The Scientification of Agriculture

Abstract

It may seem that ammonia synthesis was a scientific triumph for chemistry that also had unexpected consequences for agriculture, much in line with Max Planck’s dictum: “Understanding must precede application.” However, in historical hindsight things often happen just the opposite: in the case of ammonia synthesis, practice and experience preceded theory. Without observations from agriculture and a general understanding of the role of fertilizer—and of nitrogen and the nitrogen cycle in particular—certain essential chemical insight would not have been considered valuable.

It may seem that ammonia synthesis was a scientific triumph for chemistry that also had unexpected consequences for agriculture, much in line with Max Planck’s dictum: “Understanding must precede application (Planck 1919).”¹ However, in historical hindsight things often happen just the opposite: in the case of ammonia synthesis, practice and experience preceded theory. Without observations from agriculture and a general understanding of the role of fertilizer—and of nitrogen and the nitrogen cycle in particular—certain essential chemical insight would not have been considered valuable.

From the Middle Ages until the mid-nineteenth century, agriculture in Europe was increasingly strained, broken periodically by plague or war which limited the growing population. The three-field system was the main strategy to meet the increasing need for nourishment, while the development of farmland from forests, along with land to the East and in the Americas, also alleviated pressure. However, crops were also increasingly transported away from their place of origin so that local fields lost their nutrient base and the use of animal excrement as fertilizer became less effective. It led to a vicious spiral. By the turn of the nineteenth century, the situation was dire in many countries. In Great Britain, despite the ongoing “agricultural revolution,” experts were aware of the strain on land resources. The economist Adam Smith, for example, stressed the economic consequences of depleted fields while another economist, Thomas Robert Malthus, warned of the difficulties of feeding an ever-growing population. Germany also experienced challenges: during the first two-thirds of the nineteenth century, the country’s population grew by 1%, a threatening rate according to contemporary observers.

Agricultural crises and famine remained widespread until the Industrial Revolution keeping about three-quarters of a community’s labor force devoted to food production. Difficulties with division of labor kept this ratio high. At the beginning of the nineteenth century, many politicians and economists contended that the problems could not be solved with the tools at hand; population growth would eventually overwhelm the maximum productivity of available arable land. Mid-century Europe, particularly Ireland, witnessed widespread famine. Nevertheless, industrialization and continued optimization of agrarian production limited major disasters—but the strategy was not a permanent fix (Gray 1990), (Wrigley 2004, pp. 212–216), (Vanhaute et al. 2007).

How, then, did agriculture modernize into the efficient and calculable system we have today? Like many developments heavily reliant on chemistry, it began in the second half of the eighteenth century during the “chemical revolution”² of Antoine de Lavoisier and later John Dalton (Scholz 1987). Their work enabled the first isolation of elements, including carbon, hydrogen, oxygen, and nitrogen. It was then that researchers, most notably the pharmacist and chemist Carl Wilhelm Scheele, were able to identify organic compounds, and understand that these were common to both plants and animals (Cassebaum 1982). Known to be generally different than inorganic compounds, the study of these substances was, not surprisingly, referred to as “plant and animal chemistry.” What followed was an increase in identification and even synthesis of organic materials. A finer understanding of these substances and their constitution also strengthened the idea of conservation of matter in chemical reactions. The distinctions between plant chemistry and animal chemistry blurred as animal respiration was shown to be analogous to combustion: animals “burned” carbon for energy and required plants to replenish their carbon supplies. It was later understood that plants, too, exchange gases with the atmosphere (Gräbe 1920, pp. 1–16), (Browne 1977, pp. 170–171), (Holmes 1985, pp. 91–128, 151–198, 291–326). This increasingly scientific view of plants and animals, and their interactions with their environments, developed along with changing cultural factors.

Agricultural methods in the middle of the seventeenth century, especially in central Europe, were based largely on the dissemination of “useful” or “how-to” through what was called Hausväterliteratur or Hausbücher. Implemented in the previous century, these books were written for aristocratic heads of estates and based on societal conceptions from antiquity. The household was considered the main societal unit and each strove for individual self-sufficiency. At the end of the century, however, parallel to the developments in plant and animal chemistry, a new emphasis on husbandry emerged. Influenced by the Enlightenment, the literature focused on agriculture, industry, and commerce, asserting the claim that agriculture was part of an overall economy that would not only feed the population, but could also create wealth. As free market capitalism came to Europe, discussions of agricultural strategy entered political circles, marking a shift from pre-liberal to liberal thought. Although the ideas did not catch on immediately, the approach to agricultural was becoming increasingly based on material and scientific terms (Jones 2017, pp. 6–25), (Gray 1990).

