Even on the verge of Haber’s breakthrough, scientists still stood in wonder of the behavior of nitrogen and its compounds. Quoting Ostwald from 1903 (Ostwald 1903),

Among the chemical elements from which the body of living things, both the lowest and the highest, assembles itself, nitrogen plays a special, aristocratic role. While the other elements, oxygen, carbon, hydrogen, sulfur, iron, etc., are as willing to form chemical compounds as they are to dissociate from them, nitrogen forms compounds only with difficulty and is very inclined to leave them. An expression of this characteristic is the fact that free nitrogen makes up four-fifths of the atmosphere while fixed [nitrogen], which is mainly found in the solids and liquids of the Earth’s crust, likely amounts to less than a millionth of it.Footnote 1

The wonderment was not new (Sheppard 2020, p. 154).

By the end of the eighteenth century, ammonia was the subject of scientific inquiry and had been produced from other compounds. The substance itself had long been uniquely identified–it is readily detectable by its smell or “odeur très piquant”—but it was only after the chemical composition was determined that the idea of a synthesis from the elements could be considered (Mittasch 1951, pp. 1–24), (Timm 1984). Ammonia could be dissociated easily, but despite the simplicity of the apparently analogue reaction of water formation, the combination of H2 and O2, ammonia synthesis from the elements was subjected to a century of confusion and erroneous reports of success. The production of ammonia was repeatedly shown to have come not from direct combination of nitrogen and hydrogen but from the atmosphere, impurities in water, or other materials such as iron. The combination of the two elements must be an altogether different kind of reaction. The mystery was exacerbated by the pervasive presence of ammonia in nature, such as in the presence of rotting organic matter. What could nature do that man could not?

With our knowledge today, we can look back at the earliest experiments and know they were doomed to failure. Attempts to synthesize ammonia in the nineteenth century were repeatedly plagued by the same factors: temperatures and pressures were too low. Many believed direct synthesis was impossible (Haber 1920), a plausible position under the assumption that chemical reactions proceeded in only one direction. Before the fundamental ideas of equilibrium and reversibility were formulated, there could be no adequate understanding of the role of the catalyst. In cases where ammonia was produced with certainty from other compounds containing nitrogen and hydrogen, it was attributed to a particular state of matter called status nascens. Absolute proof of the direct combination of nitrogen with hydrogen could, however, not be found.

This is not to say that all experiments suffered from all of these deficiencies. By mid-century, there was regular use of higher temperatures, increased pressure, and catalysts, and in the last quarter of the nineteenth century elements of the emerging physical chemistry began to appear in experimental reports. Here our story converges to focus on these particular physical characteristics of ammonia production. It was the formation and dissociation of the ammonia molecule itself—the energetic nature and strength of its bonds and the bonds of its constituents, N2 and H2—when in contact with a catalyst that came under direct scrutiny. Despite the new scientific focus, however, the desperation of politicians, economists, scientists, and farmers to find a reliable source of fixed nitrogen remained.

As we have seen, knowledge of the nitrogen cycle had shown the element (in fixed form) to be the limiting factor in crop growth. With some success, the once closed nitrogen cycle between soil, crops, and manure had been opened through the use of fertilizer, transport of biomass, and drainage systems. A solution seemed feasible through further human control: scientists were intrigued by nature’s ability to fix nitrogen biologically as well as through lightning and at the end of the nineteenth century imitation ensued (von Nagel 1991, pp. 9–15), (Stoltzenberg 1994, pp. 133–139).Footnote 2 While the attempts to directly breed and market nitrobacteria cultures failed, the multi-step, bacterial fixation mechanism was emulated in the cyanamide and other processes into the first quarter of the next century. But low yields could not be improved. Echoing the force of lightning, advances in electrical engineering made it possible to bind nitrogen through a new technique: the electric arc. Although the latter was also used well into the twentieth century, even cheap sources of electricity from hydropower in Scandinavia enabled only limited industrial and commercial application.

