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Soil

SOIL

CONCEPT

If there is anything on Earth that seems simple and ordinary, it is the soil beneath our feet. Other than farmers, people hardly think of it except when tending to their lawns, and even when we do turn our attention to the soil, we tend to view it as little more than a place where grass grows and earthworms crawl. Yet the soil is a complex mixture of minerals and organic material, built up over billions of years, and without it, life on this planet would be impossible. It is home to a vast array of species that continually process it, enriching it as they do. Nor are all soils the same; in fact, there are a great variety of soil environments and a great deal of difference between the soil at the surface and that which lies further down, closer to the bedrock.

HOW IT WORKS

The Beginnings of Soil Formation

It has taken billions of years to yield the soil as we know it now. Over the course of these mind-boggling stretches of time, the chemical elements on Earth came into existence, and the uniformly rocky surface of the planet gradually gave way to deposits of softer material. This softer matter, the earliest ancestor of soil, became enriched by the presence of minerals from the rocks and, over a longer period, by decaying organic matter.

After its formation from a cloud of hot gas some 4.5 billion years ago, Earth was pelted by meteorites. These meteorites brought with them solid matter along with water, forming the basis for the oceans. There was no atmosphere as such, but by about four billion years ago, volcanic activity had ejected enough carbon dioxide and other substances into the air to form the beginnings of one. The oceans began to cool, making possible the earliest forms of lifethat is, molecules of carbon-based matter that were capable of replicating themselves. (For more on these subjects, see Sun, Moon, and Earth and Geologic Time. On the relationship between carbon and life-forms, see Carbon Cycle.)

All of these conditionsEarth itself, an atmosphere, waters, and life-formswent into the creation of soil. Soil has its origins in the rocks that now lie below Earth's surface, from which the rain washed minerals. For rain to exist, of course, it was necessary to have water on the planet, along with some form of atmosphere into which it could evaporate. Once these conditions had been established (as they were, over hundreds of millions of years) and the rains came down to cool the formerly molten rock of Earth's surface, a process of leaching began.

Leaching is the removal of soil particles that have become dissolved in water, but at that time, of course, there was no soil. There were only rocks and minerals, but these features of the geosphere, along with the chemical elements in the atmosphere and hydrosphere, were enough to set in motion the development of soil. While the atmosphere and hydrosphere supplied the falling rain, with its vital activity of leaching minerals from the rocks, the minerals themselves supplied additional chemical elements necessary to the formation of soil. (The chemical elements are discussed in several places, most notably Biogeo-chemical Cycles. See also Minerals and Rocks.)

THE FIRST PLANTS.

Among the elements leached from the rock by the falling rains were potassium, calcium, and magnesium, all of which are essential for the growth of plant life. Thus, the foundation was laid for the first botanical forms, a fact that had several important consequences. First and most obviously, it helped set in motion the formation of the complex biosphere we have around us today. Not only did the simplest algae-like plants serve as forerunners for more complex varieties of plant and animal life to follow, but they also played a major role in the beginnings of an atmosphere breathable by animal life. As the plants absorbed carbon dioxide from their surroundings, there gradually evolved a process whereby the plant received carbon dioxide and, as a result of a chemical reaction, released oxygen.

In addition, plant life meant plant death, and as each plant died, it added just a bit more organic materialand with it nutrients and energyto the ground. Notice the word ground as opposed to soil, which took a long, long time to form from the original rock and mineral material. Indeed, the processes we are describing here did not take shape over the course of centuries or millennia but over whole eonsthe longest phases of geologic time, stretching for half a billion years or more (see Geologic Time). Only around the beginning of the present eon, the Phanerozoic, more than 500 million years ago, did soil as such begin to take shape.

What Is Soil?

As the soil began to form, processes of weathering, erosion, and sedimentation (see the entries Erosion and Sediment and Sedimentation) slowly added to the soil buildup. Today the soil forms a sheath over much of the solid earth; just inches deep or nonexistent in some places, it is many feet deep in others. It separates the planet's surface from its rocky interior and brings together a number of materials that contribute to and preserve life.

Though its origins lie in pulverized rock and decayed organic material, soil looks and feels like neither. Whether brown, red, or black, moist or dry, sandy or claylike, it is usually fairly uniform within a given area, a fact for which the organisms living in it can be thanked. Under the surface of the soil live bacteria, fungi, worms, insects, and other creatures that continually churn through it and process its chemical contents.

A filter for water and a reservoir for air, soil provides a sort of stage on which the drama of an ecosystem (a community of mutually interdependent organisms) is played out. It receives rain and other forms of precipitation, which it filters through its layers, replenishing the groundwater supplies. This natural filtration system, sometimes augmented by a little human ingenuity, is amazingly efficient for leaching out harmful microorganisms and toxins at relatively low levels. (Thus, for instance, septic tank drainage systems process wastewater, with the help of soil, before returning it to the water table.)

By collecting rainwater, soil also gives the rain a place to go and thus helps prevent flooding. Water is not the only substance it stores; soil also collects air, which accounts for a large percentage of its volume. Thus, oxygen is made available to the roots of plants and to the large populations of organisms living underground. The creatures that live in the soil also die there, providing organic material that decays along with a vast collection of dead organisms from aboveground: trees and other plants as well as dead animalsincluding humans, whose decomposed bodies eventually become part of the soil as well.

Factors That Influence Soil

The processes that formed soil over the eons and that continue to contribute to the soil under our feet today are similar to those by which sedimentary rock is formed. Sedimentary rocks, such as shale and sandstone, have their origins in the deposition, compaction, and cementation of rock that has experienced weathering. Added to this is organic material derived from its ecosystemfor example, fossilized remains of animals.

Both sedimentary rock and soil are made up of sediment, which originates from the weathering, or breakdown, of rock. Weathered remains of rocks ultimately are transported by forces of erosion to what is known as a depositional environment, a location where they are sedimented. (See Sediment and Sedimentation for more about these processes.) The nature of the "parent material," or the rock from which the soil is derived, ranks among five key factors influencing the characteristics of soil in a given environment. The others are climate, living organisms, topography, and time.

PARENT MATERIAL, CLIMATE, AND ORGANISMS.

Minerals, such as feldspars and micas, react strongly to natural acids carried by rain and other forms of water; therefore, when these minerals are present in the rock that makes up the parent material, they break apart quite easily into small fragments. On the other hand, a mineral that is harderfor example, quartzwill break into larger pieces of clastic, or rock, sediment. Thus, the parent material itself has a great deal to do with the initial grain of the sediment that will become soil, and this in turn influences such factors as the rate at which water leaches through it.

The release of chemical compounds and elements from minerals in weathering provides plants with the nutrients they need to grow, setting in motion the first of several steps whereby living organisms take root in, and ultimately contribute to, the soil. As the plant dies, it leaves behind material to feed decomposers, such as bacteria and fungi. The latter organisms play a highly significant role in the biogeochemical cycles whereby certain life-sustaining elements are circulated through the various earth systems.

In addition, still-living plants provide food to animals, which, when they die, likewise will become one with the soil. This is achieved through the process of decomposition, aided not only by decomposers but by detritivores as well. The latter, of which earthworms are a great example, are much more complex organisms than the typically single-cell decomposers. Detritivores consume the remains of plant and animal life, which usually contains enzymes and proteins far too complex to benefit the soil in their original state. By feeding on organic remains, detritivores cycle these complex chemicals through their systems, causing them to undergo chemical reactions that result in the breakdown of their components. As a result, simple and usable nutrients are made available to the soil.

