What is Perlite? – How Volcanoes Can Help Your Garden

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What is perlite?

Perlite is volcanic glass with a relatively high water content. It’s usually formed by water mixing with obsidian. It is also packed with silica. It expands greatly when heated enough, growing from four to twenty times larger.

My geology’s not the best. What is obsidian?

Obisidian is a hard, dark, glass-like volcanic rock. It’s formed when lava solidifies so quickly it doesn’t have the chance to form crystals. It’s used in jewellery and as blades for surgery. TV’s Game of Thrones fans will know it as ‘dragonglass’, a substance known to kill the fearsome White Walkers. You can find out more at Geology – Obsidian.

And silica?

Silica is the name given to a group of minerals composed of silicon and oxygen. It’s a key raw material in industries such as glass-making and ceramics. It’s also used in production of silicon chips in computers.

Why is perlite so white?

The expanded material is a brilliant white, due to the reflectivity of trapped bubbles.

Expanded perlite

Why is it helpful to gardeners?

In horticulture it makes composts more open to air, while still having good water retention properties. It therefore helps to make a good medium for hydroponics. Add it to your soil to stimulate root growth, improve aeration, drainage and moisture retention.

How powerful is it?

Perlite provides aeration and optimum moisture retention for superior plant growth. Studies show outstanding yields when used in hydroponic systems.It would have been the perfect nutrient for Matt Damon’s character in the film The Martian when he started gardening there.

Tip: If you’re rooting cuttings then dip them in perlite first.

Where else can it be used?

You can use this volcanic product in:

  • greenhouses
  • landscaping
  • house plants


  • neutral pH
  • sterile and weed free
  • Lightweight so ideal for use in container growing
  • Can be used as a carrier for fertilizer, herbicides and pesticides and for pelletizing seed

What is perlite and how to add it to compost – video

How to buy perlite

Here at Fertile Fibre we sell different grades and quantities. Just take a look at our perlite selection.

Note for Game of Throners:  it should not be used as a White Walker repellent.

What is perlite? Photo of white perlite

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