their durability and ability to be polished to an attrac - tive gloss finish. Examples include large building or- naments, countertops, and headstones for graves. Sand and gravel are found in areas with past or ex- isting streamflow. In many cases, the segregating as- pects of streamflow have sized the sand and gravel so that further sizing is minimized. Crushed stone, how- ever, is produced in mining operations, and the oper- ator has control over the size range of the finished product. For the most part,these operations are at the surface and are known as quarries. Some stone is pro- duced in underground mining operations, with the high quality of the stone justifying the additional ex- pense of this type of operation. Many quarries operat- ing in a particularly desirable deposit of stone con- tinue to pursue it by going underground. Preparations for Mining Before any significant expense is committed to locat- ing and developing an aggregate production site, a market study is performed to determine the amount and quality of materials needed within a certain ra- dius of transportation. Experienced aggregate pro- ducers usually have a “sixth sense” about the need for and location of their products. After the marketing study has been completed, a quarry site is located by geological exploration techniques such as identifica- tion of surface outcrops of material or inference, through regional studies of rock type. Even surface vegetation can indicate the type of material beneath the surface. Limestone terrains, for example, do not support acidic-soil vegetation because of their high pH values. The potential source of aggregate is identified and outlined more precisely through the digging of test pits for samples of the materials or by drilling test holes and analyzing the samples obtained. Again, the inherent size of the naturally occurring materials and the estimated expense of preparing them to market specifications are critical points that help determine whether a site is to be developed. If an aggregate site is to be produced, mineral leases are obtained from the mineral owners, who usually own the aggregate as well as deeper minerals. Sometimes the aggregate producer purchases the property outright if the location is environmentally sensitive or if a particularly long-lived aggregate source is identified. If the property is leased, the lease is usually for a fixed term, perhaps ten years, with op - tions to continue the lease for additional terms. A roy - alty is paid to the owner of the minerals according to the market value of the products. The royalty usually varies from 2 percent to 10 percent of the aggregate’s selling price. After the quarry dimensions have been verified, applications to develop the site are made to the local, state, and federal agencies that will regulate all aspects of site operation. The effects of the mining operation on air, groundwater and surface water, wildlife, ar- chaeological sites, and various natural processes must be determined. This process can be extremely expen- sive and consume several years. An environmental im- pact study and report may be necessary in certain ar- eas. This a very detailed and comprehensive report that incorporates all the environmental factors to be considered in locating the pit or quarry. Aggregate Mining Operations When all applicable permits and releases have been obtained, mining of the aggregate begins. For sand and gravel, the products can be recovered using sim- ple excavation equipment such as front-end loaders, pan scrapers, or dredges. The stone in quarries, how- ever, has inherent strength such that it is usually nec- essary to use explosives to fragment the material and make it suitable for bulk transport from the quarry. Blasting is used to dislodge stone from the “face,” or quarry wall. Almost any commercial explosive can be used for this purpose, but a mixture of ammonium ni- trate and fueloil has emerged overthe past several de- cades as the most cost-efficient explosive. Dynamite is still used but in smaller quantities than before. Several rows of blast holes are drilled at predeter- mined distances from the free quarry face. These dis- tances are engineered to ensure efficient rock break- age without unnecessary “fly” rock, or rock that is propelled outside the desired blasting area. Explosive cord is also extensively used in blasting operations. This product, with an almost instantaneous ignition rate, is especially useful when “delay” blast rounds are used. In delay blasting, millisecond delay sequences cause the first row of blast holes to be initiated, then subsequent rows rearward from the free face to be ig- nited after predetermined delays. This technique, now standard in nearly all quarry blasting proce- dures, results in much greater blasting efficiency. Dimension stone may also be removed by explo- sives, but the drill holes are very closely spaced, and the explosive is limited to the amount necessary to cut the block of dimension stone to the desired size and 10 • Aggregates Global Resources not