Uses of Soda Ash A fundamental industrial chemical, soda ash is used in the manufacture of glass, ceramics, glazes, enam- els, chemicals, soaps and detergents, and paints. It serves as a flux in assaying and as a cleansing agent. It is also used in processing wood pulp to make paper, refining aluminum, desulfurizing pig iron, purifying petroleum, and softening water. Karen N. Kähler Further Reading Dyni, John R., and Richard W. Jones. Proceedings of the First International SodaAsh Conference: Utilizationof Nat- ural Resources ofSodium Carbonate into the Next Century. Laramie: Wyoming State Geological Survey, 1998. Garrett, Donald E. Natural Soda Ash: Occurrences, Pro- cessing, and Use. New York: Van Nostrand Reinhold, 1992. Jensen, Mead L., and Alan M. Bateman. Economic Min- eral Deposits. 3d ed. New York: Wiley, 1979. Kogel, Jessica Elzea, et al., eds. “Soda Ash.” In Indus - trial Minerals and Rocks: Commodities, Markets, and Uses. 7th ed. Littleton, Colo.: Society for Mining, Metallurgy, and Exploration, 2006. Royal Society of Chemistry. Sodium Carbonate: A Versa- tile Material. London: Author, 2000. Web Site U.S. Geological Survey Soda Ash: Statistics and Information http://minerals.usgs.gov/minerals/pubs/ commodity/soda_ash See also: Carbonate minerals; Evaporites; Glass; Pa- per; Sedimentary processes, rocks, and mineral de- posits. Soil Category: Ecological resources Soil is a product of the physical and chemical break- down of the Earth’s surface into small fragments, in- cluding sand, silt, and clay. Soil is also the product of organic matter decomposition—the composting of dead plant and animal debris. Soils are classified on the basis of soil profile and soil formation. They can be grouped according to a number of characteristics, in- cluding agronomic use, color, organic matter content, texture, and moisture condition. Background Typical soil is about 45 percent minerals and about 5 percent organic matter. The other 50 percent of soil consists of pores that hold either water or air. The liq- uid portionof soil contains dissolved minerals and or- ganic compounds, produced by plants and microor- ganisms. The gases found in soil often are the same as those found in the air above it. Soil can support plant life if climate and moisture are suitable. It is a changing and dynamic body, adjusting to conditions of climate, topography, and vegetation. In turn, soil influences plant and root growth, available moisture, and the nu- trients available to plants. While “the soil” is a collec- tive termfor all soils, “a soil” means one individual soil body with a particular length, depth, and breadth. Soil Profile In a typical soil, the top layer is usually dark with de - composing organic matter; the layers below are sand, 1098 • Soil Global Resources Glass 49% Chemicals 30% Soap & detergents 8% Other 13% Source: Mineral Commodity Summaries, 2009 Note: Data from the U.S. Geological Survey, .U.S.GovernmentPrinting Office, 2009. “Other” includes flue gas desulfurization, water treatment, pulp and paper, distributors, and miscellaneous uses. U.S. End Uses of Soda Ash silt, clay, or some combination of the three. Soil scien - tists classify soils on the basis of soil profile and soil for- mation. Typically the top soil layer is called the O horizon, or organic matter horizon. It has rotten logs, leaf lit- ter, and other recognizable bits of plants and animals. Underneath the O horizon is the A horizon. It is char- acterized by thoroughly decomposed organic matter. Water passing through the A horizon carries clay par- ticles and organic acids through it into the B horizon. Clay or organic substances passing into the B horizon glue soil particles together, forming soil aggregates. Soil aggregates—granular, columnar, and so on—are indicators of a mature, healthy soil. The lowest level of the soil profile is the C horizon. It containsbedrock or soil parent material that shows little or no evidence of plant growth or soil formation. Soil Formation Soil formation takes hundreds, even thousands, of years. Parent material, climate, organisms, topogra- phy, and time all contribute. Sources of parent mate- rial include igneous, sedimentary, and metamorphic rocks (fragments of which may be deposited by water, wind, and ice), and plant and animal deposits. Soil formation is the result of the physical, chemi- cal, and biochemical breakdown of parent material. It also reflects the processes of weathering and change within the soil mass. Many substances are added to soil—rain, water from irrigation,nitrogenfrom bacte- ria, sediment, salts, organic residues, and a variety of substances created by humans. However, many sub- stances are also removed from the soil—water-soluble minerals, clay, plants, carbon dioxide, and nitrogen. Other transformations also are occurring: Organic matter is decomposing, and