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 alkaline.
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).
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) fluctuations, 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%.
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 using 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 pH Up.
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 below for an alternative method.
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 make-up water.
Adjusting pH by trial-and-error can be tedious, especially for large reservoir volumes. A more convenient alternative can be to pre-calculate the pH Down (or pH Up) requirement by “titrating” a sub-sample of the nutrient solution to the target pH value. Note, the following procedure is relevant where pH Down is required. It can however be adapted for calculating pH Up requirements:
# 1. Take a 1.0 litre sub-sample (or known volume) of working nutrient (“S1”).
# 2. Add a few drops of pH indicator (Fig 18.6a).
# 3. While stirring this solution, measure (“titrate”) the volume** of acid (“T1”) required to turn this solution yellow (Fig 18.6b). ‘Yellow’ indicates ~pH 6 with most broad range liquid indicators.
# 4. Calculate the total volume of acid (“T2”) required to adjust the entire volume down to ~pH 6.0. Hence:
** If this volume is very small (most likely if a highly concentrated acid is used), you will require the use of a finely graded pipette to ensure accuracy. A better method is to prepare a portion of acid specifically for conducting this estimation. Thus, dilute a portion of the strong acid 10-fold with tap water and titrate using this diluted solution. Ensure to compensate for this dilution when calculating how much of the concentrated acid to add to the reservoir.
PH MEASUREMENT & CONTROL: Measuring pH with pH Meters | Measuring pH with 'indicators'