soils It is possible to define soil in many ways. A useful, well-established, and all-embracing definition proposed by J. S. Joffe in 1945 is that soil is a natural body consisting of layers or horizons of mineral and/or organic constituents of variable thickness which differ from the parent material in their morphology, physical, chemical, and mineralogical properties and their biological characteristics. A soil can be described in terms of its profile, which is the vertical arrangement of the soil horizons down to parent material. The fact that soils can be grouped into broad classes with similar properties suggests that all soils are influenced by a universal set of factors. These are known as state factors.

Factors of soil formation

Soils develop as a result of the interplay of five factors of soil formation: parent material, climate, organisms, topography, and time. These were incorporated by Hans Jenny in 1941, in a state factor equation:S or s = f (c, o, r, p, t)

where S denotes the soil, s is any soil property, c is the climate factor, o is the organism or biotic factor, r is the relief or topography factor, p is the parent material, and t is the time factor. Jenny defined these factors in the following terms. The climate factor represents the regional climate, with precipitation and temperature being considered as separate functions. The organism or biotic factor is essentially vegetation and is the summation of the plant matter reaching the soil. The topographic factor includes the shape and slope of the landscape, the aspect of the slope and the height of the water-table, the latter usually being related to topography. Parent material includes both weathered and unweathered material from which the soil formed. This includes material both in situ and transported and may also include a pre-existing soil. Time is the time elapsed since the deposition of material, the exposure of material at the surface, or the formation of the slope.

A number of attempts have been made to show that some factors are more important than others. In the early days of soil science, rock type was thought to be the most important factor. Early Russian workers stressed the importance of climate, and many soil classification schemes and world soil maps were based on climate characteristics. Each factor is, however, essential; none can be considered generally to be more important than any other, although locally one factor may exert a strong influence (see soils and topography). These factors define the ‘state’ of the soil system and provide the framework within which soil processes operate.

Soil characteristics

Undisturbed soil is a mixture of organic and inorganic solid particles and interconnected voids containing varying amounts of soil, water and gases. The appearance of a soil is influenced by a large number of characteristics, of which only the more significant ones can be examined here. Colour is often the most obvious characteristic and may indicate some of the processes that are, or were, operating within the soil. Dark colours near the surface usually indicate an accumulation of organic matter. Yellow-brown to red colours generally indicate the presence of iron (ferric) oxides, and greyish colours are usually due to ferrous iron compounds formed under reducing (gleying) conditions. White or light grey colours characterize a horizon from which leaching has removed oxides or hydroxides of aluminium or iron.

The textures of soils reflect the proportion of sand, silt, and clay sizes within that portion of an inorganic soil fraction that is less than 2 mm. The relative combinations of sand, silt, and clay are the formal basis of the various soil textural classes (Fig. 1). Texture will affect processes operating within the soil, and will affect chemical exchange because surface area per unit volume increases greatly as particle size decreases. Organic matter, which is usually concentrated near the surface, ranges from undecomposed plant and animal tissue to humus—a relatively resistant mixture of brown and dark-brown amorphous and colloidal substances modified from the original organic matter by various soil organisms. Carbon usually makes up over one-half of organic matter and the carbon–nitrogen ratio is a good indication of the amount of decomposition of the original organic material. The ratio is high (greater than 20) in plant tissue and low (less than 10) in humus. Soil organic matter increases the water-holding capacity of soils and the cation-exchange capacity. Carbon dioxide is released during the formation of humus, which aids the formation of carbonic acid, increases soil acidity, and enhances weathering.

Soil structure reflects the way in which individual particles are aggregated together. The individual aggregates, called peds, are classified into a number of basic types (Fig. 2). Texture can be an important control here; clay content is important in the creation of blocky, prismatic, and columnar structures. The movement of water through the soil and surface erosion are very much affected by structure. Some aggregates are unstable in water, breaking up and allowing clay to be removed by percolating water; some of this clay may be redeposited as clay skins on ped surfaces lower down the profile.

Water is an important constituent of soils. It is held in the soil by adhesive forces between water molecules and organic and inorganic particles, and by cohesive forces between adjacent water molecules. Moisture content is usually expressed as a percentage of the oven-dry soil weight, but it can also be related to its availability to plants. When the forces holding water films on soil-particle surfaces equal the forces of downward gravitational pull, the soil is said to be at field capacity. Water is removed easily from a soil by evaporation and transpiration through the vegetation. As more water is removed from the soil, water films become thinner and are held by ever-increasing forces of attraction. Eventually, the water is held so strongly that roots cannot extract it. The water content at which this occurs is called the permanent wilting point. Water retention is largely related to the organic matter and clay contents, whereas water movement is influenced by bulk density, porosity, and permeability; bulk density is the weight of soil per unit volume; porosity is the percentage of voids to the total volume of soil; and permeability is a measure of the ease with which water can pass through the soil.

Soil acidity, or pH, is an important characteristic and normally varies from 5 to 9. A value of 7 indicates neutral conditions, below 7 indicates acid conditions, and above 7 indicates alkaline conditions. Soil acidity is largely a function of organic matter and the type and amount of cations. Large amounts of organic matter tend to produce acid conditions unless counterbalanced by basic cations. Hydrogen and aluminium ions are largely responsible for soil acidity; aluminium is released by the weathering processes of hydrolysis and solution, and the production of free hydrogen ions increases the acidity. pH tends to decrease as rainfall increases because leaching of basic cations is then increased.

