Soil systematics

Lithological Discontinuity

         When two or more genetically unrelated (contrasting) materials are present in a profile as in the case of alluvial or colluvial soils, then the phenomenon is known as lithological discontinuity. This is indicated by the use of Roman letters as prefixes to the master horizons.

Soil horizon

A layer of soil or soil material approximately parallel to land surface and differing from adjacent layers in physical, chemical and morphological properties.

Soil Monolith - A vertical section of a soil profile removed from the field and mounted for display or study.

Soil Solum - Upper and most weathered part of the soil profile representing A and B horizons

Soil Sequum - Continuity and discontinuity along different axis of soil profile

Soil section – 9 inches thickness of soil profile.

Regolith – Inclusive term for all the loose materials above bed rock

Ped – Unit of soil structure

Intergrade horizon – No clear cut boundary between the horizons

Standard horizon – Clear cut boundary between the horizons

Horizonation – Pathways leading to the development of horizons

Haploidisation- No horizon formation due to pro isotropic processes (Weathering of sedimentary rocks).

Pedon

• A pedon is the smallest volume that can be called " a soil ". At the same time, it must be large enough volume of soil to be observable and to exhibit a full set of horizons. It is  three dimensional; roughly hexagonal in shape. Its area ranges from 1 to 10 m2 upon soil variability. It is a unit for soil sampling. The pedon includes the rooting zone of most native perennial plants.

• For soil surveys, a practical lower limit of the pedon is bedrock or a depth of about 2 m, whichever is shallower. It includes much of the volume of soil penetrated by plant roots, and it permits reliable observations of soil properties. Its lateral dimensions are large enough to represent the nature of the horizons and their variability.

Limitations of pedon soil mapping

The pedon, as a ‘point’ concept, is not useful for mapping. It has two fundamental limitations:

• It has a very small extent, so can’t show any ‘macro’ landscape features, most notably local landform (slope, curvature), landscape position, surface stoniness, erosion… which are essentially area concepts.

• It does not exhibit any spatial variability

Polypedon

These are the group of contiguous (adjacent and in close contact with) similar pedons bounded on all sides by not soil or by pedons of unlike characters. It is a real physical soil body which has a minimum area of more than one sq. km and an unspecified maximum area. The polypedon are the real soil bodies that we classify into series and higher categories. .  It is the link between the pedon and the taxonomic unit (the soil series).  Since  pedon is considered too small to exhibit more extensive features, such as slope and surface stoniness, the polypedon is presented in Soil Taxonomy as a unit of classification.

The polypedon is the soil body, homogeneous at the series level, and big enough to exhibit all the soil characteristics considered in the description and classification of soils 

Soil individual

A soil individual is a soil body that may be defined in terms of profile features whose arrangements and combinations over a geographic area are unique.

 Control section

It is a portion of soil profile used to spell out a property of a soil and delimited interms of an arbitrary depth or depth range in cm or inches. Eg-In USA 25 to 100 cm in soils with no textural B ( Argillic horizon) or the top 50 cm of that Argillic horizon. Control section of the pedon is used for the series category of soil. The Series Control Section (SCS) refers to a vertical section of soil that is considered in distinguishing series with families. The upper limit of SCS is below the plough or surface layer (20 – 25 cm), as the surface is subject to modifications by natural or anthropogenic activities. The lower limit is extended upto the zone of biological activity (upo 1.0 m) or  sometimes upto 1.5 m.

Soil three dimensional sequence

Soil is recognized as three dimensional natural bodies having length breath (area) and depth.

Vertical: Soil surface to depth where not affected by pedogenic processes (Z axis)

Lateral: Succession of contiguous soil bodies in horizontal direction (X and Y axis)

Soil profile

• The vertical cross section of soil exposing different horizons of a soil individual.

• It is a two dimensional feature. Profile exploration is done upto the parent material or 2 m depth whichever is earlier. The vertical cut is made upto water table in the case of waterlogged soils. Width of profile ranges from 1 m to several metres. 

• The description of entire pedon or a sample within it, should record the kinds of layers, their depth and thickness and the properties of each layer. Horizons or layers are studied in both horizontal and vertical dimensions. For selecting a profile site, the following points should be considered.

• It should be typical of the soil taxa (soil series or soil family) within the mapping unit

• It should be away from a tree, an irrigation channel / ditch / river / human settlements, road etc., as these prevent the normal development of a soil

• If possible, a virgin area should be preferred. 

 Soil profile – Nomenclature of soil horizons

• Master horizons - O, A, E, B, C,R

• Subordinate horizons -  t, a, i, e, o, b, e, …..etc.

• Transitional horizons - AE, EA, EB, BE ….etc.  

O - Organic horizon of mineral soils      

Oa -   Partly decomposed organic matter

Oe -  Highly decomposed organic matter

A-Mineral horizon (A) formed at or near the surface with well mixed organic matter

Ah-Uncultivated with high organic matter (> 1%)

Ap-Mixed by ploughing or other disturbances

Ag-Partially gleyed due to intermittent waterlogging, rusty mottles along root channels

E-Eluvial horizon of low organic matter content from which clay and humus have moved to lower horizons

E-Like Ag above but overlies a Bt horizon

B-Subsurface horizon showing typical colour, texture, structure etc., due to illuviation of material from the overlying horizons and the weathering of parent material.

   By-Accumulation of gypsum

     Bz-Accumulation of salts

     Bg-Partially gleyed, blocky or prismatic structure; variable black MnO2 mottles

   Bh-Translocated organic matter with some Fe and Al

     Bs-Enriched with sesquioxides usually by illuviation; orange to red in colour

     Bt-Accumulation of translocated clay as evidenced by clay coatings on ped faces

     Bw-Alterations of parent materials by leaching, weathering and structural reorganization

C-Parent material, excluding bed rock from which solum is believed to have formed

    Ck-Accumulation of secondary CaCO3 as concretions or coatings by 1 % or more (also occurs as Ak, Bk)

    Cy-Secondary accumulations of CaSO4 or gypsum crystals

Cm-Continuously cemented,  other than by a thin iron-pan

Cr-Weak consolidation but dense enough to prevent root penetration

Cx-Fragipan characteristics; dense but un-cemented; firm when dry and brittle when moist

D-Bed rock without decomposed by soil forming processes

Subordinate horizons – Lower case letters are used to designate the subordinate horizons

                      a                               Organic matter highly decomposed

                      o                                Accumulation of Fe and Al oxides

                      b                                Buried soil horizon

                      p                                Plowing or other disturbance

                      c                                 Concretions or nodules

                     q                                 Accumulation of silica

                     d                                 Dense unconsolidated material

                     r                                  Weathered or soft bed rock

                     e                                 Organic matter intermediate decomposition

                     s                                 Illuvial accumulation of organic matter and Fe and Al oxides

                     f                                 Frozen soil

                     t                                 Accumulation of silicate clays

                     g                                Strong gleying (mottling)    

                     v                                Plinthite (high iron, red material)

                     h                                Illuvial accumulation of organic matter                    

                     w                               Distinctive colour or structure

                     i                               Organic matter slightly decomposed

                     x                               Fragipan (high bulk density)

                     k                               Accumulation of carbonates

                     y                               Accumulation of gypsum

                     m                             Cementation

                     z                               Accumulation of soluble salts

                     n                               Accumulation of sodium