Soil Classification Systems Quiz: USDA, FAO

The two most internationally prominent soil classification systems alongside United States Soil Taxonomy are the World Reference Base for Soil Resources, published by the International Union of Soil Sciences, and the earlier FAO-UNESCO Soil Classification System from which the WRB evolved. The WRB is now the most widely adopted international reference framework and is used by the Food and Agriculture Organization for the World Soil Map.

The World Reference Base for Soil Resources is an international soil classification system developed under the auspices of the International Union of Soil Sciences. Its primary purpose is to provide a globally applicable reference framework and shared vocabulary for communicating about soils across different national systems. It uses 32 reference soil groups as its highest level of classification, facilitating correlation between different national systems such as Soil Taxonomy, the Russian system, and others.

The FAO-UNESCO Soil Map of the World, published in a series of sheets between 1971 and 1981, was a landmark achievement representing the first globally systematic attempt to map and classify the world's soils using a consistent legend. It provided the foundation for subsequent revisions including the revised FAO legend of 1988 and ultimately the development of the World Reference Base for Soil Resources as the current international reference system.

The WRB uses 32 reference soil groups as its highest classification level, compared to 12 soil orders in Soil Taxonomy. The WRB groups reflect a combination of dominant soil-forming processes and diagnostic properties. Below the reference group level, WRB uses principal and supplementary qualifiers to modify group names and capture additional soil properties, producing a flexible nomenclature rather than a strictly hierarchical multi-level system like Soil Taxonomy.

Vasily Dokuchaev is widely regarded as the father of modern soil science. His systematic study of Russian steppe soils in the late 1800s led him to recognize soils as natural bodies formed by the interaction of five soil-forming factors: climate, organisms, parent material, relief, and time. These five factors, later formalized by Hans Jenny as the CLORPT model, remain the conceptual foundation of pedology and influenced all subsequent international classification systems.

The WRB includes Chernozems for fertile grassland soils, Ferralsols for highly weathered tropical soils with iron and aluminum oxide dominance, and Podzols for eluviated soils with spodic B horizons under cool humid forest. These correspond approximately to Mollisols, Oxisols, and Spodosols in Soil Taxonomy. Arenasols is a fabricated name and is not a recognized reference soil group in any established international classification system.

United States Soil Taxonomy organizes soils into six hierarchical levels: order, suborder, great group, subgroup, family, and series, each adding more specific criteria. The WRB uses a different philosophy, classifying soils into 32 reference groups at the primary level and then refining classification using principal qualifiers for dominant properties and supplementary qualifiers for additional characteristics. This qualifier system is more flexible than the rigid hierarchy of Soil Taxonomy.

The Canadian System of Soil Classification was developed specifically to reflect the soils of Canada, which include extensive areas of cryosols with permafrost and organic soils in boreal wetlands. It uses criteria and thresholds calibrated for Canadian soil conditions, and while it parallels Soil Taxonomy in some respects, it differs in the specific diagnostic criteria, order names, and classification thresholds used for cold-climate soils, reflecting Canada's unique pedoclimatic environment.

Vasily Dokuchaev coined the term Chernozem from the Russian words for black earth to describe the remarkably dark, fertile soils of the Ukrainian and Russian steppes he systematically studied beginning in the 1870s. His characterization of Chernozems as natural bodies shaped by climate and vegetation became one of the founding observations of modern pedology, and the name Chernozem is now a recognized reference soil group in the World Reference Base.

International classification systems face several limitations. Diagnostic criteria developed in one region may not translate cleanly to another national system, making correlation uncertain. Language and conceptual barriers introduce classification inconsistencies across regions. Tropical soils with high variability and unique properties are sometimes poorly represented in systems developed primarily from northern hemisphere experience. No global system perfectly captures all soil diversity without some ambiguity or category overlap.

Both Soil Taxonomy and the World Reference Base define diagnostic horizons with quantitative, measurable criteria including minimum thickness, chemical composition thresholds, and morphological requirements. This shared quantitative approach allows approximate correlation of classification units between systems, facilitating international research collaboration, soil data exchange, and comparison of soil resource inventories across national boundaries despite differences in classification structure.

The Australian Soil Classification, developed by the CSIRO and formalized in the 1990s, uses 14 orders defined by a key-based approach that emphasizes morphological and chemical properties relevant to Australian soil conditions. While conceptually parallel to Soil Taxonomy in using orders as the highest level, it differs in the number of orders, diagnostic criteria, and the specific properties emphasized, reflecting the unique suite of soils found across Australia's predominantly arid and semi-arid landscape.

Correlation tables between Soil Taxonomy and the WRB provide approximate equivalences between major classification units, but exact one-to-one correspondence is rarely achieved. The two systems use different diagnostic criteria, thresholds, and classification philosophies, meaning that a single WRB reference group may overlap with several Soil Taxonomy orders and vice versa. Correlation is a useful approximation for general communication but cannot substitute for applying each system's own criteria independently in detailed soil surveys.

Multiple classification systems persist because national systems are tailored to local soil conditions, agricultural priorities, and scientific traditions. Decades of published surveys, soil maps, and management recommendations are tied to existing national systems, making replacement costly. Different systems also serve genuinely different purposes: Soil Taxonomy excels for detailed national surveys while the WRB functions as an international reference language. A single system would not eliminate research needs since classification challenges remain in all systems.

The International Union of Soil Sciences World Congress of Soil Science, held every four years, is the premier global forum for soil science including classification. It provides the venue where the World Reference Base is formally debated, revised, and endorsed between editions. Scientists present correlation studies between national and international systems, facilitating the ongoing harmonization of soil nomenclature and classification criteria that underpins global soil information sharing and scientific communication.