What is pH?



pH is a measurement of the concentration of hydrogen and hydroxyl ions in a water-based solution. It is measured on a scale of 0-14, the lower numbers below 7 representing an acidic solution and the numbers above 7 representing a basic or alkaline solution.

In an acidic solution, there are more hydrogen ions present than hydroxyl ions, and in a basic or alkaline solution the opposite is true. A small change in pH corresponds with a major change in the concentration of hydrogen ions.

To put this in perspective a solution that has a pH of 6 is 10 times more acidic than a solution with a pH of 7. Some plants have a different pH level requirement, so when you are considering setting up a hydroponics system it is important to select plants that all thrive in a similar pH range.

Generally plants grow better in a solution with a pH between 6 and 7.5. The wrong pH for a plant can affect its ability to absorb essential nutrients, and in odd cases can cause death. pH levels can fluctuate as the plants uptake nutrient and therefore it is necessary to test the pH of the nutrient solution at least one a day.

There are three different pH testing methods: liquid kits, paper strips otherwise known as ‘litmus paper’ and meters. The litmus papers which are coated in a pH-sensitive dye are the most inexpensive method, but they are also relatively inaccurate.
The paper strips are dipped into the nutrient solution, and change colour based on the pH of the solution. Compare the colour of the strip against the colour chart supplied with the strips to determine the pH.

Liquid kits are slightly more expensive than paper strips, but they are also easier to read and more accurate. All that is required is to add a few drops of dye to the sample of your nutrient solution and compare the resulting colour to the chart supplied.

The best way to ascertain the pH is with a meter. The meter is dipped into the nutrient solution for about a minute (until the reading stabilises) and it that point you have an accurate pH reading.  Always calibrate your instrument on a regular basis.  Generally this is done using a pH 7 and then a pH 4 calibration solution.  For accuracy it is important to calibrate using both solutions.  Make sure that you always clean the probe prior to calibrating using unperfumed Jif, rinse thoroughly with clean water and tissue after cleaning and do not touch with skin as the oils from the skin can cause reading issues.

Always check the pH of your nutrient solution AFTER adding your nutrients to the solution.
If the pH of your nutrient solution is not quite where it should be, there are chemicals that can be added to correct it. The most popular are phosphoric acid, nitric acid and potassium hydroxide.

Both Nitric and Phosphoric Acids reduce pH levels, while potassium hydroxide increases them. Add these chemicals in small amounts until you have successfully corrected the pH level. pH Lower and pH Raise are non-commercial packs using diluted forms of these materials.

Be very careful with these substances as they can easily burn you and your plants if they come into direct contact in a concentrated form. Always wear gloves and eye protection when dealing with all acids and hydroxides.

It is not recommended to use solely Phosphoric Acid in the summer. Sole use usually results in a build up of phosphate on gullies and dripper systems. A usual ratio of 3:1 Nitric to Phosphoric is recommended diluted to make a 20% acid solution with water. Acid should always be added to water NOT water to acid

When using Potassium Hydroxide a 10% solution with water should be created and the correct amount of this added to the solution. Failure to dilute the Potassium Hydroxide before addition can create a ‘jelly’ that can cause issues and blockages.

It is important to stay vigilant in maintaining the correct pH. Plants will grow and thrive and stress less which in turn will result in less root disease issues.


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