Soil conservation trials in Texas, Missouri, and Illinois slowed erosion by a factor of two to a thousand and increased crop yields by up to a quarter for crops like cotton, corn, soybeans, and wheat. Soil conservation is not radical new territory. Many of the most effective methods have been recognized for centuries.

Despite compelling evidence that soil erosion destroyed ancient societies, and can seriously undermine modern societies, some warnings of an impending global soil crisis and food shortages have been overblown. In the early i98os agricultural economist Lester Brown warned that modern civilization could run out of dirt before oil. Failure of such alarming predictions to play out over the past several decades helped conventional resource economists downplay the potential for soil erosion to compromise food security. Yet such views are shortsighted when erosion removes soil from agricultural fields faster than it forms. Arguing about whether soil loss will become an acute crisis in 2oio or 2100 misses the point.

a.n.a.lysts offer many reasons for lack of progress in the global war on poverty, but almost every region of acute poverty shares a deteriorating environment. When the productive capacity of the land fails, those living directly off the land suffer most. While land degradation results from economic, social, and political forces, it is also a primary driver of those forces. Increasingly, land degradation is becoming a princ.i.p.al cause of poverty in the developing world. Realistically, the war on poverty simply cannot be won by methods that further degrade the land.

But soil loss is not inevitable. There are productive, profitable farms in every state-and probably every country-that operate with no net loss of topsoil. Despite substantial progress and advances in soil conservation in the past half century, society still prioritizes production over long-term stewardship of the land. The direct costs to farmers from soil erosion in the form of reduced crop yields are typically negligible in the short run, which means conservation measures may never be adopted even if they make economic sense over the long run. We therefore remain in the awkward situation that many highly productive farms mine their own future productivity.

The lessons of the Dust Bowl and the Sahel make a strong case for governments to coordinate, prioritize, and invest in soil conservation. Individuals don't necessarily have an incentive to protect humanity's investment in the soil because their short-term interests need not align with society's long-term interests. A key problem therefore lies in how we view the business of farming. It is the foundation for all other businesses, yet we increasingly treat farming as simply another industrial process.

During the nineteenth century, expansion of the area under cultivation more than kept pace with population growth as pioneer farmers plowed up the Great Plains, the Canadian prairies, the Russian steppe, and vast areas of South America and Australia. Even early in the twentieth century it was clear that further population growth would have to come from increasing crop yields rather than plowing more land.

Together John Deere's plows and Cyrus McCormick's reapers allowed farmers to work far more land than a single farm's livestock could reliably manure. To expand the scope of cultivation and take full advantage of the new equipment required farmers either to continue the pattern of depend ing on access to fresh land or find a subst.i.tute for the eighty head of cattle needed to keep a quarter-section of land well manured. The potential to cultivate far larger areas with new labor-saving machinery provided a ready market for fertilizers. No longer would the scale of an agricultural operation be limited by the capacity of a farm to recycle soil fertility.

DIRTY BUSINESS

A nation that destroys its soils, destroys itself.

FRANKLIN D. ROOSEVELT.

SEVERAL YEARS AFTER SEEING THE RAPID PACE of soil destruction in the lower Amazon I found the ant.i.thesis while leading an expedition in eastern Tibet. Driving the region's rough dirt roads I saw a thousand-year-old agricultural system along the valley of theTsangpo River. We were there to study an ancient ice-dammed lake that drained in a cataclysmic flood down the Himalayan gorge through which the river slices to join the Ganges. Looking for outcrops of ancient lakebeds we drove through villages full of chickens, yaks, and pigs. All around the towns, low silt walls trapped soil in fields of barley, peas, and yellow flowers with seeds rich in canola oil.

After a few days it became obvious that corralling dirt was only part of the secret behind ten centuries of farming the lakebed. Following an unsupervised daily rhythm, Tibetan livestock head out to the fields during the day, fend for themselves, and come home at night. Driving back through towns at the end of each day's fieldwork, we saw pigs and cattle waiting patiently to reenter family compounds. These self-propelled manure dispensers were prolific; even a brief rain turned fields and roads to flowing brown muck.

The night after finding the remains of the glacial dam that once impounded the lake, we stayed at a cheap hotel in the end-of-the-road town of Pai. Homemade sleeping platforms served as beds in sleeping stalls barely separated by unfinished plank walls. The proprietor advised us on our way in that the backyard would serve as our bathroom. That the pigs clean up the yard bothered me during our pork dinner. Still, I had to appreciate the efficiency of pigs eating waste and fertilizing the soil, and then people eating both crops and pigs.

Overlooking the obvious public health issues, this system sustained soil fertility. Other than the occasional satellite dish protruding from the side of a house, villages along the Tsangpo looked much as they had soon after the lake drained. Controlling soil erosion and letting livestock manure the ground allowed generation after generation to plow the same fields.

But Tibetan agriculture is changing. On the road leading out of Lhasa, immigrant Chinese farmers and enterprising Tibetans are setting up irrigated fields and greenhouse complexes. Throughout history, technological innovation has periodically increased agricultural output since the first farmers began turning the earth with sticks before planting. Plows evolved from harnessing animals to pull bigger sticks. Heavy metal plows allowed farmers to cultivate the subsoil once the topsoil eroded away. This not only allowed growing crops on degraded land, it brought more land under cultivation.

