In a field of sugar beets outside Cambridge, England, Simon Kelly stands above a narrow trench gouged into the rusty earth, roughly 15 feet deep and 30 feet long. “Welcome to the pit,” says Kelly, a bespectacled, white-bearded geologist in a straw hat and khaki shirt. “You’re seeing something that hasn’t been seen in a long time.”
The rock layers exposed in the trench date back more than 100 million years, to when England lay submerged beneath a warm, shallow sea. Kelly—a researcher at a nonprofit geology consultancy—specializes in marine fossils of that era (“Dicranodonta vagans!” he exclaims when I find a stone pocked with the impressions of tiny clam-like shells, which he asks to keep). That’s why he had an excavator dig this trench in 2015, and why he has spent countless hours since then sifting through its trove of treasures. “Going out to Simon’s hole, are you?” Kelly’s wife deadpanned when I picked him up on the morning of my visit.
I had come because “Simon’s hole” also contained objects of more recent historical significance: dull, round pebbles that once helped feed the United Kingdom. By the 1800s, centuries of cultivation had sapped Britain’s soils of nutrients, including phosphorus—an essential element for crops. At the time, manure and bones were common sources of phosphorus, and when the country exhausted its domestic reserves, it looked elsewhere for more.
“Great Britain is like a ghoul, searching the continents,” wrote Justus von Liebig, the German chemist who first identified the critical role of phosphorus in agriculture. “Already in her eagerness for bones, she has turned up the battlefields of Leipzig, of Waterloo, and of the Crimea; already from the catacombs of Sicily she has carried away the skeletons of many successive generations.”
Then, in the 1840s, geologists discovered phosphorus-rich stones buried in the fields around Cambridge—the same smooth, coffee-colored rocks welded into the walls of Kelly’s trench. “This is what they were after,” he says, pointing to a layer of bean-to-buckeye-size lumps.
These nodules were initially believed to be fossilized feces, and became known as coprolites, meaning “dung stones.” Most turned out to be chunks of mineralized sediments, but that did not diminish their utility as fertilizer.
“In the remains of an extinct animal world, England is to find the means of increasing her wealth in agricultural produce,” Liebig wrote. “May her excellent population be thus redeemed from poverty and misery!” And it was.
Over the ensuing decades, workers extracted 2 million tons of coprolites, transforming the fields and fens of southeast England into a warren of pits and trenches that dwarfed Simon’s hole. Coprolites were sorted, washed, and transported by buggy, train, and canal barge to processing facilities, where they were milled and treated with acid to make superphosphate—the world’s first chemical fertilizer.
The rocks helped Britain boost its food supply and consummate the so-called Second Agricultural Revolution (the first “revolution” being the rise of agrarian civilization). Coprolites and other geologic deposits of phosphorus also raised the tantalizing possibility that humans had at last broken free of an age-old biological constraint. For billions of years, life on Earth had struggled against a stubborn lack of phosphorus. Finally, that was about to change.
Life as we know it is carbon based. But every organism requires other elements, too, including nitrogen and phosphorus. Nitrogen is the basis of all proteins, from enzymes to muscles, and the nucleic acids that encode our genes. Phosphorus forms the scaffolding of DNA, cell membranes, and our skeletons; it’s a key element in tooth and bone minerals.
Too little of either nutrient will limit the productivity of organisms, and, by extension, entire ecosystems. On short timescales, nitrogen often runs out first. But that scarcity never lasts long, geologically speaking: The atmosphere—which is about 80 percent nitrogen—represents an almost infinite reservoir. And early in the course of evolution, certain microbes developed ways to convert atmospheric nitrogen into biologically available compounds.
Alas, there is no analogous trick for phosphorus, which comes primarily from the Earth’s crust. Organisms have generally had to wait for geologic forces to crush, dissolve, or otherwise abuse the planet’s surface until it weeps phosphorus. This process of weathering can take thousands, even millions, of years. And once phosphorus finally enters the ocean or the soil, where organisms might make use of it, a large fraction reacts into inaccessible chemical forms.
