Potting mixtures do not have to contain soil in order to grow plants. Soilless mixtures are often more sterile than soil mixtures, as well as being finer and lighter, which aids drainage and prevents the soil from becoming soggy. One of the ingredients in many commercial soilless potting mixtures is perlite. Perlite is also used in mixes that contain soil. Perlite is a volcanic, silica-type mineral. When crushed and heated, perlite becomes light and fluffy, and resembles foam. These attributes aid drainage in the potting mix.
Place moist compost, for a soilless mix, or garden soil, for a soil mix, into a roasting pan. Cover with foil and insert a food thermometer. Bake at 250 F until the thermometer reads 180 F. Bake at this temperature for 30 minutes to sterilize the soil or compost.
Fill a bucket with lukewarm water while the soil is sterilizing. Place the peat moss in the water and allow it to soak for 30 minutes to one hour to rehydrate it. Drain out the excess moisture once the peat is done soaking.
Mix 1 part peat moss and 1 part soil or compost with 1 part of the perlite. Mix the potting soil together thoroughly, as the perlite will float to the surface when watering if it is all near the top of the mix.
Alternately, mix 1 part perlite with 1 part peat moss for a soilless, compost-less mix. This mix works especially well for seed starting as it is very fine and completely sterile.