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Stephen Powers/Washington State University

This global map of manure could help save farming as we know it

To grow the world’s wheat, corn, and beans, farmers need phosphorus—an essential nutrient that comes from bird and bat droppings and rock deposits. But the global supply of easily mineable phosphorus is dwindling; to stave off the coming drought, scientists are exploring an alternative: recycling animal manure for its phosphorus content. Now, they’ve come up with the world’s first map of this underappreciated resource, which shows that most manure is exactly where farmers need it—in their own backyards.

To make their map (above), researchers used data on livestock density and calculated the annual amount of phosphorus excreted by cattle, pigs, chickens, sheep, and goats globally—as much as a whopping 130,000 kilograms per square kilometer, they report in an upcoming issue of Earth’s Future. (Various estimates put total global production between 15 million to 20 million metric tons per year.) The researchers found “hot spots,” areas in which manure-based phosphorus is a widely available, but underused, on every continent except Antarctica. Unsurprisingly, many of those hot spots are near farming communities and river deltas where agricultural runoff abounds.

But reusing old phosphorus is easier said than done. To process pig and cow poo, farmers must break it down with bacteria or use special equipment to crystallize its struvite—the same phosphate mineral that makes up some kidney and bladder stones. These processes are already used by many commercial farms, which together help recycle about half the global supply of manure. But they are costly for small family farms, which supply most food in parts of Asia, sub-Saharan Africa, and Latin America.

The researchers hope their map will encourage countries, including India, Brazil, China, and the United States (which together use 66% of the world’s phosphorus fertilizer), to support phosphorus recycling. Not only would more recycling reduce imports, but it would also help the environment by eliminating manure—and its phosphorus—from the water supply. It could also put a few more years on our phosphorus clock.