For these reasons, the writer and chemist Isaac Asimov, in a 1959 essay, dubbed phosphorus “life’s bottleneck.” Noah Planavsky, a geochemist at Yale University, says scientists have reached the same conclusion: “It’s what really limits the capacity of the biosphere.”
One of the lingering mysteries about the origin of life, in fact, is how the earliest organisms got hold of enough phosphorus to assemble their primitive cellular machinery. Some scientists think they must have evolved in environments with abnormally high concentrations of phosphorus, like closed-basin lakes. Others have suggested that bioavailable phosphorus came to Earth in comets or meteorites—a celestial gift that helped kick-start life.
A chronic shortage of phosphorus might also explain why it took so long for oxygen to build up in Earth’s atmosphere. Phytoplankton first began belching out the gas about 2.5 billion years ago, with the advent of photosynthesis. But they might not have had enough phosphorus to ramp up production, according to research by Planavsky and others, because the element kept getting bound up in iron minerals in the ocean, helping trap the world in a low-oxygen state for more than a billion years longer.
That we breathe oxygen today—and exist at all—might be thanks to a series of climatic cataclysms that temporarily freed the planet from phosphorus limitation. About 700 million years ago, the oceans repeatedly froze over and glaciers swallowed the continents, chewing up the rock beneath them. When the ice finally thawed, vast quantities of glacial sediment washed into the seas, delivering unprecedented amounts of phosphorus to the simple marine life forms that then populated the planet.
Planavsky and his colleagues propose that this influx of nutrients gave evolution an opening. Over the next 100 million years or so, the first multicellular animals appeared and oxygen concentrations finally began to climb toward modern levels. Scientists still debate exactly what happened, but phosphorus likely played a part. (To Planavsky, it’s “one of the most fascinating unresolved questions about our planet’s history.”)
Another group of scientists, led by Jim Elser of Arizona State University, speculate that such a pulse of phosphorus could have had other evolutionary consequences: Since too much phosphorus can be harmful, animals might have started building bones as a way of tying up excess nutrients. “Mind-blowing, right?” Elser says. “If true.”
What’s clear is that after this explosion of life, the phosphorus vise clamped down again. Geologic weathering kept doling out meager rations of the nutrient, and ecosystems developed ways to conserve and recycle it. (In lakes, for instance, a phosphorus atom might get used thousands of times before reaching the sediment, Elser says.) Together, these geologic and biologic phosphorus cycles set the pace and productivity of life. Until modern humans came along.
Over the course of several weeks in 1669, a German alchemist named Hennig Brand boiled away 1,500 gallons of urine in hopes of finding the mythical philosopher’s stone. Instead, he ended up with a glowing white substance that he called phosphorus, meaning “light bearer.” It became the 15th element in the periodic table, the incendiary material in matches and bombs, and—thanks to the work of Liebig and others—a key element in fertilizer.
Long before phosphorus was discovered, however, humans had invented clever ways of managing their local supplies, says Dana Cordell, who leads the food-systems research group at the University of Technology Sydney, in Australia. There and in the Americas, for example, Indigenous people managed hunting and foraging grounds with fire, which effectively fertilized the landscape with the biologically available phosphorus in ash, among other benefits. In agrarian societies, farmers learned to use compost and manure to maintain the fertility of their fields. Even domestic pigeons played an important role in biblical times; their poop—containing nutrients foraged far and wide—helped sustain the orchards and gardens of desert cities.
But human waste was perhaps the most prized fertilizer of all. Though we too need phosphorus (it accounts for about 1 percent of our body mass), most of the phosphorus we eat passes through us untouched. Depending on diet, about two-thirds of it winds up in urine and the rest in feces. For millennia, people collected these precious substances—often in the wee hours, giving rise to the term night soil—and used them to grow food.
