Dissolve 1 cup of salt in 1 gallon of water and add in 1 tablespoon of dish soap. Mix well, pour into a water sprayer, and spray away. Spraying this mixture will kill all plant growth it comes into contact with, so be sure you're spraying only the poison ivy. Also, it's best to do this on a sunny day since rain can wash the solution right off. It might take a few applications to do the trick, but it'll work.