Soil horizons

The soil characteristics discussed above are not distributed randomly in the soil profile but are generally organized to produce a definite vertical and lateral structure to the system. Horizons are layers differentiated vertically in the soil body that differ in their physical, chemical, and biological attributes. The distinctiveness of soil type is usually related to the properties of its horizons, and horizon designations are an element in the definition of soil units and in the description of representative profiles.

A consistent set of master horizons, designated by capital letters, H, O, A, E, B, C, and R, with distinctive characteristics can be described (Fig. 3). The upper part of a soil is usually an organic horizon (O), formed or forming from accumulations of organic matter deposited at the surface. It may be subdivided into fresh litter (L) deposited during the most recent phases of litter fall and the partly decomposing litter (F) remaining from earlier periods of litter fall; this is also known as the fermentation layer. In the next layer (H) the organic matter is well decomposed and original plant structures cannot be discerned.

The A horizon is the first dominantly mineral horizon, in which humified organic matter is mixed with the mineral fraction. The organic matter is either distributed as fine particles or occurs as coatings on the mineral particles. A horizons are usually quite dark in colour because of the organic content. An E horizon, which usually underlies an O, H, or A horizon, is an ‘eluvial’ horizon (see soil development), having lost compounds of iron and aluminium (sesquioxides) and silicate clay by leaching and translocation. It contains a lower organic content than the A horizon and is usually paler in colour. The B horizon is commonly a mineral horizon characterized by weathering of the original parent material in situ. It may possess an ‘illuvial’ (see soil development) concentration of silicate clay, iron, aluminium, or humus, alone or in combinations. It is also characterized by the release of sesquioxides by weathering and is the zone where large-scale granular, blocky, or prismatic structures are formed. B horizons are the most variable of soil horizons. The C horizon is the parent material from which the soil has been derived. It lacks the properties of A and B horizons but includes weathering, as indicated by mineral oxidation, accumulation of silica, carbonates, or more soluble salts, and is often gleyed. The R horizon is usually hard, unweathered bedrock.

It is sometimes necessary to qualify the master horizon description with a suffix, or suffixes, to indicate additional properties of a horizon. Thus Ap indicates that the A horizon has been mixed by ploughing, Ag signifies gleying, and Ah denotes an accumulation of organic matter. Suffixes are used extensively in the B horizon to represent alteration by weathering in situ (Bw), illuvial concentration of clay (Bt) and sesquioxides (Bs), or a cemented layer (Bm). In this way a detailed description of a soil profile is also an interpretation of the processes that have shaped the profile. The nature and arrangement of horizons also form the basis for most soil classifications.

Soil classification

It is necessary to stress that there is no international agreement about the classification of soil types and there are at least ten different systems in use in various countries. Early Russian workers established that there was a close relationship between soils and vegetation and between soils and climate. This led the famous Russian soil scientist Dokuchaev to propose a classification based on broad vegetation zones, such as boreal, taiga, forest–steppe, steppe, etc. Such soils were called zonal soils, but it was soon apparent that soil types were not completely uniform over these broad areas. Because of this, soils that were formed as a result of the influence of a specific local factor, such as parent material or topography, were called intrazonal soils and young or poorly developed soils, such as those developed on river alluvium, were termed azonal soils. The division into zonal, intrazonal, and azonal soils was the basis for the soil classification used in the USA until 1960. It contains soil types, such as podzols, chernozems, brown earths, chestnut soils, etc., familiar to many people. Because it was based on zonal concepts this classification was very attractive to soil mappers, geographers, and ecologists. But such a scheme has severe limitations. Soils, such as podzols, which are zonal in some parts of the world might be intrazonal in other areas. Also, the system was based on environmental factors rather than on the essential characteristics of the soils. This type of classification has now been superseded by the United States Soil Conservation Service's Soil Taxonomy scheme and by an FAO–UNESCO classification.

The Soil Taxonomy scheme has six levels of categorization: order, suborder, great group, subgroup, family, and series. There are ten orders, differentiated by the presence or absence of certain diagnostic horizons or features that demonstrate which soil-forming processes have been dominant. It is a complicated system which often requires detailed measurement of the soil profile and quite sophisticated laboratory determinations, but it is a flexible system which is being adapted continuously. The FAO–UNESCO scheme was developed for use as the basis for the production of a world soil map at a scale of 1 : 50 000 00, which was completed in 1981. In this scheme, soils were divided into 26 major groups at the first level of the classification and into 106 soil units at the second level. This classification is also based on observable or measurable attributes of soils. The scheme was revised in 1988, and the number of major groups was then increased to 28 and the number of soil units to 153. These changes demonstrate the fluid nature of soil classifications and the great variability of soil types, which reflects the interaction of the soil-forming factors discussed above.

John Gerrard

Bibliography

FAO–UNESCO (1974) Soil map of the world, Vol. 1. FAO–UNESCO, Paris.
FitzPatrick, E. A. (1980) Soils: their formation, classification and distribution. Longman, Harlow.
USDA (1975) Soil taxonomy. Agricultural Handbook No. 18. USDA, Washington, DC.

