ledtime,
I use the Fafhard 'red bag' mix when I can get it, MG soil otherwise. The soil likes to go acidic. I'm still battling a mid-flower (after about 4 weeks of 12/12) yellowing issue with pH adjustment and flushing. Trying to dial things in.

The stuff I buy when the garden center has it (no continuous release fertilizer) - Potting Soil : : Fafard

Now the stuff I use is probably not the "professional" mix you use. What I've done to mitigate the soil going weird is to mix 1 to 2 tablespoons of fine lime in with the soil and perlite before potting. Then I watch runoff pH and top-dress with lime as needed. If you use the 'professional' stuff (I need to find a source of it) you may find this email I received from fafhard regarding the pH of their products:

<quote>
Fafard professional potting mix (no CRF) should have a pH outcome of 5.8 to 6.2 with a once weekly application of liquid fertilizer for up to 10 weeks. Liquid organic fertilizers tend to contain a high degree of ammonium and urea which drive the pH down. Apply clear water in between fertilizer applications.



Top dress with a quarter teaspoon of Osmocote for 4 inch pots and a half teaspoon for 6 inch pots if organic fertilizers are not used. Dolomitic limestone can be topdressed at 8 to 10 weeks at a teaspoon per pot (water in well). Limestone prevents low pH and keeps calcium in the system to prevent blossom end rot.




In summary even organic fertilizers cause concern of plant quality for homeowners.
</quote>

Hope this helps!