TOPOGRAPHY AND TIME.

Then there is the matter of topography, or what one might call landscapethe configuration of Earth's surface, including its relief or elevation. Soil at the top of a hill, for instance, is liable to experience considerable leaching and loss of nutrients. On the other hand, if soil is located in a basin area, it is likely to benefit from the vitamins and minerals lost to soils at higher elevations, which lose these nutrients through leaching and erosion.

In addition, topography influences the presence or absence of organic material, which is vital if the soil is to sustain plant life. Organic matter in mountainous areas accounts for only 1% to 6% of the soil composition, while in wet lowland regions it may constitute as much as 90% of soil content. Because erosion tends to bring soil, water, and organic material from the highlands to the lowlands, it is no wonder that lowlands are almost always more fertile than the mountains that surround them.

Finally, time is a factor in determining the quality of soil. As with everything else that either is living or contains living things, soil goes through a progression from immaturity to a peak to old age. In the earth sciences, age often is measured not in years, which is an absolute dating method, but by the relative dating technique of judging layers, beds, or strata of earth materials. (For more about studying rock strata as well as relative dating techniques, see Stratigraphy.)

REAL-LIFE APPLICATIONS

Layers in the Soil

If you dig down into the dirt of your backyard, you will see a miniature record of your regions's geologic history over the past few million years. Actually, most homes in urban areas and suburbs today have yards made of what is called fill dirtloose earth that has been moved into place by a backhoe or some other earthmoving mechanism. Even though the mixed quality of fill dirt makes it difficult to discern the individual strata, the soil itself tells a tale of the long ages of time that it took to shape it.

Better than a modern fill-dirt yard, of course, would be a sample taken from an older community. Here, too, however, human activities have intervened: people have dug in their yards and holes have been filled back up, for instance, thus altering the layers of soil from what they would have been in a natural state. To find a sample of soil layers that exists in a fully natural state, it might be necessary to dig in a woodland environment.

In any case, anyone with a shovel and a piece of ground that is reasonably untouchedthat is, that has not been plowed up recentlycan become an amateur soil scientist. Soil scientists study soil horizons, or layers of soil that lie parallel to the surface of Earth and which have built up over time. These layers are distinguished from one another by color, consistency, and composition. A cross-section combining all or most of the horizons that lie between the surface and bedrock is called a soil profile. The most basic division of layers is between the A, B, and C horizons, which differ in depth, physical and chemical characteristics, and age.

TOPSOIL.

At the top is the A horizon, or topsoil, in which humusunincorporated, often partially decomposed plant residueis mixed with mineral particles. Technically, humus actually constitutes something called the O horizon, the topmost layer. Examples of humus would be leaves piled on a forest floor, pine straw that covers a bare-dirt area in a yard, or grass residue that has fallen between the blades of grass on a lawn. In each case, the passage of time will make the plant materials one with the soil.

Owing to its high organic content, the soil of the A horizon may be black, or at least much darker than the soil below it. Between the A and B horizons is a noticeable layer called the E horizon, the depth of which is a function of the particulars in its environment, as discussed earlier. In rough terms, topsoil could be less than a foot (0.3 m) deep, or it could extend to a depth of 5 ft. (1.5 m) or more.

In any case, the E horizon, known also as the eluviation or leaching layer, is composed primarily of sand and silt, built up as water has leached down through the soil. The sediment of the E horizon is nutrient-poor, because its valuable mineral content has drained through it to the B horizon. (The E horizon is just one of several layers aside from the principal A, B, and C layers. We will mention only a few of these here, but soil scientists include several other horizons in their classification system.)

SUBSOIL, REGOLITH, BEDROCK.

The appearance and consistency of the soil change dramatically again as we reach the B horizon. No longer is the earth black, even in the most organically rich environments; by this point it is more likely to exhibit shades of brown, since organic material has not reached this far below the surface. Yet subsoil, which is the consistency of clay, is certainly not poor in nutrients; on the contrary, it contains abundant deposits of iron, aluminum oxides, calcium carbonate, and other minerals, leached from the layers above it.

The rock on the C horizon is called regolith, a general term for a layer of weathered material that rests atop bedrock. Neither plant roots nor any other organic material penetrate this deeply, and the deeper one goes, the more rocky the soil. At a certain depth, it makes more sense to say that there is soil among the rocks rather than rocks in the soil.

Beneath the C horizon lies the R horizon, or bedrock. As noted earlier, depths can vary. Bedrock might be only 5-10 ft. deep (1.5-3 m), or it might be half a mile deep (0.8 km) or perhaps even deeper. Whatever the depth, it is here that the solid earth truly becomes solid, and for this reason builders of skyscrapers usually dig down to the bedrock to establish foundations there.

Life Beneath the Surface

The ground beneath our feetthat is, the topmost layer, the A horizonis full of living things. In fact, there are more creatures below Earth's surface than there are above it. The term creatures in this context includes microorganisms, of which there might be several billion in a sample as small as an acorn. These include decomposers, such as bacteria and fungi, which feed on organic matter, turning fresh leaves and other material into humus. In addition, both bacteria and algae convert nitrogen into forms usable by plants in the surrounding environment (see Nitrogen Cycle).

WORMS.

We cannot see bacteria, of course, but almost anyone who has ever dug in the dirt has discovered another type of organism: worms. These slimy creatures might at first seem disgusting, but without them our world could not exist as it does. As they burrow through soils, earthworms mix organic and mineral material, which they make available to plants around them. They also may draw leaves deep into their middens, or burrows, thus furnishing the soil with nutrients from the surface. In addition, earthworms provide the extraordinarily valuable service of aerating the soil, or supplying it with air: by churning up the soil continuously, they expose it to oxygen from the surface and allow air to make its way down below as well.

Nor are these visible, relatively large worms the only ones at work in the soil. Colorless worms called nematodes, which are only slightly larger than microorganisms, also live in the soil, performing the vital function of processing organic material by feeding on dead plants. Some, however, are parasites that live off the roots of such crops as corn or cotton.

ANTS AND LARGER CREATURES.

Likewise there are "bad" and "good" ants. The former build giant, teeming mounds and hills that rise up like sores on the surface of the ground, and some species have the capacity to sting, causing welts on human victims. But a great number of ant species perform a positive function for the environment: like earthworms, they aerate soil and help bring oxygen and organic material from the surface while circulating soils from below.

In some areas, much larger creatures call the soil home. Among these creatures are moles, who live off earthworms and other morsels to be found beneath the surface, including grubs (insect larvae) and the roots of plants. As with ants and earthworms, by burrowing under the ground, they help loosen the soil, making it more porous and thus receptive both to moisture and air. Other large burrowing creatures include mice, ground squirrels, and prairie dogs. They typically live in dry areas, where they perform the valuable function of aerating sandy, gravelly soil.

Soils and Environments

In discussing our imaginary journey through the depths of the soil, it has been necessary to use vague terms concerning depths: "less than a foot," for instance. The reason is that no solid figures can be given for the depth of the soil in any particular area, unless those figures are obtained by a soil scientist who has studied and measured the soil.

Depth is just one of the ways that the soil may vary from one place to another. Earlier we mentioned five factors that affect the character of the soil: parent material, climate, living organisms, topography, and time. These factors determine all sorts of things about the soilmost of all, its ability to support varied life-forms. Collectively, these five factors constitute the environment in which a soil sample exists.

