When it comes to the natural processes of plants, photosynthesis tends to hog attention. If researchers could efficiently copy the ability to convert sunlight to energy, chemists promise, our energy problems would be over. They have not managed it yet.

They have had more luck with harnessing and mimicking the less-heralded, but just as important, process of nitrogen fixation — the conversion of nitrogen from the air into ammonia, which can be used by plants to make DNA, RNA and proteins, and by industry to make fertilizers and explosives. On page 84, chemists announce the discovery of an important piece of the puzzle. Jonas Peters and his colleagues at the California Institute of Technology (Caltech) in Pasadena have identified a small iron complex that efficiently catalyses the conversion.

The discovery comes a full century after the chemist Carl Bosch opened his nitrogen works in Oppau, Germany, and in doing so sealed the deaths of millions of people, and the birth and survival of billions more. Bosch had worked out how to scale up a laboratory reaction to combine nitrogen from the air and hydrogen from natural gas into synthetic ammonia. Textbooks talk about the Haber process, named after German chemist Fritz Haber, who made the theoretical breakthrough, but it is more properly called the Haber–Bosch process. Both Haber and Bosch won Nobel prizes for their work.

It is hard to overstate the impact of the Haber–Bosch process. A figure published in 2008 (the centenary of Haber’s patent) shows how the increase in world population since 1960 has kept step with increases in the use of nitrogen fertilizer (J. W. Erisman et al. Nature Geosci. 1, 636–639; 2008). Population growth through access to fertilizer and therefore food was one of Haber’s goals in developing his process. The other was to give Germany mastery of the science of munitions. Both goals demanded that the industrial supply of fixed nitrogen grew from the few hundreds of thousands of tonnes available per year at the start of the twentieth century, when it relied on natural resources such as guano and mineral saltpetre (potassium nitrate and sodium nitrate).

Bosch had to treat nitrogen and hydrogen under massive pressure and heat to make the conversion to ammonia. In industry, the process is still done in the same expensive and energy-intensive way.

Crucially, the synthesis described by the California group unfolds under mild, environmentally friendly conditions, just as it does in nature. (Well, when the conversion is done in the soil — another way to fix nitrogen naturally is through the searing flash of a lightning strike.) Peters and his colleagues examined the enzymes and cofactors that make ammonia among the roots of plants such as legumes. Iron has for decades been known to be important in these cofactors, but exactly how and why have been a mystery. For a while, attention switched to molybdenum, which chemists showed could also help to make ammonia, but biochemical and spectroscopic data have renewed the focus on iron. The finding from the Caltech scientists supports this: the iron complex they identify can do the job with no need for molybdenum.

It took less than five years for Bosch to commercialize Haber’s discovery, and to revolutionize the industrial supply of ammonia. It will probably take longer for researchers to build on the latest work, but at least now they have a platform.

The stakes have always been high. In the nineteenth century, Peru and Chile fought a war over guano. When Germany was denied access to Chile’s saltpetre during the First World War, the Haber–Bosch process gave it — and the world — an alternative, which it grasped with both hands. All the time, legumes such as alfalfa, peanut and clover have been quietly and efficiently doing their thing. A century after Bosch, they could help to write a new chapter in the ammonia story. And photosynthesis? Watch this space.