Here's where I got the tip about using them in a soil grow- http://www.overgrow.com/growfaq/1649

Here's what it says on the bag-
Hydroton: This Lightweight Expanded Clay Aggregate (L.E.C.A.) is manufactured exclusively at high-tech kilns in Germany and is used as a soil replacement in hydroponics. Composed of shale that is pelletized and fired, Hydroton holds water extremely well and transmits it effectively. The pellets are uniform in size and have an attractive, natural appearance. Hydroton is chemically inert, has neutral pH, is reusable, clean and odorless.

* Expanded clay is much easier to clean. The round balls of clay have a smooth surface coating that can be cleaned of roots and bacteria.
* Chemically inert and reusable
* Different sizes available
* Lighter than lava rock
* They are more uniform in size & shape. They can be poured into containers, and they fill containers evenly.

* The round balls do not compress to touch. The uniform space between the pebbles provides even aeration and wetting throughout, allowing roots to fully grow into the medium. The clay surface repels water, but surface tension coats each clay ball in a thin coat of water â?? perfect conditions for roots. Cracked balls absorb water like a sponge.

* The round balls have more surface area than the flatter lava rocks, proving more area for roots to cling to.