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NC State Soil Science: SSC 551 Soil Morphology, Genesis and Classification: Unit 17

Unit 17: Inceptisols

Reading Assignment: P.293-300


Topic Outline

1. Overview

2. Learning Objectives

3. Discussion

4. Summary

5. Homework

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Unit 17: Inceptisols: Reading Assignment: P.293-300

Topics: Inceptisols

  1. At one time the Inceptisols order was called ‘the trashcan of Soil Taxonomy’ but that label is less appropriate today. At the time Soil Taxonomy was first being developed soil scientist held strong concepts of ‘Zonal soils’ i.e. soils with mature profile development that usually included what we now identify as argillic, spodic or calcic horizons and/or mollic or umbric epipedons and could be spatially associated with climatic or vegetative factors of formation. They also had a strong concept of ‘Azonal soils’ i.e. soils that had few profile features considered to have been formed by pedogenic processes and located on recent flood plains or steep slopes. Soils that did not ‘fit’ into either of these concepts were considered ‘Intrazonal soils’ in the process of developing into some kind of Zonal soil. Such soils were seldom studied except in the context that they were early stages of a Zonal soil. Thus the name Inceptisols (from the word inception) was adopted. Soils that were little known in the familiar landscapes of Europe and the United States such as soils formed on volcanic ash and in very cold regions found themselves classified as Inceptisols. Between 1975 and 1999 the recognition and definition of Andisols and Gelisols greatly ‘cleaned-up‘ the diverse range of soils previously classified as Inceptisols.
  2. Inceptisols must have some pedogenic development identified as a diagnostic horizon, feature, or epipedon other than an ochric epipedon if such features are not diagnostic for another order. Common soil property limits for Inceptisols are:
    1. They have subsoil textures that are very fine sand, loamy very fine sand or finer (cambic horizon limit).
    2. They are mineral soils without andic soil properties.
    3. Their mean annual soil temperature is above 0° C.
    4. They do not have an aridic soil moisture regime.
    5. They do not have argillic, kandic, or natric horizons.
    6. They may have ochric, histic, plaggen, umbric or mollic epipedons.
    7. They have more than 10% weatherable minerals in the sand fraction and/or an apparent CEC7 of the clay >16 cmol kg-1.
  3. . A fundamental premise of Inceptisols formation is a supply of weatherable minerals in the parent material. The primary concept of the cambic horizon, diagnostic for many Inceptisols, is that some degree of the primary material alteration has taken place. Thus it is implicit that minerals that can be altered in the soil environment be present. Where weatherable mineral content of the parent material is low little or no alteration of minerals within the pedon can be detected. i.e. the Oxisols.
  4. A common thread in the genesis of Inceptisols is vertical instability of the soil surface. On sloping land processes of erosion lower the surface at such a rate that lessivage processes do not accumulate enough clay to meet the definition of an argillic or kandic horizon (Figure 17.1); (Figure 17.2). Landslides are frequent (in the context of a pedogenic time scale) in humid mountainous terrain resulting in accumulations on the lower part of the slopes and often stone-lines are evident as previous slide areas are subsequently buried by more recent slides (Figure 17.3); (Figure 17.4); (Figure 17.5). On level, depositional surfaces the surface of the soil moves upward at such a rate that lessivage transfers of clay to the subsoil fail to deposit sufficient clay within a specific thickness to form an argillic or kandic horizon. Inceptisols in these locations are often dark in color but have sufficient development of blocky structure to identify a cambic horizon (Figure 17.6). Intrasolum calcification, salinization and alkalization processes forming calcic, salic and sodium rich subsoils in soil moisture regimes more humid than aridic appear to be more rapid than lessivage. In the aridic soil moisture regimes calcic, salic, and cambic horizons are diagnostic for Aridisols but where these horizons are present within specific depths in other soil moisture regimes they are diagnostic for Inceptisols. Thus many Inceptisols are very similar to some Aridisols except for soil moisture regime.

    Figure 17.1: Photo of landscape in the Smoky Mountains of North Carolina where Inceptisols are the predominant soils present.

    Figure 17.2: Photo of an Inceptisol profile on a steep slope in the Smoky Mountains of North Carolina. Note the saprolite (Cr horizons) below 100 cm by tape and the horizontal position of the channer at 30 cm. The horizontal positioning of channers is frequently observed in these soils and believed to result from creep movement of material downslope.

    Figure 17.3: Photo view down slope taken about two months after a landslide occurred in the Smoky Mountains of North Carolina. The slide area extended a linear distance of about 500 meters straight upslope and resulted in damming of the stream at the bottom of the slope. After the water broke through the dam and removed most of debris a pile of soil material is visible on the far foot-slope.

