Bread From Air

Nitrogen is 78 per cent of the air. It is also the thing plants run out of first, and the reason is a small piece of chemistry: the two nitrogen atoms in a molecule of air are held together by a triple bond, one of the strongest in nature, and almost nothing alive can break it.

A few bacteria can, slowly. Lightning can, occasionally. For the whole of human history before the twentieth century, that was the entire supply. Every field was limited by how much nitrogen the bacteria in it and the manure on it could provide, and every population was limited by the fields.

Then two Germans worked out how to break the bond in a factory, and the German phrase for what they had done was Brot aus Luft. Bread from air.

What the reaction is

Fritz Haber demonstrated it at bench scale in 1909. Take nitrogen from the air and hydrogen from somewhere, force them together over a catalyst at high pressure and high temperature, and they combine into ammonia.

The chemistry was the smaller half of the problem. The conditions needed are brutal, several hundred atmospheres and several hundred degrees, and in 1909 there was no vessel in the world that would hold that without failing. Carl Bosch at BASF spent the next four years solving it, which meant inventing the high pressure steel vessel, the catalyst that would survive continuous use, and the plant to run the whole thing at scale. Both men later received Nobel prizes, Haber in 1918 and Bosch in 1931.

Nothing about the process is subtle. It is force applied to a molecule until it gives way.

Half of you

The consequence is easier to state than to absorb.

Around half the nitrogen atoms in a human body today were fixed by this reaction rather than by anything living. The protein in your muscles was assembled from amino acids that came from plants or from animals that ate plants, and the nitrogen in those plants was put there by a factory.

Estimates vary by method, and the number moves depending on where you live and what you eat, but the order of it is not disputed: without synthetic nitrogen the world could not produce roughly half of the food it currently produces. The population that food supports is the population that exists.

That is a strange thing to be true and almost nobody knows it, including most people who have strong views about agriculture.

What it costs to run

Making ammonia is one of the largest industrial energy consumers on earth.

The International Energy Agency's roadmap on the industry puts it at about 2 per cent of total final energy consumption worldwide, and 1.3 per cent of the carbon dioxide emissions from the entire energy system. In absolute terms that is around 450 million tonnes of carbon dioxide a year from this one reaction.

About 70 per cent of the ammonia goes into fertiliser. The rest becomes explosives, plastics, refrigerants and industrial chemicals.

The hydrogen is the problem. Hydrogen does not occur usefully on its own, so it is stripped out of something else, and overwhelmingly that something is natural gas. Roughly a fifth of world production instead uses coal, which produces about twice the emissions per tonne. Either way the carbon comes off the feedstock, not the fuel, which is why the emissions are so hard to remove by making the plant more efficient. They are not a side effect of the heat. They are the other half of the reaction.

What it costs India

This is not an abstraction here.

India is the world's largest consumer of urea, the solid form in which most nitrogen fertiliser is applied. Annual consumption runs at something like 35 million tonnes, and about 30 per cent of that has been imported, though domestic capacity has been rising.

The price a farmer pays is fixed by the government. A 45 kilogram bag of urea costs 242 rupees, and has been held near that level for years while the cost of making it moved with the price of gas. The gap is the subsidy, and it is enormous. The 2025-26 budget put fertiliser subsidy at about 1.68 lakh crore rupees, of which roughly 1.19 lakh crore was urea alone, and the revised estimate raised the total to about 1.86 lakh crore.

That is one of the largest single lines in the Union budget, and it exists because the price of bread from air is set in gas markets on the other side of the world while the price of a bag of urea in a village is set in Delhi.

It also has an agronomic cost. Because urea is cheap and other nutrients are not, farmers apply too much nitrogen relative to phosphorus and potassium, which is bad for soil and for the rivers the excess runs into. The subsidy is not only expensive, it distorts what goes on the field.

The two things it is

Haber's own life is the reason this story is usually told with a shudder, and it should be told honestly. The same man who worked out how to feed the world also directed Germany's chemical weapons programme in the First World War and was personally present at the first large scale chlorine attack. His wife, Clara Immerwahr, herself a chemist, killed herself days afterwards. He received the Nobel prize for the ammonia work while some of the committee's peers were arguing he should be tried instead.

The usual moral drawn from that is about the ambiguity of science. The more useful observation is narrower. Both things came out of the same capability. Fixing nitrogen gives you fertiliser and it gives you explosives, because the chemistry of a nitrate that feeds a plant and a nitrate that detonates is closely related, and Germany's immediate strategic reason for wanting the process in 1914 was that a naval blockade had cut it off from imported nitrate for munitions.

The plants that were built to keep an army supplied are the ancestors of the plants that now keep several billion people fed.

What replaces it

Nothing, for now, at the scale required.

The routes being worked on are the same reaction with cleaner hydrogen: capture the carbon dioxide at the plant and store it, or make the hydrogen by splitting water with electricity from something that does not burn. Both work technically. Both are more expensive than taking hydrogen out of cheap gas, and the industry's product is sold to farmers at a price a government has fixed.

Which is the real difficulty, and it is not chemical. Every serious decarbonisation path for ammonia raises the cost of a bag of urea, in a country where the political settlement of the last fifty years has been that the bag does not get more expensive.

Two per cent of the world's energy, four hundred and fifty million tonnes of carbon dioxide, and half the nitrogen in your body, all resting on a subsidy line item.

Sources

• International Energy Agency, Ammonia Technology Roadmap, as reported by the Ammonia Energy Association, "IEA publishes Ammonia Technology Roadmap" (ammonia production accounting for around 2 per cent of total final energy consumption and 1.3 per cent of carbon dioxide emissions from the energy system; around 450 million tonnes of carbon dioxide emitted annually; approximately 70 per cent of ammonia going to fertiliser; about 22 per cent of production relying on coal, which generates roughly twice the emissions of natural gas based production; and the projections and investment requirements to 2050).
• Vaclav Smil, "Nitrogen cycle and world food production", World Agriculture (the share of nitrogen in the world's food proteins originating in the Haber-Bosch synthesis of ammonia, and the dependence of roughly half of current world food production on synthetic nitrogenous compounds).
• Fertiliser India, "India FY 2025-26 budget: fertilizer subsidies, urea production and agriculture growth" (the budgeted fertiliser subsidy of about 1.68 lakh crore rupees for 2025-26, the urea share of about 1.19 lakh crore and the nutrient based subsidy of about 49,000 crore, and the revised estimate raising the total).
• Reclimatize, "India's fertiliser subsidy and the green transition" (the distortion in nutrient ratios that follows from urea being held cheap relative to phosphatic and potassic fertilisers, and the exposure of Indian supply to imported gas and finished product).
• Moneylife, "Why India's Rs 1.67 Lakh Crore Fertiliser Subsidy Bill Is Unsustainable" (the scale of the subsidy bill, the fixed price of a 45 kilogram bag of urea at 242 rupees, and the fiscal pressure created by holding it there).
• Business Today, "Supply shock at sea, subsidy strain at home: fertiliser imports threaten India's fiscal math" (India's position as the largest importer of urea, annual consumption of around 35 million tonnes with about 30 per cent imported, the rise in domestic self sufficiency, and the exposure of the subsidy bill to international prices).