How do I know when to add the "S" to the end of the word CHIROPRACTIC?

I know when saying "chiropractics is more than just back adjustments" that there should be an "s" at the end of the word "chiropractic". and I know not to add the "s" when saying "chiropractic philosophy" or the "chiropractic ociation". But what is the grammatical rule here? How do I explain to people of a non-english speaking background?