fragment it further. High-pres - sure water jets or rock-cutting saws are also used in removing dimension stone from the quarry. Since the value of the dimension stone depends on the largest physical units minable, the stone must be gently dislodged so that it remains intact. Blasting, while indispensable to the crushed stone industry, is also one of the predominant problems as- sociated with it. The noise, vibration, and dust resulting from blasting op- erations, combined with the fact that quarries are usually located near populated areas, frequently bring ongoing conflict between producers and nearby residents. As a result, blasting is done carefully, and its side effects are measured to ensure that it is done within the guidelines set and enforced by regulatory agencies. The blasted material, after it is removed from the quarry face, is loaded by front-end loaders or shovels into large trucks for transportation to the surface preparation plant. The timing of the quarry floor loading operation is a continually challenging prob- lem involving machinery size and speed, operator ef- ficiency, and positioning. Occasionally it is necessary to reduce the sizes of blasted fragments that are too large to beloaded and transported easily with existing equipment, and small amounts of explosives may again be necessary on the quarry floor. Transporta- tion from the quarry floor is usually by truck, belt conveyor, or bucket conveyor, depending on the oper- ating system in effect at the quarry. Removal of dimen- sion stone poses a special problem because the large sizes demand gentle handling and require vehicles ca- pable of transporting the large, heavy units of stone. Large track-mounted crawlers, rubber-tired vehicles, or cranes are frequently used for this task. Sizing the Material Blasted materials are transported from the quarry to be reduced in size (“comminuted”) using adjust- able crushers, sorted into various sizes to suit the cus- tomer’s specifications, washed to remove unwanted extraneous materials such as clays, and stockpiled awaiting shipment. Typically, crushers operate by chewing on the material (“jaw crushers”) or passing the material through rollers (“roll crushers”). When crushed, the material assumes sizesvarying from dust- sized particles to pieces equivalent to the crusher open- ing. Most of these crushedproducts,eventhefinesizes, have some value, so the sizing operation is indispens- able to an efficient quarry serving multiple customers. Sizing may be based on a number of physical prin- ciples, such as the ability to pass through or be re- jected by sized openings (sieving), the tendency to drop out of a fluid stream (liquid classification), or the ability to be propelled through the air to a certain distance (air classification). If the materials have par- ticular physical or chemical characteristics (magne- tism, for example), other sizing operations can be de- signed to take advantage of them. Sieves (screens) frequently have a vibration device attached to them to prevent clogging and expedite the sorting process. Screening can be fully wet or dry, but merely moist materials cannot be separated in a screening opera- tion. The end resultofsizingishavingseveralsizedprod- ucts in which the size range within a sized product is quite narrow. If necessary, sized materials can be fur- ther classified by washing techniques designed to re- move the fine sizes inherent in the crushing process. Practical sizes of quarry products range from dust sizes of a few microns in diameter to cubic-shaped di - mension stone having dimensions exceeding 6 or 9 Global Resources Aggregates • 11 The sizesofsand canvary dramatically.The large-grain sand,from England, isapproxi- mately 2 millimeters in diameter. The small-grain sand is from Tunisia. meters. For most applications,however, practical sizes include sands from 1.65 millimeters through gravels around 0.6 centimeter to 1.2 centimeters in diameter, to stone sizes 2.5 centimeters to 7.6 centimeters in diameter, to fist-sized material used for foundation bases in construction, to football-sized riprap. The ul- timate goal of a stone and rock producer is to market 100 percent of the products generated in its opera- tion. While this is a practical impossibility, the most efficient producers have been able to come remark- ably close to this goal. Environmental and Citizenship Concerns Virtually all the activities of quarry operation have some undesirable side effects. Siltation problems are inherent in stream-based operations. Blasting gener- ates noise, dust, and vibrations. Trucking activities do the same, and theypresenta hazard to mixed automo- bile traffic if public roads are used for transportation. Surface facilities are prone to emitting dust, and fluid classification equipment produces sludge that must be handled. While not toxic in themselves, these by- products from aggregate production are objection- able and must be dealt with according to stringent regulations from municipal, state, and federal agen- cies. The closer an aggregate