Tilling the soil breaks up the ground for planting, helps control weeds, and promotes crop emergence. Although it helps grow desired plants, plowing also leaves the ground bare and unprotected by vegetation that normally absorbs the impact of rainfall and resists erosion. Plowing allows farmers to grow far more food and support more people-at the cost of slowly depleting the supply of fertile dirt.

Agricultural practices evolved as farming methods improved through trial and error. Key innovations included experience with manure and regionally adapted crop rotations. Before mechanized agriculture, farmers cultivated a variety of crops, often by hand on small farms that recycled stubble, manure, and sometimes even human waste to maintain soil fertility. Once farmers learned to rotate peas, lentils, or beans with their primary crops, agricultural settlements could persist beyond the floodplains where nature regularly delivered fresh dirt.

In the Asian tropics, the first few thousand years of rice cultivation involved dryland farming, much as in the early history of wheat. Then about 2,500 years ago, people began growing rice in artificial wetlands, or paddies. The new practice helped prevent the nitrogen-depletion that had plagued tropical farmers because the sluggish water nurtured nitrogen-fixing algae that functioned as living fertilizer. Rice paddies also provided ideal environments for decomposing and recycling human and animal wastes.

A phenomenally successful adaptation, wetland rice cultivation spread across Asia, catalyzing dramatic population growth in regions ill suited for previous farming practices. Yet even though the new system supported more people, most still lived on the brink of starvation. Greater food production didn't mean that the poor had more to eat. It usually meant more people to feed.

Geographer Walter Mallory found no shortage of ideas for addressing China's famines in the early i92os. Civil engineers proposed controlling rivers to alleviate crop-damaging floods. Agricultural engineers suggested irrigation and land reclamation to increase cultivated acreage. Economists proposed new banking methods to encourage investment of urban capital in rural areas. Others with more overtly political agendas wanted to move people from densely populated regions to the wide-open s.p.a.ces of Mongolia. Focused on treating symptoms, few addressed the root cause of overaggressive cultivation of marginal land.

In 19206 China it took almost an acre (0.4 hectares) of land to feed a person for a year. A third of all land holdings were less than half an acre-not enough to feed a single person, let alone support a family. More than half of individual land holdings covered less than an acre and a half, a reality that kept the Chinese at perpetual risk of starvation. A bad year-failure of a single crop-brought famine. China was at the limit of its capacity to feed itself.

Obtaining food consumed 70 to 8o percent of an average family income. Even so, the typical diet consisted of two meals of rice, bread, and salt turnips. People survived from harvest to harvest.

Still, Mallory was impressed that peasant farmers maintained soil fertility despite intensive cultivation for more than four thousand years. He contrasted the longevity of Chinese agriculture with the rapid exhaustion of American soils. The key appeared to be intensive organic fertilization by returning human wastes from cities and towns to the fields. Without access to chemical fertilizers Chinese peasants fertilized the land themselves. By Mallory's time, soil nutrients had been recycled through more than forty generations of farmers and their fields.

In the i92os famine-relief administrator Y. S. Djang investigated whether people in provinces with abundant harvests ate more food than they needed. It was considered an issue of national concern that some provinces gorged while their neighbors starved.

One remarkable practice Djang found was prevalent in the province of Shao-hsing (Shaoxing), where crops were reliable and abundant. He re ported that people routinely ate more than twice the rice they could digest, stuffing themselves with as many as three "double-strength" portions of rice a day. So the region's human waste made superb fertilizer-and there was lots of it. Even after abundant harvests the population would not sell to outside buyers. Instead, these practical farmers built and maintained elegant public outhouses that served as rice-recapture facilities. They routinely ate surplus crops, reinvesting in their stock of natural capital by returning the partially digested excess to the soil.

Figure 23. Chinese farmers plowing sand (courtesy of Lu Tongjing).

Today about a third of China's total cultivated area of 130 million hectares is being seriously eroded by water or wind. Erosion rates in the Loess Plateau almost doubled in the twentieth century; the region now loses an average of more than a billion and a half tons of soil a year. Fully half of the hilly area of the Loess Plateau has lost its topsoil, even though labor-intensive terracing during the Cultural Revolution helped halve the sediment load of the Yellow River.

From the 1950s to the 1970s China lost twenty five million acres of cropland to erosion. Between 20 to 40 percent of southern China's soil has lost its A horizon, reducing soil organic matter, nitrogen, and phosphorus by up to 9o percent. Despite growing use of synthetic fertilizers, Chinese crop yields fell by more than io percent from 1999 to 2003. With China starting to run out of farmland, it is unsettling to wonder what might happen were a billion people to start squabbling with their neighbors over food. On a more optimistic note-as we ponder whether agriculture will be able to keep up with the world's population-we might take comfort in the amazing twentieth-century growth in agricultural production.

Until the widespread adoption of chemical fertilizers, growth in agricultural productivity was relatively gradual. Improvements in equipment, crop rotations, and land drainage doubled both European and Chinese crop yields between the thirteenth and nineteenth centuries. Traditional agricultural practices were abandoned as obsolete when discovery of the elements that form soil nutrients set the stage for the rise of industrial agrochemistry.

Major scientific advances fundamental to soil chemistry occurred in the late eighteenth and early nineteenth centuries. Daniel Rutherford and Antoine Lavoisier respectively discovered nitrogen and phosphorus four years before the American Revolution. Humphrey Davy discovered pota.s.sium and calcium in i8o8. Twenty years later Friederich Wohler synthesized urea from ammonia and cyanuric acid, showing it was possible to manufacture organic compounds.

