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Soil Conservation

SOIL CONSERVATION

CONCEPT

With the rise of the environmentalist movement in the 1960s and afterward, it has become common to speak of conserving natural resources such as trees or fossil fuels. Yet long before humans recognized the need to make responsible use of things taken from the ground, they learned to conserve the ground itselfthat is, the soil. This was a hard-won lesson: failure to conserve soil has turned many a fertile farmland into temporary dust bowl or even permanent desert. Techniques such as crop rotation aid in conservation efforts, but communities continue to face hazards associated with the soil. There is, for instance, the matter of leaching, the movement of dissolved substances through the soil, which, on the one hand, can benefit it but, on the other hand, can rob it of valuable nutrients. Issues of soil contamination also raise concerns that affect not just farmers but the population as a whole.

HOW IT WORKS

billions of Years in the Making

Earth's present wealth of soil is the result of hundreds of millions of years' worth of weathering, erosion, and sedimentation. Once, long ago, there was no soil, only rock, and it took eons' worth of weathering to dislodge particles of those rocks. These rocks, when combined with organic materials, became the basis for soil, but before the soil could even begin to take shape, a number of things had to fall into place. Chief among these was the formation of something that, at first glance, at least, does not seem to have a great deal of bearing on the soil: the atmosphere.

In combination with water in the hydrosphere (e.g., streams and rivers) as well as water in the form of evaporated moisture and precipitation in the air itself, the blanket of gases we call our atmosphere has been essential to the formation and sustenance of Earth's soil. This importance goes beyond the obvious point that rain transports water to the soil, thus making possible the abundance of plant life that grows in it. Rain, of course, is of unquestionable importance, but it is only one of several factors associated with the atmosphere (including the water vapor it contains) that have a role in shaping soil as we know it.

To move weathered rocks from highlands to lowlands, where they can become sediment and eventually begin to form soil, it is necessary to subject the rocks themselves to a process of erosion. And erosionaside from erosion caused by gravity, which usually is considered weatheringcan take place only when an atmosphere exists, along with water in the air and on the land. The chief agents of erosion are wind, water (both flowing and in the form of precipitation), and frozen water in the form of icy glaciers, all of which depend on an atmosphere or water or both (see Glaciology).

Erosion transports not only rock sediment but organic material as well. Together, these two ingredients are as essential to making soil as tea bags and water are to making tea. Obviously, the greater the organic content, the richer the soil, and here again the air plays a part. It is important that deeper layers of soil receive a supply of air from the surface to sustain the life of subterranean organisms, who not only process nutrients through the soil but (by their burrowing activities) also aerate it, or make air available to it.

A Product of Its Environment

Soil, like most people, is a product of the environment in which it was formed. That environment has five major influencing factors: the nature of the "parent material," or the rock from which the soil was derived; the local climate; the presence of living organisms; local topography; and the passage of time.

Specific classes of mineral break apart in characteristic ways, and the size of the pieces into which the original weathered rock is broken has a great deal to do with the character of the soil that it forms. This does not mean, however, that relatively large rock pieces necessarily will yield the worst soils, since erosive forces will continue to work on the rock, pulling out its nutrient-rich mineral wealth and gradually acting to break it apart.

As for climate, it is clear that rain and sun are essential for the growth of plant matter, but, of course, too much of either or both is harmful. (See Soil for a discussion of soils in rainforests.) Plants aid the soil by dying and feeding it with more organic material, but they are not the only types of organism in the soil. Indeed, the soil constitutes an ecosystem in and of itself, a realm rich in biodiversity, in which various biogeochemical cycles are played out, and through which energy flows as part of the operation of the larger Earth system.

The underground world teems with creatures ranging from bacteria to moles and prairie dogs (in some regions), each of which fulfills a function. These functions include aerating the soil by burrowing; processing material though ingestion and elimination of waste, thus converting compounds into nutrients that the soil can use; and mixing organic material with minerals. Organisms' final contribution to the soil comes when they die, as their bodies become material that feeds the earth through decomposition.

Topography, or elevation, plays a major role in making possible erosion, itself a process that can be either beneficial or detrimental. The question of whether it is one or the other may be a matter of perspective, or rather elevation. From the standpoint of lowland areas, which receive the wealth of the upland areas in the form of nutrient-rich runoff carried by gravity or flowing media, such as wind or water, erosion is a good thing. Matters do not look as good from the viewpoint of the mountains, which lose much of their best soil to low-lying areas.

The influence of time in shaping soilsas well as much else about the soil itselfcan be appreciated by studying soil horizons, the various strata, or layers, of soil that lie beneath the surface. The most basic division of layers is between the A, B, and C horizons, which differ in depth and physical and chemical characteristics as well as age.

SOIL HORIZONS.

Above the A horizon, or topsoil, lies humus, decomposing organic material that eventually will become soil. The A horizon itself contains a large amount of organic matter, and thus it may be black, or at least much darker than the soil below it. Between the A and B horizons is a sandy, silty later called the E horizon. Then comes the B horizon, or subsoil, which starts at a depth as shallow as 1 ft. (0.3 m) or deeper than 5 ft. (1.5 m).

Lacking a great deal of organic material but still rich in nutrients, the B horizon has a sizable impact on the A horizon. Mineralsboth healthful and harmfulmay rise up from the B to the A horizon, and the ability of the B horizon to hold in moisture from above greatly affects the moisture of the A horizon soil. (See Soil for a discussion of how salt deposits in the B horizon affect topsoil in deserts.) Together, A and B horizons constitute what is called the solum, or true soil.

The C horizon is called regolith. It is the home for the rocks of the parent material, which has given up much of its nutrient riches in fortifying the soil that lies above it. This far below the surface, there is no sign of plant or animal life, and below the C horizon is the R horizon, or bedrockthe top of the layers of rock and metal that descend all the way to the planet's core. Once again depths vary, with bedrock as shallow as 5-10 ft. (1.5-3 m) or as deep as 0.5 mi. (0.8 km) or more.

Differences Between Soils

The depth of the soil is a measure of wealthwealth, that is, in terms of natural resources. A sheath over much of the solid earth, soil separates the planet's surface from its rocky interior and preserves the lives of the plants and animals that live on and in it. It receives rain and other forms of precipitation, which it filters through its layers, as we discuss later, in the context of leaching. Thus, it not only provides water to organisms above and below its surface but also helps prevent flooding by acting as a reservoir.

