Guide to pH control in hydroponics
It is important to maintain the pH of nutrient solutions between 5.0 and 6.5. This helps ensure all nutrients are available for root up-take. It also minimizes the risk of plumbing blockages.
The working nutrient pH should be checked at the following times:
1. When the nutrient solution is first made.
2. After the addition of top-up water or additives – especially if either are highly alkaline (Fig 9.1).
3. In recirculating systems, pH should be checked on a daily basis because the uptake of water and nutrients causes pH to change.
How to adjust pH
Step 1. Measure the pH: Use 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.
How to minimize pH fluctuations (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) 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 30 deg C (86 deg F) and relative humidity above 50%.
pH Electrode Maintenance
Calibrating pH meters
It is essential to calibrate using at least two pH buffers. pH buffer 4.0 and 7.0 are ideal for nutrient solutions. Some manufacturers claim that their pH meters require calibration using only pH buffer 7.0. Professional practice however, stipulates using two buffers because this will confirm or otherwise that the correct calibration “slope” is being obtained i.e. that both the electrode and meter are ranging correctly.
This causes dehydration of the glass electrode and the precipitation of salts within the interstices of the frit itself (Fig 9.4). The consequences are slow response times and loss of accuracy and precision. To avoid these problems the electrode tip must be permanently stored (immersed) in a specially formulated storage solution (Fig 9.3). Note, pH buffers 7.0 or 4.0 and distilled water are NOT suitable for this.
pH electrode contamination
Unless the ‘frit’ and glass tip of a pH meter electrode is properly rinsed after use, it will invariably become contaminated with impurities (Fig 9.2). This causes similar symptoms to dry storage. Contamination may be so severe that re-calibration is not possible without prior cleaning or replacement. To prevent contamination, always avoid measuring harsh chemicals such as highly caustic solutions (e.g. pH Up and silica additives), or concentrated nutrient.
© Andrew M Taylor (FloraMax)
For more see page 49 of the FloraMax Hydroponics Manual.