When to adjust pH
The working nutrient pH should be checked at
the following times:
1. When first made.
2. After the addition of top-up water or
additives - especially if either is highly
3. In recirculating systems, pH should be
checked on a daily basis because the uptake of water and nutrients causes
pH to change (Fig 10.2).
It is best to adopt a pH maintenance regime
that helps prevent pH from getting too high. If pH is left too high for a
long period of time, the resultant precipitate is usually not easily
re-dissolved (Fig 10.1).
How to minimize pH
fluctuation in recirculating systems
1. Use a nutrient brand that is highly pH
buffered, particularly when using very alkaline water.
2. Supply at least 10 litres (2.5 gal) of
nutrient for each large plant. Failure to do this will magnify pH (and EC)
especially during hot and dry weather when water uptake and
evaporation are both excessive. To avoid excess water uptake and high
evaporation rates, keep the air temperature below 30OC (86OF)
and relative humidity above 50%.
How to adjust pH
Step 1. Measure the pH: Use either a
liquid pH indicator or an electronic
pH meter. Before measuring the pH, ensure that
the nutrient is well stirred and that the sampling container is clean.
Step 2. Choose a target pH: It is
inconvenient and unnecessary to hold pH at a single point value so choose
a target pH that provides both a safety margin and minimizes the amount of
pH maintenance. For example:
- If your pH tends to continuously rise (the
most common trend), then at each adjustment, reduce the pH to about 5.0
pH Down. This will give you
a much larger pH "safety" margin than, for example, 5.8.
- If pH tends to fall, at each adjustment
increase the pH to about 6.0 using
Step 3. Adjust the pH: Add a small
amount (e.g. 1ml per 50 litres) of pH Down / pH Up. Then stir well and
check pH. Repeat this process until the target pH is reached. When using
pH Up, ensure to pre-dilute the dose at least 100-fold with water before
adding to the nutrient.
Adjusting pH by trial-and-error can be
tedious, especially for large reservoir volumes.
Refer to Note 1 for an
Pre-adjusting the pH of water
Do not pre-adjust the pH of raw water before
adding nutrient unless the make-up water has high alkalinity. If you do
attempt this procedure, because natural waters have a low buffering
capacity you will typically get wild pH swings either side of the pH
target without ever landing on the target value.
Note that the pH values being discussed in
this chapter are the values of the working nutrient solution - not the