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).
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