This is the most basic and important question. In layman terms, pH is a measure that tells you if a solution is acid or basic, with values of pH over 7 being basic, and values below 7 being acid. Going a little bit deeper into detail, pH is just the result of applying the operator "p" over H (which symbolizes the concentration of H3O(+) ions within a solution). The operator "p" is just getting the negative decimal logarithm of a number. Since H3O(+) concentrations appear usually in really small magnitudes, like 0,00000001 M, using the logarithm let's us express this in more humanly understandable numbers, like 9.
Why is 7 the neutral pH ?
Seven is the neutral pH value because the concentration of H3O(+) ions in solution is determined by the self dissociation constant of water which is 1x10e-14 and equals the product of H3O(+) and OH(-) concentrations. If H3O(+) concentrations are equal to OH(-) concentrations you have that H3O(+) concentration should equal 1x10e-7 which after applying "p" turns into 7.
Why is pH so important in hydroponics ?
This variable is very important in hydroponic gardening because it determines the form in which nutrients are present inside the solution. In pH values which are too acid or too basic, nutrients assume forms which are different from the ones which plants can assimilate. Therefore, an adequate pH value needs to be maintained in order to ensure that all nutrients are present as the right species.
How do I measure pH correctly ?
First of all, pH meters need to be calibrated prior to each measurement. In order to calibrate any pH instrument, at least two different buffer solutions must be used, one with pH 7.0 and the other with any other known pH value. The measurement should be taken with enough time for the reading on the instrument to stabilize.
How can I correct pH changes ?
Bases or acids can be added to hydroponic solutions in order to increase or decrease the pH value of a solution. Bases and acids should be added as solutions and the amount added must be recorded in order to know how nutrients are changed. For example, if a potassium hydroxide solution is added to increase the pH of a solution, the amount of solution added needs to be recorded in order to know how much potassium was added to the solution (since this is a nutrient). Common acids to lower nutrient solution pH values are nitric acid, phosphoric acid and citric acid. I would recommend the use of citric acid to reduce pH and potassium carbonate to increase pH.
What is the ideal pH value ?
It depends on the specific plant you are cultivating. Most crops grow very well with pH values between 5.5 and 6.0, although there are some plants which require more basic or slightly more acid pH values.
How can I stop pH from changing ?