Dictionary:

soil¹

  (soil)

1. The top layer of the earth's surface, consisting of rock and mineral particles mixed with organic matter.
2. A particular kind of earth or ground: sandy soil.
3. Country; land: native soil.
4. The agricultural life: a man of the soil.
5. A place or condition favorable to growth; a breeding ground.

[Middle English, from Anglo-Norman, a piece of ground (influenced in meaning by Latin solum, soil), from Latin solium, seat.]

1. To make dirty, particularly on the surface.
2. To disgrace; tarnish: a reputation soiled by scandal.
3. To corrupt; defile.
4. To dirty with excrement.

To become dirty, stained, or tarnished.

1. 1. The state of being soiled.
   2. A stain.
2. Filth, sewage, or refuse.
3. Manure, especially human excrement, used as fertilizer.

[Middle English soilen, from Old French souiller, from Vulgar Latin *suculāre (from Late Latin suculus, diminutive of Latin sūs, pig) or from souil, pigsty, wallow (from Latin solium, seat; see soil¹).]

1. To feed (livestock) with soilage.
2. To purge (livestock) by feeding with green food.

[Origin unknown.]

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 life—that 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 conditions—Earth itself, an atmosphere, waters, and life-forms—went 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 material—and with it nutrients and energy—to 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 eons—the 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 animals—including 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 ecosystem—for 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 harder—for example, quartz—will 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 landscape—the 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 dirt—loose 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 untouched—that is, that has not been plowed up recently—can 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 humus—unincorporated, often partially decomposed plant residue—is 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 feet—that is, the topmost layer, the A horizon—is 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 soil—most 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 supply—for example, the varieties of cactusthat grow in the American Southwest—are 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 black—always 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 water—the 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>.

Finely divided rock-derived material containing an admixture of organic matter and capable of supporting vegetation. Soils are independent natural bodies, each with a unique morphology resulting from a particular combination of climate, living plants and animals, parent rock materials, relief, the groundwaters, and age. Soils support plants, occupy large portions of the Earth's surface, and have shape, area, breadth, width, and depth. Soil, as used here, differs in meaning from the term as used by engineers, where the meaning is unconsolidated rock material. See also Pedology.

Origin and classification

Soil covers most of the land surface as a continuum. Each soil grades into the rock material below and into other soils at its margins, where changes occur in relief, groundwater, vegetation, kinds of rock, or other factors which influence the development of soils. Soils have horizons, or layers, more or less parallel to the surface and differing from those above and below in one or more properties, such as color, texture, structure, consistency, porosity, and reaction (see illustration). The succession of horizons is called the soil profile.

Photograph of a soil profile showing horizons. The dark crescent-shaped spots at the soil surface are the result of plowing. The dark horizon is the principal horizon of accumulation of organic matter that has been washed down from the surface. The thin wavy lines were formed in the same manner. 1 in. = 2.5 cm.

Soil formation proceeds in stages, but these stages may grade indistinctly from one into another. The first stage is the accumulation of unconsolidated rock fragments, the parent material. Parent material may be accumulated by deposition of rock fragments moved by glaciers, wind, gravity, or water, or it may accumulate more or less in place from physical and chemical weathering of hard rocks. The second stage is the formation of horizons. This stage may follow or go on simultaneously with the accumulation of parent material. Soil horizons are a result of dominance of one or more processes over others, producing a layer which differs from the layers above and below. See also Weathering processes.

Systems of soil classification are influenced by concepts prevalent at the time a system is developed. The earliest classifications were based on relative suitability for different crops, such as rice soils, wheat soils, and vineyard soils. Over the years, many systems of classification have been attempted but none has been found markedly superior. Two bases for classification have been tried. One basis has been the presumed genesis of the soil; climate and native vegetation were given major emphasis. The other basis has been the observable or measurable properties of the soil.

The Soil Survey staff of the U.S. Department of Agriculture and the land-grant colleges adopted the current classification scheme in 1965. This system differs from earlier systems in that it may be applied to either cultivated or virgin soils. Previous systems have been based on virgin profiles, and cultivated soils were classified on the presumed characteristics or genesis of the virgin soils. The new system has six categories, based on both physical and chemical properties. These categories are the order, suborder, great group, subgroup, family, and series, in decreasing rank. The orders and the general nature of the included soils are given in the table. The suborder narrows the ranges in soil moisture and temperature regimes, kinds of horizons, and composition, according to which of these is most important. The taxa (classes) in the great group category group soils that have the same kinds of horizons in the same sequence and have similar moisture and temperature regimes. The great groups are divided into subgroups that show the central properties of the great group, intergrade subgroups that show properties of more than one great group, and other subgroups for soils with atypical properties that are not characteristic of any great group. The families are defined largely on the basis of physical and mineralogic properties of importance to plant growth. The soil series is a group of soils having horizons similar in differentiating characteristics and arrangement in the soil profile, except for texture of the surface portion, and developed in a particular type of parent material.

