There may be as many definitions of soil as there are people with an opinion about it. The following goes beyond the purely physical to a more functional understanding of soils.

Soil is a living, dynamic resource that supports plant life. It is made up of different size mineral particles (sand, silt, and clay), organic matter, and numerous species of living organisms. Soil has biological, chemical, and physical properties that are always changing. (from National Soil Survey Center et al.)

Dirt is often called soil out of place, but dirt does not have the ability to support plants, or the intricate interplay of biology and physio-chemistry that makes soil unique.