The "potential of hydrogen", more commonly referred to as the pH level, is most critical to the health of hydroponic plants. pH refers to how alkaline or acidic a nutrient solution is. The pH scale runs from 0 to 14; 7.0 being neutral, above 7.0 being alkaline, and below 7.0 being acidic.
pH should also come into consideration when choosing a grow media. For example, grow media such as perlite, rockwool and L.E.C.A. have a neutral pH and will not alter your nutrient solution; while other grow media such as saw dust and vermiculite are not as stable and can effect the pH of your nutrient solution and the life of your plants.
Three most common methods to test the pH levels of your hydroponic systems include the following:
Litmus Paper - reads ph levels when the end is dipped into the nutrient solution. Litmus paper will change color after a few seconds. The pH color on the litmus paper is compared to a ph chart and will determine if your plant's pH is acceptable.
pH Pen - the end of a pH pen is dipped into the nutrient solution and will give a digital reading of the pH level.
pH Kit - a few drops of nutrient solution are taken from the plant's water system and dripped on the pH indicator. After a few seconds the indicator will change color. This pH color is compared to the pH chart.
The pH level of your nutrient solution should always be checked before you apply it to your hydroponic plants. The pH level of your hydroponic reservoir should also be tested every second day.