The air above you is four-fifths nitrogen — but the atoms are bonded so tightly that turning them into fertiliser took a century of chemistry, and still accounts for roughly 2% of global energy use.
You almost certainly ate some of its work today.
The journey
Ammonia is made by tearing nitrogen out of the air and bolting it onto hydrogen, at pressures many times greater than a car tyre and temperatures hot enough to make steel glow red. The nitrogen is free and everywhere; the hard part is that its two atoms cling together so fiercely that only extreme conditions and a metal catalyst will persuade them to let go and grab hydrogen instead.
Two cheap feedstocks: natural gas (methane) for the hydrogen, and ordinary air for the nitrogen — air is 78% nitrogen (Royal Society).
Ammonia is built from just nitrogen and hydrogen — but where those come from decides everything about its cost and its carbon.
Nitrogen and hydrogen are the only two elements in ammonia (NH₃) — by weight it's 82% nitrogen, 18% hydrogen — yet almost the entire economic and environmental story lives in the third, invisible ingredient: the energy to get the hydrogen.
Free, inexhaustible, 78% of every breath. In the usual gas route it's carried in with a blast of air during reforming — the oxygen burns off, the nitrogen stays; coal plants use a dedicated air-separation unit instead. Either way, separating nitrogen from air is relatively easy (Royal Society). The easy ingredient.
Almost all of it stripped from methane by steam reforming — actually several reactors and purification stages in series — where ~90% of the carbon and most of the energy go (Royal Society). Ammonia is the single largest use of hydrogen on earth — about 33 of the world's ~94 Mt of hydrogen a year (Saygin). The hard, dirty ingredient — and the one everyone is trying to change. See the challenge →
Making a tonne at best practice takes about 8 MWh (28 GJ; Royal Society), but the global-average plant uses about 11 MWh (41 GJ; IEA) — roughly a typical UK household's total energy for a year.
Nitrogen from the air is almost unreactive: its two atoms are triple-bonded, one of the strongest chemical bonds found in nature. Haber–Bosch beats that bond into submission with pressure and heat — a triumph of engineering, not just chemistry.
The problem. Air isn't already fertiliser because N₂ is held by a triple bond that plants and most chemistry can't break. For centuries, farming depended on the nitrogen that lightning and soil bacteria fixed naturally.
The solution. To break that bond at scale you need three things together — high pressure, high temperature, and a catalyst. The reactor runs at 150–300 bar and 350–500 °C over an iron catalyst (Royal Society). Only then does nitrogen let go and pair with hydrogen.
The consequence. Even under that assault the reaction is stubborn and reversible — only a fraction converts each pass — so the unreacted gas is recycled round the loop again and again. That recycling, and the pressure, are why ammonia plants are among the most energy-hungry factories on earth: about 8 MWh per tonne at best practice, about 11 MWh on the global average (Royal Society; IEA).
The famous reaction isn't the dirty part. Cleaning up ammonia means cleaning up hydrogen. Source: Royal Society, 2020.
Most ammonia becomes fertiliser. The rest quietly runs through explosives, plastics, refrigeration and cleaning — most people meet ammonia's products daily without ever seeing ammonia.
Roughly 70–80% of ammonia becomes nitrogen fertiliser — sources draw the line differently, from about 70% (IEA) up to about 80% (Royal Society; IRENA; IFA). Of that fertiliser, about 55% becomes urea (Hydrogen Europe, citing IEA). The remainder spreads across industry: explosives (ammonium nitrate), nitric acid via the Ostwald process, plastics and synthetic fibres, refrigeration, and the ammonium sulphate that ties it to the sulphuric-acid story.
The gold-outlined slice marks the urea share within fertiliser — not a separate quantity. Shown as a band because sources disagree: IEA ~70/30, Royal Society / IRENA ~80/20 (in the US, USGS puts fertiliser nearer 88%). Share of ammonia demand by use — IEA / Royal Society, 2020–2023.
Ammonia is one of the most-produced chemicals on earth — second only to sulphuric acid — and one of the dirtiest to make: a paradox at the centre of the food system.
The carbon twist: China makes ~30% of the world's ammonia but roughly 45% of its CO₂ — because most Chinese ammonia is made from coal, the dirtiest route. The country that feeds a third of the world's crops also runs the dirtiest ammonia. Bars scaled to the largest share; figures give the true percentages. Share of world production — USGS MCS 2026; China share corroborated by IEA (30%, 2020).
Ammonia's problem isn't the famous reaction — it's the hydrogen feeding it. Change where the hydrogen comes from and you can decarbonise nearly the whole industry; but the clean routes still cost more than natural gas, and that gap is the whole fight.
The "colours" of ammonia are really a story about the hydrogen (the Royal Society's framing). Same molecule, same Haber–Bosch reactor — what changes is where the hydrogen is torn from, and what that costs in carbon.
Carbon falls a long way either route; the price is the fight. And it's still a small fight: as of 2025 only about 6 Mt of low-emissions ammonia is operating or firmly committed (plus ~13 Mt earlier in the pipeline) against more than 190 Mt made in total — clean ammonia is real, but barely started. Figures: Shin, Zang et al. (2026), across 63 countries; pipeline from IEA Breakthrough Agenda (2025).
Why anyone cares beyond fertiliser. Because ammonia is already shipped and stored worldwide, and packs energy at ~3 kWh/litre without hydrogen's extreme cold or pressure, it's eyed as a way to move clean energy itself — a carbon-free shipping fuel, and a carrier that lets sunny, windy regions export energy as liquid (Royal Society; Saygin). The infrastructure already exists: the US alone has about 4,830 km of ammonia pipelines and more than 10,000 storage sites (Royal Society), and ~10% of all ammonia is already traded internationally (IEA). The IEA and IRENA expect demand from new energy uses to grow ammonia roughly 3–4×, to 560–665 Mt by 2050 (Saygin/IRENA). The catch that keeps the story honest: burnt or leaked carelessly, ammonia forms nitrogen oxides and harms ecosystems — so any clean-energy future depends on handling that releases only harmless nitrogen gas (Royal Society).
The same molecule that lets the planet feed itself may help it decarbonise — if the clean hydrogen it needs can ever be made as cheaply as the dirty kind.