What is Vermiculite? Should You Use It?
Vermiculite is a versatile rock with applications ranging from construction, insulation, and furnaces, to potting mix and soil conditioner. As someone who has worked with it several times, I can attest that there are things that you still need to know before buying vermiculite.
Generally, vermiculite is a brownish, odorless, lightweight mineral with a neutral pH. It improves soil aeration, absorbs water, holds nutrients, and does not change physically due to heat. Medium, coarse, and extra coarse grades are ideal for gardening. It is hard to create DIY alternatives for it, but perlite substitutes work well.
Buying fine vermiculite was one of the mistakes I learned a lot from. I thought it was the best option because it was the most accessible grade. But I was mistaken! What went wrong for me? This story will unfold at the end of this article.
1. Formed From Biotite
Mined from natural deposits around the world, vermiculite is made from mica which is a rock composed of silicate minerals. Vermiculite develops over a million years as a result of biotite weathering.
For us to know how vermiculite was formed, we must start by knowing who biotite is.
Biotite is a mineral that has magnesium, aluminum, iron, and silicate. Do not worry, you do not have to take those elements in mind. This is just like me introducing you to a new friend!
When biotite is hydrated, vermiculite is formed. When it is heated, it expands. This expanded vermiculite is the one we use as a potting mix and soil conditioning agent.
Fun Fact: For the past century, there has been a vermiculite ore mine in Libby, Montana but it was contaminated by asbestos fibers—causing its shutdown in 1990. Today, vermiculite is mined in different countries like Kenya, Uganda, South Africa, Zimbabwe, Brazil, Bulgaria, Russia, Australia, and the United States of America.
2. Brownish, Odorless, and Lightweight
Vermiculite’s color ranges from light tan to golden brown. Once heated, the mineral wriggles, expands, and becomes lightweight. During this process, sterility is also favored—making vermiculite odorless.
Fun Fact: When vermiculite is heated, it wriggles like a worm. Its name is derived from the Latin term vermiculus which means little worm! Amazing, isn’t it?
I created the table below to summarize vermiculite’s physical properties.
|Appearance||Flaky, Brownish, Shiny|
|Melting Point||Around 1350 °C|
|Water Solubility||Not Soluble|
How can this information be useful to you as a gardener?
In the garden landscaping world, growing medium color is a big deal as it elevates the aesthetics of your garden.
Now knowing vermiculite’s appearance, one might not use it for plants with colorful and shiny leaves as well like croton. They may not complement each other in terms of attractiveness because it is preferable to pair shiny leaves with a matte-looking growing medium.
Vermiculite is also fit for indoor gardens since it does not give any off-odor.
Unlike other growing mediums that are heavy (e.g. gravel and loam soil), vermiculite is lightweight. Because of this, it is a growing medium that is easy to work with.
Assume you need to move a sack of your growing medium to the other side of your garden. When compared to loam soil and gravel, that would be a much easier job with vermiculite!
4. Melting Point
Vermiculite’s high melting point is a reflection of its compatibility with hot environments. With this information, we can say that a hot temperature in your growing room will not physically affect vermiculite.
3. Neutral pH
The pH of vermiculite is approximately 7.0–7.5. This indicates that using it as a growing medium will not easily sway the pH which is optimum for the plant’s growth.
One of the biggest issues for growing mediums is their pH. Luckily, we do not have that problem with vermiculite. This is the reason why most gardeners prefer neutral growing mediums like vermiculite, gravel, and clay pebbles.
Some mediums such as peat moss have an acidic pH. When this growing medium is used, it may adversely affect the plant if it does not grow well in acidic conditions. Thus, you need to check which pH is best for your desired plant.
Pro Tip: You can ask the following questions.
1. Does my plant prefer acidic soil?
If yes, you can use peat moss as a substrate since it lowers the pH due to an abundance of organic matter. Some plants that love low pH are daffodils, azalea, and magnolias.
2. Does my plant prefer basic soil?
If yes, you can use limestone grit as a growing medium because it has a high pH. Plants that love basic pH are mostly vegetables such as broccoli and lettuce.
3. What is the pH of the growing mediums that I have in mind?
This is the question that most gardeners skip to answer. Always remember that providing the best growing environment for plants will always pay off! This starts with providing a good foundation.
