Horizons and Boundaries of Soil

 

As you observe a soil profile pit or a roadside cut, you will see various layers in the soil. These layers are called soil horizons, whereas the arrangement of these horizons in a soil is known as a soil profile.

A pedologists or an experienced soil scientist, observe and describe soil profiles and soil horizons to classify and interpret the soil for various uses.

Soil horizons differ in a number of easily seen soil properties such as color, texture, structure, and thickness. Other properties may be less visible.

Properties, such as chemical and mineral content, consistence, and reaction require special laboratory tests. The distinction between a mineral and an organic horizon is by the organic carbon content.

Layers which contain > 20 % organic carbon and are not water saturated for periods more than a few days are classed as organic soil material.

If a layer is saturated for a longer period it is considered to be organic soil material if it has:

·≥12 % organic carbon and no clay, or

·≥18 % organic carbon and ≥ 60 % clay, or

· 12 – 18 % organic carbon and 0 – 60 % clay.

These properties are used to define types of soil horizons. Soil generally consists of visually and texturally distinct layers, which can be summarized as follows from top to bottom:

Soil generally consists of visually and texturally distinct layers, which can be summarized as follows from top to bottom:

Horizons

In this article, you will be able to

·Identify surface diagnostic (master) horizons

·Know subordinate distinctions with master horizons

·Identify transitional horizons

·Know and identify subsurface horizons

 

Important Facts to Know

1. Master horizons (major horizons) are designated by capital letters, such as O, A, E, B, C, and R.

2. Some O layers may be under composed or partially decomposed litter, such as leaves, twigs, moss, and lichens, that has been decomposed on the surface; they are either on the top of either mineral or organic soils. Other O layers, are organic materials that were deposited in saturated environments and have undergone decomposition.

3. A horizons are mineral horizons that formed at the surface or below an O layer, that exhibition obliteration of all or much of the original rock or depositional structure (in the case of transported materials).

4. B Horizons have some features such as; An illuvial concentration of silicate clay, iron, aluminum, carbonates, gypsum, or humus; Removal of carbonates; A residual concentration of sesquioxides or silicate clays, alone or mixed, that has formed by means other than solution and removal of carbonates or more soluble salts; Coatings of sesquioxides adequate to give darker, stronger, or redder colors than overlying and underlying horizons but without apparent illuviation of iron.

5. C Horizons are mineral horizons that are little altered by soil forming processes. They lack properties of O, A, E, or B horizons.

6. Transitional horizons are layers or the soil between two master horizons designated as AB, BA, EB, BE. The first letter indicates the material of greatest volume in the transitional horizon. E.g. A/B, B/A, E/B or B/E.

  

Soil Horizon and Boundaries

Soil Master Horizons

1. O Horizons:They are dominated by organic material. Some O layers consist of undecomposed or partially decomposed litter, such as leaves, twigs, moss, and lichens, that has been decomposed on the surface; they may be on the top of either mineral or organic soils.

Other O layers, are organic materials that were deposited in saturated environments and have undergone decomposition. The mineral fraction of these layers is small and generally less than half the weight of the total mass. 

In the case of organic soils (peal, muck) they may compose the entire soil profile. Organic rich horizons which are formed by the translocation of organic matter within the mineral material are not designated as O horizons.

2. A horizons:Mineral horizons that formed at the surface or below an O layer, that exhibition obliteration of all or much of the original rock or depositional structure (in the case of transported materials). A horizons show one or more of the following:

· An accumulation of humified organic matter intimately mixed with the mineral fraction and not dominated by characteristic properties or the E or B horizon or,

·Properties resulting from cultivation, pasturing or other similar kinds of disturbance.

3. E horizons:Mineral horizons in which the main feature is loss of silicate clay, iron, aluminum, or some combination of these, leaving a concentration of sand and silt particles and lighter colors. The horizons exhibit obliteration of all or much of the original rock structure.

4. B horizons:Horizons in which the dominant features (s) is one or more of the following:

· An illuvial concentration of silicate clay, iron, aluminum, carbonates, gypsum, or humus

·Removal of carbonates

· A residual concentration of sesquioxides or silicate clays, alone or mixed, that has formed by means other than solution and removal of carbonates or more soluble salts

·Coatings of sesquioxides adequate to give darker, stronger, or redder colors than overlying and underlying horizons but without apparent illuviation of iron.

· An alteration of material from its original condition that obliterates original rock structure, that form silicate clay, liberates oxides, or both, and that forms a granular, blocky, or prismatic structure.

·Any combination of these.

