Vermiculite is a mica-like mineral that serves as a soil amendment. It is mined out of the ground, exfoliated, treated with extreme heat and pressure to force it to expand. The resulting worm-like, curly appearance is responsible for its name, which comes from the Latin vermiculus, meaning "insect larva" (ultimately from the Latin vermis, meaning "worm").
After being heated, not only does it curl, but it also expands and comes away sterilized. Both of these traits are important to gardeners.
Due to its expansion, vermiculite's capacity to retain water is increased. Plus, the fact that it is sterile means that gardeners do not have to worry that vermiculite is harboring pathogens that could kill their plants. The porous surface is great for retaining moisture and nutrients. The non-toxic mineral won't deteriorate in the soil, meaning it lasts a long time. It helps improve the structure of the soil, with the unique shape trapping water and nutrients for plants to access as needed. It improves drainage and lightens heavy soils, providing a more conducive environment for plant roots.
Vermiculite is silvery-gray and flaky. In fact, it is so light that you can move the little particles by blowing on them. The product does not contain any fertilizer, so you would have to mix it with something offering nutrients (such as compost) if you will not be applying fertilizer periodically.
Vermiculite is a great addition to potting soil in containers, helping to improve drainage and moisture retention. It's an ideal medium to use for storing bulbs or root crops over winter, as it absorbs any moisture that might cause decay. It's a good addition to recently seeded lawns. A 1/4-inch layer of vermiculite helps keep grass seed from drying out while it germinates. It's even a good addition to worm bins, providing grit that helps worms digest food.
Why Vermiculite Is Used in Soilless Potting Mixes
Vermiculite boasts several characteristics that make it useful to gardeners. The product is:
- Highly absorbent. Its particles absorb water and nutrients (which must still be supplied) and retain them in a soilless potting mix. Whenever the plants' roots are ready to use the water and nutrients, they are able to absorb, thanks to the vermiculite.
- Neutral in terms of soil pH, so that you do not have to worry about compensating for pH alterations due to the presence of the soil amendment.
How to Use Vermiculite in a Soilless Potting Mix
You can buy soilless potting mixes that already contain vermiculite. But you may find that you can save money by buying the vermiculite separately and mixing it yourself. In that case, here is a general-purpose formula to go by:
How Vermiculite Differs From Perlite
Perlite is that white material in soilless potting mixes that looks like little chunks of styrofoam. In its natural state, it has a pearl-like shine to it that accounts for the name, "perlite." While vermiculite and perlite are two completely different substances, they do have some things in common; both are:
- mined out of the ground (perlite is a volcanic mineral)
- later heated, causing them to expand
- used as soil amendments, both in soilless potting mixes indoors and to improve garden soil outdoors
- have a neutral pH
- contain no nutrients
But whereas vermiculite is especially valued for its capacity to retain water, the value of perlite lies much more in its ability to provide aeration.
When to Use Vermiculite vs. Perlite
Because it retains water so well, add more vermiculite than perlite to your mix when you are creating a growing medium for plants that dry out easily. But vermiculite is useful to gardeners beyond functioning as one component in a mixed growing medium. It is also the preferred material for covering seeds when you are starting seeds indoors.
There are three main reasons why vermiculite serves this function so well (better than perlite, peat moss, regular soil, etc.):
- When you are starting seeds indoors, they need to be kept consistently moist. If they are allowed to dry out, they will not germinate. The capacity that vermiculite has to retain water is greater than that possessed by perlite, making the former an ideal medium for keeping seeds moist.
- Not all seeds should be covered, but those that do need to be covered by a medium consisting of very small particles. This allows the young plants to easily push up through after sprouting. Vermiculite fits the bill perfectly, being finer than perlite. There are different grades of vermiculite, so choose the finest grade for covering seeds.
- Vermiculite is a sterile medium, making it great as a medium for seed starting due to its lack of bacterial or fungal problems.
Straight vermiculite can also be an excellent choice for a growing medium in which to root cuttings. Select a medium grade vermiculite for this purpose.
Because it does such a good job at aeration, perlite is the go-to product in mixes for plants that do not mind drying out and crave outstanding drainage. So add more perlite than vermiculite (or exclusively perlite) to your mix when you are creating a growing medium for plants such as cacti.