A great deal of soil's volume is air, for which it also acts as a reservoir. Underground creatures depend on this air and also help circulate it by burrowing. This circulation, in turn, provides oxygen to the roots of plants and makes the soil more hospitable to growth. Even though soil performs these and other life-preserving functions, it would be a mistake to assume that all soils are the same. In fact, the U.S. Department of Agriculture has identified 11 major soil orders, each of which is divided into suborders, groups, subgroups, families, and series.

The specificity of soil types, as reflected in the identification and naming of soil series, illustrates the complexity of what at first seems a very simple thing. In fact, soils can be extremely specific, with names that reflect local landmarks. If soils share enough similarities, they are grouped together in a soil series, but it is safe to say that there are thousands of individual soil types on Earth.

Conserving Soil

On a broad level, there are certain types of environment more or less favorable to the formation of rich soil. Some of these types are discussed in the essay Soil, and specific examples of environmental problems are provided later in this essay. Yet almost any environment can become unfavorable to plant growth if proper soil-conservation procedures are not observed.

The phrase soil conservation refers to the application of principles for maintaining the productivity and health of agricultural land by control of wind-and water-induced soil erosion. For the remainder of this essay, we examine the dangers involved in such erosion and the use of measures to prevent it. In so doing, we give the matter of soil conservation a somewhat larger scope than the preceding definition might suggest. Since soil affects the world far beyond farms, it seems only fitting to approach it not as a concern merely of agriculture but of the environment in general.

Erosion is spoken of here in a general sense, but for a more in-depth discussion of erosive processes, see Erosion. Mass Wasting examines dramatic erosion-related phenomena, such as landslides. Biogeochemical Cycles contains some discussion of erosion, inasmuch as it helps circulate life-sustaining chemical elements throughout the various earth systems. Indeed, it is important to remember that erosion is not always negative in its results; on the contrary, it is a valuable process by which landforms are shaped. The erosive processes we explore here, however, generally contribute to the loss of soil health and productivity.

REAL-LIFE APPLICATIONS

The Dust Bowl

When people mismanage agricultural lands or when natural forces otherwise conspire to destroy soil, the results can be devastating. One of the most dramatic examples occurred in what came to be known as the dust bowl. This was the name given to a wide area covering Texas, Oklahoma, Kansas, and even agricultural parts of Colorado during the years 1934 and 1935. Over the course of a few months, once-productive farmlands turned into worthless fields of stubble and dust, good for almost nothing and highly vulnerable to violent wind erosion.

And wind erosion came, scattering vast quantities of soil from the Great Plains of the Midwest to the Atlantic Seaboard. The classic 1939 film The Wizard of Oz sets its fantastic, otherworldly story against this backdrop, and to viewers in the late 1930s the tornado that swept Dorothy from her Kansas farmland into the world of Oz was all too real. The only difference was that no magical adventure awaited victims of the real-life tornadoes and other windstorms.

The fate of the dust bowl farmers, many of whom lost everything, was dramatized in the novel The Grapes of Wrath by John Steinbeck in 1939 as well as in the acclaimed motion picture that followed a year later. A perhaps equally eloquent tribute appeared in the form of the American photographer Dorothea Lange's photographs of dust bowl refugees. The images etched by Lange are unforgettable: in one a woman stares into the distance, her face a landscape of despair, as her children huddle next to her, their eyes hidden from the camera. In another a man, obviously exhausted from months or years of overwork, hardship, and fear, sits behind the wheel of a truck, gazing somewhere beyond the camera lens. Like the woman, he seems to be looking into a future that offers scant hope.

CAUSES OF THE DUST BOWL.

What happened? The sad fact is that in the years leading up to the early 1930s, the future dust bowl farmlands had seemed remarkably productive, and farmers continued to be pleasantly surprised, year after year, at the abundant yields they could draw from each field. In fact, farmers were unwittingly preparing the way for vast erosion by overcultivating the land and not taking proper steps to preserve its moisture against drought. This was particularly unfortunate because farmers in the 1930s had long known about the principle of crop rotation as a means of giving the soil a rest in order to restore its nutrients. Yet the farmers of the plains tried to push their crops to yield more and more, and for a time it worked, though at great future expense to the land.

One is tempted to see in the agricultural world of the U.S. Midwest parallels to the foolhardy attitude that, just a few years earlier, created a boom on Wall Street, followed by the devastating stock market crash of October 29, 1929, that ushered in the Great Depression. Certainly the ravages of the dust bowl, when they came, were particularly unwelcome in a land already reeling from several years of widespread unemployment and a sagging economy. And though there was no cause-effect relationship between the Wall Street crash and the dust bowl, there is no question that both were brought about in large part by a lack of planning for the future and by a naive belief that it is possible to get "something for nothing"that is, to get more out of the world (whether the world of finances or the natural world) than one puts into it.

In some places farmers alternated between wheat cultivation and livestock grazing on particular plots of land. Thus, the hooves of the cattle damaged the soil, which had been weakened by raising wheat. The land was therefore ready to become the site of a full-fledged natural disaster, and, at the height of the depression, natural disaster came in the form of high winds. The winds in some cases removed topsoil as much as 3-4 in. (7-10 cm) thick. Dunes of dust as tall as 15-20 ft. (4.6-6.1 m) formed, turning acreage that once had rippled with wheat into desertlike waste-lands.

Erosion Control in Action

Today the farmlands of the plains states long since have recovered, and American farmers have benefited from the lessons learned in the dust bowl. Out of the dust bowl years came the establishment, in 1935, of the Soil Conservation Service, a federal agency charged with implementing erosion-control practices. (The Soil Conservation Service was the predecessor of the modernday Natural Resources Conservation Service.) In the wake of the legislation creating the agency, signed into law by President Franklin D. Roosevelt (1882-1945), states passed laws creating nearly 3,000 local soil conservation districts.