minerals are solubilizing and changing chemical form. Clays and soluble salts that move along with the soil water cause color and chemical changes in the soil. Parent material is a primary determinant of soil type or soil classification. All soils at the lowest cate- gory of soil classification are distinct if the parent ma- terial differs. The differences in parent materials— weathering rates, the plant nutrient content, and soil texture resulting from parent material breakdown— contribute to the formationof distinctive soils. For ex- ample, sandstone yields sandy soil with low fertility. Effects of Climate Soils slowly change color and density as a result of wet- ting and drying, warming and cooling, and freezing and thawing. During weathering—the rubbing, grinding, and moving of rocks by water, wind, and gravity—rocks are split into smaller and smaller frag- ments. Soil is composed of fragments 2 millimeters or less in diameter. The expansion force of water as it freezes is suffi- cient to split minerals. However, water also is involved in chemical weathering—solution, hydrolysis, car- bonation, reduction, oxidation, and hydration. A sim- ple example of solution, the dissolving of minerals in liquid, is the dissolving of salt in water. The salts then move along with the liquid. In hot arid climates, salts can move to the surface as water evaporates, creating Global Resources Soil • 1099 O horizon A horizon B horizon C horizon Bedrock Soil Horizons O = organic material (loose leaves and other plant debris); A = top - soil (primarily mineral in composition but containing humus and high biological activity); B = subsoil (containing deposits such as clay, iron oxide, and carbonate calcium which have percolated down via water from A horizon); C = bedrock. salt flats. In wetter climates, salts can move through the soil, depleting it of necessary plant nutrients and contaminating groundwater. Hydrolysis is the splitting of a water molecule to form hydroxides and soluble hydroxide compounds, such as sodium hydroxide. Hydration is the addition of water to minerals in rock. When a mineral such as hematite (an oxide of iron) hydrates, it expands, soft- ens, and changes color. Carbonation is the reaction of a compound with carbonic acid, a weak acid pro- duced when carbon dioxide dissolves in water. Water often contains carbonic acid and other organic acids produced by organic matter decomposition. These acids increase the power of the water to disintegrate rock.Oxidation is the additionof oxygen to amineral, and reduction is the removal of oxygen from a min- eral. Climate also influences soil formation indirectly through its action on vegetation. Soils in arid climates have sparse vegetation, less organic matter, and little soil profile development. Wet soil, however, usually has thick vegetation and high organic matter, and therefore a deep soil profile. Biological Weathering Biological weathering is a combination of physical and chemical disintegration of rocks to produce soil. The roots of plants can crack rocks and break them apart. Plant roots also produce carbon dioxide, which combines with water to produce carbonic acid. Car- bonic acid dissolves certain minerals, speeding the breakdown of parent material and chemically chang- ing the soil. Plants and animals also add humus (organic mat- ter) to soil, increasing its fertility and water-holding capacity and speeding rock weathering. Animals such as earthworms, ants, prairie dogs, gophers, and moles also contribute to soil aeration and fertility by mixing the soil. In areas where animal populations are large, they can influence both the formation and destruc- tion of soil. Topography The shape of the land is referred to as its topography. Each landform—valleys, plains, hills, and mountains— is covered with a crazy quilt of different soil types. For example, the steep sides of the Sandia Mountains near Albuquerque, New Mexico, which are severely eroded by wind and summer rains, contain a varietyof soil types—forest soils, sandy soils, and rocky soils. Sand, silt, and clay eroded from the mountains and nearby extinct volcanoes combine in the moist and fertile Rio Grande Valley. The valley has deep sandy soils, layered sand and clay soils, and soils eroded by flash floods. Soils located in similar climates that develop from similar parent material on steep hillsides usually have thin A and B horizons because less water moves through the soil. Similar materials on shallower slopes allow more water to pass through them. Topography and climate work together either to allow or to pro- hibit plant growth and organic matter deposition. Without moisture, plants cannot grow to impede soil erosion, and soil development is slow. With moisture, plants can grow, hold the soil in place, add organic matter to the soil, and speed soil