Why the Latest Soil Classification System Makes Progress and Outshines Its Predecessors

Soil classification is an intricate discipline, influenced by myriad factors such as parent material, age, climate, vegetation, and drainage. This complex classification problem is critical for various client groups, including farmers, engineers, geologists, and land-use planners. The evolution of soil classification systems reflects the continuous advancement of scientific knowledge and the increasing complexity of soils, particularly those shaped by human activities.

The U.S. Soil Classification System

Over my career, the U.S. soil classification system has undergone significant changes. Initially comprising ten classes, it expanded to twelve, a move that allowed soil scientists to innovate and classify soils based on new criteria. For instance, the Soil Conservation Service introduced categorizations for soils derived from volcanic ash and those on permafrost. While these updates were justified, some could see the adoption of a larger number of classes as a relatively trivial reason, such as printing soil calendars with unique soil families on each page. Interestingly, one memorable incident involved a colleague's attendance at a family emergency, during which I handed over a class to a soil scientist who distributed calendars with what history remembers as "dirty pictures."

Global Soil Classification Systems

Global soil classification systems, particularly the one developed by the United Nations Food and Agriculture Organization (FAO), are more detailed due to the variety of soil types encountered worldwide. In regions beyond the U.S., a significant factor is the presence of anthropogenic soils, influenced by centuries of human activity. The FAO system includes many more subdivisions, capturing the nuances of these diverse soil types. The earliest soil classification systems, pioneered by the Russians, introduced terms like chernozem (black earth).

The Genetic Elements in Soil Classification

Arno Kleber eloquently points out that the FAO system incorporates genetic elements, based on assumptions about how the soil formed. While this approach has its merits, there is a growing trend across the scientific community to eliminate genetic terminology in classification systems. Geologists, for example, prefer terms like reverse fault over thrust fault, and strike-slip fault over wrench fault. Similarly, the geosciences have continuously revised and refined their classification systems over the past century.

Continuous Evolution in Soil Science

Classification systems in all sciences are subject to constant revisions. For instance, the stellar classification system has added new classes at the cooler end of the Main Sequence due to advancements in technology. In biological taxonomy, there is a recognition of new levels of complexity between traditional taxonomic levels like species and genus. Despite these complexities, some disciplines still rely on well-established systems. For instance, mineralogy textbooks continue to use Dana’s classic system, but more complex systems based on atomic structure are also in use.

The key point here is that while older systems remain useful, the latest soil classification systems aim to incorporate their strengths and build upon them. This ensures that classifications better reflect the current understanding of soil formation and structure, making them more useful for practical applications and decision-making.