Humphrey Davy endorsed the popular theory that manure helped sustain harvests because organic matter was the source of soil fertility. Then in 1840 Justus von Liebig showed that plants can grow without organic compounds. Even so, Liebig recommended building soil organic matter through manure and cultivation of legumes and gra.s.ses. But Liebig also argued that other substances with the same essential const.i.tuents could replace animal excrement. "It must be admitted as a principle of agriculture, that those substances which have been removed from a soil must be completely restored to it, and whether this restoration be effected by means of excrements, ashes, or bones, is in a great measure a matter of indifference. A time will come when fields will be manured with a solution ... prepared in chemical manufactories. "I This last idea was revolutionary.

Liebig's experiments and theories laid the foundation of modern agrochemistry. He discovered that plant growth was limited by the element in shortest supply relative to the plant's needs. He was convinced that crops could be grown continuously, without fallowing, by adding the right nutrients to the soil. Liebig's discovery opened the door to seeing the soil as a chemical warehouse through which to supply crop growth.

Inspired by Liebig, in 1843 John Bennet Lawes began comparing crop yields from fertilized and unfertilized fields on Rothamsted farm, his family's estate just north of London. An amateur chemist since boyhood, Lawes studied chemistry at Oxford but never finished a degree. Nonethe less, he experimented with agricultural chemistry while running the farm. After investigating the influence of manure and plant nutrients on crop growth, Lawes employed chemist Joseph Henry Gilbert to test whether Liebig's mineral nutrients would keep fields fertile longer than untreated fields. Within a decade it was clear that nitrogen and phosphorus could boost crop yields to match, or even exceed, those from well-manured fields.

An enterprising friend aroused Lawes's curiosity and commercial instincts by asking whether he knew of any profitable use for industrial waste consisting of a mix of animal ashes and bone. Turning waste into gold was the perfect challenge for a frustrated chemist. Natural mineral phosphates are virtually insoluble, and therefore have little immediate value as fertilizer-it takes far too long for the phosphorus to weather out and become usable by plants. But treating rock phosphate with sulfuric acid produced water-soluble phosphates immediately accessible to plants. Lawes patented his technique for making superphosphate fertilizer enriched with nitrogen and pota.s.sium and set up a factory on the Thames River in 1843. The dramatic effect of Lawes's product on crop yields meant that by the end of the century Britain was producing a million tons of superphosphate a year.

Bankrolled by substantial profits, Lawes split his time between London and Rothamsted, where he used his estate as a grand experiment to investigate how crops drew nutrition from the air, water, and soil. Lawes oversaw systematic field experiments on the effects of different fertilizers and agricultural practices on crop yields. Not only was nitrogen necessary for plant growth, but liberal additions of inorganic nitrogen-based fertilizer greatly increased harvests. He saw his work as fundamental to understanding the basis for scientific agriculture. His peers agreed, electing Lawes a fellow of the Royal Society in 1854, and awarding him a royal medal in 1867. By the end of the century, Rothamsted was the model for government-sponsored research stations spreading a new agrochemical gospel.

Now a farmer just had to mix the right chemicals into the dirt, add seeds, and stand back to watch the crops grow. Faith in the power of chemicals to catalyze plant growth replaced agricultural husbandry and made both crop rotations and the idea of adapting agricultural methods to the land seem quaint. As the agrochemical revolution overturned practices and traditions developed and refined over thousands of years, large-scale agrochemistry became conventional farming, and traditional practices became alternative farming-even as the scientific basis of agrochemistry helped explain traditional practices.

Nineteenth-century experiments showed that grazing animals process only a quarter to a third of the nitrogen in the plants they ingest. So their dung is full of nitrogen. Still, manure does not return all the nitrogen back to the soil. Without fertilizers, periodically cultivating legumes is the only way to retain soil nitrogen and still harvest crops over the long run. Native cultures around the world independently discovered this basic agricultural truth.

In 1838 Jean-Baptiste Boussingault demonstrated that legumes restored nitrogen to the soil, whereas wheat and oats could not. Here at last was the secret behind crop rotations. It took another fifty years to figure out how it worked. In 1888 a pair of German agricultural scientists, Hermann h.e.l.lriegel and Hermann Wilfarth, published a study showing that in contrast to grains, which used up the nitrogen in the soil, legumes were symbiotic with soil microbes that incorporated atmospheric nitrogen into organic matter. By the time the pair of Hermanns figured out the microbial basis for the nitrogen restoring properties of beans, peas, and clover, the agrochemical philosophy was already entrenched, spurred on by the discovery of large deposits of guano off the Peruvian coast.

Peruvians had known of the fertilizing effects of guano for centuries before the conquistadors arrived. When scientific explorer Alexander von Humboldt brought a piece collected from the Chincha Islands back to Europe in 1804 the curious white rock attracted the attention of scientists interested in agricultural chemistry. Situated off the arid coast of Peru, the Chincha Islands provided an ideal environment where huge colonies of nesting seabirds left tons of guano in a climate rainless enough to preserve it. And there was a lot-in places the Chincha guano deposits stood two hundred feet thick, a mountain of stuff better than manure. Phosphaterich guano also has up to thirty times more nitrogen than most manures.

