In July 1909, at the Karlsruhe Technical Institute in Germany, Fritz Haber, a 40-year-old chemist, demonstrated a tabletop apparatus to representatives from BASF, a major chemical company. The setup was deceptively simple: a steel reaction tube, a compressor to achieve roughly 200 atmospheres of pressure, a heating element to elevate the temperature to approximately 500 degrees Celsius, and a catalyst made of osmium metal. The chemistry was straightforward in notation — 3 H2 + N2 → 2 NH3 — yet notoriously difficult to actualize. Haber fed hydrogen gas and nitrogen from the air into the tube's top, and from the bottom, a modest stream of liquid ammonia emerged, about one hundred millilitres per hour. This seemingly modest breakthrough, however, was monumental, for atmospheric nitrogen, which forms 78 per cent of our air, is notoriously stable, its N2 molecules bound with a formidable triple bond that had defied fixation attempts. Haber's solution was as elegant as it was forceful: deploy enough pressure, heat, and the right catalyst, and the bond yields. Among those observing were Carl Bosch and Alwin Mittasch, who would spend the next four years and roughly 200 million reichsmarks scaling this experiment to industrial proportions. By September 1913, the first Haber-Bosch plant was operational at Oppau, allowing ammonia to be synthesized at a scale that would change the world. The advent of the First World War, just ten months later, saw ammonia's dual potential: as a precursor for both fertilisers and explosives.

Why nitrogen was scarce

Nitrogen is indispensable for life, one of the elemental quartet — along with carbon, hydrogen, and oxygen — that constitute the bulk of living organisms. Plants require nitrogen to synthesize proteins and chlorophyll, while animals obtain it through consuming plants. The nitrogen content in crops like wheat is directly proportional to what the soil provides during the growing season. Prior to the Haber-Bosch process, fixed nitrogen, crucial for plant growth, was notoriously limited. Its sources were threefold: biological fixation by bacteria in legume roots, atmospheric fixation via lightning, and mined nitrate deposits. The first, occurring naturally, yielded around 110 million tonnes per year globally. The second, an incidental result of lightning strikes, contributed a mere 10 million tonnes annually. The third, geological deposits — prominently Chilean caliche — provided about 2.5 million tonnes by the early 1900s, heavily reliant on expensive, monopoly-controlled exports. This finite supply constrained agricultural output, and population growth threatened to surpass food production capacity. William Crookes, in his 1898 presidential address, highlighted this impending crisis, forecasting that without a new nitrogen source, wheat production would soon fail to keep pace with the rising population.

Haber's breakthrough in 1909 was thus not merely a technical triumph but a critical response to a looming global food crisis. By industrializing nitrogen fixation, he addressed the bottleneck that had long restricted agricultural productivity. His method was a chemical answer to Crookes’s dire prediction, transforming what had been a scarce resource into an abundant commodity. The availability of synthetic nitrogen fertilisers promised to support a growing population, setting the stage for the agricultural revolutions that followed.

What the process does

The Haber-Bosch process, upon industrialisation, enabled the conversion of atmospheric nitrogen and hydrogen into ammonia on a massive scale. Ammonia became the cornerstone for two major industrial streams: fertilisers and explosives. As fertiliser, ammonia or its derivatives — ammonium nitrate, urea, and ammonium sulphate — are crucial in augmenting soil fertility, particularly for cereal grains like wheat, maize, and rice that constitute the staples of human nutrition. Conversely, ammonia is also vital for producing ammonium nitrate, the primary component of many explosives, as well as nitric acid, from which high explosives like TNT are derived. Thus, the same chemical pathway bolsters both agricultural yields and military arsenals.

The duality of ammonia's use is underscored by its production growth: from about 7,000 tonnes in 1913 to approximately 180 million tonnes in 2024. While the agricultural application predominates, accounting for roughly 80 per cent of ammonia usage, the balance serves as a stark reminder of its role in warfare. During the First World War, the Oppau plant’s output was swiftly redirected from fertilisers to nitrates for explosives, a pivot that extended Germany's war efforts by three years. Without Haber-Bosch, Germany would have exhausted its nitrate supplies within a year of conflict's commencement.

The agricultural revolution

The industrial production of synthetic fertilisers via the Haber-Bosch process has been fundamental to the agricultural productivity advances of the 20th century. The so-called 'green revolution' of the 1940s to 1970s, which significantly boosted crop yields, was heavily reliant on the availability of synthetic nitrogen fertilisers. High-yielding crop varieties, such as those developed by Norman Borlaug and his contemporaries, required nitrogen inputs that natural soil fertility could not provide. According to Vaclav Smil in his book 'Enriching the Earth', by 2000, Haber-Bosch nitrogen fed about 40 per cent of the world's population. Subsequent updates to this estimate have suggested that this figure is now around 50 per cent.

