Wikipedia
Soil series as established by the National Cooperative Soil Survey of the United States Department of Agriculture (USDA) Natural Resources Conservation Service are a level of classification in the USDA Soil Taxonomy classification system hierarchy. The actual object of classification is the so-called soil individual, or pedon. Soil series consist of pedons that are grouped together because of their similar pedogenesis, soil chemistry, and physical properties. More specifically, each series consists of pedons having soil horizons that are similar in soil color, soil texture, soil structure, soil pH, consistence, mineral and chemical composition, and arrangement in the soil profile. These result in soils which perform similarly for land use purposes.
The soil series concept was originally introduced in 1903. Soil series were originally intended to consist of groups of soils which were thought to be the same in origin but different in texture. Soils were thought to be alike in origin if they were derived from the same kind of rocks or if they were derived in sediments derived from the same kind of rocks and deposited at the same time.
A soil series name generally is derived from a town or landmark in or near the area where the soil series was first recognized. For example, the Haugan Series was first identified near Haugan, Montana. The distribution of a given series is not necessarily restricted to the boundaries of only one county or state—for example, the Hagerstown Series was first described near Hagerstown, Maryland, but has also been found as far away as Tennessee and Kentucky.