Is it ok to compost potting mix? (MIX--not SOIL; just the standard store-bought kind containing mostly peat & perlite) Does it make a difference if I added a little vermiculite & some ADDITIONAL perlite to it? (I'm mainly wondering if the perlite is ok to compost...) And, does it matter if it's leftover from LAST year? (stored on a shelf in a greenhouse in zone 5)
Well, first off, strictly speaking, you can't compost potting soil because composting is a process of decomposition, and most potting soil won't decompose. It doesn't consist of the right material to do that. in particular, perlite is essentially inert in compost and soil. However, you can certainly add potting soil and/or perlite it to the compost pile, and in my opinion it does some nice things to the texture of the compost. It shouldn't matter how old it is.