Apart from frequent food shortages, which threatened to rile the populace, two conflicts were integral in establishing the new views. First, the aftermath of the French Revolution legitimatized new liberal thought. The entrenched feuding interests of the past centuries could be openly scorned and governments were able to break the lord-peasant relationship. The capitalist ideas in England of Adam Smith and Arthur Young began to spread across the continent. Second, the Napoleonic Wars (1803–15) showed the vulnerability of a nation’s food supplies and land. Desperate politicians were forced to consider possible solutions that they may not have otherwise entertained. It was in this context that Daniel Albrecht Thaer began working for the Prussian State service in 1804 and set up one of the earliest agricultural stations at Möglin in 1806 (Jones 2017, p. 164). Unlike some of his fellow colleagues in the German lands and the rest of Europe, Thaer embraced the modern approach to agriculture and can be counted among “the advocates of ‘rational’ husbandry [who] were convinced that the time had now come to equip agriculture with a scientific methodology rooted in experimentation (Jones 2017, p. 27).”

Thaer’s was not the first agricultural research station (Lavoisier performed experiments at his estate at Fréchines in the 1770s and 80s), but it was one of the earliest to perform work based on scientific knowledge and profited from the first chemical-based breakthroughs in agricultural science. In 1804, Nicolas de Saussure announced his findings that atmospheric CO₂ provided the main source of food for plants. Combined with his belief in the importance of soil rich in nutrients, this report was an early step toward a mineral theory of plant nutrition (de Saussure 1804). It was also around this time that the cultivation of the sugar beet began, which ultimately helped advance the concept of crop maximization and the acceptance of “artificial” fertilizers (Jones 2017, pp. 171–175). The science of “agronomy,” based on the ideas of Thaer and de Saussure, was gaining acceptance. A regular publication, Archive of Agricultural Chemistry for Thinking Farmers,³ appeared committed to agricultural chemistry. Lecture series were also popular: at the Royal Institution in London in 1803 (published in 1813) Humphrey Davy stated (Davy 1813, p. 607),

Agricultural chemistry has for its objects all those changes in the arrangements of matter connected with the growth and nourishment of plants, the comparative values of their produce as food; the constitution of soils; the manner in which lands are enriched by manure, or rendered fertile by the different processes.

However, he continued,

[while] agricultural chemistry has not yet received a regular systematic form […] it is scarcely possible to enter upon any investigation in agriculture, without finding it connected, more or less, with doctrines or elucidations derived from chemistry.

A long and fruitful development of knowledge centered on improved field research and science-based education had begun (Harwood 2005, pp. 77–109). At the turn of the nineteenth century, the state of agricultural chemistry was different in Great Britain than on the Continent, though neither region had developed an adequate system (Russel 1966, pp. 66–69). Better analysis methods were needed at the research stations before the quantitative results could be achieved which finally shifted attention from crop rotation to soil improvement and fertilizer. Such methods included the use of control plots and work with dung, mathematical models and improved access to methods determining the composition of soils and plants (Jones 2017, pp. 164–170). Especially the latter received a boost from advances in organic chemistry in the first three decades of the nineteenth century with the development of accurate quantitative analysis and the further isolation of organic substances. Primitive superficial analysis techniques, such as combustion and distillation, were improved upon until reliable quantitative results, that is, the amount of hydrogen, carbon, oxygen, nitrogen, etc., in a sample could be confidently obtained (Fig. 2.1). The culmination of these devices, Justus von Liebigs’ Kaliapparat from 1831, coincided with the birth of modern research stations across Europe (von Liebig 1837), (Ihde 1964, pp. 89–94, 165–179). It was also a period of economic transition to the post-Malthusian Regime. As a higher standard of living changed household activity, more wealth became available for peripheral activities, such as science and technology in agriculture (Galor and Weil 2000; Gray 1990).

Fig. 2.1

Instruments for determining the composition of substances (von Liebig 1837). Hydrogen is converted to water and carbon to different oxides through combustion (the combination of the organic components of the sample with oxygen). Figures 1–4 show parts of an apparatus for removing water from samples prior to combustion. Figure 5 allows the removal of air from around samples that are especially difficult to dry. The dried sample can then be weighed in the small tube in Fig. 6. The apparatus in Fig. 10 also allows the sample to be heated and dried at reduced pressures. It is already in the combustion tube, C, where it is mixed with copper oxide prior to combustion. Water created during combustion is collected in the calcium chloride tube (Fig. 8) which is attached directly to the combustion tube (Fig. 9). An alternative form of the combined combustion-calcium chloride tube is shown in Fig. 10. Oxidized carbon is captured in the Kaliapparat in Fig. 11; its components, assembly, and preparation are sketched in Figs. 12–14. Figure 15 shows a view from above of the iron sheet oven in which combustion takes place, and Fig. 16 shows the front view. The full assemblage appears in Figs. 18 and 21. Source: Niedersächsische Staats- und Universitätsbibliothek, Göttingen, signature: 8 CHEM II 5351

One important research station was Jean-Baptiste Boussingault’s facility at Bechelbronn in Alsace where he published results beginning in 1836. Boussingault went beyond chemical analysis of plants and fixed soil content by extending the quantitative analysis of de Saussure to analyze how varying quantities of nutrients impacted plant growth. Among these was nitrogen, the exact role of which had long been a mystery. Boussingault demonstrated the importance of the element’s assimilation and replenishment in plants, namely that the amount of nitrogen was essentially proportional to plant growth. It was also during these investigations that he noted the soil’s improved nitrogen content after the growth of legumes (Browne 1977, pp. 239–251), (Jones 2017, p. 181).

