pH is the measure of the acidity or alkalinity of a solution based on hydrogen ion activity. pH is measured on a scale of 0-14. 0-7 represent acids, with 0 being the strongest. 7-14 are bases, with 14 being the strongest. The scales overlap at 7, which is considered neutral. As nutrients are added or absorbed by the roots, hydrogen and hydroxide ions will become active, constantly changing the pH of your solution. Frequent testing must be done to ensure an environment that will promote nutrient absorption by the roots.
Ideal pH for Plant Growth
It is hard to give a strict number that is best for your plant's growth because every plant type is different. You should do more specialized research on the kind of plant you will be growing to find out exactly what pH you want. However, the general consensus seems to be between 5.0 and 7.0 for most plants, with 6.0 to 6.5 being the best.
Be Careful When Handling Chemicals
Solutions between 3 and 10 are usually safe to handle. Use caution when handling solutions that fall outside that range, as they are likely to be very strong acids or bases. Of course, you should always wear gloves and eye protection when working with any type of chemicals.
Controlling pH Level
You should get in the habit of checking the pH every time you add nutrients to your solution. If you notice that the solution is becoming too acid more quickly, you will need to change your solution more frequently. The absorption of nutrients by the plant will make the solution more acidic which can lead to nutrient "lock up" where the plant becomes unable to absorb the fertilizer. Be sure to check your pH whenever you see signs of nutrient deficiency. The problem may be with your water and not your nutrient solution.
There are various types of pH test kits available. They vary in accuracy, but usually the more expensive tests are the more accurate. Litmus paper or liquid pH test kits similar to the type used in home swimming pools are inexpensive ways to get a rough reading of your garden's pH. This will work for most plants, but you may need to invest in a pH metering system if your plant has specific pH needs. If your pH is off, there are chemicals called pH Up and pH Down that will adjust your solution as needed.