The reverse occurs at a more alkaline pH to produce a carbonate compound and release hydrogen ions and CO2 back into the water, which can create carbonic acid that once again lowers the pH.
To buffer this swing, a carbonate acts by binding excess acid in the form of hydrogen ions to create bicarbonate. The reverse occurs at a more alkaline pH to produce a carbonate compound and release hydrogen ions and CO2 back into the water, which can create carbonic acid that once again lowers the pH. The CO2 released in this reaction can also be consumed by plants. Carbonates are also a source of carbon for plants when normal dissolved CO2 and carbonic acid is low or absent. When KH, and therefore carbonate levels, is very low, usually below 3 dKH or about 50 ppm, and all available CO2 in the water is used up by plant growth, carbonates will be rapidly used up and the buffering capacity will vanish. Without any buffering ability, plant cycles can now cause dangerous pH fluctuations.