development. The age of a soil may be reckoned in tens, hun- dreds, or thousands of years. Under ideal conditions, a soil profile may develop in two hundred years; how- ever, under less favorable conditions soil develop- ment may take several thousand years. Soil Classification Scientists have identified and classified soils for hun- dreds of years. Soils can be grouped according to ag- ronomic use, color, organic matter content, texture, moisture condition, and other characteristics. Each of these groupings serves a particular purpose. U.S. soil scientists adopted a system of soilclassification on Jan- uary 1, 1965, that was based on the knowledge they had about soil genesis, morphology, and classifica- tion. The U.S. system is divided into six categories: or- der, suborder, great group, subgroup, family, and se- ries. (Soil taxonomy is patterned after the worldwide system of plant and animal taxonomy, which contains phylum, class, order, family, genus, and species.) Changes to the system have proceeded through a number of major revisions or approximations. The system can be used to classify soils anywhere in the world. Soil classification is based on similar physical, chemical, and mineralogical properties. The mini- mum volume of soil that scientists consider when they classify a soil is the pedon, which can range from 1 to 10 meters square and is as deep as roots extend into a soil. The U.S. soil classification system recognizestwelve soil orders. The differences among orders reflect the dominant soil-forming processes and the degree of soil formation.Each order is identified by a word end - ing in “-sol.” Each order is divided into suborders, pri - 1100 • Soil Global Resources marily on the basis of properties that influence soil genesis, are important to plant growth, and reflect the most important variables within the orders. The last syllable in the name of a suborder indicates the order. An example is “aquent,” meaning water, plus “-ent,” from “entisol.” Suborders are distinctive to each order and are not interchangeable between orders. Each suborder is divided into great groups on the basis of additional soil properties and horizons resulting from differences in soil moisture and soil Global Resources Soil • 1101 The Twelve Soil Orders in the U.S. Classification System Soil Order Features Alfisols Soils in humid and subhumid climates with precipitation from 500 to 1,300 millimeters (20 to 50 inches), frequently under forest vegetation. Clay accumulation in the B horizon and available water most of the growing season. Slightly to moderately acid soils. Andisols Soils with greater than 60 percent volcanic ash, cinders, pumice, and basalt. They have a dark A horizon as well as high absorption and immobilization of phosphorus and very high cation exchange capacity. Aridisols Soils that exist in dry climates. Some have horizons of lime or gypsum accumulations, salty layers, and A and slight B horizon development. Entisols Soils with no profile development except a shallow A horizon. Many recent river floodplains, volcanic ash deposits, severely eroded areas, and sand are entisols. Gelisols Soils that commonly have a dark organic surface layer and mineral layers underlain by permafrost, which forms a barrier to downward movement of soil solution. Common in tundra regions of Alaska. Alternate thawing and freezing of ice layers results in special features in the soil; slow decomposition of the organic matter due to cold temperatures results in a peat layer at the surface in many gelisols. Histosols Organic soils of variable depths of accumulated plant remains in bogs, marshes, and swamps. Inceptisols Soils found in humid climates that have weak to moderate horizon development. Horizon development may have been delayed because of cold climate or waterlogging. Mollisols Mostly grassland soils, but with some broadleaf forest-covered soils with relatively deep, dark A horizons, a possible B horizon, and lime accumulation. Oxisols Excessively weathered soils. Oxisols are over 3 meters (10 feet) deep, have low fertility, have dominantly iron and aluminum oxide clays, and are acid. Oxisols are found in tropical and subtropical climates. Spodosols Sandy leached soils of the cool coniferous forests, usually with an organic or O horizon and a strongly acidic profile. The distinguishing feature of spodosols is a B horizon with accumulated organic matter plus iron and aluminum oxides. Ultisols Strongly acid and severely weathered soils of tropical and subtropical climates. They have clay accumulation in the B horizon. Vertisols Soils with a high clay content that swell when wet and crack when dry. Vertisols exist in temperate and tropical climates with distinct dry and wet seasons. Usually vertisols have only a deep self-mixing A horizon. When the topsoil is dry, it falls into the cracks, mixing