Where the Nitrogen Needed by Fields Came From
Every grain harvest removes some nitrogen-containing nutrients from farmland. Farmers replenished the soil with manure, straw, and green manure and incorporated legumes into crop rotations. Symbiosis between legumes and nitrogen-fixing microorganisms introduces new nitrogen, while recycling organic matter returns existing nitrogen to the fields. As populations and urban demand for food grew, the nutrients supplied by this cycle increasingly limited production gains.
In the nineteenth century, guano, Chilean nitrate, and ammonium salts recovered as by-products of coke production entered the fertilizer market. Farmland began to depend on mineral deposits, ocean shipping, and industrial production. Although air contains abundant nitrogen gas, ordinary crops cannot use it directly: the strong chemical bond in the nitrogen molecule makes it difficult to react. Converting atmospheric nitrogen into usable compounds promised agriculture a nutrient source whose production could be expanded.
Haber Put the Reaction into a Circulating System
In Karlsruhe, Fritz Haber studied the chemical equilibrium between nitrogen, hydrogen, and ammonia. Higher pressure favors ammonia formation, and lower temperatures improve equilibrium yield but slow the reaction. Working with Robert Le Rossignol, he improved high-pressure experimental equipment and sought catalysts that could accelerate the reaction at lower temperatures, finding useful activity in osmium and uranium. This required repeated experiments balancing reaction speed, yield, and equipment demands: experimental pressures reached about two hundred atmospheres, with temperatures of several hundred degrees Celsius. Catalysts enabled the gases to approach equilibrium more quickly, while cooling and recirculation turned the reaction's output into a continuous product stream.
By 1909, the team demonstrated a continuous ammonia-production system to BASF. Nitrogen and hydrogen passed over a heated catalyst; the ammonia formed was separated by cooling, and unreacted gas returned to the reaction zone. Recycling allowed gas that was only partly converted on each pass to be used repeatedly. The bench-scale apparatus thus demonstrated a production route. To run it reliably every day, a factory still needed inexpensive feed gases, a durable catalyst, and equipment able to withstand heat and pressure.
Solving Separate Problems of Catalysts and Pressure Vessels
BASF entrusted the scale-up to Carl Bosch. Osmium's scarcity made the experimental catalyst unsuitable for a large plant, so Alwin Mittasch led a team through many thousands of screening tests to develop an iron-based catalyst containing promoters. These additives improved activity and maintained an effective structure. Catalyst formulation and preparation therefore became an industrial technology in their own right; the laboratory process could not be reproduced merely by selecting a metal with the same name.
Another obstacle faced by Bosch lay inside the steel tubes. Under high temperatures and pressures, hydrogen penetrated carbon-containing steel, causing decarburization and cracking, so experimental tubes gradually lost their load-bearing capacity. He used metallurgical analysis to trace the damage and separated functions between a soft-iron lining and a pressure-bearing steel shell, providing routes for hydrogen to escape. Meanwhile, engineers redesigned compressors, valves, seals, and measuring equipment and purified the feed gases to remove impurities that poisoned the catalyst. Reactor reliability depended on all these supporting elements meeting the necessary standards together.
Oppau Joined the Experiments into a Factory
On September 9, 1913, BASF's plant at Oppau on the Rhine produced synthetic ammonia. Gas generation, purification, compression, catalytic reaction, and product recovery were linked in one production process. Coal and coke helped provide hydrogen and energy, while air supplied nitrogen. The ammonia was further processed into ammonium sulfate, a fertilizer that could be stored, shipped, and sold. Gas pipelines linked multiple workshops within the plant, and ammonia output depended on the entire process. Inadequate feed-gas supply or increased impurities upstream could affect fertilizer deliveries downstream even when the synthesis reactor itself remained intact.
Moving from a small apparatus to a factory also changed the division of labor. Specialists handled gas separation, purification, coal gasification, and fertilizer production. Operators monitored pressure, temperature, and gas flow, while maintenance workers dealt with leaks and corrosion. Smil records that even ammonium sulfate production, already an established industrial process, suffered ammonia leaks and acid overflows during the early operation. Producing ammonia on the first day was only a beginning; continuous production required reducing these losses one by one.
The Same Nitrogen Stream Fed Fields and Battlefields
After the First World War began, Germany's access to imported nitrate was obstructed, and the military value of synthetic ammonia rapidly increased. Ammonia could be oxidized to nitric acid, supplying nitrate and explosives production. Bosch helped advance this conversion, and industrial nitrogen fixation became important to sustaining German munitions output. Equipment originally intended for agricultural markets was drawn into the war through government procurement and wartime expansion.
Thereafter, the nitrogen-fertilizer industry supported rising food production alongside plant breeding, irrigation, and agricultural machinery. Whether farmers could obtain fertilizer, land, and water, and whether food could be transported and sold to those who needed it, still determined how output became adequate nourishment. Meanwhile, nitrogen not absorbed by crops could enter groundwater, rivers, and lakes, promoting eutrophication, or reach the atmosphere as ammonia, nitrous oxide, and other compounds. The productive capacity inaugurated at Oppau expanded food supplies while making energy conservation and reduced nutrient loss enduring agricultural challenges.