Lauw asked, Sorry! Here are some definitions: Horticultural pumice is gray to white volcanic rock, very light in weight, porous, absorbs water. It is crushed to about 0.7 cm maximum diameter particles which are irregular and sharp. It can be purchased either washed (fine particles removed) or unwashed (fines remain). I think the Icelandic pumice available in western Europe is similar. Roots will grow right into pumice particles, as they will into limestome tufa (see below). In my area we can also buy scoria, which is a dark red, somewhat denser volcanic rock that is crushed and used mostly for mulch and in some areas for grit on roadbeds. It is said to be high in plant nutrients. Scoria does not absorb much water. To answer Shmuel's inquiry, the term "tuff" is sometimes applied to the lighter kind of pumice here, as is "tufa." Actual calcium carbonate tufa occurs rarely in the northwestern USA, but there is a quarry in British Columbia that has sold a lot of it (I have two beds made of it), and I once got some from a quarry near Baker City, Oregon. It is not good to use as grit in potting mixes because when crushed it tends to disintegrate completely. When you plant things such as Saxifraga into it, you use the dust to pack around the roots in the hole. The crushed basalt I mentioned for horticultural use is a dense, heavy, dark volcanic rock, high in iron, and the kind we use for horticultural purposes consists of sharp particles about 1 cm maximum diameter. If used in potting soil it should be washed; the unwashed product is used mostly for surfacing paths. The washed product does not absorb water. The horticultural pumice is packaged and sold in various parts of the USA, but I think the packaged product is always washed. Because these products are quarried near where I live (Pacific Northwest USA), we can buy them in various forms, including bulk by the cubic yard (slightly less than a cubic metre), and they are also available in potting soils made up by local companies for retail sale or, by the truck load, for the many nurseries around here. Some nurseries have formulated special mixes that are made up by a company, ProGrow, but these are not available packaged as far as I know. Near my home is an "indoor garden" supplier (i.e., a store with a lot of customers who grow Cannabis sativa) where I have my choice of more than 20 different prepared potting soils in large bags. I buy a kind that contains no bark or sawdust, and that has pumice instead of perlite, and mix it with sharp sand and pumice for several uses. So if you really want to customize your soil, Oregon is the place to be! Jane McGary Portland, Oregon, USA At 05:54 AM 10/24/2012, you wrote: >Hello, >Can anyone explain (to us foreigners) what is ment by "pumice" and >"crushed basalt" and what are the sizes of the particles used for potting >soil?