FAQ – controlling, adjusting and knowing pH in Hydroponic Gardening

Even though there has been great effort by many people to show hydroponic growing as something that can be done by anyone with little knowledge, it has come to my attention that many novice and commercial gardeners fail because of their inability to properly interpret the chemical phenomena around them. One of the variables that is primordial in hydroponic culture and that is grossly oversimplified in most literature about hydroponic gardening is the treatment of pH. For this reason, I decided to create this pH FAQ post in order to answer (in a basic but scientific way) the questions most people have (or should have anyway) about the science of hydroponics.
What is pH anyway ?

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 ?

Please refer to the article I wrote about controlling the pH of your nutrient solution with buffers in order to effectively prevent pH variations inside your hydroponic nutrient solution.

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6 Responses

  1. Marc
    Hi Daniel, I just wanted to pass on something I learned the other day in dealing with pH meter calibrations. There are some meters on the market that only require a "one-point" calibration, i.e. 7 calibration solution only. Both Milwaukee and Hanna will provide this information in the Users Manual. Just wanted to pass that on. According to Milwaukee, trying to calibrate a "one-point" with two solutions is, as they told me, "a waste of money on a 4.0 solution". Thanks for the website Marc
  2. Hi Is the ph of the nutrient solution affected by temperature? If so how is the characteristic? Best Regards Miko
    • admin
      Hi Miko, The pH is affected by temperature. In reality pH is just -log([H3O+]) where [H3O+] is the molar concentration of hydronium ions in solution. The concentration of these ions is determined by the equilibrium of all reactions in solution involving this ion. These reactions are related to temperature via their equilibrium constants and the delta of their gibbs free energy as follows: delta G = -RT Ln(K). Where T is temperature and K is the equilibrium constant of the reactions. So how does temperature affect pH? Depends. If there are reactions that generate hydronium ions that have a negative delta G then the pH will tend to decrease while if the opposite happens - if there are reactions with a negative delta G that consume hydronium ions - it will increase. It all depends on what you have in solution and the thermodynamics of the reactions involved. It can be complicated but wonderful if you want to get into it :o) I hope this answers your question, Best Regards, Daniel
      • Wow I did not expect it to be so complex. Does that mean I can control the ph through temperature without having to use ph up or ph down?
        • admin
          No, the pH will change too little as a function of temperature. With plants you can probably change solution temperature from around 10-30°C and this range is too small to cause any large changes in pH.
          • Thanks daniel, your answer is very good. Maybe I should create an automated system to be able to add a ph up or ph down instead of an automated system that regulates temperature. Best Regards, Miko

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