The History of Nitrogen Fertilizer

Nitrogen is not a rare substance. About 78% of Earth's atmosphere is nitrogen gas. There is far more of it than oxygen, and it floats endlessly above our heads right at this very moment. Yet for a very long time, humanity starved because of a lack of nitrogen. This paradox is where our story begins.

Nitrogen is so vital because it is an essential component of both protein and DNA. Among the building blocks of living bodies, almost nothing can be made without it. Crops draw nitrogen up from the soil, and humans obtain nitrogen by eating those crops. That is why, once a harvest is over, the field is depleted of just that much nitrogen. If it isn't replenished, the next year's harvest shrinks.

The problem lies in the bond. Atmospheric nitrogen consists of two atoms tightly locked together by a triple bond, and breaking that bond requires 945.41 kJ/mol. It is one of the strongest locks in all of chemistry. Most living organisms cannot open it. Bacteria living on the roots of legumes and lightning strikes are among the few exceptions. Unlocking this bond to convert nitrogen into forms that living things can use, such as ammonia or nitrates, is called nitrogen fixation.

Why, of all nutrients, did nitrogen hold us back? In 1840, Justus von Liebig published a book. His core message was that plants absorb inorganic nutrients like nitrogen, phosphorus, and potassium from the soil, and that the single most deficient nutrient determines crop yields. In a wooden barrel made of staves of varying lengths, water can only rise to the height of the shortest stave. In most farmland, that shortest stave was nitrogen. That was why replenishing nitrogen alone caused crop yields to surge noticeably.

However, Liebig himself changed his stance several times during his lifetime over whether nitrogen fertilizer was truly necessary. At one point, he believed atmospheric ammonia alone was sufficient, before later returning to his original view. Despite his nickname as the "Father of the Fertilizer Industry," he too wavered on this question for a long time.

The history of farming was therefore also a history of where to find nitrogen. Livestock manure, ash, and crop rotation incorporating legumes—all of these were methods for moving nitrogen that was already circulating on the surface from one place to another. They were not ways to increase the total amount.

The first method to increase the total amount was digging it up. The second method was capturing it directly from the air. This story is about what unfolded between those two methods.

In November 1802, Alexander von Humboldt arrived at the Peruvian port of Callao. On the offshore islands, there were thick layers of seabird droppings that had hardened over thousands of years. Because it rarely rained along this coast, they were never washed away and remained completely preserved. Humboldt surveyed the substance and introduced it to Europe. That material was guano.

Calling it a "discovery" would be incorrect. Indigenous peoples of the Andes had already been applying guano to their fields for hundreds of years. What Humboldt did was connect its existence to European markets. Then, with the publication of Liebig's book in 1840, European farmers learned what their soil was lacking and what they needed to buy and pour into it. Demand exploded.

Guano was not a primitive fertilizer, but a highly concentrated nitrogen mineral resource. And these earnings represented the state budget itself for Peru. At one point, guano from the Chincha Islands generated about 60% of the Peruvian government's annual revenue.

How that money was made must also be documented. In 1849, the Peruvian government authorized the importation of Chinese indentured laborers, and by 1874, about 100,000 of them had entered Peru. More than two-thirds of them died before finishing their contract terms. The Chincha Islands became known as "Hell's Islands."

Individual records survive as well. Calculations as of 1860 show that among the 4,000 laborers sent to the Chincha Islands up to that point, not a single one survived. While this was a contemporary estimate rather than an exact census, it gives a clear picture of the staggering mortality rate. By around 1852, two-thirds of the mining workforce were Chinese indentured laborers, and floggings, riots, and suicides were recorded in the archives.

Once nitrogen became profitable, nations mobilized. On August 18, 1856, the United States passed the Guano Islands Act. It allowed American citizens who discovered guano on an unclaimed island to claim that island on behalf of the United States. By the following year, over 100 islands were claimed, and by 1903, 66 locations were recognized as U.S. territories. This law remains on the books today. However, most of the islands claimed back then were later settled or relinquished, leaving 10 remaining today.

