Really? Cool!
Leave it a long while, then take the earthy stuff off the bottom (best if you have a solid base rather than being on earth) and boil it up in some water. Filter off the gunk.
Keep boiling to reduce the liquid, then cool. Remove the white salt crystals, dissolve, filter to clean, and reduce, recrystallise. Should be Saltpetre or similar. This acts as a very effective oxidising agent.
Carbon is easy to obtain. Not sure about ready natural sources of sulphur (or should I say sulfur).
A fun book to read is 'Caveman Chemistry'.
..d