Maintaining pH in Hydroponic Systems

Check pH levels in your tank weekly.

Decide the pH you want to maintain. We recommend between pH 5.8 and 6.5.

Test the pH of the nutrient solution in your tank using an approved test kit e.g, Nutriculture liquid pH test kit or a digital pH meter.

If the pH of the solution is in the correct range 5.8 – 6.5 do nothing.

If the pH of the solution is above the correct range add phosphoric acid (pH down).

If the pH is below the correct range add potassium hydroxide (pH up).

Add a very small amount of phosphoric acid or potassium hydroxide to a glass of warm water and stir well before adding it to the nutrient solution. Once in the solution stir well again, or leave for 75 minutes before re-testing pH.

It is easy to overdose with pH down and pH up. It is more damaging to add too much pH down or pH up than for the solution not to be in the ideal range, so use the pH down or pH up very sparingly. Adjust pH to your chosen level over several days.

Take a note of the amount of phosphoric acid used to adjust your first tank of nutrient solution, it will make it easier when you need to do it at solution changes.

In soft water areas very little pH adjustment (if any) will be needed but in hard water areas the pH will rise gradually. Do not be tempted to add extra acid to compensate, simply adjust pH back down to preferred level.

Avoid large changes in pH and nutrient strength.

Safety First: Phosphoric acid is strongly acidic and potassium hydroxide is strongly alkaline. Always follow the safety instructions on the container. Never mix phosphoric acid with potassium hydroxide. You should never need both materials on the same site.

Useful tips

Phosphoric acid and potassium hydroxide are very dense chemicals which do not mix readily with water. To improve mixing so as to get the best results from your pH test kit add phosphoric acid or potassium hydroxide very sparingly to a glass of warm water stirring vigorously before adding it to the nutrient solution. Once in the solution stir vigorously again, or allow ample circulation of the nutrient (say over 1 hour) before retesting pH.

It is very easy to overdose with either chemical. It is not critical for your plants that the pH should always be in the determined range, while overdosing is always very harmful so go easy and use either chemical very sparingly. Be satisfied to adjust pH to your chosen level over several days.

Take a note of the amount of phosphoric acid used to adjust your first tank of nutrient solution it will make it much easier when you need to do it again at solution changes. 

Avoid large changes in either pH or nutrient strength and do not let the solution level in your tank drop significantly. On larger systems an automatic top-up device can be fitted to prevent this happening.

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