The article is pretty explicit about it being as cheap to produce and more effective.

"iron sulfate, a readily available compound that steel foundries throw away by the tons."

"Fleming said iron sulfate in fertilizer adds iron and acidifies soil. “It does good things for soil health. It takes alkaline soil and makes it more neutral, closer to an ideal pH level,” he said. “The closer you get a neutral pH, the more crops grow. Crop yield would improve significantly.

“And iron-containing fertilizer added to the soil would be taken up in crops and help fight anemia and other iron deficiencies in people who eat them.”

The soil in Afghanistan is alkaline with a high pH, and could benefit from an ammonium nitrate/iron sulfate fertilizer, Fleming said. “What they use now, ammonium nitrate with calcium carbonate — which makes soil more alkaline — doesn’t make sense,” he said."

"But he said there are some ideas about how to get the non-detonable formula, which would not cost more to produce, into the marketplace. “We could give the formula to a neutral party and let them work with the Afghans, Pakistanis and others,” he said. “They could set up side-by-side demonstrations to see which fertilizer works better. Prove it to them gradually."