Entisol
Entisols are one of the twelve soil orders in the United States Department of Agriculture (USDA) Soil Taxonomy system, characterized by little or no evidence of pedogenic horizon development, making them the least developed soils among the orders.[1] They typically form in recent parent materials such as floodplains, dunes, or erosional surfaces where ongoing geological processes prevent substantial profile differentiation, often consisting primarily of A and C horizons without subsurface diagnostic features like argillic or spodic horizons.[1][2] These soils exhibit high diversity in texture, drainage, and chemical properties due to their varied origins and limited pedogenesis; for instance, many are sandy with low cation exchange capacity and high leaching potential, while others are wet or human-disturbed.[1][2] Entisols are subdivided into suborders reflecting specific environmental conditions.[3] Globally, Entisols occupy about 16% of the ice-free land surface and about 12% in the United States; they are widespread, particularly in arid and semi-arid regions, river valleys, coastal plains, and areas of recent glaciation or volcanism, prominent in U.S. states like Nebraska, California, and along the Mississippi River.[4][5][1][2] Despite their youth, Entisols support diverse land uses such as rangeland, cropland, forestry, and urban development, though they often require intensive management like irrigation, fertilization, and erosion control to mitigate limitations from poor nutrient retention and drainage variability.[1][2]Definition and Characteristics
Definition
Entisols constitute one of the twelve soil orders in the U.S. Department of Agriculture's (USDA) Soil Taxonomy system, a hierarchical classification framework established in 1975 and revised periodically thereafter to incorporate advances in soil science. This system, developed under the leadership of Guy D. Smith, provides a quantitative basis for categorizing soils based on diagnostic horizons, properties, and other morphological features observable in soil profiles.[6] The defining criterion for Entisols is that they are mineral or organic soils exhibiting little or no morphological evidence of horizonation or pedogenic alteration, specifically lacking any diagnostic subsurface horizons such as the argillic (clay accumulation), spodic (illuvial organic matter and iron-aluminum), or other pedogenic features required for other soil orders.[3] Instead, Entisols typically consist of an A horizon or similar surface layer overlying unweathered parent material, with minimal development due to insufficient time, environmental constraints, or frequent disturbance.[4] This places Entisols at the most incipient stage of soil formation within the taxonomy. In distinction from other soil orders, Entisols represent the least developed soils globally, contrasting sharply with more mature orders like Mollisols, which feature a thick, dark mollic epipedon indicative of grassland influences, or Ultisols, which display pronounced subsurface clay accumulation and low base saturation from prolonged weathering in humid climates. The term "Entisol" was coined in the 1960s by Guy D. Smith during the formulation of the 7th Approximation, an early comprehensive soil classification system that laid the groundwork for modern Soil Taxonomy by emphasizing observable soil properties over genetic theories.[7]Key Morphological Features
Entisols are defined by their minimal pedogenic development, typically displaying a simple A/C profile where a thin or absent A horizon overlies the C horizon representing unaltered parent material. The surface may feature an ochric epipedon, a light-colored, low-organic layer less than 18 cm thick with low base saturation, but Entisols lack diagnostic subsurface horizons such as cambic, argillic, or spodic horizons that indicate more advanced soil formation.[8] In terms of texture and structure, Entisols exhibit high variability inherited from their parent materials, often consisting of sandy, silty, or loamy textures with little to no structural development, appearing massive or single-grained. Colors are weakly expressed and directly reflect the underlying parent material, ranging from light grays and browns in dry settings to grayer or bluish hues in saturated conditions, while consistence is generally friable to firm when moist, becoming loose in sandy variants.[8] Organic matter accumulation is typically low throughout the profile, often less than 1% in mineral horizons, though it can be higher in wet environments where histic epipedons form with greater than 12-18% organic carbon. Representative examples include alluvial Fluvents on floodplains, which show stratified layers of recent sediments without significant alteration, preserving the depositional structure.[8]Formation and Distribution
Processes Limiting Soil Development
Entisols exhibit limited soil development primarily due to recent depositional processes that outpace pedogenesis, such as the accumulation of alluvium, eolian sands, or volcanic ash, which continually reset the soil profile by burying older material.[8] These dynamic geomorphic activities, including flooding in river valleys or wind-driven dune formation, deposit new parent material at rates that exceed typical soil formation velocities. Consequently, horizon differentiation remains minimal, as the influx of unweathered sediments prevents the accumulation and alteration necessary for structured soil layers.[8] Erosion represents another dominant barrier, particularly on steep slopes or in areas subject to geologic or human-induced disturbance, where surface material is removed faster than it can form, maintaining the soil in an immature state.[8] In such settings, rates of denudation surpass pedogenic buildup, ensuring that Entisols retain properties closely resembling their parent material.[9] Environmental constraints further impede development, including permafrost and cryoturbation in cold regions that disrupt horizon stability through frost heaving, or persistently high water tables in wetlands that inhibit oxidation and organic matter decomposition.[8] Arid climates with limited moisture also restrict chemical weathering, while saturated conditions in tidal marshes prevent aeration essential for pedogenic reactions.[8] The temporal dimension underscores these limitations, as Entisols are characteristically younger than 10,000 years, often originating from post-glacial landscapes or catastrophic events like volcanic eruptions that initiate soil formation from scratch.[8] Within the CLORPT model of soil-forming factors—climate (cl), organisms (o), relief (r), parent material (p), and time (t)—Entisols are dominated by relief and parent material, which override the influences of time and biota due to ongoing instability.[8][10] This imbalance results in soils where topographic position and recent lithology control development trajectories, frequently leading to thin A horizons without subsurface differentiation.[8]Global and Regional Distribution
