Mikal E. Saltveit
It is difficult for people living in an advanced industrialized society to fully comprehend the life of a modern farmer, much less the life of farmers living before the Industrial Revolution. Up until the end of the eighteenth century, the vast majority of people were farmers who, as described by the English philosopher Thomas Hobbes in Leviathan, lived lives that were “solitary, poor, nasty, brutish and short.” And so it had been since the beginning of time for the vast majority of people, until the advent of an agricultural revolution that started in Great Britain during the early 1700s, reached North America by the mid-1800s, and continues to this day in all but the most benighted of nations. Agriculture had been changing since it had first appeared thousands of years earlier, but the pace quickened during the start of the Industrial Revolution in the eighteenth century, and changes that had previously taken centuries and generations began to occur within decades. By 1750, the best English agriculture was the best in the world. The most technologically advanced agriculture, it was also fully integrated into a market economy. The dominance of the British Empire in world affairs during the eighteenth and nineteenth centuries ensured that these agricultural improvements were widely distributed.
The Europe-centered Industrial Revolution of the eighteenth and nineteenth centuries accelerated an ongoing revolution in agriculture. In the industrialized West, animal power and human labor were first augmented and than almost completely replaced with mechanized sources of power. This was only part of the changes that gathered momentum in the 1700s and then transformed the world. Spectacular developments in all areas of science and nearly two centuries of exploration and conquest in the New World caused many Europeans in the early nineteenth century to reevaluate their relationship with nature. They developed an expanded worldview; it appeared to them that humanity in general, and Europeans specifically, had acquired the wisdom, knowledge, and scientific technology to dominate nature. This conviction was reinforced by the publication in 1859 of the Origin of Species by Charles Darwin. While the new attitude initially produced colonialism, rampant environmental pollution, and the exploitation of poorly organized workers, it also permitted the manipulation of the entire farming environment to an extent inconceivable to previous generations, and later produced for common people a standard of living previously available only to the aristocracy.
This new point of view stripped plants, animals, and soil of their mystical “vital” attributes and made of them machines to be molded to fit human needs. Physics, chemistry, and biology became tools to dissect, examine, and then reconstruct agricultural systems to make them better than any that had previously existed on earth. This process started with improved methods of crop production, advances in livestock breeding, and the invention of new farm equipment during the latter part of the Industrial Revolution. Soil, for example, ceased to contain a vital essence that must be periodically replenished by removing it from cultivation (fallowing), and became an aggregation of mineral and organic structures and chemicals whose fertility could be maintained by the application of scientific management—crop rotation, fertilizers, irrigation, pesticides, and other new methods. Adoption of new power sources, such as steam, and increased use of chemicals followed. Today, improved plant and livestock breeding through genetic engineering promises to continue the revolution well into the future.
Agricultural advances after the Industrial Revolution greatly increased food production, but increased urbanization required food to be transported long distances from producers to consumers. As with the citizens of ancient Rome, inhabitants of the ever-expanding cities became increasingly dependent on the transport, preservation, and storage of food. People starved in one part of the world, while abundant harvests spilled from granaries in another. The storage and distribution of food, rather than its production, became crucial to millions of people.
There have been periodic learned pronouncements of impending mass starvation since the English economist Thomas Malthus first proposed in his 1798 Essay on the Principle of Population that food production could only increase arithmetically, while population would increase geometrically. Malthus wrote that population would exceed food production at some point, and an apocalypse would ensue. While population growth has borne out his thesis for the past two hundred years, a number of scientific advances have allowed agricultural production to keep pace with it. A few major and thousands of minor refinements in agricultural practices have steadily improved productivity.
Before the 1700s, the problem of soil fertility had been met by letting half or a third of the land go fallow for a year in two-and three-field rotations. A new four-field rotation was now based on growing specific kinds of crops in a sequence that took from or added to the soil different nutrients. Part of the field did not have to be left fallow, and the continuous use of the land greatly increased the production of forage crops used to support livestock through the winter, thereby vastly increasing the availability of meat and dairy products. The diet of even the poorest improved as they could now afford to augment their daily bread with meat and cheese.
Another major change was a rapid acceleration in the fencing of large tracts of land to produce more efficient units of production. The enclosure movement in England did away with many traditional smallholdings, combining the land into larger tracts that could be more efficiently farmed. Earlier subdividing of land among generations of sons had produced a patchwork-quilt distribution of fields. A farmer may have had access to sufficient crop area (around twenty acres) to support his family, but it would be in small strips scattered among the holdings of other farmers, in a number of fields. This was because it was thought that each field had to remain fallow for a year to recover its fertility. From 1750 to 1831, enclosures consolidated these smallholdings into fields whose size could benefit from the application of modern methods of crop production.
