The Nitrogen Problem
Plants need nitrogen to build proteins and chlorophyll. The air is about four-fifths nitrogen gas, yet plants cannot use it in that form, because the two atoms in each nitrogen molecule are held together by a very strong triple bond. Plants take up nitrogen as compounds such as ammonium and nitrate. In nature, certain bacteria, many living in the root nodules of legumes such as clover, peas and beans, convert atmospheric nitrogen into usable forms, a process called nitrogen fixation. Lightning also fixes a small amount.
For most of history, farmers dealt with this by rotating crops, planting legumes, and spreading manure, compost and other organic waste. These methods work, but they limit how much food an acre can produce, and they depend on having animals or land to spare.
Looking for New Sources
In the 19th century, the growing populations of Europe and North America increased demand for more fertilizer. Imported guano, the accumulated droppings of seabirds from islands along the Peruvian coast, became a major trade item, as did sodium nitrate mined in Chile. Both were finite, and guano deposits in particular were depleted within a few decades. Chemists and public figures began to warn that the world might hit a nitrogen limit on food. In 1898 the British scientist William Crookes gave a well-known speech arguing that new sources of fixed nitrogen would be needed to feed growing wheat-eating populations.
Some inventors tried using electric arcs to combine nitrogen and oxygen from air. A Norwegian process developed in the early 1900s did work, but it needed very large amounts of cheap hydroelectric power, so it was not widely adopted.
Haber and Bosch
The solution that eventually spread combined nitrogen from the air with hydrogen to make ammonia. The reaction is easy to write down but difficult to carry out: the molecules react slowly, and the process needs high pressure, elevated temperature and a catalyst.
Fritz Haber, a German chemist working at Karlsruhe, demonstrated a laboratory ammonia synthesis in 1909, using high pressure and an osmium-based catalyst to produce small but measurable amounts. BASF, a German chemical company, backed the work. Carl Bosch, an engineer at BASF, led the effort to scale it up. That meant building reactors and pipes that could withstand hydrogen under very high pressure, which attacks steel, and finding a cheaper, more durable catalyst. Alwin Mittasch and colleagues tested thousands of materials and arrived at iron-based catalysts that are still the basis of the method. The first large plant, at Oppau in Germany, began production in 1913.
Haber received the Nobel Prize in Chemistry in 1918 and Bosch shared one in 1931. The name Haber-Bosch honors both men, and the process is an early and important example of industrial chemistry that combined laboratory chemistry with heavy engineering.
From Ammonia to Fertilizer
Ammonia can be applied directly as a gas stored under pressure, but more often it is converted into solid or liquid products such as urea, ammonium nitrate and ammonium sulfate, which are easier to handle. The hydrogen in the process originally came from coal and now usually comes from natural gas, with the carbon ending up as carbon dioxide. This makes fertilizer production energy-intensive, and the climate effects are a continuing concern.
Synthetic nitrogen was not adopted instantly everywhere. Germany used its output heavily for munitions in World War I, since nitrates are also needed for explosives. Fertilizer use expanded in the 1920s and 1930s, and then dramatically after World War II, when wartime ammonia plants were converted to peacetime production. The “Green Revolution” in the 1960s and later combined high-yielding, short-strawed crop varieties with fertilizer and irrigation, and these varieties were bred to respond to added nitrogen without falling over.
Place in the Food Supply
A widely cited estimate from the researcher Vaclav Smil is that roughly half of the nitrogen in the bodies of people alive today came from the Haber-Bosch process. Estimates of this kind involve assumptions, and the exact figure varies by method and time, but the broad point is accepted: synthetic nitrogen supports a large share of world food production and, by extension, population.
Fertilizer works together with other elements of modern farming, such as improved seeds, irrigation, pest control and farm machinery. It is difficult to credit any one factor alone.
Costs and Limits
The downsides are significant, and they are a major reason the topic remains debated:
- Water pollution. Nitrogen not taken up by crops can run off into rivers and coastal waters, where it feeds algal blooms that deplete oxygen, creating “dead zones” in places such as the Gulf of Mexico.
- Drinking water. High nitrate levels in groundwater are a health concern, especially for infants.
- Greenhouse gases. Production releases carbon dioxide, and soils treated with nitrogen can emit nitrous oxide, a potent greenhouse gas.
- Inefficiency. In many regions a considerable portion of applied nitrogen is lost rather than used by the crop, while some regions with poor access to fertilizer have low yields.
Researchers and farmers are looking at ways to use nitrogen more precisely, grow more legumes, improve manure handling and develop cleaner ways to make hydrogen and ammonia. Haber himself also led German chemical weapons development in World War I, a reminder that the same chemical science can serve very different ends.
The Haber-Bosch process is a case where one industrial invention changed the basic limits of agriculture, with benefits and costs that are both large and still being managed.