Perlite

Perlite has a distinctive feel, lightweight and crispy, but what is it and how do we use it in the garden?


(This one's for you, Jim!)

Perlite is the featherweight white stone found in potting soil. Public Domain

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What is perlite?
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Perlite is actually a form of obsidian. Obsidian is a dark volcanic glass that forms when lava cools quickly. Obsidian is very brittle and extremely sharp. It was commonly used to make cutting and hunting tools by primitive peoples. [Back in the early 80’s, I had an archeology professor at Seattle Central Community College. He loved to tell us how, when he needed chest surgery, he found a surgeon who was willing to use obsidian tools. Being sharper than surgical scalpels, the obsidian left a scar that was practically invisible!]


How perlite is made

Coming out of a volcano, obsidian is less than 1% water. As it comes into contact with rain and groundwater, it starts to absorb moisture. This hydrated obsidian is mined and then baked in a 3,000°F oven where it pops like kernels of popcorn, growing to twenty times its original size! These glassy kernels look more like a froth of bubbles, but the outer bubbles are broken, leaving jagged edges.


How perlite is used

Perlite has many industrial uses, including insulation, mortar, plaster, and ceiling tiles. The broken glass bubbles of horticultural perlite are used to aerate soil, increasing porosity. Perlite does this by increasing the number of macropores and micropores that carry and hold air and water for plant roots. Perlite can hold 3 to 4 times its weight in water. Perlite is also used in hydroponic garden systems as a filtration medium. (Did you know that perlite is used to filter beer?) Perlite is also found in many planting mixes. You will see them as small white chunks. Sometimes these white particles are pumice, like the volcanic foot scrubbing stone you buy at the drugstore. Sometimes those white bits are styrofoam. Both perlite and pumice improve soil structure, but styrofoam does not. If you are going to use perlite to improve heavy clay soil, don’t just pour it on top. You will need to dig it in a bit, otherwise you will simply end up with a layer of perlite on top of your clay.


Perlite is an excellent soil additive for roof gardens, balcony plants, and extra large planting  containers, because it is so lightweight. Perlite is sterile, inert, and incombustible. Mold and mildew won’t grow on it and pests won’t eat it. Adding perlite to your soil can lighten heavy clay and it can help sandy soils retain more water and nutrients.