“Many of our life’s activities and pursuits are related to and influenced by the behavior of the soil around our houses, roads, septic and sewage disposal systems, airports, parks, recreation sites, farms, forests, schools, and shopping centers. What is put on the land should be guided by the soil that is beneath it. Like snowflakes, no two soils are exactly the same.
Surface and subsurface soil features change across landscapes. A grouping of soils having similar properties and similar behavioral characteristics is called a series. More than 21,300 soil series and 285,200 soil map units have been named and described in the United States, and more are being defined each year.
When soils are mapped, soil series are further divided into phases according to properties that are important to soil use, such as texture of the surface layer and slope. These phases of soil series all have a characteristic behavior. The behavior of the individual phase is applicable no matter where the soil is observed.”
From “The Soil Down”, USDA NRCS, second edition.