In 1864, war broke out when Spain occupied the Chincha Islands. Over islands covered in bird droppings, the old European imperial master sent an entire naval fleet.

Yet any resource that must be dug out has a bottom. Entering the 1870s, the guano of the Chincha Islands was virtually depleted. Seabirds fled the islands from the noise of excavation, and the rate of fresh accumulation could never keep up with the speed of extraction. It was a boom that ended in just over 30 years.

As guano began to run out, the next contender stepped up: Chilean saltpeter. These were deposits of sodium nitrate nestled in the Atacama Desert. Because it rarely rains there, the water-soluble nitrates were never washed away and remained intact. In 1872, the Santa Laura and La Palma saltpeter works were established in the Atacama. La Palma would later become better known as Humberstone.

Saltpeter had an even wider range of uses than guano. Spread it across fields, and it was fertilizer; send it to munitions factories, and it was the raw material for explosives. The fact that both of these crucial uses depended on a single mineral deposit sealed the fate of this desert.

On February 14, 1878, the Bolivian congress approved a tax of 10 centavos per quintal on a Chilean company mining saltpeter along its coast. In terms of the amount alone, it seemed like a trivial dispute. But this tax became the fuse. On March 1, 1879, the War of the Pacific broke out. It was a clash between Chile and the allied forces of Bolivia and Peru, and at its core lay the saltpeter and guano deposits of the Atacama. Peru's guano exports had already plummeted from 575,000 tons in 1869 to less than 350,000 tons in 1873.

The war dragged on for 5 years. Casualties were recorded at about 25,000 for the allied forces and 2,425 to 2,791 for Chile. Under the Treaty of Ancón on October 20, 1883, Peru ceded Tarapacá, its richest saltpeter region, to Chile. The following year, under the Truce of Valparaíso, Bolivia lost its entire coastline. Bolivia has been a landlocked country ever since.

What the victor won was no ordinary desert. By around 1883, Chile controlled about 80% of the world's nitrogen supply. The world's farms and the world's gunpowder now depended entirely on a single desert in one country. How much wheat a European farmer could harvest hung on the borders and port controls along the western coast of South America.

There was no need to explain just how precarious this arrangement was. The deposits were finite, the source lay on the opposite side of the globe, and the sea lanes connecting them could be cut off by anyone commanding a navy. Nitrogen became a matter of geopolitics not in the 20th century, but in the 19th.

In 1898, William Crookes delivered his presidential address to the British Association for the Advancement of Science. He warned that the world's wheat-eating nations would run out of food by the 1930s. His calculations showed that virtually all arable land was already in use, and the remaining sources of nitrogen were strictly finite. He then issued a challenge to the world's chemists: find a way to fix nitrogen from thin air.

It was a rare case where a prophecy of doom actually summoned new technology into existence. Yet Crookes was not the very first to try. Efforts to capture atmospheric nitrogen were already underway.

The first was the Frank-Caro process. Perfected between 1895 and 1899 by German chemists Adolph Frank and Nikodem Caro, it reacted calcium carbide with nitrogen at about 1,000°C to produce calcium cyanamide. It was the world's first commercially viable method for atmospheric nitrogen fixation. Marketed as lime nitrogen, it was actually spread onto fields.

The second was the Birkeland-Eyde process. Developed for practical use in 1903 by Kristian Birkeland and Sam Eyde in Norway, the idea was to mimic lightning. By heating air to over 3,000°C with an electric arc, nitrogen and oxygen fused into nitric oxide, which was then oxidized and absorbed to produce nitric acid.

The principle was elegant, but the electricity bill was a nightmare. Producing 1 ton of nitric acid required about 15 MWh of energy. To run a plant this way, electricity had to be virtually free. That is why they built the facilities in Notodden and Rjukan, Norway—right next to hydroelectric power stations. A technology that only works where electricity is dirt cheap could never feed the world's fields. The process was pushed aside in the 1910s and 1920s.

