When you use containers for gardening, you know there are two extremes: dry, dusty soil and wet, mushy soil. Luckily, soil additives like perlite and vermiculite can help you to avoid these two extremes.
So, should you use perlite or vermiculite? Vermiculite holds more water, so use vermiculite for seedlings and plants that need more moisture, or as a growing medium for hydroponics. Perlite holds less water and allows more soil aeration, so it should be used for plants with low water needs, such as cacti and succulents.
Of course, the decision to use perlite or vermiculite does not have to be all-or-nothing. You can mix perlite and vermiculite in a 50-50 ratio (or any other ratio you desire) to get a combination that retains more water than pure perlite, but less water than pure vermiculite.
Depending on your gardening needs, you can also look to the various alternatives, such expanded clay, expanded shale, pumice, and peat moss.
Let’s start off by taking a look at why you should use perlite and vermiculite and what they are. Then, we’ll go into more detail about the alternatives, and answer some common questions about perlite and vermiculite.
Why Should You Use Perlite or Vermiculite?
The main reason to use perlite and vermiculite is that they help soil to retain both water and air. This is especially useful when gardening in pots and containers, since natural drainage and aeration methods may be absent.
Perlite and Vermiculite Retain Water
Perlite does absorb water and helps to retain it in soil, but it dries out quickly as well. This means that perlite takes up moisture when you water your plants, and readily gives it up when conditions become dry.
This benefits your plants by helping them to keep the soil moist, but not too wet. So, perlite is best when you want the soil to dry out completely between waterings.
Adding perlite to the soil in your containers will work well for cacti, succulents, and plants that have low water needs. It will also help to “correct” your watering if you tend to overdo it. For more information, check out my article on over watering your plants.
Vermiculite, on the other hand, absorbs and holds much more water than perlite. In fact, it can hold 3 to 4 times its volume in water. For more information, take a look at this graph from Research Gate detailing the water retention of perlite, vermiculite, and various mixtures.
Vermiculite is best when you want the soil to stay moist consistently, such as when you are starting seedlings.
Perlite and Vermiculite Help Keep Soil Aerated
Another benefit of perlite and vermiculite is that they aerate soil and prevent compaction. When soil is compacted, it has difficulty holding air and water, which are both necessary for plants to survive and thrive.
As mentioned above, vermiculite holds more water than perlite. However, the flip side is that perlite will provide better soil aeration than vermiculite.
What Is Perlite?
According to Wikipedia, “perlite is an amorphous volcanic glass that has relatively high water content, typically formed by the hydration of obsidian.” It contains a large amount of silicon, along with some aluminum and other elements.
The perlite you see in potting soil looks like small white stones with holes in them. It is sometimes mistaken for Styrofoam balls, since it has a similar appearance and you can crush it easily between your fingers.
For more information, check out this article on perlite from Wikipedia.
What Is Vermiculite?
According to Wikipedia, “vermiculite is a hydrous phyllosilicate material. It undergoes significant expansion when heated.” Like perlite, vermiculite contains significant amounts of both silicon and aluminum, along with other elements.
Its name comes from Latin vermiculare (“to breed worms”) and indeed, small pieces of vermiculite look like worms when they are soaked with water. Vermiculite may come in fine or coarse grades, and its texture is soft and spongy.
For more information, check out this article on vermiculite from Wikipedia.
What Are Some Alternatives To Perlite and Vermiculite?
If your goal is to improve the aeration or water retention of your soil, then there are several alternatives to perlite and vermiculite that can work for your purposes.
According to Wikipedia, “lightweight expanded clay aggregate (LECA) is a lightweight aggregate made by heating clay to around 1200 degrees Celsius in a rotary kiln.” One of its uses is in agriculture, such as in hydroponics systems.
Expanded clay can also be blended with soil to improve drainage, retain water during droughts, and aerate soil. It usually appears as clay pebbles that look brownish or gray.
For more information, check out this article on expanded clay from Wikipedia.
