Inceptisols

 

Inceptisols

Summary:

  • Vegetation: not specified
  • Climate: variety of climates excluding arid
  • Soil moisture regime: variety of soil moisture regimes except aridic
  • Major soil property: few diagnostic features
  • Diagnostic horizons: cambic but no spodic, argillic, kandic, natric, and oxic horizon
  • Epipedon: ochric, umbric, histic, or plaggen (mollic)
  • Major processes: mass movement, soil erosion, deposition
  • Characteristics: Environmental conditions inhibit soil-forming processes

 

Inceptisols - Environmental Conditions

Climate: Inceptisols form under a variety of climates except aridic conditions. Soil moisture regimes can be variable ranging from poorly drained soils to well-drained soils on steep slopes. By definition, Inceptisols cannot have an aridic soil moisture regime. Climate which inhibits soil development such as low temperatures or low precipitation favors the development of Inceptisols. The suborder of Aquepts requires higher soil moisture conditions compared to the other suborders of Inceptisols.

Vegetation: Inceptisols occur under forested ecosystems, grassland or agricultural land. Although Inceptisols are not limited to forest environments, most of the soils classified into this order occur under forest ecosystems. Some Inceptisols (Umbrepts) were probably developed under prairie vegetation. Present use may be restricted by the shallowness of the solum (e.g. on steep slopes) or by poor drainage (e.g. in depression areas). Those Inceptisols are suited only to forestry and/or wildlife habitat.

Relief: Most Inceptisols develop on steep slopes where soil erosion removes parts of the topsoil continuously. Other Inceptisols are formed on convex toeslope areas where slope is level to gently rolling. These Inceptisols develop in deep colluvium where sediment has been / is deposited.

Parent Material: Inceptisols are extensive in areas of glacial deposits or on recent deposits in valleys or deltas. Where they occupy upland positions on young geomorphic surfaces, both primary and secondary minerals are present. Most Inceptisols are present on geologically young sediments (e.g. alluvium, colluvium, loess). Parent materials which is highly calcareous or resistant to weathering inhibit soil development but favor the development of Inceptisols.

Time: Most Inceptisols are formed on young landscapes (< Holocene), where time limited the development of soil diagnostic features. There are Inceptisols where the solum is permanently altered by loss of soil particles due to erosion or by the deposition of soil particles. These processes might be acting smooth but continuously or sporadically in space and time. In tropical zones the speed of development of Inceptisols into other soil orders is greater than in temperate or cold zones, it may be slowed down by retarded weathering of resistant rocks.

 

Inceptisols - Processes

Virtually many pedogenic processes are active to some extent in Inceptisol profiles but none predominates. The genesis of Inceptisols includes multiple pathways depending on the processes occurring on a given landscape and geographic area. Environmental factors can slow down weathering (e.g. low temperatures, low precipitation, or resistant parent material) and soil development to form other soil orders is retarded or even inhibited.

Soil erosion on steep slopes can alter the topsoil extremely. When erosion has leveled the slope erosion rates become lower and more distinct pedogenic features like argillic horizons are formed. Usually Inceptisols are formed in underlying volumes of parent material as erosion lowers the landscape by removing the volume of material that was soil. Long time periods and high erosion rates are necessary to develop an Inceptisol on a steep slope (shallow soil, AC horizons) to a further developed soil (deep soil profile, ABC horizons).

Inceptisols form also in colluvium at the base of steep slopes. Processes to form colluvium are mass movement, soil creep (slow mass movement), and deposition. Due to the hillslope processes and weathering morphological features are being formed and destroyed continuously.

Inceptisols may be also found on alluvial deposits where temporary flooding alters the soil profile due to the deposition of soil particles on the soil surface and the soil profile becomes saturated. For example, Inceptisols in the southern Mississippi River Valey are developed on alluvial deposits. A high water table favors the reduction of iron and aluminium oxides.

In depression areas or valley bottoms poorly drained Inceptisols are found where gleyzation produces redoximorphic features. In those areas leaching may be more extensive than in other landscape positions, but the process of lessivage and thus argillic horizon formation is somewhat retarded, probably because the soils do not undergo frequent desiccation. In areas of acid rocks, soils formed in landscape depressions tend to be more leached and somewhat lower in base content than soils in surrounding areas. In landscapes of high base status soils, the associated poorly drained Inceptisols in depression areas usually have higher base status than the surrounding soils. This can be attributed to the enrichment of the low-lying parts of the landscape by lateral processes such as transport of bases attached to soil particles, in surface runoff, or lateral subsurface flow. In some materials saturated with brackish water sulfides may accumulate and sulfuric horizons may be formed. When oxidized, usually by artificial drainage, sulfuric acid is formed. These unique Inceptisols are commonly known as 'cat-clays'.

