Global Population and the Nitrogen Cycle

Part I 

During the 20th century, humanity has almost quadrupled its numbers. Although many factors have fostered this unprecedented expansion, its continuation during the past generation would not have been at all possible without a widespread—yet generally unappreciated— activity: the synthesis of ammonia. The ready availability of ammonia, and other nitrogen-rich fertilizers derived from it, has effectively done away with what for ages had been a fundamental restriction on food production. The world’s population now has enough to eat (on the average) because of numerous advances in modern agricultural practices. But human society has one key chemical industry to thank for that abundance— the producers of nitrogen fertilizer. Why is nitrogen so important? Compared with carbon, hydrogen and oxygen, nitrogen is only a minor constituent of living matter. But whereas the three major elements can move readily from their huge natural reservoirs through the food and water people consume to become a part of their tissues, nitrogen remains largely locked in the atmosphere. Only a puny fraction of this resource exists in a form that can be absorbed by growing plants, animals and, ultimately, human beings. Yet nitrogen is of decisive importance. This element is needed for DNA and RNA, the molecules that store and transfer genetic information. It is also required to make proteins, those indispensable messengers, receptors, catalysts and structural components of all plant and animal cells. Humans, like other higher animals, cannot synthesize these molecules using the nitrogen found in the air and have to acquire nitrogen compounds from food. There is no substitute for this intake, because a minimum quantity (consumed as animal or plant protein) is needed for proper nutrition. Yet getting nitrogen from the atmosphere to crops is not an easy matter. The relative scarcity of usable nitrogen can be blamed on that element’s peculiar chemistry. Paired nitrogen atoms make up 78 percent of the atmosphere, but they are too stable to transform easily into a reactive form that plants can take up. Lightning can cleave these strongly bonded molecules; however, most natural nitrogen “fixation” (the splitting of paired nitrogen molecules and subsequent incorporation of the element into the chemically reactive compound ammonia) is done by certain bacteria. The most important nitrogen fixing bacteria are of the genus Rhizobium, symbionts that create nodules on the roots of leguminous plants, such as beans or acacia trees. To a lesser extent, cyanobacteria (living either freely or in association with certain plants) also fix nitrogen.

 

A Long-standing Problem

Because withdrawals caused by the growth of crops and various natural losses continually remove fixed nitrogen from the soil, that element is regularly in short supply. Traditional farmers (those in pre-industrial societies) typically replaced the nitrogen lost or taken up in their harvests by enriching their fields with crop residues or with animal and human wastes. But these materials contain low concentrations of nitrogen, and so farmers had to apply massive amounts to provide a sufficient quantity. Traditional farmers also raised peas, beans, lentils and other pulses along with cereals and some additional crops. The nitrogen-fixing bacteria living in the roots of these plants helped to enrich the fields with nitrogen. In some cases, farmers grew legumes (or, in Asia, Azolla ferns, which harbor nitrogen-fixing cyanobacteria) strictly for the fertilization provided. They then plowed these crops into the soil as so-called green manures without harvesting food from them at all. Organic farming of this kind during the early part of the 20th century was most intense in the lowlands of Java, across the Nile Delta, in northwestern Europe (particularly on Dutch farms) and in many regions of Japan and China. The combination of recycling human and animal wastes along with planting green manures can, in principle, provide annually up to around 200 kilograms of nitrogen per hectare of arable land. The resulting 200 to 250 kilograms of plant protein that can be produced in this way sets the theoretical limit on population density: a hectare of farmland in places with good soil, adequate moisture and a mild climate that allows continuous cultivation throughout the year should be able to support as many as 15 people. In practice, however, the population densities for nations dependent on organic farming were invariably much lower. China’s average was between five and six people per hectare of arable area during the early part of this century. During the last decades of purely organic farming in Japan (which occurred about the same time), the population density there was slightly higher than in China, but the Japanese reliance on fish protein from the sea complicates the comparison between these two nations. A population density of about five people per hectare was also typical for fertile farming regions in northwestern Europe during the 19th century, when those farmers still relied entirely on traditional methods. The practical limit of about five people per hectare of farmland arose for many reasons, including environmental stresses (caused above all by severe weather and pests) and the need to raise crops that  were not used for food—those that provided medicines or fibers, for example. The essential difficulty came from the closed nitrogen cycle. Traditional farming faced a fundamental problem that was especially acute in land scarce countries with no uncultivated areas available for grazing or for the expansion of agriculture. In such places, the only way for farmers to break the constraints of the local nitrogen cycle and increase harvests was by planting more green manures. That strategy preempted the cultivation of a food crop. Rotation of staple cereals with leguminous food grains was thus a more fitting choice. Yet even this practice, so common in traditional farming, had its limits. Legumes have lower yields, they are often difficult to digest, and they cannot be made easily into bread or noodles. Consequently, few crops grown using the age-old methods ever had an adequate supply of nitrogen.

