escapee wrote:

Vermiculite is inert, has no nutritive value to speak of and is used merely by
the greenhouse industry as a potting medium and seed germination product. It is
used because it is very lightweight and deliveries are easier and trucks can
haul more...



Vermiculite is a mica-like mineral. When heated, the laminae break apart
and the mineral expands, which makes it light and also makes it able to
hold water. The primary value in growing media is its water holding
capacity, coupled with improved drainage. That sounds contradictory, but
it's not. The water is held in the vermiculite particles, but the
particles are large, creating spaces in the medium between the particles
through which excess water can travel for drainage. Since roots need
both water and air, vermiculite is a valuable addition for starting
plants. I believe the basic composition is related to silica, so it has
no significant nutritive qualities.

As a mineral, it is extracted from mines.