Soil is a dynamic, living system that supports nearly all life on Earth. It is a complex blend of minerals, organic matter, water, air, and countless organisms, from microscopic bacteria to larger invertebrates. This intricate mixture provides the foundation for plant growth, filters water, and plays a role in regulating the planet’s atmosphere. To better understand and manage this resource, scientists classify soils into various categories, with soil orders representing the broadest level of organization.
What Defines a Soil Order
Soil orders are distinguished by characteristics that reflect the dominant soil-forming processes and the long-term interaction of environmental influences. These influences include climate, living organisms, topography, the parent material from which the soil develops, and the time over which these processes occur. These factors shape the unique physical, chemical, and biological properties found within soil layers.
A defining element for classifying soil orders involves “diagnostic horizons,” which are specific layers within the soil profile exhibiting distinct properties. For example, a mollic epipedon is a dark, organic-rich surface horizon characteristic of certain fertile soils, while an argillic horizon indicates a subsurface layer where clay has accumulated. Beyond horizons, “diagnostic features” refer to specific characteristics within these layers, such as the presence of permafrost, high salt concentrations, or particular mineral compositions. These measurable attributes provide the basis for differentiating one soil order from another.
How Soil Orders Are Classified
Soil orders are classified primarily by the USDA Soil Taxonomy system, widely adopted globally. This system organizes soils into a hierarchy, with soil orders at the most general level. There are 12 recognized soil orders, each named with a word ending in “-sol.”
Below the order level, the system further subdivides soils into suborders, great groups, subgroups, families, and series. This hierarchical structure allows for increasingly specific classifications based on more detailed soil properties. Classification relies on quantifiable soil attributes, including depth, moisture content, temperature, texture, structure, and the presence of specific diagnostic horizons and features.
Major Categories of Soil Orders
The diversity among soil orders reflects the wide array of environmental conditions across the globe. For instance, soils with minimal development, like Entisols, are found in areas where soil formation is slower than processes like erosion or deposition, such as on steep slopes or floodplains. In contrast, Inceptisols show slightly more development but still lack the pronounced features of older soils, occurring in various climates.
Soils influenced by extreme cold include Gelisols, characterized by permafrost within a short distance of the surface, often found in polar regions or high elevations. Histosols are organic soils formed in wetlands where the slow decomposition of plant material leads to significant accumulation of organic matter, such as in bogs and swamps.
Soils shaped by specific parent materials include Andisols, which originate from volcanic ash and are known for their ability to retain water and nutrients. Vertisols are clay-rich soils that expand when wet and shrink when dry, forming deep cracks, which can be challenging for construction. Mollisols are typically found in grasslands and are recognized by their thick, dark, organic-rich surface layer, making them highly fertile.
Other orders reflect varying degrees of weathering or moisture regimes:
- Aridisols: Dry soils common in deserts, often accumulating salts and calcium carbonate.
- Alfisols: Moderately weathered soils with subsurface clay accumulation, supporting forests and agriculture.
- Ultisols: Acidic, highly weathered soils found in humid tropics and subtropics.
- Oxisols: The most weathered soils, prevalent in intertropical regions and low in natural fertility.
- Spodosols: Acidic soils under coniferous forests in cooler climates, characterized by a distinct subsoil layer of accumulated organic matter, aluminum, and iron.
Why Understanding Soil Orders is Important
Understanding soil orders provides meaningful insights for various practical applications. In agriculture, this knowledge helps determine which crops are best suited for particular soil types, guiding decisions on irrigation, fertilization, and overall land management to maximize productivity.
Beyond farming, soil classification is valuable for land use planning, informing decisions about urban development, infrastructure placement, and conservation efforts. Recognizing the properties of a Vertisol, for example, helps engineers design appropriate foundations for buildings due to its shrink-swell characteristics. In environmental management, knowing soil orders aids in assessing nutrient cycling, water filtration capabilities, and potential for pollutant transport, which directly impacts water quality.
Soil orders also play a role in climate studies, particularly concerning carbon sequestration. Soils are significant reservoirs of organic carbon, and different orders store varying amounts, influencing global carbon budgets. This understanding supports efforts to manage soils in ways that can help mitigate climate change.
Overall, a grasp of soil orders enables informed decisions for sustainable resource utilization and environmental stewardship.