Vermiculite is a mined ore that is heated at very high temperatures to create particles with high moisture retention properties. It is useful in gardening because it provides air space for plant roots and stores captured nutrients for the roots to absorb. The heating process makes vermiculite inert and an excellent medium for starting seeds.
Vermiculite is capable of holding and releasing potassium and magnesium in a form plants can use. Vermiculite, with the help of microorganisms, can transform ammonium nitrogen to a form plants can absorb. Combined with organic materials, such as peat moss, composted bark materials or natural soils, vermiculite will help promote faster root growth. Vermiculite and these organic materials will help retain air, nutrients and moisture around the plant's roots.
Vermiculite's primary function in the garden is to act as a wedge that separates soil particles, increasing soil porosity and aeration. Gardeners who practice square-foot gardening frequently use vermiculite as a main component for their planting mixture. Gardeners distribute a mixture of 1/3 vermiculite, 1/3 peat moss and 1/3 compost among square-foot boxes laid out in grids as small as 4 square feet. Proponents of this gardening method claim that the compost provides all of the nutrients for the plants, while the vermiculite and peat moss keep the soil moist, but loose.
Various studies and tests have found some mined vermiculite has been contaminated with asbestos. However, the U.S. Environmental Protection Agency's (EPA) position is that vermiculite represents a low risk to consumers of asbestos exposure. The greatest danger asbestos poses is when it is airborne. To minimize the risk of exposure, the EPA recommends gardeners and other users work outdoors when using vermiculite and keep it moist while working.
Vermiculite is used frequently to store bulbs, tubers and rhizomes. According to the Schundler Company, one of the largest producers of vermiculite in the United States, "the absorptive power of vermiculite acts as a regulator that prevents mildew and moisture fluctuation during the storage period. It will not absorb moisture from the inside of stored tubers, but it does take up free water from the outside, preventing storage rot."
Thomas H. Webb is credited with being the first to observe the properties of and naming vermiculite in 1824. When Webb exposed the particles to a flame, they took on a shape that reminded him of small worms. It is believed that is the reason he named it "worm breeder." The first commercially successful mine was started in Libby, Montana in 1923. Today, vermiculite is brought in flake form from several large deposits in the United States and South Africa. It is transported to processing centers where it is exposed to 1,000- to 1,500-degree Fahrenheit temperatures that flash the trapped water to steam, causing the flakes to expand into particles useful for horticulture and many fields.