Another purpose for growing medium, is moisture retention. This is the moisture that the growing medium can absorb when the system is flooded. When the system is drained it’s important that the medium retains some of the moisture, this way the plants can continue to drink up the water as well as the nutrients that are in the nutrient solution. How well it retains this moisture also plays a big part in your watering cycles. You don’t want to deprive the root system of the oxygen/air they need for healthy growth, but you don’t want to deprive them from moisture either.
Growing medium needs to have good drainage. Too much moisture in the root system can be a problem, well it’s actually the lack of air/oxygen to the root system (because of the excess water) that would actually be the problem, commonly referred to as over watering, and root rot. Some plants are more sensitive to excess water in the root 
system than others are, this is normally referred to as wet feet. In other words some plants can tolerate wet feet more than others. So when deciding on the growing medium you plan to use, it would be a good idea to take into consideration how your plants tolerate wet feet for best results. Some plants cant tolerate dry feet much either, and depending on the growing medium that you use, it may need more frequent watering cycles.
Your growing medium should also be pH stable/neutral and can be checked by using a pH meter. If it’s not, you may be constantly changing the pH balance of your nutrient solution. Also being in direct contact with the plants root system, a unstable growing medium will most likely have a negative affect in the way the plants are able to absorb the nutrients in the nutrient solution. Most growing mediums that are sold specifically for hydroponics are going to be pH neutral/stable, you will just want to look into that aspect when using other materials.
Some of the most commonly used Growing Mediums
Grow Rock (Hydrocorn)
Grow rocks are a Lightweight Expanded Clay Aggregate (L.E.C.A.). This clay aggregate is formed into small pebble shaped rocks. This is made from a type of clay which is super-fired to create a porous medium. It’s also heavy enough to provide support for your plants’ root systems. Grow rocks are a non-degradable, sterile growing medium that will hold moisture. Hydrocorn is also recyclable, it can be cleaned, serialized and then reused. Although on large scale growing setup, cleaning and sterilizing a lot of Hydrocorn can be time consuming and tedious. Grow rock is often the growing medium of choice for novices and professionals alike because it’s easy to use, and reusable.
Rockwool
This is a sterile, porous, and non degradable medium. It’s primarily composed of a granite or limestone, which is then melted down and spun like cotton candy. The Rockwool is then compressed and formed into blocks, sheets, cubes, slabs, or flocking. Rockwool absorbs moisture without holding nutrients, and even when it is completely saturated it still generally retains around 20% air for the roots.
Perlite
Perlite is primarily composed of minerals that are subjected to intense heat, then they expand and become very absorbent. This material is lightweight and can float to the top of the water when used all by itself in some situations, like in a flood and drain system. But it’s very porous and is used in a wide variety of hydroponic systems because of its ability to hold moisture and nutrients as well as air. It’s also easy to use and inexpensive. It works best in drip systems, especially when it’s added to other growing mediums. It adds moisture retention, and can then still support the plants weight.
Coco Fiber and Coco Chips
Coconut fiber is the first “organic” medium to offer high performance in hydroponic systems. Coco fiber has the 
consistency of potting soil, and coco Chips have a larger particle size (much like wood chips for your BBQ/smoker). Coconut chips are made from the same material (coconut husks), but are in chunks so they don’t fall through the openings in the growing baskets the way coco fiber can. It also does not become as compacted, allowing better aeration to the root system. Coconut fiber/chips hold more oxygen than rockwool and is pH neutral. It usually comes as a compressed brick, then when it’s soaked in water, it expands to about 6 times its compressed size. Many growers find that a 50/50 mix of coconut fiber/chips and L.E.C.A is the perfect organic medium, but the coco fiber/chips are not reusable like the grow rocks. So they can be difficult to separate when it comes time to clean the grow rocks.
Vermiculite
Vermiculite is Similar to perlite, except it has a relatively high cation exchange capacity (meaning it can hold nutrients for later use). And like perlite, vermiculite is very lightweight and has a tendency to float. So like perlitte it may not be the best choice to use in EBB and Flow (Flood and Drain) system, unless it’s mixed with other growing mediums. And like perlite, it’s quite suitable for drip systems.
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