The sewage of the Aztec empire fed its famous floating gardens. Excreta became so valuable that authorities in 17th-century Edo, Japan, outlawed toilets that emptied into waterways. And in China the industry of collecting night soil became known as “the business of the golden juice.” In Shanghai in 1908, a visiting American soil scientist named Franklin Hiram King reported that the “privilege” of gathering 78,000 tons of human by-products cost the equivalent of $31,000.
King, a forefather of the organic-farming movement who briefly worked at the U.S. Department of Agriculture, admired this careful reuse of waste and lamented that he saw nothing like it at home. This, King wrote, was an unfortunate side effect of modern sanitation, which “we esteem one of the great achievements of our civilization.”
The so-called Sanitation Revolution followed close on the heels of the Industrial Revolution. In the 1700s and 1800s, Europeans and Americans moved to cities in unprecedented numbers, robbing the land of their waste and the phosphorus therein. This waste soon became an urban scourge, unleashing tides of infectious disease that compelled leaders in places like London to devise ways to shunt away the copious excretions of their residents.
Liebig and other Victorian thinkers argued that this sewage should be transported back to the countryside and sold to farmers as fertilizer. But the volumes involved posed logistical challenges, and critics raised concerns about the safety of sewage farms—as well as their smell. Thus, waste ultimately was sent to rudimentary treatment centers for disposal or, more often, dumped into rivers, lakes, and oceans.
This created what Karl Marx described as the “metabolic rift”—a dangerous disconnect between humans and the soils on which they depend—and effectively sundered the human phosphorus cycle, reshaping its loop into a one-way pipe.
“That single disruption has caused global chaos, you could argue,” Cordell says. For one thing, it forced farmers to find new sources of phosphorus to replace the nutrients lost every year to city sewers. To make matters worse, agricultural research in the late 1800s suggested that plants required even more phosphorus than previously thought. And so began a frantic race for fertilizer.
Spain and the United States laid claim to uninhabited islands in the Pacific Ocean, where workers harvested towering accumulations of bird droppings. (Among them was Midway Atoll—later a U.S. naval station.) Back home on American soil, fertilizer companies scoured bat caves for guano and processed the bones of the countless bison slaughtered by hide hunters on the Great Plains.
In the course of these exploits, humans reached across vast distances to secure phosphorus. The discovery of coprolites in British fields allowed humans to reach back in time, too, seizing nutrients from another era and short-circuiting the geologic phosphorus cycle altogether. We saw a way to turn the stubborn trickle into a torrent, and that’s exactly what we did.
Until the late 1800s, the “stinking stones” that dotted the fields of South Carolina were considered a nuisance. But as the cost of imported guano soared and the Civil War reshaped southern agriculture, scientists discovered that these nodules of phosphate rock could be processed into decent fertilizer. By 1870, the first U.S. phosphate mines opened near Charleston and along the coast, tearing up fields, forests, and swamps to reach the bedrock below.
A decade later, geologists discovered even larger deposits in Florida. (To this day, most of the phosphorus on American fields and plates comes from the southeastern U.S.) Other massive formations of phosphate rock have since been identified in the American West, China, the Middle East, and northern Africa.
These deposits became increasingly important in the 20th century, during the Green Revolution (the third revolution in agriculture, if you’re keeping track). Plant breeders developed more productive crops to feed the world and farmers nourished them with nitrogen fertilizer, which became readily available after scientists discovered a way of making it from the nitrogen in air. Now, the main limit to crop growth was phosphorus—and as long as the phosphate mines hummed, that was no limit at all. Between 1950 and 2000, global phosphate-rock production increased sixfold, and helped the human population more than double.
But for as long as scientists have understood the importance of phosphorus, people have worried about running out of it. These fears sparked the fertilizer races of the 19th century as well as a series of anxious reports in the 20th century, including one as early as 1939, after President Franklin D. Roosevelt asked Congress to assess the country’s phosphate resources so that “continuous and adequate supplies be insured.”
