There are many types of media or substrate combinations that can be used to grow plants hydroponically, and the best choice is largely dependent upon the type of system and the size and type of plant that is to be grown. The only issue when choosing is that it be totally inert and sterile, meaning no organic matter or chemical additives. Also it must not contain any fertilisers, as this will throw out the levels of the nutrients (plant foods) you start with. The following are the main types of media used in hydroponic systems.
Perlite: Perlite is a volcanic rock, which when heated in excess of 1000 degrees C, expands into lightweight particles. In hydroponics approximately 3mm particles size should be used. The medium is inert and sterile and is used as a lightweight support medium. It does not have a high capacity for water retention and has no nutrient holding capacity. Perlite can be used on its own or with vermiculite, depending upon specific plant needs.
Note - The perlite needs to be pre-dampened before use as the dust can be irritating.
Vermiculite: An exfoliated mica, vermiculite is light and has a very high capacity for water retention. Because of its flaky structure, vermiculite eventually breaks down into a paste and requires replacement every 2 - 4 years.
A 50/50 mix of perlite/vermiculite is recommended as an ideal growing medium for the home gardener. It is advisable not to mix sand, scoria or crushed granite with perlite or vermiculite as they will separate when wet due to differences in densities.
Expanded Clay: The medium is totally inert, but has a relatively low water retention capacity. The medium can be re-used over a period of time. Expanded clay can be used to cover perlite pots to retain moisture and prevent algae blooms from forming on the white perlite. Expanded clay is mostly used in flood and drain or drip systems and has an approximate particle size of 8 to 16mm.
Rockwool: Horticultural rockwool consists of a mat of long, fine fibres, spun from molten natural rock. It is inorganic and inert, very light when dry, sterile, is not biodegradable and has a very high water and air holding capacity. Rockwool is mainly use in hydroponic flower production and generally is not widely used by the home grower.