual aggregates by such gentle mechanical actions as agitation in water. In contrast, rock is a natural aggregate of mineral grains held together by strong and permanent cohesive bonds. The bonds resist rupture even when subjected to vigorous mechanical actions such as striking the rock with a hammer. What the agronomist terms soil extends downward only to about the depth of root penetration, which is seldom more than a few feet. What the engineer terms soil extends from the ground surface down to its contact with a layer of hard rock, which in many localities is hundreds of feet below the surface. Engineers who deal with soils are a specialized group of civil engineers known as geotechnical engineers. The branch of physical science dealing with the mechanical behavior of soils is called soil mechanics.

Human use of soil as a building material dates back to the beginning of modern civilization. Only in recent years has the use of soils as a construction material or as a base for structures been treated scientifically. In large part, this change was brought about by the publication in 1925 of the book Erdbaumechanik (Soil Mechanics) by Karl Terzaghi. Prior to this publication, only a few scattered theories existed relating to the behavior of soils. Most engineering design in soils was based on local experience and rules of thumb. Terzaghi's work introduced scientific methods for predicting the behavior of soils and developed a basis for rational engineering design.

The primary reason for the delay in applying scientific analyses to soil is the more complex behavior of soils compared with that of other construction materials such as concrete and steel. A few relatively easily measured properties are adequate to define the behavior of steel or concrete under almost any loading condition. Numerous properties are required to similarly define the behavior of a soil under load. Strength, compressibility, and permeability of the soil are the properties of particular importance to geotechnical engineers. These properties in turn depend on such factors as grain size and shape, density, mineral types, amount of water in the voids, and forces that have previously acted on the soil body. In addition, natural soils are usually nonhomogeneous. Soil properties may thus vary greatly from layer to layer and from point to point within a layer. Some properties also vary directionally within the same soil element. Permeability, for example, is commonly greater in a horizontal direction than in a vertical direction. Consequently, soil properties to be used in engineering calculations ordinarily can be only approximately defined. As a result, less precise calculations can be made for soils than for manufactured materials such as steel. The following two design applications illustrate typical uses of soil-mechanics principles.

Foundations. The first application might be exemplified by a large building to be built on several unconnected slab footings, with one footing beneath each column in the building. Two possible modes of failure must be considered in formulating the design. In the first mode, the soil beneath the footings could shear and allow the footings to punch into the ground. To prevent this type of failure the footing must be designed to provide adequate bearing capacity. The determination of the bearing capacity involves the engineer's investigation of the subsurface soils with the aid of borings and soundings; the measurement of the strength and density of test specimens using laboratory and field tests; the estimation of the in-place soil strength from this information; and then the calculation of the amount of load that can be placed on the footing as a function of footing size. The engineer then selects the size and type of footing required to bear the column load.

The other possible mode of failure results from ground settlement caused by compression of soil layers beneath the building. This compression occurs as a result of the additional load applied to the soil by the building. Such compression and settlements commonly continue to occur at a decreasing rate over a period of years. If the settlement is large and uneven, the building will be distorted and may eventually fail. To determine the amount of settlement beneath the building, the engineer determines the configuration of soil layers beneath the building with the aid of subsurface borings. Using laboratory tests, the amount and rate of compression is determined from test specimens taken from the soil layers. Appropriate field or design values are then estimated for these parameters. Finally, the amount of settlement to be expected and the period of time over which it will occur is calculated. If the settlements are too large, the design of the foundation must be altered. For example, a pile foundation might be recommended rather than footings.

Dams. For the second typical application, consider an earthen dam to be constructed to impound a large reservoir. The weight of the embankment, the force of the water pressing against the dam, and the pressures in the water seeping through the dam all contribute forces that tend to cause the embankment to slump and fail. The strengths of the soils in the embankment provide forces resisting failure. To design a safe dam, the engineer calculates the forces in the dam that tend to cause failure. This can be done from analyses of the weight and geometry of the dam, the depth of water in the reservoir, and the rate and distribution of water seeping through various segments of the dam. These forces are then summed and compared with the forces tending to resist failure. The geometry of the dam, soil types used in various segments of the dam, and configurations of drains built into the structure are then adjusted to provide an economical and safe design.

Other applications of soil mechanics include embankments, pavements and roadways, and tunnels.

T. Leslie Youd