Alkalinity and Acidity also known as pH is extremely important in Hydroponic gardening. The scale for pH readings ranges from 1 to 14. 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 vital for hydroponic growing, even though the difference may seem slight between an ideal pH and actual pH, the numbers- if not understood make a huge difference.

The requirements of Soil ph is not the requirements of Hydroponics ph ,do not confuse the two. Optimal pH for a nutrient solution is 5.6  (between 5.5-6.0), although most plants can still survive in an environment with a pH of between 5.0 and 7.5.  If your nutrient solution or growing medium is too alkaline or too acidic many of the vital nutrients will be wasted , unabsorbed by the plant.

A solution that is too acidic would create a calcium deficiency which would inevitably damage the root system, and lead to root rot as well as leaf fall off . If the solution is too alkaline, the plants can't absorb iron. This can generally cause chlorosis, a disease that makes plants produce insufficient chlorophyll. No chlorophyl, no photosynthesis the plant will die if the deficiency becomes severe.
 

The initial pH of the water you will use for your hydroponic garden should be determined before any nutrients are added. Basic H2O registers a completely neutral reading of seven, when the water is less than pure it will have a pH either higher or lower than 7.

 Inexpensive Nitrazine paper  {Also see Litmus Paper} is readily available and is used to quickly test the pH level. Nitrazine paper and similar test strips are impregnated with a pH sensitive dye which will change color when dipped into the nutrient solution. The strip is then compared to a color chart to determine the pH level . These test strips are inexpensive, but can be difficult to read as the color differences are very subtle.

 Liquid Test kits are also available, they work by adding a few drops of pH sensitive dye to a sample of the nutrient solution and then comparing the color of the solution with a color chart. They are more expensive than paper test strips, but are much easier to read and more accurate.

Digital meters are the most advanced method. These meters naturally are more expensive but are very accurate when properly calibrated and cared for. pH meters are actually very sensitive volt meters and are susceptible to problems with the electrode. pH meters generally need to be calibrated frequently, as they can drift and to insure accuracy you should check calibration frequently.
 

Due to the fact that pH meters have a reputation of breaking down without warning it is a good idea to keep an emergency backup for checking pH (paper test strips or a liquid pH test kit), just in case.

To lower a high pH small amounts of distilled white vinegar will work, some people also use aspirin, although I've never tried it myself.

A low pH can be corrected {increased} by adding sodium hydroxide or potassium hydroxide to the water. Small amounts of one or the other should be used. Never touch hydroxides with wet hands. Handle them as little as possible and when you do, use sturdy water resistant gloves .

IF possible, test any pH adjustments on a single plant before you continue to make adjustments to your entire nutrient solution.

The pH of any nutrient solution will fluctuate. It has a tendency to go up as the plants use the nutrients. As a result the pH needs to be checked periodically and adjusted accordingly. In the beginning you should check hydroponic pH on a daily basis. Each hydroponic system will change pH at a different rate depending on a variety of factors. The growing medium used, the temperature, the type and condition of plants all effect the pH variations.