Perlite Soil - What is Perlite & Why is it Important?
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Author: Beth Sears is a freelance writer and gardener in the Midwest.
Well-drained soil is a must-have for seed-starting and growing many plants. For many gardeners, perlite plays a key role in improving soil drainage and aeration. Made from transformed volcanic glass, perlite is the small, white balls often found in potting soil and seed-starting mixes. Lightweight and pH neutral, it doesn't decompose. Read on to learn more about perlite, how it is made and how it is useful for gardeners.
Perlite is a naturally occurring mineral that is added to garden soil to improve aeration, water retention, and drainage. It looks like small, white Styrofoam balls and is commonly found in potting soil and seed-starting mixes. Gardeners often use perlite in vegetable gardens and flower beds to improve water drainage and aeration. Sold in bags of various sizes, perlite is available online and in garden stores. The National Organic Standards Board allows its use in certified organic agriculture.
In its natural form, perlite is a dark-colored, dense volcanic glass that is composed of about 70 percent silicon dioxide. Its other components include:
- Aluminum oxide
- Calcium oxide
- Iron oxide
- Magnesium oxide
- Potassium oxide
- Sodium oxide
In its natural form, volcanic glass, or obsidian, contains a lot of water and is heavy. After it is extracted out of the earth, the material is crushed.
To turn it into the lightweight material used for gardening, natural perlite is put in industrial ovens where it is quickly heated to temperatures of around 1600 degrees F. The water in perlite reacts much like popcorn does when heated. The perlite "pops" and can expand to up 40 times its original size.
After the heat transformation, perlite is lightweight and looks like white balls. The lightweight balls are extremely porous and hold their shape. They're also stable and do not decompose.
Perlite may then be bagged and sold at gardening stores as either coarse, medium or fine grade perlite. It may also be mixed into potting soils and seed-starting mixes.
- Perlite has a neutral pH level, gardeners can mix it with their soil and not worry about it changing the soil's pH level.
- Because it's made from naturally occurring compounds found in the soil, it contains no toxic chemicals. This also makes it a top choice for organic gardening.
- Once processed, perlite retains its shape, even when pressed, and is physically stable. This makes it great to use in clay or compacted soils.
- In its processed form, it's very porous. When added to soil, it improves aeration, water retention, and drainage.
When perlite is added to the soil, it improves the soil's drainage capabilities. Its porosity helps in both water retention and drainage. So it holds onto some water while allowing excess water to drain away, making plants less likely to drown from excessive watering.
Perlite is often used in seed-starting and potting soil mixes because it keeps the soil loose, well aerated and well draining. To make your own soil mix, use 1 part perlite, 1 part loam and 1 part peat moss and mix well.
In the garden, perlite can be scattered on top of the surface. It'll act as a wicking agent and eventually work its way into the soil. Perlite can also be mixed into the soil when preparing raised beds or planting beds.
Instead of using just water for root cuttings, use water and perlite. It encourages better root growth.
Perlite, along with vermiculite, can be used in hydroponic gardening, too.
Like perlite, vermiculite is a naturally occurring mineral that is extracted from the earth, crushed and heated at high temperatures to expand and create a lightweight material. The brown-gold flakes absorb water and expand.
Both perlite and vermiculite are sold in bags at garden centers. They're both used in potting soils and seed-starting mixes. However, there are times when you'll want to use one rather than the other.
Because vermiculite absorbs water, it's best used with plants, such as many tropical plants, that need more moisture. Because perlite drains and aerates, it's often used with cacti, succulents and plants that need to dry out between watering. Perlite is also good for mixing with clay soil in the garden.
With seed-starting, vermiculite can protect seedlings from fungal diseases and retain moisture. Often perlite is used when the seedlings are moved to separate pots.
Perlite is sold in three different grades: course, medium and fine. Course perlite has the largest particles while fine has the smallest. The type to choose depends on how you want to use it.
Fine perlite is best to use for seed-starting and root cuttings. While it can be scattered on top of the soil, the fine particles can easily be affected by the wind.
Medium perlite is often used for potted seeds and seedlings. It can also be scattered on top of the soil.
Coarse perlite is the best for mixing into clay garden soil or scattering on top of the soil. It's also used for succulents and orchids.
Because water drainage and aeration is so important to plant health, understanding how perlite is used and can improve your soil can help you grow stronger, healthier plants. This soil additive is so useful that many gardeners keep a bag of perlite in their gardening shed so that is available when needed.
