How To Use NPK 19 19 19 For Indoor Plants ? If you have ever picked up a fertilizer bag and seen three numbers like “19-19-19” on it, you may have wondered what they mean. Don’t worry, it is simple once you understand it. In this guide, I will explain what NPK 19-19-19 is, how to use it, and how much to give your plants — in plain, easy words. What Does NPK 19-19-19 Mean? NPK stands for three nutrients that every plant needs: N — Nitrogen: helps leaves grow green and strong P — Phosphorus: helps roots and flowers …