pH is the measurement of the acidity or basicity of a solution. A pH of 0 is the most acidic, 7 is neutral, and 14 is the most basic.
Plants need a specific rootzone pH to make plant nutrients available for the roots to take up. In field soils, the best pH for most plants is 6-6.8. For container grown and hydroponically grown plants, the best pH is usually slightly lower at 5.5-6.5. Some plants like blueberries, conifers, and some tropical plants prefer a slightly lower than normal pH. Plants can usually live a bit outside of their optimum pH range, but it may slow their growth.
Measuring pH: To measure pH, there are test strips, liquid indicators, and pH probes. I usually use a liquid indicator like API Wide Range. Test strips can be difficult to read, and pH probes can malfunction if not properly cared for. The only pH probe I use is the one on the pH controller because it is one of the few that can be permanently submerged.
Adjusting pH: To adjust pH, I recommend using a product like Bloom City pH Up and Down. Use a pipet and add small amounts of pH adjuster until the desired pH is reached. Sometimes calcium hydroxide can be used, in aquariums for example. This raises pH as well as adds calcium and alkalinity to the water; creating a better environment for fish and crustaceans. Our Quick Dissolve can also be used to raise pH and alkalinity. The alkalinity creates buffer and prevents rapid changes in pH.