producer is to populated areas, the greater the inherent problems are (and the harder it is for the producer to be considered a good community citizen), but moving farther from these areas brings increased transportation costs and may make the producer uncompetitive. Moreover, most quarries are below the water table, which means that a constant inflow of groundwater must be pumped to a nearby creek or river. Although this water is fre- quently used in the crushing and sizing processes, it must still be pumped, routed, stored, and eventually disposed. Desludging equipment may also be neces- sary, further increasing the complexity of the opera- tion. Most successful aggregate producers have recog- nized and come to terms with the environmental con- sequences of their businesses. Blasting procedures are carefully engineered to minimize unwanted side effects and are scheduled at times that are least objec- tionable to those living or working nearby. A bag- house (a type of giant vacuum-cleaner bag) traps dusts produced by crushing operations and, in turn, frequently creates its own marketable product. Elec- trostatic precipitators are also used to remove and col- lect dust-sized material. Fines (powder or very small particles) from classification facilities are collected in sediment ponds that can be periodically “mined” to yield a marketable product. In addition to being a good environmental citizen, an aggregate producer should have a visually pleasing facility if it is located in or near populated areas. Well-kept grounds, clean equipment, and storage areas free of debris are marks of the conscientious operator. 12 • Aggregates Global Resources Crushed Stone Sold or Used in the United States, 2007 Number of Quarries Metric Tons Total Value (dollars) Unit Value (dollars) Calcareous marl 3 2,820,000 18,800,000 6.68 Dolomite 137 72,500,000 562,000,000 7.75 Granite 384 241,000,000 2,620,000,000 10.88 Limestone 2,123 1,020,000,000 8,280,000,000 8.13 Marble 21 7,580,000 71,100,000 9.38 Sandstone and quartzite 188 47,700,000 398,000,000 8.35 Shell 5 2,850,000 24,200,000 8.47 Slate 33 3,820,000 37,000,000 9.68 Traprock 361 105,000,000 1,030,000,000 9.82 Volcanic cinder and scoria 46 6,630,000 48,800,000 7.36 Miscellaneous stone 435 94,100,000 781,000,000 8.30 Source: Data from U.S. Bureau of Mines, Mineral Yearbook, 2007. U.S. Government Printing Office, 2009. Operational Life Span Although a large sand and gravel pit or stone quarry may remain open for decades, there is an ultimate life span to each location. Besides the restrictions of min- ing to the edges of the deposit or to the legal property limits of the miners’ leases, there is a practical depth beyond which a quarry must either go underground or face significantlyincreased costs of lifting themate- rial to the surface. If the decision is made to abandon the mine site, a large excavation remains that may en- compass many hectares and be as much as 60 to 90 meters in depth. With the closing of the pit or quarry, the excavation quickly fills with water, and an im- poundment, sometimes having very steep sides, be- comes part of thelandscape.Topsoil initially removed to expose the minable aggregate must be hauled away or stored nearby. Spoil that was removed and dis- carded during the quarrying operation may now be visually objectionable and must be remedied. If all these remedial activities are included as a part of the overall life cycle of the pit or quarry, then a desirable residential location may be created. It is common to see upscale housing developments built around old pits andquarries, withthe impoundments becoming favorite targets of sport fishermen. They are also frequented by migratory birds and other wild- life favoring large bodies of fresh water. Charles D. Haynes Further Reading Blatt, Harvey, Robert J. Tracy, and Brent E. Owens. Pe- trology: Igneous, Sedimentary, and Metamorphic. 3d ed. New York: W. H. Freeman, 2006. Chatterjee, Kaulir Kisor. Uses of Industrial Minerals, Rocks, and Freshwater. New York: Nova Science, 2009. Grotzinger, John P., et al. Understanding Earth. 5th ed. New York: W. H. Freeman, 2007. Hockensmith, Charles D. The Millstone Industry: A Summary of Research on Quarries and Producers in the United States, Europe, and Elsewhere. Jefferson, N.C.: McFarland, 2009. Kogel, Jessica Elzea, et al., eds. “Stone and Rock.” In- dustrial Minerals and Rocks: Commodities, Markets, and Uses. 7th ed. Littleton, Colo.: Society for Mining, Metallurgy, and Exploration, 2006. Philpotts, Anthony R., and Jay J. Ague. Principles of Ig- neous and Metamorphic Petrology. 2d ed. New York: Cambridge University Press, 2009. Raymond, Loren A. Petrology: The Study of Igneous, Sedi - mentary, and Metamorphic Rocks. 2d ed. Boston: McGraw-Hill, 2002. Smith, M. R., and L. Collis, eds. Aggregates: Sand, Gravel, and Crushed Rock Aggregates for Construction Purposes. 3d ed. Revised by P. G. Fookes et al. Lon- don: Geological Society, 2001. Tarbuck, Edward J., and Frederick K. Lutgens. Earth: An Introduction to Physical Geology. 