Unfortunately, managing pH fluctuations is just one of the things you have to do. The good news is that it is relatively simple. Fluctuations in pH are caused by a number of things but mainly by the growing processes which are going on.
When plants are actively growing in the vegetative phase, they use a lot of nitrogen. This high nitrogen use changes the pH of the nutrient solution either up or down, depending on the type of nitrogen. The two main forms used are nitrate and ammonium. If the nitrate form is being taken up, then the pH of the solution will rise. If it’s the ammonium form, then the pH will drop.
The form available naturally depends on what is in the nutrient solution. Most commercial nutrient solutions contain predominantly the nitrate form. So, this means that the pH solution will be rising, sometimes rather sharply. If this happens, it means that your plants are healthy and growing strongly.
If the nutrient is mainly ammonium, then the pH will drop sharply. This form of nitrogen is not used widely, though; commercial growers use a mix of the two to try to limit the rises. These same commercial growers, however, are happy to see the rise-it means their crop is growing well.The majority of commercial nutrients contain mainly nitrate with a bit of ammonium to try to limit the rise. As there is desirable nitrate to ammonium ratio, though, generally there’s not enough ammonium to make the solution pH drop.
During the bloom phase, the plant is taking up a lot more potassium which will tend to make the pH drop. However, it is still taking up some nitrate (pH rise) so the net result is usually a slow pH drift one way or the other.
Plant roots also contribute carbon dioxide to the medium or solution which has a slight acidifying effect.