Mixing iron sulfate, a waste product from steel foundries, with ammonium nitrate fertilizer leads to changes in its chemical composition that keep it from detonating in homemade bombs, say researchers at Sandia National Lab in Albuquerque, New Mexico. The researchers devised the formula in response to a request from the Department of Defense for ways to combat the use of improvised explosive devices.

Ammonium nitrate is commonly used to make powerful bombs in Afghanistan and other countries, and it was used in the 1995 Oklahoma City bombing that killed 168 people.

In a press release, the researchers said that the new mixture could be even better as a fertilizer than conventional fertilizer, since it could improve the pH of soil and increase the levels of iron in food. If someone tries to alter the iron sulfate and ammonium nitrate mixture to make a bomb, ammonium nitrate turns into ammonium sulfate and iron nitrate, which can’t be detonated, they said.

The researchers won’t patent the formula for the new fertilizer—instead they’ll make it freely available in the hope that that will speed its adoption. They say the new mixture won’t cost more to make than conventional fertilizer. Still, with ordinary ammonium nitrate available in huge quantities all over the world, it could be some time before the advance has a significant impact.