Mike wrote:

No it doesen't make it more flammable but rather speeds up the decay of the
stump.

Thanks for clearing that up.

As a kid, I remember my dad always had a bag of potassium nitrate
around, which he would somewhat regularly put into holes in the numerous
stumps we had in the yard. My understanding was it made the stumps rot
away more quickly.

In recent years, I bought a container of ???Green Light??? stump
remover from a box store. The directions were just what the original
poster described. Said to add into holes, wait a specified time, then
burn out the stump with kerosene. I had thought maybe dear old Dad had
been missing the last step all those years ago, as he never burned any.
Now I see that he was smarter than me. No big surprise.