Ammonia plants—which produce 90 percent of the fertilizer used worldwide—haven’t broken ground in the U.S. in more than 20 years. But in the next three to five years, there are as many as 14 ammonia plants proposed in the U.S., with nearly 12 million tons of new capacity and $10 billion of expected investment. Several older plants are also being recommissioned and upgraded. Louisiana, Iowa, North Dakota, Texas and Indiana are among the proposed sites. This boom, driven by low prices for natural gas—the main ingredient in ammonia production—will drive a corresponding surge in the industry’s already substantial carbon footprint.