If one passes through agricultural lands today, one is likely to see signs identifying the local conservation district. Even more important, the lands themselves are a testament to principles put into practice as an outgrowth of the dust bowl years. For instance, instead of alternating one year of wheat with one year in which a field lies fallow, or unused, farmers in the dust bowl region discovered that a three-year cycle of wheat, sorghum, and fallow land worked much better. They also planted trees to serve as barriers against wind.

EROSION CONTROL LEGISLATION.

Concerns over soil conservation in America did not end with the dust bowl. As United States farm production soared in the 1970s, American farms enjoyed such a great surplus that U.S. farmers increasingly began to sell their crops overseasmost notably, to the Soviet Union. While some Americans were upset to see the farmers of the Midwest selling wheat to the Communists in Moscow, others saw in this act a testament to the failure of the Soviet agricultural system and to the strength of U.S. farming. In the wake of these increased exports, farmers were encouraged to cultivate even marginal croplands to increase profits, thus heightening the vulnerability of their lands to erosion.

What followed was not another dust bowl, however; instead, the experience of the 1970s and 1980s shows just how much American farmers, legislators, and others had learned from the 1930s. Environmental activists in the 1970s, concerned over water quality, helped return public interest to the problem of soil erosion. They called attention to the flow of nutrients from croplands into water resources, most notably leaching of nitrogen and phosphorus that choked lakes with eutrophication (see Biogeochemical Cycles). As a result of public concerns over these and related issues, Congress in 1977 passed the Soil and Water Resources Conservation Act, mandating the conservation of soil, water, and other resources on private farmlands and other properties.

In 1985 the Food Security Act further served to encourage steps toward the reduction of soil erosion. Some 45 million acres (18 million hectares) of land vulnerable to erosion were removed from intensive cultivation by the act. The legislation also forbade the conversion of rangelands into agricultural fields, which would have raised great potential for erosion and depletion of already vulnerable soil. In addition, the act required farmers to develop and maintain practices for the control of erosion on lands susceptible to that threat.

BARRIER AND COVER.

Soil-conservation practices fall under two headings: barrier and cover. Under the barrier approach, various structures act as a wall against water runoff, wind, and the movement of soil. Among such structures are banks, hedgerows, walls of earth or other materials, and silt fences such as one sees at construction sites. The cover approach is devoted to the idea of maintaining a heavy soil cover of living and dead plant material. This is achieved through the use of mulch, cover crops, and other techniques.

Local governments and property owners in non agricultural lands often apply both the cover and barrier approaches, planting trees as well as grass not simply to beautify the land but also to hold the soil in place. Land has to have some sort of vegetative protection to stand between it and the forces of wind and water erosion, and the two approaches together serve to protect soil against nature's onslaught.

Leaching

Like erosion, leachingthe movement of dissolved substances with water percolating through soilcan be both positive and negative. For any plot of land, assuming the rate of water input is greater than the rate of water loss through evaporation, water has to go somewhere, so it leaves the site by moving downward. Eventually it either reaches the deep groundwater or passes through subterranean springs to flow into the surface waters of streams, rivers, and lakes.

Along the way, the leached water carries all sorts of dissolved substances, ranging from nutrients to contaminants. The threat of the latter has led to widespread concern in the United States over the leaching of toxins into water supplies, and in 1980 this concern spurred a massive piece of legislation called CERLA (Comprehensive Environmental Response, Compensation, and Liability Act), better known as Superfund. Six years later, in 1986, Congress updated CERLA with the Superfund Amendments and Reauthorization Act. These laws provided for a vast array of measures directed toward environmental cleanup, including the removal of chemicals and other toxins in soil.

Drastic measures such as those outlined in CERLA and other legislation may be required for the cleanup of artificial materials introduced into soils and groundwater. But for human waste and other more natural forms of toxin, nature itself is able to achieve a certain amount of cleanup on its own. In a septic-tank system, used by people who are not connected to a municipal sewage system, bacteria process wastes, removing a great deal of their toxic content in the tank itself. The waste-water leaves the tank and passes through a filtration system, in which the water leaches through layers of gravel and other filters that help remove more of its harmful content. As the wastewater percolates from the filtration system through the soil (usually well below the A horizon by this point), it is purified further before it enters the groundwater supply.

Not only does leaching help purify the water that passes through the soil, it also is an important part of the soil-formation process, inasmuch as it passes nutrients to the depths of the A horizon and into the B horizon. Its ability to pass along nutrients is not always beneficial, and in some ecosystems, large amounts of dissolved nitrogen are lost to soil as a result of leaching. In such a situation, soil typically is fertilized with nitrate, a form of the element with which soil often has difficulty binding (see Nitrogen Cycle). For this reason, nitrate tends to leach easily, leading to an overabundance of nitrogen in the lower levels of the soil and in the groundwater. This condition, known as nitrogen saturation, can influence the eutrophication of waters (see Biogeochemical Cycles for an explanation of eutrophication) and can cause the decline and death of trees on the surface.

Desertification

Much of North Africa lies under the cover of a vast desert, the Sahara. By far the world's largest desert, the Sahara today spreads across some 3.5 million sq. mi. (9.06 million sq km), an area larger than the continental United States. Only about 780 acres (316 hectares) of it, or little more than 1 sq. mi. (2.6 sq km), is fertile. The rest is mostly stone and dry earth with scattered shrubsand, here and there, the rolling sand dunes typically used to depict the Sahara in movies.

Given the forbidding moonscape of the Sahara today, it might be surprising to learn that just 8,000 years agothe blink of an eye in terms of geologic timeit was a region of flowing rivers and lush valleys. For thousands of years it served as a home to many cultures, some of them quite advanced, to judge from their artwork. Though they left behind an extraordinary record in the form of their rock-art paintings and carvings, which show an understanding of realistic representation that would not be matched until the time of the Greeks, the identity of the early Saharan peoples themselves remains largely a mystery.

Instead of identifying them by the name of a nationality or empire, archaeologists divide the phases of the early Saharan culture according to a set of four names that collectively tell the story of the region's progressive transformation into a desert. First was the Hunter period, from about 6000 to about 4000 b.c., when a Paleolithic, or Old Stone Age, people survived by hunting the many wild animals then available in the region. Next came the Herder period, from about 4000 to 1500 b.c. As their name suggests, these people maintained herds of animals and also practiced basic agriculture.

As the Sahara became drier and drier, however, there were no more herds. Egyptians began bringing in domesticated horses to cross the desert: hence the name of the Horse period (ca. 1500-ca. 600 b.c.) By about 600 b.c., not even horses could survive in the forbidding climate. There was only one creature that could survive: the hardy, seemingly inexhaustible camel. Thus began the Camel era, which continues to the present day.

ATTEMPTS TO CONTROL DESERTIFICATION.

As with the dust bowl, the first question one wants to ask when confronted with a story such as that of the Sahara, is "What happened?" The answer is much more complex, just as the effects of desertificationthe slow transformation of ordinary lands to desertare much more permanent than those of the erosion associated with the dust bowl. Desertification does not always result in what people normally think of as a desert. It is rather a process that contributes toward making a region more dry and arid, and because it is usually gradual, it can be reversed in some cases. But doing so represents a vast challenge.

In 1977 the United Nations (UN), in the form of the UN Conference on Desertification in Nairobi, Kenya, set out to address the spread of the Sahara into the Sahel, an arid region that stretches south of the desert. Some 700 delegates from almost 100 countries adopted a number of measures designed to halt the spread of desertification in that region and others by the year 2000.

Even though there have been some successes, the Sahel region today remains a blighted area where famine is common, and this state of affairs is not entirely the result of the natural causes addressed in the conference's resolutions. Poor government management and a near-constant state of civil war in such countries as Ethiopia have played at least as important a role in spreading famine as nature itself. During the 1980s, in fact, the government of Ethiopia (at that time a Marxist-Leninist state) deliberately withheld food supplies, shipped to it from the West, as a way of exerting pressure on rebel factions and other groups it wished to subdue.

THE EXAMPLE OF IRAQ.

The arid regions of Iraq provide another example of how human influences can result in desertification. Once that country, known in ancient times as Mesopotamia, was among the greenest and most lush places in the known world. For this reason, historians today use the name Fertile Crescent to describe an arc from the deltas of the Tigris and Euphrates rivers in Mesopotamia to the mouth of the Nile in Egypt. Today, of course, Iraq is mostly a dust-colored land of bare trees and brush.

What happened? Agricultural mismanagement certainly played a role, as did the simple exhaustion of the soil by some 6,000 years of human civilization. Indeed, since the Fertile Crescent was perhaps the first area settled by agricultural societies long before the beginning of full-fledged civilization as such in about 3500 b.c., it is safe to say that the region has been under cultivation for several thousand years longerperhaps 8,000 or even 10,000 years. Direct human action and malice also may have played a role: some historians believe that the Mongols, during their brutal invasion in the 1250s, so badly devastated the farmlands and irrigation channels of Iraq that the land never recovered.

SOME CAUSES OF DESERTIFICATION.

With regard to human involvement in the desertification process, it is not necessary for a society to be advanced agriculturally to do long-term damage to the soil. The Pueblan culture of what is now the southwestern United States depleted an already dry and vulnerable region after about a.d. 800 by removing its meager stands of mesquite trees. And though human causes, in the form of either mismanagement or deliberate damage, have contributed toward desertification, sometimes nature itself is the driving force.

Long-term changes in rainfall or general climate as well as water erosion and wind erosion such as caused the dust bowl can turn a region into a permanent desert. An ecosystem may survive short-term drought, but if soil is forced to go too long without proper moisture, it sets in motion a chain reaction in which plant life dwindles and, with it, animal life as well. Thus, the soil is denied the fresh organic material necessary to its continued sustenance, and a slow, steady process of decline begins.

WHERE TO LEARN MORE

Bear, Firman E., H. Wayne Pritchard, and Wallace E. Akin. Earth: The Stuff of Life. Norman: University of Oklahoma Press, 1986.

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

Bright Edges of the World: The Earth's Evolving Drylands (Web site). <http://www.nasm.edu/ceps/drylands/drylands.html>.

Cherrington, Mark. Degradation of the Land. New York: Chelsea House, 1991.

"Desertification." United States Geological Survey (Web site). <http://pubs.usgs.gov/gip/deserts/desertification/>.

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

Natural Resources Conservation Service (Web site). <http://www.nrcs.usda.gov/>.

Pittman, Nancy P. From the Land. Washington, DC: Island Press, 1988.

Soil and Water Conservation Society (Web site). <http://www.swcs.org/>.

"Voices from the Dust Bowl: The Charles L. Todd and Robert Sonkin Migrant Worker Collection, 1940-1941." Library of Congress (Web site). <http://memory.loc.gov/ammem/afctshtml/tshome.html>.

KEY TERMS

A HORIZON:

Topsoil, the upper mostof the three major soil horizons.

AERATE:

To make air available to soil.

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.

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 earth system, 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;however, 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.

EUTROPHICATION:

A state of heightened biological productivity in a body of water, which is typically detrimental to the ecosystem in which it takes place. Eutrophication can be caused by an excess of nitrogen or phosphorus in the form of nitrates and phosphates, respectively.

HUMUS:

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

LANDFORM:

A notable topographicalfeature, such as a mountain, plateau, or valley.

LEACHING:

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

MASS WASTING:

The transfer of earth material down slopes by processes that include creep, slump, slide, flow, and fall. Also known as mass movement.

MINERAL:

A naturally occurring, typically inorganic substance with a specific chemical composition and a crystalline structure.

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 fragments 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.

SEDIMENTATION:

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

SOIL CONSERVATION:

The application of principles for maintaining the productivity and health of agricultural land by control of wind-and water-induced soil erosion. The term also may be applied more broadly to encompass the maintenance and protection of non agricultural soils.

SOIL HORIZONS:

Layers of soil, parallel to the surface of the earth, which have built up over time. These layers 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.

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 conservation

soil conservation The protection of the soil by careful management, to prevent physical loss by erosion and to avoid chemical deterioration (i.e. to maintain soil fertility).

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soil conservation

soil conservation The protection of the soil by careful management to prevent physical loss by erosion and to avoid chemical deterioration (i.e. to maintain soil fertility).

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soil conservation

soil conservation The protection of the soil by careful management to prevent physical loss by erosion and to avoid chemical deterioration (i.e. to maintain soil fertility).

