Weapons and Fertilizer: The Haber-Bosch Process of Nitrogen Fixation

The Haber-Bosch process of fixing nitrogen produces a lot of fertilizer--but its first application was for weapons.

Cut open one of the nodules on the roots of any legume, and the inside is moist and pink. It looks surprisingly like blood, for good reason—that pink color comes from leghemoglobin, a molecule that binds oxygen just like hemoglobin. In an intact nodule, the leghemoglobin serves a very important purpose—creating the anaerobic conditions necessary for the bacterial enzyme nitrogenase to fix nitrogen. Nitrogen fixation also takes a lot of energy. But compared to synthetic nitrogen fixation, biological nitrogen fixation is a very efficient and nontoxic process.

Scientists knew the general equation for synthetically fixing atmospheric nitrogen back in the 1880s. Just react nitrogen and hydrogen gas to make ammonia. It sounded simple, but trying to copy the process that was going on in every legume root nodule proved to be really, really difficult. The first attempt—the arc process—tried to duplicate the effect of lighting by running high voltage electric currents through nitrogen and oxygen gas to make nitric oxide. This method did fix nitrogen, but used way too much energy to be a viable commercial process. Another early method of synthesizing nitrogen—the cyanamide process—used a lot less energy than the electric arc process. But cyanamide wasn’t a good fertilizer because it was slightly toxic to plants and released nitrogen slowly.

The breakthrough in nitrogen fixation came in Germany in the early twentieth century when Fritz Haber successfully synthesized ammonia by reacting nitrogen and hydrogen gas. While the reaction was similar to the one used by nitrogen-fixing bacteria, it was a whole lot harder without the nitrogenase enzyme. Haber could only make it work at high temperatures and pressures, and the catalyst that worked best, osmium, was too rare to use on a commercial scale. It took four more years for BASF engineer Carl Bosch to develop a commercially viable process. The first factory to use what became known as the Haber-Bosch process of nitrogen fixation opened in Oppau near the end of 1913—just in time to fuel the German army in World War I. Some historians estimate that Germany’s ability to fix nitrogen using the Haber-Bosch process prolonged the war up to two years, greatly increasing the amount of casualties and destruction.

By World War II, the secret was out—everybody on both sides had factories that used the Haber-Bosch process. Now it was possible to make far more explosives than ever before. The bombing raids of World War II wouldn’t have been nearly as deadly without the practically unlimited supply of nitrates now available from synthetic nitrogen fixation. Even the most technologically optimistic writers, who claim that it’s impossible to feed the world without the Haber-Bosch process, acknowledge that it was used for war before it was used to make an appreciable amount of fertilizer. The subtitle of Thomas Hager’s 2008 book The Alchemy of Air highlights this ambiguity: A Jewish Genius, a Doomed Tycoon, and the Scientific Discovery That Fed the World but Fueled the Rise of Hitler. Vaclav Smil also mentions the paradox in his 2001 book Enriching the Earth: Fritz Haber, Carl Bosch, and the Transformation of World Food Production.

If even the strongest promoters of synthetic nitrogen fixation acknowledge its use in weapons, why are they so adamant in insisting that it’s necessary to feed the world? Is it simply the fact that 75 percent of the 150 million metric tons of ammonia produced by the Haber-Bosch process each year is used for fertilizer? If everyone switched to organic farming, three-quarters of the world’s nitrogen-fixing factories would have to close. Is that the only reason they claim the world can’t be fed without the Haber-Bosch process? Or is there something else going on?