- Rockwool is one of the most powerful of all hydroponic substrates, and its ability to hold significant amounts of both water and air have long made it a favorite with both hobbyists and commercial gardeners. The air space that rockwool is able to retain provides excellent drainage--certainly an important consideration when it comes to soil-free gardening. In addition, rockwool holds water efficiently, and that ability to regulate moisture allows for superior root growth compared to other types of substrates.
- Perlite is derived from volcanic rock, and that origin helps to give this substrate many advantages over other alternatives for hydroponic gardening. Perlite is one of the most lightweight of substrate materials, and it is able to provide excellent aeration for any type of garden. Traditional gardeners often add perlite to their soil to increase the amount of airflow, and the same concept is at work when it comes to hydroponic gardening. Perhaps the biggest disadvantage of perlite is the fact that it does not retain water well, so gardeners must water their plants more frequently when using perlite as a substrate material.
- Expanded clay pellets make an intriguing substrate alternative for hydroponic gardeners. These pellets are created by baking clay in a kiln to create a substrate material that ranges from 1mm to 18mm in size. These inert pellets provide excellent aeration because each pellet contains many tiny air pockets. These air pockets also give clay pellets an excellent drainage capacity, but that superior drainage is a double-edged sword. Clay pellets do not retain moisture well, so this hydroponic substrate is most suitable for systems that receive frequent waterings.
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