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Soil Conservation

Soil conservation

Soil conservation is the protection of soil against excessive loss of fertility by natural, chemical, or artificial means. It encompasses all management and land-use methods protecting soil against degradation, focusing on damage by erosion and chemicals . Soil conservation techniques can be divided into six categories, crop selection and rotation, fertilizer and lime application, tilth , residue management, contouring and strip cropping, and mechanical (e.g., terracing ).

While the potential dangers of chemical degradation and soil erosion were recognized as early as the American Revolution, it was not until the early 1930s that soil conservation became a familiar term. The soil conservation movement was a result of the droughts during the 1930s, the effects of water erosion, the terrific dust storms created by wind erosion in the Great Plains, and by the urging of Hugh Hammond Bennett.

Dr. Bennett, a soil scientist from North Carolina, recognized the erosion damage to previously arable land in the Southeast, Midwest, and elsewhere in the 1920s and 1930s. In 1929, he published a bulletin entitled "Soil Erosion, A National Menace" and started a successful personal campaign to get federal support, beginning with a $160,000 appropriation by Congress to initiate a national study. In 1933, the U.S. Department of the Interior named Bennett as head of the Soil Erosion Service, which conducted soil erosion control demonstrations nationwide. In 1935, the Soil Conservation Service , led by Bennett, was established as a permanent agency of the U.S. Department of Agriculture .

Soil degradation problems addressed include soil compaction , salinity build-up, and excessive soil acidity. Because soil and water are so intimately related, the program also deals with water conservation and water quality . Under M. L. Wilson, then Assistant Secretary of Agriculture, the Soil Conservation Service established conservation districts guided by elected officials assisted by Soil Conservation Service personnel. Currently there are more than 3,000 districts in the United States.

Sharing the cost of conservation became federal policy with the passage of the Soil Conservation and Domestic Allotment Act of 1936. The act funded the shift of croplands to "soil-building" crops and established soil conservation practices on croplands and grasslands . The Great Plains Conservation Program, enacted by Congress in 1956, sought to shift some of the highly erodible land from cropland to grassland. The Water Bank and Experimental Rural Clean Waters Programs were an attempt to resolve disputes over drainage of "potholes" in the Midwest and Great Plains and demonstrate the influence of soil and water conservation practices on water quality.

As early as the 1930s, the federal government began to purchase "submarginal" lands outright. The Conservation Reserve segment of the Soil Bank (1956-1960) bought substandard farmland to conserve soil and alleviate surplus crop production. The current Conservation Reserve Program , authorized in a 1985 farm bill, paid farmers to convert land from cropland to grassland or trees under a long-term lease. Currently the "sodbuster," "swampbuster," and conservation compliance programs attempt to force farmers to comply with soil and water conservation programs to be eligible for other government programs, such as price supports.

The role of the Soil Conservation Service has changed over the years, but its central mission is still to provide technical information for good land use . Today the Soil Conservation Service is highly concerned with environmental problems, water quality, wetland preservation, and prime farm land protection, as well as urban concerns related to their mission. The soil conservation movement has spawned a number of professional societies, including the Soil and Water Conservation Society and the World Association of Soil and Water Conservation.

See also Conservation tillage; Contour plowing; Dust Bowl; Environmental degradation; S oil organic matter; Strip-farming; Sustainable agriculture

[William E. Larson ]


RESOURCES

BOOKS

National Academy of Sciences. Soil Conservation. 2 vols. Washington, DC: National Academy Press, 1986.

Wilson, G. F., et al. The Soul of the Soil: A Guide to Ecological Soil Management. 3rd ed. Agaccess, 1996.

Yudelman, M., et al. New Vegetative Approaches to Soil and Water Conservation. Washington, DC: World Wildlife Fund, 1990.

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Soil Conservation

Soil Conservation

History

How soil erodes

Soil conservation methods

Barrier approaches

Cover approaches

Resources

Soil conservation refers to the control of soil erosion in order to maintain agricultural productivity. Soil conservation practices are based on an analysis of land use needs and the suitability of land for different uses.

Erosion is any process by which soil is transported from one place to another. At naturally occurring rates, land typically loses about one inch (2.5 cm) of topsoil in 100 to 250 years. A tolerable rate of soil erosion is considered to be 48 to 80 lb of soil per acre (55-91 kg per hectare) each year. Natural weathering processes that produce soil from rock can replace soil at about this rate. Cultivation, construction, and other human activities have greatly increased the rate of soil erosion in most regions. Some areas of North America are losing as much as 18 tons of soil per acre (40 tonnes per hectare) per year.

Soil erosion not only results in the loss of soil particles, but also organic matter and nutrients. The first 7 to 8 in (18-20 cm) of soil is the surface layer (topsoil) that provides most of the nutrients needed by plants. Because most erosion occurs from 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 (this is known as sedimentation).

History

Human activities have caused increases of soil erosion since the beginning of agriculture more than 5,000 years ago. Plentiful land and a scarcity of labor in some countries encouraged farmers to intensively farm a piece of land, abandon it, and then move on to more fertile ground. This practice is still common in some developing countries, in the form of shifting cultivation or so-called slash and burn agriculture. In slash and burn agriculture, farmers burn part of a forest and plant crops among the ashes. After several years, the farmer moves to another area of forest and repeats the process. Although shifting cultivation is commonly considered to be a major cause of soil erosion, if sufficient time is allowed between clearings, soil fertility can maintain itself over the longer term.

Practices to protect the land from erosion have existed for several thousand years, particularly in the tropics and subtropics. For example, Chinese artifacts dating from about 4,500 years ago (2500 BC) depict terraces used to control erosion on cultivated slopes. Similarly, terraces have been used to grow rice in the Philippines for more than 1,000 years.

In the United States, abusive agricultural practices in combination with drought caused the great dust storms of 1934 and 1935, which carried large amounts of soil from the Great Plains to the Atlantic Ocean. Soil conservation became a practice of national importance as a result of those storms. President Franklin Roosevelt signed bills in 1935 that established the Soil Conservation Service, an agency responsible for implementing practices to control soil erosion. Individual states also passed laws establishing nearly 3,000 local soil conservation districts.