Until the 1700s and 1800s peasants in most European countries could not actually own the land they farmed, but held ancestral rights to work land belonging to the proprietors of large estates. When laws were passed that allowed British landowners to abrogate these traditional agreements and combine many smallholdings, thousands of farm families were displaced. Migrating to urban areas, they furnished the labor that fueled the Industrial Revolution, and the wretched characters who populated many of Charles Dickens’s novels.
Throughout Europe, similar changes produced millions of restless people with a deep-seated desire to own land. Many emigrated to North America, Australia, and New Zealand, where a variety of homestead laws granted land to those who settled and worked a farm or ranch for a number of years. Based on these policies, the family farm became an institution in North America during the nineteenth and twentieth centuries, and a social and political force that continues to shape our national character. A combination of economic and demographic changes, however, has led to the steady decline in the number of functioning family farms. Although the U.S. population doubled between 1930 and 2000, the number of farms fell from 7 to 2 million. After 1987, this decline stabilized at about one percent annually. The price farmers got for commodities such as corn and soybeans remained virtually constant between 1970 and 2000, while the price they paid for everything they bought kept pace with inflation. This produced a farming population 60 percent of whom had farm incomes below the poverty line. It also discouraged the young so much that the number of farmers under the age of twenty-five decreased 50 percent between 1990 and 2000.
The large capital inputs needed to start and run a farm have transformed the production of many agricultural crops into large businesses. Ninety percent of U.S. farms, however, continue to be classified as individual operations, accounting for 71 percent of farmland and 74 percent of gross farm sales. Partnerships and corporations comprise a very small share of American farms, and people related by blood or marriage own 90 percent of them. Average acreage is higher for corporate farms (1,165 acres) and for partnerships (856 acres) than for individual operations (373 acres). Whereas the United States in the year 2000 had 2 million farms, just 60,000 of them produced 75 percent of the nation’s farm output. Fewer and fewer farms were thus producing more and more of what Americans ate. The 1.3 million farms with incomes below $20,000 comprised 60 percent of the total number of farms, but cultivated only 17 percent of the total U.S. farm acreage. In contrast, the 60,000 farms with sales over $250,000 comprised only 7 percent of the total number, yet cultivated almost 30 percent of the acreage.
The many improvements in crop production up to 1935 produced only modest increases in average yield per acre in America because of deteriorating soil fertility and poor water use. Control of erosion, soil conservation, extensive government-backed irrigation developments, and better water-use efficiency reversed this trend, leading to large increases in production.
Since 1950, the gap in efficiency between the most productive mechanized agricultural systems and the least productive manual farming systems has increased twentyfold. While in small part the result of reduced soil, water, and environmental quality in developing countries, this change really reflects the spectacular advances in agricultural technology in industrialized counties. Transference of this technology to farmers throughout the world should be a major goal, and will be a major challenge to the fortunate few in the coming decades.
Before the eighteenth century, raising animals was slow and costly; thus, meat and dairy products were usually scarce and expensive before the Industrial Revolution. The lack of sufficient forage to keep large numbers of animals over the winter often led to the slaughter of most livestock in the fall. Celebrants at many late fall festivals not only consumed the bountiful harvest of the field, but also the animals whose progeny could have supplied them with fresh meat, milk, and cheese the following year. Each spring, herds had to be rebuilt from the survivors.
Farmers had used their intuition and observations to breed animals for millennia, but the process was slow and haphazard because the inheritance of desirable traits was poorly understood. In the late 1700s Robert Bakewell, an English farmer, showed how intensive breeding for desirable traits could produce improved cattle, horses, and sheep. In Europe, sheep had been raised mainly for wool because they fattened too slowly to provide an economic source of meat. Bakewell’s Leicester breed fattened quickly and could therefore be raised for both wool and slaughter. The cost of mutton dropped so low that it became the most popular meat in England, Australia, and New Zealand.
