- Vermiculite expands when heated, forming a lightweight aggregate that is used to hold air, water and nutrients in potting mix. It contains magnesium and calcium and has a neutral pH. It is available in different grades. Medium grade is most often used in seed starting mixes. Coarser grades are suitable for potting mixes for established plants. Vermiculite is fragile and requires careful handling. Rough handling will cause it to compact and lose its ability to hold air, which is necessary for healthy root growth.
- Vermiculite is used in soil-based potting media to hold air, water and nutrients in the soil. It is available in many commercial potting mixes, but many of these contain ingredients not approved for organic growers. For this reason many growers and home gardeners prefer to make their own potting mix with sterilized loam soil, sphagnum peat moss and vermiculite.
- Peat-based potting mixes do not contain any soil. They are commonly used for seed germination because they are lightweight and sterile. The light texture is ideal for seed germination and healthy root development. Equal parts of sphagnum peat moss and vermiculite are mixed together and moistened with water. Peat-based potting media require the addition of small amounts of ground limestone and fertilizer. Soil-less seed starting mixes are readily available in most garden centers.
- Between 1919 and 1990, 80 percent of the world's vermiculite came from a mine in Libby, Montana, that was found to have a large deposit of tremolite asbestos. The vermiculite ore contained 2 to 3 percent asbestos fibers. According to some sources, all vermiculite contains some asbestos. However, studies conducted by the United States found that commercial vermiculite currently in use for horticultural purposes in commercial greenhouses and by home gardeners presents no significant risk of asbestos exposure.
- Vermiculite is hazardous if swallowed and causes eye, skin and respiratory irritation. If it is ingested, inhaled or comes into contact with the skin or eyes, follow first aid instructions on the package and seek medical attention. Work with vermiculite in well-ventilated areas. Wear a dust mask, gloves and protective glasses at all times. Wet it as soon as possible and mix with other ingredients to minimize airborne particles that can be inhaled. Keep vermiculite in a sealed container in a cool, well-ventilated area. Dispose of vermiculite in accordance with federal, state and local regulations.
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