Decomposition, humification, and mineralization result in the accumulation of organic matter. The soil organic matter is higher in the suborders Umbrepts and Aquepts compared to the suborder Ochrepts.

 

Inceptisols - Properties

The cambic subsurface diagnostic horizon of Inceptisols is composed of very fine sand, loamy fine sand or finer texture, with some weak indication of either an argillic or spodic horizon, but not enough to qualify as either. Typically, these soils have an ochric or umbric epipedon over a cambic horizon. The ochric epipedon is generally a light-colored, low organic matter horizon. The umbric epipedon is similar to the mollic epipedon except for having a base saturation less than 50 %. Some poorly drained Inceptisols have a histic epipedon where organic matter content is high. Soils with mollic epipedons are Inceptisols when base saturation at pH 7 is less than 50 % in some horizon between the mollic epipedon and a depth of 180 cm or a lithic or paralithic contact if shallower.Shallow Inceptisols show only few horizons, for example AC, AR or ABC. Due to erosion the development of soil structure is weak.

 

Inceptisols - Classification

The requirements to qualify for an Inceptisol are the following:

  • Usually a cambic diagnostic horizon is present but no spodic, argillic, kandic, natric, or oxic horizon
  • Soils that lack subsoil development but have umbric, histic, or plaggen epipedons
  • Soil texture: loamy or finer textured mineral soils
  • They exhibit profile development sufficient to exclude them from Entisols but lack features though to represent mature soil formation
  • No andic soil properties are permitted in any layer thicker than 35 cm within the top 60 cm
  • No aridic soil moisture regime is allowed

The suborders of Inceptisols are distinguished by soil moisture, epipedon properties, and soil temperature regime.

Suborders:

Aquepts: They show redoximorphic features and are saturated with water at some period in the year. Aquepts usually have cambic horizons and commonly in the US, they have fragipans. Aquepts are found in the Flood Plains of the Mississippi River Valley, the lacustrine regions in the Midwest, and the lower Coastal Plain along the Atlantic and Gulf Coast.

Plaggepts: They have dark brown or black plaggen epipedons. Plaggepts were formed by anthropic activity mainly in Europe and are of small extent.

Tropepts: They are formed in isomesic or a warmer iso soil temperature regime.

Ochrepts: They have an ochric epipedon or if their soil temperature regime is mesic or warmer they have thin (< 25 cm) mollic or umbric epipedons. Their soil organic matter content is low.

Umbrepts: They have umbric, mollic, or anthropic epipedons. They are freely drained Inceptisols that are acid, dark reddish or brownish, and high in organic matter.

Soil properties, soil temperature and moisture regimes distinguish the great groups and subgroups of Inceptisols.

Cryic or pergelic soil temperature (e.g. Cryaquepts), ustic moisture regime (e.g. Ustochrepts, Ustic Humitropepts), and aridic moisture regime (e.g. Aridic Ustochrepts) is considered in the Inceptisol Order.

A sulfuric horizon is considered on the great group (e.g. Sulfaquepts) and subgroup level (e.g. Sulfic Cryaquepts). The presence of a fragipan (e.g. Fragiaquepts, Fragic Epiaquepts, or Fragic Xerochrepts), a duripan (e.g. Durochrepts), or plinthite (e.g. Plinthaquepts) are considered. Carbonates within the soil profile of Inceptisols or a high base status define the Eutrochrepts. Inceptisols low in bases are common on the great group level (e.g. Dystropepts) and on the subgroup level (e.g. Dystric Eutrochrepts, Dystric Xerochrepts).

Shallow soil profiles are found in several subgroups, for instance, Lithic Ustochrepts, Lithic Cryaquepts, and Lithic Endoquepts. Vertic characteristics such as cracks and the extensibility of the mineral component of the soil define several subgroups of Inceptisols (e.g. Vertic Ustochrepts, Vertic Eutrochrepts, or Vertic Humitropepts).

Fluvial parent material is considered in several subgroups, for example, Fluventic Humitropepts, Fluventic Ustochrepts, or Fluventic Xerumbrepts. Inceptisols formed on volcanic material (e.g. volcanic glass) which does not meet the criteria of the Andisol order are considered by the formative element 'vitrandic' (e.g. Vitrandic Humitropepts, Vitrandic Durochrepts) or 'andic' if the fine-earth fraction exhibits a bulk density of 1.0 g/cm 3 or less (e.g. Andic Durochrepts, Andic Fragiochrepts).

 

Inceptisols - Distinguishing Characteristics

Inceptisols include soils that have some subsoil development but lack features of other soil orders. They are excluded from the Aridisol order by soil moisture regime, from the Vertisol order by lack of argilli-pedoturbative features, and from the Andisol orderby andic parent material. In temperate climate and increased precipitation Mollisols or Alfisols are formed. In tropical and subtropical climate Ultisols or Oxisols are formed.