Traditionally, people worldwide have used gravel at the bottom of containers for potted plants. The reason for this is to improve drainage. Perlite, on the other hand, is mixed into potting soil to aid drainage. One advantage is that it doesn’t break down and decompose over time, but there are many more.
Perlite and gravel are not interchangeable. However, both have properties that will generally aid drainage when mixed or used with potting soil for container planting. It is, though, important to know why you would use perlite.
What is perlite?
Perlite is a substance that has many different uses including agricultural and horticultural applications. It is also known as volcanic glass.
In its natural state, perlite is a volcanic rock that cools rapidly, trapping water within the rock. The lava, which is the molten rock from the volcano, forms into a glass-like structure.
Perlite is a silicate rock with a high percentage of silica and contains 2-3% water. Once it’s been processed, it bears no resemblance to the rock it once was.
Something else to be aware of is that perlite is a non-organic substance.
How is perlite processed?
The U.S is one of the world’s largest producers and users of both crude and expanded perlite.
Here, perlite rock is mined using open-pit methods that include blasting and ripping. It is then broken and sometimes crushed and transported to processing plants.
When it’s heated, it can expand up to 20 times its original volume. Even though the raw volcanic rock is usually gray, and sometimes brown, green, blue, and even red, it becomes a light gray or white color once it’s been heated.
So, when you buy perlite, you’ll be getting a light-colored, very lightweight material.
Where do you use perlite?
Perlite is an efficient, versatile material. It is sterile, fireproof, insulating, and lightweight.
It is commonly used in the construction, horticultural, environmental, insulation, industrial, and filtration industries.
For horticultural uses, it is mixed with soil to increase water retention and increases the amount of oxygen in the soil. But interestingly, this represents only about 10% of the annual consumption of perlite.
It has major value in the construction industry where it is used primarily for roof insulation and ceiling tiles. It is also used to make refractory bricks, for pipe insulations, and as a filling for masonry block structures. It reduces noise in buildings and increases fire resistance.
Perlite used for gardening
Perlite is a useful component for home gardeners. It is especially useful for adding to potting mixes for succulents and other houseplants that don’t need frequent watering.
An advantage is that it is physically stable and doesn’t decompose over time. It also allows water to drain better. It works particularly well when mixed with potting soil you’re using to start cuttings or seedlings that need to be established before being moved into the garden.
Generally, it will cost you less to add perlite to potting soil rather than buy special potting soil for succulents and so on.
What are the disadvantages of perlite?
Probably the most obvious disadvantage of perlite is that it isn’t a renewable resource. But, by all accounts, we haven’t used much of it in the past 60 years, and it’s inexpensive. Reports say we haven’t even used 1% of the world’s perlite ore … yet!
Another disadvantage is that, because perlite is composed of such small particles, aggressive root systems can block the soil mix. This reverses the advantageous effect it has on making the soil drain better.
Is perlite a good substitute?
Perlite can be used in the same way as several other materials. One perlite substitute is granite gravel that looks a lot like perlite and can be used in a similar way.
Both perlite and granite gravel is processed from porous rocks and both are great for retaining moisture in the soil. They also both keep the soil aerated and prevent the soil from becoming overly compacted.
Pea gravel, on the other hand, doesn’t have any water retention qualities. It is completely different from the angled gravel that is often used for underground structural construction and drainage applications.
Nevertheless, all types of gravel are heavier than perlite and they won’t wash away.
Sometimes perlite can be used instead of sand, but it’s not as heavy and the drainage it provides is a bit different from perlite. This is why it’s best to add perlite to potting soil. Don’t use it on its own.
Another problem with perlite sand is that it tends to be salty.
Can I use perlite instead of rocks?
Often people use little rocks or pebbles to line the bottoms of containers. The idea is to form a base beneath the soil that will allow the water to drain away.
While it can work, rocks will make the container super-heavy. This may not be an issue at first, but over time, it may make it a lot more difficult for you to move your potted plants.
Also, sometimes people describe perlite mixed into potting soil mixes as little white rocks. But, of course, even though perlite started off as volcanic rock, by the time we use it in potting soil, there’s nothing hard or rocky about it!
Even when the rocks are crushed to form gravel, they aren’t the same. However, mixing fine pea gravel into the soil can also aid drainage.
There are times when you can use perlite instead of gravel. Mostly it will be when you want to add something to your soil to make it drain better.
Perlite is a lot lighter than gravel, but it can do the same job of helping to make the soil drain better. Perlite is also a really great option for anyone growing succulents.
But before you add perlite to your soil, make sure that you need it to drain really well. Some plants need soil that has more bodies that will retain water better.