4. Improves Soil Aeration
Due to its small particles, vermiculite can enhance the porosity when added to another growing medium such as loam soil. The flaky shape of vermiculite gives way to the formation of small air pockets that can accommodate both air and water resources for the plant.
Let me explain porosity using a dishwashing sponge.
We usually use a sponge as a cleaner because it absorbs and holds water right? The sponge works well because it is a porous material.
The same thing can be said about good soil. For a growing medium to be ideal in potted plants, it must hold enough water for a long time.
The longer a growing medium holds water, the longer the plant has access to resources (i.e. nutrients and water) to fuel biological processes that make them grow.
5. Interacts Well With Heat
Aside from its use in horticulture, vermiculite is also a prominent insulator for houses and furnaces because it is fire resistant. It can effectively hold heat while not being altered physically and chemically, which is helpful in hydroponic gardening since it will not transfer the heat to the plants and damage them.
Is vermiculite safe in the oven?
Because vermiculite has a high melting point, it is safe in the oven! This is actually one of the interesting features of vermiculite. It is used for furnaces! Vermiculite is poured throughout the burner to ensure even heat distribution.
Fun Fact: You can even use the vermiculite from a gardening shop in a natural gas fireplace! This will help you have a controlled temperature around the fireplace and radiate the hot temperature away from your living area.
6. Absorbs and Retains Water Effectively
Vermiculite can absorb and hold water effectively due to its high water holding capacity and capillary action. Hence, it does not float. Thus, vermiculite is best used for houseplants that do not require daily watering.
1. Does vermiculite absorb water?
Unlike perlite, which is another prominent growing medium, vermiculite can absorb water. This means that the water can be soaked inside the material, and not only at the surface of the particles.
2. Does vermiculite hold water?
Vermiculite’s ability to hold water lies in its soaking capability and capillary action properties.
To explain capillary action, have you ever noticed water trapped inside your drinking straw? This is the simplest illustration of capillary action. Instead of being pulled by the gravity downwards, a force (i.e. capillary action) holds the water in its place.
The same thing happens in vermiculite. Water is trapped in between the spaces of each particle, thereby making water accessible for a longer time.
3. Does vermiculite float?
Because vermiculite absorbs and holds water inside and on its surface, it does not float.
7. Holds Nutrients Well
Vermiculite holds nutrients well due to its high cation exchange capacity (CEC). This feature gives the plant’s roots longer access to nutrients that support growth and development.
From the statement above, I bet some words might seem unfamiliar to you. So let us go through them.
These are positively-charged ions, on a molecular level. As you will notice, batteries have positive and negative signs. So just think of cations as small particles that have a positive sign on them.
In the plant world, nutrients such as calcium, magnesium, phosphorus, nitrogen, and potassium are essential. These nutrients carry cations.
2. Exchange Capacity
Naturally in the soil, these particles are present. The cation exchange capacity is a measure of how many negative ions soil can hold.
Remember that positive and negative ions attract each other. So if there is a higher amount of negative ions in the soil (high CEC), more cations (positive ions) can be accommodated. Thus, when the growing medium (e.g. vermiculite) has a high cation exchange capacity, it can hold nutrients efficiently and make them accessible to the plant’s roots.
This characteristic is useful if you are planning to build a hydroponic system!
Head to our article on how to choose a hydroponic growing medium if that is something you would venture into.
8. Available in 3 Grades
Vermiculite is available in the market in 3 horticultural grades namely, medium vermiculite (2-4 mm), coarse vermiculite (3-6 mm), and extra coarse vermiculite (5-8 mm). These are known commercially as Grades 2, 3, and 4, respectively.
Which one is the best? Well, to answer that, each grade is appropriate to a certain purpose.
1. Medium vermiculite
Medium (Grade 2) is mainly used as a germination medium, either alone or in combination with peat. Seedlings grown in vermiculite have dense root growth and establish quickly in the growth cycle.
Grade 2 vermiculite can also be used for propagating root cuttings.
2. Coarse vermiculite
Coarse vermiculite (Grade 3) is an effective soil amendment, especially for high-density soils.
To provide appropriate soil porosity, gardeners usually use a 1:1 ratio of coarse vermiculite and soil. Using this improves moisture and nutrient availability especially in sandy soil, resulting in rapid growth and soil permeation for the root system of plants.
