Expanded clay aggregate

  • Suitable for hydroponic systems in which all nutrients are carefully controlled in water solution.
  • re-usable because they can be cleaned and sterilized, typically by washing in:
    • solutions of white vinegar
    • chlorine bleach
    • hydrogen peroxide
    • and rinsing completely.
  • but: best not re-used even when they are cleaned, due to root growth that may enter the medium.


  • Made from glass waste
  • more air and water retention space than perlite and peat
  • holds more water than parboiled rice hulls.

Coir peat

  • also known as coir or coco
  • colonized with trichoderma fungi, which protects roots and stimulates root growth
  • extremely difficult to over-water
  • high cation exchange, meaning it can store unused minerals to be released to the plant as and when it requires it.
  • Coir is available in many forms

Rice husks


  • volcanic rock that has been superheated
  • used loose or in plastic sleeves immersed in the water.
  • similar properties and uses to vermiculite but, in general, holds more air and less water and is buoyant.


  • mineral that has been superheated until it has expanded into light pebbles
  • natural “wicking” property that can draw water and nutrients in a passive hydroponic system.
  • If too much water and not enough air surrounds the plants roots, it is possible to gradually lower the medium’s water-retention capability by mixing in increasing quantities of perlite.

Pumice stone


  • does not hold water very well
  • must be sterilized between uses


  • same type that is used in aquariums
  • any small gravel can be used, provided it is washed first.
  • easy to keep clean, drains well and will not become waterlogged
  • if the system does not provide continuous water, the plant roots may dry out.

Wood fibre

  • produced from steam friction of wood
  • keeps its structure for a very long time
  • wood fibre may have detrimental effects on “plant growth regulators”.

Sheep wool

  • little-used yet promising
  • greatest yield out of the tested substrates

Rock wool


  • most widely used medium in hydroponics.
  • typically used only for the seedling stage, or with newly cut clones
  • can remain with the plant base for its lifetime
  • can be engineered to hold large quantities of water and air
  • good mechanical structure to hold the plant stable


  • possible skin irritancy (mechanical) whilst handling (1:1000)
  • naturally high pH of mineral wool makes them initially unsuitable to plant growth and requires “conditioning” to produce a wool with an appropriate, stable pH.

Brick shards

  • similar properties to gravel.

Added disadvantages:

  • possibly altering the pH
  • requiring extra cleaning before reuse.

Polystyrene packing peanuts


  • inexpensive
  • readily available
  • excellent drainage
  • used mainly in closed-tube systems
  • non-biodegradable polystyrene peanuts must be used


  • too lightweight for some uses
  • biodegradable packing peanuts will decompose into a sludge
  • Plants may absorb styrene and pass it to their consumers