If you have ever read a list of ingredients on a bag or bale of potting soil or looked at a recipe for making your own growing medium online, you have probably encountered either the word “perlite”, or “vermiculite”, and probably both. You may have wondered what exactly they were, or if they were two different terms for the same material (which they are not); they are in fact quite different. Each has different properties and are selected for use in various growing media for their unique abilities to enhance a soil blend. Perlite is also widely used in the cultivation of mushrooms. Let’s take a look at what makes each distinctive from the other, how similar their commercial and industrial applications are, and how they each play a part in growing strong, healthy, and productive plants.
What is Perlite?
The name “perlite” is given to this material, due to the fact that it breaks into small, white, pearl-like orbs, resembling tiny pearls. Perlite is a lightweight, white, granite-like material that is often mistaken for Styrofoam pellets. It is in fact a volcanic glass that forms from lava cooling quickly after erupting.. Because of the rapidity of the cooling, moisture is trapped inside, constituting between two and five percent of its total makeup. It also contains high amounts of silica (Si). Its pH is generally neutral though it can range from slightly acidic, to slightly alkaline (between 6.6 and 7.5).
This silicate rock is mined in several parts of the world including the US, China, and the Mediterranean countries, and is used most often in agricultural, packaging, and medicinal applications, among several other commercial and industrial uses. As an industrial product, it is primarily used in roof and pipe insulation. It is desirable to the construction industry because it is non-combustible, and a great heat and noise insulator. Perlite is also used in some water filtration systems, including applications for potable water, swimming pool water, and even storm runoff. Many common alcoholic beverages and juices are filtered with perlite. It is, even added to some soaps as an abrasive. In the automotive industry, and in environmental hazard responses, perlite is used to help absorb chemical spills.
The raw material, which does not look anything like the finished product, is highly processed by crushing and heating to form the small, white pellets that are sold as amendments for growing media. It undergoes exposure to temperatures of 1600 F, at which temperature the raw material pops like corn, expands up to 20 times its size and turns white.
What is Vermiculite?
Like perlite, vermiculite is a lightweight, naturally occurring, mined rock product. The name derives from the worm-like appearance of this material when it is heated and formed before being processed. “Vermi”- is the root word, meaning “worm”, such as in the term “vermiculture”, “vermicide”, or “vermin”. Vermiculite is technically a clay which is in the mica family and is most often found in state as laminated flakes. Unlike perlite, it can expand when moist and can hold water, nutrients and even air. Conversely, it can compact, which will compromise its holding capacities, so it is important not to compress it if it is used for horticultural or packing purposes.
Vermiculite is brown to gold in color, is sterile, and has a pH range between 6.5 and 7.2. It is fairly abundant and is found on all of the world’s continents. It is specifically mined in African countries like Uganda, Kenya and South Africa, Eastern European countries like Russia and Bulgaria, in the Americas where it is mined in the US and Brazil and is mined in Australia as well. It is typically mined using strip or open cast methods, where ore is separated from the waste products.
When vermiculite is heated up to 1800 degrees F (980 C) , its structure expands in a process known as “exfoliation”. This occurs when the water that is trapped within its layers converts to steam as it heats up. It is sold as four different particle sizes from small to large flakes. Larger sizes are better for soil mixes as they offer better aeration, but each size has its place in different applications.
Vermiculite is soft and does not cling to itself but can “ferry” other materials. As such, it is great as a medium to carry nutrients and vitamins. This has applications for plant growth, but also in the animal feed industry. It holds fat concentrates and syrup-like concoctions such as molasses very well and is safe for animals to ingest.
Industrially, vermiculite is used in materials that are exposed to friction such as car brake linings. Before realizing how dangerous a material it was, asbestos was used for such friction linings; now, vermiculite is used. It has other automotive applications as well due to its shape, heat resistance, oxidation resistance and torque retention. It can be found in clutch linings, gaskets, and other automotive seals. Ground vermiculite is also used in automotive paints and finishes.
Because of its heat and fire resistance, as well as its low density, it is also used in construction applications as solid or loose-fill insulation, building boards, plasters, concrete flooring, concrete roofing, and forms the core of fire-rated doors. It has become a favored material because in addition to its insulation and heat retention, additionally vermiculite will not rot or deteriorate over time and does not attract rodents and pests. It is also odorless and potentially most valuable to the industry because it is far more economical to use than many other materials.
Vermiculite is valuable to the shipping and packing industry as well. Its lightweight nature makes it an economical alternative to many other packing materials. It can be poured around items to be shipped, and will not scratch them, but will keep them from moving around much in transit. Because it is so absorbent, it is often used as packing material for liquids, and even around hazardous materials that could leak during shipping. Because it is non-flammable, it is also the preferred shipping medium for items that could potentially explode or catch on fire when in transit if handled improperly.
Horticultural Uses of Each
Perlite’s primary contribution to soil mixes is that it does a great job with aerating the mix. This allows for air and water to circulate freely around a plant’s root system, preventing the media from retaining too much moisture. Perlite is also used as an alternative to sand in some mixes because, like sand, it provides drainage to a soilless medium, but it is much lighter in weight than sand and holds more air. It is also sterile, so it does not pose a risk to adding unwanted biological material to a soilless mix. One drawback to using it though, is that perlite can float to the top of a fully saturated mix and potentially be lost to overwatering if too much water is inadvertently added. It is also recommended that growers adding perlite to a mix, moisten it before pouring it out of the bag as its dust can be a lung irritant when it is dry.
