The U.S. system includes a category named suborder that is not in the Canadian system, and the FAO soil units are arranged into only two categories. Thus the categorical levels of the related taxa are generally not equivalent. The closest approximation to equivalence of taxa is in the case of Organic soils (Canadian) and Histosols (U.S.). Great groups and subgroups of Organic soils are nearly equivalent to the corresponding suborder and great groups of Histosols. This correspondence resulted from United States and Canadian pedologists working together to develop taxonomy for Organic soils. However, even in this case there are differences. For example, the organic matter content required for Histosols differs somewhat from that required for soils of the Organic order.

In other orders, differences in the approximately equivalent taxa are greater. For example, most Chernozemic soils are Mollisols, but some are Aridisols. Many Mollisols are Chernozemic soils, but some are Gleysolic soils, others are Solonetzs, and still others are Melanic Brunisols. There are also basic differences between Podzolic soils and Spodosols. Most Podzolic soils are Spodosols, but a significant proportion are Inceptisols because a spodic horizon must be dominated more by amorphous material than a podzolic B horizon. Conversely, a few Podzolic Luvisols are Spodosols. All Vertisolic soils would qualify as Vertisols. However, all Vertisols would not be Vertisolic because Vertisolic soils must have a layer affected by argillipedoturbation as well as slickensides and cracks. At lower levels in the systems, differences become progressively greater.

Tables 6, 7, and 8 provide some idea of how soil horizon designations and soil taxa are related at the upper levels in the three systems. The tables have been revised from those prepared by J.S. Clayton for previous editions of the Canadian System of Soil Classification. Revisions were required because of some changes in horizon designations and in the definitions of horizons and taxa in both the Canadian and U.S. systems. Correlation at the subgroup level is not presented.

The tables are not adequate for correlating either the horizon designations or the taxonomy of a given pedon as expressed in one system with those of another system. Adequate correlation requires knowing the criteria and definitions in the systems involved.

• Table 6: Correlation of Horizon Definitions and Designations

• Table 7: Correlation of U.S. and FAO Diagnostic Horizons with nearest Canadian Equivalents

• Table 8 Taxonomic Correlation at the Canadian Order and Great Group Levels