9th ed. Illustrated by Dennis Tasa. Upper Saddle River, N.J.: Pearson Prentice Hall, 2008. Weiss, Norman L., ed. SME Mineral Processing Hand- book. 2 vols. New York: Society of Mining Engineers of the American Institute of Mining, Metallurgical, and Petroleum Engineers, 1985. Web Sites Natural Resources Canada Canadian Minerals Yearbook, Mineral and Metal Commodity Reviews http://www.nrcan-rncan.gc.ca/mms-smm/busi- indu/cmy-amc/com-eng.htm U.S. Geological Survey Crushed Stone: Statistics and Information http://minerals.usgs.gov/minerals/pubs/ commodity/stone_crushed/index.html#mcs U.S. Geological Survey Dimension Stone: Statistics and Information http://minerals.usgs.gov/minerals/pubs/ commodity/stone_dimension/index.html#mcs See also: Abrasives; Cement and concrete; Dimen- sion stone; Granite; Igneous processes, rocks, and mineral deposits; Limestone; Marble; Metamorphic processes, rocks, and mineral deposits; Quarrying; Sand and gravel; Slate. Agricultural products Categories: Plant and animal resources; products from resources Agricultural products encompass all commodities de- rived from the production of animals and the growing of crops to be used by humans for food, fiber, shelter, me - dicinal applications, or aesthetic purposes. Global Resources Agricultural products • 13 Background A nation’s ability to sustainan agricultural system with the capacity to feed, shelter, and clothe its population is perhaps its greatest natural resource. The human domestication of certain animals and cultivation of certain food crops predate written history. In modern society, the diversity of products that can be produced via agricultural technology is enormous. Major agri- cultural commodities can be divided into those that come from animals and those that are derived from plants. Agricultural Products from Animals The major animal-derived products can be divided into edible and inedible red meat products, milk and milk products, poultry and egg products, and wool and mohair. Red meat products refer primarily to those products that come from cattle (beef and veal), swine (pork), sheep (lamb and mutton), goats (chevon), and, to a lesser extent, other animals such as horses and buffalo. Other than the meat itself, edible products from red meat animals include meat scraps used to make processed meats such as frankfurters and bologna; organ meats such as liver, tail, tongue, tripe (stomach), and sweetbread (thymus); and tallow or lard. The major inedible red meat prod- ucts include rendered fat, which is used to make soap and formula animal feeds; bone meal, used in fertilizer and animal feeds; manure; and hides or skins, which are tanned and used to make leather products. Milk and milk products, also referred to as dairy products, are major components of the hu- man diet in many countries. Whole milk with part or all of the fat removed is sold as low-fat or skim milk. Evaporated and condensed milk refers to milk from which approximately 60 percent of the water has been removed, and dry or powdered milk has had at least 95 percent of the water re- moved. Cultured milk products such as butter- milk, yogurt, sour cream, and cottage cheese are produced via fermentation by the addition of ap- propriate bacterial cultures to fluid dairy prod- ucts. Cream products are separated from liquid milk and have a milk fat content of not less than 18 percent. Butter is made by separating the but- terfat from either milk or cream and contains at least 80 percent by weight milk fat. Cheese is pro - duced through the microbialaction of a variety of bacteria and fungi on whole milk. Ice cream, ice milk, sherbet, and frozen custard are produced by freezing a variety of liquid milk products in combination with sugar and other ingredients. Poultry products are nutritious and relatively inex- pensive, and they are used by humans throughout the world. Broiler chickens provide most of the world’s poultry meat, but turkeys, roaster chickens, mature laying hens (fowl), ducks, geese, pigeons, and guinea hens are also consumed and may be more important than chickens in some parts of the world. Much of the poultry meat is processed into preformed products such as poultry rolls or nuggets, canned products, or cured and ground products such as frankfurters, bo- logna, turkey ham, or salami. Other than meat, poul- try also provides eggs. The majority of the world’seggs are now produced by chickens specifically selected to lay large numbers of eggs, but eggs from the other birds listed above are sometimes eaten. Eggs can be 14 • Agricultural products Global Resources Harvested corn is loaded onto a grain-hauling truck. (AP/Wide World Photos) further processed and sold as liquid eggs or dried eggs. There is also an industry based on the meat and eggs fromtheratitebirds,suchastheostrichandemu. The hair covering the skin of some farm animals is also considered an agricultural product. The two most important of these are wool from sheep and mohair from angora goats. Wool or mohair can be sheared annually from the same sheep or goat. The cleaned and