Both methods succeeded in using air as their raw material, but both were detours. One required synthesizing an intermediate substance called calcium carbide, while the other required tethering an entire power plant to the factory. The only remaining path was to combine nitrogen and hydrogen directly into ammonia, but this reaction stubbornly refused to cooperate. Raising the temperature sped up the reaction, but the newly formed ammonia broke apart again; lowering the temperature slowed the reaction to a crawl. The only tool left was pressure.

In the summer of 1909, Fritz Haber and his assistant Robert Le Rossignol succeeded in continuously producing ammonia with a tabletop apparatus. The operating conditions were about 500 °C, 200 atmospheres, and an osmium catalyst. The production rate was about 125 mL per hour—a trickle so slow it took a long time just to fill a single glass bottle. They demonstrated this before representatives from BASF, and the company made the decision to commercialize it.

Why was 125 mL such a big deal? Because until then, no one had managed to run this reaction at any practical speed. The person who built the device capable of withstanding 200 atmospheres was Le Rossignol. His name was included on Haber's patent and he received royalties, but the Nobel Prize went to Haber alone.

The catalyst had to be replaced as well. Osmium and uranium were scarce and expensive. In 1909, Alwin Mittasch at BASF began systematically searching for an affordable iron-based catalyst. Because sources cite the number of trials anywhere from thousands to tens of thousands of times, it is difficult to pin down an exact figure. What is certain is the result: a recipe mixing iron oxide with small amounts of potassium, calcium, aluminum, and silicon oxides that remains in use almost unchanged to this day.

And from here on, the job belonged to someone else. Carl Bosch took on the challenge of turning "it works" into "it works as a factory." Enduring 200 atmospheres for a brief moment in a laboratory is an entirely different matter from enduring it uninterrupted for months in a plant. Bosch's team redesigned the reactor, engineered compressors that could continuously generate that staggering pressure, and built out a system to supply hydrogen in bulk.

In 1913, the world's first industrial ammonia production began at BASF's Oppau plant. The following year, output reached 20 tons a day. Early production capacity is cited differently across sources, ranging from 20 to 40 tons a day, so it can roughly be read as tens of tons per day. It took 4 years to go from 125 mL per hour to an operating factory, and just one year from there to reach that scale. These 4 years are the true missing chapter in 20th-century food history.

The backbone of today's process remains the same. It runs at 450–550 °C and 150–250 bar, and only about 15% of the gas converts to ammonia in a single pass. The rest is cycled back. By repeating this recirculation, the overall conversion rate climbs to about 97%. Thanks to this loop design—recycling continuously instead of trying to convert everything in one go—the factory runs under conditions where the reaction does not even proceed halfway. It was Bosch's team that built the compressors to drive that relentless recycling on an industrial scale.

What Haber proved was the reaction; what Bosch proved was the factory. Lump the two men together, and half of chemical engineering disappears. They received separate Nobel Prizes, too. Haber was named the recipient of the 1918 Nobel Prize in Chemistry for ammonia synthesis, though the actual award ceremony took place in 1919. Bosch received the Chemistry Prize in 1931 alongside Friedrich Bergius for the development of high-pressure chemical methods. That is a 13-year gap, and the reasons for their awards were completely different.

What the Haber-Bosch process makes is ammonia. Ammonia itself can be used as a fertilizer, but the fork in our story lies right after that.

In 1902, Wilhelm Ostwald patented a process. It was a method of oxidizing ammonia over a platinum catalyst to convert it into nitric acid. For a long time, this process remained just a patent and sat idle, because the raw material—ammonia—was expensive. Then, in 1913, as cheap ammonia began pouring out of Oppau, it finally became economically viable. At that moment, a complete path came into being: air → ammonia → nitric acid.

