Vermiculite, a naturally occurring mineral, is mined in many parts of the world, including South Africa, China, Brazil, and the United States. It is often mixed with peat moss to make soil-less planting mediums used for hydroponic growing, potted plants, and square-foot gardens. Gardeners appreciate vermiculite's aerating and water-retaining properties. Additionally, vermiculite has an almost neutral pH and is disease-free. In 1990, the Libby mine, one of the major vermiculite suppliers in the United States, was closed due to asbestos contamination. Since then, many retailers stopped carrying the product or only offer it in small bags.
Determine how much vermiculite you need. You'll probably need one quart of vermiculite if you're making a potting mix for a few potted plants. For square-foot gardening or hydroponic gardening, you'll need more, depending on the size of your garden.
Determine the grade needed. Vermiculite is sold in fine, medium, and large grades. The grade refers to the size of the vermiculite particles. Larger particles provide good aeration and are better for outdoor applications like gardens. Smaller particles hold water, but may become compacted. Fine grades are most often used as a seeding medium in seed trays.
Buy vermiculite in quantities of four quarts, a half bushel, or 1 1/2 bushel bags. Or order vermiculite in bulk from distributors. You can find vermiculite at local nurseries, big box stores, or online.