When to adjust pH

Nutrient pH should be checked at the following times:

1.  When working nutrient solutions are first made.

2.  After the addition of top-up water or additives - especially if they are highly alkaline.

3.  In re-circulating systems, pH should be checked on a daily basis because the uptake of water and nutrients causes pH to change (Fig 1.10).

Best practice is to adopt a pH maintenance regime that prevents pH from getting too high and the associated precipitation problems.  If pH is too high for a long enough period of time, the resultant precipitate usually cannot be re-dissolved (see Fig 1.10).  Note, when the addition of high pH additives causes a white precipitate, the initial particle size is small and provided the pH is immediately re-adjusted it will usually re-dissolve quickly. 

How to minimize pH fluctuation 

1.  Use a nutrient brand that is highly pH buffered, particularly when using highly alkaline water.

2.  Supply at least 10 litres (2.5 gal) of nutrient for each large plant.  Failure to do this will exacerbate pH (and EC) fluctuations, especially during hot and dry weather where water uptake and evaporation are excessive.  Note, to avoid excess water uptake and evaporation, keep air temperature below 30 deg C (86 deg F) and humidity above 50% RH.

How to adjust pH

Step 1.  Measure the pH:  Before measuring the pH, ensure that the nutrient is well stirred and that the sampling container is clean.

Step 2.  Choosing a target pH:  Note, it is inconvenient and unnecessary to hold pH at a single point value.  Therefore, choose a target pH that minimizes the amount of pH maintenance:

+  If your pH tends to continuously rise (the most common trend), then at each adjustment reduce the pH to about 5.0 using pH Down.  This will give you a much larger pH "safety" margin than if adjusting to, say, 5.8. 

+  If pH tends to continuously fall, at each adjustment increase the pH to about 6.0 using pH Up.

Step 3.  Adjusting the pH:  Add a small amount of pH Down / Up*.  Then stir well and check pH.  Repeat this process until the target pH is achieved.   Adjusting pH is largely a process of trial and error, and can be a difficult task (see section below - Handy hints for adjusting pH) especially for larger system volumes.

* It is essential to pre dilute the pH UP dose at least 5-fold with raw water. Then stir the nutrient as you add this mixture. Failure to do this may cause permanent precipitation of essential nutrients.  Also, when using pH UP, precipitation problems can be minimized by ensuring the nutrient mixture is stirred rapidly near the point where the reagent strikes the surface of the nutrient.  Also, if accidental overdosing to above 6.5 occurs, to prevent permanent precipitation and loss of essential elements, reduce the pH back to below pH 6.0 as quickly as possible using pH Down.

Handy hints for adjusting pH

1.  A simple way to estimate the required volume of acid: Take a 1.0 litre sub-sample (or known volume) of working nutrient, place a few drops of pH indicator (Fig 1.12a) and measure the volume of acid required to turn the solution color yellow (Fig 1.12b) i.e. ~pH 6 with most broad range liquid indicators. Multiply the volume of acid** by the volume of nutrient in your tank and this is the volume of acid required to adjust the entire volume i.e.

Total volume of nutrient ÷ Volume of sub-sample x volume of acid

** If this volume is very small (most likely if a highly concentrated acid is used), to ensure accuracy you may require the use of a finely graded pipette. To further enhance accuracy, allocate a portion of acid specifically for conducting this calculation and dilute it by a known amount - for example 10-fold. Ensure to compensate for this dilution when calculating how much of the concentrated acid to add to the tank.

2.  Add “high pH” (alkaline) additives before adding nutrient: See Fig 1.13. Note that essentially all additives will affect nutrient pH at least slightly. The best technique to adopt with those that elevate pH significantly is to add them to the water and adjust the pH down to ~6 prior to adding the nutrient.

The less preferred but common alternative is to pre dilute the additive in a separate volume of raw water prior to adding to the nutrient solution, then quickly lower the pH to below 6.5 once this solution is added. Note, a white cloudy precipitate (calcium sulphate) may form when the pre diluted additive initially merges with the nutrient solution (Fig 1.13a). However, because the initial particle size is small and provided the pH is immediately re-adjusted, it will usually re-dissolve.