Vermiculite

Vermiculite is a brownish silicate mineral commonly used to help create air spaces in soil, which improves drainage and nutrient movement.

Vermiculite (Wikipedia) CC BY-SA 3.0

Seed starting

A layer of vermiculite is often placed over newly planted seeds, in pots. This helps retain moisture, which improves germination without encouraging fungal diseases, such as damping off disease.

Soilless growing medium

Vermiculite is also a component of soilless growing medium. Combined with materials such as peat or composted bark, this mixture provides a very light medium, filled with water, air, and nutrients, making it easy for new roots to become established.

Root and bulb storage

If you store root crops or bulbs over the winter, keeping them in a bed of vermiculite can stabilize moisture levels, preventing rot.

Soil conditioner

Because it is so light, vermiculite can be mixed with heavy clay to create more macropores and micropores. This improves soil structure, making it easier for roots to move through the soil.

With all of those good characteristics, you would expect more people to use vermiculite, right?

Unfortunately, until 1990, many sources of vermiculite contained asbestos. Pure vermiculite does not contain asbestos, but there is no way to be 100% sure that the vermiculite in the bag at the garden center hasn’t absorbed asbestos or some other harmful chemical. Using it in small doses is probably fine. I prefer renewable resources, such as aged compost, which I know is healthy for everyone involved.