the soil to the depth of the cracks. temperature. Great groups are denoted by a prefix that indicates a property of the soil. An example is “psammaquents” (“psamm” referring to sandy tex- ture and “aquent” being the suborder of the entisols that has an aquic moisture regime). Soil scientists have identified about 190 great groups in the United States. Great groups are distinguished on the basis of differing horizons and soil features. The differing soil horizons include those with accumulated clay, iron, or organic matter and those hardened or cemented by soil cultivation or other human activities. The differ- entiating soil features include self-mixing of soil be- cause of clay content, soil temperature, and differ- ences in content of calcium, magnesium, sodium, potassium, gypsum, and other salts. Each great group is divided into three subgroups: a typic subgroup, an intergrade subgroup, and an extragrade subgroup. The typic subgroup represents the central spectrum of a soil group. The intergrade subgroup represents soils with properties like those of other orders, suborders, or great groups. The ex- tragrade subgroup represents soils with some prop- erties that are not representative of the great group but do not indicate transitions to any other known kind of soil. Each subgroup is identified by one or more adjectives preceding the name of the great group. Families are established within a subgroup on the basis of physical and chemical properties and other characteristics that are important to plant growth or that are related to the behavior of soils that are impor- tant for engineering concerns. Among the properties and characteristics considered are particle size, min- eral content, temperature regime, depth of the root zone, moisture, slope, and permanent cracks. A fam- ily name consists of the name of a subgroup preceded by terms that indicate soil properties. About forty-five hundred families have been identified in the United States. Soil Texture, Structure, and Consistency Soil texture is determined by the percent of sand, silt, and clay in a soil sample. Most fertile or productive soils have a loam texture, or about equal amounts of sand, silt, and clay, and a high organic matter con- tent (about 5 to 10 percent). Soil texture determines the water-holding and nutrient-holding capacity of a soil. Thus, clay soils have a high nutrient-holding ca - pacity, but they waterlog easily. Sandy soils have a lower nutrient-holding capacity but dry out easily. Farmers base their plans of how to fertilize and irri - gate their crops partly on the texture of the soil. Soil structure refers to how soil particles are glued together to form aggregates. During soil formation, soil particlesare glued together with clay, dead micro- organisms, earthworm slime, and plant roots, and they form air and water channels. Plants need these channels so they can absorb nutrients, water, and air. Soil structure may be destroyed when farmers culti- vate wet or waterlogged soils with heavy farm machin- ery; destroying soil structure makes a soil unsuitable for plant growth. Soil consistency is the “feel” of a soil and the ease with which a lump can be crushed in one’s fingers. Common soil consistencies are loose, friable, firm, plastic, sticky, hard, and soft. Clay soils, for example, are sticky or plastic when they are wet, but they be- come hard or harsh when they are dry. The best time to work a clay soil is when it is soft or friable. Sandy soils, on the other hand, do not become plastic or sticky when they are wet or hard or harsh when they are dry. They have a tendency to stay loose, which makes them easier to work. Loam and silt loam soils are intermediate in behavior. When farmers are try- ing to determine whether to work the soil or wait for better soil moisture conditions, they usually check the soil consistency. Soil Aeration and Soil Moisture Soil aeration relates to the exchange of soil air with at- mospheric air. Growing roots need oxygen and are constantly expiring carbon dioxide. Unless there is a continuous flow of oxygen into soil and carbon diox- ide out of the soil, oxygen becomes depleted. When their oxygen supply is cut off, the roots will die. Soil moisture refers to water held in soil pores. A plant draws water from soil the same way a child draws water from a cup with a straw. When the cup is full, it is easy for the child to draw up the water, but as the cup empties, the child must work harder to get water. Sim- ilarly, plants draw water from soil easily when the soil has plenty of water. As the soil dries, however, plants must work harder to pull water out of the soil until they reach a wilting point. Soil Fertility Plants absorb many of the nutrients they need from soil, including phosphorus, potassium, calcium, mag - nesium, sulfur, boron, chlorine, cobalt, copper, iron, manganese, molybdenum, and