Recognition of the fertilizing properties of guano led to a nineteenthcentury gold rush on small islands composed almost entirely of the stuff. The new system worked well-until the guano ran out. By then the widespread adoption of chemical fertilizers had shifted agricultural practices away from husbandry and nutrient cycling in favor of nutrient application.

The first commercial fertilizer imported to the United States inaugurated a new era in American agriculture when John Skinner, the editor of the American Farmer, imported two casks of Peruvian guano to Baltimore in 1824. Within two decades, regular shipments began arriving in New York. The guano business boomed. England and the United States together imported a million tons a year by the 185os. By 1870 more than half a billion dollars' worth of the white gold had been hauled off the Chincha Islands.

Figure 24. Lithograph of mountainous Chincha Islands guano deposit, circa 1868 (American Agriculturist [1868] 27:20).

As much as conservative agricultural societies scoffed at the notion that bird droppings could revive the soil, farmers who tried it swore by the results. Given the cost and difficulty of obtaining the stuff, the steady spread of guano from Maryland to Virginia and the Carolinas attests to its effect on crop yields. Widespread adoption of guano opened the door for the chemical fertilizers that followed by breaking any dependence on manure to sustain soil fertility. This transformed the basis for farming from a reliance on nutrient recycling into a one-way transfer of nutrients to consumers. From then on nothing came back to the farm.

In the end, only so much guano could be mined from South American islands. Peruvian imports peaked in 1856. By 1870 all the high-quality Chincha guano was gone. In 1881 Bolivia-now the only landlocked country with a navy-lost its Pacific coastline to Chile in a war fought over access to guano islands. Within a few years guano taxes financed the Chilean government. Demonstrated to greatly enhance harvests, guano rapidly became a strategic resource.

The government of Peru maintained tight control over its guano monopoly. American farmers frustrated over the rising price of Chincha Islands guano agitated for breaking the Peruvian monopoly. President Millard Fillmore admonished Congress in 1850 that it was the duty of the government to ensure guano traded at a reasonable price. Entrepreneurs scoured whaling records to rediscover unclaimed guano islands where the stuff could be mined freely. After President Franklin Pierce signed the 1856 Guano Island Act, making it legal for any U.S. citizens to claim any unoccupied guano island as their personal property, several dozen small tropical islands became the United States' first overseas possessions. Paving the way for later global engagements, these diminutive territories helped lead to the development of the modern chemical fertilizer industry.

Industrializing European nations that lacked phosphate deposits raced to grab guano islands. Germany annexed phosphate-rich Nauru in 1888, but lost the island after the First World War when the League of Nations placed it under British administration. In 1901 Britain annexed Ocean Island-a pile of phosphate eight and a half miles square. The Britishowned Pacific Islands Company wanted to sell the stuff to Australia and New Zealand, which lacked cheap phosphates. For an annual payment of 150 the company bought the mining rights for the whole island from a local chief with dubious authority. Too lucrative to be inconvenienced by such formalities, the Ocean Island phosphate trade reached one hundred thousand tons a year by 1905.

After the First World War the British Phosphate Commission bought the Pacific Islands Company and increased phosphate mining from Nauru sixfold. In response to the islanders' protests that stripping the island of vegetation and soil was destroying their land, the British government con fiscated the remaining lands that could be mined. Shortly thereafter deep mining operations began throughout the island. After that a million tons of phosphate left for commonwealth farms each year. Although Nauru gained independence in 1968, the phosphate deposits are mostly gone and the government is virtually bankrupt. Once a lush paradise, this island nation-the world's smallest republic-has been completely strip-mined. The few remaining islanders live on the coast surrounding the barren moonscape of the island's mined-out interior.

Ocean Island is no better off. Phosphate deposits were exhausted by i98o, leaving the inhabitants to eke out a living from land made uninhabitable to bolster the fertility of foreign soils. The island now specializes as a haven for tax shelters.

Large phosphate deposits were discovered in South Carolina on the eve of the Civil War. Within two decades South Carolina produced more than a third of a million tons of phosphate a year. Southern farmers began combining German potash with phosphoric acid and ammonia to create nitrogen, phosphorous, and pota.s.sium based fertilizer to revive cotton belt soils.

The emanc.i.p.ation of slaves spurred the rapid growth in fertilizer's use because plantation owners could not otherwise afford to cultivate their worn-out land with hired labor. Neither could they afford to have large tracts of taxable land lie idle. So most plantation owners rented out land to freed slaves or poor farmers for a share of the crop or a fixed rent. The South's new tenant farmers faced constant pressure to wrest as much as they could from their fields.

Merchants saw tenant farmers trying to work old fields as a captive market for new commercial fertilizers. They were too poor to own livestock, yet their fields would not produce substantial yields without manure. When merchants began lending small farmers the supplies needed to carry them from planting to harvest, experience quickly showed that paying off high-interest, short-term loans required liberal use of commercial fertilizers. Conveniently, bulk fertilizer could be purchased from the merchants who provided the loans in the first place.

Just before the Civil War, Mississippi's new state geologist Eugene Hilgard spent five years touring the state to inventory its natural resources. His i86o Report on the Geology and Agriculture of the State of Mississippi gave birth to modern soil science by proposing that soil was not just leftover dirt made of crumbled rocks but something shaped by its origin, history, and relationship to its environment.

