Something to know about horticultural pumice is that it comes in two versions, washed and unwashed. The washed kind is what one finds in little bags in garden centers; it has had the fines (dust) washed out. It's used primarily as an alternative to Perlite and has the same disadvantage: it "floats" to the top of the pot unless mixed with something of the same general dimensions and density (i.e., fine bark). I find it horrible as an ingredient for bulb potting soil, and it is less likely to give the plants a nutrient boost, which the unwashed type certainly does. However, the washed pumice is a decent soil amendment in the garden and is useful in growing demanding alpines. I use only the unwashed type now, which is harder to find. I mix it with forest loam and very sharp coarse sand. I don't know if unwashed pumice is available except near the source. I think the place I get it from buys it from a nearby plant that uses it to manufacture building blocks. Jane McGary Northwestern Oregon