zinc. They may obtain 1102 • Soil Global Resources carbon, hydrogen, and nitrogen from the air and water. Soil testing services give farmers specific fertilizer and lime recommendations based on soil texture and chemical analysis. Farmers use soil tests to determine if their soil has enough essential nutrients for a crop to grow. The absence of one essential nutrient can limit overall crop growth. Nitrogen, phosphorus, and potassium are commonly applied to the soil as com- mercial fertilizer and manure. Calcium and magne- sium are applied as lime, which is also used to reduce the acidity of soil and to increasethe solubility of some minerals. Manure and other organic matter added to soils increase water-holding and nutrient-holding ca- pacity and therefore boost crop yields. Agricultural extension services offer guidelines for the maximum amounts of manure, sewage sludge, fertilizer, and other chemicals that farmers should ap- ply to soils. Farmers are encouraged to apply nitrogen fertilizer in small applications at times when plants are growing rapidly. This soil management practice decreases deep percolation losses that could pollute groundwater. With an understanding of soil characteristics, farm- ers and gardeners can learn to manage a wide variety of soils. Some soils are naturally fertile and need few amendments to promote high crop yields. Other soils, whether because of their parent material or climate, are naturally infertile and might best be used for pur- poses other than agriculture. Like the water and the air, the soil is a crucial natural resource. From an air- plane, all soils look about the same, but from an ant’s view, soils are all different. Differences in soil type make huge differences to plants, animals, humans, and the environment. Judith J. Bradshaw-Rouse Further Reading Ashman, M. R., and G. Puri. Essential Soil Science: A Clear and Concise Introduction to Soil Science. Malden, Mass.: Blackwell, 2002. Brady, Nyle C., and Ray R. Weil. The Nature and Prop- erties of Soils. 14th ed. Upper Saddle River, N.J.: Prentice Hall, 2008. Davies, Bryan, David Eagle, and Bryan Finney. Re- source Management: Soil. Brighton, Ont.: Diamond Farm Book, 2001. Donahue, Roy L., Roy Hunter Follett, and Rodney W. Tulloch. Our Soils and Their Management: Increasing Production Through Environmental Soil and Water Con - servation and Fertility Management. 6th ed. Danville, Ill.: Interstate, 1990. Esch, Neal S., et al. Soil Science Simplified. 5th ed. Illus- trated by Mary C. Bratz. Ames, Iowa: Blackwell, 2008. Miller, Raymond W., and Roy L. Donahue. Soils: An In- troduction to Soils and Plant Growth. 6th ed. Engle- wood Cliffs, N.J.: Prentice-Hall, 1990. Paul, Eldor A., ed. Soil Microbiology, Ecology, and Bio- chemistry. 3d ed. Boston: Academic Press, 2007. Plaster, Edward J. Soil Science and Management. 5th ed. Clifton Park, N.Y.:Delmar Cengage Learning,2009. Sposito, Garrison. The Chemistry of Soils. 2d ed. New York: Oxford University Press, 2008. White, Robert E. Principles and Practice of Soil Science: The Soil as a Natural Resource. 4th ed. Malden, Mass.: Blackwell, 2006. See also: Agriculture industry; Erosion and erosion control; Farmland; Fertilizers; Groundwater; Leach- ing; Lime; Nitrogen cycle; Soil degradation; Soil test- ing and analysis. Soil degradation Category: Environment, conservation, and resource management Soil degradation is a decline in soil quality, productiv- ity, and usefulness because of natural causes, human activities, or both. Degradation may be caused by unfa- vorable alterations in one or all of a soil’s physical, chemical, and biological attributes. Background In 1992, for the first global study of soil degradation, the World Resources Institute in Washington, D.C., reported that 1.2 billion hectares of land worldwide had been seriously degraded from World War II to the present. It also stated that 9 million hectares of once usable land could no longer support crops. Natural Processes and Human Activities Of the total lands lost to soil degradation, almost two- thirds is in Asia and Africa; most of the loss is at- tributable to water and wind erosion resulting from agricultural activities, overgrazing,deforestation, and firewood collection. There are also seriously degraded Global Resources Soil degradation • 1103 soils in Central America, where degradation is caused primarily by deforestation and overgrazing. In Eu- rope, industrial and urban wastes, pesticides, and other substances have poisoned soils in much of Po- land, Germany, Hungary, and southern Sweden. In the United States, the U.S. Department of Agriculture estimates that a quarterof the nation’s croplands have been depleted through deep plowing, removal of crop residue, conversion to permanent pasture, and other conventional agricultural