Nitric acid goes to two places. On one side are fertilizers like ammonium nitrate; on the other are explosives like TNT and nitroglycerin. It is the very same molecule—only the pipe it travels through is different. The branching point was not chemistry, but plumbing.

In 1914, a British naval blockade cut off Germany's imports of Chilean saltpeter. That was the saltpeter from the Atacama Desert we saw in the previous chapter. Germany, which had relied on imports for the nitric acid needed to produce gunpowder, plugged that hole by converting synthetic ammonia into nitric acid via the Ostwald process. The blockade also severed the fertilizer supply, and as remaining stockpiles were diverted to munitions production, it played a part in the collapse of German agricultural output. It meant that nitrogen destined for the fields went into artillery shells instead.

Oppau alone was not enough. On May 25, 1916, construction began on a second large-scale ammonia plant in Leuna, central Germany. The reason was that the demand for explosives had outstripped existing production capacity, and the site was chosen because it was beyond the flight range of French aircraft. In April 1917, the first ammonia tank car rolled out of the factory.

Here, we need to pause for a moment. The statement that 'without this technology, Germany would have run out of munitions within a few years and the war would have ended early' is very widely quoted. However, this is not a fact established by historical records, but a counterfactual interpretation. Even in academic literature, it is often framed cautiously as 'it has long been said that...' Only two things can be confirmed: that the blockade cut off Chilean saltpeter, and that Germany replaced it with synthetic nitric acid.

The fact that fertilizer and explosives are the same thing was proven not on the battlefront, but right in a factory yard. On September 21, 1921, at 7:32 AM, a fertilizer mixture of ammonium sulfate and ammonium nitrate exploded in a silo at the Oppau plant. About 10% of the 4,500 tons in storage detonated. At least 560 people were killed and about 2,000 were injured. The crater measured 90×125m with a depth of 19m, and the blast was heard over 300km away in Munich and northeastern France. The cause was the routine work practice of using dynamite to break up compacted piles of fertilizer.

Ammonium nitrate is a fertilizer with 34% nitrogen. At the same time, when mixed with fuel oil, it becomes an explosive called ANFO, which accounts for 80% of all explosive use in North America. A fertilizer warehouse and an ammunition depot are effectively stocking chemically identical materials.

Fortunately, this is not the mainstay of the world's nitrogen fertilizer today. In 1922, Bosch and Wilhelm Meiser patented a process to make urea from ammonia and carbon dioxide. Urea is a solid, does not explode, and its nitrogen content of 46% is the highest among commonly used solid nitrogen fertilizers. It is also the very same substance that Friedrich Wöhler synthesized from inorganic matter in 1828, bringing down vitalism.

In this chapter, I will simply record the dates and events, without judgment.

In 1911, Haber took office as the director of the Kaiser Wilhelm Institute for Physical Chemistry and Electrochemistry in Dahlem, Berlin. This institute would later become the headquarters for poison gas research during World War I.

Around 5:00 p.m. on April 22, 1915, during the Second Battle of Ypres, the German army released 168 long tons—that is, 171 tons—of chlorine gas from 5,730 cylinders across a 6.5 km front line. Haber proposed this operation and supervised the installation. Otto Hahn, James Franck, and Gustav Hertz were there with him. Casualty counts for that day vary widely across sources, making it impossible to state a definitive figure.

Here, we need to clearly separate the chemistry. The chlorine gas used that day is a substance produced during the electrolysis of brine. It has no connection to the Haber-Bosch process, neither in raw materials nor in chemical procedures. The explosives route we saw in the previous chapter ran from ammonia to nitric acid, and chlorine was not even plumbed into those pipes. Saying that 'the Haber-Bosch process created poison gas' is chemically false. The accurate statement is simply that the man named Haber did both things.

On May 2, 1915, Clara Immerwahr took her own life in the garden of their home in Dahlem, Berlin, using her husband's military service pistol. It was shortly after Haber returned from Ypres. She was a chemist who, in 1900 at the University of Breslau, became the first German woman to earn a doctorate in chemistry.