    Figure 17.4: Photo of a debris pile upon which Inceptisols are present, believed formed in a fashion similar to that described in Figure 17.3, at the base of a mountain slope in western North Carolina. The grass vegetation probably is the result of recent cultivation and pasturing.

    Figure 17.5: Photo of stone-line in a recently exposed excavation near the base of a mountain slope in Eastern Madagascar. The stone-line appears to have resulted when an erosion surface was buried by colluviums. The soils are Inceptisols.

    On level, depositional surfaces the surface of the soil moves upward at such a rate that lessivage transfers of clay to the subsoil fail to deposit sufficient clay within a specific thickness to form an argillic or kandic horizon. Inceptisols in these locations are often dark in color but have sufficient development of blocky structure to identify a cambic horizon (Figure 17.6).

    Figure 17.6: Photo of an Inceptisol profile in a well-drained depositional landscape position in The Peoples Republic of China. Note the strong angular blocky structure of the cambic horizon (20 to 60 cm depth).

  5. A common misconception that Inceptisols are precursory to more ‘developed‘ soils is fostered by the presence of weakly developed subsoil, cambic horizon properties and of course the name Inceptisols derived from inception. The vertical instability of the soil due to rapid (geological time scale) erosion or deposition determines that the vertical volume we examine and study today will not be the volume that is present at some distant future date. As landscapes are altered by erosion slope angles decrease and as depositions fill depressions in the landscape the surface of the soil rises. As these geomorphic processes slow the intensity of intrasolumn soil forming processes increases relative to the intensity of erosion and deposition processes and argillic, kandic spodic or calcic horizons may form but in material that is at a different vertical location than the present Inceptisols. (Review Unit 7.) Ignoring the vertical spatial changes inherent to the ‘steady state‘ conditions in which Inceptisols are present several authors have advanced the concept that with time Inceptisols will develop into Alfisols. This developmental concept also ignores that many Inceptisols formed in acid igneous materials have base saturation percentages far to low for consideration as Alfisols. (See ‘Dystro‘ and ‘Eutro‘ great group discussion in paragraph 7 of this unit.)
  6. The presence of weatherable minerals in Inceptisols in many cases seems to contribute sufficient fertility to attract subsistent farmers. In humid, Udic and Ustic soil moisture regimes Inceptisols on steep slopes, especially on lower portions of the slopes where colluvial material is the site of thicker solums than further up the slope, are frequent sites for farmers to construct small level bench terraces on which water can be added and controlled for paddy rice growing (Figure 17.7); (Figure 17.8). Level areas of Inceptisols, often with aquic soil moisture regimes are drained and used extensive production of food and fiber crops (Figure 17.9).

    Figure 17.7: Photo of a farmer tilling for paddy rice on the lowest of several terrace level recently formed by hand labor in Inceptisols in Indonesia.

    Figure 17.8: Photo looking down on several leveled bench terraces constructed on a steep hillside in Sri Lanka. The rice plants have recently been transplanted and each terrace will be flooded (note raised area on the down hill side of each terrace to retain water).

    Figure 17.9: Photo of a cotton field on Inceptisols in Louisiana.

  7. The classification of Inceptisols was greatly altered in 1999. Soil moisture regimes have replaced older criteria in most suborders. By eliminating the suborder Tropepts, (the criteria for ‘Trop‘ was simply ‘iso‘ soil temperature regimes) the redundancy between the family criteria and the suborder criteria was removed. The former epipedon criteria for identifying organic carbon content and color of the epipedon used as criteria for Ochrepts and Umbrepts has been shifted to the subgroup category and former Plaggepts are now a great group in the suborder Anthrepts where soil material such as mine tailings and other material deposited by human activity but having been in place long enough to develop some subsoil characteristics such as a cambic horizon are now classified.

One of the most significant properties for agricultural use of Inceptisols, base saturation percentage is identified at the great group category by the formative elements ‘Eutro‘ for high base saturation percentage and ‘Dystro‘ for low base saturation percentage. Unfortunately ‘Eutro‘ does not appear as a formative element in the great groups of either the Ustepts or Xerepts where the formative element ‘Haplo‘ in effect identifies ‘Eutro‘ because it follows ‘Dystro‘ in the keys to great groups. (Perhaps that should be changed to make provide better connotation of soil properties via great group names.)

The unique advantages of the formative elements is demonstrated in (Figure 17.10) where the properties identified in the old name of the soil 'Typic Eutropept' can easily be recognized in a new name 'Typic Eutrudepts'. The former connotation of 'Trop' would be identified as an 'iso-' soil temperature regime at the family level and the suborder 'Ud' (udic soil moisture regime) is clearly identified whereas in the older name it was identified by default in the Subgroup name Typic.

Figure 17.10: Photo of a Typic Eutropept (former classification) now a Typic Eutrudepts with an iso-soil temperature regime. The profile is in Porto Rico.

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