5. C horizons:Mineral horizons that are little altered by soil forming processes. They lack properties of O, A, E, or B horizons. The designated C is also used for saprolite, sediments, or bedrock not hard enough to qualify for R. the material designated as C may be like or unlike the material from the A, E, and B horizons are thought to have formed.

6. R Layers:Consolidated bedrock (hard bedrock), such as granite, basalt, quarzite, sandstone, or limestone. Small cracks, partially or totally filled with soil material and occupied by roots, me frequently present in the R layers.

  

Subordinate Distinctions within Master Horizons (Letter Suffixes)

Lower case letters are used to designate specific features within master horizons. They are listed in alphabetical order below:

·  a: Highly decomposed organic material. The ‘a’ is used only with the O master horizon (Oa). The rubbed fiber content < l7 % of the volume.

·  b: Buried genetic horizon. It is not used in organic soils or to identify a buried O master horizon.

·  c: Concretions of hard non-concretionary nodules. This symbol is used only for iron, aluminum, manganese, or titanium cemented nodules or concretions.

·  d: Physical root restriction. It is used to indicate natural occurring or humanly induced layers such as basal till, plow pans, and other mechanical compacted zones. Roots do not enter except along facture planes.

·  e: Organic material of intermediate decomposition. This symbol is only used in combination with an O master horizon with rubbed fiber content between 17 – 40 % of the volume.

·  d: Frozen soil. The horizon must contain permanent ice.

·  g: Gleying: This symbol is used in B and C horizons to indicate low chroma color (< = 2), caused by reduction of iron in stagnant saturated conditions. The iron may or may not be present in the ferrous form (Fe2+). The g is used to indicate either total gleying or the presence of gleying in a mottled pattern. 

It is not used in E horizons, which are commonly of low chroma, or in C horizons where the low chroma colors are inherited form the parent material and no evidence of saturation is apparent.

·  h: illuvial accumulation of organic matter: Used only in B horizon. The h indicates an accumulation of illuvial, amorphous, dispersible organic matter with or without sequioxide component. If the sequioxide component contains enough iron so that the color value and chroma exceed 3 additionally a s is used (hs). The organoosequioxide complexes may coat sand and silt particles, of occur as discrete pellets, or fill voids and cement the horizon (use of m).

·  i: Slightly decomposed organic material. Used only in Combination with an O master horizon to designate that the rubbed fiber content is > 40 % or the volume.

·  k: Accumulation of carbonates, usually calcium carbonate. Used with B and C horizons.

·  m: Cementation or induration: Used with any master horizon, except R, where > 90 % of the horizon is cemented and roots penetrate only through cracks. Tire cementing material is identified by the appropriate letter.

·km: carbonate

·qm: silica

·sm: iron

·ym: gypsum

·kqm: both lime and silica

·zm: salts more soluble than gypsum

·  n: Accumulation of sodium: This symbol is used on any master horizon showing morphological properties indicative of high levels of exchangeable sodium.

·  o: Residual accumulation of sesquioxides.

·  P: Tillage or other cultivation disturbance (e.g. plowing, hoeing, discing). This symbol is only used in combination with the master horizon A or O.

·  q: Accumulation of silica: This symbol is used with any master horizon, except R, where secondary silica has accumulated.

·  r: Weathered soft bedrock: This symbol is only used in combination with the master C horizon. It designate saprolite or dense till that is hard enough that roofs only penetrate along cracks, but which is soft enough that it can be dug with a spade or shovel.

·  s: illuvial accumulation or sesquioxides and organic matter. This symbol is only used in combination with B horizons. It indicates the presence or illuvial iron oxides. It is often used in conjunction with h when the color is = < 3 (chroma and value).

·  ss: Presence of slicken sides. They are formed by shear failure as clay material swell upon wetting. Their presence is an indicator of vertic characteristics.

·  t: Accumulation of silicate clay: The presence of silicate clay forming coats on ped faces, in pores, or on bridges between sand-sized material grains. The clay coats may be either formed by illuviation or concentrated by migration within the horizon. Usually used ill combination with B horizons, but it may be used in C or R horizons also.

·  v: Plinthite: This symbol is used in B and C horizons that are humus poor and iron rich. The 5 material usually has reticulate mottling of reds, yellows and gray colors.

·  w: Development of color and structure. This symbol is used for B horizons that. have developed structure or color different, usually redder than that of the A or C horizons, but do not have apparent illuvial accumulations.

·  x: Fragipan character: This symbol is used to designate genetically developed firmness, brittleness, or high bulk density in B or C horizons. No cementing agent is evident.

