DIAGNOSTIC HORIZONS - SURFACE HORIZONS

A diagnostic horizon is one, which is formed as a result of pedogenic processes and having distinct propertiesor features that can be measured in terms of measurable soil properties. The diagnostic horizons are largely used not only for identifying soils but also in classifying soils at various categoric levels, especially great groups.

A number of diagnostic horizons have been defined in soil taxonomy. Based on location in soil profiles, these horizons have been divided into two categories, that is surface (epipedons) and subsurface (endopedons). The diagnostic surface horizons are called epipedons. The epipedons are the upper most soil horizons and include the upper part of the soil darkened by organic matter. They are not synonymous with “A”-horizon. Seven epipedons, viz., Mollic, Umbric, Anthropic, Ochric, Histic, Plaggen and Melanic are recognized but only three viz., Mollic, Ochric and Umbric are of importance of the soils of India. The diagnostic subsurface horizons are called endopedons. The endopedons include the lower part of the soil where soil materials accumulate. Nineteen endopedons, viz, Argillic, Natric, Agric, Spodic, Sombric, Cambic, Kandic, Oxic, Sulphuric, Salic, Placic, Albic, Glossic, Calcic, Gypsic, Duripan, fragipan, Petrocalcic, Petrogypsic are recognized. Of these eight, that is Argillic, Natric, Cambic, Kandic, Oxic, Salic, Calcic and Gypsic are commonly observed in India.

EPIPEDON (DIAGNOSTIC SURFACE HORIZONS)

Mollic epipedon

A thick, dark coloured, mineral horizon with high (>50%) base saturation and strong structure. It contains 1 per cent or more organic matter (when mixed to a depth of 17.5 cm) with colour values darker than 5.5 dry and 3.5 moist and chroma less than 3.5 moist. Soil structure cannot be massive and hard, very hard or extremely hard when dry. Base saturation is over 50 per cent and the epipedon is not naturally dry in all parts for more than nine months per year.

Anthropic epipedon

A surface horizon, like the mollic but formed under long continued system of farming that involves large addition of organic matter (compost), containing more than 250 ppm of citric acid soluble P2O5.

Umbric epipedon

A surface horizon like the mollic, but is low (<50%)in base saturation (dominantly saturated with H+) with high CN ratioand is notnaturally dry for more than three months per year.

Melanic epipedon

A thick black horizon at or near the soil surface which contain six per cent or more organic carbonas weighted average and / or 4 per cent or more organic carbon in all the layers. Usually associated with short range order minerals or alluvium humus complex.Colour value moist and chroma of 2 or less throughout and a melanic index of 1.70 or less throughout.

Histic epipedon

A thin organic horizon ( if virgin ) or horizon with very high organic matter content depending on clay content, that remains saturated with water for 30 days a some season of the year unless artificially drained. It is thinner than 30 cm , if drained or 45 cm if not drained.

Ochric epipedon

A surface horizon that is light in colour(colour values > 5.5 dry or > 3.5 moist) contains less than 1 per centorganic matter or is hard or very hard and massive when dry or dry for more than three months per year.

Plaggen epipedon

A thick (>50 cm) man made surface horizon, produced by a long and continued manuringwith sod. In addition the following designations are used for thick, sandy surface horizons

Grossarenic horizon

A sandy (loamy fine sand or coarser ) horizon, 100 cm or more thick over an argillic horizon.

DIAGNOSTIC SUBSURFACE HORIZONS (ENDOPEDONS)

Argillic horizon

A silicate clayenriched horizon formed by illuviation of clay. The fine clay is carried downward by percolating water and deposited as clay skins or cutans on ped faces and on the walls of pores. In general this is a B horizon that has atleast1.2 times as much clay content as some horizon above or 3per cent more clay content (if the eluvial layer has <15 per cent clay) or 8 per cent more clay if eluvial layer has > 40 per cent clay. It should be atleast 1/10 as thick as all overlying horizons or more than 15 cm whichever is thinner.

Umbric epipedon

A surface horizon like the mollic, but is low (<50%)in base saturation (dominantly saturated with H+) with high CN ratioand is notnaturally dry for more than three months per year.

