The commercially available plant growing medium commonly referred to as potting soil is, in fact, soilless. It may contain several natural materials, both plant-derived and mineral, but soil is not used unless it is first sterilized by a heat treatment.
Farm and garden soils---earth---are a vibrantly alive environment in which millions of organisms live, excrete and die. Many prey and feed upon other inhabitants of the soil, from bacteria and fungi to worms and beetles.
In a natural outdoor environment, this soil food web lives and thrives, while providing plant-available nutrients to food crops and ornamental plantings.
A single handful of healthy garden soil contains billions of organisms. When that soil is removed from its natural environment and brought indoors in a plant pot or other container, the balance of the soil's food web is fatally disturbed. The soil is no longer a beneficial medium for plant growth.
The inhabitants of healthy soil are constantly moving around, opening up tiny channels through which water can drain. As the soil food web falters and dies in the unfamiliar indoor conditions, water no longer drains freely. Surplus water remains bound up in the soil, pushing out oxygen and creating an anaerobic environment which can have strongly negative effects on plant roots. Another consequence, changing pH (soil acidity), can also be harmful to plants.
The inhabitants of the soil food web have a symbiotic relationship with the roots of plants. Bacteria receive substances from roots that are vital to their survival, and their life cycle makes essential plant nutrients available to the roots. When soil is brought indoors, that symbiosis rarely survives. Eventually the plant will probably require feeding with a supplemental fertilizer.
Potting Mix Ingredients
Potting mixes are made from a combination of materials that can physically support plant growth, and hold moisture and nutrients while draining well. Potting mixes are not surgically sterile, but they are designed not to harbor disease organisms.
The physical structure---coarse or fine---and the exact contents of the potting mix are often tailored to meet the exact needs of a particular plant or species.
Ingredients include sphagnum peat moss, vermiculite, perlite, coconut coir, clay, sand, gravel, tree bark, wood chips and screened compost.