Another leading investigator and supporter of field research was Carl Sprengel (Sprengel 1819). After assisting Thaer for seven years at research stations in Celle and Möglin, he began teaching agricultural chemistry at the university level in 1829. Sprengel was an early adherent to the mineral theory of plant nutrition and aware of the work of de Saussure. In 1831, he moved to Braunschweig to help establish and run a modern research facility, the Ducal Institute for Agriculture and Forestry,⁴ but it never came to fruition. Only small field experiments resulted. In 1839, Sprengel moved to Prussia and the Pommeranian Economic Society ⁵ where he established an agricultural academy. It was here that he made his most significant theoretical and educational contributions to agricultural chemistry (Wendt 1950), (Browne 1977, pp. 231–239), (Frielinghaus and Dalchow 2003).

One of Sprengel’s most controversial contentions was the rejection of long-standing beliefs regarding the role of humus (decayed organic matter in the top soil) in plant growth. The theory contending the importance of humus can be traced to ancient alchemical ideas and was supported by evidence that manure and plant matter had a positive effect on crops (Brock 1992, pp. 14–20), (Bensaude-Vincent and Stengers 1996, pp. 1–24). It was believed that humus was the primary contributor to soil fertility and, therefore, the basis of plant nutrition, specifically because of it’s high carbon content. Plants purportedly extracted humus from the earth and changed it into plant tissue through combination with water. Supporters of the humus theory thought minerals played, at most, a minor role in plant growth, perhaps acting as a stimulant. The belief persisted well into the nineteenth century, even after the role of minerals in plant growth had been experimentally verified. De Saussure himself believed that humus, in addition to atmospheric CO₂, acted as a source of carbon as well as nitrogen, and other essential nutrients. Supporters of the mineral theory, in contrast, believed plants could be nourished, in principle, with purely inorganic minerals (Waksman 1942). In 1826, Sprengel made one of the earliest clear rejections of the humus theory, arguing that nutrients needed for plant growth are completely supplied from the environment and the lack of any essential mineral will limit further growth (Sprengel 1828), (Browne 1977, p. 231).

De Saussure, Boussingault, and Sprengel did much to advance the mineral theory. They firmly established that the fertility of farmland could only be maintained if the mineral nutrients removed with the harvest were replaced. The actual mechanism of plant nourishment itself, however, remained unexplained. The common attitude was that the nitrogen in plants was drawn directly from diatomic nitrogen in the atmosphere. The mineral theory of plant nourishment was not yet complete. It did not achieve its breakthrough until further knowledge of the role of nitrogen was gained and Justus von Liebig pushed it into the mainstream of agriculture and agricultural education in the 1840s (Finck 2003; Gorham 1991; Gottwald and Schmidt 2003).

Liebig was born in 1803 and received an appointment as professor at the Ludwig’s University in Giessen in 1824 at age of 21. He deeply influenced German education in the field of chemistry by establishing the now famous “Liebig School,”⁶ which still embodies the turn toward increasing competency in chemistry education in the first half of the nineteenth century (Scholz 1987). The school’s effect on the development of chemists and the chemical industry lasted for generations.

Between 1839 and 1845 Liebig and Sprengel published several editions of seminal works in the agrarian sciences. Their regular emphasis on science, especially chemistry, in developing larger crop yields as well as their reliance on experimental observation established definitively the modern mineral theory of agriculture. In 1839, Sprengel published Die Lehre vom Dünger (Fertilizer Studies, Fig. 2.2, left) with a second expanded edition appearing in 1845 (Sprengel 1839, 1845). Liebig, on the other hand, published the first edition of his Agrikulturchemie (Agricultural Chemistry) in 1840. However, it was only after the publication of his Thier-Chemie (Animal Chemistry) (von Liebig 1842) that he felt comfortable enough in his knowledge of dietary cycles of animals and their waste to publish the fifth and definitive edition of Agrikulturchemie in 1843 (von Liebig 1843, p. 7) (Fig. 2.2, right).

