The sun was about to set when Robert L. Shirley drove his beige pickup onto the Pamlico River ferry.

He was joined by fellow Potash Corp. employees who had just finished the day shift mining what scientists say could be the "gravest natural resource shortage you've never heard of."

Often overlooked, phosphorus is one of three elements needed to make fertilizer. The others, nitrogen and potash, are readily available with no shortages projected. But phosphate rock — the primary source of phosphorus in fertilizer — isn't as plentiful.

Scientists have estimated that minable supplies may not be sufficient to meet worldwide demand within decades. The situation could lead to higher food prices, famine and worse.

"There will be wars over water and oil. And right along with that, there will be wars over phosphorus," said Mark Edwards, a marketing professor and co-organizer of Arizona State University's Sustainable Phosphorus Initiative.

Shirley isn't so sure. The thought seemed distant, if not inconceivable, that afternoon as he read the paper and occassionally scanned pelicans and pines trees on the horizon. His employer estimates that the Aurora mine will be productive for at least 49 years, and while some people can think about the global implications of a possible shortage, Shirley focuses on how crucial phosphorus is to the local economy.

"This is a good company to work for," he said, adding, "There's very few jobs around here."

What FDR said

Like oil, phosphate rock is a finite, non-renewable natural resource created millions of years of ago beneath Earth's surface.

Intensive mining of the element began last century after PresidentFranklin D. Rooseveltwarned that phosphorus content in United States soil, after generations of cultivation, had "greatly diminished," threatening the nation's ability to produce crops.

"I cannot overemphasize the importance of phosphorus not only to agriculture and soil conservation but also to the physical health and economic security of the people of the nation," Roosevelt told Congress in 1938.

The element, which is found in every body cell, is most concentrated in human bones and teeth. It is essential to life and, at the present time, irreplaceable.

At the time of Roosevelt's speech, worldwide phosphate rock production hovered around 10 million tons. It doubled by 1950 and climbed to 100 million tons during the 1970s. Production reached 191 million tons last year, according toU.S. Geological Surveyestimates.

Reliance on the element is an "underappreciated aspect" that helped the world population grow by 4.2 billion people since 1950, according to a 2009 Foreign Policy magazine article.

"Peak phosphorus"

In 2009, a pair of Australian scientists published studies suggesting that demand for the element could exceed supplies as early as 2035. Using the term "peak phosphorus," an analogy to peak oil, they relied partly on a Geological Survey estimate that the world had 16 billion tons of minable phosphate rock.

The agency revised its estimate in 2010 to 71 billion tons after a massive deposit was proven in Morocco and the western Sahara. The region has the most reserves followed by Iraq, China and Algeria. With 1.4 billion tons, the U.S. is thought to have the world's eighth-largest reserves.

Jim Elser, who co-wrote the Foreign Policy article, said the new African reserves, if mined, would stave off peak phosphorus for decades. Nevertheless, it shouldn't prevent the world from reassessing how it uses phosphorus.

"Look at it like this," he wrote in an email to the Daily Press, "You're in a hotel and the fire alarm goes off. You get moving to exit your room when the phone rings. It's the front desk telling you that the fire isn't on your floor, it's actually five or six floors below you and won't reach you for another hour or so.

"Do you go back to bed to try to catch some more rest? Of course you don't."

The fertilizer industry did its own analysis and found there to be 300 years worth of phosphate rock worldwide, said Kathy Mathers, a spokeswoman for The Fertilizer Institute, which represents U.S. fertilizer businesses.

The Aurora mine

The Aurora mine, about 160 miles south of Newport News in Beaufort County, N.C., is one of 12 phosphorus mines in the U.S. The others are in Florida, Idaho and Utah.

A large, open-pit quarry, it stretches for miles along the Pamlico River, where ancient sea beds, sometimes only 40 feet beneath the surface, contain phosphate rock — a dirt-like mixture of clay, silt, fine quartz and phosphate sand.

Canada-based Potash Corp. declined to allow the Daily Press inside the mine; however, promotional material states that large excavators cut into the Earth exposing the rock. High-pressure water then breaks apart the rock to create a slurry, which is pumped through pipes into a nearby processing plant.

Course materials, clay and sand are removed from the slurry to obtain the phosphorus, which is then mixed with sulfur, sulfuric acid, ammonia and other chemicals.

Potash Corp. uses lower grade phosphorus to create phosphoric acid, a versatile compound used in toothpaste, soft drinks, anti-freeze and numerous other products. Some higher grade phosphorus is used as an animal feed supplement, but most is turned into fertilizer.

The mine can produce six million tons of phosphate rock annually.

Steady paycheck

A Texas native and mechanic, Shirley had a variety of jobs before he was lured to Aurora by the promise of steady work. That was 23 years ago.

He is one of 1,100 employees at the mine, the region's largest employer.

"We're basically just a poor, rural county," said Jerry Langley, chairman of the Beaufort County Board of Supervisors. "Without the mine, our tax base would be gone."

Not everyone is happy with Potash Corp. The Pamlico-Tar River Foundation, a local environment watchdog, is contesting a 30-year permit that Potash Corp. received in 2009 to expand mining operations.

The mine, which opened in 1965, has a poor environmental track record, said Heather Deck, the group's riverkeeper. Past problems including phosphate leaching into the river and the destruction of ecologically sensitive wetlands.

Potash Corp. spokesman Tom Pasztor said the company rebuilds two acres of wetlands for every acre it destroys. Deck countered that new wetlands are not as efficient as existing wetlands.

"Eventually they may provide the same value as the one destroyed, but it takes time," she said.

There are at least 49 years worth of minable reserves at Aurora, Pasztor said — a number that Shirley takes comfort in.