 

A Fertile Place for Science

As their knowledge of chemistry expanded, 19th-century scientists began to understand the critical role of nitrogen in food production and the scarcity of its usable forms. They learned that the other two key nutrients—potassium and phosphorus—were limiting agricultural yields much less frequently and that any shortages of these two elements were also much easier to rectify. It was a straightforward matter to mine potash deposits for potassium fertilizer, and phosphorus enrichment required only that acid be added to phosphate rich rocks to convert them into more soluble compounds that would be taken up when the roots absorbed water. No comparably simple procedures were available for nitrogen, and by the late 1890s there were feelings of urgency and unease among the agronomists and chemists who were aware that increasingly intensive farming faced a looming nitrogen crisis. As a result, technologists of the era made several attempts to break through the nitrogen barrier. The use of soluble inorganic nitrates (from rock deposits found in Chilean deserts) and organic guano (from the excrement left by birds on Peru’s rainless Chincha Islands) provided a temporary reprieve for some farmers. Recovery of ammonium sulfate from ovens used to transform coal to metallurgical coke also made a short-lived contribution to agricultural nitrogen supplies. This cyan amide process— whereby coke reacts with lime and pure nitrogen to produce a compound that contains calcium, carbon and nitrogen— was commercialized in Germany in 1898, but its energy requirements were too high to be practical. Producing nitrogen oxides by blowing the mixture of the two elements through an electric spark demanded extraordinary energy as well. Only Norway, with its cheap hydroelectricity, started making nitrogen fertilizer with this process in 1903, but total output remained small. The real breakthrough came with the invention of ammonia synthesis. Carl Bosch began the development of this process in 1899 at BASF, Germany’s leading chemical concern. But it was Fritz Haber, from the technical university in Karlsruhe, Germany, who devised a workable scheme to synthesize ammonia from nitrogen and hydrogen. He combined these gases at a pressure of 200 atmospheres and a temperature of 500 degrees Celsius in the presence of solid osmium and uranium catalysts. Haber’s approach worked well, but converting this bench reaction to an engineering reality was an immense undertaking. Bosch eventually solved the greatest design problem: the deterioration of the interior of the steel reaction chamber at high temperatures and pressures. His work led directly to the first commercial ammonia factory in Oppau, Germany, in 1913. Its design capacity was soon doubled to 60,000 tons a year—enough to make Germany self-sufficient in the nitrogen compounds it used for the production of explosives during World War I. Commercialization of the Haber- Bosch synthesis process was slowed by the economic difficulties that prevailed between wars, and global ammonia production remained below five million tons until the late 1940s. During the 1950s, the use of nitrogen fertilizer gradually rose to 10 million tons; then technical innovations introduced during the 1960s cut the use of electricity in the synthesis by more than 90 percent and led to larger, more economical facilities for the production of ammonia. The subsequent exponential growth in demand increased global production of this compound eightfold by the late 1980s. This surge was accompanied by a relatively rapid shift in nitrogen use between high- and low-income countries.

to be continue..................................