PH is a way of measuring the acidity or alkalinity of a substance. The scale for pH readings ranges from 1 to 14. Unadulterated water registers a completely neutral reading of seven. A reading of seven means that the number of acid ions is totally equal to the number alkaline ions present in the measured solution. Any reading above seven indicates that alkaline ions are in greater number, and any reading below seven indicates that there are more acid ions. Each number above or below seven indicates a tenfold increase in either acidity or alkalinity. So a substance with a pH reading of five would be ten times more acidic than a substance with a reading of six. This is important to remember for hydroponic growing, because even if the difference seems slight between your ideal pH and your actual pH, the difference may actually be a huge problem.