Substrate

In hydroponics and aquaponics the term substrate is used for the soil-less anorganic medium, that's used to anchor the roots of growing plants and to give them access to water, nutrients and oxygen. The Substrate itself ideally contains no nutrients. These are only dissolved and transported by water that is pumped through the system

The most common materials used as medium are gravel and Expanded clay, but there are many alternative materials that can be used according to specific needs or budget. A medium has to be anorganic and water resistant. it must not dissolve. Certain plastics do dissolve components in water without showing visible decay.

Ideally it has a somewhat porous surface to help nitrifying bacteria and other microorganisms colonizing the substrate.

Comparison
This is a comparison table that shows some of the material that have been used successfully. It is not complete. If you did use another material, feel free to add it to the table. You hesitate to write wikitable syntax? Then add some Information to the talk page and the next wikigardener will copy it to the table.