Synopsis Classification schemes of natural objects seek to organize knowledge so that the properties and relationships of the objects can be most easily remembered and understood for some specific purpose. Ultimately, soil taxonomy and other soil classification schemes seek to make knowledge useful in satisfying the human needs that depend on soils. This book briefly considers the nature and categories of Soil Taxonomy, which was published by the U.S. Department of Agriculture in 1975, and the Canadian System of Soil Classification, published in 1978.

Diagnostic soil horizons are used to classify soils into orders – the highest category. Diagnostic horizons formed at the surface are called diagnostic surface horizons, or epipedons; six horizons are recognized: mollic, umbric, anthropic, ochric, histic, and plaggen. Subsurface diagnostic horizons form below the surface of the soil and, in some cases, just immediately below a layer of leaf litter. Diagnostic subsurface horizons may be exposed by truncation of the soil. The diagnostic subsurface horizons include cambic, argillic, nitric, spodic, oxic and agric.