Marguerite and Cathy mentioned crushed volcanic rock and scoria. We use a great deal of these products here in the Pacific Northwest of the USA (because they're local and cheap, and also because they're great for plants). The crushed horticultural pumiceis a pale gray or white pumice and is the kind most people use. A geologist told me that it will break down quickly (i.e., in 5 years or so) to a claylike substance if combined with an acid ingredient such as peat. It is nearly neutral in pH and contributes potassium to the soil, as well as being a superb physical amendment -- hospitable to fine roots, retains just enough water, and is angular for good drainage. European and British gardeners can get a similar product that is mined in Iceland. I use several cubic yards/meters of this pumice every year, purchasing it in bags that weigh 60-70 pounds and cost $7 apiece. It is cheaper bought in bulk but you need a dumptruck and a way to keep the weed seeds out of it once it is dumped. My bulb potting mix is: 1 part crushed pumice, 1 part humus, and 2 parts very coarse, sharp river sand. This is suitable for all the smaller bulbs I grow but would be too lean for, e.g., lilies. Scoria is a harder type of lava that is usually dark in color -- what we get here is dark red. It is typically used as a mulch. I can't find any crushed small enough to use in potting soil, but Loren Russell reports that he goes to the rock yard and scrapes up the dust and grit that falls to the bottom of the redrock bins. Jane McGary NW Oregon