practices. Although unwise management practices contribute significantly to soil degradation, soil degradation also involves three natural soil processes: physical, chemical, and biological degradation. Physical Soil Degradation Physical soil degradation involves deterioration in soil structure, leading to compaction, crusting, acceler- ated erosion, reduced water-holding capacity, and de- creased aeration. Soil compaction is the compression of soil particles into a smaller volume. Excessively compacted soil suffers from poor aeration and re - duced gas exchange, which can restrict the depth of root penetration. Soil compaction also causesacceler- ated runoff and erosion of soils. Crustingis the formationof a hard layer afew milli- meters or a few tensof millimeters thickat the soil sur- face. Crusts affect drainage, leading to waterlogging at the soil surface and to salinity or alkalinity prob- lems. Once crusts called “duricrusts” form, soil mois- ture recharge declines, and vegetation cannot root. Sheet and gully erosion increases as the land fails to absorb precipitation. Hard layers can also form below the cultivation depth and are called hardpans (other names are plow soles, traffic pans, and plow pans). These compacted layers can restrict root growth, mak- ing cropsand trees vulnerable to drought and lodging (falling over). Chemical Degradation Chemical degradation comprises changes in soil’s chemical properties that regulate nutrient availability. 1104 • Soil degradation Global Resources Soil degradation in areas such as this one in Niger has lead to widespreadfood shortagesindevelopingcountries. (AP/Wide WorldPhotos) Nutrient depletion is the major fac - tor in chemical soil degradation. Soil nutrient depletion may be caused or exacerbated by many factors, includ- ing monocropping, leaching of nu- trients, and salt buildup. A historic example of nutrient de- pletion is the depletion of soilsin the southeastern United States by the growing of cotton. As late as 1950, “King Cotton” was the most valuable farm commodity produced in Ala- bama, Arkansas, Georgia, Louisiana, Mississippi, South Carolina, Tennes- see, and Texas. In the eighteenth and nineteenth centuries, the grow- ing of cotton ruined soil fertility as it spread westward from the Atlantic to the Texas panhandle.Cotton growth without regard to topography in hilly regions contributed to soil erosion. Topsoil was eventually removed from many fields, which further depleted nutrients. One reason that peanuts became a major crop in the South is that they were nitrogen-fixing plants that could grow in soils depleted of nitrogen by cotton. Nutrient leaching is another problem. Continuous irrigation can leach nutrients and cause salt buildup in soils where drainage is poor. Leaching can move es- sential but soluble nutrients past the root zone deeper into the soil and into groundwater. In addition, the water used to irrigate soil often contains salts that can accumulate to toxic levels and inhibit plant growth where evaporation occurs readily. Thick crusts of salt on farmland in Pakistan, Australia, Ethiopia, Sudan, and Egypt have made soil unfit for crops. Laterization refers to the product and process of wetting and drying that leads to the irreversible con- solidation and hardening of aluminum- and iron-rich clays (plinthitic materials) into hardpans, sometimes of great thickness. In Greek plinthos means “brick.” Laterization is particularly common in the humid and subhumid tropics. Biological Degradation The loss of organic matter and soil nutrients needed by plants can occur in any environment, but it is most dramatic in hot, dry regions. Organicmatter is impor - tant in maintaining soil structure, supporting micro- organisms, and retaining plant nutrients. Because or- ganic matter is near the soil surface, it is generally the first soil component to be lost. Organic matter may be lost through brushfires, stubble-burning, overgraz- ing, or the removal of crops, fodder, wood, and dung. Loss of organic matter can be accelerated when soil moisture is reduced, when soil aeration is increased, or both. For example, peat soils that are drained de- compose rapidly and subside. In drier climates, the loss of organic matter reduces the soil’s moisture- holding capacity and lowers soil fertility, which leads to lower crop yields and thus to less organic matter be- ing returned to the soil. Tropical forests such as those of the Amazon basin in South America seem lush, so people widely assume tropical soils to be fertile and high in organic matter. Although tropicalforests do produce considerable or- ganic matter, the amount that stays in the soil is sur- prisingly small, and the soils actually have low nutrient levels. Soil microorganisms in the rain forest break down the organic matter and release nutrients that Global Resources