processed fibers are woven into yarn, which can then be used to make carpet or cloth. Agricultural Products from Plants Agricultural products that are derived from plants are also diverse. They can be subdivided into timber products, grain crops, fiber crops, fruit crops, nut crops, vegetable crops, beverage crops, spice and drug crops, ornamental crops, forage crops, and other cash crops such as sugarcane, tobacco, artichoke, and rub- ber. Timber products include those materials derived from the trees of renewable forests. The two major products are lumber for building and paper, but other products such as pine tar, resin, and turpentine are also extracted from trees. More cultivated land is devoted to the production of grain crops, also known as cereal grains, and fiber crops than to any other agricultural crop. Cereal grains are the edible seed from a variety of grasses. Throughout history, these crops have been primarily responsible for sustaining large human populations and domestic animal herds. The products from the world’s major grain crops include corn, wheat, oats, barley, rye, rice, and derivatives such as grain-based cereals and flours. Rice is the leading seed crop in the world and is the principal food crop of about half the world’spopulation.Corn, barley, rye, and rice are also used to produce alcoholic beverages. Grainsorghums are also produced in large quantities in the United States, but this grain is primarily used as feed for live- stock. Cotton, flax, and hemp are the principal fiber plants grown in the United States, although less im- portant crops such as ramie, jute, and sisal are also grown. While these plants are grown primarily for their fiber, which is used to make textiles, rope, twine, and similar products, other products such as cotton- seed oil and linseed oil from flax are also produced. Fruits from a variety of perennial plants are har- vested for their refreshing flavors and nourishment. Fruits can be subdivided into temperate, subtropical, and tropical crops. Grapes, a temperate fruit grown for table use, winemaking, and raisins, account for ap - proximately 25 percent of the fruit production of the world. Examples of other temperatefruits include the pome fruits (apple and pear), stone fruits (plum, peach, apricot), kiwi fruit, and berry fruits (straw- berry, raspberry, blackberry, cranberry, and blue- berry). Tropical and subtropical fruit crops include banana, pineapple, mango, papaya, avocado, date, fig, olive, and citrus fruits (orange, lemon, and grape- fruit). In addition to providing fresh fruit, the prod- ucts from fruit crops can be canned, dried, converted to juices, and used to make jams and jellies or special products such as olive oil. Nut crops refer to those woody plants that produce seed with firm shells thatseparate them from an inner kernel. Nuts are generally considered to be luxury food. They are often eaten as a delicacy (either fresh or canned) or consumed with candy or other sweets. Major temperate nut crops include walnut, almond, chestnut, pistachio, pecan, and hazelnut. Tropical nut crops include the coconut, cashew,Brazil nut,and macadamia. Nuts are characteristically rich inoil.The inedible tungnut is grown exclusively for its oil. Vegetable crops are extremely diverse and range from starchy calorie sources (potato) to food that sup- plies vitamins and minerals (broccoli). Examples of the major vegetable crops include the edible legumes (beans, soybeans, peas, lentils, and peanuts); roots, tubers, and bulbs (potato, sweetpotato, taro, yam, cas- sava, onion, sugar beet, and carrot); solanaceous fruits (tomato, eggplant, and chili pepper); salad plants (lettuce, endive, chicory, parsley, and garden cress); cole crops (cabbage, cauliflower, broccoli, brussels sprouts, and radish); and vine crops (cucumbers, wa- termelon, cantaloupe, pumpkin, squash, and gourd). Like fruit crops, vegetable produce can be served fresh, canned, frozen, or in an assortment of juices. The world’s three most popular nonalcoholic bev- erages—coffee, tea, and cocoa—are produced in the tropics. These products represent a major proportion of the tropical world’s agricultural exports, and their production is the major industry in many tropical countries. Many plants or plant parts that possess strong aroma and flavor are grown for the purpose of producing spices. Because of the diverse biochemis- try of plants, some crops are cultured for the produc- tion of medicinal drug compounds. There are more than two hundred spice and drug plants; some of the more common products are peppermint, nutmeg, garlic, vanilla, allspice, cinnamon, black pepper, mus - tard, opium, quinine, belladonna, and digitalis. Global Resources Agricultural products • 15 Ornamental and forage crops are not consumed by humans. Ornamental crops are grown for aesthetic purposes and are divided into florist crops (flower and foliage plants) and landscape crops (nursery plants). Common ornamental crops include rose, or- chid, carnation, chrysanthemum, and a variety of shrubs. Forage crops are grown to feed livestock and include a host of small grain grasses, clover, alfalfa, and a variety of straw crops for haymaking. D. R. Gossett Further Reading Akinyemi, Okoro M. Agricultural Production: Organic and Conventional Systems. Enfield, N.H.: Science Publishers, 2007. Brody, Aaron L., and John B. Lord, eds.DevelopingNew Food Products for a Changing Marketplace. 