There were also cautionary tales: Large deposits of phosphate rock on the tiny Pacific island of Nauru bolstered Australia and New Zealand’s agricultural progress during the 20th century. But by the 1990s, Nauru’s mines had run low, leaving its 10,000 residents destitute and the island in ecological ruins. (In recent years, Nauru has housed a controversial immigrant detention center for Australia.)
These events raised a terrifying possibility: What if the phosphorus floodgates were to suddenly slam shut, relegating humanity once more to the confines of their parochial phosphorus loops? What if our liberation from the geologic phosphorus cycle is only temporary?
In recent years, Cordell has voiced concerns that we are fast consuming our richest and most accessible reserves. U.S. phosphate production has fallen by about 50 percent since 1980, and the country—once the world’s largest exporter—has become a net importer. According to some estimates, China, now the leading producer, might have only a few decades of supply left. And under current projections, global production of phosphate rock could start to decline well before the end of the century. This represents an existential threat, Cordell says: “We now have a massive population that is dependent on those phosphorus supplies.”
Many experts dispute these dire predictions. They argue that peak phosphorus—like peak oil—is a specter that always seems to recede just before its prophecy is fulfilled. Humans will never extract all of the phosphorus from the Earth’s crust, they say, and whenever we have needed more in the past, mining companies have found it. “I don’t think anybody really knows how much there is,” says Achim Dobermann, the chief scientist at the International Fertilizer Association, an industry group. But Dobermann, whose job involves forecasting phosphorus demand, is confident that “whatever it is is going to last several hundred more years.”
Simply extracting more phosphate rock might not solve all of our problems, Cordell says. Already, one in six farmers worldwide can’t afford fertilizer, and phosphate prices have started to rise. Due to a tragic quirk of geology, many tropical soils also lock away phosphorus efficiently, forcing farmers to apply more fertilizer than their counterparts in other areas of the world.
The grossly unequal distribution of phosphate-rock resources adds an additional layer of geopolitical complexity. Morocco and its disputed territory, Western Sahara, contain about three-quarters of the world’s known reserves of phosphate rock, while India, the nations of the European Union, and many other countries depend largely on phosphorus imports. (In 2014, the EU added phosphate rock to its list of critical raw materials with high supply risk and economic importance.) And as U.S. and Chinese deposits dwindle, the world will increasingly rely on Morocco’s mines.
We have already glimpsed how the phosphorus supply chain can go haywire. In 2008, at the height of a global food crisis, the cost of phosphate rock spiked by almost 800 percent before dropping again over the next several months. The causes were numerous: a collapsing global economy, increased imports of phosphorus by India, and decreased exports by China. But the lesson was clear: Practically speaking, phosphorus is an undeniably finite resource.
I first heard about the potential for a phosphorus catastrophe a few years later, when a farmer friend mentioned casually that we consume mined phosphorus every day and that those mines are running out. The more I learned, the more fascinated I became by the story, not only because of its surprising and arcane details—eating rocks! mining poop!—but because of its universality.
Phosphorus is a classic natural-resource parable: Humans strain against some kind of scarcity for centuries, then finally find a way to overcome it. We extract more and more of what we need—often in the name of improving the human condition, sometimes transforming society through celebrated revolutions. But eventually, and usually too late, we discover the cost of overextraction. And the cost of breaking the phosphorus cycle is not just looming scarcity, but also rampant pollution. “We have a too-little-too-much problem,” says Geneviève Metson, an environmental scientist at Linköping University in Sweden, “which is what makes this conversation very difficult.”
At nearly every stage of its journey from mine to field to toilet, phosphorus seeps into the environment. This leakage has more than doubled the pace of the global phosphorus cycle, devastating water quality around the world. One 2017 study estimated that high phosphorus levels impair watersheds covering roughly 40 percent of Earth’s land surface and housing about 90 percent of its people. In more concrete terms, this pollution has a tendency to fill water bodies with slimy, stinking scum.