Melanic epipedon

A thick black horizon at or near the soil surface which contain six per cent or more organic carbonas weighted average and / or 4 per cent or more organic carbon in all the layers. Usually associated with short range order minerals or alluvium humus complex.Colour value moist and chroma of 2 or less throughout and a melanic index of 1.70 or less throughout.

Histic epipedon

A thin organic horizon ( if virgin ) or horizon with very high organic matter content depending on clay content, that remains saturated with water for 30 days a some season of the year unless artificially drained. It is thinner than 30 cm , if drained or 45 cm if not drained.

Ochric epipedon

A surface horizon that is light in colour(colour values > 5.5 dry or > 3.5 moist) contains less than 1 per centorganic matter or is hard or very hard and massive when dry or dry for more than three months per year.

Plaggen epipedon

A thick (>50 cm) man made surface horizon, produced by a long and continued manuringwith sod. In addition the following designations are used for thick, sandy surface horizons

Grossarenic horizon

A sandy (loamy fine sand or coarser ) horizon, 100 cm or more thick over an argillic horizon.

DIAGNOSTIC SUBSURFACE HORIZONS (ENDOPEDONS)

Argillic horizon

A silicate clayenriched horizon formed by illuviation of clay. The fine clay is carried

downward by percolating water and deposited as clay skins or cutans on ped faces and on the walls of pores. In general this is a B horizon that has atleast1.2 times as much clay content as some horizon above or 3per cent more clay content (if the eluvial layer has <15 per cent clay) or 8 per cent more clay if eluvial layer has > 40 per cent clay. It should be atleast 1/10 as thick as all overlying horizons or more than 15 cm whichever is thinner.

Natric horizon

A high sodium clay enriched horizon with columnar or prismatic structure. The horizon meets all the requirements of an argillic but has 15 per cent or more of the exchange complex saturated with Na+ or more exchangeable Mg2+ + Na+ than Ca2+ plus exchange acidity at pH 8.2.

Agric horizon

An illuvial horizon of clay, silt and humus formed directly under the plough due to long and continued cultivation.

Spodic horizon

A humus and or sesquioxides enriched horizon with or without iron. There are many specific limits dealing with Al, Fe, organic matter and clay ratio, depending on whether the overlying horizon is virgin or cultivated. It is generally formed in cold humid regions.

Sombric horizon

A free draining horizon located not under an albic horizon; has colours (darkness) and base status like an umbricepipedon, and has formed due to illuviation of humus and not of aluminium or sodium.

Cambic horizon

A colour or structural B horizon formed due to alteration by physical movement or by chemical weathering. The pedogenic processes have altered the material enough to form structure(if the texture is suitable) to liberate free iron oxides to form silicate clays or both and to obliterate most evidences of the original rock structure. The alteration is not enough to completely destroy volcanic glass, allophone, glass, feldspars, micas, similarly illuviation of oxides, humus or clay should not be enough that will quality it for argillic or  spodic horizon. The horizon is extremely variable in mineralogy because of its pedogenic youthness, occurs under widely differing environments and may develop in the presence or absence of fluctuating ground water.

Kandic horizon

A horizon of low activity clays with or without clay skins. It has < 16 cmol (P+) kg-1 clay CEC at pH 7, and <12 cmol (p+) kg-1 clay ECEC. It shows a clay content increases at its upper boundary of >1. 2 x clay with in a vertical distance of < 15 cm that is abrupt or clear textural boundary.

Oxic horizon

A horizon enriched with Fe and Al oxides with dominance of 1:1 type clay minerals and from where silica has leached. It is at least 30 cm thick and is sandy loam or finer in texture. It has dominance of low charge 1:1 clays with clay CEC at pH 7 of 16 or less cmol (p+) kg-1 and apparent ECEC of 12 or less cmol (P+) kg-1 clay. The clay content increase is gradual that in kandic horizon. It contains less than 10 per cent weatherable minerals in the soil sand fraction.

Sulphuric horizon

A mineral or organic soil horizon that has a pH of <3.5 is toxic to plant roots, and has yellow mottles of jarosite.

