I used to run my pro-mix at 6.2- 6.3 but now run at 6.3-6.5 and I think the plants do better. It's really about quality of fertilizer, which is basically a salt which lowers pH over time. Lime raises it, so they kind of offset each other. Over time, lime is used up, and salt slowly builds up in the medium. So, if you don't flush occasionally, your soil pH and therefore the runoff will fall. Enzymes (Prozyme, Cannazyme, etc.) help break down left over fertilizer in the medium so it's available to be used by the plants- and therefore helps reduce salt buildup.
Organics vs. Chemical fertilizers- organics produces much less salts and is healthier for the medium. Chemical ferts will toxify the soil faster by what is left over, or from over fertilization.
Also depends on how much lime is in the medium..... I use 3 3/4 cups of lime per 3.8 cu. ft. bale. Use powered lime, it breaks down faster.
Some growers vary the pH and let it go up and down lightly to increase and decrease the availability of certain nutrients. So, 6.2 pH one fertilizing, 6.4 the next and so on. Personally, I try to keep it constant right at 6.4. I think you can get good results anyway you go as long as you are ALWAYS in the 6.2- 6.5 range.
Lastly, when mixing your fertilizer, give it 10 minutes to stabilize. Some people mix it and when it gets to 6.3 they start watering. And then a bit later, the pH has drifted off a few 1/10ths as the meter needed time to adjust and read the pH properly.
Easiest way is to run a few plants at different pH levels.....and see what plants do better and worse. Also, some strains like higher pH and others like lower. That's why you often have to grow a strain a few times to get to know what it likes....
