pH is the measurement of water quality. Soil or water can be too acid or too alkaline. A pH of 7 is neutral. Generally plants want a pH between 5.5 and 6.5. pH-Up and pH-Down adjust the acid/alkaline balance. pH that is too high or low will prevent plants from absorbing nutrients. To make the most out of your nutrients, use pH-Up and pH-Down to regulate your pH.