Her death is often portrayed as a suicide in protest against her husband's chemical weapons work, but because there was no suicide note and contemporary records are sparse, her motive remains unconfirmed. Interpretations attributing it to opposition to chemical warfare, frustration with a marriage that stifled her scientific career, or the view that the motive simply cannot be known all coexist. Even local newspapers at the time reported that the reason was unknown.

Haber was selected as the recipient of the 1918 Nobel Prize in Chemistry, and the award was presented in 1919. The stated reason was the synthesis of ammonia. There was fierce criticism over awarding the prize to someone who had led a chemical weapons program, and the Nobel Committee stated that it judged the synthesis of ammonia to have conferred the greatest benefit to humankind.

On October 1, 1933, Haber resigned from his post as institute director amid Nazi antisemitic measures. On January 29, 1934, he died of heart failure in a hotel in Basel, Switzerland. He was 65 years old. Shortly before his death, he had accepted the directorship of the Sieff Institute in Rehovot, Palestine.

A pest control research organization he established in 1917 developed cyanide-based pesticides, and a descendant of that product line was Zyklon B. Haber did not invent it himself. Starting in 1942, Zyklon B was used to murder people in the gas chambers of extermination camps, including Auschwitz-Birkenau. Approximately 1.1 million people were murdered at Auschwitz, and among them, about 900,000 were Jews killed in the gas chambers shortly after arrival. Haber had died 8 years earlier. His half-sister's daughter, Hilde Glücksmann, and her family perished in a concentration camp. In 1945, Bruno Tesch and Karl Weinbacher, from the firm that supplied Zyklon B to the SS, were convicted by a British military tribunal and executed, on the grounds that they knowingly supplied the chemical to murder humans.

This is what happened.

You might think that simply pouring on more nitrogen fertilizer would keep increasing yields, but that is not what actually happens. When traditional varieties of wheat and rice receive heavy doses of nitrogen, they shoot up tall and their grain heads grow heavy, causing them to collapse to the ground. Fertilizer alone runs straight into a wall.

In July 1944, Norman Borlaug arrived in Mexico under a Rockefeller Foundation program and began his wheat research. At the time, Mexico was importing half of its wheat needs—that was the situation in 1943. The country achieved self-sufficiency in 1956, and by 1964, it was exporting 500,000 tons of wheat a year.

The key was plant height. In 1953, Borlaug introduced the Japanese semi-dwarf wheat variety Norin 10 and began using it for crossbreeding. Developed by Gonjiro Inazuka, this variety had short stalks, so it did not fall over even when the grain heads grew heavy. Farmers could now apply fertilizer to their hearts' content. Semi-dwarf varieties and synthetic nitrogen fertilizer are an inseparable pair.

The same thing happened with rice. In 1966, the International Rice Research Institute (IRRI) released IR8. Even without fertilizer, it yielded about 5 tons per hectare, and under optimal conditions, about 10 tons. It was publicized as yielding about 10 times more than traditional varieties, but because that multiple depends on which traditional variety is used for comparison, it is hard to take literally. There are clearer numbers to gauge its scale: rice yields in Asia rose from about 2 tons per hectare in the 1960s to about 6 tons by the mid-1990s. On March 8, 1968, William Gaud, Administrator of the U.S. Agency for International Development (USAID), gave this trend a name: the 'Green Revolution.'

In 1970, Borlaug received the Nobel Peace Prize. By that time, India's wheat harvest had grown from 12.3 million tons in 1965 to 20.1 million tons in 1970, and Pakistan's had expanded from 4.6 million tons to 7.3 million tons. India's wheat harvest reached 76.4 million tons in 2000.

Borlaug is almost always accompanied by the phrase that he 'saved one billion people from starvation.' But that number was an estimate from an article in 1997—a projection, not a tabulated tally. What can actually be verified goes only as far as the harvest statistics above.