2d ed. Boca Raton, Fla.: CRC Press/Taylor & Francis, 2008. Field, Thomas G., and Robert E. Taylor. Scientific Farm Animal Production: An Introduction to Animal Science. 9th ed. Upper Saddle River, N.J.: Prentice Hall, 2008. Janick, Jules. Horticultural Science. 4th ed. New York: W. H. Freeman, 1986. Ward, Janet D., and Larry T. Ward. Principles of Food Science. Tinley Park, Ill.: Goodheart-Willcox, 2002. Web Sites Agriculture and Agri-Food Canada Producers http://www4.agr.gc.ca/AAFC-AAC/display- afficher.do?id=1165871799386&lang=eng U.S. Department of Agriculture Animal Production http://www.usda.gov/wps/portal/!ut/p/_s.7_0_A/ 7_0_1OB?navid=ANIMAL_PRODUCTION&pare ntnav=AGRICULTURE&navtype=RT U.S. Department of Agriculture Crop Production http://www.usda.gov/wps/portal/!ut/p/_s.7_0_A/ 7_0_1OB?navid=CROP_PRODUCTION&parent nav=AGRICULTURE&navtype=RT See also: Agriculture industry; Corn; Horticulture; Monoculture agriculture; Plant domestication and breeding; Plant fibers; Plants as a medical resource; Rice; Rubber, natural; Sugars; Wheat; Wood and timber. Agriculture industry Categories: Plant and animal resources; obtaining and using resources The ability to produce sufficient food and fiber to feed and clothe its population is the most important natu- ral resource a nation can have. In modern urban soci- eties, it is also the natural resource that is most often taken for granted. Background The beginnings of agriculture predate the written his- tory of humankind. No one knows when the first crop was cultivated, but at some time in the distant past hu- mans discovered that seeds from certain wild grasses could be collected and planted in land that could be controlled, the end product of which could later be gathered for food. Most authorities believe this oc- curred at about the same time in both the Old World and New World, some eight thousand toten thousand years ago. The earliest attempts at growing crops were primarily to supplement the food supply provided by hunting and gathering. However, as the ability to pro- duce crops increased, people began to domesticate animals, and their reliance on hunting and gathering decreased, allowing the development of permanent human settlements. As far back as six thousand years ago, agriculture was firmly established in Asia, India, Mesopotamia, Egypt, Mexico, Central America, and South America. The earliest agricultural centers were located near large rivers that helped maintain soil fertility by the deposition of new topsoil with each annual flooding cycle. As agriculture moved into regions that lacked the annual flooding of the large rivers, people began to utilize a technique known as slash-and-burn agri- culture. In this type of agriculture, a farmer clears a field, burns the trees and brush, and farms the field. After a few years, soil nutrients become depleted, so the farmer must repeat the process. This type of agri- culture is still practiced in some developing countries and is one reason that the tropical rain forests are dis- appearing at such a fast rate. Until the nineteenth century, most farms and ranches were family-owned, and most farmers prac- ticed sustenance agriculture: Each farmer or rancher produced a variety of crops sufficient to feed himself and his family as well as a small excess to be sold for 16 • Agriculture industry Global Resources cash or bartered for other goods or services. Agricul- tural tools such as plows were made of wood, and al- most all agricultural activities required human or ani- mal labor. This situation placed a premium on large families to provide the help needed to tend the fields. The Industrial Revolution changed agriculture just as it did almost allother industries. Eli Whitney invented the cotton gin in 1793. The mechanical reaper was in- vented by Cyrus McCormick, and John Lane and John Deere began the commercialmanufactureofthesteel plow in 1833 and 1837, respectively. These inventions led the way for the development of the many different types of agricultural machinery that resulted in the mechanization of most farms and ranches. By the early part of the twentieth century, most agricultural enterprises in the United States were mechanized. The Industrial Revolution produced a significant change in society. The industrialized nations were gradually transformed from agrarian societies into ur - ban societies. People involved in agricultural produc - tion left the farms to go to the city to work in the facto- ries. At the same time, there was no longer a need for large numbers ofpeople to produce crops. As aresult, fewer people were required to produce more agricul- tural produce for an increasing number of consum- ers. This trend