Sorry to bump an old thread, but just want to make sure I have these calculations correct. I called Pro-Mix and finally spoke with someone who was able to tell me that the Potting & Container Mix (retail blue bag) contains 12% perlite.
We're aiming for the 20-25% range, so let's call it 22.5%.
We know that the 3 parts Pro-Mix contains 12% perlite and the 2 parts bark fines contain 0%. So the question is, how many parts of perlite (100%) do we need to add to get to 22.5%?
It's been a while since algebra class, so please check my math:
( 3(0.12) + 2(0.00) + x(1.00) ) / 6 = 0.225
3(0.12) + 2(0.00) + x(1.00) = 0.225 * 6
3(0.12) + 2(0.00) + x(1.00) = 1.35
0.36 + x = 1.35
x = 0.99
... so in other words, almost exactly one part perlite for each 3 parts Pro-Mix and 2 parts bark fines. Using the same math, I found that using 1 part perlite for the Pro-Mix BX (with its 17% perlite content) + bark fines gets you to just over 25.2%.
Sound right?
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