What is the White Stuff in Potting Soil?

At some point, all new gardeners wonder “…what is the white stuff in potting soil?”…

Most often, the white stuff in potting soil is Perlite – a manufactured granular product made by heating up little bits of naturally occurring glass until they pop like popcorn. That’s why these little white lumps are also known as “volcanic popcorn”! Check out the photos below to see perlite both in potting mix and on its own.

Perlite is made of volcanic rock that is crushed into small pieces and then heated to the point where it expands. Read on to learn more about perlite and its use in container gardening growing media.

what is the white stuff in potting soil
What is the white stuff in potting soil, anyways?

Perlite: A Container Gardener’s Best Friend

While perlite may look like bits of popcorn or styrofoam, it’s made of rock. When lava from a volcano cools, the rock traps a small amount of water. This volcanic bedrock is mined, crushed, and heated in big ovens to the point where the heated water in the rock causes the grains of rock to expand. The water content of the perlite expands and causes this to happen. When popped, the end product of perlite is about 40% less dense than its original form.

Uses for Perlite

Perlite has a wide range of applications and uses outside of horticulture. Perlite can take the place of a variety of lightweight construction materials such as clay, shale, pumice, and vermiculite. Each of these materials has its own benefits and disadvantages, with most having their own niche market. When it comes to perlite, it’s a lightweight material that’s preferred for a variety of lightweight construction products, product fillers, and filtration.

Is Perlite Organic?

A chemist would say that perlite is made of rock (which isn’t living) and is therefore not considered an organic compound. In terms of gardening, however, plain perlite is OMRI-Listed as allowed for use in organic gardening. Since the volcanic rock its made of occurs naturally and doesn’t contaminate the soil, perlite is allowed in organic growing methods.

Medium-Grain Perlite in a Mason Jar

Types of Perlite

Perlite is available in coarse, medium, and fine-grade types. Each type of perlite has its own uses. The coarse version has the highest porosity, giving it the most drainage capability. This is popular among people who grow orchids that need a great deal of drainage. The finer grades of perlite are often included in seed-starting mixes and other potting soil products.

The White Stuff in Potting Mix Isn’t ALWAYS Perlite…

Always check the ingredients in your potting soil. A low-quality gardening soil could potentially contain styrofoam or other imitation fillers. Styrofoam should not be used in potting mix instead of perlite. Polystyrene foam should be sent for proper recycling into new styrofoam packaging rather than shredded and added to soil as hydrocarbon-based litter.

Perlite can also be confused for gypsum. Both gypsum and perlite are used in building construction and as soil additives. Gypsum is added to soil as a type of fertilizer while perlite is added as an aggregate amendment only (not a fertilizer – it doesn’t add nutrients). Gypsum is a mineral while perlite is a volcanic glass. Both gypsum and perlite are mined, but they are made of different materials and have different internal structures.

Keep asking “what is the white stuff in this potting soil?”… and make sure its actually perlite and not a cheaper additive!

Bag of perlite for sale at garden centre

Why Use Perlite in the Garden?

The primary use of perlite in soil is to aerate the soil. The roots of plants require access to oxygen just as they require access to nutrients and water. Perlite allows for air pockets in the soil, giving terrestrial plants the oxygen access they need. This is especially important in container plants, which have limited soil access and no worms to make tunnels!

Perlite has a similar usage to vermiculite, although the two materials may not be interchangeable. One of the benefits of using perlite in soil is that it is a porous material and will allow water to drain easily. In contrast, vermiculite holds on to water and may cause problems with too much moisture for some plants.

Finally, when growing plants with perlite in the soil, be aware that its interaction with the environment can cause fluoride burn on some houseplants. This isn’t common, but may appear as brown tips on leaves. Always observe your houseplants for health.

“The absorption of fluoride by the crops was small and did not appear to be greatly effected by the nature of the growth media.”

Studies Of Fluoride Absorption By Plants Grown In Perlite, by George J. Doss, Leigh E. St. John Jr., and Donald J. Lisk, Departments of Vegetable Crop and Food Science, Pesticide Residue Laboratory, New York State College of Agriculture and Life Sciences, Cornell University
Cactus with Perlite Mulch Top Dressing

Can Perlite Be Used Alone?

Although it’s most common to use perlite as a part of a potting mix, it can be purchased separately. On its own, it works particularly well for rooting cuttings. It can also be used as a top dressing for cacti in containers (it looks lovely and modern).

To propagate cuttings in perlite, take cuttings and put them in a ziploc bag of moistened perlite that’s about 1/3 full. Fill the rest of the bag with air and then seal it securely. Put the bag indirect sunlight where it will grow roots over the next two or three weeks. When the roots reach about an inch long, the cuttings can be removed and planted in a container or outside as desired.

Some gardeners also like to use perlite on their lawn and in outdoor garden beds. The fine version of perlite can be lightly sprinkled on a lawn or garden. Over time, it will work its way down into the soil and improve the drainage.

Perlite used in soil preparation for a new lawn

Using Perlite in Garden Soil

Now that you have a greater understanding of perlite and what it is, it may be helpful to include it in your container gardens and even outdoors. It’s nice to finally know what the white stuff in potting soil is!

Whether starting cuttings or growing in containers, this versatile material has a large range of uses. Let’s get some air into the soil!

Further Reading: The Ultimate Gardener’s Guide to Perlite

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Mary Jane

Mary Jane is a home gardener who loves creating healthy, welcoming spaces (indoors and out!) - About Mary Jane (https://www.homefortheharvest.com/authors/about-mary-jane-duford/)

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