Flat, accordion-like flakes of mica that contain water are heated; the water turns into steam and the flakes expand, a process called exfoliation. The result is odorless, fire resistant and chemically inert vermiculite. Vermiculite has numerous non-horticultural uses because of its insulating properties. Used in potting mixes, it can store water, nutrients, pesticides and herbicides and pass them onto plants.
Although vermiculite can compact in time, it will not rot, deteriorate or turn moldy. It retains the nutrients and moisture needed by seeds, cuttings and roots.
The chemical composition, color and pH of vermiculite vary according to its source. It is usually pH 7, neutral, but has an alkaline reaction to compounds containing carbon.
Amending Potting Mixes
Combined with peat or composted pine bark, vermiculite gives quick anchorage to young roots. Potting mixes containing vermiculite retain air and release moisture and nutrients as the plants need them.
Adding vermiculite to heavy soils or clay lets the plants breathe; it allows sandy soils to hold water and air.
Vermiculite can be mixed with peat or soil or used alone to germinate seeds.
If vermiculite is used alone, feed seedlings with 1 tbsp. of soluble fertilizer per 1 gallon of water. No additional fertilizer is needed if vermiculite is mixed half and half with composted pine bark, peat or soil.
Vermiculite is sterile so the seedlings will almost never catch fungal diseases of the root and crown. The texture of vermiculite allows seedlings to form dense root balls, and they can be removed without breaking the fine roots.
Potting Mix Recipes
Extension, a cooperative agriculture extension service of 75 American universities, suggests five basic potting mixes that vermiculite:
Mix for transplant starts: Combine 2 parts of screened compost with 4 parts of sphagnum peat, 1 part Perlite and 1 part vermiculite.
Soil-less mix for starting seeds: Combine 50 to 75 percent of sphagnum peat to 25 with 50 percent of vermiculite. To each cubic yard of mix add 5 pounds of ground or superfine dolomitic lime.
Soil-less potting mix: Combine 1 part compost, 1 part peat moss and 1 part vermiculite. Screen with a ½ inch mesh screen; to increase fertility add ½ oz. each of blood meal, rock phosphate and greensand to each gallon of potting mix. Greensand is made of fossil-rich sandstone.
Seedling mix for seedling flats: 1 ½ buckets of compost, 1 ½ buckets of vermiculite, 2 to 3 gallons of Sphagnum peat moss. To this add ½ cup of fertility mix. To make the fertility mix combine ¼ cup of kelp meal, ½ cup of blood meal, 2 cups of greensand, and 2 cups of rock phosphate.
The Cornell organic potting mix: Add ½ cubic yard of vermiculite to ½ cubic yard of sphagnum peat, plus 5 lbs. of ground limestone, 2 to 4 lbs. of bone meal and 5 lbs. of blood meal.
Vermiculite and Asbestos
A vermiculite mine in Libby, Montana, that once produced 80 percent of the world supply of vermiculite was found to contain asbestos, minute particles of which were a carcinogen when inhaled. In operation since the 1920s, the Libby mine was closed in 1990. Vermiculite is now mined in Virginia, South Africa and China; those sources contain environmentally safe trace amounts of asbestos or none at all.
Here is the conclusion of the National Institute for Occupational Safety and Health: "Based on available information, there is no clear evidence that dust from vermiculite itself causes any serious health effects. Nevertheless, as with any dust, workers should avoid prolonged, high-level exposures. The observed health effects associated with asbestos-contaminated vermiculite can be attributed to contaminant fibers, rather than to vermiculite itself."