Fig. 2.2

Title pages from (left side) Carl Sprengel’s Die Lehre vom Dünger (Fertilizer Studies, first edition, 1839) (Sprengel 1839) Source: Humboldt Universität zu Berlin, signature: AD CA 1986, photographed by the author; and (right side) Justus von Liebig’s Agrikulturchemie (Agricultural Chemistry, fifth edition, 1843) (von Liebig 1843) Source: Bayerische Staatsbibliothek München, signature: Chem. 210 he, page 7, urn: nbn:de:bvb:12-bsb10073229-2

At the time, it was known the combustion of plants produced ashes containing inorganic compounds, such as carbon dioxide, ammonia and certain acidic salts, including alkalines combined with silicic acid or phosphoric acid. As Liebig remarked, some (presumably supporters of the humus theory) thought the presence of these inorganic compounds was accidental and they were not critical to a plant’s growth (von Liebig 1843, p. 83). However, both Liebig and Sprengel asserted that plants needed these inorganic chemicals to survive and bloom. Furthermore, they took a different form in plants than either before assimilation or in the ashes. In other words, organic plant tissue (and thus animal tissue) was created from purely inorganic compounds and returned to that form after the plant decayed; it explained the cycle of transformation between organic and inorganic matter. Humus could bolster plant growth but was not a necessity. The organic matter in plants did not require its nutrients, or building blocks, to be of similar organic origin. The plants required only a stable, porous medium (soil) through which water could transport dissolved salts. “If we fortify the soil [which already contains silicic acid],” Liebig wrote, “with ammonia and phosphoric acid, which is essential for wheat…we will have fulfilled the conditions necessary for a plentiful harvest because the atmosphere represents an endless reserve of carbon dioxide (von Liebig 1843, p. 174).”⁷

Even the combative and possessive Liebig saw himself as the beneficiary of a European tradition that viewed agriculture not only as a necessity for continued human civilization, but also as a problem with a potentially exact scientific solution. He dedicated his book Agricultural Chemistry to his former mentor Alexander von Humboldt (von Liebig 1843, pp. III-IV),

I don’t know whether any part [of Agricultural Chemistry] actually belongs to me; when I read the introduction which you gave 42 years ago for Ingenhousz’ work “On the Food of Plants,” it appears to me, as if I only furthered the opinions which [you] stated and justified therein.”⁸

While this may have been customary language at the time, Humboldt indeed foresaw the emergence of the mineral theory. In 1798 he wrote the introduction to the German edition of Jan Ingenhousz’ Essay on the Food of Plants and the Renovation of Soils. Humboldt’s scientific interest at this early stage of agricultural chemistry and plant physiology was not only a matter of improving harvests. Any new understanding of nature, he wrote, “will be beneficial enough if it teaches mankind to decide between competing methods, to clarify everyday yet still incomprehensible phenomena, and to gain insight into the causal connection between outcomes…(Ingenhousz 1798, p. 7)”⁹ Humboldt viewed agricultural science not only as a means to achieve technical application, but also as basic research: the information must be collected, regardless of its immediate value. Prophetically, Humboldt touched on how soil composition effects plant growth, and that rain water (which contains nitrogen-based acids) bolsters growth, whereas spring or river water does not. He also stressed the interdependence of animate and inanimate matter.

Liebig reached similar conclusions and was so sure of the consequences he attempted, as did John Bennet Lawes, to market his own line of chemical fertilizer (Cushman 2013, pp. 50–51), (Jones 2017, pp. 181–184). Although the endeavor failed (during the famine of the 1840s) his scientific achievement remained clear. “No one can deny any longer,” Liebig wrote, “that further progress in agriculture can only be expected from the field of chemistry now that the conditions for fertile soil and the support of plant life have been determined (von Liebig 1843, p. XII).”¹⁰

However, many of Liebig’s conclusions met with resistance. One notable detractor was Jöns Jacob Berzelius, a supporter of the humus theory. After Liebig sent Berzelius the first edition of Agrikulturchemie, the elder scientist responded with comments on December 11, 1840 (Reschke 1978),

…I do not completely agree with you that carbolic acid, ammonia, and water are the actual and sole nourishment of plants and that fertilizer in the soil has no other role than to supply these [compounds]. If this were actually the case, it would be possible to nourish the plant to complete maturation of the seed with these given [compounds] […] If this thesis cannot be proven through experiment, much of the substance of your teaching would fall away [Berzelius then further critiques Liebig’s work]. This way of treating science makes for tempting, entertaining reading, but it seems to me an attempt to return to the Fourcroy method, which constructed science from colorful soap bubbles that were swept away after exact examination. From these not even a drop of soap remains from which the bubbles were formed.¹¹

However, the influence of Berzelius and other supporters of the humus theory had begun to wane after they made the mistake of attributing a unique chemical nature to each of the many organic compounds found in humus (Kononova 1966). Liebig’s reputation, contrarily, was improving, as his work had found many supporters (Jones 2017, p. 184). In 1848, for example, his ideas appeared in the section Fertilizer Studies ¹² in Karl Nikolaus Fraas’ Historical-encyclopedic Compendium of Agricultural Economics,¹³ though not without some criticism (Fraas 1848, pp. 92–111). While Liebig’s Agrikulturchemie is often considered the turning point in how scientists, farmers, and politicians viewed agriculture and nitrogen, this attribution is partly a modern one; actual change was not abrupt. The dispute about plant nourishment remained in agricultural science for some time, while confusion (if not outright dissent) abounded. In 1864, 20 years after Liebig’s publication, Wilhelm Schumacher, then assistant professor at the Agricultural University of Berlin, wrote (Schumacher 1864, pp. 71–72),

