Container plants have different needs than those planted in the ground. Ordinary garden soil should not be used in containers because it packs into a dense mass that prohibits water and air penetration and it harbors microorganisms that can cause disease, especially if the plant is brought indoors. There are different commercial formulations of potting soil from which to choose depending on the type of plant to be potted.
Potting Soil Fundamentals
When you choose a potting soil, keep in mind the particular needs of container plants, specifically good drainage and aeration. The soil has to have a loose, open structure so that the water can move freely through the container to the roots, as well as containing air spaces so air can penetrate the soil and keep the roots healthy, according to "Gardening For Dummies," by Michael MacCaskey and Bill Marken. Keep in mind that potting soil has no fertility, so wet or dry fertilizer, either organic or inorganic, must be added.
Sooner or later every gardener will need to make her own potting soil to meet a specific need for a specific plant. The basic potting soil recipe is two parts peat moss, one part vermiculite and one part perlite, according to "The Complete Idiot's Guide to Gardening," by Jane O'Connor and Emma Sweeney. Do not forget to add the fertilizer.
Potting Soil Additions
There are instances when additional mediums need to be added to either commercial or homemade potting mix to enhance the mixture. These include charcoal, which helps filter out toxins; lime, which regulates soil pH; composted bark, which adds to the soil's organic matter and builder's sand, which, when used to propagate seeds can provide aeration and support for the cuttings, according to Janet Marinelli in the "Brooklyn Botanic Garden Gardener's Desk Reference."