What is used to decay stumps

There's a variety of chemicals used, ranging from saltpeter to acids. They don't speed up the decay process all that much. You can hasten the decay process by drilling several holes in the stump, covering it with soil, and keeping it moist. Mother Nature will do what she usually does, you just give her a head start by providing the decay microorganisms (in the soil) and increasing the area they have to work on (by drilling the holes).

I got a bunch of hits, with a web search.

That said, one time I heard about drilling holes, and pack the holes with powdered milk. Not sure that works, but it's cheap to buy.

Some hardware stores have stump remover chemicals.

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Normally I dig around them and build a fire on top, but I have one too close to a building.

Potassium nitrate, commonly called saltpeter. It's an oxidizing agent that accelerates the rotting (oxidizing) of the stump.

It's also a key ingredient,the oxidizer, in black powder.