POOR SOILS.

A desert environment might be one of immature soil, defined as a sample that has only A and C horizons, with no B horizon between them. On the other hand, the soil in rainforests suffers from just the opposite condition: it has gone beyond maturity and reached old age, when plant growth and water percolation have removed most of its nutrients.

Whether in the desert or in the rainforest, soils near the equator tend to be the "oldest," and this helps explain why few equatorial regions are noted for their agricultural productivity, even though they enjoy otherwise favorable weather for growing crops. Soils there have been leached of nutrients and contain high levels of iron oxides that give them a reddish color. Moreover, red soil is never good for growing crops: the ancient Egyptians referred to the deserts beyond their realm as "the red land," while their own fertile Nile valley was "the black land."

RAINFORESTS.

If soil is so poor at the equator, why do equatorial regions such as the Congo or the Amazon River valley in Brazil support the dense, lush rain-forest ecosystems for which they are noted? The answer is that the abundance of organic material at the surface of the soil continually replenishes its nutrient content. The rapid rate of decay common in warm, moist regions further supports the process of renewing minerals in the ground.

This also explains why the clearing of tropical rainforests, an issue that environmentalists called to the world's attention in the 1990s, is a serious problem. When the heavy jungle canopy of tall trees is removed, the heat of the sun and the pounding intensity of monsoon rains fall directly on ground that the canopy would normally protect. With the clearing of trees and other vegetation, the animal life that these plants support also disappears, thus removing organisms whose waste products and bodies would have decayed eventually and enriched the soil. Pounded by heat and water and without vegetation to resupply it, the soil in an exposed rainforest becomes hard and dry.

DESERTS.

In deserts the soil typically comes from sandstone or shale parent material, and the lack of abundant rainfall, vegetation, or animal life gives the soil little in the way of organic sustenance. For this reason, the A horizon level is very thin and composed of light-colored earth. Then, of course, there are desert areas made up of sand dunes, where conditions are much worse, but even the best that desertshave to offer is not very good for sustainingabundant plant life.

Only those species that can endure a limitedwater supplyfor example, the varieties of cactusthat grow in the American Southwestare able tosurvive. But lack of water is not the only problem. Desert subsoils often contain heavy deposits ofsalts, and when rain or irrigation adds water to thetopsoil, these salts rise. Thus, watering desert top-soil can make it a worse environment for growth.

RICH SOILS.

In striking contrast to the barren soil of the deserts and the potentially barren soil of the rainforest is the rich earth that lies beneath some of the world's most fertile crop-producing regions. On the plains of the midwestern United States, Canada, and Russia, the soil is blackalways a good sign for growth. Below this rich topsoil is a thick subsoil that helps hold in moisture and nutrients.

The richest variety of soil on Earth is alluvial soil, a youngish sediment of sand, silt, and clay transported by rivers. Large flowing bodies of water, such as the Nile or Mississippi, pull soil along with them as they flow, and with it they bring nutrients from the regions through which they have passed. These nutrients are deposited by the river in the alluvial soil at its delta, the place where it enters a larger body of waterthe Mediterranean Sea and the Gulf of Mexico, respectively. Hence the delta regions of both rivers are extremely fertile.

WHERE TO LEARN MORE

Bial, Raymond. A Handful of Dirt. New York: Walker, 2000.

Bocknek, Jonathan. The Science of Soil. Milwaukee, WI: Gareth Stevens, 1999.

Canadian Soil Information System (Web site). <http://sis.agr.gc.ca/cansis/>.

Gardner, Robert. Science Projects About the Environment and Ecology. Springfield, NJ: Enslow Publishers, 1999.

Scheiderman, Jill S. The Earth Around Us: Maintaining a Livable Planet. New York: W. H. Freeman, 2000.

Snedden, Robert. Rocks and Soil. Illus. Chris Fairclough. Austin, TX: Raintree Steck-Vaughn, 1999.

Soil Association (Web site). <http://www.soilassociation.org>.

Soil Science Society of America (Web site). <http://www.soils.org/>.

USDA-NRCS National Soil Survey Center (Web site). <http://www.statlab.iastate.edu/soils/nssc/>.

World Soil Resources (Web site). <http://www.nhq.nrcs.usda.gov/WSR/Welcome.html>.

KEY TERMS

A HORIZON:

Topsoil, the upper mostof the three major soil horizons.

AERATE:

To make air available to soil.

ATMOSPHERE:

In general, an atmosphere is a blanket of gases surrounding a planet. Unless otherwise identified, however, the term refers to the atmosphere of Earth, which consists of nitrogen (78%), oxygen (21%), argon (0.93%), and other substances that include water vapor, carbon dioxide, ozone, and noble gases such as neon (0.07%).

B HORIZON:

Subsoil, beneath topsoil and above regolith.

BEDROCK:

The solid rock that lies below the C horizon, the deepest layer of soil.

BIOGEOCHEMICAL CYCLES:

The changes that particular elements undergo as they pass back and forth through the various earth systems and particularly between living and nonliving matter. The elements involved in biogeochemical cycles are hydrogen, oxygen, carbon, nitrogen, phosphorus, and sulfur.

BIOSPHERE:

A combination of all living things on Earthplants, mammals, birds, reptiles, amphibians, aquatic life, insects, viruses, single-cell organisms, and so onas well as all formerly living things that have not yet decomposed.

C HORIZON:

Regolith, which lies between subsoil and bedrock and constitutes the bottommost of the soil horizons.

DECOMPOSERS:

Organisms that obtain their energy from the chemical breakdown of dead organisms as well as from animal and plant waste products. The principal forms of decomposer are bacteria and fungi.

DECOMPOSITION REACTION:

A chemical reaction in which a compound is broken down into simpler compounds or into its constituent elements. In the earthsystem, this often is achieved through the help of detritivores and decomposers.

DETRITIVORES:

Organisms that feed on waste matter, breaking organic material down into inorganic substances that then can become available to the biosphere in the form of nutrients for plants. Their function is similar to that of decomposers, but unlike decomposerswhich tend to be bacteria or fungidetritivores are relatively complex organisms, such as earthworms or maggots.

ECOSYSTEM:

A community of interdependent organisms along with the inorganic components of their environment.

EROSION:

The movement of soil and rock due to forces produced by water, wind, glaciers, gravity, and other influences. In most cases, a fluid medium, such as air or water, is involved.

FILL DIRT:

Loose earth that has been moved into place by a backhoe or some other earthmoving machine, usually as part of a large construction project.

GEOSPHERE:

The upper part of Earth's continental crust, or that portion of the solid earth on which human beings live and which provides them with most of their food and natural resources.

HUMUS:

Unincorporated, often partially decomposed plant residue that lies at the top of soil and eventually will decay fully to become part of it.

HYDROSPHERE:

The entirety of Earth's water, excluding water vapor in the atmosphere but including all oceans, lakes, streams, groundwater, snow, and ice.

LEACHING:

The removal of soil materials that are in solution, or dissolved inwater.

ORGANIC:

At one time chemists used the term organic only in reference to living things. Now the word is applied to most compounds containing carbon, with the exception of carbonates (which are minerals), and oxides, such as carbon dioxide.

PARENT MATERIAL:

Mineral fragments removed from rocks by means of weathering. Along with organic deposits, these form the basis for soil.

