Horticultural vermiculite is a light, absorbent material that is chemically inert and odourless. It is made from flakes of a mica-like hydrated silicate of magnesium-aluminium-iron. The flakes are flash heated to 482 degrees C centigrade. Water quickly evaporates as the vermiculite expands like an accordion. This process, known as exfoliation, balloons the flakes to twenty times their original size. With an essentially neutral pH of 7.0, horticultural vermiculite helps with soil aeration and retains moisture, fertilisers, other nutrients and plant treatments.
The United States has commercial vermiculite deposits in South Carolina and Virginia. A plant in Libby, Montana was closed in 1990. Vermiculite concentrate is also processed into horticultural vermiculite plants in the east coast and southwestern states, where flash heating is performed. As of 2009, South Africa was the world's largest producer of vermiculite, a ranking it has held for several years. Other mining locations include China, Brazil and Zimbabwe.
Horticultural vermiculite is versatile, clean and sterile. While its neutral pH of 7.0 trends toward alkaline because of carbonates in the product, it does not have the pH lowering effect of peat. When combined with peat or composted pine bark, the resulting mix provides faster root growth. Because of its expanded size and light weight, it helps aerate mixes while retaining moisture and nutrients. Horticultural vermiculite is also used to bulk up mixes and lessen compaction.
According to The Vermiculite Association, starting with seed germination, vermiculite can be used alone or mixed 50/50 with peat or soil. When used alone, 1 tbsp of soluble fertiliser per gallon of water is recommended. When used as a mix, no fertiliser is recommended. Root cuttings can be inserted directly into watered vermiculite. Houseplants benefit from looser soil and less watering. Moving to the outside garden, horticultural vermiculite adds air pockets and loosens the soil. For sandy conditions, vermiculite helps retain moisture.
In addition to horticultural vermiculite, several soil amendments promise to aerate soil and retain nutrients. Cost factors vary with each choice, as do desired goals. Besides vermiculite, the other two most common amendments with aeration and nutrient retention attributes are sphagnum peat and perlite. The Colorado State University Extension Service rates all of these based on permeability and water retention. For vermiculite, it has high permeability and high water retention. Perlite is rated with high permeability but low water retention. Peat has low to medium permeability, and very high water retention properties.
In 2000, the Environmental Protection Agency (EPA) began conducting tests to evaluate risks "associated with exposure to asbestos-contaminated vermiculite," as described by the National Cancer Institute (NCI) in its publication, "Asbestos Exposure and Cancer Risk." Vermiculite consumption in the United States fell from 204,000 metric tons in 2000 to 130,000 metric tons in 2003. By 2008, annual consumption stood at 168,000 metric tons. The EPA concluded exposure from some vermiculite products poses "only a minimum health risk." The EPA recommended reducing the low risk of occasional vermiculite use during gardening activities by reducing vermiculite dust. EPA suggestions included using vermiculite outdoors or in a well-ventilated area. Keep vermiculite damp when using it, do not bring dust indoors on clothing, and use premixed potting soil, which is less likely to cause dust.
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