Whoever thinks back a few years will remember the unresolved, yet abandoned dispute between the mineral theorists and the nitrogen theorists [who thought nitrogen was the limiting factor in plant growth]. The “nitrogenists,” through their perspective on the nutrition of plants, emphasized mainly ammonia and nitric acid. Everything that contained nitrogen was, as fertilizer, of the greatest importance. Indeed a famous agricultural chemist went so far as to envision the inert nitrogen in the air as accessible to the methods of the chemist so that one day fertilizer could be delivered to farmers through an atmospheric synthetic fertilizer factory. However, this was only an idea borne of inexperience; and yet, the newest science has shown us that such control of the inert atmospheric nitrogen does not belong to the realm of impossibility. Nature itself can, through simple processes, convert nitrogen into ammonia and nitric acid. Luckily, these methods of nitrogen conversion cannot, in general, be made useful for agriculture; I say luckily because the agricultural chemical industry and fertilizer industry could easily overlook the economic interests of preserving atmospheric nitrogen and pilfer the atmosphere until the air is unpalatable for future generations. The current state of the human body, and of the bodies of animals, would be difficult to maintain under a different composition of air with regard to nitrogen and oxygen. And the composition of the atmosphere would certainly change if it were robbed of its nitrogen over thousands of years without otherwise being replenished.¹⁴

While the “famous agricultural chemist” showed much foresight, Schuhmacher’s imagination seemed limited to natural processes of nitrogen fixation (these processes are bacterial and will be discussed shortly). He also overestimated the danger of extracting nitrogen from the atmosphere, which illustrates that the mechanism for replenishing nitrogen in the atmosphere had not been universally accepted or understood–but it did not remain so for long. This mechanism is part of the nitrogen cycle which, along with the law of the minimum, represents Sprengel and Liebig’s essential contribution to the understanding of fixed nitrogen in agriculture.

What is now known as Liebig’s law of the minimum was actually formulated by Sprengel beginning in 1826 and by Liebig in 1840. The law states that when a specific element or compound in arable land is limited, the addition of other minerals will have no effect on crop growth on that land. A visualization for the law of the minimum is Liebig’s barrel: every plank in the barrel represents an element and the shortest plank limits the water level in the barrel just as that element would limit the growth of a plant in a field. The law of the minimum is now considered the basis for agricultural chemistry and the fundamental idea that agricultural optimization is a quantitative problem. The growth of crops had come to be considered a question of resource inflow in a dynamic which is part of the cycle of elements, especially nitrogen, through the biosphere. In Agrikulturchemie Liebig described a cycle of life and death, decay and rebirth by broadening the concept of the nitrogen cycle and emphasizing the chemical form of the nitrogen at different points in the process. However, it took several more decades before all of the steps were understood.

Liebig concluded that all nitrogen absorbed by plants came from the atmosphere and was delivered solely via precipitation. Experiments on fertilizer had shown its nitrogen content was limited and it was clear that the vast amount of nitrogen in air was chemically inert. It must be present in another more reactive form when absorbed by plants: nitric acid formed from ammonia. Liebig believed ammonia was the final result of putrefaction and that the compound must, therefore, exist everywhere in the atmosphere, albeit in undetectable quantities. Experimentally, he had not been able to show the existence of ammonia in atmospheric air but thought it existed there as a salt with carbonic acid. The salt’s solubility in water meant that precipitation carried ammonia in higher concentrations and continuously washed it out of the atmosphere. Ammonia then decayed into nitric acid, the form of nitrogen essential for plant growth (von Liebig 1843, pp. 50–54, 58–62), (Bradfield 1942). “Ammonia in the form of its salts [nitric acid] is what delivers nitrogen to these plants,”¹⁵ and comprises the main part of the plant protein (von Liebig 1843, p. 62). Today, we refer to these compounds as forms of “fixed nitrogen” in which the triple bond between two atmospheric nitrogen atoms has been broken and the nitrogen is bound to other elements.¹⁶

Although Liebig stated decaying plant and animal matter as well as animal excrement could be used as fertilizer, he believed these offered no new sources of nitrogen. The ammonia formed during decay was simply returning to the form it had taken in the atmosphere before assimilation. Furthermore, Liebig agreed with the incorrect conclusion of Boussingault that the amount of ammonia from decaying organic matter was irrelevant compared to what was constantly available in the atmosphere (von Liebig 1843, p. 53). This conclusion was based on his observations that peas and beans could thrive without the presence of fertilizer of any kind. He did not believe that they could fix nitrogen on their own (von Liebig 1843, p. 290).

For Liebig, the nitrogen cycle comprised the following steps: ammonia, naturally present in the atmosphere in undetectable concentrations, enters the soil via precipitation (atmospheric ammonia is dissolved in higher concentrations in rain water) where it is absorbed by plants in the form of saltpeter. The ammonia later reenters the atmosphere through the decay of organic material or evaporation out of the soil. One question Liebig could not answer, however, was where the ammonia came from in the first place (von Liebig 1843, 279–292).