REGOLITH:

A general term describing a layer of weathered material that rests atopbedrock.

SEDIMENT:

Material deposited at or near Earth's surface from a number of sources, most notably preexisting rock. There are three types of sediment: rocks, or clastic sediment; mineral deposits, or chemical sediment; and organic sediment, composed primarily of organic material.

SEDIMENTARY ROCK:

One of the three major types of rock, along with igneous and metamorphic rock. Sedimentary rock typically has its basis in the deposition, compaction, and cementation of rock that has experienced weathering, though it also may be formed as a result of chemical precipitation. Organic sediment also may be a part of sedimentary rock.

SEDIMENTATION:

The process of erosion, transport, and deposition undergone by sediment.

SOIL HORIZONS:

Layers of soil, parallel to the surface of Earth, that have built up over time. They are distinguished from one another by color, consistency, and composition.

SOIL PROFILE:

A cross-section combining all or most of the soil horizons that lie between Earth's surface and the bedrock below it.

TOPOGRAPHY:

The configuration of Earth's surface, including its relief as well as the position of physical features.

WEATHERING:

The breakdown of rocks and minerals at or near the surface of Earth due to physical, chemical, or biological processes.

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Soil

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

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Soil

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 surfacebacteria, fungi, worms, insects, and small mammalsbut 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 magnesiumminerals essential for plant growthfrom 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 matterliving 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 particlesclay, silt, and sandare 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 ]

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Soils

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.

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Soil

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.

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soil

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.

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soil

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).

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Soil

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.

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soil

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.

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Soiling (Encopresis)

Soiling (Encopresis)

Why Does Soiling Happen?

How Is Soiling Treated?

Soiling, also called encopresis (en-ko-PREE-sis), is having uncontrolled bowel movements in ones 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 persons doctor, helps in developing a good behavioral treatment program that also minimizes emotional difficulties.

See also

Bedwetting (Enuresis)

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soil

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).

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soil

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.

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soil

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
.

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soil

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

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soil

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

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soil

soilboil, 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

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soil

soil
1. The natural, unconsolidated, mineral and organic material occurring above bedrock 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.

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Soil

Soil


Soil is the unconsolidated mineral material on the immediate surface of the earth that serves as a natural medium for the growth of land plants. Soil is found on all surfaces except on steep, rugged mountain peaks and areas of perpetual ice and snow.

Soil is related to the earth much as the rind is related to an orange. But this rind of the earth is far less uniform than the rind of the orange. Soil is deep in some places and shallow in others. Soil can be red in Georgia and black in Iowa. It can be sand, loam, or clay. Soil is the link between the rock core of the earth and the living things on its surface. Soil is the foothold for the plants we grow and the foundation for the roads we travel and for the buildings we live in.

Soil consists of mineral and organic matter, air, and water. The proportions vary, but the major components remain the same. Minerals make up 50% of an ideal soil while air and water make up 25% each. Every soil occupies space. Soil extends down into the planet as well as over its surface. Soil has length, breadth, and depth. The concept that a soil occupies a segment of the earth is called the "soil body." A single soil in a soil body is referred to as a "pedon." The soil body is composed of many pedons and is thus called a "polypedon."

Every soil has a profile or a succession of layers (horizons) in a vertical section down into the non-soil zone referred to as the parent material. Parent materials can be soft rock, glacial drift, wind blown sediments, or alluvial materials. The nature of the soil profile is important for determining a soil's potential for root growth, storage of water, and supply of plant nutrients.

Soil formation proceeds in steps and stages, none of which is distinct. It is impossible to determine where one step or stage in soil formation begins and another ends. The two major steps in the formation of soils are the accumulation of soil parent materials and the differentiation of horizons in the profile. Soil horizons develop due to the processes of additions, losses, translocations, and transformations. These processes act on the soil parent material to produce soil horizons. The intensity of any one process will vary from location to location due to five soil-forming factors:

  • Soil forms from the parent materials, and they can be rock, sand, glacial till, loess, alluvial sediments, or lacustrine clays.
  • Topography is the position on the landscape where the parent material is located. Positions can be summits, sideslopes, and footslopes. Erosion and the depth to the water table will influence soil formation as a result of topography.
  • Climate acts on the parent material and determines the rate of rainfall and evapotranspiration , which influence the amount of leaching in the soil profile. The greater the leaching, the faster soil horizons develop.
  • Biotic factors include vegetation and animals. Animals are important mixers of the soils and can destroy soil horizons. Vegetation influences soils by determining the amount of organic matter incorporated into the soils. Soils formed under prairie grasses will have a thick black surface layer, compared to soils formed under forests where the surface layer is thin with a light-colored leached zone under it.
  • Time refers to the number of years that the parent material has been acted on by climate and vegetation. The older a soil is, the greater the horizon development. Young soils will have very minimal horizon development. The age of a soil is thus dependent on its development and not on the total number of years. Therefore, developed soils in the Midwest may be 10,000 years old, while young soils in the valleys of California will also be 10,000 years old. Developed soils on high terraces in California can be as old as one million years.

Because the soil-forming factors vary from location to location, soils are different across the landscape. The ability to interpret where the soils will change has allowed soil scientists to make a map of these changes. The soil map is thus an interpretation of the soils that occur on a landscape. The soil map is published on a county-by-county basis and is called a soil survey . Soil surveys are very useful documents in planning land use according to the soil's potential.

Soil is an absolutely essential natural resource but one that is both limited and fragile. The soil is often overlooked when natural resources are listed. Throughout history the progress of civilizations has been marked by a trail of windblown or water-washed soils that resulted in barren lands. Continuing to use the soil without appropriate soil conservation management is very destructive to the environment . Protecting the quality of our nation's topsoil is largely within human control. To many soil scientists, saving our soil is much more important than saving oil, coal , or natural gas resources.

[Terrence H. Cooper ]


RESOURCES

BOOKS

Miller, R. W., and R. L. Donahue. Soils: An Introduction to Soils and Plant Growth. Englewood Cliffs, NJ: Prentice-Hall, 1990.

OTHER

Stefferund, A., ed. Soils: 1957 Yearbook of Agriculture. Washington, DC: U.S. Government Printing Office, 1957.

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soil

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

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soil

soil, surface layer of the earth, composed of fine rock material disintegrated by geological processes; and humus, the organic remains of decomposed vegetation. In agriculture, 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 clay, silt, sand, gravel, 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 crop; rotation of crops). Minimum-tillage systems, often entailing herbicide use, avoid erosion and maintain soil structure. Soil fertility and agricultural productivity can also be improved, restored, and maintained by the correct use of fertilizer, either organic, such as manure, 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.

Bibliography

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.

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Soil

Soil

Soil formation

Soil profiles and horizons

Aging soils

Soil categories

Soil groups and agriculture

Life in the soil

Resources

Soil is a complex mixture of pulverized rock and decaying organic matter, which covers most of the terrestrial surface of the Earth. Soil not only supports a huge number of organisms below its surfacebacteria, fungi, worms, insects and small mammals, which all play a role in soil formationbut it is essential to all life on Earth. Soil provides a medium in which plants can grow, supporting their roots and providing them with nutrients for growth. Soil filters the skys precipitation through its many layers, recharging the aquifers and groundwater reserves from which we drink. Slowing the movement of rainfall by absorption, soil prevents damaging floods. By holding air in its pores, soil provides oxygen to plant roots and to the billions of other organisms inhabiting soil. Soil receives and thrives on organic matter as it dies, assuring that it returns to a form useful to subsequent living organisms. Soil has built up over eons on top of bedrock, the solid rock layer that makes up the crust of Earth, as exposed rocks have weathered and eroded and organic matter, including plant and animal life, have decomposed and become part of the soil. The word soil comes from the Latin word for floor, solum.