For the next several decades, U.S. farmers produced consistent surpluses of agricultural commodities. They had little incentive to push the land for higher yields. However, in the 1970s grain exports increased, especially to the Soviet Union. Farmers were encouraged to cultivate marginal lands to fill the export quotas. Those areas, amounting to almost two million acres (800,000 hectares), included land on slopes and wetter areas that are relatively vulnerable to erosion.

The concern of the environmental movement about water quality in the 1970s helped to return attention to the problem of soil erosion. Excessive amounts of phosphorus and nitrogen occurred in streams and lakes as a result of agricultural fertilization practices, and this added to public criticism of soil conservation programs. Congress passed the Soil and Water Resource Conservation Act to evaluate and conserve soil, water, and related resources on non-federal land. Erosion can also contribute to increased turbidity levels in streams, which inhibit the development of such endangered or threatened fish as salmon.

The 1985 Food Security Act encouraged land management practices that were intended to reduce soil erosion. The Act removed up to 45 million acres (18 million hectares) of erosion-prone land from intensive cultivation. It also prevented the conversion of rangelands into cultivated fields through its sodbuster provision. The Act withdrew some commodity (feed grain, wheat, rice, upland cotton, etc.) acreage from production, through multiyear acreage set-asides and conservation easements. It also required farmers to develop plans and apply management practices that would keep soil erosion on highly erodible lands within acceptable limits.

How soil erodes

Soil erosion is caused primarily by water and wind. There are several different types of water-caused erosion: sheet, rill, gully, and stream channel. In sheet erosion, the flow of water over the surface of the soil detaches and transports particles in thin layers. Concentrated flows of water form small channels or grooves (rills), and eventually develop larger gullies that carry away large amounts of soil. Sometimes, underground tunnels are formed by erosion of the subsoil. Eventually, the tunnel roof falls in to form deeper gullies. Stream channels erode when soil is removed from the fringing banks, or from within the channel of the stream itself.

Soil erosion is influenced by several variables, especially climate, soil type, density and types of plants and animals, and topography. Climatic factors include precipitation, evaporation, temperature, wind, humidity, and solar radiation. Frequent and extreme changes in these conditions, such as freezes and thaws and severe rainstorms, often increase the rate of erosion.

Soil conditions that affect erosion include detachability and transportability. Detachability is the tendency of soil particles to separate from each other. Detachability increases as the size of soil particles increases. Transportability is the ease with which soil is carried from one location to another. Transportability increases as the size of soil particles decreases.

Vegetation helps to reduce erosion by intercepting rainfall, decreasing the surface velocity of runoff, physically restraining soil movement, improving the porosity of the soil so that percolation is rapid, and by decreasing the amount of runoff, by evaporating water to the atmosphere through planttranspiration.

Soil topography features that influence soil erosion include the degree, shape, and length of the slope, and the size and shape of the watershed. Erosion increases rapidly with increasing steepness and length of slope.

Soil conservation methods

In addition to erosion control, comprehensive soil conservation includes the maintenance of organic matter and nutrients in soil. Soil conservation practices also prevent the buildup of toxic substances in the soil, such as salts and excessive amounts of pesticides. Soil conservation maintains or improves soil fertility, as well as its tilth, or structure. These all increase the capacity of the land to support the growth of plants on a sustainable basis.

There are two basic approaches to soil erosion control: barrier and cover. The barrier approach uses banks or walls such as earthen structures, grass strips, or hedgerows to check runoff, wind velocity, and soil movement. Barrier techniques are commonly used all over the world.

The cover approach maintains a soil cover of living and dead plant material. This cover lessens the impact and runoff of rain water, and decreases the amount of soil carried with it. This may be done through the use of cover crops, mulch, minimum tillstage, or agroforestry.

Barrier approaches

Terracing is the construction of earthen embankments that look like long stair-steps running across the slope of rolling land. A terrace consists of a channel with a ridge at its outer edge. The channel intercepts and diverts downhill runoff. Terraces help to prevent soil erosion by increasing the length of the slope, thereby reducing the speed of overland water flow to allow for greater infiltration. The channels redirect excess runoff to a controlled outlet. Terraces help prevent the formation of gullies and retain runoff water to allow sediment to settle.

Extensive systems of irrigated terraces have long been used in numerous countries, including Yemen, the central Andes, the southwestern United States, Ethiopia, Zimbabwe, and northern Cameroon. Soil terraces occur widely in Southeast and South Asia, New Guinea, East Africa, and Nigeria.

The construction of reservoirs, usually ponds, is another barrier method for intercepting the surface runoff of water and sediment. Reservoirs increase soil moisture, thereby improving the resistance of soil to erosion. Water stored in reservoirs is also available for use in irrigation.

Contouring is plowing, planting, cultivating, or harvesting across the slope of the land, instead of up and down the hillside. Contouring reduces the velocity of surface runoff by impounding water in small depressions.

Cover approaches

Strip or alley cropping grows alternate strips of different crops in the same field. For example, rows of annual cultivated crops such as corn or potatoes, which have the most potential to cause erosion because of frequent plowing, are rotated with small grains such as oats that allow less erosion, and also with dense perennial grasses and legumes such as lespedeza and clover, which provide the best erosion control because the soil is not disturbed very often.

A combination of contouring and strip cropping provides relatively efficient erosion control and water conservation. Both contour and strip crops can be planted with shrubs and trees, known as windbreaks or shelterbelts, that form perennial, physical barriers to control wind erosion. In addition, shrubs and trees produce litter that increases soil cover, while helping to accumulate soil upslope to eventually develop terraces, and stabilizing the soil with their root systems.

Protective cover cropping and conservation tillage are systems of reduced or no-tillage that leave crop debris covering at least 30% of the soil surface. Crop residues on the surface decompose more slowly than those that are plowed into the soil, and they release nitrogen more uniformly and allow plants to use it more efficiently. Crop residues also reduce wind velocity at the surface, trap eroding soil, and slow down surface and subsurface runoff of water. Residues also attract earthworms to the surface, whose burrows act as drains for the percolation of runoff water during heavy rains. Crop residues also provide insulation that lowers spring and summer soil temperatures, and increases soil moisture by reducing evaporation. In areas that are more productive under irrigation, conservation tillage reduces water requirements by one-third to one-half, compared with conventionally tilled areas.

