Well I'm no soil pro but in googling Pro Mix it's called a peat based medium.....peat is certainly not an inert medium so I think you'd have to call it soil and treat it the same way.

7.1 is too high.....i'd wonder about overfert/fert buildup as being as issue as well as those conditions can raise your ph too. Aim for 6.2-6.8. Again....photos could make a huge difference.
Weedhound Reviewed by Weedhound on . Need help with nutrient lockout Hope somebody can clear this up for me. I am almost 100% that I have nutrient lockout and that it is being caused by a ph problem. Sorry I don't have any pics to show, but from what I've seen and read, I am sure it is a nutrient lockout. I have 8 plants. 3 of them have burnt tips and edges and turning yellow. 2 of them show an Iron deficiency, and the other 3 look very healthy. The medium I am using is Promix. I water with regular tap water, the ph on the tap water is 7.2 to 7.3. This Rating: 5