Soil degradation • 1105 Permafrost 6% Soil too shallow 22% Soil too wet 10% Soil too dry 28% Chemical problems 23% Soil that can be farmed without being irrigated, drained, or otherwise improved 11% Soil Limits to Agriculture, by Percentage of Total World Land Area Source: United Nations Food and Agriculture Organization (FAOSTAT Database, 2000). are absorbed by growing plants. However, warm tem - peratures and high rainfall cause accelerated nutrient loss if plants are absent. Nutrients that would buffer the pH of the soil are lost. Consequently, the clearing of rain forests exposes the soil to erosion, leaching, acidification, and rapid nutrient depletion. Judith J. Bradshaw-Rouse Further Reading Ashman, M. R., and G. Puri. Essential Soil Science: A Clear and Concise Introduction to Soil Science. Malden, Mass.: Blackwell, 2002. Brady, Nyle C., and Ray R. Weil. The Nature and Prop- erties of Soils. 14th ed. Upper Saddle River, N.J.: Prentice Hall, 2008. Braimoh, Ademola K., and Paul L. G. Vlek, eds. Land Use and Soil Resources. London: Springer, 2008. Davies, Bryan, David Eagle, and Bryan Finney. Re- source Management: Soil. Brighton, Ont.: Diamond Farm Book, 2001. Donahue, Roy L., Roy Hunter Follett, and Rodney W. Tulloch. Our Soils and Their Management: Increasing Production Through Environmental Soil and Water Con- servation and Fertility Management. 6th ed. Danville, Ill.: Interstate, 1990. Esch, Neal S., et al. Soil Science Simplified. 5th ed. Illus- trated by Mary C. Bratz. Ames, Iowa: Blackwell, 2008. Gerrard, John. “Soil Degradation.” In Fundamentals of Soils. New York: Routledge, 2000. Hannam, Ian, and Ben Boer. Legal and Institutional Frameworks for Sustainable Soils: A Preliminary Report. Gland, Switzerland: International Union for Con- servation of Nature, 2002. Lal, Rattan, et al. Soil Degradation in the United States: Ex- tent, Severity, and Trends. Boca Raton, Fla.: Lewis, 2004. _______, eds. Methods for Assessment of Soil Degradation. Boca Raton, Fla.: CRC Press, 1998. Miller, Raymond W., and Roy L. Donahue. Soils: An In- troduction to Soils and Plant Growth. 6th ed. Engle- wood Cliffs, N.J.: Prentice-Hall, 1990. Paul, Eldor A., ed. Soil Microbiology, Ecology, and Bio- chemistry. 3d ed. Boston: Academic Press, 2007. Plaster, Edward J. Soil Science and Management. 5th ed. Clifton Park, N.Y.:Delmar Cengage Learning,2009. Raman, Saroja. “Soil Degradation and Its Impact on Agriculture.” In Agricultural Sustainability: Princi - ples, Processes, and Prospects. New York: Food Prod - ucts Press, 2006. Singer, Michael J., and Donald N. Munns. “SoilDegra - dation.” In Soils: An Introduction. 5th ed. Upper Sad- dle River, N.J.: Prentice Hall, 2002. Sposito, Garrison. The Chemistry of Soils. 2d ed. New York: Oxford University Press, 2008. White, Robert E. Principles and Practice of Soil Science: The Soil as a Natural Resource. 4th ed. Malden, Mass.: Blackwell, 2006. Zimmerer, Karl S. “Environmental Discourses on Soil Degradation in Bolivia: Sustainability and the Search for Socioenvironmental ‘Middle Ground.’” In Liberation Ecologies: Environment, Development, So- cial Movements, edited by Richard Peet and Michael Watts. 2d ed. New York: Routledge, 2004. Web Site University of Michigan, Global Change Program Land Degradation http://www.globalchange.umich.edu/ globalchange2/current/lectures/land_deg/ land_deg.html#soil%20loss%20processes See also: Dust Bowl; Erosion and erosion control; Farmland; Rain forests; Slash-and-burn agriculture; Soil; Soil management. Soil management Category: Environment, conservation, and resource management Soil management refers to the collection of tillage, con- servation, and cropping practices that are used to pre- serve soil resources while optimizing soil use. Definition Soils are managed differently depending on their in- tended use. Soil management groups are soil types with similar adaptations or management requirements for specific purposes, such as use with crops or crop- ping rotations, drainage, fertilization, forestry, high- way engineering, and construction. For agriculture, soil management includes all tillage and planting operations, cropping practices, fertilization, liming, irrigation, herbicide and insecticide application, and other treatments conducted on or applied to the soil surface for the production of plants. 