I had a question like this also and I think the amount agreed upon was 25%. I think I was with Stinkyattic on this one. This ratio was going into a commercial potting soil.
Dr. Bloor Reviewed by Dr. Bloor on . My soil good to use? IN CALIFORNIA AND GEORGIA, THIS MIX CONTAINS 68%-72% SPHAGNUM PEAT MOSS, VERMUCULITE, AND PERLITE AND LESS THAN 1% FERTILIZER AND LIME FOR PH BALANCE GUARANTEDD ANALYSIS Total Nitrogen (N) 0.09% 0.04% Ammonical 0.05% Water insoluble Available Phosphate (P2 O5) 0.05% Soluble Potash (K2 O) 0.09% F-1431 :dance::gunfighter2::chainsaw: Rating: 5