Lauw,Pumice is volcanic rock which is full of gas bubbles and is so light that it will float on the ocean.Basalt is also from volcano's,but is a very heavy, dense rock,made below the earths surface.Basalt is often crushed,for its multiple minerals. Del --- On Wed, 10/24/12, contact <contact@bulbargence.com> wrote: From: contact <contact@bulbargence.com> Subject: Re: [pbs] Water retained by various soil ingredients To: "Pacific Bulb Society" <pbs@lists.ibiblio.org> Date: Wednesday, October 24, 2012, 12:54 PM 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? Kind greetings (sorry for my ignorance) Lauw de Jager South of France ( busy potting up bulbs)