Good potting soil usually consists of no soil at all. Soil from the ground provides poor aeration and becomes heavy when wet if used as a potting soil. Potting soil consists of ingredients that try to replicate what nature provides, using ingredients that aid in drainage and aeration while providing critical nutrients plants need to grow.
The best potting soil consists of weed and pest-free ingredients meant to provide enough density to hold seedlings and water while offering porosity to get rid of excess water. Most potting soil consists of organic matter such as peat moss or bark, and inorganic matter such as sand, vermiculite or perlite.
Potting soil contains a variety of interesting ingredients including ground-up pine bark, peat moss and perlite or vermiculite. Perlite looks like tiny little Styrofoam balls, but actually, the material consists of heat-puffed volcanic glass. Vermiculite comes from a mica-like mineral that expands with heat, forming a lightweight material that helps aerate soil and hold water and fertilizer in the potting soil. Peat moss comes from bogs where decomposing moss accumulates. The moss is harvested from these bogs and dried before it gets sent to a processing facility for packaging. While peat moss provides no nutrients itself, it helps aerate the soil while retaining water and improving drainage.
Unless stated on the bag, most potting soils contain little or no fertilizer. Gardeners need to add soluble or time-released fertilizers to the potting soil after planting. Or, they can use compost instead of fertilizer, thoroughly mixing the compost in with the rest of the potting soil, then planting seeds or plants in the soil.
Prepackaged potting soil comes in four different types including all-purpose, premium, professional and plant-specific. All-purpose potting soil requires the addition of plant food. Premium potting soil uses perlite or vermiculite to aid in water drainage and aeration. Some premium potting soils include water-soluble or slow-release fertilizers. Professional mixes contain higher quality ingredients often used by nurseries. Plant-specific potting soils use formulas geared for growing certain plants such as roses, cactus, African violets and orchids.
Organic potting soils use ingredients that remain uncontaminated from pesticides, herbicides and other chemicals. Organic soil sometimes contains topsoil as well as sand, composted pine bark, and sphagnum pea moss or other forms of peat moss. Since organic growers do not use synthetic fertilizer, they rely on compost to provide nutrients to the plants. Some packaged organic potting soils contain compost, or compost can be added to the mix. True organic potting soils avoid the use of coir dust, a mixture of fibers that form as a by-product of the coconut fiber industry. Other alternative ingredients include newspaper, alfalfa, sawdust, clay, perlite and vermiculite.