KEY TERMS

Contouring Plowing along a slope, rather than up and down it, to create furrows that catch soil and water runoff.

Fertility The capacity of the soil to support plant productivity.

Minimum tillage A farming method in which one or more planting operations is eliminated so as to reduce the exposure of the soil to erosion by wind and water.

Strip cropping A farming method in which alternating bands of soil are planted in crops that are prone to soil erosion and others that prevent it.

Terracing The creation of steplike basins on hilly ground in order to irrigate crops grown there.

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

Degrees of conservation tillage range from no-till, in which the soil is not plowed and seeds are planted by a drilling technique, to varying degrees of tillage. However, during tillage the soil is not completely turned, as it would be if a moldboard plow was used. Weeds and pest insects are controlled using herbicides and insecticides, respectively. Conservation tillage eliminates the need to let fields lie fallow (unplanted) for a year to rest. Fallow acreage is somewhat prone to soil erosion and to becoming dominated by intruding vegetation.

Another cover approach can provide temporary erosion control, such as that needed at construction sites. When certain chemical substances known as polymers are added to the soil, they form aggregates with the soil particles. These additives have no toxic effect, but stabilize the soil to provide temporary erosion control until a longer-lived plant cover can be established.

See also Contour plowing; Slash-and-burn agriculture.

Resources

BOOKS

Bloom, A.L. Geomorphology: A Systematic Analysis of Late Cenozoic Landforms. 3rd ed. Long Grove, Illinois: Waveland Press, 2004.

Morgan, R.C.P. Soil Erosion and Conservation. Malden, Mass.: Blackwell, 2005.

Karen Marshall

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Soil Conservation

Soil conservation

Soil conservation refers to maintaining the productivity of agricultural land by control of the erosion of soil by wind or water . Soil conservation practices use the land according to its needs and capabilities.

Erosion is any process by which soil is transported from one place to another. At naturally occurring rates, land typically loses about one inch (2.5 cm) of topsoil in 100-250 years. A tolerable rate of soil erosion is considered to be 48-80 lb of soil per acre (55-91 kg per hectare) each year. Natural weathering processes that produce soil from rock can replace soil at about this rate. However, cultivation, construction, and other human activities have greatly increased the rate of soil erosion in most regions. Some areas of North America are losing as much as 18 tons of soil per acre (40 tonnes per hectare) per year.

Soil erosion not only results in the loss of soil particles, but also organic matter and nutrients . The first 7-8 in (18-20 cm) of soil is the surface layer (topsoil) that provides most of the nutrients needed by plants. Because most erosion occurs from 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 (this is known as sedimentation).


History

Human activities have caused increases of soil erosion since the beginning of agriculture more than 5,000 years ago. Plentiful land and a scarcity of labor in some countries encouraged farmers to "wear out" a piece of land, abandon it, and then move on to more fertile ground. This practice is still common in some developing countries, in the form of shifting cultivation or "slash and burn." This involves farmers cutting down an area of forest, burning the downed vegetation, and planting their crops among the ashes. After several years, the farmer moves to another area of forest and repeats the process. Although shifting cultivation is commonly considered to be a major cause of soil erosion, if sufficient time is allowed between clearings, soil fertility can maintain itself over the longer term.

Practices to protect the land from erosion have existed for several thousand years, particularly in the tropics and subtropics. For example, Chinese artifacts dating from about 4,500 years ago (2500 b.c.) depict terraces used to control erosion on cultivated slopes. Similarly, terraces have been used to grow rice in the Philippines for more than 1,000 years.

In the United States, abusive agricultural practices in combination with drought caused the great dust-storms of 1934 and 1935, which carried huge quantities of soil from the Great Plains to the Atlantic Ocean. Soil conservation became a practice of national importance as a result of those storms. President Franklin Roosevelt signed bills in 1935 that established the Soil Conservation Service, an agency responsible for implementing practices to control soil erosion. Individual states also passed laws establishing nearly 3,000 local soil conservation districts.

For the next several decades, U.S. farmers produced consistent surpluses of agricultural commodities. They had little incentive to push the land for higher yields. However, in the 1970s grain exports increased, especially to the Soviet Union. Farmers were encouraged to cultivate marginal lands to fill the export quotas. Those areas, amounting to almost two million acres (800,000 hectares), included land on slopes and wetter areas that are relatively vulnerable to erosion.

The concern of the environmental movement about water quality in the 1970s helped to return attention to the problem of soil erosion. Excessive amounts of phosphorus and nitrogen occurred in streams and lakes as result of agricultural fertilization practices, and this added to public criticism of soil conservation programs. Congress passed the Soil and Water Resource Conservation Act to evaluate and conserve soil, water, and related resources on non-federal land.

The 1985 Food Security Act encouraged land management practices that were intended to reduce soil erosion. The Act removed up to 45 million acres (18 million hectares) of highly erosion-prone land from intensive cultivation. It also prevented the conversion of rangelands into cultivated fields through its "sodbuster" provision. The Act withdrew some commodity (feed grain, wheat , rice, upland cotton , etc.) acreage from production, through multiyear acreage set-asides and conservation easements. It also required farmers to develop plans and apply management practices that would keep soil erosion on highly erodible lands within acceptable limits.

How soil erodes

Soil erosion is caused mainly by the actions of water and wind. There are several different types of water-caused erosion: sheet, rill, gully, and stream channel. In sheet erosion, the flow of water over the surface of the soil detaches and transports particles in thin layers. Concentrated flows of water form small channels or grooves (rills), and eventually develop larger gullies that carry away large amounts of soil. Sometimes, underground tunnels are formed by erosion of the subsoil. Eventually, the tunnel roof falls in to form deeper gullies. Stream channels erode when soil is removed from the fringing banks, or from within the channel of the stream itself.

Soil erosion is influenced by several variables, especially climate, soil type, density and types of plants and animals, and topography. Climatic factors include precipitation , evaporation , temperature , wind, humidity , and solar radiation . Frequent and extreme changes in these conditions, such as freezes and thaws and severe rainstorms, often increase the rate of erosion.