Then there was the direct synthesis of ammonia from the elements. The process, intriguing and elegant in theory, gained attention as the need for nitrogen fertilizer intensified. In practice, however, it remained unfeasible. Ammonia, while quickly dissociated catalytically, could not be produced in useful quantities because key physical insight was missing. All the while, a permanent solution for food production for the growing population had become a pressing topic in official circles. Crops were more and more constrained to specific geographical locations with increasing expectations on shrinking plots of arable land; there were also warnings that the nitrogen supplies in South America would soon be exhausted. In fact, these sources of fixed nitrogen, while having the potential to alleviate hunger, did more to better the lives of the wealthy while contributing to global instabilities, environmental destruction, and dismal working conditions; modern, high-throughput farming was now dependent on this stock resource (Cushman 2013, pp. 72–74). The limited quantities of ammonia from coking plants and other local supplies caused a permanent, synthetic source of fixed nitrogen to be viewed as a great necessity. In what has become a pivotal, historic appeal, the chemist William Crookes, also a student of August Wilhelm von Hofmann’s, warned the British Association for the Advancement of Science in 1898 that the civilized nations were in danger of producing inadequate quantities of foodstuffs.Footnote 3 He pointed to the fixation of atmospheric nitrogen as a solution and his words also remind us of the mysterious nature of the process and its challenges (Crookes 1917, pp. 2–3),

My chief subject is of interest to the whole world–to every race–to every human being. It is of urgent importance to-day, and it is a life and death question for generations to come. I mean the question of Food supply. Many of my statements you may think are of the alarmist order; certainly they are depressing, but they are founded on stubborn facts. They show that England and all civilised nations stand in deadly peril of not having enough to eat. As mouths multiply, food resources dwindle. Land is a limited quantity, and the land that will grow wheat is absolutely dependent on difficult and capricious natural phenomena. I am constrained to show that our wheat-producing soil is totally unequal to the strain put upon it […] It is the chemist who must come to the rescue of the threatened communities. It is through the laboratory that starvation may ultimately be turned into plenty.

He continued (Crookes 1917, pp. 37–38),

For years past attempts have been made to effect the fixation of atmospheric nitrogen, and some of the processes have met with sufficient partial success to warrant experimentalists in pushing their trials still further; but I think I am right in saying that no process has yet been brought to the notice of scientific or commercial men which can be considered successful either as regards cost or yield of product. It is possible, by several methods, to fix a certain amount of atmospheric nitrogen; but to the best of my knowledge no process has hitherto converted more than a small amount, and this at a cost largely in excess of the present market value of fixed nitrogen.

The fixation of atmospheric nitrogen therefore is one of the great discoveries awaiting the ingenuity of chemists […] This unfulfilled problem, which so far has eluded the strenuous attempts of those who have tried to wrest the secret from nature, differs materially from other chemical discoveries, which are in the air so to speak, but are not yet matured.

At the time of Crookes speech, nearly two decades before the First World War, the geopolitical dangers in the world were palpable. The Great Powers, Germany, France, and England were not only jostling for influence amongst each other, but also feared the world order that centered power on Europe was pivoting toward other nations: Russia, the United States, and Japan (Kennedy 1987, pp. 191–197), (Mackinder 1904; Wilkinson et al. 1904). Military and economic tensions were rising and “industrial productivity, with science and technology, became an ever more vital component of national strength (Kennedy 1987, p. 197).” Fixed nitrogen was used for many purposes besides fertilizer, including dyes and explosives for military, mining, and construction (Tamaru 1991). Not only were the resources limited, but the sea routes for importing nitrates from South America could easily be blocked. A new industrial method to produce fixed nitrogen would serve several purposes at once (Schwarte 1920, pp. 537–551), (Haber 1971, p. 85).

The final step, however, was not political. The industrial capital and infrastructure for chemical production was already in place. Fritz Haber would successfully synthesize ammonia only seven years later and the beginning of industrial upscaling was only a decade away. It was a development in pure science that offered the solution to the mystery of ammonia synthesis: the emergence of physical chemistry.