Vermiculite is a mined mineral with certain properties that make it useful for gardeners. The mineral, after being treated, is often added to potting soil, though the process that transforms it into a useful soil additive also makes vermiculite expensive. Another potential downside is that the vermiculite could be mined from a place containing asbestos. Thus, make sure to use a mask when handling vermiculite. Also look for packaging stating the vermiculite contains no asbestos and no products taken from mines containing asbestos.
Vermiculite's ability to retain moisture is particularly helpful for young plants, which need dependable moisture to thrive.
The inherent nutritional properties of vermiculite are somewhat mild. The vermiculite might provide some potassium (K), magnesium (Mg) and trace minerals. However, the moisture-retaining qualities of vermiculite mean that nutrients dissolved in the water are also retained by vermiculite instead of draining away.
Vermiculite is made by heating the mined mineral to about 1,800 degrees F, which causes the vermiculite to expand as the water contained within it changes to steam. The result is a product that can lighten soil and provide aeration. Different sizes of vermiculite are available; to get the lightening benefits of vermiculite, choose the largest sizes.
Because of vermiculite's expanded area, adding it to a soil can increase the soil's volume without adding to its weight. Vermiculite's ability to help soil retain air along with the mineral's ability to absorb water make vermiculite an oft-used medium for sprouting seeds.
The pH of vermiculite hovers around neutral, which is 7 on the pH scale, with above 7 being alkaline and below 7 being acid. Most plants prefer a slightly acid soil environment, so vermiculite won't upset their well-being.
As an inorganic mineral, vermiculite is not subject to problems with opportunistic diseases and pests. Additionally, it won't be sprouting any weeds. Because vermiculite is inorganic, it won't decay in dry storage.