Expanded Clay Pellets
This man-made product is often called grow rocks and is an extremely good growing medium. It is made by baking clay in a kiln. The inside of the clay pellets is full of tiny air pockets (much like lava rock) which makes this a light weight medium (some of the pellets even float).
The pellets are great for ebb & flow systems or other systems that have frequent watering cycles (clay pellets do not retain much water so they need to be watered often so that the roots of your plants do not dry out). The rocks are often mixed with other growing medium(s) to increase oxygen retention.
Expanded clay pellets are rather expensive but they are one of the few kinds of growing medium that is easily reusable, which makes them a good choice for the long term. After you harvest your crop you can wash the clay rocks to remove all the old roots and then sterilize them with a 10% bleach and water mix (one part bleach to 9 parts water). The grow rocks can also be sterilized by using a mixture of Hydrogen Peroxide and water (use 1 or 2 teaspoons of 35% food grade hydrogen peroxide per gallon of water).
The Hanging Gardens of Babylon were thought to be sand-based hydroponic systems (what else would you use if you're stuck in the middle of the desert?). This is probably the first hydroponic growing medium ever used, and it is still being used successfully.
Sand has a tendency to pack tightly together reducing the amount of air available to the roots. So you should use a coarse builders sand or mix the sand with perlite or other material that will increase aeration.
A completely natural medium that is used as a major ingredient in most soiless mixes. Sphagnum moss can also be used by itself in a hydroponic system. Sphagnum moss makes a good fluffy growing medium that retains a high percentage of air and retains water well also.
The major problem with this growing medium is that it can decompose over time and you can get small particles that can plug up your pump and (or) drip emitters if you are using a recovery type hydroponic system.
I have never personally used fiberglass insulation but I have known several people that have used it in their hydroponic systems. There were mixed reviews, and none of those people are still using it.
The most common complaint that I have heard is that it retains too much water, not leaving room for enough air around the roots, which can cause problems with the plant. I have heard that sometimes the insulation is treated with chemicals for fireproofing, etc., so unless you want to experiment, the Professor would not recommend using fiberglass insulation.
People talk about hydroponic systems that do not use any growing medium at all. As far as I know, that would be impossible. The plants roots would have to growing in a complete vacuum. This would instantly kill the plant.
Air on the other hand is frequently used as a growing medium. Aeroponic systems have the plants roots hanging in air and are periodically sprayed with a nutrient solution.
The biggest advantage to growing in air is the roots get all the oxygen they could possibly need (roots need plenty of oxygen). Another major advantage to air is it's cost (Free is hard to beat!). There is no disposal problems as with some other mediums either.
The biggest problem associated with aeroponics is it's susceptibility to power failures and pump or timer failures. There is NO buffer.The roots could start to dry out within minutes and the loss of the total crop can come very quickly.