·  y: Accumulation of gypsum. This symbol is used in B and C horizons to indicated genetically accumulated gypsum.

·  z: Accumulation of salts more soluble than: gypsum. This symbol is used in combination with B and C horizons.

Note:Arabic numerals can be added as suffixes to the horizon designations to identify subdivisions within horizons. For example, Btl – Bt2 – Bt3 indicated three subsamples of the Bt horizon.

 

 Transitional Horizons

Transitional horizons are layers or the soil between two master horizons. There are two types of transitional horizons:

a. Horizons dominated by properties of one master horizon that also have subordinate properties of an adjacent master horizon.

The designation is by two master horizon capital letters:

·The first letter indicates the dominant master horizon characteristics

·The second letter indicated the subordinate characteristics For example, an AB horizon indicates a transitional horizon between the A and B horizon, but one that is more like the A horizon than the B horizon. An AB or BA designation can be used as a surface horizon if the master A horizon is believed to have been removed by erosion.

b. Separate components of two master horizons me recognizable in the horizon and at least one of the component materials is surrounded by the others. The designation is by two capital letters with a slash in between. The first letter designates the material of greatest volume in the transitional horizon. For example A/B, B/A, E/B or B/E.

 

Diagnostic Subsurface Horizons

The accumulation of substances such as silica, iron, aluminum, carbonate, and other salts can result in cementer layers, which change the physical, chemical, and biological behavior of the soil. For example, a cemented layer retards percolation, and restrict root activity.

Furthermore, the availability of nutrients for plant growth is reduced, i.e., the cation exchange capacity is reduced. There are accumulations in the soil which show the enrichment of one substance and / or the depletion of another substance.

This can be expressed by diagnostic subsurface horizons, which are listed in alphabetically order below. It should be stressed that some characteristics can be measured only in the laboratory and not in the field.

a. Agric horizon: This is formed directly under the plow layer and has silt, clay, and humus accumulated as thick, dark lamallae.

b. Albic horizon: Typically this is a light-colored E horizon with the color value > = 5 (dry) or > = 4 (moist).

c.  Argillic horizon: It is formed by illuviation of clay (generally a B horizon, where the accumulation of clay is denoted by a lower case ‘t’) and illuviation argillans are usually observable unless there is evidence of stress cutans. Requirements to meet an argillic horizon are:

· 1/10 as thick as all overlying horizons

· > = 1.2 times more clay than horizon above, or:

· lf eluvial layer < 15 % clay, then > = 3 % more clay, or:

· If eluvial layer > 40 % clay, then > = 8 % more clay.

d. Calcic horizon: This layer has a secondary accumulation or carbonates, usually of calcium or magnesium. Requirements:

· > = 15 cm thick

· > = 5 % carbonate than an underlying layer

e. Cambic horizon:This subsurface often shows weak indication of either an argillic or spodic horizon, but not enough to qualify as either. H may be conceptually regarded as a signature of early stages of soil development, i.e. soil structure or color development.

Requirements:

· Texture: loamy very fine sand or finer texture

· Formation of soil structure

· Development of soil color

f.Duripan: It is a subsurface horizon cemented by illuvial silica. Air-dry fragments from more than 50 % of the horizon do not slake in water or HCl but do slake in hot concentrated KOH.

g. Fragipan:These subsoil layers are of high bulk density, brittle when moist, and very hard when dry. They do not soften on welling, but can be broken in the hands. Air-dry fragments slake when immersed in water. Fragipan genesis as outlined in Soil Taxonomy is largely dependent all physical processes and requires a forest vegetation and minimal physical disturbance.

Desiccation and shrinking cause development of a network of polygonal cracks in the zone of fragipan formation. Subsequent rewetting washes very fine sand, silt, and clay-sized particles from the overlying horizons into the cracks. Upon wetting, the added materials and plant roots growing into the cracks result in compression or the interprism materials.

Close packing and binding of the matrix material with clay is responsible for the hard consistence of the dry prisms. Iron is usually concentrated along the bleached boundaries of the prisms. It has also been postulated that clay and sequioxides cements to be binding agents in fragipans.

h. Glossic horizon: It occurs usually between an overlying albic horizon and an underlying argillic, kandic, or natric horizon or fragipan.

Requirements:

· > = 5 cm thick

· Albic material between 15 % to 85 %, rest: material like the underlying horizon

i.Kandic horizon: It is composed of low activity clays, which are accumulated at its upper boundary. Clay skins mayor may not be present It is considered that clay translocation is involved in the process of kandic formation, however, clay skills may be subsequently disrupted or destroyed by physical and chemical weathering, or they may have formed in situ.