SOIL

SOIL is a mixture of weathered rocks and minerals, organic matter, water, and air in varying proportions. Soils differ significantly from place to place because the original parent material differed in chemical composition, depth, and texture (from coarse sand to fine clay), and because each soil shows the effects of environmental factors including climate, vegetation, macro-and microorganisms, the relief of the land, and time since the soil began forming. The result of these factors is a dynamic, living soil with complex structure and multiple layers (horizons). Soils have regional patterns, and also differ substantially over short distances. These differences have shaped local and regional land use patterns throughout history. Because of this, historians have studied soil for clues about how people lived and for explanations of historical events and patterns.

Soil Classification and Mapping

The basis of the modern understanding of soil formation is attributed largely to work in the 1870s by the Russian V. V. Dokuchaev and colleagues. The Russians classified soil based on the presumed genesis of the soils and described the broadest soil categories. Simultaneously but separately, soil scientists in the United States were mapping and classifying soils based on measurable characteristics and focused on the lowest and most specific level of the taxonomy—the soil series. The Russian concepts did not reach the United States until K. D. Glinka translated them into German in 1914, and the American C. F. Marbut incorporated Glinka's ideas into his work. The U.S. system of soil classification that eventually developed considers the genetic origins of soils but defines categories by measurable soil features. Soils are divided into 12 soil orders based on soil characteristics that indicate major soil-forming processes. For example, Andisols is an order defined by the presence of specific minerals that indicate the soils' volcanic origin. At the other end of the taxonomic hierarchy, over 19,000 soil series are recognized in the United States. Research data and land management information are typically associated with the soil series.

Some U.S. soils were mapped as early as 1886, but the official program to map and publish soil surveys started in 1899 by the U.S. Department of Agriculture (USDA) Division of Soils, led by Milton Whitney. The effort was accelerated in 1953 when the Secretary of Agriculture created the National Cooperative Soil Survey, a collaborative effort of states, local governments, and universities led by the USDA Natural Resources Conservation Service. As of 2000, mapping was complete for 76 percent of the contiguous United States, including 94 percent of private lands.

Soil Fertility

Ancient writings demonstrate awareness of the positive effect of manure and certain crops on soil productivity. Modern agricultural chemistry began in eighteenth-century England, France, and Germany, and was dominated by scientists from these countries through the nineteenth century. In the 1840s, the German scientist Justus von Liebig identified essential plant nutrients and the importance of supplying all of them in soil, but this led to a concept of soil as a more or less static storage bin of nutrients and failed to reflect the dynamic nature of soil in relation to plants.

In 1862, state agricultural colleges were established by the Morrill Act, and the USDA was created. The Hatch Act of 1888 created experiment stations associated with the colleges. These developments led to the expansion of research plots that established the value of fertilizer in crop production and defined the variations in soil management requirements across the country.

Soil fertility can change because agriculture and other human activities affect erosion rates, soil organic matter levels, pH, nutrient levels, and other soil characteristics. An example of this is the change in distribution of soil nutrients across the country. In the early twentieth century, animal feed was typically grown locally and manure was spread on fields, returning many of the nutrients originally taken from the soil with the crop. Since farms became larger and more specialized toward the end of the twentieth century, feed is commonly grown far from the animals and manure cannot be returned to the land where the feed was grown. Thus, nutrients are concentrated near animal lots and can be a pollution problem, while soil fertility may be adversely affected where feed crops are grown.

Technology and Soil Management

Soil characteristics influence human activity, and conversely, human land use changes soil characteristics. Many technologies have changed how people use soil and have changed the quality of U.S. soils. The plow is one of these technologies. In 1794, Thomas Jefferson calculated the shape of the plow that offered the least resistance. Charles Newbold patented the cast iron plow in 1796. John Deere's steel plow, invented in 1837, made it possible for settlers to penetrate the dense mesh of roots in the rich prairies, and led to extensive plowing. Aeration of soil by plowing leads to organic matter decomposition, and within decades as much as 50 percent of the original soil organic matter was lost from agricultural lands. Until about 1950, plowing and other land use activities accounted for more annual carbon dioxide emissions than that emitted by the burning of fossil fuels. Fossil fuel emissions have grown exponentially since then, while net emissions from land use held steady and have declined recently.

Soil drainage systems expanded rapidly across the country in the early twentieth century in response to technological advances and government support. Drainage made it possible to farm rich lands in the Midwest that were previously too wet to support crops, and it allowed the use of irrigation in arid lands where irrigated soils quickly became saline when salts were not flushed away. The extensive drainage systems radically changed the flow of water through soil and altered the ability of land to control floodwater and to filter contaminants out of water.

A third critical soil technology was the development of manufactured fertilizers. During World War I (1914– 1918), the German chemist Fritz Haber developed a process to form ammonia fertilizer. Nitrogen is commonly the most limiting nutrient for intensive crop production. Phosphorus, another important limiting nutrient in some soils, became readily available as fertilizer in the 1930s. The use of these and other manufactured fertilizers made it possible to grow profitable crops on previously undesirable lands, and made farmers less dependent on crop rotations and nitrogen-fixing plants to maintain soil productivity.

A fourth technology was the development of herbicides beginning after World War II (1939–1945), combined with the refinement of"no-till" farm machinery in the 1970s. No-till is a method of crop farming that eliminates plowing and leaves plant residue from the previous crop on the soil surface. This residue protects the soil and can dramatically reduce erosion rates. The system also requires less fuel and labor than conventional tillage and thus allows a single farmer to manage more acres. The result has been a substantial reduction in erosion rates around the country and an increase in the amount of organic matter stored in the soil. The organic matter and associated biological activity improve productivity and reflect the sequestration of carbon dioxide from the atmosphere into the soil.

