Vermiculite is a mica-like mineral that expands into a porous, absorbent material when heated. Large amounts of it are used as insulation for homes, as packing material, and as a soil additive. Because of its light weight and ability to absorb water and nutrients, it makes an excellent potting mix or soil additive.
For many years, most of the world’s supply of vermiculite was mined near Libby, Montana. This mine was contaminated with asbestos, a potent carcinogen. As a result, as many as 35 million homes in the United States are insulated with contaminated material. The Libby mine closed in 1990. Current sources of vermiculite have no asbestos contaminants or only barely detectable traces.
The Environmental Protection Agency recommends that gardeners take these precautions when using vermiculite:
- Use vermiculite outdoors or in a well-ventilated area.
- Keep vermiculite damp while using it to reduce the amount of dust created.
- Avoid bringing dust from vermiculite use into the home on clothing.
- Use premixed potting soil, which usually contains more moisture and less vermiculite than a pure vermiculite product, and is less likely to generate dust.
- Use other soil additives such as peat, sawdust, perlite, or bark.