Entisols cover approximately 16% of the Earth's ice-free land surface, making them one of the most widespread soil orders globally.[11] This extensive distribution is primarily associated with young, dynamic landscapes where soil formation is minimal due to ongoing geological activity.[12] These soils are prevalent in various key regions shaped by recent depositional environments. In floodplains, such as the Mississippi Delta in North America and the Amazon Basin in South America, fluvial processes deposit fresh sediments that prevent horizon development.[1] Deserts feature Entisols in areas like the Sahara's dune fields, where aeolian deposition dominates.[13] Mountainous terrains, including the steep slopes of the Andes, host these soils due to erosion and instability.[1] Additionally, arctic tundra regions exhibit Entisols in areas influenced by glacial retreat and periglacial processes.[11] In the United States, Entisols occupy about 12.3% of the land area, with concentrations in Alaska's permafrost-influenced terrains, the alluvial deposits of the Great Plains, and the arid Southwest.[12] Their distribution is closely tied to active geomorphic processes, including fluvial deposition in river valleys, aeolian transport in windy arid zones, and glacial activity in polar and high-altitude settings.[1]Classification System
Suborders
Entisols are subdivided into six suborders primarily based on moisture regimes, parent materials, and minor diagnostic features that reflect limited pedogenic development. These suborders are Aquents, Arents, Fluvents, Orthents, Psamments, and Wassents.[14] Aquents are characterized by aquic conditions, including saturation and reduction within 50 cm of the surface, with redoximorphic features such as low chroma (≤2) and more than 20 consecutive days of saturation during normal years.[14] These soils occur in poorly drained environments like wetlands and floodplains, where water saturation inhibits horizon formation. Arents result from human alteration, featuring a layer 50 cm or more thick that has been mixed or transported by human activity, with irregular boundaries and no natural horizon sequence, often due to mixing from activities like plowing or grading.[14] They lack aquic conditions within 50 cm and are common in urban or agricultural settings. Fluvents form in recent alluvial deposits on floodplains, with stratified layers, irregular decreases in organic carbon with depth (e.g., ≥0.2% at 125 cm), and without aquic conditions within 50 cm.[14] This stratification reflects ongoing sediment deposition that resets soil development. Orthents represent the most common suborder globally, defined by the absence of significant diagnostic horizons and no aquic conditions, often on steep, eroded uplands with dry conditions.[14] Psamments are sandy soils with a texture of loamy fine sand or coarser (≥85% sand) throughout the upper 100 cm or to bedrock, containing less than 35% coarse fragments and low organic matter, typically in dunes or coastal areas.[14] Wassents exhibit saturation with aquic conditions within 50 cm and saturation within 100 cm, often in tidal flats or coastal wetlands inundated to depths allowing rooted vegetation (less than 2.5 m), with the water table 2 cm or more above the surface for more than 21 hours daily.[14] In terms of relative abundance, Orthents are the most widespread Entisol suborder worldwide, covering steep slopes and eroded landscapes, while Psamments are prominent in sandy environments such as coastal dunes.[14] The suborder classification evolved from the keys in the 1999 second edition of Soil Taxonomy, with minor updates in the 2014 twelfth edition of Keys to Soil Taxonomy to incorporate organic variants and refine moisture criteria.[14] The classification was further refined in the 13th edition (2022) of Keys to Soil Taxonomy, particularly for urban Arents with new subgroups for construction fill and compaction.[3]Higher and Lower Taxa
Entisols are subdivided below the suborder level into great groups, subgroups, families, and series within the U.S. Soil Taxonomy system, allowing for increasingly specific classification based on soil properties such as texture, moisture, temperature, and human alteration.[3] Great groups represent the primary divisions under each suborder, defined by diagnostic horizons, saturation conditions, or material origins, with approximately 50 great groups recognized across the Entisol suborders.[3] Key examples of great groups include Xerorthents under the Orthents suborder, characterized by dry moisture regimes in well-drained soils with minimal development; Torripsamments under Psamments, typical of hot desert environments with sandy textures; Cryaquents under Aquents, found in cold, wet settings with aquic conditions; and Haplarents under Arents, reflecting human-transported materials in altered landscapes.[3] Other notable great groups encompass Fluvaquents for alluvial deposits in wet areas, Quartzipsamments for soils dominated by quartz sand, and Sulfaquents for those with sulfidic materials.[3] A comprehensive enumeration of great groups by suborder is provided below for clarity:| Suborder | Great Group Examples |
|---|---|
| Aquents | Cryaquents, Duraquents, Endoaquents, Epiaquents, Fluvaquents, Gelaquents, Haplaquents, Humaquents, Hydraquents, Psammaquents, Sulfaquents |
| Arents | Cryarents, Durarents, Haplarents, Torrarents, Xerarents |
| Fluvents | Cryofluvents, Gelifluvents, Haplofluvents, Torrifluvents, Udifluvents, Ustifluvents, Xerofluvents |
| Orthents | Cryorthents, Gelorthents, Haplocryorthents, Torriorthents, Udorthents, Ustorthents, Xerorthents |
| Psamments | Cryopsamments, Haplocryopsamments, Quartzipsamments, Torripsamments, Udipsamments, Ustipsamments, Xeropsamments |
| Wassents | Fluventic Wassents, Frasiwassents, Haplowassents, Hydrowassents, Psammowassents, Sulfiwassents |