Erosion and Conservation

Soil degradation can take many forms, including loss of organic matter, poor biological activity, contamination with pollutants, compaction, and salinization. The most prominent form of land degradation is erosion by wind or water. Erosion is a natural process that is accelerated by over grazing and cultivation. In Conquest of the Land Through 7,000 Years (1999), W. C. Lowdermilk attributed the loss of numerous civilizations to unsustainable agricultural practices that caused erosion, resulting in silting of irrigation systems and loss of land productivity.

The first English colonists in America faced heavily forested lands but gradually cleared the land of trees and planted tobacco, cotton, and grain year after year in the same fields. In the eighteenth century there were references to worn-out land, and by 1800 much farm acreage along the coast had been abandoned. In 1748 Jared Eliot, a Connecticut minister and physician, published a book of essays documenting his observation of the connection between muddy water running from bare, sloping fields and the loss of fertility. John Taylor, a gentleman farmer of Virginia, wrote and was widely read after the Revolution (1775–1783) on the need to care for the soil. Perhaps the best known of this group of pre–Civil War (1861– 1865) reformers was Edmund Ruffin of Virginia. Clean-cultivated row crops, corn and cotton, according to Ruffin, were the greatest direct cause of erosion. He urged liming the soil and planting clover or cowpeas as a cover crop. His writings and demonstrations were credited with restoring fertility and stopping erosion on large areas of Southern land.

After the Civil War farmers moved west, subjecting vast areas to erosion, although interest in the problem seemed to decline. In 1927, Hugh Hammond Bennett of the U.S. Department of Agriculture urged, in Soil Erosion: A National Menace, that the situation should be of concern to the entire nation. In 1929, congress appropriated funds for soil erosion research.

The depression of the early 1930s led to programs to encourage conservation. The Soil Erosion Service and the Civilian Conservation Corps began soil conservation programs in 1933 with work relief funds. The dust bowl dust storms of 1934 and 1935 influenced Congress in 1935 to establish the Soil Conservation Service (SCS). Within a few years the service was giving technical assistance to farmers who were organized into soil conservation districts. These districts, governed by local committees, worked with the SCS to determine the practices to be adopted, including contour cultivation, strip farming, terracing, drainage, and, later, installing small water facilities. By 1973, more than 90 percent of the nation's farmland was included in soil conservation districts. The SCS was renamed the Natural Resources Conservation Service in 1994.

According to USDA Natural Resources Inventory data, erosion rates declined significantly during the 1980s, largely due to widespread adoption of reduced tillage practices. In the mid-1990s, erosion rates leveled off to about 1.9 billion tons of soil per year.

BIBLIOGRAPHY

Brady, Nyle C. The Nature and Properties of Soils. Upper Saddle River, N.J.: Prentice Hall, 2001.

Helms, Douglas. "Soil and Southern History." Agricultural History 74, no. 4 (2000): 723–758.

History of the Natural Resources Conservation Service. Available at http://www.nrcs.usda.gov/about/history/

Lowdermilk, W. C. Conquest of the Land Through 7,000 Years. Agriculture Information Bulletin No. 99. USDA Natural Resources Conservation Service, 1999.

Simms, D. Harper. The Soil Conservation Service. New York: Praeger, 1970.

U.S. Department of Agriculture, Yearbook (1938, 1957, 1958).

AnnLewandowski

Soil

Soil, which covers most of the land surface of Earth, is a complex mixture of weathered rock debris and partially decayed organic (plant and animal) matter. Soil not only supports a huge number of organisms below its surface—bacteria, fungi, worms, insects, and small mammals—but it is essential to all life on the planet. Soil provides a medium in which plants can grow, supporting their roots and providing them with water, oxygen, and other nutrients for growth.

Soil now covers Earth in depths from a few inches to several feet. Soils began to form billions of years ago as rain washed minerals out of the molten rocks that were cooling on the planet's surface. The rains leached or dissolved potassium, calcium, and magnesium—minerals essential for plant growth—from the rocks onto the surface. This loose mineral matter or parent material was then scattered over Earth by wind, water, or glacial ice, creating the conditions in which very simple plants could evolve. Plant life eventually spread and flourished.

As these early plants died, they left behind organic residues. Animals, bacteria, and fungi fed on this organic matter, breaking it down further and enriching the parent material with nutrients and energy for more complex plant growth. Over time, more and more organic matter mixed with the parent material, a process that continues to this day.

Soil is generally composed of 50 percent solid material and 50 percent space. About 90 percent of the solid portion of soil is composed of tiny bits of rock and minerals. These solid particles range in size from fine clay to mid-range silt to relatively large, coarse sand. The remaining 10 percent is made up of organic matter—living plant roots and plant and animal remains, residue, or waste products.

Words to Know

Bedrock: Solid layer of rock lying beneath Earth's surface.

Clay: Portion of soil comprising the smallest soil particles.

Horizons: Layers of soil that have built up over time and lie parallel to the surface of Earth.

Humus: Fragrant, spongy, nutrient-rich material resulting from the decomposition of organic matter.

Leaching: Downward movement through soil of chemical substances dissolved in water.

Loam: Soil that contains a balance of fine clay, medium-sized silt, and coarse sand particles.