With a global population reaching approximately 8 billion in 2026, this implies that roughly 4 billion people depend on Haber-Bosch nitrogen for their sustenance. The process has thus become integral to modern agriculture — withdraw it, and the global food supply would halve within a few growing seasons. This is no hyperbole but rather a reflection of the fundamental shift in soil nitrogen budgets that Haber-Bosch induced. The process has allowed humanity to transcend the natural limitations of pre-industrial agriculture, reshaping the global food landscape and underpinning the dramatic population growth of the last century.

Haber's war

With the outbreak of the First World War in August 1914, Fritz Haber rapidly offered his expertise to the German military, assuming command of its chemical weapons program. By December of that year, he was developing chlorine gas as a combat agent, and in April 1915, he personally oversaw the first chlorine gas deployment at Ypres, Belgium. The attack resulted in approximately 5,000 fatalities among French and Algerian colonial troops, with overall chemical warfare deaths during the conflict reaching an estimated 90,000. Haber justified his actions by arguing that chemical weapons would expedite the war's conclusion and ultimately save lives. Yet this view starkly contrasted with that of his wife, Clara Immerwahr, who vehemently opposed his military endeavors. Ten days after the Ypres attack, she ended her own life with his service revolver, an act of profound despair and protest that coincided with a celebration of Haber's promotion.

Despite the personal tragedy, Haber continued his work unabated, receiving the 1918 Nobel Prize in Chemistry for ammonia synthesis — a decision that remains contentious due to his wartime activities. The Nobel committee’s citation notably omitted any mention of his military contributions, focusing solely on his chemical achievements. This duality in Haber's legacy — as a pioneer of life-sustaining technology and a harbinger of chemical warfare — encapsulates the complex moral dimensions of his life. He was a German nationalist, a chemist of extraordinary ability, and a man whose choices and contributions have had profound and contradictory impacts.

What came after

The subsequent chapters of Haber's life were marked by professional accomplishments and personal losses. Leading the Kaiser Wilhelm Institute for Physical Chemistry in Berlin, he maintained his position as a central figure in the scientific community. However, his attempt to extract gold from seawater to alleviate Germany's reparations burden ended in futility, showcasing the limitations of even his considerable talents. The institute became a hub for international scientific collaboration, yet Haber's legacy grew increasingly complicated with the rise of the Nazi regime.

In 1933, as antisemitic policies took root, Haber, despite his conversion to Lutheranism and his patriotic service, was forced to dismiss Jewish colleagues. He refused and resigned, leaving Germany permanently. He passed away in 1934 en route to an academic appointment in Cambridge. His son Hermann, who had moved to the United States, took his own life in 1946. The industrial legacy of Haber's work endured, as the technology and knowledge he developed were later repurposed for horrific ends in Nazi concentration camps. The chemical pathways pioneered by Haber, albeit adapted posthumously, became a tool of genocide, further entrenching the complex moral narrative of his contributions.

The honest accounting

By 2026, the total impact of the Haber-Bosch process is staggering. It has facilitated a population increase from 1.8 billion in 1900 to 8 billion today, by underpinning the food supply with synthetic nitrogen fertilisers. It has also supplied the ammonia for the production of about 90 per cent of all military explosives and propellants since 1915. The dark side includes ammonium nitrate's role in approximately 40 per cent of modern terrorist bombings, the creation of dead zones in coastal waters from nitrogen runoff, and its contribution to greenhouse gas emissions from fertiliser manufacture, which accounts for a significant share of global emissions.

The arithmetic of Haber-Bosch’s legacy is stark: lives sustained by nitrogen fertilisers surpass those lost through its weapons applications by orders of magnitude. Yet, both the benefits and the detriments are undeniable. A thorough evaluation of Haber-Bosch necessitates an appreciation of its full history, its dual-purpose nature, and the ethical complexities it embodies. It stands as a testament to the ambivalence of technological progress and the profound impact of scientific innovation.

Today, about 600 ammonia plants operate worldwide, spread across 80 countries, with the largest in China, India, Russia, and the United States. Each plant traces its lineage to the Oppau facility of 1913. Despite improvements in efficiency and reactor design, the core chemistry remains that which Fritz Haber demonstrated at Karlsruhe. These plants sustain an estimated 4 billion people, while indirectly supplying explosives responsible for the deaths of millions. The fertilisers they produce also contribute to significant environmental challenges. Haber's grave in Basel, alongside Clara's, bears an inscription that captures the ambiguity of their legacy: 'In war and peace, while there is life, in the service of mankind.' It is a fitting epitaph for a life and work that defy simple judgments.