Soil conditions that affect erosion include detachability and transportability. Detachability is the tendency of soil particles to separate from each other. Detachability increases as the size of soil particles increases. Transportability is the ease with which soil is carried from one location to another. Transportability increases as the size of soil particles decreases.

Vegetation helps to reduce erosion by intercepting rainfall, decreasing the surface velocity of runoff, physically restraining soil movement, improving the porosity of the soil so that percolation is rapid, and by decreasing the amount of runoff, by evaporating water to the atmosphere through plant transpiration .

Soil topography features that influence soil erosion include the degree, shape, and length of the slope, and the size and shape of the watershed . Erosion increases rapidly with increasing steepness and length of slope.


Soil conservation methods

Comprehensive soil conservation is more than just the control of erosion. It also includes the maintenance of organic matter and nutrients in soil. Soil conservation practices also prevent the buildup of toxic substances in the soil, such as salts and excessive amounts of pesticides. Soil conservation maintains or improves soil fertility, as well as its tilth, or structure. These all increase the capacity of the land to support the growth of plants on a sustainable basis.

There are two basic approaches to soil erosion control: barrier and cover. The barrier approach uses banks or walls such as earthen structures, grass strips, or hedgerows to check runoff, wind velocity, and soil movement. Barrier techniques are commonly used all over the world.

The cover approach maintains a soil cover of living and dead plant material. This cover lessens the impact and runoff of rain water, and decreases the amount of soil carried with it. This may be done through the use of cover crops, mulch, minimum tillage, or agroforestry.


Barrier approaches

Terracing is the construction of earthen embankments that look like long stair-steps running across the slope of rolling land. A terrace consists of a channel with a ridge at its outer edge. The channel intercepts and diverts downhill runoff. Terraces help to prevent soil erosion by increasing the length of the slope, thereby reducing the speed of overland water flow to allow for greater infiltration. The channels redirect excess runoff to a controlled outlet. Terraces help prevent the formation of gullies and retain runoff water to allow sediment to settle.

Extensive systems of irrigated terraces have long been used in numerous countries, including Yemen, the central Andes, the southwestern United States, Ethiopia, Zimbabwe, and northern Cameroon. Soil terraces occur widely in Southeast and South Asia , New Guinea, East Africa , and Nigeria.

The construction of reservoirs, usually ponds, is another barrier method for intercepting the surface runoff of water and sediment. Reservoirs increase soil moisture, thereby improving the resistance of soil to erosion. Water stored in reservoirs is also available for use in irrigation .

Contouring is plowing, planting, cultivating, or harvesting across the slope of the land, instead of up and down the hillside. Contouring reduces the velocity of surface runoff by impounding water in small depressions.


Cover approaches

Strip or alley cropping grows alternate strips of different crops in the same field. For example, rows of annual cultivated crops such as corn or potatoes, which have the most potential to cause erosion because of frequent plowing, are rotated with small grains such as oats that allow less erosion, and also with dense perennial grasses and legumes such as lespedeza and clover, which provide the best erosion control because the soil is not disturbed very often.

A combination of contouring and strip cropping provides relatively efficient erosion control and water conservation . Both contour and strip crops can be planted with shrubs and trees, known as windbreaks or shelter-belts, that form perennial, physical barriers to control wind erosion. In addition, shrubs and trees produce litter that increases soil cover, while helping to accumulate soil upslope to eventually develop terraces, and stabilizing the soil with their root systems.

Protective cover cropping and conservation tillage are systems of reduced or no-tillage that leave crop debris covering at least 30% of the soil surface. Crop residues on the surface decompose more slowly than those that are plowed into the soil, and they release nitrogen more uniformly and allow plants to use it more efficiently. Crop residues also reduce wind velocity at the surface, trap eroding soil, and slow down surface and subsurface runoff of water. Residues also attract earthworms to the surface, whose burrows act as drains for the percolation of runoff water during heavy rains. Crop residues also provide insulation that lowers spring and summer soil temperatures, and increases soil moisture by reducing evaporation. In areas that are more productive under irrigation, conservation tillage reduces water requirements by one-third to one-half, compared with conventionally tilled areas.

Degrees of conservation tillage range from no-till, in which the soil is not plowed and seeds are planted by a drilling technique, to varying degrees of tillage. However, during tillage the soil is not completely turned, as it would be if a moldboard plow was used. Weeds and pest insects are controlled using herbicides and insecticides , respectively. Conservation tillage eliminates the need to let fields lie fallow (unplanted) for a year to "rest." Fallow acreage is somewhat prone to soil erosion and to becoming dominated by intruding vegetation.

Another cover approach can provide temporary erosion control, such as that needed at construction sites. When certain chemical substances known as polymers are added to the soil, they form aggregates with the soil particles. These additives have no toxic effect, but stabilize the soil to provide temporary erosion control until a longer-lived plant cover can be established.

See also Contour plowing; Slash-and-burn agriculture.


Resources

books

Hallsworth, E. G. Anatomy, Physiology and Psychology of Erosion. New York: John Wiley & Sons, 1987.

Lake, Edwin B. and Aly M. Shady. "Erosion Reaches Crisis Proportions." Agricultural Engineering. (November 1993): 8-13.

Michaelson, E.L., J. Carlson, and R.L. Papendick. Conservation Farming in the United States. CRC Press, 1998.

Schwab, Glenn O., et al. Soil and Water Conservation Engineering. 4th ed. New York: John Wiley & Sons, 1993.

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

Troeh, Frederick R., J. Arthur Hobbs, and Roy L. Donahue. Soil and Water Conservation. 2nd ed. Englewood Cliffs, NJ: Prentice-Hall, 1991.

Young, Anthony. Agroforestry for Soil Conservation. Wallingford, UK: C.A.B. International, 1989.


Karen Marshall

KEY TERMS

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Contouring

—Plowing along a slope, rather than up and down it, to create furrows that catch soil and water runoff.

Fertility

—The capacity of the soil to support plant productivity.

Minimum tillage

—A farming method in which one or more planting operations is eliminated so as to reduce the exposure of the soil to erosion by wind and water.

Strip cropping

—A farming method in which alternating bands of soil are planted in crops that are prone to soil erosion and others that prevent it.

Terracing

—The creation of steplike basins on hilly ground in order to irrigate crops grown there.

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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