Organic matter: Remains, residues, or waste products of any living organism.

Parent material: Loose mineral matter scattered over Earth by wind, water, or glacial ice or weathered in place from rocks.

Sand: Granular portion of soil composed of the largest soil particles.

Silt: Medium-sized soil particles.

Soil profile: Combined soil horizons or layers.

Topsoil: Uppermost layer of soil that contains high levels of organic matter.

The proportion of solid material in soil determines the amount of oxygen, water, and nutrients that will be available for plants. Since smaller particles stick together when wet, soil with a lot of clay holds water well, but drains poorly. Clay particles also pack together tightly, allowing for little air space. As a result, plant roots suffer from a lack of oxygen. Sand particles do not hold water or nutrients well. The best soil for plant growth is one in which all three types of particles—clay, silt, and sand—are in balance. Such a soil is called loam.

Soil horizons and profile

Once soil has developed, it is composed of horizontal layers with differing physical or chemical characteristics and varying thickness and color. These layers, called horizons, each represent a distinct soil that has built up over a long time period. The layers together form the soil profile. Soil scientists have created many different designations for different types of soil horizons. The most basic soil layers are the A, B, and C horizons.

The A horizon, the top layer, includes topsoil. The A horizon generally contains organic matter mixed with soil particles of sand, silt, and

clay. The amount of organic matter varies widely from region to region. In mountainous areas, organic matter is likely to make up only a small portion of the soil, from 1 to 6 percent. In low wet areas, organic matter may account for as much as 90 percent of soil content. Because it contains organic matter, the A horizon is generally darker in color than the deeper layers. The surface of the A layer is sometimes covered with a very thin layer of loose organic debris.

Below the A layer is the subsoil, the B horizon. This layer usually contains high levels of clay, minerals, and other inorganic compounds as water forced down by gravity through the A horizon carries these particles into the B horizon. This natural process is called leaching.

The A and B horizons lie atop the C horizon, which is found far enough below the surface that it contains little organic matter. Fragmented rocks and small stones make up most of the C horizon. Beneath this horizon lies bedrock, the solid layer of rock that lies underneath all soil.

Life in the soil

Soils teem with life. In fact, more creatures live below the surface of Earth than above. Among these soil dwellers are bacteria, fungi, and algae, which exist in vast numbers (bacteria are the most abundant). Three-hundredths of an ounce (one gram) of soil may contain from several hundred million to a few billion microorganisms. These microscopic organisms feed on plant and animal remains, breaking them down into humus, the dark, crumbly organic component of soil present in the A horizon. Humus cannot be broken down any further by microorganisms in the soil. It is a very important aspect of soil quality. Humus holds water like a sponge, serves as a reservoir for plant nutrients, and makes soil particles clump together, helping to aerate the soil.

Ants abound in soils. They create mazes of tunnels and construct mounds, mixing soils and bringing up subsurface soils in the process. They also gather vegetation into their mounds, which become rich in organic matter as a result. By burrowing and recolonizing, ants can eventually rework and fertilize the soil covering an entire prairie.

Earthworms burrow through soils, mixing organic material with minerals as they go and aerating the soil. Some earthworms pull leaves from the forest floor into their burrows (called middens), enriching the soil. Almost 4,000 worms can inhabit an acre of soil. Their burrowing can bring 7 to 18 tons of soil to the surface annually.

Larger animals inhabit soils, including moles, which tunnel just below the surface eating earthworms, grubs, and plant roots. In doing so, they loosen the soil and make it more porous. Mice also burrow, as do ground squirrels, marmots, and prairie dogs. All bring tons of subsoil material to the surface. These animals all prefer dry areas, so the soils they unearth are often sandy and gravelly.

Soil erosion

Erosion is any process that transports soil from one place to another. At naturally occurring rates, land typically loses about 1 inch (2.5 centimeters) of topsoil in 100 to 250 years. A tolerable rate of soil erosion is considered to be 48 to 80 pounds of soil per acre (55 to 91 kilograms per hectare) each year. Weathering processes that produce soil from rock can replace soil at this rate. However, cultivation, construction, and other human activities have increased the rate of soil erosion. Some parts of North America are losing as much as 18 tons of soil per acre (40 metric ton per hectare) per year.

The surface layer of soil (topsoil) provides most of the nutrients needed by plants. Because most erosion occurs on the surface of the soil, this vital layer is the most susceptible to being lost. The fertilizers and pesticides in some eroded soils may also pollute rivers and lakes. Eroded soil damages dams and culverts, fisheries, and reservoirs when it accumulates in those structures as sediment.

[See also Erosion ]

Soils

Soil on a suspect's shoe or splattered inside a car fender can provide forensic scientists with information about the travels of suspects and crime victims.

