Perlite and Vermiculite are both used as soil additives and they seem to do the same thing. They make soil lighter, hold water and nutrients, and increase drainage. If they are so similar, why do we need both? In this post I will compare the two products and make it clear when and why you should use these soil amendments.
What is Perlite?
I did a thorough review of perlite in a previous post called Perlite Myths – Should It Be Used in Gardens and Potted Plants?
Perlite starts as a mined volcanic glass-like rock that is heated to high temperatures to make it pop into the light weight material you are familiar with. It absorbs water, has a low CEC (cation exchange capacity) so it does not hold nutrients very well, and it’s inert adding few nutrients to soil.
What is Vermiculite?
Vermiculite is mined silicate, a types of clay, that is heated to high temperatures to expand the layers in it to form the product used in horticulture. It looks similar to mica with thin layers of material.
Vermiculite is light weight, chemically inert, holds a significant amount of water and nutrients, and adds some nutrients to soil.
Asbestos in Vermiculite
Some vermiculite was contaminated with asbestos a number of years ago. The mine producing this material has been closed for over 20 years and todays vermiculite is mostly asbestos free. In 2000 the EPA tested commercial horticultural products to see if they contained asbestos. “Only 15 percent (8/54) of these products contained enough asbestos to allow EPA to quantify the percentage of asbestos reliably”. They concluded that, “the likelihood of the asbestos becoming airborne, during routine use of these products, indicated that this potential exposure poses a minimal health risk to consumers.” The Vermiculite Association has set a tolerance is .01% which is nearly undetectable and current vermiculite is well below this value (communication with industry insider).
The risk for asbestos in horticultural products is minimal.
Myths About Vermiculite
Several gardening sources claim that vermiculite prevents damping off in seedlings. I found no support for this claim but the fungus may grow better in soil.
One online garden source claimed vermiculite can be composted but that is not true.
Another claim – vermiculite does not add drainage. This is not true. It does add drainage and aerates soil but it is not as effective as perlite. It also compresses over time, reducing it’s ability to improve drainage.
Properties of Perlite and Vermiculite
Perlite and vermiculite are both lightweight, improve drainage, are odorless, seed-free and neither will rot or decompose.
Both are fairly stable. Perlite can be crushed in your hand, but it is usually remains unchanged in potting media. Vermiculite however is easily compressed even by the weight of soil. This is why media containing vermiculite loses it porosity over time, decreasing both the amount of air and drainage.
Neither product produces harmful dust, but perlite tends to have more fine dust.
pH of Perlite and Vermiculite
Perlite has a pH of 7.0 to 7.5. Common gardening literature pegs the pH of vermiculite as neutral but in actual fact it has a pH in the range of 6.0 to 9.5. Vermiculite holds on to minerals better than perlite and these minerals can affect the pH. If the vermiculite manufacturer does not provide the pH it should be checked before it is used. I checked a few products on Amazon and none provided the pH.
Unlike perlite, vermiculite has a strong buffering capacity. This means it will affect the pH of the added water or fertilizer solution.
CEC of Perlite and Vermiculite
CEC is a measure of the ability to hold on to nutrients. The CEC for peat moss, perlite and vermiculite are 110, 15 and 150 meq/100 grams. This gives the impression that vermiculite holds more nutrients than peat moss but when these numbers are converted to volume we get 10, 0.15 and 2.3 meq/ml. Since gardeners make up mixes on a volume basis these latter numbers are more realistic. Peat moss is much better than vermiculite, which is better than perlite.
Water Holding Capacity
The water holding capacity of peat moss, perlite and vermiculite are approximately 76%, 38% and 71% (v/v%). Mixtures of peat moss and either perlite or vermiculite result in a water holding capacity between these limits. These numbers are approximate and are affected by particle size as well as other properties.
I see a lot of comments on line that say adding vermiculite to a peat-based soilless mix will make it wetter. Based on the above numbers, that is not true. In fact vermiculite makes peat slightly drier than peat moss alone.
Vermiculite will increase the water holding capacity of well drained sandy soil, but it won’t increase it in clay soil.