Something must also be recorded with equal weight: across the 1970s and 1980s, criticisms of the Green Revolution mounted. First, the Green Revolution was not a single seed; it was a package. Semi-dwarf varieties only proved their worth with generous doses of nitrogen; getting that nitrogen into the plant required timely irrigation; and with that came the need for chemical pesticides. Buying only the seeds accomplished nothing—fertilizer, pesticides, and irrigation had to be in place all together. The outcome hinged on whether a farmer could pay for all four at once, and smallholders who could not afford the package were shut out from the very start. Second, around 1980 in the Philippines, credit programs created specifically to address that financing hurdle ended up benefiting wealthy landowners while sinking poor farmers into debt. Third, monoculture concentrated around just a few varieties reduced biodiversity. Fourth, pesticides damaged river ecosystems, and excessive use of nitrogen fertilizer drove up emissions of nitrous oxide. We will encounter this last point again in the next chapter.

It is a fact that yields increased, and it is also a fact that seeds did not decide where that extra harvest would go. If you leave out either one of these two statements, it becomes impossible to understand what really happened back then.

How much nitrogen does the world use today? In 2023, global agricultural fertilizer consumption was 190 million tons on a nutrient basis, with nitrogen making up 58% at 112 million tons. The largest consumers were China at 18.7 million tons, India at 11.9 million tons, and the United States at 9.1 million tons.

There is a trap when reading these figures. The 112 million tons is the weight of pure nitrogen nutrient. If you count by total product weight, the number becomes vastly larger. That is because urea is only 46% nitrogen, and ammonium nitrate is 34%. There is another dimension here as well. Global ammonia production was tallied at 240 million tons in 2023, which is based on the weight of the ammonia molecule itself. This figure varies widely across sources. It shifts depending on whether one counts production or production capacity, and whether it is measured by ammonia weight or nitrogen nutrient content. If you mix these three criteria in a single paragraph, you will inevitably end up with contradictory statements.

Not all the nitrogen applied to fields reaches the crops, either. The global nitrogen use efficiency of crops is less than 50%. That means more than half leaks elsewhere. It dissolves in water as nitrate and leaches into groundwater, volatilizes into the air as ammonia, or turns into nitrous oxide and nitrogen gas through microbial denitrification. Nitrate does not cling well to soil particles, making it especially easy to wash away.

When this escaped nitrogen travels down rivers and reaches the ocean, algae blooms explosively. Then, the bacteria decomposing their remains consume all the oxygen at the bottom. This creates hypoxic zones where fish cannot survive—dead zones. The Gulf of Mexico dead zone reached 22,730 km² in 2017, the largest ever recorded. About 70% of the driving nutrients flow in from the Mississippi River basin. However, because dead zones are seasonal phenomena that appear and disappear in summer, their surface area fluctuates sharply each year. The 2017 figure is a record maximum, not an annual average.

Some nitrogen escapes into the atmosphere as well. Nitrous oxide has a global warming potential about 300 times that of carbon dioxide for an equivalent mass. While the exact figure is updated in each report, it is currently ranked as the third-largest contributor to global warming, after carbon dioxide and methane.

There is also a bill on the manufacturing side. In its 2021 Ammonia Technology Roadmap, the International Energy Agency estimated that ammonia production accounts for about 2% of global final energy consumption and 1.3% of CO2 emissions from the energy system. According to the same report, about 70% of the ammonia produced goes toward fertilizer, while the remainder goes into plastics, synthetic fibers, explosives, and more. Because this share is measured slightly differently in each dataset, you need to check which agency compiled it and in what year.

'Planetary Boundaries' is an assessment framework that defines thresholds Earth can safely withstand across various dimensions to evaluate where we currently stand. In the 2025 update, nitrogen remained an item that significantly exceeded its boundary. The safe operating limit for industrial and intentional nitrogen fixation is 62 Tg N per year, but the actual value stands at 165 Tg N per year. Tg stands for teragram, where 1 Tg is 1 million tons, and N indicates it is measured by nitrogen nutrient content. That is about 2.7 times the threshold.