Surveys

Soil surveys include those researches necessary (1) to determine the important characteristics of soils, (2) to classify them into defined series and other units, (3) to establish and map the boundaries between kinds of soil, and (4) to correlate and predict adaptability of soils to various crops, grasses, and trees; behavior and productivity of soils under different management systems; and yields of adapted crops on soils under defined sets of management practices. Although the primary purpose of soil surveys has been to aid in agricultural interpretations, many other purposes have become important, ranging from suburban planning, rural zoning, and highway location, to tax assessment and location of pipelines and radio transmitters. This has happened because the soil properties important to the growth of plants are also important to its engineering uses.

Two kinds of soil maps are made. The common map is a detailed soil map, on which soil boundaries are plotted from direct observations throughout the surveyed area. Reconnaissance soil maps are made by plotting soil boundaries from observations made at intervals. The maps show soil and other differences that are of significance for present or foreseeable uses.

Physical properties

Physical properties of soil have critical importance to growth of plants and to the stability of cultural structures such as roads and buildings. Such properties commonly are considered to be: size and size distribution of primary particles and of secondary particles, or aggregates, and the consequent size, distribution, quantity, and continuity of pores; the relative stability of the soil matrix against disruptive forces, both natural and cultural; color and textural properties, which affect absorption and radiation of energy; and the conductivity of the soil for water, gases, and heat. These usually would be considered as fixed properties of the soil matrix, but actually some are not fixed because of influence of water content. The additional property, water content—and its inverse, gas content—ordinarily is transient and is not thought of as a property in the same way as the others. However, water is an important constituent, despite its transient nature, and the degree to which it occupies the pore space generally dominates the dynamic properties of soil. Additionally, the properties listed above suggest a macroscopic homogeneity for soil which it may not necessarily have. In a broad sense, a soil may consist of layers or horizons of roughly homogeneous soil materials of various types that impart dynamic properties which are highly dependent upon the nature of the layering. Thus, a discussion of dynamic soil properties must include a description of the intrinsic properties of small increments as well as properties it imparts to the system.

From a physical point of view it is primarily the dynamic properties of soil which affect plant growth and the strength of soil beneath roads and buildings. While these depend upon the chemical and mineralogical properties of particles, particle coatings, and other factors discussed above, water content usually is the dominant factor. Water content depends upon flow and retention properties, so that the relationship between water content and retentive forces associated with the matrix becomes a key physical property of a soil. See also Erosion; Ground-water hydrology; Soil mechanics.

verb

1. To make dirty: befoul, begrime, besmirch, besoil, black, blacken, defile, dirty, smudge, smutch, sully. See clean/dirty.
2. To contaminate the reputation of: befoul, besmear, besmirch, bespatter, blacken, cloud, denigrate, dirty, smear, smudge, smut, spatter, stain, sully, taint, tarnish. Idioms: give a black eye to, slingthrowmud on. See attack/defend, clean/dirty.
3. To make morally impure: contaminate, corrupt, defile, infect, pollute, taint. See clean/dirty.

Definition: dirty
Antonyms: clean, keep clean

v

Definition: make dirty
Antonyms: clean

The naturally occurring, unconsolidated, upper layer of the ground consisting of weathered rock which supplies mineral particles, together with humus; the most common medium for plant growth. The five major factors affecting the formation of a soil are: climate, relief, parent material, vegetation, and time.

For more information on soil, visit Britannica.com.

1. Sediments or other unconsolidated accumulations of solid particles produced by the physical and chemical disintegration of rocks; may or may not contain organic matter.
2. Same as sewage.

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).

Material on the surface of the Earth on which plants can grow. (See topsoil.)

The earth, origin of all plant growth and the basis of all animal agriculture. Its characteristics of chemical composition, physical structure, especially porosity and water retaining capacity, its humus content, pH and salinity exert enormous effects on its productivity.

• s. analysis — an essential activity in densely farmed farms. Measures the soil content of total and available amounts of each of the important soil minerals.
• s. contaminated herbage — either from dust storms or in loose soil by hoof movement may contribute to dental attrition in sheep, or sand colic in horses.
• s. eating — a form of pica; caused by salt deficiency.
• s. fumigants — are used to prepare fields for planting and may cause poisoning in animals grazing them or eating crops harvested from them. See methyl bromide.
• s. type — includes clay, sand, loam.

(see Regolith) The upper layers of sediment on Earth that support plant growth.

The thin layer of weathered rock particles and organic matter, containing water and tiny air spaces, that covers the earth and provides support and nutrients for plants.