Soil formation

Soils began to form billions of years ago as rain washed minerals out of the once molten rocks that were cooling on the planets surface. The rains leached potassium, calcium, and magnesiumminerals essential for plant growth from the rock, creating the conditions in which very simple plants could evolve. Plant life eventually spread and flourished, and as each plant died and decomposed, it added nutrients and energy to the mineral mixture, making the soil more fertile for new plants.

Soil now covers Earth in depths from a few inches to several feet, and these soils are constantly forming and changing. Soils are created from parent material, loose earthy matter scattered over Earth by wind, water, or glacial ice, or weathered in place from rocks.

Parent material is turned into soil as other reactions take place on exposed rock surfaces. Water-borne acids react with elements in the rock and slowly change them into soil components. Minerals that break down relatively easilyfeldspars and micasbecome clay, the smallest soil particles with diameters less than 0.0002 mm, while harder minerals like quartz turn into sand (0.05-2.0 mm) and silt (0.0002-0.05 mm).

As the parent material weathers, the nutrients necessary for plant growth are released, and plants

begin to establish themselves. As they die, they leave behind organic residues on which animals, bacteria, and fungi feed. Their consumption breaks down the organic matter further, enriching the parent material for plant growth. Over time, more and more organic matter mixes with the parent material.

Wherever soil is found, its development is controlled by five important factors: climate, parent material, living organisms, topography, and time.

A regions climate determines the range and fluctuation of temperature and the amount of precipitation that falls to Earth, which in turn controls the chemical and physical processes responsible for the weathering of parent materials. Weathering, in turn, controls the rate at which plant nutrients are released. Nutrient flow, along with temperature and precipitation, determines the types of plants a region can support.

A soils parent material plays an important role in determining the chemistry and texture of soil (the size and shape of soil particles). The rate at which water moves through soil is controlled in part by the texture of the soil. Soils from some parent materials weather more or less quickly than others. Soils derived from quartz minerals, for example, weather more slowly than those derived from silicate materials.

The numbers and kinds of living organisms in a given region help determine the chemical composition of soil. Grassland soils are chemically different from those that develop beneath forests, and even within these broad categories of vegetative cover, soil profiles can differ; for example, different soils develop under conifers than under deciduous trees.

Topography, the configuration of Earths surface, affects soil development because it determines the rate at which precipitation washes over soil and how soils erode. Smooth, flat lands hold water longer than hilly regions, where water moves more quickly down slopes. Swamps, marshes, and bogs are formed as low-lying areas hold water over time. Soil erodes, or wears away, more quickly on sloping land than flat.

Time plays an important role in soil development: soils are categorized as young, mature, or old, depending on how the above factors are combined, and the rate at which they work.

Soil profiles and horizons

Below the surface of Earth lie layers of soil that are exposed when people dig into the earth, or by natural forces like earthquakes. These cross-sections of soil, called soil profiles, are composed of horizontal layers or horizons of soil of varying thickness and color, each representing a distinct soil that has built up over a long time period. Soil horizons contain soils of different ages and composition, and soil scientists can tell a lot about a regions climate, geography, and even agricultural history by reading the story of the regions soils through these layers.

A soil horizon is a horizontal layer of soil with physical or chemical characteristics that separate it from layers above and below. More simply, each horizon contains chemicals, such as rust-like iron oxides, or soil particles that differ from adjacent layers. Soil scientists generally name these horizons (from top to bottom) O, A, B, C, and R, and often subdivide them to reflect more specific characteristics within each layer. Considered together, these horizons constitute a soil profile.

Horizons usually form in residual soils: soils not transported to their present location by water, wind, or glaciers, but formed in place by the weathering of the bedrock beneath them. It takes many thousand to a million years to achieve a mature soil with fully developed horizons.

The O horizon (sometimes known as the A0) consists of freshly dead and decaying organic matter mostly plants but also small (especially microscopic) animals or the occasional rigid cow. A gardener would call this organic matter (minus the cow) compost or humus. Below the O lies the A horizon, or topsoil, composed of organic material mixed with soil particles of sand, silt, and clay. Earthworms, small animals, and water mix the soil in the A horizon. Water forced down through the A by gravity carries clay particles and dissolved minerals (such as iron oxides) into the B horizon in a process called leaching; therefore, the A is known as the Zone of Leaching. These tiny clay particles zigzag downward through the spaces (pores) between larger particles like balls in a Japanese pachinko game. Sometimes the lower half of the A horizon is called the E (Eluvial) horizon, meaning it is depleted of clay and dissolved minerals, leaving coarser grains.

The leached material ends up in the B horizon, the Zone of Accumulation. The B horizon, stained red by iron oxides, tends to be quite clay like. If the upper horizons erode, plant roots have a tough time penetrating this clay; and rain which falls on the exposed clay can pool on the surface and possibly drown plants or flood basements.

Sometimes the top of the B horizon develops a dense layer called a fragipana claypan (compacted by vehicles) or a hardpan (cemented by minerals). In arid climates, intense evaporation sucks water and its dissolved minerals upward. This accumulation creates a hardpan impenetrable to any rain percolating (sinking) downward, resulting in easily evaporated pools or rapid runoff. If the hardpan is composed of the calcium-rich mineral calcite, it is called caliche. If composed of iron oxides, it is called an ironpan. Fragipans are extremely difficult for crop roots and water to penetrate. The A and B horizons together make up the solum, or true soil.

Partially weathered bedrock composes the C horizon. Variously sized chunks of the rock below are surrounded by smaller bits of rock and clay weathered from those chunks. Some of the original rock is intact, but other parts have been chemically changed into new minerals.

The R layer (D horizon) is the bedrock, or sometimes, the sediment from which the other horizons develop. Originally, this rock lay exposed at the surface where it weathered rapidly into soil. The depth from the surface to the R layer depends on the interrelationships between the climate, the age of the soil, the slope, and the number of organisms. Most people do not consider the R layer as soil, but include it in the profile anyway, since the weathering of this bedrock usually produces the soil above it.

In a perfect world, all soils demonstrate these horizons, making the lives of soil scientists and soil students blissful. In reality, however, some soils, like transported soils (moved to their present locations by water, wind, or glaciers), lack horizons because of mixing while moving or because of youth. In other soils, the A and B rest on bedrock, or erosion strips an A, or other complicated variations. Around the world, scientists classify soils by these horizonal variations.

Aging soils

Like all living things, soils age. Exposure to wind, rain, sun, and fluctuating temperatures combine to push soils through four stages of development: parent material, immature soil, mature soil, and old-age soil.

As noted above, parent materials are loose materials weathered from rocks. As plants establish themselves in parent material, organics accumulate, and the upper soil layer becomes richer and darker, and evolves into an A horizon. At this point, the soil has only A and C horizons and is in the immature stage, which it usually reaches in less than 100 years.

Through continued weathering and plant growth, the soil gathers more nutrients, and can support more demanding species. Soils break down into smaller particles such as clay, and as water moves down through the matrix, it carries these fine soil particles with it. As they accumulate in the underlying layer, these particles form a B horizon. Soils that have A, B, and C layers are described as mature.

