Soil is a complex, living natural body that covers the Earth’s land surface. This medium forms through the slow interaction of climate, organisms, relief, parent material, and time. Because these factors vary dramatically across the globe, a massive diversity of soil types exists. Scientists use classification systems to organize and study these soils, which helps determine the most geographically common type across the planet.
Understanding Global Soil Classification Systems
Scientists categorize the world’s soils using frameworks based on physical and chemical properties. The two most widely adopted systems are the USDA Soil Taxonomy and the World Reference Base for Soil Resources (WRB). Both systems are hierarchical, classifying soils into increasingly specific levels, with the broadest level identifying major soil types.
The USDA Soil Taxonomy organizes soils into 12 major categories known as soil orders. These orders are differentiated primarily by the presence or absence of specific diagnostic horizons, or layers, within the soil profile. These horizons reflect the degree of soil development and the processes that have shaped the soil over time.
Identifying the Most Extensive Soil Type
Based on global classification systems, the most geographically widespread soil type is the Entisol soil order. Entisols are defined as young soils that show little to no evidence of development beyond a surface layer, lacking sufficient time to form distinct subsurface layers. This lack of development is the primary reason Entisols rank as the most extensive. Globally, they occupy approximately 16 to 18 percent of the world’s ice-free land area, securing their position as the most common soil order on Earth.
Defining Features and Global Occurrence
The defining characteristic of an Entisol is its minimal profile development, lacking any diagnostic subsurface horizons. They typically consist only of an A horizon, or topsoil, directly over a C horizon, which is the unconsolidated parent material. This structure means the soil is essentially unaltered from the original sediment or rock it formed from.
Entisols are found in environments where the processes of soil formation are constantly interrupted, inhibited, or renewed. They frequently occur in areas with steep slopes, where erosion removes the soil nearly as fast as it forms. They are also common in floodplains and river deltas, where the frequent deposition of new sediment continually buries developing layers.
The limited development of Entisols also makes them widespread in areas with extreme conditions, such as arid deserts or polar regions. In deserts, the lack of water inhibits chemical weathering, while in high-altitude or high-latitude regions, cold temperatures slow down the soil-forming processes.
Suborders
A common suborder, Fluvents, is found in river valleys and is often fertile due to fresh sediment deposition. Psamments are sandy Entisols found in dunes and beach deposits.
While some Entisols, like those in active floodplains, can be highly productive for agriculture, many others have limited utility. The high sand content in some Entisols, or their occurrence on steep terrain, often results in poor nutrient retention and low water-holding capacity. Using these soils for extensive crop production often requires intensive management practices, such as irrigation, fertilization, and careful erosion control.