Seeking out virgin soils, Hilgard soon realized that different soils had different characteristic thickness that corresponded to the depth of plant rooting. He described how soil properties changed with depth, defining topsoil and subsoil (what soil scientists now call the A and B horizons) as distinct features. Most radically, Hilgard conceived of soil as a dynamic body transformed and maintained by interacting chemical and biological processes.

Both geologist and chemist by training, Hilgard argued that the secret to fertile soil lay in retaining soil nutrients. "No land can be permanently fertile, unless we restore to it, regularly, the mineral ingredients which our crops have withdrawn." 2 Hilgard admired the Asian practice of returning human waste to the fields to maintain soil fertility by recycling nutrients. He considered America's sewers conduits draining soil fertility to the ocean. Refusing to contribute to this problem, he personally fertilized his own backyard garden.

In an address to the Mississippi Agricultural and Mechanical Fair a.s.sociation in November 1872, Hilgard spoke of how soil exhaustion shaped the fate of empires. "In an agricultural commonwealth, the fundamental requirement of continued prosperity is ... that the fertility of the soil must be maintained.... The result of the exhaustion of the soil is simply depopulation; the inhabitants seeking in emigration, or in conquest, the means of subsistence and comfort denied them by a sterile soil at home." Hilgard warned that improvident use of the soil would lead America to the same end as Rome.

Armed with better implements of tillage it takes but a short time to "tire" the soil first taken in cultivation.... If we do not use the heritage more rationally, well might the Chickasaws and the Choctaws question the moral right of the act by which their beautiful parklike hunting grounds were turned over to another race, on the plea that they did not put them to the uses for which the Creator intended them.... Under their system these lands would have lasted forever; under ours, as heretofore practiced, in less than a century more the State would be reduced to the condition of the Roman Campagna.3 Hilgard captivated the audience with his conviction and compelling delivery-until he explained that maintaining soil fertility required applying marl to acidic fields and spreading manure year after year. All that sounded like more trouble than it was worth.

Hilgard rightly dismissed the popular idea that the source of soil fertility lay in the organic compounds in the soil. Also rejecting the western European doctrine that soil fertility was based on soil's texture and its ability to absorb water, he believed that clays retained nutrients necessary for plant growth and considered reliance on chemical fertilizers a dangerous addiction that promoted soil exhaustion.

Hilgard recognized that certain plants revealed the nature of the underlying soil. Crab apple, wild plum, and cottonwoods grew well on calciumrich soil. Pines grew well on calcium-poor soil. Hired by the federal government to a.s.sess cotton production for the i88o census, he produced two volumes that divided regional soils into distinct cla.s.ses based on their physical and chemical differences. Hilgard stressed understanding the physical character of a soil, as well as its thickness and the depth to water, before judging its agricultural potential. He thought that phosphorus and pota.s.sium in minerals and nitrogen in soil organic matter controlled soil fertility. Hilgard's census report noted that aggressive fertilizer use was starting to revive agriculture in the Carolinas.

He also reported how Mississippi hill country farmers concentrated on plowing valley bottoms where upland dirt had piled up after cotton plantations stripped off the black topsoil. Great gullies surrounded empty manors amidst abandoned upland fields. Hilgard thought that a permanent agriculture required small family farms rather than large commercial plantations or tenant farmers seeking to maximize each year's profits.

With a view of the soil forged in the Deep South, Hilgard moved to Berkeley in his early forties to take a professorship at the new University of California. He arrived just as Californians began shaking off gold rush fever to worry about how to farm the Central Valley's alkali soils-salty ground unlike anything back East. Newspapers were full of accounts of crops that withered mysteriously or produced marginal yields.

The extent of alkali soils increased as irrigation spread across the golden state. Every new irrigated field raised the local groundwater table a little more. Each summer, evaporation pumped more salt up into the soil. Hilgard realized that, like a lamp's wick, clay soils brought the salt closer to the surface. Better drained, sandy soils were less susceptible to salt buildup. Hilgard also realized that alkali soils could make excellent agricultural soils-if you could just get rid of the salt.

Hilgard fought the then popular idea that salty soils resulted from seawater evaporated after Noah's flood. The ancient flood idea simply didn't hold water; the dirt was full of the wrong stuff. California's soils were rich in sodium sulfate and sodium carbonate, whereas seawater was enriched in sodium chloride. The salts in the soil were weathering out of rocks, dissolving in soil water, and then reprecipitating where the water evaporated. He reasoned that drier areas had saltier soil because rain sank into the ground and evaporated in the soil. So just as greater rainfall leached the alkali from the soil, repeated flooding could flush salts from the ground.

Collaborating with farmers eager to improve their land, Hilgard also advocated mulching to reduce evaporation of soil moisture. He experimented with using gypsum to reclaim alkali soils. On New Year's Eve 1893 the San Francisco Examiner trumpeted Hilgard's successful transformation of "alkali plains to fields of waving grain." Later that year, on August 13, the Weekly Colusa Sun went so far as to a.s.sert that Hilgard's work was worth "the whole cost of the University."

