Beautiful info @farside05Sunshine and ProMix should be pretty much interchangeable. They are both peat based soil less mixes. Sunshine claims their mix is pH balanced between 5.8-6.2. ProMix claims their product is stabilized at 5.5-5.8. I use ProMix because it's easier to find in my area.
As for the pH of your feed, I believe a lot of that has to do with your fertilizer and how it's designed. There are 3 different sources of nitrogen typically used in fertilizers. Urea (cheapest and normally found in low grade fertilizers), Amonical Nitrogen (causes pH to drop as the plant uses it), or Nitrate Nitrogen (causes pH to increase as the plant uses it). Some companies will use a blend of Amonical and Nitrate, usually in a 1:2 ratio to balance the effects. I was using Dyna Gro with ProMix and it uses a combination of Amonical and Nitrate. I would feed it at 6.5 and have no issues. Remember ProMix has a slightly lower buffered pH to start with than Sunshine, so the pH would drift downward through the range when feeding. Sunshine having a higher pH, and feeding at 6.5, it may not be drifting as much causing some lockout. Especially if your fertilizer is all Nitrate Nitrogen (I can't find a guaranteed analysis to see what your nutes contain). I'm just starting a new grow and have changed fertilizers. Because the new one is all Nitrate Nitrogen rather than a blend, I'm gonna feed this batch at 6.0 rather than the 6.5 I did in the past. Might want to give a couple feedings at 6.0 and see if they improve.
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