How, then, did the age-old observation of the benefit of fertilizer on plant growth—a practice at least as old as agriculture itself—fit into the process? Dung and mineral fertilizers had been recycled or brought in from outside ecosystems and used in widespread and varied strategies for thousands of years in Egypt, Peru, and North America. However, logistical difficulties and lack of understanding of its mode of function limited success (Mazoyer and Roudart 2006, pp. 61–64). Although some causal relationships began to emerge around the seventeenth century, conclusions remained dubious. Through trial and error, some farmers were able to optimize their own fields, but their strategies assured nothing for neighboring lands. In some cases, special salt mixtures were known to help. The actual function of fertilizer only began to emerge in the middle of the eighteenth century when it was tied to increased knowledge of the composition of soils. The definitive discovery was made in England where, despite little acceptance of the mineral theory early on, significant scientific strides had been made by mid-century (Jones 2017, p. 170, 182).

Building on results from Boussingault, John Bennet Lawes, an English landowner, and Joseph Henry Gilbert, a chemist from the Liebig School, began large scale experiments at Lawes’ agricultural station in Rothamsted, England in 1843 (Fig. 2.3). The investigations ranged from several years to more than a decade and examined the effect of differing amounts of fertilizer on wheat harvests. Through the use of nitrogenous manures containing ammonia salts, they were able to increase the total amount of nitrogen in crop yields by about 40% compared to crops receiving no fertilizer (Lawes et al. 1861). Despite Liebig’s authority and staunch defense of his belief, some experimental conclusions contradicted his position and the question of whether plants received all of their nitrogen from the atmosphere remained a matter of debate. Lawes and Gilbert finally proved that plants were able to receive a large portion of their nitrogen from sources that were altogether non-atmospheric. The result uncovered the existence of a new channel into the nitrogen cycle: fertilizer. Crops, indeed, required fixed nitrogen, but plants such as wheat could neither fix the element from the atmosphere themselves, nor did atmospheric ammonia satiate them.

Fig. 2.3

Aerial photograph of Hoosfield at Rothamsted from 1925 showing parceled land. Lawes and Gilbert began an experiment studying the effects of nutrients on spring barley here in 1852. The experiment still looks much the same as it did when it was started. Source: Rothamsted Research

Bolstered by this knowledge, agricultural production in Europe and the United States had managed to stave off the expected famine in the middle of the nineteenth century, but the expansion had made farmers crucially reliant on nitrate imports. The main source of this fixed nitrogen in fertilizer was saltpeter from Chile and guano (millennia-old deposits of sea bird excrement) from Peru (Fig. 2.4) (Honcamp 1930, pp. 3–21), (Leigh 2004, pp. 77–80), (Cushman 2013, chapters 1, 2), (Slotta 2015). Used for centuries in South America, guano first came to Europe in 1803 after Alexander von Humboldt and French botanist Aimé Bonpland returned with samples gathered during a research trip in Peru. However, it was not until two decades later, after global unrest that led to the independence of much of Spanish America, that interest grew and its chemical makeup was assessed: guano contained large amounts of nitrogen. Furthermore, it was soluble in water, making it fast acting. Commercial imports began in the mid-1830s but only became profitable the following decade. Over the next 40 years, the guano reserves were intensively harvested in Peru, mainly for export and often under harsh conditions for the laborers (Clarke and Foster 2009). Although many countries, including France, Germany, Brazil, and Australia, had their own import markets, the chief destinations were Great Britain and the United States. In America, the imports were spurred by competition between farms in the East and new land obtained during westward expansion, but the high prices limited availability. The results of the substance on farm fields in Great Britain, however, were so impressive that guano became their most important fertilizer; imports to Britain climbed to 95,000 tons in 1850 and 300,000 tons in 1858. The international “guano rush” was also helped by outspoken support among prominent farmers and agriculturalists, including in Liebig’s magazine Organic Chemistry. Worldwide guano imports amounted to 342,000 tons in 1856 and 522,000 tons in 1871 before the best deposits were depleted; shipments declined over the next two decades as the quality of the guano waned and it was eclipsed by nitrates from Chile. The falloff was exacerbated by the mineral theory and its emphasis on the role of nitrogen, more readily available in saltpeter. Farmers also turned to local sources while the Kali Syndikat in Europe and similar organizations in the United States further decreased demand (Mathew 1862), (Skaggs 1994, pp. 1–15, 150–152).