Q. Dear Mr. McGrath: My wife is an avid listener of your program, planning her Saturdays so that she can be home when your show is on. She suggested I contact you with my small concern that the perlite you often recommend isn't "organic" in any sense that I recognize as a chemist—or probably in any sense that non-scientists use the word, as it is a form of glass http://www.perlite.net/. Now, this doesn't take away from it being a useful component for gardening as you have advised; the advice is still fine. I would just suggest adjusting your justification of "it's organic." Oh, and rest assured that I am also a listener who enjoys your show. Keep up the good work!
---Harry in Havertown, PA
A. I have to thank Harry for bringing up an important point. In the world of technical chemistry, the word 'organic' has a very different meaning than it does in the world of contemporary food and agriculture. But that's not unusual, as many English words have multiple meanings, like 'bulb', which can refer to an underground plant part, something that gives off light or even a squeezable thing, like the back end of a turkey baster. Context always reveals which version is in play (at least you hope it does, and people aren't burying CFLs in their garden or screwing tulip bulbs into table lamps.)
Let's take a look at the roots of the word 'organic' as we use it in modern gardening and grocery shopping. Back in 1940, J. I. Rodale, the founder of Rodale Press (now known as Rodale, Inc. or just 'Rodale') and originator of ORGANIC GARDENING magazine (titled "Organic Farming and Gardening" during its early years), chose 'organic' from a slew of words that the British were using to try and describe a form of agriculture that shunned the toxic chemical pesticides and fertilizers of the day; things like 'arsenate of lead' and ammonium nitrate—and later, famously, DDT. In its earliest and simplest uses 'organic' meant anything that occurred naturally—like seaweed, manure, rock dusts…
…and perlite, the misunderstood mineral. Harry is correct that perlite is a form of glass; specifically a volcanic glass that is mined and then heated in big ovens until it 'pops' into a round, white material that's used in seed-starting and potting mixtures to lighten the soil, allow more air around the roots of plants and to both help retain water and improve drainage.
A lot of people think that those little white balls are some kind of Styrofoam-type artificial substance—heck, that's what I thought they were my first couple of years gardening! But they are a mined mineral popped into little balls. Little balls that our bulb-happy friend Art Wolk (author of "Bulb Forcing for beginners and the seriously smitten") has raved about on our show, during his lectures at places like the Philadelphia Flower Show, and in his publications.
One of Art's missions as an educator is to beat the importance of air in the soil into gardener's heads, whether they're forcing bulbs in the ground or growing plants in garden beds—and perlite is a great way to do that. You may also recall that the great Four-Season farmer Eliot Coleman listed 'air in the soil' as one of his "three essential keys" in the seaweed piece we did a few shows back.
And I am proud to add that I personally use a lot of perlite. I buy a couple of big bags of it every season and mix lots into my seed-starting mixes>; however much perlite they already contain, I add more. And when I began to worry that the soil in some of my raised beds seemed to be getting a little heavy a few years back, I started mixing perlite into what's otherwise now mostly 20 years of compost. And I add a lot of perlite to my garlic planting beds in the fall to make sure the bulbs don't get waterlogged over winter.
And remember a few weeks back when we reported that Eliot Coleman said that seaweed was the only input he would buy if he didn't live right near a cold ocean where he could get it for free? Perlite is my choice in that category. And since I don't happen to live near an old volcano that's conveniently located next to a giant oven, I do have to buy mine. And I buy and use more of it every year. "Air in the soil"; it's a gardener's second best friend. (After compost, of course!)
And you can be certain that it's organic, because it's a mined mineral; like rock dust and seaweed, it's naturally occurring. It does need to be 'popped' but that only makes it more useful, not unnatural.
And you don't have to believe just me…
…because it's also officially organic.
In the early 1990s, the state of California did the hard work of making the first official decisions on what inputs and products would be allowed in what would come to be called 'certified organic agriculture'. Shortly afterwards, the NOSB—the National Organic Standards Board—was formed to codify the Federal regulations that exist today. And part of that 'standard' is that all of the 'materials' or 'inputs' allowed in organic production must be on what's called the "OMRI list"—the official listing of allowed products as issued by the Organic Materials Review Institute. Access their website, type in the name of the product or material you're considering and it'll give you a thumbs up or down.
And with perlite, the thumb is up: "Status: Allowed in certified organic agriculture; a mined mineral."
Case closed. Now, where's my big bag of perlite….?