According to the Pro-Mix datasheet, the BX contains approx 17% perlite. When you factor in the microbark which obviously does not contain any perlite, then adding in the one scoop in the 3:2:1 combo Mix brings the perlite percentage back up to about 23%.
If you are not going to use microbark in the Combo Mix, I then recommend cutting the Pro-Mix BX at a 5 scoops mix to 1 cup added perlite. This increases the percentage of perlite which provides added aeration and drainage which tends to make up for the lack of microbark.
Raybo
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