In hydroponics, the substrate has different functions, adapted to the use: houseplant, garden or patio plant, horticulture…
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:
Expanded clay granules or pellets are suitable for houseplants and patio plants.
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
Lava rock is very porous stone that consists of basalt which is, generally, chemically inert.
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, also known as mineral wool or slag wool, is the most used medium in horticulture.
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.
Perlite is a volcanic rock that at high heating will change into very light, expanded glass stones.
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.
Vermiculite, like perlite, is a volcanic rock that is heated at high temperatures and forms expanded glass pebbles.
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.
Sand is inert, inexpensive and easy to find.
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.
Gravel are small stones with a very bad water absorbing capacity.
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 have similar properties as gravel but they can cause problems with the pH and are difficult to clean.
Polystyrene packing peanuts:
Polystyrene packing peanuts are inexpensive, readily available and have excellent drainage.
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.
Coir, also known as or coco peat, is the remaining material after the fibers are removed from the outer shell of the coconut.
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.
Wood fiber is a very efficient biological substrate for hydroponics. It has the advantage that it its structure retains a very long time.
It is also possible to hold the plants in a ring or a hole in a plate, without substrate, so that the roots hang in the air and they are directly sprayed with the solution of water and nutrients.