pH Up and pH Down are used to adjust the water or fertilizer that is being fed to the plants. pH is very important to plant nutrient uptake and pH that is too high or too low can cause your plant to not absorb nutrients. Without pH Up and Down the pH of what you're feeding your plants may be off. After checking the pH of your plants you'll add pH Up or pH Down to raise or lower the pH to the proper level based on your initial reading. If you haven't been adjusting your pH, give it a try and the difference may surprise you!