How to Use Vermiculite
Vermiculite is a soil amendment often used interchangeably with perlite to improve soil drainage by keeping organic soil elements from clumping. Both of these inorganic materials are processed under high heat, which expands the small particles, adding air pockets that can hold water. Vermiculite, however is a flake-form clay related to mica that can hold both water and nutrients throughout its structure, while perlite is volcanic glass that holds water only on its surface. Vermiculite can be used in several ways throughout the garden -- indoors and out -- depending on the soil in your garden.
Prepare an outdoor work area with good ventilation and pour a small amount of water into the vermiculite bag or container to dampen it before working with the material, which can contain low levels of asbestos, a carcinogen. According to the U.S. Environmental Protection Agency, the amount of asbestos in bags of vermiculite poses a low risk to gardeners. However, the agency recommends against breathing the dust that rises from the dry product and bringing particles into your home.
Combine 1 part moistened vermiculite with 1 part sterilized, coarse sand and 1 part milled peat moss in a wheelbarrow or smaller container. Fill sterile seed-starting trays or small pots with the mixture to germinate seeds indoors.
Top-dress garden rows with a light layer of vermiculite when starting seeds outdoors. This helps prevent the garden soil from forming a crust that can be difficult for emerging seedlings to crack.
Mix equal parts vermiculite, peat moss and sterilized or purchased garden soil in the wheelbarrow or container to create a potting mix suitable for indoor or outdoor plants. For a soilless mix, use equal parts of vermiculite and peat moss, supplemented with a handful of slow-release fertilizer granules.
Amend garden soil in small plots with 50 percent volume of vermiculite to either improve the water- and nutrient-holding capacity of sandy soil or to increase drainage in clay soil. This may not be practical for large plots as it would require a 3-inch deep layer of vermiculite to be tilled or dug into the top 6 inches of garden soil to make a significant difference. University of Colorado Extension recommends vermiculite only be used in sandy soils as it can increase moisture retention in clay soils, though similar sources say it can improve either.
- Wheelbarrow or container
- Seed-starting trays or pots
- Use only horticultural vermiculite, as grades meant for construction do not perform in the same way.
- Oregon State University Extension: Add Organic Matter to Improve Garden Soil
- Penn State University Department of Horticulture: Homemade Potting Media
- Purdue University Plant and Pest Diagnostic Laboratory: Down the Garden Path - EPA Study Shows Minimal Risk to Consumers Who Use Vermiculite in Gardening
- University of California Gardener's Corner: Starting Seeds Inside
- University of California Cooperative Extension of Placer and Nevada Counties: The Curious Gardener - Fall Planting
- Comstock Images/Comstock/Getty Images