The Environmental Defense Fund opened an office near Walmart's headquarters in Bentonville, Ark., 10 years ago. It was part of a carefully plotted strategy to persuade the giant retailer that going green could be good for business. If it worked, it certainly could be good for the planet — Walmart's revenues are bigger than the entire economy of most countries.

"We really saw that working with companies could be transformative at a scale that was pretty unmatched," says Suzy Friedman, a senior director at EDF.

If you're looking for evidence that the strategy is working, there's this: Last year, Walmart unveiled Project Gigaton, a plan to reduce emissions of greenhouse gases by a billion tons of carbon between now and 2030. That's almost as much carbon as what's released from the country's entire fleet of passenger cars and trucks in a year.

The cuts will come from the company's suppliers: the vast galaxy of companies that make the products it sells.

Even before unveiling that pledge, Walmart had been calculating the climate price tags of those products, estimating the greenhouse gases that are released in the process of making each one. Laura Phillips, Walmart's senior vice president for sustainability, was startled to see the climate price of simple food items, like baked goods, that don't seem like they'd require burning a lot of fossil fuels.

"Why is that?" she wondered. "Why are we seeing bread have high emissions?"

Other food companies are asking the same question. Many of them, including General Mills and Kellogg, have made their own commitments to reduce greenhouse emissions. To get a better grasp of the task, they joined forces and set up an organization called Field to Market to measure and reduce the environmental impact of their operations.

Allison Thomson, the group's research director, says "it has been a process of discovery, mapping out the emissions and understanding that there's a huge footprint that comes from the farm."

That's right: on the farm. Not just factories or fleets of trucks.

Down on the farm, the most important greenhouse source is something that doesn't normally get a lot of attention. It's the fertilizer — mainly nitrogen — that farmers spread on their fields to feed their crops.

Nitrogen is the most important nutrient for plants. It's the fuel that drives modern food production. Every year, American farmers spread millions of tons of it on corn fields alone.

Manufacturing nitrogen fertilizer is energy-intensive, burning lots of fossil fuels and releasing carbon dioxide. What's just as damaging, and perhaps even more so, is what happens when it's spread on a field. Bacteria feed on it and release a super-powerful greenhouse gas called nitrous oxide.

These bacteria are naturally present in the soil, says Philip Robertson, a researcher at Michigan State University, "but once they get exposed to nitrogen fertilizer, they really light up" and pump out nitrous oxide.

If you add it all up, fertilizer is the biggest part of the global warming price tag of a loaf of bread or a box of corn flakes. According to one study, carried out by the consulting group Deloitte, greenhouse emissions from fertilizer are the biggest single piece of the global warming price tag for almost half of the top-selling items on the shelves at Walmart.

Yet it's a climate driver that Walmart can't easily control. "We don't make the product ourselves," Phillips says. "We would want to work with our suppliers" to reduce the climate cost of fertilizer.

In fact, even Walmart's suppliers, the companies that deliver meat and baked goods, don't control fertilizer use. Bakers just buy the grain that the farmers grow; meat packers buy the cattle that eat that grain. They're a step removed from the farmers who grow the grain and decide how much fertilizer to put on fields.

This long supply chain threatens to undermine the Environmental Defense Fund's carefully plotted strategy to enlist Walmart as a partner in environmental progress.

"That was a really big eye-opener," Friedman says. "This is a lot more complicated than we thought in the beginning."

"In the beginning, you had the idea that Walmart can just do it?" I ask.

"I think that even Walmart had that idea in the beginning," Friedman says. "We learned that you really need to engage the whole supply chain."

In the spring of 2014, though, way down at the other end of that long supply chain, a man named Matt Carstens was paying attention. Carstens was a fertilizer dealer; he worked, at the time, for a company called United Suppliers, in Iowa.

Carstens had been reading about Walmart's interest in cutting greenhouse emissions — specifically emissions from fertilizer applied to corn fields in the Midwest.

"It got pretty specific what they were targeting, and that kind of hit close to home," Carstens says.

He tried to set up a meeting with Walmart, but couldn't get anyone to return his calls. So he called EDF instead. The environmental group had been quoted in those stories he was reading.

Before long, he was on a flight to Washington, D.C., to meet with the environmental group, to hear their concerns about fertilizer use on farms.

"You can't help but sit back, as somebody deeply involved in agriculture, and go, 'We've got to understand this,' " Carstens says. "You can take two approaches at that point. You can try to fight it, or you can try to be part of whatever solutions are out there.

After those meetings, on the flight back to Iowa, Carstens decided that he knew some potential solutions: Technology like chemicals that farmers can mix with nitrogen fertilizer to keep it from washing away so quickly; computer programs that show farmers how much nitrogen is in their soil, so they don't add more than they need.

But an equally important part of the solution, he realized, was his own connection to farmers. Walmart and EDF didn't have that connection.

"They knew where the issue was, but how do you reach that farmer?" Carstens says. "Everybody wants to talk to the farmer, but the trusted adviser of the farmer is their ag retailer, in most cases." An ag retailer is a business, like his own, that supplies seeds and chemicals to farmers, along with advice about how to use them.

Carstens imagined building a business devoted to selling those solutions. It could work, he thought, if those tools put more money in farmers' pockets, by saving them money that they'd otherwise spend on fertilizer. "You can't go to the farm and just say, 'You have to do this, because,' " Carstens says. "You have to put it in a way that's economical or profitable for them."

After that, things moved quickly. While he was still working for United Suppliers, Carstens turned that brainstorm into a program called SUSTAIN, which sells those nitrogen-saving tools to farmers. In 2015, Land O'Lakes, an agricultural cooperative that spans the country, bought United Suppliers and adopted SUSTAIN as its own.

Land O'Lakes has promised Walmart that it will enroll 20 million acres of farmland in SUSTAIN by 2025, and Walmart is counting on it to help meet the goals of Project Gigaton.

Next:We go to Iowato find out how farmers are receiving this message of better fertilizer management, and whether it's making much of a difference.

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