Sponge wasn't the right word to use. yes, you're right, it's like a brittle rock and can be crushed between the fingers. That's what I should have said. Vermiculite holds moisture, but anytime I've read about perlite, the info stated the purpose was to hold moisture. More reading shows that it's used to retain water and air.
"In horticultural applications, perlite is used throughout the world as a component of soilless growing mixes where it provides aeration and optimum moisture retention for superior plant growth."
It really can't do both at the same time, which I guess is why vermiculite is added.
polishpollack Reviewed by polishpollack on . My turn - Stunted growth? Using coco, but this form seems more appropriate than the hydro form. What is your experience level? (first timer, novice, experienced...) Noob (first grow) Your Equipment: .1) Type and wattage of lights. (MH, HPS, CFL's, tube fluorescents, LED's) T5 Florescent 4-2ft tubes in fixture .2) Distance from tops? 4-6 inch .3) Reflector type? (cool tube set-up, bat wing, enclosed reflector, bare bulb...) built-in (fixture) .4) Is there a consistent fresh air supply? Maybe not -- it's in Rating: 5