Fig. 2.4

“Strata of Guano, Chincha Islands” from Rays of Sunlight from South America by Alexander Gardner (1865). Source: The New York Public Library Digital Collections

Natural and man-made saltpeter, on the other hand had been traded throughout the world for centuries. It was first used in Asia, before spreading westward via the Dutch East India Company. The European markets consumed much of it for the production of gun powder but its fertilizing power was not realized until the beginning of the nineteenth century (Malanima 2009, p. 64). As demand grew, imports from East India initially dominated the market until imports from South America, especially from the mountains of northern Chile, overwhelmed those from Asia. The process had begun between 1809 and 1812 when a Bohemian researcher, Tadeáš Haenke, developed a method for extracting saltpeter from what was then Peruvian territory. The reputation of saltpeter as a fertilizer quickly caused the small mining operation to grow and exports began in earnest in the 1830s. The extraction in 1812 was about 1000 tons and grew to 23,000 tons in 1850, and 330,000 tons in 1875 by which time the largest company was Chilean and English-owned. Amidst growing instability in the international nitrate market, tensions between Chile and allied Bolivia and Peru led to the War of the Pacific; nitrate output declined until 1883, when Chile won the saltpeter province of Tarapaca. Chile then held a monopoly on the world nitrate market as exports reached into the millions of tons. The importance of nitrates extended beyond their use as fertilizer: they were also a key ingredient in the production of sulfuric acid, a substance central to the rise of the chemical industry (discussed later). For example, sulfuric acid was necessary to extract ammonia sulfate from coking operations, an important source of fixed nitrogen. It also found use in the Leblanc process and the production of nitric acid and nitroglycerine. Only after the First World War did the synthetic production of ammonia from atmospheric nitrogen displace Chile saltpeter as the world’s main source of fixed nitrogen. As if to underscore the changing times, Germany, the world’s largest pre-war consumer of Chile saltpeter, was the place of origin of ammonia synthesis (O’Brien 1982, pp. 9–18).

In the second half of the nineteenth century, Chile saltpeter and guano were used in tandem with continuously improving technical and strategic methods of preparing farmland to reinvigorate the fields of Europe. However, Chile saltpeter and guano were stock supplies—a limited natural resource. An independent and permanent source of fixed nitrogen was needed.

Returning to the nitrogen cycle, the mineral nitrogen stocks of South America had vastly increased the cycle’s input from fertilizer as the theoretical picture continued to improve. The final input mechanism, a biological one, would soon remind Europeans of a natural phenomenon humans could not replicate. This was the legume. It had been well established that crop rotations should include legumes, because crops planted after the legume harvest showed increased yields. Some scientists, including Boussingault and Lawes, had already observed that legumes could grow well without the addition of fertilizer and that the earth was enriched with nitrogen after their harvest. The investigations continued and grew in complexity into the mid-1880s, but apart from recommendations on how to take advantage of leguminous crops, the mechanism could not be explained. It was attributed to the legumes’ possible ability to assimilate nitrogen or to effect a more favorable distribution of the nitrogen remaining in the soil for the following crop.

Advances in plant physiology, chemistry, and, especially, bacteriology were needed before the question could be answered. From the 1850s to 1870s, a debate arose regarding the nature of legume root nodules. Some thought they were a kind of fungus, others that they were of bacterial origin. Finally, in 1886 at the Meeting of the Society of German Natural Scientists and Doctors ¹⁷ in Berlin, Hermann Hellriegel announced the answer and published the definitive paper two years later (Hellriegel 1886; Hellriegel and Wilfarth 1888). Legumes were able to assimilate more nitrogen for their growth than was directly available from the soil and the secondary source could be inhibited by soil purification: the presence of bacteria working in symbiosis with the roots’ nodules fixed atmospheric (or diatomic) nitrogen (Fred et al. 1932, pp. 1–11).¹⁸ Experimental verification of Hellriegel’s results soon followed. Apart from providing a further avenue into the nitrogen cycle, the question of whether plants could fix free nitrogen on their own or whether they only assimilated the fixed nitrogen supplied to them could also be answered: it depended on the plant in question. In fact, the action of fixing atmospheric nitrogen for growth, called diazotrophy, is carried out mainly by the bacteria of the genera frankia and rhizobium in symbiotic relationships with plants. Some of the species in these genera can fix nitrogen without symbiosis, instead forming a small group of free-living diazotrophs. There are also phototrophic diazotrophs, the largest group of which are cyanobacteria, formerly called blue-green algae. They have the ability to store chemical energy through oxygenic photosynthesis as well as fix nitrogen for growth in a spatially separated cell (heterocyst) because the production of oxygen generally inhibits nitrogen fixation. Nitrogen fixing bacteria are found in a wide array of environments including large bodies of water as well as moist, arctic, or arid desert soils (Eady 1992, pp. 534–553), (Stal 2015).

Other bacteria, those responsible for the processes of nitrification and denitrification, are vital to understanding the complete nitrogen cycle. In 1873, the German scientist Alexander Müller had noted that nitrates remained stable in sterile solutions, while they disappeared in solutions with untreated water. Four years later, Theophile Schloesing and Achille Müntz refined these observations through their own experiments (Schloesing and Müntz 1877a,b). They passed sewage through a tube containing a mixture of incinerated sand and chalk, and detected ammonia. After twenty days, they detected nitrates and the ammonia content fell to zero. Upon passing chloroform through the pipe the nitrates disappeared and ammonia was again detected. Heating the tube to 100 ^∘C also stopped the transformation of ammonia into nitrates; the process could be renewed and cycled. In the following years it was confirmed that biological processes were responsible for nitrification of the soil. Ammonia is oxidized through a two-step process into a form plants can assimilate: the bacterium Nitrosomonas oxidizes ammonia into nitrites, while Nitrobacter oxidizes nitrites to nitrates (McKee 1962, pp. 103–112).