Breeding of livestock is now a science in industrial societies, with genetic analysis an integral aspect. Accurate monitoring to detect estrus, or its induction by hormones, and the use of artificial insemination allow complete control over the reproductive cycle of most livestock. Removal of ova, their in vitro fertilization, and embryo implantation promise to allow a further level of control and manipulation of the reproductive process. For example, separation of the cells resulting from the first divisions of the fertilized ovum (zygote) may be used to produce a number of embryos that, when implanted, give rise to whole herds of genetically identical animals. These techniques are being coupled with the genetic engineering of DNA in specific chromosomes, or the replacement of the entire nucleus in a zygote with a nucleus from another individual of the same, or a related, species (cloning). While possibly replete with ethical conundrums, these procedures will surely transform livestock into units of production whose fecundity, efficiency, and vigor would marvel our ancestors.
Once Mendelian genetics was rediscovered in 1900, science gave plant and animal breeders a clear understanding of how traits were controlled by genes on chromosomes, and how they could be altered by selective breeding. Breeders made full use of this knowledge to steadily improve livestock and crops. Plants such as maize, soybeans, tomatoes, and peanuts became dietary staples in many parts of the world after their introduction from the Americas, Africa, and Asia. Identifying the sites of their origin helped locate ancestral forms of many crops, and these plants provided additional genetic resources to improve commercial varieties.
The upper limit of plant productivity is imposed by the quantum efficiency of photosynthesis and the energy content of sunlight. The photosynthate translocated from leaves not only produces the harvestable commodity, but is also used for all other plant functions. Reducing the drain of these other functions can increase yield. For example, symbiotic microorganisms in nodules on soybean roots can fix atmospheric nitrogen. It is often cheaper, however, to supply nitrogen fertilizers fixed by processes involving fossil fuels than to incur the loss of yield that would result from the soybean plant fixing a similar amount of nitrogen. Modification of the basic biochemistry and physiology underlying crop and livestock production will require levels of scientific knowledge and technical sophistication currently unavailable.
Economics of industrial-scale production require that most agriculture is monoculture, involving vast fields and herds of nearly genetically identical crops and livestock. This uniformity simplifies all aspects of production, but it also invites epidemics of plant disease. The devastating outbreak of bacterial southern corn blight in the United States in 1970, and the 2001 epidemic of viral foot-and-mouth disease in England, are examples of the seriousness of this problem. A major goal of plant and animal breeders is to stay ahead of chronic or exotic pathogens that can decimate crops and herds.
Genetic engineering has the potential to quickly create crops and livestock with unique characteristics. Rapid release of genetically modified organisms is slowed, however, because they require the same extensive field testing as new strains derived from traditional breeding. Consumer wariness has slowed the introduction of GMOs in Europe, but American consumers have readily accepted them. Most U.S. consumers have been unaware of the presence of GMOs in their food, and when they become aware, they are willing to accept claims by scientific and government sources that GMOs are safe to eat and environmentally benign. In Europe, a greater level of environmental activism and skepticism in government and scientific pronouncements has contributed to consumers’ doubts about the safety and environmental impact of GMO crops. A large percentage of U.S. corn, soybean, and cotton production uses GMOs that possess pest resistance. Tailoring GMOs to the specific needs of farmers in developing countries may be the only way for food production to keep pace with their rapidly increasing populations. Crops designed with increased disease and drought resistance and better use of nutrients in the soil could supplant the strains that have been developed for use in industrialized countries and that require expensive irrigation, pesticides, and fertilizers that are unavailable in developing countries.
Development of hybrid corn was a watershed in plant breeding, heralding a change in concept from a straightforward selection of desirable characteristics to the employment of a deeper understanding of the genetics involved. Experiments by G. H. Shull in 1906 showed that crossing could reverse reductions in vigor resulting from inbreeding. Using the strategy of double-cross hybrids suggested by D. F. Jones in 1918, the first commercial corn hybrid was released in 1921. About 95 percent of the corn now grown is hybrid, and use of double-cross hybrids allows 20 percent more corn to be produced on 25 percent fewer acres than when hybrid corn first became widely available in 1930. Hybrids of many other agronomic and horticultural crops have since been developed.
The Green Revolution. The Green Revolution, a sterling example of how the development of strains suited for developing countries, and a multifaceted approach to agriculture, can greatly increase food production, was a planned international effort funded by the Rockefeller and Ford Foundations and the governments of many developing countries. In the early 1950s, wheat production in Mexico had encountered an insurmountable yield barrier because the varieties being grown became too tall, top-heavy, and lodged (fell over and were difficult to harvest) when heavily fertilized. Using short-stalked lines developed years earlier by the U.S. Department of Agriculture, Norman E. Borlaug led an effort to develop broadly adapted, short-stemmed, disease-resistant wheats that excelled at converting fertilizer and water into high yields. Mexico went from importing half its wheat in 1964 to exporting half a million tons annually within two decades.
