The surface layer and such features as slope, stoniness, degree of erosion, and topographic position may vary unless these factors are associated with significant differences in the arrangement of horizons. Soil series are differentiated on all the differentia of the higher categories plus those additional and significant characteristics in the series control section. Some of the characteristics commonly used to differentiate series are the kind, thickness, and arrangement of horizons and their structure, color, texture, reaction, consistence, content of carbonates and other salts, content of humus, content of rock fragments, and mineralogical composition. A significant difference in any one of these can be the basis for recognizing a different series. Very rarely, however, do two soil series differ in just one of these characteristics.
Most characteristics are related, and generally several change together.