Perlite is used in some applications to boost humidity. This may seem counterintuitive because its primary role in horticulture is for drainage, but because of its shape, which is full of holes, it holds water without absorbing it, so it releases it as it evaporates off of its surface. It can also be used in the garden to loosen clay soils and avoid puddling in compacted soils if it is properly worked in. Its insulating properties also help to mitigate some amount of temperature fluctuations in the garden.
It should be noted that growers are not advised to grow plants in straight perlite. In some cases, this can cause fluoride burn on plants. This shows up as brown tips on plants. Mix in other media when growing with perlite.
If perlite is not available, there are alternative materials that can perform the same function to varying degrees. Sand performs the same function for aeration and drainage, but it is much heavier than perlite, making mixes more difficult to work with. Rice hulls, as long as they are sterilized so that all weed, and rice seeds are killed can be a viable alternative. Uncooked rice should not be used as an alternative to perlite. Polystyrene, commonly known by most as “Styrofoam” is a viable alternative to perlite. Indeed, many people initially believe that the small, white pellets in potting mixes are indeed Styrofoam, until they learn about perlite. It does give soilless mixes lightweight properties and helps with aeration, but unlike perlite, it will compress over time, reducing its effectiveness.
Vermiculite on the other hand, does hold water very well (absorbing three to four times its volume in water), and nutrients too. It is comprised of hydrogen (H), magnesium (Mg), iron (Fe), aluminum-silicon (Al-Si), and oxygen (O). It can contribute some amounts of these elements to the soil mix. It can also add some amount of calcium (Ca), Potassium (K), and small amounts of other trace minerals to the mix. Its consistency promotes a slow and steady release of absorbed nutrients back into the media.
Of the various grades of coarseness and particle sizes, typically a medium grade is most useful to incorporate into seeding and seedling mixes and a coarser grade is more suitable for more mature plants. It also has the benefit of protecting young seedlings from some types of fungus that could easily otherwise kill them.
Like perlite, vermiculite can also be incorporated into garden soil. It will help sandy or fast-draining soil types to retain some amount of moisture. Because it is a rock, it won’t break down and decompose like compost, but instead will be a useful soil additive for many years. It should not however be used in heavy, clay soils as it could cause too much moisture around your plant roots, leading to oxygen depletion and rot.
When vermiculite cannot be obtained, other materials can be used to perform some of the same functions. Pine bark, when finely ground (often called “pine fines”), can serve some of the same roles of water retention in a soil mix. It can however, lower the pH of the mix, which will need to be factored in and potentially counteracted with lime or other alkaline materials. Coir, which is finely ground coconut husks can be used in mixes where vermiculite is not available as it too can hold moisture and nutrients well. Another natural material which is not often used but can be a suitable substitute is the leavings from cotton processing. Composted cotton stems, leaves, and hulls can be used in place of vermiculite as a water retentive substance.
It should be noted that vermiculite mined prior to the 21st century can have asbestos in it. The reason for this is that up to 80% of the world’s vermiculite supply was mined in a location in the United States where asbestos was also mined. This often led to cross-contamination of the vermiculite that left that particular mine. Strict safety protocols have since been put in place nationally, eliminating the chance of cross-contamination and as a result, some vermiculite packaging contains messaging indication that it is 100% asbestos free or a similar claim. Any bags of vermiculite that may still be hanging around unused from pre-2000, should not be used, and instead, should be disposed of in a safe manner.
Uses in Mushroom Cultivation
Perlite can be used as the primary component of some mushroom growing systems. It is used to maintain the high humidity levels required for mushroom grow chambers. The large surface area on each perlite “pebble” means that the evaporation process is prolonged, creating a more humid environment. Perlite is also used in mushroom cultivation, because just like with plant cultivation, it is valued because of its aerating and moisture retaining properties as well as being sterile.
To effectively use perlite to grow mushrooms, the growing chamber or terrarium (or whatever container is used) should have a layer of wet perlite at the bottom. It can be used right out of the bag if it is sealed. If it has been stored in an open bag or container it should be boiled first or otherwise heated to ensure sterilization. Alternatively, or in addition to, a small amount of Hydrogen Peroxide (.5%) can be added, so long as it does not come in direct contact with the developing mushrooms.
Water should then be added to a level about ¼ to ½ the depth of the perlite. After the perlite is ready to go, then, colonized “cakes” or blocks can be placed right on top of it and then the container should be sealed. If the cakes are touching water, then either more perlite should be added or water should be removed. It is important that there is enough water to keep the perlite moist and humidity high, but not so much that it touches the colonizing mushrooms. After all of that is squared away, making sure that the perlite remains moist and off-gassing the CO2 daily is about the extend of this growing method.
After the mushrooms have developed and been fully harvested, the container will need to be sterilized between uses. Avoid the temptation to clean and reuse the perlite. Sterility is vital to the production of mushrooms and a new layer of perlite should be added to each batch.
It should be noted that some folks do use vermiculite in mushroom cultivation, but most growers would advise against it. It can be useful during the “flush” phase of growth. Incorporating it into other substrates can also help avoid clumping. Most mushroom producers do not however use it as it can cause the media to get too “mushy”.