Soil is the product of biological, chemical, and physical alteration of materials at Earth's surface. Soils form in horizons, or layers, that are approximately parallel to the surface, have distinct properties, and are denoted by uppercase letters. The uppermost O horizon consists of decaying organic matter. It is underlain by the A horizon, or topsoil, which consists of a mixture of mineral and organic material. Beneath the A horizon is the B horizon, which consists of slightly altered mineral material, and the C horizon, which consists of the unaltered but loose parent material from which the soil developed (for example, sand). If intact rock is present, it can comprise an R horizon. Desert soils rich in calcium carbonate can also contain Bk or K horizons (the K is used to avoid confusion with the C horizon) that range from light accumulations of calcium carbonate to so-called petrocalcic horizons that are limestone formed in place. The term soil is also used loosely to refer to virtually any unlithified material at Earth's surface regardless of whether it has undergone the soil forming process known as pedogensis. Examples of materials that do not fall under the strict definition of soil include sand in dunes or along beaches and mud deposited by a recent flood. Because soils form by a complicated process that is influenced by factors such as temperature, precipitation, the mineralogical and chemical composition of the parent material, and even the nature of particles that may be washed out of the air during rainstorms, soil from different locations can have different physical and chemical characteristics that are useful to forensic scientists.

Soil recovered from shoes, clothes, and automobiles can be analyzed in order to determine if a suspect was or was not in a particular location. This is done by carefully comparing the color, particle size and shape, mineralogical composition, and biological components of a soil sample obtained from a suspect to those of soil from a known location. Particle sizes and shapes can be compared using reflected light microscopes . The chemical and mineralogical composition of the soil can be compared using techniques such as x-ray diffraction, in which a pulverized soil sample is subjected to x rays that produce patterns indicative of the crystal structure of minerals in the soil. Soils that are, or once were, adjacent to water may also contain distinctive shell fragments. The presence of soil unique to a particular area can show that a suspect must have traveled to that area, just as the absence of soil can be used to disprove an alibi. In some situations, layers of soil or mud can be used to establish presence at a sequence of locations.

The fictional British detective Sherlock Holmes is generally credited with the first use of soils as forensic evidence in the late nineteenth century, and soils have been employed as real life forensic evidence since the early years of the twentieth century. Holmes possessed the ability to distinguish different soil types and, using that information, make inferences about the travels of suspects. Real-life German chemist Georg Popp used goose droppings, sandstone fragments, and three different kinds of dust on a suspect's shoes to link to the same materials found at a murder victim's home, the place where the body was found, and the place where the murder weapon was found. Just as importantly, Popp used the absence of distinctive quartz crystals to disprove the suspected murderer's alibi he was walking in a specific field near his home when the crime occurred.

In more recent times, soil analysis was used in an attempt to track down the killers of Italian prime minister Aldo Moro in 1978. Investigators matched sand found on Moro's body to that found on an 6.8 mile (11-km) long beach north of Rome, which helped to focus their investigation. Another high profile case involved United States drug enforcement agent Enrique Camerena Salazar and his pilot Alfredo Zavala Avelar, who were killed by Mexican federal police in 1985. Their bodies were reported to have been found at the scene of a shootout between police and known drug dealers, implicating the drug dealers as murderers. Close examination of soil samples taken from the bodies, which contained an unusual combination of mineral and volcanic glass particles, revealed that the bodies had originally been buried in a remote mountainous area far from the shootout. This, combined with other forensic evidence, eventually showed that the federal police had been involved in the kidnapping, torture, and murder of the two.

Soil analysis is not restricted to cases involving politics and international intrigue. Soil found with a body inside a plastic garbage bag in New Jersey was identified as material that had been dredged from Newark Bay and used as fill to create new land along the shore. This clue led investigators to the victim's wife and daughter, who had killed him and temporarily buried the body beneath their home, which was built on the fill. California authorities were confounded when soil found in a murder suspect's car partially, but not completely, matched the soil around an oil well where the victim's body had been dumped. Further research showed that gravel from a different location had been spread around the well, explaining why the soil from the car was not an exact match with the natural soil in the area.

Small fragments of chert, a sedimentary rock made of silica, in cow manure collected from the back of a truck were used to prove that a herd of cattle had been rustled in Missouri and taken to Montana. Although the cattle rustlers had altered the brands on the cattle in an attempt to cover their tracks, they did not realize that the manure contained evidence that could have come only from Missouri. Another example of agricultural soil forensics is the comparison of soil samples to determine whether valuable plants were removed from protected government land and sold for landscaping.

see also Geology; GIS; Minerals.

Soil

One of the first distinctions made by a soil scientist is that "soil" and "dirt" are not the same. Dirt is what collects on the car or in the corner of the bedroom when it has been months since the last time it was vacuumed. Soil, on the other hand, is a highly structured matrix of inorganic and organic particles that form the substrate for terrestrial ecosystems . The substrate is the foundation where plants, ranging in size from minute ferns to tall trees, are rooted. The inorganic particles are formed from minerals in rock through weathering; a process that produces them by physical means (for example, erosion, freezing and thawing, and wind abrasion) or chemical means (for example, oxidation , dissolving crystals, or the action of acids). The organic particles originate from plant and animal tissues through fragmentation, decomposition, and chemical transformation.

The climate, rainfall, and temperature determine the pattern of soil weathering in a particular area. The weathering process often produces horizontal layers of soil of varying thickness called soil horizons because each layer is roughly parallel to Earth's horizon. The uppermost horizon often contains the most organic matter and others have differing nutrient contents and physical properties.

Soil provides physical support for plants, and the pores between particles provide spaces that contain water used by the plants and animals living within the soil. Oxygen from air diffuses into the pores when the water drains through the soil. This allows plant roots, aerobic microorganisms, and invertebrates to survive. Root systems may be located just below the surface, or may penetrate many meters deep. Too much water prevents air from reaching roots. Because of this, too much continuous water can kill many species of plants just as effectively as the absence of water during an extended drought. Only certain specially adapted plants are successful in water-saturated soils.