The Green Revolution is an agricultural success story. It increased food production in Mexico tenfold from 1960 to 1990 through the use of new crop varieties, irrigation, fertilizers, pesticides, and mechanization. At the same time, famine decreased 20 percent, caloric consumption per capita increased 25 percent, and incomes and standards of living increased. The successes in Mexico led to the establishment of a rice-breeding center in the Philippines. Working at about eighteen such centers worldwide, plant breeders have produced high-yielding varieties of virtually every major crop, including potato, sorghum, maize, cassava, and beans. Increased population growth and poor husbandry of natural resources, however, have eroded many of these gains since the 1980s.
Preparing the soil with a plow, planting seeds, cultivation, harvesting, and threshing are some of the most important steps in crop production, and some of the most labor-and energy-intensive. Inventions in the eighteenth and nineteenth centuries transferred much of farm labor to machines. Before Jethro Tull invented the precision seed drill in 1701, seeds were inefficiently planted by scattering them over a prepared field. Stands were thus often erratic and almost impossible to cultivate. The uniform placement of seeds in straight rows allowed horse-drawn cultivators to move easily up and down the rows for the control of weeds.
Little cotton was grown in the United States before the late 1790s because of the difficulty of separating the lint from the seed. Development of the cotton gin by Eli Whitney in 1793 greatly reduced the cost of producing cotton fiber for the rapidly increasing British textile industry. Slumps in production of tobacco, indigo, and rice during the 1790s had undermined the economic justification for slavery, but increased production of sugarcane and upland cotton still relied on the institution. The production of cotton in America now jumped twentyfold, from 2 million pounds in 1790 to 40 million pounds in 1800, while exports increased over 1000 percent in the same period. In less than a generation, cotton became the major crop grown in the U.S. South, and it revived the moribund slave-worked plantation system.
The first successful harvester or reaper was invented by Cyrus McCormick in 1834, as was the first modern thresher, by Hiram and John Pitts. The outbreak of the American Civil War in 1861 and subsequent conscription depleted farm labor, and forced many wheat farmers to buy reaping machines. The harvesting and threshing functions were later integrated into one machine, the combine, which did not become widely used until the early 1900s. Replacement of the cast-iron plow by the self-polishing steel plow, invented by John Deere in 1837, reduced the energy needed to plow a field because soil did not adhere to the smooth surface. To reach their full potential, most of these inventions depended on mechanical sources of power, which did not become readily available until the early twentieth century.
The major functions of a machine were often perfected over years of experimentation and modification. Sometimes mechanical limitations prevented further refinements, or an existing machine could not be easily modified to accommodate a new crop. When that occurred, it might be found easier to modify the crop to fit existing machines than to build entirely new machines. Dwarf sorghum was developed so it could be harvested with only slight modifications to an existing combine, and soybeans were developed that bore pods higher on the stalk so they could more easily be harvested. Tomatoes for processing were developed that had a more uniform set and were tough enough for mechanical harvesting using existing technology.
Integration of computers, sensors, and global positioning satellites into field equipment promises to revolutionize the planting, cultivation, and harvesting of many crops. For example, the location of each corn or tomato plant in a field can be identified using sensors and GPSs, and stored in computer memory. Subsequent operations such as weeding, applications of pesticides and fertilizers, and harvesting can then be positioned to maximize the effectiveness of every operation, thereby increasing yield and quality while at the same time reducing expenditures of time, fuel, and chemicals.
Energy and Information
In essence, farming is the conversion of sunlight and other sources of energy into food. For most of recorded history the energy to plant, cultivate, harvest, and process crops was supplied by the farmer and by domesticated draft animals (such as oxen and horses). Necessarily, the energy captured by the plant and harvested as food calories had to exceed that expended in its production. Preindustrial agriculture generally returned around twenty times more calories in the food consumed than was expended during its production. This efficiency decreases as the consumed food product requires additional processing (as for white bread versus grain potage, or cheese versus milk), conversion to other forms (cornflakes versus corn used to produce beefsteak), and shipment to distant consumers. As agriculture became mechanized, greater and greater amounts of energy were expended for each unit of food produced. Currently, mechanized agriculture in developed countries uses ten times as much energy to produce food as is returned in the food consumed. But agricultural mechanization has increased productivity so much that today’s farmer can feed almost 150 people, while at the beginning of the twentieth century a farmer could feed only 2.5 people. The vast input of fossil fuels to synthesize the required fertilizers and pesticides, and to power the machinery that cultivates the fields, harvests the crops, processes them, and transports them to the consumer has so increased production that a small percentage of the population can raise enough food to keep most Americans overweight.
