Media
A good hydroponic medium should support the plants in the system, not break down, absorb and retain moisture, be porous to allow air circulation and proper drainage, and protect plant roots from temperature extremes. Different media are suitable for different crops. Commonly used media for hydroponic culture are:
A fairly new hydroponic medium, coconut fiber is porous, long-lasting, all-natural, and features good oxygen and water-retention properties. It's much like rockwool, but it doesn't irritate the skin when dry. Also, it's biodegradable, so disposal doesn't pose a problem.
If you buy a drip or ebb-and-flow system kit from a dealer, it will probably come equipped with expanded clay pellets for the medium. Like perlite, clay pellets are porous and retain water and air well. Expanded clay is dusty, so it will need to be rinsed before it goes into your system.
Perlite is lightweight, stable, and has good water-retention properties. Like most media, perlite must be rinsed before you fill your system with it.
Rockwool is an inorganic, sterile, inert growing medium made from a combination of basalt rock, limestone, and silica. It's available in various-sized propagating blocks, wrapped cubes, and large slabs. Commercial growers prefer rockwool over other media because it's light, relatively inexpensive, safe for immediate use, and has the best water- and air-holding capacities. Although rockwool is nontoxic, when dry it can cause skin irritation. If this occurs, simply wash your hands for instant relief.
Although sand is one of the oldest hydroponic media, it's heavy when wet and tends to dry out quickly. However, it's long-lasting, easy to keep clean, inexpensive, and in plentiful supply. If you choose sand for those reasons, be sure to buy coarse sand, not fine. The latter drains poorly.
When used in hydroponics, vermiculite is often mixed 1:5 or 1:10 with perlite or clay pellets. The problem with vermiculite--and the main reason it's not used alone--is that it tends to break down after a year or so, which leads to clogging and stagnation.
Some other media used are: