Scientists have been pointing out the “broken” phosphorus cycle for more than a decade: Humanity has unearthed huge quantities of the element, which winds up in waterways instead of returning to cropland.
The problem comes down to crap. People and livestock eat crops and excrete phosphorus as a result. (A University of Iowa researcher calculated that the state’s livestock produce a load of excrement equivalent to a nation of 168 million people.) But most of it won’t end up feeding plants again. Waste treatment can loop sludge or manure back to being fertilizer, but transporting and treating it is often impractical, so it may sit in stockpiles and “dry stacks” without the chance to boost another crop.
Or the system may be leaky: Sewage, septic tanks, stockpiles, and eroded soil drip phosphorus into oceans and rivers, where it dilutes to oblivion while degrading those ecosystems. For instance, phosphorus runoff drives the harmful algal blooms that have killed Florida’s seagrass, starving thousands of manatees.
Demay’s model determined that in a 67-year span, humans pumped almost a billion tons of nonrenewable phosphorus into food systems. Her team’s figures are derived from statistical data from the Food and Agriculture Organization of the United Nations. The global data, broken up by country, reported agricultural yields—like the amount of wheat grown, or headcounts of pigs and cows—from 1961 to 2017. (Data from 1950 to 1961 came from other data sets.)
Her team also broke down use trends. In 2017, Western European, North American, and Asian reliance climbed to nearly 60 percent of the total plant-ready phosphorus available in each region’s soil. Brazil, China, and India are quickly increasing their use, to 61, 74, and 67 percent respectively. The numbers for France and the Netherlands are no longer rising, because they’ve replaced some use of phosphate rock with manure; now they sit at roughly 70 and 50 percent. Yet in African countries like Zimbabwe, a lack of soil phosphorus limits crop yields. Demay’s estimates pin mineral fertilizer use in Zimbabwe to the 20 to 30 percent range, which is even lower than the 32 percent average for all of Africa.
To Elser, this illuminates a global inequity: Poorer countries access far less fertilizer, despite needing it more. And wealthy countries have been able to amass stockpiles from the rock reserves for decades, while countries that struggle with food security can’t afford to do the same.
This raises concerns over who will control the future of fertilizer. Nearly 75 percent of the world’s supply sits in the mines of Morocco and the Western Sahara. Economists get anxious when a commodity is consolidated in the hands of a few powerful people. (OPEC controls roughly the same fraction of the world’s oil, but with 13 member states.)
And it’s not entirely clear how long supplies will last. In 2009, Cordell estimated that a global “peak phosphorus” moment could happen as soon as 2030, which would leave 50 to 100 years of dwindling reserves. Today, she and Elser agree that the peak will likely come later, although it’s hard to predict when, because demand may skyrocket for other uses, like lithium iron phosphate batteries. Elser notes that new analyses now put the maximum supply at around 300 to 400 years.