Before the Industrial Revolution, there was localized use of wind and water power for milling grain and pumping water, but these sources were stationary, and of no use in planting or plowing a field, or harvesting a crop. The earliest tractors were basically large stationary engines equipped with a drive system. At first, steam engines were immobile and of little use for field operations because of their enormous weight. Even in the mid-1800s steam-powered tractors were so expensive and difficult to operate that most farmers continued to use horses and mules to power farm machines. The introduction of high-pressure boilers in the 1850s lightened engines, and steam tractors enjoyed significant usage between 1885 and 1914. Tractors with internal-combustion engines eventually supplanted steam tractors because they had several advantages: they were cheaper, easier to operate, and less prone to explosions and fires.
World War I (1914–1918) did for the tractor what the American Civil War had done for the reaping machine. Soon after it began, German U-boats were sinking so many British ships that it was necessary to increase food production by bringing thousands of acres of new farmland into production or face food shortages. There were not enough horses to plow this new land, and only five hundred tractors in all of Britain. The five thousand tractors ordered by the British government from Henry Ford were delivered within five months, and were soon at work on British farms. Almost overnight, British farmers, and later their conscripted American visitors, became accustomed to seeing tractors displacing teams of horses.
The labor shortage and guaranteed market for crops during World War I stimulated U.S. tractor design and manufacture; massive industrialization led to greatly increased production. It was not until after World War II, however, that tractors became widely accepted. Many farmers bought early tractors, particularly the smaller, lighter machines that could do varied field work, but the worldwide depression of the 1930s and the fact that tractors still couldn’t compete with the agility of horses in the field led to the demise of many tractor companies. Most farmers would have been thrilled to be rid of draft animals and to use mechanical devices. During the period 1908–1927 Ford built over 15 million automobiles with the Model T engine. Many farmers had a car or truck that was used to supply power for nonfield operations and for trips to town, yet continued to use draft animals for fieldwork; the Sears catalog of the time listed hundreds of accessories that could be used with the drive train of a car or truck to do everything from threshing grain and pumping water to churning butter, sawing wood, and washing clothes. Only when tractors appeared with enough power and maneuverability for fieldwork and the flexibility to furnish power for other nonfield operations were they readily adopted by most farmers. It was not until the 1920s that the all-purpose tractor made its appearance and gradually replaced steam-powered machines and draft animals. By the 1930s, seven tractor companies controlled over 90 percent of the market, and North America led the world in tractor design and production. Today, tractors using gasoline or diesel engines are ubiquitous on farms throughout the world.
Most farms in Europe and Japan had electrical power by the mid-1930s. Only 10 percent of U.S. farms were so supplied, however, at the time the Federal government established the Rural Electrification Administration in 1935. The REA supplied economic incentives that stimulated rural electrification; by 1960, over 97 percent of American farms had electricity.
Electricity brought with it better communication through the telegraph, telephone, radio, television, and eventually the Internet, all of which in turn have had tremendous effects on the farmer’s life. Farms are no longer isolated from the mainstream of society. Information on weather and on commodity prices is readily available, and can assist in better planning. Farming has become such a capital-, energy-, and information-intensive business that most farmers need assistance in managing it all. Most industrialized countries have government-sponsored agricultural-research and extension services. These not only engage in practical research of immediate and local importance, but also provide significant levels of basic research to address future problems. They assist farmers, marketers, and distributors through publications, interactive websites, classes, and farm visits and demonstrations. Almost more than any other aspect of modern agriculture, the extension services are responsible for today’s unmatched levels of food production, nutrition, and safety.
All living things are groups of simple and complex chemicals functioning together in specific and unimaginably complex ways. Since all living things are interrelated through evolution, they are very similar at the molecular level. This means that many require roughly the same resources (thus, all plants need sunlight, carbon dioxide, water, and a few common minerals), and can be food for one another (thus, the starch stored by a potato can be used for its future growth or consumed by humans). Many organisms therefore compete for the same scarce resources and develop elaborate strategies to avoid being eaten. Weeds are simply plants that out-compete crops for the resources that limit plant growth and that we apply in profusion to cultivated crops. During domestication, the elimination of many natural defense mechanisms to produce a more easily grown, harvested, or palatable crop also produces a crop more vulnerable to pests. Reintroduction of specific natural defense mechanisms through selective genetic engineering could drastically reduce the dependence of agriculture on synthetic pesticides.
