How Is Fertiliser Made? Taking Nitrogen Out of the Air
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The atmosphere is nearly four-fifths nitrogen and almost none of it is available to plants, because the two atoms in a nitrogen molecule are bound by one of the strongest bonds in chemistry. Breaking that bond industrially is the single reaction that lets the world feed itself, and something like half the nitrogen atoms in a typical person's body passed through it.
Why nitrogen is the limit
Plants need nitrogen to build proteins and nucleic acids, and they cannot use the gas form. Naturally, nitrogen becomes available in only a few ways: bacteria in the root nodules of legumes fix it enzymatically, some free-living microbes do the same, and lightning fixes a small amount. That supply capped agricultural yields for the whole of history, which is why crop rotation with legumes mattered so much and why farming depended on returning manure to the soil. By the nineteenth century the shortage was acute enough to drive an international trade in mined nitrogen, first in Peruvian guano, accumulated seabird droppings on rainless islands, which was exhausted within decades, then in Chilean nitrate deposits, over which a war was fought. In 1898 William Crookes told the British Association that the wheat-eating world faced starvation unless chemistry could fix atmospheric nitrogen, which framed the problem as the central scientific challenge of the age.
The Haber-Bosch process
Fritz Haber demonstrated in 1909 that nitrogen and hydrogen could be combined into ammonia over a catalyst, and Carl Bosch at BASF turned the laboratory result into an industrial process within a few years, which required solving materials problems that were unprecedented:
- •High pressure, typically one to two hundred atmospheres, because the reaction produces fewer molecules than it consumes and pressure therefore drives it forward
- •High temperature, around four hundred to five hundred degrees, which is a compromise, since heat speeds the reaction and simultaneously shifts the equilibrium the wrong way
- •An iron-based catalyst, found by testing thousands of candidates, which lowers the energy needed to break the nitrogen triple bond
- •Recycling, since only a fraction converts on each pass, so unreacted gas is separated and sent round again
- •Vessels able to contain hydrogen at those conditions without embrittling, which was Bosch's principal engineering contribution and required a new approach to steel
- •Hydrogen supplied by steam reforming of natural gas, which is where most of the energy and nearly all of the carbon emissions of the process come from
From ammonia to a bag of fertiliser
Ammonia is the starting point rather than the product. Some is applied directly as anhydrous ammonia, injected into soil, which is efficient and hazardous to handle. Most is converted further. Oxidising ammonia over a platinum catalyst gives nitric acid, and reacting nitric acid with more ammonia gives ammonium nitrate, a high-analysis fertiliser that is also an explosive, which is why its storage is regulated and why accidental detonations have levelled parts of cities, including Texas City in 1947 and Beirut in 2020. Reacting ammonia with carbon dioxide gives urea, now the most widely used solid nitrogen fertiliser worldwide, which is stable and cheap and loses nitrogen to the air as ammonia if left on the surface. Phosphorus and potassium, the other two major nutrients, are not synthesised but mined, as phosphate rock and potash, and phosphate is a genuinely finite resource concentrated in a small number of countries, which is a long-term supply question with no chemical solution.
What it changed and what it costs
The consequences are hard to overstate in either direction. Population rose from around 1.6 billion in 1900 to several times that, and analyses attribute the food supply for roughly half of it to synthetic nitrogen, which makes the process arguably the most consequential invention of the twentieth century. Haber received a Nobel prize and also directed Germany's chemical weapons programme in the First World War, personally overseeing the first large chlorine attack, and the ammonia plants that fed crops also fed explosives production, which extended that war considerably. The environmental costs are substantial and ongoing: the process consumes something like one to two percent of world energy and produces a comparable share of carbon dioxide; nitrogen applied in excess runs off into rivers and coastal waters, causing algal blooms and oxygen-depleted dead zones; nitrous oxide released from fertilised soil is a potent greenhouse gas and depletes stratospheric ozone; and nitrate contaminates groundwater. Responses include precision application, nitrification inhibitors, better crop breeding, and producing the hydrogen by electrolysis with renewable electricity rather than from natural gas.
The takeaway
Atmospheric nitrogen is unusable by plants because of the strength of its triple bond, which capped yields until Haber demonstrated the synthesis of ammonia in 1909 and Bosch industrialised it. The process combines nitrogen with hydrogen from natural gas at high pressure and temperature over an iron catalyst, and ammonia is then converted to nitric acid, ammonium nitrate or urea. It feeds roughly half the world's population, consumes one to two percent of world energy, and its runoff causes coastal dead zones.