continues in developing nations with more rural dwellers, adding to the overcrowding in urban centers. Modern Agriculture As populations continued to grow, there was a need to select and produce crops with higher yields. The Green Revolution of the twentieth century helped to make these higher yields possible. Basic informa- tion supplied by biological scientists allowed the agri- cultural scientists to develop new, higher-yielding vari- eties of numerous crops, particularly the seed grains that supply most of the calories necessary for main - tenance of the world’s population. These higher- yielding crop varieties, along with improved farming Global Resources Agriculture industry • 17 A farm manager practices weed control in a soybean field as an aspect of the farm’s weed-tillage system. (United States Department of Agriculture/Keith Weller) methods, resulted in tremendous increases in the world’s food supply. The new crop varieties also led to an increased reliance on monoculture. While the practice of growing only one crop over a vast number of hectares has resulted in much higher yields than planting multiple crops, it has also decreased the ge- netic variability of many agricultural plants, increased the need for commercial fertilizers, and produced an increased susceptibility to damage from a host of bi- otic and abiotic factors. These latter two develop- ments have resulted in a tremendous growth in the agricultural chemical industry. Today’s modern ag- ricultural unit requires relatively few employees, is highly mechanized, devotes large amounts of land to the production of only one crop, and is highly reliant on agricultural chemicals such as fertilizers and pesti- cides. Agricultural Diversity With all the diversity that has occurred in modern ag- riculture, the industry is subdivided into many differ- ent specialties. On the animal side of the industry, there is the beef industry, which deals with the pro- duction of beef cattle; the dairy industry, which fo- cuses on the production of dairy cattle, milk,andmilk products; the horse industry, which produces horses for work, sport, or pleasure; the sheep and goat indus- try; the swine industry, which deals with the produc- tion of pigs and hogs; and the poultry industry, which is concerned with the production of commercial birds and bird products such as eggs. Those agricultural in- dustries that deal with plants include agronomy, the production of field crops (wheat, cotton, and so on); forestry, the growth and production of trees; and hor- ticulture. Horticulture is subdivided into pomology, the growth and production of fruit crops (oranges, apples, and so on); olericulture, the growth and pro- duction of vegetable crops (tomatoes, lettuce); land- scape horticulture, the growth and production of trees and shrubs that are used in landscape design; and floriculture, the growth and production of flow- ering plants used in the floral industry. The various agriculture industries produce a tre- mendous number of different agricultural products. Those agricultural products that are derived from plants can be subdivided into timber products (such as lumber, furniture), grain products (wheat, oats), fiber products (cotton, flax), fruit products (grapes, peaches), nut crops (pecans, hazelnuts), vegetable products (lettuce, cabbage), beverage products (tea, coffee), spice and drug crops (garlic, mustard, opium, quinine), ornamental crops (carnation, chry- santhemum), forage crops (alfalfa, clover), and other cash crops,suchassugarcane,tobacco,artichoke, and rubber. The animal industries provide products such as red meats from cattle (beef and veal), swine (pork), sheep (lamb and mutton), and goats (chevon). Milk and milk products, also referred to as dairy products, in- clude milk, ice cream, and cheeses. Broiler chickens provide most of the world’s poultry meat, but turkeys, roaster chickens, mature laying hens (fowl), ducks, geese, pigeons, and guinea hens are also consumed and may be more important than chickens in some parts of the world. Other than meat, poultry also pro- vides eggs. The hair covering the skin of some farm animals is also considered an agricultural product. The two most important of these are wool from sheep and mohair from angora goats. Impact on Other Natural Resources While there have been tremendous increases in agri- cultural productivity through the use of modern agri- cultural practices, these same practices have had a sig- nificant impact on some other natural resources. Soil is one of the most overlooked and misunderstood re- sources. Most people think of soil as an inert medium from which plants grow. In reality, topsoil—that up- per 15 to 25 centimeters of the Earth’s terrestrial sur- face in which nearly all plants grow—is a complex mixture of weathered mineral materials from rocks, partially decomposed organic molecules, and a large number of living organisms. The process of soil for- mation is very slow. Under ideal conditions, topsoil can form at a rate sufficient to produce a layer of about 1 millimeter