I was "visiting" the garden center of my local Lowe's store the other day (Lowe's is my second home!). I wanted to purchase a large bag of coarse perlite. I couldn't find any and asked one of the workers whether they had some stored somewhere. He checked his inventory and said the perlite was on back-order but that he had vermiculite. He assured me that vermiculite and perlite were similar products and that I could just substitute one product (vermiculite) for the other (perlite). In one regard, he was telling me the truth, but in another, he was totally off-base. Not knowing the difference could ruin your project.
Both vermiculite and perlite are inorganic products, both are relatively sterile, and both are often used as soil additives. Vermiculite is a soft,spongy material made from super-heating mica. Perlite is a hard, highly porous material made by super-heating volcanic glass. Typically, vermiculite will be tan/brown in color and perlite will be white. Whereas vermiculite absorbs water in its plate-like structure, perlite traps water in its very porous, undulated structure. Vermiculite is close to a neutral pH whereas perlite has a slightly more alkaline pH. Both are very light-weight.
Though both products do in fact aerate the potting soil, the fact that vermiculite holds moisture longer than perlite makes their usage different.
Vermiculite retains more water and retains it for longer periods than perlite. Vermiculite affords slightly less aeration than perlite. For plants that thrive growing in a more-wet soil, vermiculite would do well as a potting soil additive. Many plants are more sensitive to alkaline conditions, so in that case, vermiculite would be a better choice than perlite. Since many foliage plants appreciate more water-retentive potting soil, they do well in a vermiculite-added potting soil. Also, simply because of its water retention/nutrient retention, vermiculite makes a good seed-starter medium.
Perlite only traps water on its large surface area, consisting of nooks and crevices, and thus releases its water more quickly. In doing so, it can help raise the humidity around plants. While vermiculite would be a better choice for starting seeds, perlite would be a better choice when rooting cuttings. Cuttings would tend to rot more easily in vermiculite. For plants that need a quickly draining soil, a soil that does not retain much moisture, a soil that is extremely well aerated, and a soil that could have a higher pH, perlite would be the best additive. Cacti and non-cacti succulents would be such plants. Also, because most epiphytes like to have their roots dry out quickly between watering and also appreciate higher humidity, perlite would be the best choice.
Thus, although vermiculite and perlite have some similar properties, they are not the same. When plants need more water-retentive soil or for seed-starting, use vermiculite. For plants that prefer quick-drying soil or for starting cuttings, use perlite.
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