Vermiculite -- Vermiculite is essentially heated (about 1100° C), popped mica chips. It has a high cation exchange capacity (i.e., can remove positively charged ions from the soil and slowly release them) and contains magnesium and potassium. It is available in variable sizes, the larger sizes are better for larger containers. Vermiculite can absorb over 15 times its weight in water and provides plenty of air spaces so it increases water retention and to a point aeration. I say to a point because it can hold too much water, you should not use fine grade vermiculite in containers larger than one gallon or it will get too soggy. Adding sand or gravel with the vermiculite will improve drainage. Vermiculite breaks down into smaller pieces over time. You can use 100% pure fine grade vermiculite for small containers/seedlings. One hundred percent vermiculite can be used for seed germination, rooting cuttings, outdoor transplanting as well as bulb and tuber storage. Vermiculite is even used to incubate turtle eggs at the Darwin research center in the Galapagos islands.
Vermiculite is a finite resource. Horticulture grade vermiculite is mined largely in Georgia and is near neutral in pH. You should know that building or insulation grade vermiculite is alkaline, coarser and often treated with chemicals; it should not be used to grow plants. You should avoid breathing the dust from vermiculite. Keep it wet when working with it just like perlite above

Good Growing
Dr. E. R. Myers

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