pH is the measurement of water quality. Soil or water can be too acidic or too alkaline. A pH of 7 is neutral. In general plants respond best to a pH between 5.5 and 6.5 for maximum nutrient absorbency. pH-Up and pH-Down adjust the acid/alkaline balance of your water. pH that is either 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. A pH meter or strips can be used to measure the pH of water, nutrient solution and soil.