Too much phosphorus—or nitrogen—jolts aquatic ecosystems long accustomed to modest supplies, Elser says, triggering algal blooms that turn the water green, cloudy, and odorous. The algae not only discourage people from recreating in lakes and rivers (people “like to see their toes,” Elser observes) but also can produce toxins that harm wildlife and disrupt drinking-water supplies. And when the algae die, decomposition sucks oxygen out of the water, killing fish and creating devastating dead zones.
Indeed, pollution may be the strongest argument for reducing our dependence on mined phosphorus. “If we take all the phosphorus in the ground and move it into the system—ooh, we’re done,” Elser says. Some researchers have calculated that unchecked human inputs of phosphorus, combined with climate change, could eventually push much of the ocean into an anoxic state persisting for millennia. “I’m pretty sure we don’t want to do that,” Elser says, chuckling. Such events have occurred numerous times over Earth’s history and are thought to have caused several mass extinctions—for instance, when land plants evolved and sent a pulse of newly weathered phosphorus into the ocean.
The clear consensus among phosphorus experts is that humans must start mending the phosphorus cycle to reduce the environmental damage caused by pollution and to waste less of an increasingly scarce resource. Or, as a button I once saw Elser wear put it, save the p(ee).
Even industry has gotten on board: Yara, one of the world’s largest purveyors of fertilizer, recently announced a partnership with the European waste giant Veolia to recycle phosphorus from agricultural and food waste. Dobermann says that for many companies, sustainability “has increasingly taken over as a priority.”
Recycling human waste offers the most direct way of closing the phosphorus loop. A Canadian company called Ostara has installed systems to extract phosphorus from wastewater at municipal treatment plants in more than 20 cities around the world, including Chicago and Atlanta. Switzerland and Germany have even passed laws mandating the recovery of phosphorus from sewage that will take effect over the next decade.
The potential of recapturing phosphorus from animal manure is even greater. “If you get 40 Ph.D.s in a room, we always end up talking about cow shit,” Elser says. That’s because there’s a lot of it. And because the last great disruption to the phosphorus cycle involved livestock.
Throughout most of human history, farmers raised crops and animals side by side, which allowed them to easily recycle manure as fertilizer. During the 20th century, however, agricultural specialization separated livestock operations and grain growers, often by distances too large to transport manure.
This geographic rift effectively severed the last remaining strand of the human phosphorus cycle. And it led to a surplus of phosphorus in areas of intense animal agriculture, exacerbating pollution problems in places like the Chesapeake Bay, the waterways of Wisconsin’s dairy country, and Lake Erie. According to a recent study by Metson and others, 55 pounds of phosphorus are released into the environment for every pound of phosphorus consumed in U.S.-raised beef, more than half of which comes from manure. (For wheat, the ratio is roughly 2 to 1.)
In theory, recapturing this phosphorus could make a big difference. Metson and others estimate that the waste of American livestock contains more than enough phosphorus to support the entire U.S. corn crop; another analysis found that recycling all manure could halve global demand for phosphate rock. We have to change our mindset, says Graham MacDonald, Metson’s collaborator and an agricultural geographer at McGill University. “These aren’t waste streams,” he says. “These are resource streams.”
One cool day in December, Joe Harrison and I stand six feet apart, wearing masks, in a fenced gravel lot at Washington State University’s Puyallup Research and Extension Center. Harrison is a nutrient-management expert at WSU, and we first met at a sustainable phosphorus conference in 2018, where he told me about the mobile recycling unit he was developing to extract phosphorus from manure.
Now, the contraption sits before me: an 18-foot-long metal funnel folded on a flatbed trailer surrounded by green scaffolding, electrical panels, and an assortment of tubes. Over the past several years, Harrison and his colleagues have towed the unit to dozens of dairies across Washington for trial runs. First, a pump sucks liquid manure into huge plastic tanks, where it gets treated with acid. The slurry then flows through a thick hose into the base of the funnel, where it mixes with other chemicals and begins to form struvite, a pearly white phosphorus-bearing mineral (the researchers add some seed crystals beforehand, to help the reaction along). As the manure is processed, the struvite settles to the bottom for collection.