Gradually, as weathering continues, plant growth and water percolation remove nearly all of the mineral nutrients from soil, and acidic by-products begin to develop. When a soil lacks the nutrients or contains enough acids that plant growth is slowed, the soil has reached old age.

Soil categories

Soil scientists have developed a number of systems for identifying and classifying soils. Some broad systems of soil classification are used worldwide, and one of the most widely applied is that developed by the U.S. Department of Agriculture. It includes 11 major soil orders: alfisols, andisols, aridisols, entisols, histosols, inceptisols, mollisols, oxisols, spodisols, ultisols, and vertisols. Each major order is subdivided into suborders, groups, subgroups, families, and series.

Soils are also classified at an extremely specific level: soils are named after a local landmark such as a town, school, church, or stream near where the soil is first identified. There are soils named Amarillo and Fargo, for example, identifying their origins in northwestern Texas and North Dakota, respectively. Soils that share characteristics that fall within defined limits share the same name, and these soils form a soil series. (Local names for soil series are usually used within countries but not across boundaries, even though soils on different continents share the same characteristics.)

Soil groups and agriculture

Plants have adapted to the globes variety of soils and can grow in almost every soil and under all variations of weather, yet plants grow better in some places than others, especially in places where nutrients are most readily available from the soil.

The tropical belt around Earths equator contains the globes oldest soils. Under heavy rainfalls and high temperatures, most nutrients have leached out of these soils. They generally contain high levels of iron oxides, which is why most tropical and subtropical (lateritic) soils are red in color. Yet many tropical soils are able to support rich, dense forests because organic matter is readily available on the surface of the soil as tropical vegetation falls to the ground and decays quickly. When tropical forests are cleared, the hot sun and heavy rains destroy the exposed organics, leaving very hard, dry soil that is poor for cultivation.

Soils in desert regions are usually formed from sandstone and shale parent rocks. Like tropical soils, desert soils contain little organic matter, in this case because the sparse rainfall in arid regions limits plant growth. Desert soils are generally light in color and shallow. Desert subsoils may also contain high levels of salts, which discourage plant growth, and rise to the surface under rains and irrigation, forming a white crust as the water evaporates.

Tundra (a Finnish word meaning barren land) soils, dark mucky soils, cover treeless plains in arctic and subarctic regions. Below the A horizon lie darker subsoils, and below that, in arctic regions, lies perma-frost. While these soils are difficult to farm because of their high water content and because permafrost prevents plant roots from penetrating very deeply, tundras naturally support a dense growth of flowering plants.

Below the great flat plains of the Midwestern United States and the grassy plains of South Africa,

KEY TERMS

Alluvial soils Soils containing sand, silt, and clay, which are brought by flooding onto lands along rivers; these young soils are high in mineral content, and are the most productive soils for agriculture.

Bedrock The unweathered or partially weathered solid rock layer, which is exposed at the Earths surface or covered by a thin mantle of soil or sediment.

Clay The portion of soil comprising the smallest soil particles, those with diameters less than 0.002 mm, which is composed mainly of hydrous aluminum silicates and other minerals.

Horizons Layers of soil that have built up over time and lie parallel to the surface of Earth; these are composed of soils of varying thickness, color, and composition.

Nutrients The portion of the soil necessary to plants for growth, including nitrogen, potassium, and other minerals.

Organic matter The carbonaceous portion of the soil that derives from once living matter, including, for the most part, plants.

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

Percolation The movement of water down through soil layers, through which minerals and nutrients are moved through soil.

Sand Sediment particles smaller than pebbles and larger than silt, ranging in size from 1/16 of a millimeter to 2 millimeters.

Silt Soil particles derived mainly from sedimentary materials that range between 0.00020.05 mm in size.

Soil series Soils that share a defined set of characteristics and share the same name.

Topsoil The uppermost layer of soil, to a depth of approximately 7.1-7.9 in (18-20 cm), which is the primary feeding zone for agricultural plants.

Russia, and Canada lie deep layers of black soil atop a limestone like layer, which has leached out of the soil into the subsoil. These soils are termed chernozem soils, a term that comes from the Russian word for black earth. These soils are highly productive.

The most productive soils for agriculture are alluvial soils, which are found alongside rivers and at their mouths, where floods bring sediments containing sand, silt, and clay up onto the surrounding lands. These are young soils high in mineral content, which act as nutrients to plants.

Life in the soil

Soils teem with life. In fact, more creatures live below the surface of Earth than live above. Among these soil dwellers are bacteria, fungi, and algae, which feed on plant and animal remains breaking them down into humus, the organic component of soil, in the process. The numbers of these microscopic soil organisms is vasta gram of soil, which would fit into a peanut shell, can contain from several hundred million to a few billion microorganisms. The importance of their actions to the health of the soil is equally large.

Bacteria are the most abundant life form in most soils and are responsible for the decay of the residue from crops. Certain bacteria convert ammonia in soils into nitrogen, a fundamental plant nutrient. Some algae perform the same function (assuring a nitrogen supply) in rice paddy soils. Algae are numerous on the surfaces of moist soil. Fungi teem in soils, and range from several celled fungi to the large wild mushrooms that grow on moist soil. Fungi are capable of decomposing a greater variety of organic compounds than bacteria.

Nematodes are also abundant in most soils, and these eel-shaped, colorless worms are slightly larger than bacteria, algae, and fungi. An acre of soil may hold as many as 1 million nematodes. Most nematodes feed on dead plants, but some are parasites, and eat the roots of crops such as citrus, cotton, alfalfa, and corn.

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, as a result, become rich in organic matter. By burrowing and recolonizing, ants can eventually rework and fertilize the soil covering an entire prairie.

Earthworms burrow through soils, mixing organics 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. The burrowing of the 4,000 or so worms that can inhabit an acre of soil turns and aerates soil, bringing 7-18 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, loosening the soil and making it more porous. Mice also burrow, as do ground squirrels, marmots, and prairie dogs; all bringing tons of subsoil material to the surface. These animals all prefer dry areas, so the soils they unearth are often sandy and gravelly.

See also Land use; Soil conservation.

Resources

BOOKS

Hamblin, W.K., and Christiansen, E.H. Earths Dynamic Systems. 9th ed. Upper Saddle River: Prentice Hall, 2001.

Middleton, Gerard V., and Celestina V. Cotti Ferrero. Encyclopedia of Sediments & Sedimentary Rocks. Boston: Kluwer Academic Publishers, 2003.

Spearks, Donald L. Environmental Soil Chemistry. 2nd ed. New York: Academic Press, 2002.

Beth Hanson

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Soil

Soil

Soil is a complex mixture of pulverized rock and decaying organic matter , which covers most of the terrestrial surface of the Earth . Soil not only supports a huge number of organisms below its surface—bacteria, fungi , worms, insects and small mammals , which all play a role in soil formation—but it is essential to all life on Earth. Soil provides a medium in which plants can grow, supporting their roots and providing them with nutrients for growth. Soil filters the sky's precipitation through its many layers, recharging the aquifers and groundwater reserves from which we drink. Slowing the movement of rainfall by absorption, soil prevents damaging floods. By holding air in its pores, soil provides oxygen to plant roots and to the billions of other organisms inhabiting soil. Soil receives and thrives on organic matter as it dies, assuring that it returns to a form useful to subsequent living organisms. Soil has built up over eons on top of bedrock , the solid rock layer that makes up the crust of the earth, as exposed rocks have weathered and eroded and organic matter, including plant and animal life, have decomposed and become part of the soil. The word soil comes from the Latin word for floor, solum.