Salic horizon

A horizon with secondary accumulation of water soluble salts(NaCl, Na2SO4, etc.) at some depth in the soil profile. The horizon at least 15 cm thick with secondary soluble salts enrichment of over 2 per cent and the multiple of thickness and per cent salt is 60 or more.

Albic horizon

A bleached E horizon of podzols and planosols. It has typical colour values of >5 (dry) or <4 (moist).

Glossic horizon is a horizon in which albic horizon characteristics gradually intrudes into an argillic, a kandic or a natric horizon. It may be 5 cm or more thick and consists of an eluvial part which constitutes 15 to 85 per cent (by volyme ) of the glossic horizon and an illuvial part (i.e. partially altered argillic,kandic or natric horizon).

Calcic horizon

A horizon with secondary Ca and /or Mg- carbonate enriched materials. It is 15 cm or

more thick, has 15 per cent or more of secondary accumulation of carbonate and contains

atleast 5 per cent more carbonate than any underlying layer horizon.

Gypsic horizon 

Calcium and / or magnesium sulphate- enriched horizon. It is more than 15 cm thick,

and contains at least 5 per cent more calcium sulphate than the underlying horizon / layer.

Petrocalcic horizon

An indurated calcic horizon that has hardness of 3 or more (Mohs scale) and whose one-half or more of dry fragments break down in acid, but not in water.

Petrogypsic horizon

A strongly cemented gypsic horizon whose dry fragments do not slake in water.

Placic horizon

A thin (2 to 10 mm thick), slowly-permeable, dark reddish brown to black coloured iron or manganese pan that lies within 50 cm of the surface.

Other diagnostic soil characteristics 

Several other layers or horizons and macro features are recognized and named in Soil Taxonomy. These features are usually used as diagnostic criteria at a somewhat lower level in the classification system than the subsurface diagnostic horizon and epipedons.

Abrupt textural change

Refers to a boundary between a surface (ochric epipedon or albic horizon) and an underlying subsoil (argillic horizon). It can be established using the following criteria.

If the surface material has <20 per cent clay, clay content doubles within 7.5 cm; if the surface (ochricepipedon) horizon has 20 per cent or more clay, an absolute increase of at least 20 per cent clay content is required within 7.5 cm.

Durinodes

Are weakly cemented to indurated nodules, cemented by SiO2.

Duipan

It is a subsurface horizon at least half cemented by SiO2. The air dry pedsdo not slake in water or HCl but are destroyed by hot KOH after acid washing.

Fragipan

Subsoil layers of high bulk density. It is brittle when moist, and very hard when dry. It does not soften on wetting, but can be broken in the hands. The air dry fragments slake in water.

Low chroma mottles

These are moist soil colours of chroma-2 or less; value4 or more (often represent gley conditions).

Permafrost

It is a layer where soil temperature is always <0°C. It may either be very hard or loose.

Plinthite

It is a humus-poor, sesquioxide - rich horizon, which hardens irreversibly to ironstone hardpans or aggregates with repeated wetting and drying. The red, indurating portions of the layer are usually mottled with yellowish, greyish, or white materials.

Soft powdery lime 

It is authigenic lime translocated within the soil, normally present as coating on the ped surfaces.

Tonguing

It is used when albic horizon material (of at least 5 cm deep and 5 mm wide) penetrates into an underlying argillic or natric horizon.

Diagnostic contacts (to "non soil" material)

A root-restricting layer wherein roots generally do not penetrate, especially in lithic material.

Lithic contact

A boundary between soil and continuous coherent, underlying material that has a hardness of >3 on the Moh scale.

Paralithic contact

A boundary between soil and underlying coherent material, with a hardness of <3 on the Moh scale. The roots may penetrate at irregular and infrequent intervals of >10 cm.

Petroferric contact

A boundary between soil and an indurated layer of iron cemented material.

SOIL MOISTURE AND TEMPERATURE REGIMES

The soil temperature and moisture regimes control the formation of soil. As these two parameters are measurable and can be quantified, they play a major role in soil classification at different categoric levels.