1106 • Soil management Global Resources Overview The most basic aspect of soil management is the way in which it is cultivated or tilled for crop growth. Till- age is the mechanical manipulation of the soil profile to modify soil conditions, manage crop residues or weeds, or incorporate chemicals for crop production. Tillage can be exhaustive or minimal. Conventional tillage uses multiple tillage operations to bury exist- ing crop residue and prepare a uniform, weed-free seedbed for planting. This method breaks up soil ag- gregates in the process and destroys soil structure. Consequently, it can result in excessive wind and water erosion. Conservation tillage, or minimum till- age, involves soil management practices that leave much more crop residue on the soil surface andcause much less soil disruption. As a result, the soil is less susceptible to erosion, and the plant residue acts as a mulch to protect the soil surface from the destructive impact of rainfall as well as to reduce evaporation. No tillage, or chemical tillage, is a soil management practice adapted to sloping soils in which herbicides rather than tillage are used to controlweeds, while the disruptionofsoil structure is limited to a narrow slit in the soil surface in which the seeds are planted. Soil management extends to the way in which soils are manipulated. Terraces, for example, are raised horizontal strips of earth constructed along the con- tour of a hill to slow the movement of downward- flowing water. Tile drains are perforated ceramic or plastic pipes buried in poorly drained soils that act as underground channels to carry water away, lower the water table, and allow a soil to drain faster after rain- fall. The benefit of managing potentially erodible soils on hill slopes as permanent pastures has gained increasing recognition. Likewise, the value of retain- ing wet soils as wetlands has been acknowledged. Wet- lands provide wildlife habitat, assist in flood control, and act as buffers to protect surface waterways from nutrient and soil runoff from cultivated fields. Soil management also involves the addition of chemicals to soil: lime to make acid soils more neu- tral; fertilizers to increase the nutrient level; herbi- cides and insecticides to control weed and insect pests; and soil conditioners to improve soil aggrega- tion, structure, and permeability. A growing technol- ogy is the use of mobile global positioning system (GPS) units attached to the equipment that applies these chemicals to soil. Called “site specific manage - ment,” it uses computer technology to regulate chem - ical addition based on the exact position in a field and previous yield or fertility maps that indicate whether the soil needs to be amended. The goal of site specific management is to optimize chemical use and profit while minimizing potential chemical loss to other en- vironments by only applying the chemicals to areas where they are needed. Judith J. Bradshaw-Rouse and Mark S. Coyne See also: Agriculture industry; Agronomy; Erosion and erosion control; Farmland; Fertilizers; Land man- agement; Leaching; Nitrogen cycle; Soil; Soil degra- dation; Soil testing and analysis. Soil testing and analysis Category: Environment, conservation, and resource management Understanding soil propertiesis crucial to a large vari- ety of agricultural, geological, and soil science applica- tions as well as to geotechnical, environmental, and foundation engineering projects. Soil testing can be conducted in situ or at the laboratory. Background Soil is the top part of the Earth’s crust; it comprises loose, unconsolidated materials that have been formed by physicochemical weathering and disinte- gration of rocks. Soils are usually composed of mix- tures of inorganic minerals, organic matter (humus), water, and air. Inorganic soils are mainly classified as coarse (granular) soils and fine (cohesive) soils. Coarse soils include boulders, cobbles, gravel, and sand. Fine soils include silts and clays that have a mean particle diameter of less than 0.062 millimeter. The properties of coarse soils are affected mainly by me- chanical forces such as gravity, buoyancy, drag, and in- ertia. In addition to the mechanical forces, fine soils are affected by electrochemical phenomena such as Van der Waalsforces (weak molecularforces based on electric polarization) and electric surface forces. Depending on the purpose of the study, there are a number of in situ and laboratory soil tests. In general, soil testing and analysis can be categorized as physical or chemical. Some of the tests are focused directly on soil properties, while others investigate the behavior of the soil relative to the water content. Global Resources Soil testing and analysis • 1107 . purifying petroleum, and softening water. Karen N. Kähler Further Reading Dyni, John R., and Richard W. Jones. Proceedings of the First International SodaAsh Conference: Utilizationof Nat- ural Resources ofSodium. Ecological resources Soil is a product of the physical and chemical break- down of the Earth’s surface into small fragments, in- cluding sand, silt, and clay. Soil is also the product of organic. decomposition—the composting of dead plant and animal debris. Soils are classified on the basis of soil profile and soil formation. They can be grouped according to a number of characteristics, in- cluding