Here, two statements must be placed side by side. If we keep our current diets and farming methods, it is hard to feed the world without synthetic nitrogen. And that synthetic nitrogen is also an item that has vastly exceeded its planetary boundary. Both are true. If you tell this story relying on only one of them, you will inevitably end up with a false narrative.

Nitrogen comes from the air, but making ammonia requires hydrogen. And most of that hydrogen comes from natural gas. Gas serves as the raw material as well as the fuel to generate high temperatures and high pressure. So, when gas prices rise, fertilizer prices follow right along.

That very thing happened in 2021. Global fertilizer prices jumped 80% in a single year. Ammonia plants across Europe cut production because they could not bear the gas costs, and on February 2, 2022, Russia temporarily banned exports of ammonium nitrate. According to a summary by the World Bank on May 11, 2022, prices climbed another 30% through May of that year, pushing urea past its 2008 peak. Remember, this figure is strictly as of May 2022. Prices came down afterward, so reading it as today's price would be incorrect.

Why is it shaken so easily? Because supply is concentrated in just a few countries. As of May 2022, Russia accounted for about 16% of global urea exports and about 12% of phosphate fertilizers; combined with Belarus, the two held about two-fifths of potash fertilizer exports. This means that even fertilizer pulled from thin air is not free from geopolitics.

There is also a stark case showing what happens when fertilizer is cut off. In April 2021, the Sri Lankan government announced an outright ban on imports of chemical fertilizers and pesticides, declaring a transition to an all-organic nation. Within six months, rice production fell 20%, shattering self-sufficiency and forcing the country to import $450 million worth of rice. A drop in tea production cost another $425 million. In November of that year, the policy was revoked.

However, this measure was not the sole cause of Sri Lanka's economic crisis. A foreign exchange crisis, the collapse of tourism, and tax cuts all played a part together. Reducing the cause down to a single factor turns it into an inaccurate story too.

You do not need to predict the future. Instead, let us walk through what signals to watch so you can spot the board shifting before anyone else.

First is the lost half. The fact that more than half of applied nitrogen escapes the field means, looking at it the other way, there is just as much room to achieve the same harvest while applying less. Research is moving forward on two fronts: pinpointing precisely when, in what form, and how much to apply, and breeding crop varieties that lose less nitrogen.

Second is the production method. If hydrogen is obtained by splitting water using electricity from renewable energy instead of natural gas, carbon dioxide emissions drop dramatically. This is known as green ammonia. It is easy to confuse this with the Birkeland-Eyde process, which was pushed aside earlier due to electricity costs, but the two are different methods. That old process used electricity to heat air up to 3,000°C to oxidize nitrogen directly, whereas green ammonia uses electricity only to produce hydrogen and still leaves the job of binding it with nitrogen to the Haber-Bosch process. The share of work electricity must carry has become far smaller. Even so, the standard in ammonia plants today remains obtaining hydrogen from natural gas.

Third is prices and borders: gas prices, export controls, and which country's dinner table feels the shock first. Fourth is the benchmark behind the numbers: whether it refers to nutrient content or total product weight, which year's tally it is, and whether it is an estimate or a direct measurement. This topic is notoriously prone to mixed-up standards.

Let me add one final scene. In 1960, the saltpeter towns of Humberstone and Santa Laura in the Atacama Desert shut their doors. Between the flood of cheap synthetic nitrogen and the Great Depression, an industry that once supplied 80% of the world's nitrogen collapsed entirely. Here, too, the cause was not just one thing. Those ghost towns left behind in the desert became a UNESCO World Heritage site in 2005. They stand as a lasting reminder that when technology shifts, an entire geopolitical landscape can lose its value overnight.