The main role is mechanical: the plant grows in water but that gives him no support. So it needs something that gives him support.
Further, the substrate will or will not provide transport of the solution of water and nutrients to the roots by capillarity.
Expanded clay granules:
The clay is formed into round pellets and fired in rotary kilns at 1,200 °C. This ensures that the clay expands like popcorn and becomes porous. It is light in weight and can not be compacted.
The porosity ensures that the solution of water and nutrients is brought up to roots of the plant by capillarity.
Manufacturers argue that the clay pellets are an ecological, sustainable and recyclable growing medium because they can be washed, sterilized and used again.
But, in practice, the plant grows micro-roots which grow inside of the clay pellets and are impossible to remove.
The clay pellets are inert, pH neutral and have no nutritional value. They are more expensive than potting soil but last longer.
Seramis consists of clay granules.
Its main feature is its very high water absorbing capacity: 100% of its own weight, without any change in volume.
It is particularly suitable for orchids, bonsai and cactus.
For other plants, it should be used with a moisture indicator.
See detailed article: Seramis
Sometimes lava rock contains trace elements of metals that disturb the balance and pH of the nutrient solution.
Lava rock is especially suitable for ebb and flow systems.
See detailed article: Lava rock
Rock wool is made from rock, slag or basalt, which is heated to 1,400 °C. The molten rock is then introduced into a spinner and hurled away, thus forming thin filaments, as in the case cotton candy.
Subsequently, these fibers are mixed with a binding agent to form mats that bring the solution to the roots by capillarity.
Rock wool is inert, pH neutral and has no nutritional value.
It is similar to vermiculite but holds more air and less water.
Because it is so light, it can drifting away and is not suitable for active hydroponics where the water flows.
It is perfect for the sowing of plants that are destined to be grown on hydroponics.
Perlite can also be mixed with ordinary potting soil to air it.
It holds more water and less air than perlite and has a better capillarity.
When there is too much water and not enough air at the roots, it can be combined with the perlite.
The capillarity of sand is not so good and, therefore, it is not suitable for a mature plants.
It is ideal for sowing very fine seeds and give weight and stability to containers with large and heavy plants.
It can only be used in dynamic hydroponics setups where the solution is flushed by the roots.
In static hydroponic setups the plants could dry out.
It is the same type as gravel for aquariums. It's cheap, easy to clean and reusable but it is heavy.
Brick shards:Brick shards have similar properties as gravel but they can cause problems with the pH and are difficult to clean.
Polystyrene packing peanuts:
They can, however, be too light for some applications. They are mainly used in closed-tube systems.
Notice that only polystyrene peanuts may be used since biodegradable packing peanuts will rot.
Note that polystyrene is not suitable for fruit and vegetables because it can be absorbed by the plant and eaten.
Coconut fiber is a 100% natural substrate. It is colonized by Trichoderma fungi which protect the roots and promote root growth.
It has the perfect ratio of air-water and cation exchange capacity, which means that the unused minerals are stored and released when the plant requires.
A disadvantage is that dried coconut fibers are difficult to rehydrate so that the plant risks drying out when the reserve was left empty for too long.