Vermiculite is a silicate mineral commonly used in soil and potting mixes. In its natural state, vermiculite resembles mica's shiny flakes, and is grayish or greenish in color. When heated to high temperatures, it expands in an accordion shape up to 30 times its original size. Vermiculite is mined all over the world.
Vermiculite is useful for improving growing media because it mixes easily and evenly with other materials like peat, bark or soil as well as pesticide or herbicide additions. It can also be used as a carrier for fertilizers because its porous layers hold the material, carry it evenly throughout the mixture, and release it slowly to the plants. Vermiculite improves drainage in heavy clay soils and increases water retention in sandy soils. It is usually close to pH neutral, but can help gradually increase alkalinity in acid soils because of how it reacts with other soil compounds. According to Texas A&M University, vermiculite also supplies plants with needed potassium and magnesium.
Vermiculite greatly increases soil aeration and reduces soil compaction. It is lightweight and porous, which helps the soil hold more air and makes it fluffy. This makes vermiculite a useful addition or medium for growing seedlings and root cuttings because it gives roots a lot of room to spread out and develop quickly, and the young, tender roots can be extracted from the mixture without much damage. Plants grown in vermiculite soil tend to grow faster with fuller, more developed root systems.
In addition to making soil light and airy, vermiculite also retains moisture, which keeps the aerated soil from drying out too quickly. In potted plants, less frequent watering is needed which helps preserve the nutrients in the soil. The vermiculite itself helps hold the nutrients in its pores, according to the Vermiculite Association. Seeds planted in vermiculite have a higher germination rate and there is little danger of root and stem rot from over-watering.
Vermiculite is generally considered safe to handle because it does not cause skin or eye irritation. As with any silicate dust, care should be taken not to inhale large amounts of vermiculite, but special precautions need not be taken for most home garden applications. Vermiculite has been associated with asbestos-related illness because of some of the mineral mined in Montana in the 1990s contained dangerous traces of asbestos, according to the National Institute on Occupational Health and Safety. If you have an old bag of vermiculite or vermiculite whose source you are unsure of, NIOHS advises that you do not handle it and contact your local extension service or Environmental Protection Agency office for information on proper disposal.