Vermiculite is a type of clay soil and does swell a bit as it absorbs water. Perlite does not swell.
Plant Available Water
The above mentioned water holding capacity is a measure of how much water a media can hold. For example, When a pot full of peat moss is submerged in water, allowing the peat to absorb as much water as possible and then allowed to drain, the amount of water in the pot will be around 76% of the pot volume. This is the water holding capacity of peat moss.
Over time plants absorb water and some is lost to evaporation. The amount of water slowly decreases. At some point the peat moss will still contain water, but plants will no longer be able to use it. In garden soil this is called the wilting point. The soil still contains water, but plants start to die because they cant’ use it.
The difference between the water holding capacity and the wilting point is the amount of water available to plants, called the “plant available water.” This value is much more important to gardeners than the water holding capacity since it tells us how much water is available to plants, and this varies for different kinds of media.
The following table shows this value for various types of media (data is from Sun Gro Horticulture). The data is approximate and depend on a variety of factors including particle size.
Vermiculite absorbs more water than perlite (the water holding capacity), but it also holds on to it better keeping it away from plants. Vermiculite does provide more water to plants than perlite, but not that much more, and it is certainly not as efficient as peat moss in this regard. A peat + vermiculite mixture is drier for plants than just peat moss.
The idea of adding vermiculite to media, be it peat moss, coir or bark, in order to make it wetter for plants is a myth.
If you are going to add either perlite or vermiculite, vermiculite will make a slightly wetter mix.
Nutrients in Perlite and Vermiculite
As un-fertilized products, perlite will release very few nutrients to plants, whereas vermiculite usually contains some potassium, magnesium and other cations in small amounts. As the products are used and fertilized, both will hold nutrients in the soil water solution in and around the particles. These nutrients are easily washed out in perlite, but held much longer in vermiculite due to its high CEC.
Organic Certification
Both products are mined minerals and processed so they are neither natural or organic, as so frequently miss-stated. They are however accepted in certified organic farming.
Claims that “vermiculite can be organic, but it is not always organic” don’t make any sense. Some bags may be labeled certified for organic agriculture, but all vermiculite is non-organic.
Using Perlite and Vermiculite
Both products will improve drainage and prevent compaction although perlite is better for the later. They will both improve moisture in dry sandy soils. Perlite will make a soilless mix drier than vermiculite, which can be useful for gardeners who like to overwater their plants or when the media is used for plants that like to be drier. Vermiculite holds more moisture and is better in cases where this extra water is a benefit.
Because perlite won’t compress it is a better choice for adding aeration.
Both options are good for containers and growing houseplants. They can be used in the garden, but I would not recommend them for such use. Vermiculite compressing over time and perlite tends to float away with watering. Neither is a natural product, nor are they very sustainable. Sand is usually a better option if it comes from a local source.
Don’t use perlite or vermiculite in garden soil!
Both perlite and vermiculite can be used for starting seeds or rooting cuttings. Before you use vermiculite check it’s pH to make sure it is not too alkaline.
Some DIY recipes for soilless mixes include both vermiculite and perlite. There is really no point in doing that. Any difference between the two tends to be canceled out by adding both. Such recipes clearly show that the person who designed them did not know what they are doing.
Can you elaborate on why these products should not be added to garden soil?
For one – they are real expensive considering the amount you need. Vermiculite breaks down over time, but in a garden it is there for hundreds of years. Perlite tends to wash to the surface.
In garden soil organic matter is a much better choice and has similar properties.
Great information to know. Thanks! I do enjoy reading your articles
I have a roof garden on my third floor with the boxes initially started with mainly peat and vermiculate and some soil. I have been adding compost over many years and now the soil seems to dry out too quickly. But when well watered, it is quite productive, especially with the leafey greens that resead each year.
The only downside is that I have to remove most of the soil over winter due to weight concerns if a big snowfall were to occur.
In another post you say that adding clay soil to media (coir, peat moss, composted wood etc) for use in containers means you don’t have to water them so often. Presumably this means that the plant available water for the mix is then greater than 50% for media alone?
The clay is less porous and holds water longer.
Great article, thanks! Concise and yet answered all my burning questions about these two products 👍