Modern agriculture is based on the establishment of a monoculture in which one specific crop or animal is grown or raised over large areas to the exclusion of all potentially competing organisms. Methods of planting, cultivation, and harvesting are all geared to the growth of uniform plants and animals. An orchard is usually composed of genetically identical (cloned) trees. Each tree will flower at roughly the same time, and produce fruits that look and taste the same and ripen at the same time. Fields are often modified to provide growing conditions that are as close as possible to being identical for all plants to further limit variability. Pruning, applications of pesticide sprays, cultivations, irrigation, and harvesting can be done over the entire orchard because all trees and fruit are at similar stages of growth and will respond similarly. An orchard composed of dissimilar seedlings would have trees that flowered at different times, and have fruit that were green or red, sweet or tart, or large or small, and that ripened at different times. The susceptibility of trees and fruits to various pests would be different, so different pesticides would have to be used at different times and rates of application. As with the other field operations, harvesting would have to be done a number of times, and would have to be selective since the fruit on each tree would ripen at a different rate. Fruit, or any other agricultural commodity, produced in this way would be expensive and of variable quality—two attributes abhorrent to modern consumers.
Most undisturbed natural ecosystems are stable because they contain many species of plants and animals that are genetically diverse, that can exploit all the available niches, and that interact with one another to hinder the uncontrolled growth of a single pest or disease. In a monoculture the genetic and species diversity that provides stability is lacking, and control must be exerted by the farmer to maintain the health of the crops or animals and ensure an ample, high-quality yield.
Agricultural chemistry had become a recognized discipline by the mid-nineteenth century. Fertilizers and other products of the chemical industry became widely used early in the twentieth century, and have become indispensable in maintaining the yields of modern agriculture. Reliance on chemical answers to agricultural problems, however, has often obscured their deficiencies, as well as the existence of alternative solutions. Crop rotation and cultural practices have long been used to control pests. Since the end of World War II, however, farmers and ranchers have come to rely more heavily on chemical pesticides (such as insecticides, herbicides, fungicides, and nematocides). Shortly after DDT was found to be an effective insecticide in 1939, the United States began producing large quantities of it to control vector-borne diseases such as typhus and malaria. The dramatic success of DDT in controlling over five hundred insect pests diverted attention from traditional nonchemical methods of pest control. The publication of Silent Spring by Rachel Carson in 1962, however, made the public aware of the environmental drawbacks of using too many pesticides. Since then, there has been a complete shift in research emphasis, so that, whereas in 1925 three-fourths of published studies were on chemical pesticides, almost 80 percent of USDA pesticide research in 2000 was on alternatives to chemicals.
Environmental concerns, regulatory legislation, and increased costs were driving forces in the development of Integrated Pest Management (IPM) programs throughout the United States in the early 1970s. Bringing together experts from many fields, IPM programs strive to reduce the usage of chemical pesticides by integrating knowledge about the biology of the pest, the response of the crop to infestation, and the costs involved in applying or withholding treatment. Researchers have recognized that the pest population does not have to be completely eliminated, only kept below the point at which the farmer starts to lose money because of it. Implementation of these types of programs has significantly reduced the use of chemical pesticides while maintaining yield, quality, and economic return.
Fertilizers represent the single largest use of chemicals on the farm. Although nitrogen gas comprises 80 percent of the air we breathe, it is the element most commonly limiting plant growth. Atmospheric nitrogen must be fixed into ammonia or nitrates before a plant can use it. Lightning and biological activity (such as that of symbiotic microorganisms in nodules on legume roots) can fix nitrogen. The vast majority of nitrogen used in fertilizer, however, is fixed by the Haber process, an elegant method of combining nitrogen from air with hydrogen from natural gas under high pressure and temperature to produce ammonia. This process was invented by German scientists before World War I in response to a blockade of Chilean nitrate imposed by the British Royal Navy. During the war it was used to fix nitrogen for agriculture and to produce explosives. The inorganic ammonia fixed by this process now supplies about the same amount of nitrogen for crops as is fixed by all natural organic processes. The synthesis of ammonia consumes about 2 percent of the fossil fuel used worldwide. Overuse of cheap nitrogen fertilizer can lead to excessive runoff that pollutes groundwater and to eutrophication of bodies of water. But while inorganic nitrogen fertilizers remain cheap, it will cost less to apply them to crops than to have the plant divert its photosynthate from producing a crop to fixing its own nitrogen.
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