The particles that make up the soil may occasionally be all of the same size, as in the case of river sand deposits, or a silt layer that settled out on the bottom of ancient lakes. Sand particles are fairly large, only slightly smaller than gravel used in a fish tank, while silt particles are smaller than sand grains and clay particles are even smaller, approaching the fineness of talcum powder or baker's flour. Soils that are composed predominately of one of these particle sizes are known respectively as sands, silts, and clays. However, very often there is a mixture of particle sizes and the soil is referred to as a loam (a sandy loam has a mixture of particle sizes, but is mostly composed of sand). Loams are generally the best soils for plants to grow in. The larger sand particles facilitate drainage and oxygen penetration, while the small clay or organic humus particles provide a large amount of surface area where nutrient ions can become attached. Examples of these nutrients include nitrate, potassium, calcium, phosphate, and iron. They can be provided by commercial fertilizers, but are present naturally in nutrient-rich soils. The ions are attracted to electrically charged sites on clay or fine humus particles and gradually released into the water as they are exchanged with other ions. This nutrient-rich soil solution provides nutrition to plants through the roots.

Finally, the soil is a habitat for millions of small organisms per cubic meter such as bacteria, algae, nematodes , insects, and mites. These organisms make nutrients available through metabolic activity or the production of feces. They also die and add to organic matter and in general contribute to good soil quality. Larger organisms also inhabit the soil. Earthworms are particularly important because they mix the soil and process organic matter, which passes through their intestinal tracts and is released as feces. This helps produce loose textured soils with a high organic content and nutrient-holding capacity. In addition, their burrowing increases oxygen penetration. Larger animals such as moles, rabbits, foxes, and groundhogs create burrows that provide them with amenities such as shelter and food storage areas. This allows them to survive and thrive within the subterranean part of the ecosystem.

see also Biogeochemical Cycles; Mycorrhizae; Nematode; Nitrogen Fixation; Plant Nutrition; Roots

Dean Cocking

Bibliography

Killham, Ken. Soil Ecology. New York: Cambridge University Press, 1994.

Miller, Raymond W., and Duane T. Gardiner. Soils in Our Environment, 9th ed. Upper Saddle River, NJ: Prentice Hall, 2001.

Paton, T. R., G. S. Humphreys, and P. B. Mitchell. Soils: A New Global View. New Haven, CT: Yale University Press, 1995.

soil The layer of unconsolidated particles derived from weathered rock, organic material (humus), water, and air that forms the upper surface over much of the earth and supports plant growth. The formation of soil depends on the parent material (i.e. the original material from which the soil is derived), the climate and topography of the area, the organisms present in the soil, and the time over which the soil has been developing. Soils are often classified in terms of their structure and texture. The structure of a soil is the way in which the individual soil particles are bound together to form aggregates or peds. The structure types include platy, blocky, granular, and crumbs. The texture of a soil denotes the proportion of the various particle sizes that it contains. The four main texture classes are sand, silt, clay, and loam, of which loams are generally the best agricultural soils as they contain a mixture of all particle sizes. A number of distinct horizontal layers can often be distinguished in a vertical section (profile) of soil – these are known as soil horizons. Four basic horizons are common to most soils: an uppermost A horizon (or topsoil) containing the organic matter; an underlying B horizon (or subsoil), which contains little organic material and is strongly leached; a C horizon consisting of weathered rock; and a D horizon comprising the bedrock.

soil¹ / soil/ • n. the upper layer of earth in which plants grow, a black or dark brown material typically consisting of a mixture of organic remains, clay, and rock particles: blueberries need very acid soil| fig. the Garden State has provided fertile soil for the specialty beer market. ∎  the territory of a particular nation: the stationing of U.S. troops on Japanese soil. DERIVATIVES: soil·less adj. soil² • v. [tr.] make dirty: he might soil his expensive suit | [as adj.] (soiled) a soiled T-shirt. ∎  (esp. of a child, patient, or pet) make (something) dirty by defecating in or on it. ∎ fig. bring discredit to; tarnish: what good is there in soiling your daughter's reputation? • n. waste matter, esp. sewage containing excrement.See also night soil. ∎ archaic a stain or discoloring mark. soil³ • v. [tr.] rare feed (cattle) on fresh-cut green fodder (originally for the purpose of purging them).

377. Soil

See also 5. AGRICULTURE ; 119. DIRT ; 179. GEOLOGY .

agrogeology

the branch of geology concerned with the adaptability of land to agriculture, soil quality, etc.

agrology

the branch of soil science dealing especially with crop production. —agrologist, n. —agrological, adj.

edaphology

pedology.

geopony

the science of cultivation; agriculture. —geoponist, n. —geoponic, geoponical, adj.

lithification

the process by which loose mineral fragments or particles of sand are solidified into stone.

paleopedology, palaeopedology

the branch of pedology that studies the soil conditions of past geologic ages. —paleopedologist, palaeopedologist, n. —paleopedologic, palaeopedologic, paleopedological, palaeopedological, adj.

pedology

the branch of agriculture that studies soils; soil science. —pedologist, n. —pedologic, pedological, adj.

pinguidity

the state or quality of being rich or fertile. —pinguid, adj.

soil Surface layer of loose material resting on top of the rock which makes up the surface of the Earth. It consists of undissolved minerals produced by the weathering and breakdown of surface rocks, organic matter, water, and gases. The organic remains provide the humus and the inorganic particles provide vital minerals. Soils are classified by structure and texture. The structure is determined by the aggregation of particles (peds). The four main textures of soil are sand, silt, clay, and loam. Loam soils are best for cultivation, since they are able to retain more water and nutrients. Erosion and mismanagement are the chief causes of soil infertility.

