And to belabour the point a little...
One should not confuse weight per volume of the soil with its drainage capacity... it is actually the proportion of rock particles (e.g. sand, grit, and larger - which are heavy) to a much lower proportion of the lightweight components (humus, peat, manure) that make a soil fast-draining and lean, both desirable properties for growing many alpines. And how much the soil weighs is not a problem to the plants... though to the gardener, lifting a tray of 48 small alpine pots can be fairly heavy!