Soil formation

Soils began to form billions of years ago as rain washed minerals out of the once molten rocks that were cooling on the planet's surface. The rains leached potassium, calcium , and magnesium—minerals essential for plant growth from the rock, creating the conditions in which very simple plants could evolve. Plant life eventually spread and flourished, and as each plant died and decomposed, it added nutrients and energy to the mineral mixture, making the soil more fertile for new plants.

Soil now covers the earth in depths from a few inches to several feet, and these soils are constantly forming and changing. Soils are created from "parent" material, loose earthy matter scattered over the earth by wind , water , or glacial ice , or weathered in place from rocks.

Parent material is turned into soil as other reactions take place on exposed rock surfaces. Water-borne acids react with elements in the rock and slowly change them into soil components. Minerals that break down relatively easily—feldspars and micas—become clay, the smallest soil particles with diameters less than 0.0002 mm, while harder minerals like quartz turn into sand (0.05-2.0 mm) and silt (0.0002-0.05 mm).

As the parent material weathers, the nutrients necessary for plant growth are released, and plants begin to establish themselves. As they die, they leave behind organic residues on which animals, bacteria , and fungi feed. Their consumption breaks down the organic matter further, enriching the parent material for plant growth. Over time , more and more organic matter mixes with the parent material.

Wherever soil is found, its development is controlled by five important factors: climate, parent material, living organisms, topography, and time.

A region's climate determines the range and fluctuation of temperature and the amount of precipitation that falls to the earth, which in turn controls the chemical and physical processes responsible for the weathering of parent materials. Weathering, in turn, controls the rate at which plant nutrients are released. Nutrient flow, along with temperature and precipitation, determines the types of plants a region can support.

A soil's parent material plays an important role in determining the chemistry and texture of soil (the size and shape of soil particles). The rate at which water moves through soil is controlled in part by the texture of the soil. Soils from some parent materials weather more or less quickly than others. Soils derived from quartz minerals, for example, weather more slowly than those derived from silicate materials.

The numbers and kinds of living organisms in a given region help determine the chemical composition of soil. Grassland soils are chemically different from those that develop beneath forests , and even within these broad categories of vegetative cover, soil profiles can differ; for example, different soils develop under conifers than under deciduous trees.

Topography, the configuration of the earth's surface, affects soil development because it determines the rate at which precipitation washes over soil and how soils erode. Smooth, flat lands hold water longer than hilly regions, where water moves more quickly down slopes. Swamps, marshes, and bogs are formed as low-lying areas hold water over time. Soil erodes, or wears away, more quickly on sloping land than flat.

Time plays an important role in soil development: soils are categorized as young, mature, or old, depending on how the above factors are combined, and the rate at which they work.


Soil profiles and horizons

Below the surface of the earth lie layers of soil that are exposed when people dig into the earth, or by natural forces like earthquakes. These cross-sections of soil, called soil profiles, are composed of horizontal layers or horizons of soil of varying thickness and color , each representing a distinct soil that has built up over a long time period. Soil horizons contain soils of different ages and composition, and soil scientists can tell a lot about a region's climate, geography, and even agricultural history by reading the story of the region's soils through these layers.

A soil horizon is a horizontal layer of soil with physical or chemical characteristics that separate it from layers above and below. More simply, each horizon contains chemicals, such as rust-like iron oxides, or soil particles that differ from adjacent layers. Soil scientists generally name these horizons (from top to bottom) O, A, B, C, and R, and often subdivide them to reflect more specific characteristics within each layer. Considered together, these horizons constitute a soil profile.

Horizons usually form in residual soils: soils not transported to their present location by water, wind, or glaciers , but formed "in place" by the weathering of the bedrock beneath them. It takes many thousand to a million years to achieve a mature soil with fully developed horizons.

The O horizon (sometimes known as the A0) consists of freshly dead and decaying organic matter—mostly plants but also small (especially microscopic) animals or the occasional rigid cow. A gardener would call this organic matter (minus the cow) compost or humus . Below the O lies the A horizon, or topsoil, composed of organic material mixed with soil particles of sand, silt, and clay. Frolicking earthworms, small animals, and water mix the soil in the A horizon. Water forced down through the A by gravity carries clay particles and dissolved minerals (such as iron oxides) into the B horizon in a process called leaching ; therefore, the A is known as the Zone of Leaching. These tiny clay particles zigzag downward through the spaces (pores) between larger particles like balls in a Japanese pachinko game. Sometimes the lower half of the A horizon is called the E (Eluvial) horizon, meaning it is depleted of clay and dissolved minerals, leaving coarser grains.

The leached material ends up in the B horizon, the Zone of Accumulation. The B horizon, stained red by iron oxides, tends to be quite clay-like. If the upper horizons erode, plant roots have a tough time penetrating this clay; and rain which falls on the exposed clay can pool on the surface and possibly drown plants or flood basements.

Sometimes the top of the B horizon develops a dense layer called a fragipan—a claypan (compacted by vehicles) or a hardpan (cemented by minerals). In arid climates, intense evaporation sucks water and its dissolved minerals upward. This accumulation creates a hardpan impenetrable to any rain percolating (sinking) downward, resulting in easily evaporated pools or rapid runoff. If the hardpan is composed of the calcium-rich mineral calcite, it is called caliche. If composed of iron oxides, it is called an "ironpan." Fragipans are extremely difficult for crop roots and water to penetrate. The A and B horizons together make up the solum, or true soil.

Partially weathered bedrock composes the C horizon. Variously sized chunks of the rock below are surrounded by smaller bits of rock and clay weathered from those chunks. Some of the original rock is intact, but other parts have been chemically changed into new minerals.

The R layer (D horizon) is the bedrock, or sometimes, the sediment from which the other horizons develop. Originally, this rock lay exposed at the surface where it weathered rapidly into soil. The depth from the surface to the R layer depends on the interrelationships between the climate, the age of the soil, the slope, and the number
of organisms. Most people do not consider the R layer as soil, but include it in the profile anyway, since the weathering of this bedrock usually produces the soil above it.

In a perfect world, all soils demonstrate these horizons, making the lives of soil scientists and soil students blissful. In reality, however, some soils, like transported soils (moved to their present locations by water, wind, or glaciers), lack horizons because of mixing while moving or because of youth. In other soils, the A and B rest on bedrock, or erosion strips an A, or other complicated variations. Around the world, scientists classify soils by these horizonal variations.


Aging soils

Like all living things, soils age. Exposure to wind, rain, sun , and fluctuating temperatures combine to push soils through four stages of development: parent material, immature soil, mature soil, and old-age soil.

As noted above, parent materials are loose materials weathered from rocks. As plants establish themselves in parent material, organics accumulate, and the upper soil layer becomes richer and darker, and evolves into an A horizon. At this point, the soil has only A and C horizons and is in the immature stage, which it usually reaches in less than 100 years.

Through continued weathering and plant growth, the soil gathers more nutrients, and can support more demanding species . Soils break down into smaller particles such as clay, and as water moves down through the matrix, it carries these fine soil particles with it. As they accumulate in the underlying layer, these particles form a B horizon. Soils that have A, B, and C layers are described as mature.

