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Soil & Testing

What Is Perlite For? Drainage and Air, Not Nutrition

Perlite is heat-popped volcanic glass that holds air in a mix. Where it belongs, how it differs from vermiculite, and the dust precaution.

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Perlite’s whole job is air in the root zone. It feeds nothing, holds almost no water, and never breaks down.

Perlite against vermiculite

perlitevermiculite
OriginVolcanic glass, heat-poppedExpanded mica
Effect on waterDrains it awayAbsorbs several times its weight
Effect on airIncreases air spaceReduces it
Nutrient holdingNoneHolds and releases nutrients
Best forCuttings, succulents, orchids, heavy mixesSeed starting, moisture-loving plants
Breaks downNoSlowly compresses

Mix ratios that work

useperlite by volume
General potting mix20–25%
Seed starting25–33%
Cuttings and propagation33–50%
Succulents and cacti50%+ with grit
OrchidsCoarse grade, as part of a bark mix
Garden bedsNot recommended — use compost or grit

Dampen the bag before you open it properly

Dry perlite dust is a fine silica-containing particulate that irritates eyes, nose and lungs. Mist or hose the bag, mix outdoors, and wear a mask for any large quantity. Once it is in a damp mix there is nothing left to breathe.

Grades, and which to buy

  • Fine: seed starting and fine propagation mixes
  • Medium: the standard general-purpose grade, and the one most garden centres stock
  • Coarse / chunky: orchids, succulents and anything needing maximum air space

Reusing it

  1. Sieve it out of spent compost
  2. Rinse thoroughly
  3. Sterilise with boiling water, or in an oven, if the last plant had disease or gnats
  4. Discard if the old mix was salt-crusted from over-fertilising, or if the particles have crushed to dust

Common questions

What does perlite do for plants?

It creates air space. Perlite is volcanic glass heated until it pops like popcorn, leaving a lightweight particle full of sealed air pockets with a rough surface. Mixed into compost it keeps the structure open so water drains and roots get oxygen, which prevents the dense waterlogged conditions that cause root rot. It supplies no nutrients at all, holds almost no water inside itself, and does not break down, so its contribution is purely physical and lasts for years.

What is the difference between perlite and vermiculite?

Perlite drains; vermiculite retains. Perlite's sealed air pockets shed water and hold air, making it right for cuttings, succulents, orchids and any mix that must not stay wet. Vermiculite is an expanded mica that absorbs several times its weight in water along with nutrients, which suits seed starting and moisture-hungry plants. Use perlite where drainage is the problem and vermiculite where drying out is.

How much perlite should I add to soil?

Between 10 and 50 percent by volume depending on the plant. Roughly a quarter perlite suits general potting mixes and most houseplants, a third suits seed-starting and cuttings, and up to half or more suits succulents, cacti and orchids. For garden beds it is usually the wrong product — it is expensive by volume and floats to the surface — and compost or coarse grit improves soil structure far more cheaply at that scale.

Is perlite safe to handle?

Yes, with one precaution: wet it before use. Perlite is chemically inert and non-toxic, but the dry dust is a fine silica-containing particulate that irritates eyes, nose and lungs. Dampen the bag with a hose or mist before mixing, work outdoors or in a ventilated space, and wear a dust mask if you are handling a large quantity. Once it is in a damp mix there is nothing to breathe.

Can you reuse perlite?

Yes, and that is one of its advantages — it does not decompose. Sieve it out of spent compost, rinse it, and sterilise it with boiling water or in an oven if the previous plant had disease or fungus gnats. Discard it if the old mix was heavily salt-crusted from over-fertilising. Over several reuses some particles crush down to dust, which is when a batch has reached the end of its life.

Does perlite float to the top, and does it matter?

It does float, both when you water heavily and over time as the mix settles. It is cosmetic rather than harmful in a pot — the particles that matter are the ones still distributed through the root zone. In an open bed it is a genuine drawback, since perlite migrates to the surface and blows away, which is another reason to use coarse grit or compost for in-ground work instead.

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