Dear All, I am interested in why horticultural pumice if mixed with peat would break down into a clay-like substance. Patty, can you explain this to us with your knowledge of geology. What would this do to a mix? Will Ashburner, a former participant of the IBS forum from Australia, did an air filled porosity test at a meeting a number of us in Northern California attended in his honor and concluded that most of the soil mixes we use, at least in California, were inferior to the ones available in Australia. In fact he pronounced the one he tested fit for bog plants, not bulbs. When he visited me a day later, he was unimpressed with the ingredients I was using in my mixes. The sand I had he declared too fine and said it would make my mix worse rather than better. After his visit I experimented with red lava rock in many of my mixes one year. I don't know if it is the same thing as scoria. The bag we purchased had mixed sizes. Some was rather fine and it sounds like from Jane's report that may be what Loren uses. I thought in retrospect based on Will's advice it would have been better to sift that part out. It didn't seem that things grew that well using it. I have been happier with the white kind of pumice, but have only found one place I could buy it in larger quantities and that is in Berkeley and I'm not there very often. The bags I see for sale locally are small and expensive. Interestingly Rod Saunders suggested when I was trying to grow South African ericas from seed to use a peat, smaller size pumice mix to germinate the seeds. Mary Sue