When naturally occurring, chemically inert silicon rock containing 2 to 6 percent water is heated to 1600 degrees F, the interior water vaporizes. This causes the silicon to expand from four to 20 times its original volume; it “pops” in the manner of exploding popcorn. The result is perlite, an ideal medium for rooting cuttings and seedlings and as a part of growing mixes.
Perlite, a generic term, has a pH of 6.5 to 7.5, making it essentially neutral. Each particle of lightweight perlite, white to grayish white in color, contains tiny bubbles or air cells. The surface of these particles is covered with tiny broken cavities that trap moisture and soluble plant nutrients, making them available roots.
Since perlite is sterile, it is free of disease and weeds. Perlite is lightweight, safe to handle and has no odor; since it is inorganic, it does not deteriorate.
The porous surface of perlite particles enables them to hold three to four times their weight without becoming soggy. Perlite holds five to eight times more water than ordinary soil.
Aeration and Drainage
The shape of particles of perlite allows necessary air to get to developing and mature plant roots. Perlite helps improve soil drainage and aeration. It can be mixed with clay soil to loosen it, making a medium more hospitable to plant roots.
Perlite is used in gardens, the landscape and containers. A mixture of ½ part perlite and ½ sphagnum peat moss is often used to root cuttings, although cuttings can be rooted in 100 percent perlite.
There is no set or universally accepted formula for mixing perlite with other growing media; the composition of the growing mix depends on the size of the container, the rooting habits and needs of individual plants and the climate.
A mixture of ½ perlite and ½ vermiculite is a popular soil-less medium to deliver water and nutrients to plant roots in hydroponic systems.
Use in Containers
Horticultural perlite is commonly mixed with sphagnum peat moss and composted wood shavings or bark as a growing medium in planting tubs, boxers and pots. A formula of 1/3 perlite, 1/3 peat moss and 1/3 compost is often used. A mixture of ½ perlite and ½ sphagnum peat moss may also be used. The light weight of mixes containing perlite and sphagnum moss makes containers easier to move about. Most container mixes contain at least 20 percent perlite.