Requirements:

· Within a distance of < 15 cm at its upper boundary the clay content increases by > 1.2 times

· Abrupt or clear textural boundary to the upper horizon

· A pH 7: low-activity days with CEC of < = I6 cmol/kg and ECEC (effective CEC) of < = 12 cmol/kg

j.Natric horizon: It is a subsurface horizon with accumulations of clay minerals and sodium.

Requirements:

· Same as argillic horizon

· Prismatic or columnar structure

· > 15 % of the CEC is saturated with Na+ ,or :

· More exchangeable Na+ plus Mg2+ than Ca2+

k. Oxic horizon:

Requirements:

· > = 30 cm thick

· Texture: sandy loam or finer

· At pH 7: CEC of < = I6 cmol/kg and ECEC of’ < = 12 cmol/kg (i.e., a high content or 1:1 type clay minerals).

· Clay content is more gradual than required by the kandic horizon

· < 10 % weatherable minerals in the sand

· < 5 % weatherable minerals by volume rock structure (i.e., indicative of a very strongly weathered material)

l.Petrocalcic horizon: It is an indurated calcic horizon.

Requirements:

· At least 1/2 of a dry fragment breaks down when immersed in acid but does not break down when immersed in water.

m. Petrogypsic horizon: This is a strongly cemented gypsic horizon. Dry fragments will not slake in H2O.

n. Placic horizon:This is a dark reddish brown to black pan or iron and/or manganese.

Requirements:

· 2 – 10 cm thick

· It has to lie within 50 cm of the soil surface

· Boundary: wavy

· Slowly permeable

o. Salic horizon: This is all subsurface horizon accumulated by secondary soluble salts.

Requirements:

· > = 15 cm thick

· Enrichment of secondary soluble salts such (hat electrical conductivity exceeds 30 dS/m more than 90 days each year

p. Sombric horizon: Formed by illuviation or humus (dark brown to black color) but not of aluminum or sodium. Requirements:

· At pH 7: base saturation < 50 %

· Not under an albic horizon

· Free-draining horizon

q. Spodic horizon: This horizon has an illuvial accumulation of sequioxides and/or organic matter. There are many specific limitations dealing with aluminum, iron, and organic matter content, and clay ratios, depending on whether the overlying horizon is virgin or cultivated.

r. Sulfuric horizon:this is a very acid mineral or organic soil horizon.

Requirements:

· pH < 3.5

· Mottles are present (yellow color: jarosite)

 

Boundary

The boundary between the horizons can be described considering the distinctness and topography.

Distinctness refers to the degree of contrast between two adjoining horizons and the thickness of the transition between them. 

Topography refers to the shape or degree of irregularity of the boundary.

 

Classification of Horizon Boundaries

 

Conclusion on Horizons and Boundaries of Soil

A soil horizon is a layer parallel to the soil surface, whose physical characteristics differ from the layers above and beneath.

Each soil type usually has three or four horizons. 

Horizons are defined in most cases by obvious physical features, mainly colour and texture.

There are master horizons (Surface horizons) and are designated by capital letters, such as O, A, E, B, C, and R., Diagnostic Sub surface and Transitional Horizons.

Soil horizons are identified in the field by careful observation of a standard soil profile pit. The experience of the observer will go a long way in ensuring accuracy of obtained results.

O- Horizon (Organic matter): Litter layer of plant residues in relatively undecomposed form. A-Horizons (Surface soil): Layer of mineral soil with most organic matter accumulation and soil life. This layer eluviates (is depleted of) iron, clay, aluminium, organic compounds, and other soluble constituents. When eluviation is pronounced, a lighter coloured “E” subsurface soil horizon is apparent at the base of the “A” horizon.

A-horizons may also be the result of a combination of soil bioturbation and surface processes that winnow fine particles from biologically mounded topsoil. In this case, the A-horizon is regarded as a “biomantle”.

B-Horizon (Subsoil): This layer accumulates iron, clay, aluminium and organic compounds, a process referred to as illuviation.

C-Horizon (Parent rock): Layer of large unbroken rocks. This layer may accumulate the more soluble compounds. R-Layer (bedrock): R horizons denote the layer of partially weathered bedrock at the base of the soil profile. Unlike the above layers, R horizons largely comprise continuous masses (as opposed to boulders) of hard rock that cannot be excavated by hand.

Soils formed in situ will exhibit strong similarities to this bedrock layer.