What is Vermiculite?
Have you ever noticed how some potting mixes sparkle in the sun? That’s because they contain vermiculite, a soil amendment that helps with water retention, aeration and nutrient exchange. Vermiculite is a phyllosilicate mineral that is mined from rocks formed 1.5 to 3 billion years ago. Crude untreated vermiculite is used in a variety of industrial materials. The garden variety of vermiculite is called “exfoliated” vermiculite, which has been treated with extreme heat and pressure to force it to expand. This process creates a porous surface that is great for retaining moisture and nutrients. In the past, some vermiculites contained asbestos. However, those contaminated mines were shut down and modern vermiculite is rigorously tested to make sure it is free of this carcinogen.
Vermiculite is a non-toxic mineral that will not deteriorate in your soil, so its effects last for a long time. Since it does not break down, it is not useful as a source of nutrients. Instead, it is a structural helper to promote improved soil. Its unique shape traps water and nutrients, which can be extracted by plant roots as needed. This means you need to water less often in high temperatures than you would with soil that does not contain vermiculite.
What is the Benefit of Adding Vermiculite to the Soil
- Improves moisture retention when mixed with soil.
- Helps with aeration of root systems, although if this is your primary goal in using a soil additive, you should instead choose perlite.
- Because it is a sterile medium, it is great for starting seeds and for propagating cuttings. Using plenty of vermiculite, or even straight vermiculite, can prevent bacterial and fungal problems such as damping off and root rot. It is commonly used for seed germination and in seed germination mixes.
- Improves drainage and lightens the soil in the garden, in raised beds, or in pots.
Using Vermiculite in the Garden and Landscape
- Use 1/3 to 1/2 vermiculite in your potting soil for containers or when building your raised beds, or improve your garden soil by adding it in the spring with your other soil amendments and compost.
- For new lawns, spread a 1/4 inch layer evenly around the planted area just after you seed it, then irrigate well. The vermiculite will help hold moisture near the seeds to improve germination.
- Vermiculite can also be used for storing bulbs and root crops over winter. It will retain water from the environment around the roots and bulbs, without desiccating the roots and bulbs themselves. Just layer the vermiculite and the roots or bulbs, and store in a cool, dry place away from sunlight.
- Your worm bin can also benefit from the addition of vermiculite. Just add a handful to provide the grit your worms need to digest their food.
Alternatives for Vermiculite
Vermiculite is used similarly to perlite, pumice, biochar and rice hulls. Each has its own pros and cons, and they can be used in combination to get the most benefits. Compared to these other soil amendments, vermiculite is best for areas and plants that require plenty of moisture, as it has the best water retention. It is not as good at aerating as perlite, so for heavy soils you should use perlite instead or in combination. Since it doesn’t break down like rice hulls, it is good if you want the benefits to last multiple seasons with a single application. Give your plants some vermiculite and grow organic for life!