Gradually, as weathering continues, plant growth and water percolation remove nearly all of the mineral nutrients from soil, and acidic by-products begin to develop. When a soil lacks the nutrients or contains enough acids that plant growth is slowed, the soil has reached old age.


Soil categories

Soil scientists have developed a number of systems for identifying and classifying soils. Some broad systems of soil classification are used worldwide, and one of the most widely applied is that developed by the U.S. Department of Agriculture. It includes 11 major soil orders: alfisols, andisols, aridisols, entisols, histosols, inceptisols, mollisols, oxisols, spodisols, ultisols, and vertisols. Each major order is subdivided into suborders, groups, subgroups, families, and series.

Soils are also classified at an extremely specific level: soils are named after a local landmark such as a town, school, church, or stream near where the soil is first identified. There are soils named Amarillo and Fargo, for example, identifying their origins in northwestern Texas and North Dakota, respectively. Soils that share characteristics that fall within defined limits share the same name, and these soils form a soil series. (Local names for soil series are usually used within countries but not across boundaries, even though soils on different continents share the same characteristics.)


Soil groups and agriculture

Plants have adapted to the globe's variety of soils and can grow in almost every soil and under all variations of weather, yet plants grow better in some places than others, especially in places where nutrients are most readily available from the soil.

The tropical belt around the earth's equator contains the globe's "oldest" soils. Under heavy rainfalls and high temperatures, most nutrients have leached out of these soils. They generally contain high levels of iron oxides, which is why most tropical and subtropical (lateritic) soils are red in color. Yet many tropical soils are able to support rich, dense forests because organic matter is readily available on the surface of the soil as tropical vegetation falls to the ground and decays quickly. When tropical forests are cleared, the hot sun and heavy rains destroy the exposed organics, leaving very hard, dry soil that is poor for cultivation.

Soils in desert regions are usually formed from sandstone and shale parent rocks. Like tropical soils, desert soils contain little organic matter, in this case because the sparse rainfall in arid regions limits plant growth. Desert soils are generally light in color and shallow. Desert subsoils may also contain high levels of salts, which discourage plant growth, and rise to the surface under rains and irrigation , forming a white crust as the water evaporates.

Tundra (a Finnish word meaning "barren land") soils, dark mucky soils, cover treeless plains in arctic and subarctic regions. Below the A horizon lie darker subsoils, and below that, in arctic regions, lies permafrost. While these soils are difficult to farm because of their high water content and because permafrost prevents plant roots from penetrating very deeply, tundras naturally support a dense growth of flowering plants.

Below the great flat plains of the Midwestern United States and the grassy plains of South Africa , Russia, and Canada lie deep layers of black soil atop a limestone-like layer, which has leached out of the soil into the subsoil. These soils are termed chernozem soils, a term that comes from the Russian word for "black earth." These soils are highly productive.

The most productive soils for agriculture are alluvial soils, which are found alongside rivers and at their mouths, where floods bring sediments containing sand, silt, and clay up onto the surrounding lands. These are young soils high in mineral content, which act as nutrients to plants.


Life in the soil

Soils teem with life. In fact, more creatures live below the surface of the earth than live above. Among these soil dwellers are bacteria, fungi, and algae , which feed on plant and animal remains breaking them down into humus, the organic component of soil, in the process. The numbers of these microscopic soil organisms is vast—a gram of soil, which would fit into a peanut shell, can contain from several hundred million to a few billion microorganisms . The importance of their actions to the health of the soil is equally large.

Bacteria are the most abundant life form in most soils and are responsible for the decay of the residue from crops . Certain bacteria convert ammonia in soils into nitrogen , a fundamental plant nutrient. Some algae perform the same function (assuring a nitrogen supply) in rice paddy soils. Algae are numerous on the surfaces of moist soil. Fungi teem in soils, and range from several celled fungi to the large wild mushrooms that grow on moist soil. Fungi are capable of decomposing a greater variety of organic compounds than bacteria.

Nematodes are also abundant in most soils, and these eel-shaped, colorless worms are slightly larger than bacteria, algae, and fungi. An acre of soil may hold as many as 1 million nematodes. Most nematodes feed on dead plants, but some are parasites , and eat the roots of crops such as citrus, cotton , alfalfa, and corn.

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, as a result, become rich in organic matter. By burrowing and recolonizing, ants can eventually rework and fertilize the soil covering an entire prairie .

Earthworms burrow through soils, mixing organics 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. The burrowing of the 4,000 or so worms that can inhabit an acre of soil turns and aerates soil, bringing 7-18 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, loosening the soil and making it more porous. Mice also burrow, as do ground squirrels , marmots , and prairie dogs; all bringing tons of subsoil material to the surface. These animals all prefer dry areas, so the soils they unearth are often sandy and gravelly.

See also Land use; Soil conservation.


Resources

books

Adams, John A. Dirt. College Station, TX: Texas A&M; University Press, 1986.

Brady, Nyle C. The Nature and Properties of Soils. New York: Macmillan, 1989.

Foth, Henry D. Fundamentals of Soil Science. New York: John Wiley & Sons, 1990.

Hamblin, W.K., and Christiansen, E.H. Earth's Dynamic Systems. 9th ed. Upper Saddle River: Prentice Hall, 2001.

Harpstead, M.I., F.D. Hole, and W.F. Bennet. Soil Science Simplified. Ames, IA: Iowa State University Press, 1988.

Hillel, Daniel. Out of the Earth. New York: The Free Press, 1991.

Middleton, Gerard V., and Celestina V. Cotti Ferrero. Encyclopedia of Sediments & Sedimentary Rocks. Boston: Kluwer Academic Publishers, 2003.

Spearks, Donald L. Environmental Soil Chemistry. 2nd ed. New York: Academic Press, 2002.


periodicals

Rhonda, R. J. "Spatial Heterogeneity of The Soil Moisture Content and Its Impact." Journal of Hydrometeorology 3, no. 5 (2002): 556-570.


Beth Hanson

KEY TERMS


. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Alluvial soils

—Soils containing sand, silt, and clay, which are brought by flooding onto lands along rivers; these young soils are high in mineral content, and are the most productive soils for agriculture.

Bedrock

—The unweathered or partially weathered solid rock layer, which is exposed at the Earth's surface or covered by a thin mantle of soil or sediment.

Clay

—The portion of soil comprising the smallest soil particles, those with diameters less than 0.002 mm, which is composed mainly of hydrous aluminum silicates and other minerals.

Horizons

—Layers of soil that have built up over time and lie parallel to the surface of the earth; these are composed of soils of varying thickness, color, and composition.

Nutrients

—The portion of the soil necessary to plants for growth, including nitrogen, potassium, and other minerals.

Organic matter

—The carbonaceous portion of the soil that derives from once living matter, including, for the most part, plants.

Parent material

—Loose mineral matter scattered over the Earth by wind, water, or glacial ice, or weathered in place from rocks.

Percolation

—The movement of water down through soil layers, through which minerals and nutrients are moved through soil.

Sand

—Sediment particles smaller than pebbles and larger than silt, ranging in size from 1/16 of a millimeter to 2 millimeters.

Silt

—Soil particles derived mainly from sedimentary materials that range between 0.0002–0.05 mm in size.

Soil series

—Soils that share a defined set of characteristics and share the same name.

Topsoil

—The uppermost layer of soil, to a depth of approximately 7.1-7.9 in (18-20 cm), which is the primary feeding zone for agricultural plants.

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