Cobra Effect · Chemistry

Fertiliser made from thin air

How chemists learned to pull fertiliser out of the air, and what else it made.

7 cards, read aloud in 3:54, with a test and sources.

A row of wheat stalks, each shorter and with fewer grains than the last

In 1898 William Crookes warned that the world could soon run short of wheat.

Crops need nitrogen, and every harvest carries some of it away from the soil. Farmers in Europe and North America were importing bird guano from Peru and saltpetre from mines in Chile to put it back. Crookes, speaking as president of the British Association, said those supplies could not keep up with a growing population. He called on chemists to find a way to take nitrogen from the air.

The air is mostly nitrogen, but plants cannot use it.

About four fifths of every breath is nitrogen gas. Its two atoms are held together by a triple bond, one of the strongest in chemistry. Plants cannot break that bond themselves. In nature it is split mostly by certain bacteria, with a little help from lightning. What farmers needed was nitrogen in a form plants can take up, such as ammonia.

In 1909 Fritz Haber made ammonia from nitrogen and hydrogen on a laboratory bench.

Haber was a chemistry professor at Karlsruhe, in Germany. He forced the two gases together at very high pressure, over a hot metal catalyst that sped up the reaction. Only a small part of the gas turned into ammonia on each pass, so he took the ammonia out and sent the rest round again. His small apparatus produced a steady stream of liquid ammonia, impressing the chemical company BASF.

Carl Bosch of BASF turned Haber’s bench apparatus into a factory.

A factory needed steel reactors far larger than anything used before, holding hot gases at around two hundred times the pressure of the air. The first reactors burst under the strain, and Bosch’s engineers had to design new ones. His colleague Alwin Mittasch ran thousands of experiments to find a cheap catalyst, and settled on one made mainly of iron. In 1913 the world’s first ammonia factory opened at Oppau.

Fertiliser made this way now helps feed about half the people on Earth.

Researchers estimate that without synthetic nitrogen fertiliser, the world could grow food for only around half as many people. Making ammonia uses roughly one or two percent of the world’s energy, mostly from natural gas. Fertiliser that washes off fields into rivers and seas can feed harmful blooms of algae. The process is still, at heart, the one Haber and Bosch built.

Haber also led Germany’s poison gas attacks in the First World War.

In April 1915, chlorine gas that Haber had championed was released against enemy troops at Ypres, in Belgium. When a British blockade cut off saltpetre from Chile, nitrogen from the new factories was turned into explosives, which helped Germany keep fighting. Haber won the Nobel Prize in Chemistry for making ammonia, awarded for 1918, and many scientists protested. Bosch won a Nobel Prize too, in 1931, for high pressure chemistry.

So when a discovery changes the world, ask what else it makes possible.

The wheat shortage Crookes feared never came, because chemists learned to take nitrogen from the air. The same chemistry that fed billions also supplied explosives for war. Haber, who was of Jewish descent, resigned and left Germany in 1933 after the Nazis came to power. He died the following year. Chemistry does not choose how it is used. People do.

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