This makes grade 3 vermiculite perfect for outdoor transplanting and root cuttings.
3. Extra coarse vermiculite
As discussed above, vermiculite has both great insulation and moisture control abilities. These features make it an ideal medium for bulbs (e.g. onions) and tubers (e.g. potato) storage.
To do this, pour 3-4 inches of extra coarse vermiculite into a bucket or any deep container. After that, add the bulbs or tubers. Repeat this step, layer by layer to preserve these crops.
If you want to compare vermiculite with other growing mediums, head to our article perlite vs vermiculite.
9. Hard to Do-it-Yourself
Unlike perlite, vermiculite is harder to replicate at home due to its flaky particles. Thus, vermiculite DIY options are not available.
However, if you are interested in possible alternatives for vermiculite, perlite substitutes can also provide the same characteristics as vermiculite.
If you want to learn the DIY option for perlite, go to our article about perlite facts.
10. Coarse Vermiculite is Better
Coarse vermiculite is more recommended since it drains water faster compared to finer grades. A major disadvantage of fine vermiculite is it has a higher tendency of becoming soggy. When this happens, microorganisms can grow and contaminate the growing medium, and further the plant.
This is the mistake that I was talking about at the start of this article. Back then, I used the common vermiculite being sold at garden shops. After using it as a growing medium, my plants died because of overwatering.
Fine vermiculite can absorb so much water that it may lead to other problems such as browning of leaves, softening of stems, and—worst of all—pathogen infestation. I do not want you to experience that! Thus, I recommend using coarse or extra coarse vermiculite.
11. Various Packaging (+ Costs)
Vermiculite packs that are available in the market are 1, 2, 4, and 8 quart-bags that weigh around 10 ounces, 20 ounces, 2 pounds, and 5 pounds, respectively. The price of vermiculite ranges from 10 to over 20 dollars per unit.
The one below is a 1-quart bag that weighs 10 ounces and costs around 10 bucks!
This 2-quart vermiculite bag weighs around 20 ounces. You can buy this at around 13 dollars.
A 4-quart packaging that costs around 17 dollars is also available. This package weighs around 2 pounds.
Below is an 8-quart packaging of vermiculite. This weighs 5 pounds and costs around 22 bucks!
Does organic vermiculite exist?
Vermiculite is considered organic according to the United States Department of Agriculture (USDA). This is because it naturally exists and no artificial processes are done to produce it. They are just heated and packaged as is.
Does vermiculite expire?
Because vermiculite is a mined mineral, it does not have an expiration date. However, you need to sanitize it before and after reuse to prevent the spread of disease from the previous growing cycle. To sanitize, you can use bleach, detergent, or hydrogen peroxide. Learn more about these disinfectants in our article on hydrogen peroxide vs bleach.
Summary of Vermiculite Features
Vermiculite is a growing medium that is formed from the weathering of biotite. It is brownish, odorless, lightweight, and has a neutral pH. By using it as a gardening medium, vermiculite improves soil aeration, absorbs and holds water and nutrients well, and is not adversely affected by heat.
Horticultural vermiculite is available in three grades namely medium, coarse, and extra course. Medium grade (#2) is fit for seed germination and rooting of cuttings. Coarse (#3) is recommended for transplanted seedlings and cuttings. While extra course grade (#4) is useful for the storage of bulbs and tubers.
For garden use, coarse grades are recommended because they have lower tendencies to accumulate water and become saturated. There are 1, 2, 4 , and 8-quart vermiculite packs available on the market. Unlike perlite, vermiculite is difficult to DIY because of its flakiness. But, perlite DIY options can also be done in place of vermiculite.
- “Construction Materials: Lightweight Aggregates” by Bush, A.L. in Encyclopedia of Materials: Science and Technology (Second Edition)
- “Horticultural Vermiculite” by N/A/ in J.P. Austin Associates, Inc.
- “Inorganic and Synthetic Organic Components of Soilless Culture and Potting Mixes” by Papadopoulos, A.P. et al. in Soilless Culture
- “NIOSH Recommendations for Limiting Potential Exposures of Workers to Asbestos Associated with Vermiculite from Libby, Montana” by N/A in CDC