The project is both clever and pragmatic. It seems unlikely that humans will ever go back to growing all of our food locally on diversified, small-scale farms where manure can be recycled the old-fashioned way. (“It’s a shitload of work,” Dobermann observes.) But technologies like this offer an opportunity to close the phosphorus loop even over vast distances. For instance, Harrison wants to send the struvite harvested at dairies back to the eastern Washington farms that supply them with feed. “Why don’t we capture some of this phosphorus in western Washington and ship it back east where the alfalfa’s grown?” he says.
Harrison’s unit removes up to 62 percent of phosphorus if the manure has been digested by microbes beforehand—an increasingly common practice that also reduces greenhouse-gas emissions—and 39 percent if not. He calculates that a single cow can produce roughly 50 grams of struvite every single day, which means that, in a year, eight animals could provide about enough phosphorus to fertilize an acre of crops.
Struvite is one of several promising phosphorus fertilizers made by recycling human and animal waste. And it has numerous advantages: It’s portable; it doesn’t contain pathogens and other contaminants common in waste; and, according to Harrison, it works great as fertilizer. “The alfalfa growers—they want it,” he says.
Ostara has been testing its struvite, marketed as Crystal Green, for 15 years, with encouraging results. Their trials have found that, when blended with conventional fertilizer, struvite increases the yields of many crops, including canola and potatoes, Ahren Britton, Ostara’s chief technology officer, says. And growers have noticed. “Frankly, the demand for the product has outstripped the amount that we can recover,” he says. (Harrison’s collaborator on the mobile project, Keith Bowers, has since joined the company, in part to help expand its agricultural operations.)
Ostara’s success and Harrison’s pilot project prove that on a small scale, at least, it’s possible to reconnect the phosphorus cycle. And for wastewater-treatment plants, doing so is economical; under Clean Water Act regulations, they already have to remove excess phosphorus before discharging effluent. But for farmers, most of whom aren’t subject to similar rules, phosphorus recovery is just an added cost, according to Jay Gordon, the policy director of the Washington State Dairy Federation. “There’s something there,” says Gordon, who joined Harrison and me at the research center. But “it’s a big damn Rubik’s Cube.”
Gordon has tried to broker water-quality trading deals in which cities would pay local farmers to reduce runoff, with little success. Earlier this year, he took a different tack: While hosting a tour of the state’s dairies for Starbucks executives, Gordon suggested adding phosphorus to the company’s new sustainability program. “This is a world and a national food-security issue,” Gordon told them. And farmers can be part of the solution. “I would like to see every dairy farmer be a little miniature fertilizer plant,” he says. (When contacted, a Starbucks representative could not offer any information on the impact of Gordon’s pitch.)
For the moment, however, the mobile recycling unit sits idle. Harrison says farmers in other states had expressed interest in trying the system, but the pandemic brought operations to a halt. And he’s retiring in the spring.
Inside his field lab, Harrison shows me a stack of large cardboard cylinders filled with what looks like sand. Each contains the struvite from a single dairy. The project didn’t generate enough to supply the commercial alfalfa growers Harrison had in mind. Still, he estimates that there’s about a half ton of fertilizer stored in this shed.
“To be a good guy, I oughta find a home for it,” he says, but admits that he’s already started throwing some away. Gordon, who operates a 600-acre farm, lets out a little cry of surprise. He sold his dairy herd a few years ago and now grows corn, melons, and alfalfa, among other crops. And he perks up at the mention of free phosphorus: “I know just exactly where it can go.”
Reporting in the U.K. was supported by a science-journalism fellowship from the European Geosciences Union.
This article is part of our Life Up Close project, which is supported by the HHMI Department of Science Education.