Whereas Hilgard's Mississippi work showed the importance of geology, topography, and vegetation to soil development, his California work stressed the importance of climate. In 1892 Hilgard published a landmark report that synthesized data from around the country to explain how soils formed. He explained why soils rich in calcium carbonate typical of the West were unusual in the East, and how greater temperature and moisture in the tropics leached out nutrients to produce thoroughly rotten dirt. Hilgard's report laid out the basic idea that the physical and chemical character of soils reflect the interplay of a region's climate and vegetation working to weather the underlying rocks. Soils were a dynamic interfaceliterally the skin of the earth.

Before Hilgard's synthesis, soil science was dominated by perceptions based on the humid climates of Europe and the eastern United States. Differences between soils were thought simply to reflect differences in the stuff left over from the dissolution of different rocks. By showing that climate was as important as geology, Hilgard showed that soil was worthy of study in its own right. He also championed the idea that nitrogen was the key limiting nutrient in soils based on observed variations in their carbon to nitrogen ratio and thought that crop production generally would respond greatly to nitrogen fertilization.

Now recognized as one of the founding fathers of soil science, Hilgard's ideas regarding soil formation and nitrogen hunger were ignored in agricultural colleges back East. In particular, South Carolina professor Milton Whitney championed the view that soil moisture and texture alone controlled soil fertility, maintaining that soil chemistry didn't really matter because any soil had more nutrients than required by crops. What was important was the mix of silt, sand, and clay. Based on bulk chemistry, Whitney had a point. But Hilgard knew that not everything in a soil was available to plants.

In igoi Whitney was appointed chief of the U.S. Department of Agriculture's Bureau of Soils. The new bureau launched a ma.s.sive national soil and land survey, published detailed soil survey maps for use by farmers, and exuded confidence in the nation's dirt, believing that all soils contained enough inorganic elements to grow any crop. "The soil is the one indestructible, immutable a.s.set that the Nation possesses. It is the one resource that cannot be exhausted; that cannot be used up."4 Outraged, an aging Hilgard complained about the lack of geologic and chemical information in the new bureau's surveys.

Several years before, in 1903, Whitney had published a USDA bulletin arguing that all soils contained strikingly similar nutrient solutions saturated in relatively insoluble minerals. According to Whitney, soil fertility simply depended on cultural methods used to grow food rather than the native ability of the soil to support plant growth. Soil fertility was virtually limitless. An incensed Hilgard devoted his waning years to battling the politically connected Whitney's growing influence.

A year before he published the controversial bulletin, Whitney had hired Franklin King to head a new Division of Soil Management. A graduate of Cornell University, King had been appointed in 1888 by the University of Wisconsin to be the country's first professor of agricultural physics at the age of forty. Considered the father of soil physics in the United States, King had also studied soil fertility.

King's stay in Washington was short. In his new post, King studied relations between bulk soil composition, the levels of plant nutrients in soil solutions, and crop yields. He found that the amount of nutrients in soil solutions differed from amounts suggested by total chemical a.n.a.lysis of soil samples but correlated with crop yields-conclusions at odds with those published by his new boss. Refusing to endorse King's results, Whitney forced him to resign from the bureau and return to academia where he would be less of a nuisance.

While Hilgard and Whitney feuded in academic journals, a new concept evolved of soils as ecological systems influenced by geology, chemistry, meteorology, and biology. In particular, recognition of the biological basis for nitrogen fixation helped lay the foundation for the modern concept of the soil as the frontier between geology and biology. Within a century of their discovery, nitrogen, phosphorus, and pota.s.sium were recognized to be the key elements of concern to agriculturalists. How to get enough of them was the issue.

Even though nitrogen makes up most of our atmosphere, plants can't use nitrogen bound up as stable Nz gas. In order to be used by organisms, the inert double nitrogen molecule must first be broken and the halves combined with oxygen, carbon, or hydrogen. The only living organisms capable of doing this are about a hundred genera of bacteria, those a.s.sociated with the roots of legumes being the most important. Although most crops deplete the supply of nitrogen in the soil, root nodules on clover, alfalfa, peas, and beans house bacteria that make organic compounds from atmospheric nitrogen. This process is as essential to us as it is to plants because we need to eat ten preformed amino acids we can't a.s.semble ourselves. Maintaining high nitrogen levels in agricultural soil requires rotating crops that consume nitrogen with crops that replenish nitrogen-or continually adding nitrogen fertilizers.

Phosphorus is not nearly as abundant as nitrogen, but it too is essential for plant growth. Unlike pota.s.sium, which accounts for an average of 2.5 percent of the earth's crust and occurs in rocks almost everywhere in forms readily used as natural fertilizer, phosphorus is a minor const.i.tuent of rockforming minerals. In many soils, its inaccessibility limits plant growth. Consequently, phosphorus-based fertilizers greatly enhance a crop's productivity. The only natural sources of phosphorus other than rock weathering are relatively rare deposits of guano or more common but less concentrated calcium-phosphate rock. By 19o8 the United States was the largest single producer of phosphate in the world, mining more than two and a half million tons from deposits in South Carolina, Florida, and Tennessee. Almost half of U.S. phosphate production was exported, most of it to Europe.

By the First World War serious depletion of phosphorus was apparent in American soils.

For extensive areas in the South and East the phosphorus is so deficient that there is scarcely any attempt to raise a crop without the use of phosphate compounds as fertilizers.... Western New York and Ohio, which not more than fifty or sixty years ago were regarded as the very center of the fertility of the country, are very seriously depleted in this element; and into them there is continuous importation of phosphate fertilizer.5 Early twentieth-century estimates of the amount of phosphorus lost in typical agricultural settings predicted that a century of continuous crop ping would exhaust the natural supply in midwestern soils. As phosphate became a strategic resource, calls for nationalizing phosphate deposits and prohibiting exports began to circulate in Washington.

