The pH value indicates the degree of acidity of water. Acid water has a value below 7. Basic water has a value above 7. An insight into the pH value of pond water is of great importance, because it tells us something about the CO2 content in water. Pond water generally has a pH value of well over 7. If we want to measure the pH value, we also have to know the CH value, the carbonate hardness. If both values are known, we can rather accurately read the CO2 value per litre of water. For a proper plant growth at least 5 mg of CO2 per litre of pond water are required.
At normal carbonate hardness and sufficient CO2 the pH value of the pond water will be relatively low. At a similar CH but insufficient CO2 the pH value is relatively high. The fact is that the CO2 content is not constant during twenty-four hours , however, it is fluctuating under the influence of day and night, it is the s-called assimilation process. Underwater plants absorb CO2 with the help of light and give off oxygen. The CO2 content will decrease as the day progresses. At night this process will be in reverse and plants will absorb oxygen and give off CO2.
Micro-organisms, the largest producers of CO2, are supplying CO2 day and night. Owing to these factors CO2 will accumulate at night and decrease considerably during the afternoon. This process will for that matter occur only in properly functioning water gardens, i.e. when a growing stock of underwater plants and sufficient activity of micro-organisms is at stake.
On the basis of the pH value we can inspect this process. If early in the morning the pH value is relatively low (pH 7 – 8) and measured in the evening relatively high (pH 8 – 9) the pond environment is functioning well, the plants are growing well and the water will be clear.
A too high pH value (pH 9 to pH 10) need not be a problem, however this is always the result of a badly functioning pond environment. The cause may be, for example, that the activity of the micro-organisms is zero. Another possibility is, that some biogenic decalcification is occurring in the environment owing to excessive algal growth. This process will result in reduction of the Carbonate hardness and an increasing pH value.
Measures to raise the pH value to a more favourable level for a prolonged period of time, should be based on activating the micro-organisms (bottom substrate and bacterial culture) and on placing pond plants, which, for their CO2 supply do not have to rely on the water (floating plants, water lilies and marsh plants). The use of peat granulate can expedite the reduction of the pH value.
The pH value will not decrease below pH 7. However, if the GH value should be too low, insufficient calcium is present to bind the excess of CO2. The environment will acidify and lack of oxygen will occur. The pH value will decrease in this case to values below pH 6.
A pH meter will enable you to determine the pH value quite accurately. The same accurateness and far more advantageously is achieved by a simple pH test.