Soil is defined differently by groups that use soils for varying purposes.
For the purposes of site and soil investigations for on-site systems in North Carolina, the most important definition is that contained in the Rules for Sewage Treatment and Disposal Systems:
“The naturally occurring body of porous mineral and organic materials on the land surface. Soil is composed of sand, silt, and clay sized particles that are mixed with varying amounts of larger fragments and some organic material. Soil contains less than 50 percent of its volume as rock, saprolite, or coarse-earth fraction (mineral particles greater than 2.0 millimeters). The upper limit of the soil is the land surface, and its lower limit is ‘rock,’ ‘saprolite,’ or other parent materials.”
Engineers define soil as "any unconsolidated material composed of discrete solid particles with gases and liquids between" (Sowers, 1979).
Soil, as defined by geologists, is "that material which has been so modified and acted upon by physical, chemical, and biological agents that it will support rooted plants" (AGI, 1976).
Soil, as defined by soil scientists, is a naturally occurring, three-dimensional body that has developed at the earth's surface as a result of soil-forming processes: additions, losses, translocations, accumulation, and transformations, as influenced by the five soil-forming factors: parent materia relief, organisms, climate, and time.
Many of the criteria that are used to determine soil suitability for on-site systems come directly from soil characteristics defined by the Soil Conservation Service (SCS) in Soil Taxonomy Handbook (1993) and in the Soil Survey Manual (1994).
From the North Carolina Onsite Guidance Manual