Tom Standage in More Intelligent Life:
You are what you eat. The atoms in your body come from the food and drink you consume – and, to some extent, from the air you breathe. That is not terribly surprising. What few people realise, however, is that about half the nitrogen atoms in your body have passed through something called a Haber-Bosch reaction. This chemical process, invented just before the first world war, did as much to change the world during the 20th century as the atom bomb or the microchip. Its story deserves to be more widely known, because it offers hope today for a fight whose front line is fast approaching: the battle against climate change.
The tale begins with a dispute between two German chemists, which erupted at a conference in Hamburg in 1907. At the time, solidified bird excrement from South America, known as guano, was used around the world as fertiliser. Compared with manure, it contains 30 times more nitrogen, the key ingredient. Why not extract that element from the air, which is 78% nitrogen? Alas, nitrogen molecules are so stable and unreactive that chemists were having great difficulty getting them to combine with other elements. When Fritz Haber, a German scientist, reacted nitrogen with hydrogen to make ammonia, for example, only 0.0048% of the mixture combined. Walther Nernst, another German chemist, took issue with Haber’s results. The proportion of gas that combined, he calculated, ought to have been 0.0045%. Most people would have thought Haber’s figure was close enough, but not Nernst, who demanded that Haber withdraw his results. Greatly distressed at this rebuke, Haber concluded that repeating the experiment was the only way to restore his reputation. But when he did so, he discovered that performing the reaction at a higher pressure vastly increased the amount of ammonia produced: 10% of the mixture combined. This suggested that, rather than waiting for birds to do their business, fertiliser could be made directly from the atmosphere.