thick when spread over 1 hectare per year. Under less favorable conditions, it can take thousands of years to produce this small amount of soil. With proper management, topsoil can be kept fertile and productive indefinitely. However, many ag- ricultural techniques lead to the removal of trees and shrubs, which providewindbreaks,ortothedepletion of soil fertility, which reduces the plant cover over the field. These practices have exposed the soil to in- creased erosion from wind and moving water, and as a result, as much as one-third of the world’s current croplands are losing topsoil faster than they can be re- placed. Because plants require water in order to grow, agri - culture represents the largest single global user of 18 • Agriculture industry Global Resources water. Worldwide, about 70 percent of all fresh water withdrawn from groundwater supplies, rivers, and lakes is used to irrigate crops, and almost 15 percent of the world’s croplands are irrigated. Water usage varies among countries. Some countries have abun- dant water supplies and irrigate liberally, while water is scarce in other countries and must be used carefully. Because as much as 60 percent of the water intended for irrigation is lost through old pipes and canals or to evaporation before the water reaches the field, the efficiency of water use in some countries can be very low. There is no doubt that irrigation has dra- matically increased crop production in many areas, but some irrigation practices have been detrimental. Overwatering can lead to a waterlogging of the soil. Waterlogging cuts off the supply of oxygen to the roots, and the plants die. Irrigation of crops in dry cli- mates can often result in salinization of the soil. In these climates, the irrigation water rapidly evaporates from the soil, leaving behind the mineral salts that were dissolved in the water. As the salts accumulate, they become lethal to most plants. Some experts esti- mate that as much as one-third of the world’s agricul- tural soil has been damaged by salinization. There is also an argument as to whether or not the increased usage of water for agriculture has decreased the sup- ply of potable water fit for other human uses. Plants require sunshine, water either from rainfall or irrigation, carbon dioxide from the atmosphere, and thirteen mineral nutrients from the soil. Of these, calcium, magnesium, nitrogen, phosphorus, and po- tassium are required in the greatest amounts. Cal- cium and magnesium are plentiful in soils located in dry climates, but in wetter climates, these nutrients are often leached through the soil. In these regions, calcium and magnesium are returned to the soil in the form of lime, which is primarily used to raise the soil pH. Nitrogen, phosphorus, and potassium are the nutrients most often depleted from agricultural soils, and these nutrients are often referred to as the fertil- izer elements. Because thesenutrients stimulate plant growth and usually greatly increase crop yields, it is necessary to apply them to the soil regularly in order to maintain fertility. The amount of fertilizer applied to the soil has in- creased more than twentyfold in the past fifty years. While this increase in the use of fertilizers has more than tripled the worldwide crop production, it has also caused some problems. The increased produc - tion of fertilizers has required the use of energy and mineral resources that could have been used else - where. In many cases, farmers tend to overfertilize. Overfertilization not only wastes money but also con- tributes to environmental degradation. Fertilizer ele- ments, particularly nitrogen and phosphorus, are carried away by water runoff and are eventually de- posited in the rivers and lakes, where they contribute to pollution of aquatic ecosystems. In addition, ni- trates can accumulate in underground water supplies. These nitrates can be harmful if ingested by new- borns. Modern agriculture, as it is practiced in the indus- trialized nations, consumes large amounts of energy. Farm machinery utilized in planting, cultivating, har- vesting, and transporting crops to market consumes the largest supplies of liquid fossil fuels such as gaso- line or diesel. Theenergy required to produce fertiliz- ers, pesticides, and other agricultural chemicals is the Global Resources Agriculture industry • 19 An agronomist cradles a box of broccoli on a farm in Salinas, Cali - fornia. (United States Department of Agriculture) . a waterlogging of the soil. Waterlogging cuts off the supply of oxygen to the roots, and the plants die. Irrigation of crops in dry cli- mates can often result in salinization of the soil. In these. sizes of quarry products range from dust sizes of a few microns in diameter to cubic-shaped di - mension stone having dimensions exceeding 6 or 9 Global Resources Aggregates • 11 The sizesofsand. and storage areas free of debris are marks of the conscientious operator. 12 • Aggregates Global Resources Crushed Stone Sold or Used in the United States, 2007 Number of Quarries Metric Tons Total