Soiling (Encopresis)

Why Does Soiling Happen?

How Is Soiling Treated?

Soiling, also called encopresis (en-ko-PREE-sis), is having uncontrolled bowel movements in one’s underwear.

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Encopresis

Enuresis

Young children routinely have bowel movements in their diapers or underwear, but by about age 3 most children are able to maintain good bowel control and can be toilet-trained. When people who have established bowel control begin to have a bowel movement in their pants, the condition is called soiling, or encopresis. This soiling is often a leaking and not a full bowel movement. Most people who have a problem with soiling do not even realize that it is happening, because they do not feel as if they are having a bowel movement. In the majority of cases, encopresis is a medical problem. This medical problem can have serious psychological effects, ranging from embarrassment to family stress to teasing.

Why Does Soiling Happen?

Soiling is related to constipation (kon-sti-PAY-shun). Constipation is infrequent, hard, and painful bowel movements. When food goes through the digestive system, it is broken down into a thick, sludgelike liquid. The nutrients that the body needs, such as sugars, are absorbed from this liquid in the small intestine. The rest of the material passes into the large intestine, where water is reabsorbed. The remaining solids, called feces (FEE-seez), are then passed out of the body as a bowel movement.

Soiling results when solid body waste becomes hard and compacted in the large intestine, blocking it and causing it to stretch out of shape. If softer waste (liquid stool) seeps around the blockage, it can leak out of the anus, causing soiling.

When the bowels move infrequently, the large intestine reabsorbs so much water that the feces become hard and compacted. As a result, bowel movements are painful, causing many people to try to avoid having them. This only makes the problem worse. Eventually, the mass of hard solids in the large intestine causes it to stretch out of shape. As it stretches, small amounts of liquid sludge from the small intestine seep around the hard mass of feces in the large intestine and then leak out of the body. This is the material that causes soiling.

Some adults think that children soil on purpose or that soiling is evidence of a psychological problem. In reality, soiling accidents are not intentional. In fact, people often do not know that soiling is happening until feces are noticed or others smell it. At times the person with encopresis may not even smell the accident. Sometimes children who are teased or embarrassed about soiling can have emotional or behavior problems. Generally, once the soiling is treated and stops, these problems will disappear.

How Is Soiling Treated?

There are three steps to treating soiling:

• Empty the large intestine
• Establish regular bowel movements
• Maintain regular bowel movements.

An enema or a laxative medication often is used to empty the large intestine. With an enema, liquid is pushed into the large intestine to soften the hard mass of feces and create the urge to expel it. Sometimes strong laxatives are used instead, to encourage the intestine to contract and push out the feces.

Once the large intestine is unblocked, it is important to establish regular bowel movements to keep it clear. A doctor may recommend laxatives taken by mouth, such as milk of magnesia, products that contain senna, or mineral oil. These laxatives keep waste material moving quickly through the large intestine so that it remains soft. Setting aside time each day to try to have a bowel movement (usually after breakfast or dinner) also helps establish a regular schedule.

Once a person is having regular bowel movements daily, laxatives are reduced and then gradually eliminated so that a regular schedule can be maintained without artificial assistance. Eating a high-fiber diet and drinking plenty of liquids also help maintain bowel regularity. Once feces move through the large intestine in a regular, painless way, the problem of soiling disappears. Unfortunately, it often takes time for soiling to be diagnosed correctly and properly treated. Sometimes consultation with a mental health professional, who works with a person’s doctor, helps in developing a good behavioral treatment program that also minimizes emotional difficulties.

See also

Bedwetting (Enuresis)

soil
1. The natural, unconsolidated, mineral and organic material occurring on the surface of the Earth; it is a medium for the growth of plants.

2. In engineering geology, any loose, soft, and deformable material (e.g. unconsolidated sands and clays).

soil3
A. defile, pollute XIII; sully, tarnish XVI;

B. take to water or marshy ground XV. — OF. soill(i)er, suill(i)er (mod. souiller) :- Rom. *suculāre, f. L. suculus, -ula, dim. of sūs SOW1.

soil2 †muddy place; stretch of water as refuge for a hunted animal XV; stain, pollution XVI; filth, ordure XVII. — OF. *soille, souille (mod. souille muddy place), f. souiller SOIL2
.

soil1 (piece of) ground or earth, land, country; ground as cultivated. XIV. — AN. soil land, perh. repr. L. solium seat, solum (F. sol) ground.

soil4 (dial.) feed (cattle) with green fodder, orig. for purgation. XVII. perh. a use of SOIL3.

soil •boil, Boyle, broil, coil, Dáil, Doyle, embroil, Fianna Fáil, foil, Hoyle, moil, noil, oil, roil, Royle, soil, spoil, toil, voile •parboil • trefoil • jetfoil • airfoil •cinquefoil • milfoil • tinfoil • multifoil • aerofoil • hydrofoil •counterfoil • gargoyle • turmoil •charbroil • topsoil • subsoil