i learned this in my greenhouse management class. vermiculite and pearlite sere completly diff purpose. vermiculite is mica put through what is essentially a giant popcorn popper to expand it, the purpose is to hold as much water as possible. vermiculite is designed not to hold moisture. if you are using the 2 in a potting mix for plants, the formula is 1:1:1 peat moss, vermiculite, pearlite. verm to hold water and pearlite to provide airspace