On March 12, i9oi, the United States Industrial Commission invited Bureau of Soils chief Milton Whitney to testify about abandoned farmland in New England and the South. Whitney attributed New England's abandoned farms to the falling price of crops pouring out of the Midwest on the nation's new railroads. In his opinion, New England's farmers just could not compete with cheap wheat and cattle from out West.

Whitney told the committee that growing crops poorly suited to a region's soil or climate led to abandoned farms. He described how farms established twenty years earlier in semiarid parts of Kansas, Nebraska, and Colorado had experienced boom times for a few years, and then failed after a run of dry years. Whitney was certain that it would happen again given the region's unpredictable rainfall.

Whitney also thought that social conditions affected farm productivity. Prime farmland in southern Maryland sold for about ten dollars an acre. Similar land in Lancaster County, Pennsylvania, sold for more than ten times as much. Since Whitney believed that all soils were capable of similar productivity, he invoked social factors to explain differences in land values. Pennsylvanian farmers owned their farms and grew a diverse array of crops, including most of their own food. They sold their surplus locally. In contrast, hired overseers or tenant farmers worked Maryland's farms growing tobacco, wheat, and corn for distant markets. Whitney considered export-oriented, cash-crop monoculture responsible for impoverishing Maryland, Virginia, and the southern states in general.

Whitney saw that fertilizers could greatly increase crop yields. He considered natural fertility to be sustained by rock weathering that produced soil. Fertilizers added extra productivity. "We can unquestionably force the fertility far beyond the natural limit and far beyond the ordinary limits of crop production.... In this sense the effect of fertilization is a simple addition of plant food to the soil in such form that the crops can immediately use it."6 Whitney thought fertilizers sped the breakdown of soil minerals, accelerating soil production. Pumped up on fertilizers, the whole system could run faster.

In effect, Whitney conceived of the soil as a machine that required tuning in order to sustain high crop yields. He thought that American farmers' destructive habit of ignoring the particular type of soil in their fields reflected the fact that they didn't stay on their land very long. In i9ro more than half of America's farmers had been on their land for less than five years, not long enough to get to know their dirt.

Here was where soil scientists could help. "The soil scientist has the same relation to the partnership between the man and the soil that ... the chemist has to the steel or dye manufacturer." Whitney literally considered soil a crop factory. "Each soil type is a distinct, organized ent.i.ty-a factory, a machine-in which the parts must be kept fairly adjusted to do efficient work." 7 However, he was unimpressed with how American farmers ran the nation's dirt factories. In Whitney's view, new technologies and more intensive agrochemistry would define America's future. The Bureau of Soils chief did not realize that it would be a British idea implemented with German technology.

In 1898 the president of the British a.s.sociation, Sir William Crookes, addressed the a.s.sociation's annual meeting, choosing to focus on what he called the wheat problem-how to feed the world. Crookes foresaw the need to radically restructure fertilizer production because society could not indefinitely mine guano and phosphate deposits. He realized that higher wheat yields would require greater fertilizer inputs and that nitrogen was the key limiting nutrient. The obvious long-term solution would be to use the virtually unlimited supply of nitrogen in the atmosphere. Feeding the growing world population in the new century would require finding a way to efficiently transform atmospheric nitrogen into a form plants could use. Crookes believed that science would figure out how to bypa.s.s legumes. "England and all civilised nations stand in deadly peril of not having enough to eat.... Our wheat-producing soil is totally unequal to the strain put upon it.... It is the chemist who must come to rescue.... It is through the laboratory that starvation may ultimately be turned into plenty."8 Ironically, solving the nitrogen problem did not eliminate world hunger. Instead the human population swelled to the point where there are more hungry people alive today than ever before.

In addition to being natural fertilizers, nitrates are essential for making explosives. By the early twentieth century, industrial nations were becoming increasingly dependent on nitrates to feed their people and weapons. Britain and Germany in particular were aggressively seeking reliable sources of nitrates. Both countries had little additional cultivatable land and already imported large amounts of grain, despite relatively high crop yields from their own fields.

Vulnerable to naval blockades that could disrupt nitrate supplies, Germany devoted substantial effort toward developing new methods to capture atmospheric nitrogen. On July 2, i9o9, after years of attempting to synthesize ammonia, Fritz Haber succeeded in sustaining production of liquid ammonia for five hours in his Karlsruhe laboratory. Crookes's challenge had been met in just over a decade. Less than a century later, half the nitrogen in the world's people comes from the process that Haber pioneered.

Badische Anilin- and Sodafabrik (BASF) chemist Carl Bosch commercialized Haber's experimental process, now known as the Haber-Bosch process, with amazing rapidity. A prototype plant was operating a year later, construction of the first commercial plant began in 1912, and the first commercial ammonia flowed in September the following year. By the start of the First World War, the plant was capturing twenty metric tons of atmospheric nitrogen a day.

