What Is Perlite? Why Cannabis Growers Mix It Into Their Soil

Ever opened a bag of potting soil and found little white specks all through it? That's perlite: expanded volcanic glass that growers mix into soil to improve drainage and aeration, keep the mix from compacting and get more oxygen to the roots. So how does it get there, and what exactly is it?

What is perlite?

Perlite starts life as obsidian, a volcanic glass that forms naturally when lava cools fast.

When obsidian soaks up moisture over time, the water trapped inside it turns it into perlite. Raw perlite looks nothing like the stuff you buy at the garden store, though. Manufacturers crush it and heat it to around 980 °C (1,800 °F) until it softens. That releases the water trapped inside and makes the material puff up to as much as 20 times its original size. That's how expanded perlite ends up white and porous, in a process a lot like popping popcorn.

Heads up, though: don't overdo it with perlite in your cannabis mix. Too much and the medium drains too fast and holds less water and nutrients than your plant needs, so as a general rule keep it to about 20% of your growing medium.

How does perlite help your growing medium?

Now that you know a bit about perlite, let's get into all the ways you can use it when growing weed. Perlite brings a bunch of benefits to your soil mix. But it has its downsides too.

Benefits of perlite for cannabis plants

Better drainage

Good drainage keeps your plants from drowning and heads off potentially deadly pathogens down the road. Standing water can become a hideout for smelly anaerobic bacteria and for fungal diseases like fusarium wilt, which thrive in wet conditions. Adding perlite to the soil builds a structure that drains better, cutting the risk of waterlogging and root rot.

Better aeration

Roots are busy underground taking up nutrients and water, and they need to breathe to keep doing their job. That gas exchange between the root zone and the air has to happen efficiently, and poor soil structure can really get in the way. That's where perlite shines.

Mix perlite into your soil and you create tons of little air pockets that keep the roots well supplied with oxygen, which helps the whole plant breathe.

Neutral pH

Perlite has a neutral pH, so it won't push your medium more acidic or more alkaline, which helps you avoid problems like nutrient lockout.

Less compaction

Soil compacts easily, especially when the pore spaces between particles shrink, and that can leave you with poor drainage and aeration.

Because perlite has a rigid structure full of tiny holes, it's a champ at fighting compaction and an excellent soil amendment for dirt that's already packed down.

Downsides of perlite for cannabis plants

So that's everything going for these little beads of volcanic glass. What's the catch? As useful as perlite is, it can actually pose a risk to growers and to the environment. Here are some of its drawbacks.

Irritating dust

Perlite can break down into fine dust during shipping, and those tiny, sharp particles can irritate your eyes and throat. If you're working with large amounts of perlite, wear safety goggles and a dust mask to protect your lungs. Pro tip: wetting it down before you mix keeps the dust in check.

Environmental impact

We mentioned perlite also takes a toll on the environment, and you might be wondering how. Big mining operations dig this glass out of deposits, and it's then usually shipped around the world before it hits store shelves. Plenty of gardeners love the stuff, but it doesn't sit well with a lot of environmentalists, much less with growers who want to raise their weed using permaculture methods.

How to use perlite in a cannabis grow

Now that you've got the pros and cons, here's how to use it. Perlite works great as a soil amendment. Just figure out how much soil you need, add the perlite and mix it in really well.

How much perlite should you use?

How much perlite you use depends on what you're going for. Keep this in mind:

• 10-20% perlite: this amount works really well for aerating your medium and improving drainage. Bonus: you won't need to keep adding fertilizer to your plants all the time.
• 30-50% perlite: this is the range for maximum drainage. But since perlite lets nutrients wash out of the soil, you'll need to feed more over your plant's life cycle. Don't let that scare you off: as long as you feed consistently, your plants will grow fast and healthy.