Caustic soda, also known as sodium hydroxide, is a chemical that can effectively kill tree stumps. White and odourless, it can be in solid and liquid forms. When around moisture, the chemical can burn or eat away at animal and vegetable matter, and even metals. It is an ingredient in several stump killers but also is effective when applied singularly. After applying caustic soda to a stump in a sufficient amount to kill it, rent a stump grinder to extract the remaining dead wood, or hire a professional to remove the dead stump. Always wear protective gloves when handling caustic soda.
Attach a 1/4-inch drill bit to an electric drill. Drill holes 1 inch apart all the way around the edge of the stump. Insert the entire length of the drill bit into the stump. The deeper the drill bit penetrates the better. Drill a second ring of holes in the centre of the stump, also 1 inch apart and 2 inches from the first ring. For large stumps, continue drilling holes at 2-inch intervals, working toward the centre of the stump.
Mix 2 parts water with 1 part caustic soda in a 5-gallon bucket. Slowly pour the solution over the top of the tree stump, giving the liquid time to seep into the drilled holes. Pause when the holes become full. Let them drain, then continue pouring until the solution is gone.
Wait one week. Repeat the application of the caustic soda. Wait an additional week, then repeat the application. That should be sufficient for killing the stump.