Pure and concentrated Green Planet Nutrients pH UP/DOWN is used in your reservoir to maintain the optimum pH levels in your nutrient mixture. pH levels effects the availability and absorption of essential elements needed for plant growth. Maintaining stable and safe levels of pH will result in better growth. Hydroponically grown plants will do best when the pH of the nutrient mixture is maintained between 5.8 and 6.3. When working with soiless mediums keep pH between 6.0-6.5.