According to gardeningknowhow.com, “Expanded shale is formed when the shale is crushed and fired in a rotary kiln at 2,000 F.” This treated shale contains more air than normal, making it more similar to perlite and vermiculite.
As a result, expanded shale helps to aerate the soil and retains more water than ordinary shale. For more information, check out this article on expanded shale from gardeningknowhow.com.
According to Wikipedia, “pumice, called pumicite in its powdered or dust form, is a volcanic rock that consists of highly vesicular rough textured volcanic glass…” Translation: pumice has lots of holes that help it to retain air, water, and nutrients.
This makes it an ideal alternative to perlite or vermiculite, which is why it is often used in agriculture. An added benefit of pumice is that it does not attract insects or pests.
For more information, check out this article on pumice from Wikipedia.
Peat Moss (Sphagnum)
According to Wikipedia, “sphagnum is a genus of approximately 380 accepted species of mosses, commonly known as peat moss.” Both living and dead peat moss can store water – sometimes as much as 26 times their dry weight in water!
Peat moss does not decay easily, making it useful in gardening applications due to a longer useful life. Since peat moss holds so much water, it is useful as a soil additive for dry or sandy soils that do not retain moisture well.
Remember that peat moss has a low pH, meaning it is very acidic. Adding it to your soil can lower the pH substantially, so you may want to offset this effect by adding lime (calcium carbonate).
For more information about lime, check out my article on how to raise soil pH.
For more information about peat moss, check out this article on sphaghum from Wikipedia.
Common Questions About Perlite and Vermiculite
Here are a few common questions about perlite and vermiculite, along with some answers for your reference.
Is Perlite Toxic To Humans Or Animals?
Perlite is simply small fragments of volcanic glass. It may not be toxic per se, but eating rocks is not generally recommended. If you are worried about a child or pet eating perlite, then keep it out of reach, along with any potted plants that use it in the soil.
Is Perlite A Styrofoam?
The two look similar, but perlite and Styrofoam are not the same. Perlite is a volcanic glass. You can easily break perlite pebbles between your fingers, similar to Styrofoam. They also have a similar appearance (small, white pebbles), which is why they are often mistaken for one another.
Is Vermiculite Dangerous?
Vermiculite itself is not inherently dangerous, since it is simply a rock or mineral. However, mines where vermiculite is harvested may also contain asbestos.
This was more of a concern in the past, and governments are now much stricter about what can be sold to consumers. Many houses have vermiculite insulation, and this insulation may contain asbestos.
Can You Use Sand Instead of Perlite or Vermiculite?
Sandy soil drains well, which means that it will be difficult to overwater plants in sandy soil. However, sand does not retain moisture well, so adding it to soil will cause the soil to drain and dry out faster.
Adding sand to your soil is a good idea if you are growing cacti, succulents, carrots, or other plants that need well-draining or soft soil. On the other hand, sand is not a good alternative to vermiculite if you want to start seedlings in a moist growing medium.
Can You Use Diatomaceous Earth Instead of Perlite or Vermiculite?
Diatomaceous earth has its uses, but it won’t have the same aeration and water retention properties as perlite and vermiculite.
Diatomite, or diatomaceous earth, comes from fossils of diatoms (one-cell organisms). It is a sedimentary rock which contains large amounts of silicon and also some aluminum.
It is often seen as a fine, white powder. It is used in pool maintenance, but it also has the added benefit of being an insecticide (it is actually sharp enough to cut open insects and kill them!)
Be sure to use food-grade diatomaceous earth for any gardening applications. For more information on pest control, check out my article on how to get rid of ants and my article on how to get rid of aphids.
When using diatomaceous earth, avoid breathing it in by wearing a mask and handling it carefully. Also, avoid using it on a windy day, or else use it indoors.
By now, you should have a better idea of the difference between perlite and vermiculite, when to use each one in your gardening, and the possible alternatives.
I hope you found this article helpful. If you have any questions or advice of your own about perlite, vermiculite, and alternatives, please leave a comment below.