Perlite is a generic term for naturally occurring siliceous amorphic volcanic rock. But unlike other volcanic glasses it expands from 4 to 20 times of its original volume when heated above a point(800-900 deg C). Main component of perlite is silica or SiO2(about 70-75%). The rest is the mixture of Aluminum Oxide or Al2O3, and Oxides of Sodium, Potassium, Iron, Magnesium and Calcium. 3-5& moisture are also present in Perlite.
Perlite is mined through out the world. Countries like Greece, USA, China and Turkey are the fore runners in this.
When perlite is heated above 900 deg C the moisture trapped inside the rock vaporized and escapes and causes the huge expansion of the material. After the expansion the bulk density of the perlite reduces significantly(0.03-0.15gm/cm3). The expansion also creates the most distinguishing feature of perlite its white color. While the unexpanded perlite rock may range from transparent light grey to glossy black, the expanded perlite ranges from snow white to greyish white.
Because of its low density and low price, perlite has many commercial applications. Among which main uses are building and constructions uses, horticultural uses, fillers and filter aids.
In gardening and horticulture the perlite is used for number of reasons.
Perlite encourages quicker germination than any other growing media with quick seedling growth. It can be used by mixing with peat moss or coconut coir and used as a potting mix or it can be used solely provided it kept wet all the time.
Perlite is inorganic, inert and sterile and naturally contains the different minerals which are essential for the growth of the plant. Being inorganic it is free from weeds, diseases and pests.
There are no toxicity or fire hazards known about perlite.
Unlike peat moss, perlite does not decompose very easily(if at all) so you can store and use perlite for many years.
It improves aeration and drainage of the soil mix if mixed with other growth media, or soil.
Perlite are almost pH neutral. So it can also help to reduce acidity of the soil.
There are couple of disadvantages also of using perlite as growth media.
Perlite holds water by having a large surface area and within the nooks and cervices of vast pores. But being porous and made of volcanic glass it allows the excess water to drain away much quickly than any other media. So in case you have really thirsty plants, investing in perlite might not be a good choice.
Perlite is a non-renewable resource. Which means you can not increase its availability as per your likings as in case with coconut coir.
Perlite as an excellent filtering agent. Beside horticulture, Perlite filters are used in many places to filter beer before getting bottled.
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