Plants require a couple of different things to grow. These include optimal light, because some prefer full on direct sun all day while others can do well in partial or full shade; adequate nutrients, both the macro nutrients and the trace elements; proper watering (the latter two provided by your nutrient solution in hydroponic growing); and the proper pH. pH?
What is pH?
pH was originally the shortened form of "pouvoir hydrogene" which translates roughly as the power of hydrogen. It is a way to state whether something is more acidic or more alkaline, with 1 being the most acidic, 7 being neutral and 14 being the most alkaline.
So, the reason that this is important is that plants take up nutrients differently at different pH levels. At some levels, as the chart shows, they don't take up nutrients at all.
We have noticed that in Hong Kong over the past year, the water has become more alkaline, with tap water now coming out at around 9. This means that even after adding nutrients, which will lower the pH, it can still be a challenge to get the right level.
So what can be done?
To lower your pH, you need something that will make the water more acidic. There are a lot of "natural" remedies that are actually not effective and not good for your system, like adding lemon juice (which has very little lasting effect), adding vinegar (which will ultimately breed Mother of Vinegar and give you a soupy mess), adding driftwood (didn't work for us) and adding MSG (which also didn't work and added significant salts.
We are using pH down, which is used in aquariums to manage pH so is a natural acidic treatment. Want to get some - give us a shout.