Bacteria also supply the mechanism to reduce nitrates back to diatomic nitrogen returning to the atmosphere. This process, known as denitrification, had been observed in experiments throughout the mid-nineteenth century. In the 1880s, Ulysse Gayon, a professor of chemistry at the University of Bordeaux, and Gabriel Dupetit showed that bacteria, again through a multistage process, convert oxidized nitrogen in the soil to diatomic nitrogen in the atmosphere. Their experiments also relied on heating and poisoning bacterial cultures to activate and deactivate the denitrification process (McKee 1962, pp. 116–121). In contrast to nitrification, which is carried out by two main bacteria, denitrification is caused by an array of bacteria, the most common of which are Pseudomonas, Bacillus, and Alcaligenes.

With that, the first incarnation of the nitrogen cycle was established (Fig. 2.5).

Fig. 2.5

This page: biospheric cycles of oxygen, carbonic acid, and water from Youmans’ Atlas of Chemistry (1856), illustrating the interdependence of plants and animals (Youmans 1856, p. 87). Fixed nitrogen is not included in the graphic, although the role of ammonia and “neutral nitrogenized compounds” are discussed in the text. Opposite page: the nitrogen cycle according to Delwiche in 1970 (Delwiche 1970). This depiction is still current, and includes the additional step of the nitrogen cascade (Sutton et al. 2011c). Today, the complexity of the nitrogen, carbon, oxygen, water, etc. cycles is understood to such a degree that simplicity can only be preserved if they are shown separately

Today, rather than a stepwise circular pathway, our understanding of the nitrogen cycle in soils is based on the biogeochemical concept of competition between organisms in which physicochemical forces such as diffusion, emission, and erosion supply the distribution mechanisms. The existence of the cycle itself is based on nitrogen’s ability to assume an unusually large number of oxidation states ranging from -3 to +5. The nitrogen in N₂ (oxidation state 0) can gain up to 3 electrons in reduction (fixation) processes or it can lose up to 5 electrons in oxidation (nitrification) processes. While the reduction processes (via enzyme) generally release more energy than the oxidation processes, organisms driving the cycle have evolved to extract energy from each step. The plants themselves can assimilate nitrogen in either fixed form (ammonium ions, NH$_4^+$) or as nitrates (NO$_3^-$). In cases where these inorganic compounds are limited, plants may assimilate organic nitrogen in the form of amino groups (-NH₂). In other words, local conditions dictate which forms of nitrogen are introduced into the local nitrogen cycle and determine the balance of conversion reactions.

Thus, in contrast to soils where the nitrogen cycle is dominated by bacterial processes, the cycle in the atmosphere is governed mainly by chemical reactions. Here, the input mechanisms are reduced nitrogen in the form of ammonia gas from agriculture and oxidized nitrogen (NO _x) from the burning of fossil fuels. Aquatic, marine, and coastal ecosystems, which have vast contact areas with soil and atmosphere, will have a mixture of characteristics. Knowledge of the intricate pathways in these systems has been increasing since the 1950s, thanks primarily to the use of the ¹⁵N tracer isotope (Butterbach-Bahl et al. 2011; Delwiche 1970; Durand et al. 2011; Hertel et al. 2011; Müller and Rehder 2015; Rennenberg et al. 2009; Seitzinger et al. 2015; van Groenigen et al. 2015; Voss et al. 2011).

In Liebig’s time, however, the greatest achievement of the study of the chemical basis of agriculture was to show the importance of nitrogen for plants and animals. It was this early grasp of the nitrogen cycle that allowed an understanding of the depth and dynamics of the element’s role in the biosphere. As we have seen, the knowledge was accumulated by many researchers over decades of interwoven and overlapping discoveries.

The importance of ammonia’s role in food production had far-reaching economic and technological consequences, and also impacted chemistry as a scientific field. This influence was caused by several factors. By the 1880s, the finite sources of fixed nitrogen in South America had been identified as a bottleneck for further socioeconomic development. Replacing these resources became a political, economic, scientific, and technological challenge. Also, the realities of the nitrogen cycle posed fundamental questions about nitrogen fixation, specifically about ammonia synthesis: which biological mechanism did nature perform that chemistry could not mimic? Ammonia synthesis became both an economic and a scientific problem, an interconnection that would continuously encourage development.

To better understand the circumstances of the scientific arena that made ammonia synthesis possible, we now turn to the transformation of organic chemistry from its early empirical stage to its hybridization with physics as well as the comprehension of the role of a catalyst in chemical reactions.