As feared by the German high command, the British naval blockade cut off Germany's supply of Chilean nitrates in the opening days of the war. It soon became clear that the unprecedented amounts of explosives used in the new style of trench warfare would exhaust German munitions in less than a year. The blockade also cut off BASF from its primary markets and revenue sources. Within months of the outbreak of hostilities the company's new ammonia plant was converted from producing fertilizer to nitrates for Germany's ammunition factories. By the war's end, all of BASF's production was used for munitions and together with the German war ministry the company was building a major plant deep inside Germany, safe from French air raids. In the end, however, the German military did not so much run out of ammunition as it ran out of food.

After the war, other countries adopted Germany's remarkable new way of producing nitrates. The Allies immediately recognized the strategic value of the Haber-Bosch process; theTreaty of Versailles compelled BASF to license an ammonia plant in France. In the United States, the National Defense Act provided for damming the Tennessee River at Mussel Shoals to generate cheap electricity for synthetic nitrogen plants that could manufacture either fertilizers or munitions, depending on which was in greater demand.

In the 192os German chemists modified the Haber-Bosch process to use methane as the feedstock for producing ammonia. Because Germany lacked natural gas fields, the more efficient process was not commercialized until 1929 when Sh.e.l.l Chemical Company opened a plant at Pittsburg, California to convert cheap natural gas into cheap fertilizer. The technology for making ammonia synthesis the dominant means of fixing atmospheric nitrogen arrived just in time for the industrial stagnation of the Depression.

Ammonia plant construction began again in earnest in the run-up to the Second World War. The Tennessee Valley Authority's (TVA) dams provided ideal sites for additional ammonia plants built to manufacture explosives. One plant was operating when j.a.pan bombed Pearl Harbor; ten were operating by the time Berlin fell.

After the war, governments around the world sought and fostered markets for ammonia from suddenly obsolete munitions factories. Fertilizer use in the TVA region shot up rapidly thanks to abundant supplies of cheap nitrates. American fertilizer production exploded in the 1950s when new natural gas feedstock plants in Texas, Louisiana, and Oklahoma were connected to pipelines to carry liquid ammonia north to the corn belt. Europe's bombed-out plants were rebuilt and converted to fertilizer production. Expansion of Russian ammonia production was based on central Asian and Siberian natural gas fields. Global production of ammonia more than doubled in the i96os and doubled again in the 197os. By 1998 the world's chemical industry produced more than 150 million metric tons of ammonia a year; the Haber-Bosch process supplied more than 99 percent of production. Natural gas remains the princ.i.p.al feedstock for about 8o percent of global ammonia production.

The agricultural output of industrialized countries roughly doubled in the second half of the twentieth century. Much of this newfound productivity came from increasing reliance on manufactured fertilizers. Global use of nitrogen fertilizers tripled between the Second World War and 1960, tripled again by 1970, and then doubled once more by 1980. The ready availability of cheap nitrogen led farmers to abandon traditional crop rotations and periodic fallowing in favor of continuous cultivation of row crops. For the period from 1961 to 2000, there is an almost perfect correlation between global fertilizer use and global grain production.

Soil productivity became divorced from the condition of the land as industrialized agrochemistry ramped up crop yields. The shift to largescale monoculture and increasing reliance on fertilizer segregated animal husbandry from growing crops. Armed with fertilizers, manure was no longer needed to maintain soil fertility.

Much of the increased demand for nitrogen fertilizer reflects the adoption of new high-yield strains of wheat and rice developed to feed the world's growing population. In his 1970 n.o.bel Peace Prize acceptance speech, Norman Borlaug, pioneering developer of the green revolution's high-yield rice, credited synthetic fertilizer production for the dramatic increases in crop production. "If the high-yielding dwarf wheat and rice varieties are the catalysts that have ignited the Green Revolution, then chemical fertilizer is the fuel that has powered its forward thrust."9 In 1950 high-income countries in the developed world accounted for more than 90 percent of nitrogen fertilizer consumption; by the end of the century, lowincome developing countries accounted for 66 percent.

In developing nations, colonial appropriation of the best land for export crops meant that increasingly intensive cultivation of marginal land was necessary to feed growing populations. New high-yield crop varieties increased wheat and rice yields dramatically in the i96os, but the greater yields required more intensive use of fertilizers and pesticides. Between 1961 and 1984 fertilizer use increased more than tenfold in developing countries. Well-to-do farmers prospered while many peasants could not afford to join the revolution.

The green revolution simultaneously created a lucrative global market for the chemicals on which modern agriculture depended and practically ensured that a country embarked on this path of dependency could not realistically change course. In individuals, psychologists call such behavior addiction.

Nonetheless, green revolution crops now account for more than threequarters of the rice grown in Asia. Almost half of third-world farmers use green revolution seeds, which doubled the yield per unit of nitrogen fertilizer. In combination with an expansion of the area under cultivation, the green revolution increased third-world agricultural output by more than a third by the mid-1970s. Once again, increased agricultural yields did not end hunger because population growth kept pace-this time growing well beyond what could be maintained by the natural fertility of the soil.

Between 195o and the early 1970s global grain production nearly doubled, yet per capita cereal production increased by just a third. Gains slowed after the 1970s when per capita grain production fell by more than 1o percent in Africa. By the early 198os population growth consumed grain surpluses from expanded agricultural production. In 1980 world grain reserves dropped to a forty-day supply. With less than a year's supply of grain on hand, the world still lives harvest to harvest. In developed nations, modern food distribution networks typically have little more than a few days' supply in the pipeline at any one time.