The major difference between topsoil and potting soil is the intended use of each. Topsoil is intended for spreading on the ground outdoors. Potting soil is intended for use in containers. Both contain non-soil products such as bark fines, pH adjusters and fertilizers, but potting soil also contains peat moss, perlite, sand, clay and compost.
Home Guides explains that topsoil and potting soil are not used interchangeably. Unlike topsoil that combines with existing soil, potting soil provides a separate growth environment. Topsoil does not drain well enough for container use. It lacks the aeration of potting soil. Potting soil dries out quickly if spread on the ground, resulting in lack of water for plants.
Topsoil can be the source of unwanted weeds, invasive plant species and toxic substances depending upon the location from which they were taken. Though potting soil lists the names of ingredients in decreasing order of volume, there is no regulation of the physical and chemical properties of potting soil. Some types of potting soil are not suitable for growing edibles.
According to Home Guides, the Mulch and Soil Council established standards for topsoil and potting soil in 2007, certifying products that meet those standards. However, these standards are voluntary, and sales are regulated only in some localities.