Tolerance and Availability
Let’s look at what pH means to the Plant.
More plants are, in general, tolerant of a low pH in the root than a high ph. That’s the way they’ve evolved because, in nature, low pH soils are more common than high pH soils. High pH soils tend to occur in limited areas such as limestone or arid areas. With the way things are going, climate-wise, maybe arid areas are going to become more common, though. The majority, if not all, the plants you would be concerned with as an indoor gardener are not tolerant of high pH.
What causes the intolerance? This is simply due to the unavailability of certain elements (mainly trace or minor) at varying pH levels. Elements become unavailable due to chemical reactions at high pH and/or competition from other elements. They prefer a ph range of 4-7 with an optimum of around 5.5-6.5 so if you keep to this range in your growing you won’t run into any pH related problems.
You’ve probably seen the charts that get around showing availability of elements at different pH levels. This is known to the scientific community as Truog’s chart and shows theoretical levels of availability of elements at varying levels of pH. While this chart was devised to show what happens in mineral soils, not hydroponics, it’s near enough to illustrate what goes on. You can see from this chart that most elements are most available in the 5.5-6.5 range. Occasional short excursions outside this range won’t cause any problems provided you readjust your solution.
If your pH goes higher than this for any length of time (several days), you can, in most cases, reverse the reaction. Not always, though. If that happens, change your solution.
On no account should you let you nutrient solution or medium drop below 3.5, even for a short time. You will cause physical damage to the roots that can’t be reversed. Your plant may recover if it’s in the vegetative phase but it’s unlikely to do so if it’s in the bloom phase.