Haber-Bosch Process
The reaction that pulls fertilizer out of thin air, feeds about half the people alive, and kept German artillery firing through the First World War.
About half the nitrogen atoms in your body got there through a steel pressure vessel. The Haber-Bosch process takes nitrogen out of the air, combines it with hydrogen under enormous pressure and heat, and produces ammonia, which is the raw material of synthetic fertilizer. It is the main reason world population went from 1.6 billion in 1900 to 8 billion, and it is also why Germany could keep firing artillery shells after the Royal Navy cut off its imports of Chilean nitrates in 1914. It is probably the most geopolitically consequential chemistry ever done.
The problem it solved is that plants need nitrogen in a usable form and cannot take it from the air, even though air is 78 percent nitrogen. Until the twentieth century farmers got it from manure, from rotating crops with legumes, and from imported guano and Chilean saltpeter. In 1898 the chemist William Crookes told the British Association that the world would run short of wheat within a generation unless somebody learned to fix nitrogen from the atmosphere.
Two men and a pressure vessel
Fritz Haber, working at Karlsruhe, demonstrated the reaction on a laboratory bench in 1909, running nitrogen and hydrogen over a catalyst at roughly 200 atmospheres and 500 degrees Celsius. It produced a few cubic centimeters of ammonia an hour. That was enough to prove the point.
Making it industrial was Carl Bosch's job and it was the harder half. Nothing then in existence could hold those pressures at those temperatures. Hydrogen under pressure attacks steel from the inside and makes it brittle, and Bosch's early vessels burst. He solved it with a soft iron liner inside a wound steel jacket. His colleague Alwin Mittasch tested thousands of candidate materials before settling on a cheap iron-based catalyst to replace Haber's osmium. BASF opened the first full-scale plant at Oppau in September 1913. Haber received the Nobel Prize in chemistry for 1918, Bosch in 1931.
The other half of the story
The First World War began a year after Oppau opened. Artillery ammunition needs nitric acid, and nitric acid came from the same Chilean nitrates Germany could no longer import once the British blockade closed the sea lanes. Germany had a matter of months of stock. Ammonia from Haber-Bosch could be converted into nitric acid instead, and BASF expanded enormously on state contracts. Without it Germany would have run out of shells in 1915. The process that went on to feed the world first extended a war that killed millions.
Haber went further than that. He ran Germany's chemical weapons program and personally supervised the first mass chlorine attack at Ypres in April 1915. His wife Clara Immerwahr, a chemist herself, shot herself with his service pistol days after he returned from it. Haber was Jewish. In 1933 the state he had served pushed him out, and he died in Basel the following year. The institute he built developed the pesticide later manufactured as Zyklon, a variant of which was used at Auschwitz, where members of his extended family were killed.
Now
Ammonia production consumes 1 to 2 percent of the world's energy, nearly all of it from natural gas, and produces a similar share of global carbon dioxide. Fertilizer prices therefore track gas prices, which is why European ammonia plants shut down in 2022 when gas spiked, and why fertilizer was quietly exempted from Western sanctions on Russia and Belarus, two of the largest exporters. A country that cannot buy fertilizer has a food crisis one growing season later.
The runoff is a separate problem. Nitrogen that crops do not absorb washes into rivers and creates oxygen-dead zones at their mouths, and some leaves fields as nitrous oxide, a greenhouse gas far more potent than carbon dioxide.
Nobody is going to stop. Roughly half the world eats because of this reaction. The realistic ambition is to make the hydrogen from electricity and water instead of from methane, which is what green ammonia means. The chemistry itself does not change. It was settled in 1909.