Soil Temperature Regimes (STR)

Soil temperature regime is one of the important properties that control plant growth and soil formation. Soil temperature Regimes (STRs) are the ranges in temperature classes within which biological activity of different degrees prevails. There is no biotic activity at temperature below freezing point (0°C). Between 0° and 5°C, root growth and seed germination of most plants are impossible as there is no respiration by plants and the soil is termed as biologically inactive. The lower limit of soil temperature (viz 5°C) acts as a thermal pan to root growth and germination of seeds. Like the lower limit, there exists an upper limit of soil temperature (35°C or more) above which root growth and germination of most of the mesophytic plants are severely restricted. Therefore, the temperature range from 5° to 35°C is important for determining the degree of biological and chemical activities , and physical processes going on in soils, and hence used in soil classification systems.

Soil temperature regimes play an important role in classifying soils at the Family and Suborder Categoric levels and in making land-use recommendations. The nomenclature of the different soil temperature regimes and the criteria are as follows. There are six soil temperature regimes, viz., pergelic, cryic/frigid, mesic, thermic, hyperthermic and megathermic. The prefix iso is used if the difference between the mean summer and mean winter temperatures is less than 5°C to separate tropical from other areas. A Mean Annual Soil Temperature (MAST) of 8°C is used as a limit to separate frigid (cold) soils from mesic (cool to warm) soils. Similarly, as MAST of 22°C is used to separate thermic (warm to hot)

from hyperthermic (hot) soils and a MAST of 28°C (recently proposed for some arid soils) is used as the limit to separate hyperthermic (hot) from megathermic (very hot) soils. Soil temperature is the resultant effect of absorption and reradiation of electromagnetic radiations emitted by the sun. In general, for every 10 cm increase in soil depth, the diurnal fluctuations either increase or decrease soil temperature by a factor of 0.6°C. This fluctuation coefficient is negative in summers and positive in winters. The class criteria for different soil temperature regimes are as follows

Pergelic :

Mean annual soil temperature < 00C.

Cryic:

Mean annual soil temperature 0 to 8 0C with summer temperature < 15°C.

Iso - When used as a prefix on the following temperature regimes, iso refers to soils in which the average soil temperature for the 3 warmest months differs by less than 5°C from

the average temperature of the 3 coldest months.

• Frigid and Iso-frigid O°C to < 8°C

• Mesic and Iso-mesic 8°C to < 15°C

• Thermic and Iso-thermic 15°C to < 22°C

• Hyperthermic and Iso-hyperthermic > 22°C

Soil Moisture Regimes (SMRs)

Soil Moisture Regimes (SMRs) refers to the presence or absence of water held at a tension of <1500 kPa in the soil moisture control section during difference periods of a year. Soil is considered moist when it is at moisture tension of less than 1500 kPa (15-bar) (but more than zero) and dries when the tension is 1500 kPa (15- bar) or more within the Soil Moisture Control Section (SMCS). It implies that water held at tension of 15 kPa or more is not available to keep most mesophytic plants alive. The availability of water to plants is also affected by dissolved salts. Consequently a soil is considered to be dry when it is at moisture tension of 1500kPa (15 - bar) or more, or when salt concentration reaches the level that limits moisture availability to plants. Under such conditions of high salt concentration, the soil is considered to be physiologically dry. A soil may remain moist and /or dry either throughout the year or some part of the year. This helps to determine the length of moist and / or dry period, or the length of plant growing

period in a soil.

Soil Moisture Control Section (SMCS)

The SMCS, considered important formoisture supply to crops, is defined as the depth limits of soil that regulates the moisture supply and this depends (largely) on its texture. The limits of the SMCS (upper and lower) are determined by the soil depths to which a dry soil at wilting point (kPA> 1500, but not air dry) is moistened by: 2.5 cm of water after 24 hours, and 7.5cm of water after 48 hour.

These depths exclude moistening along any crack or animal burrows, that are open to

the surface. The upper and lower boundaries of the SMCS extend to different depths, depending on

the texture as under:

• •  From 10 to 30cm in fine (clayey; fine loamy or fine silty) soils;

  •  From 20 to 60 cm in medium (loamy or coarse loamy) soils; and

  •  From 30 to 90 cm in coarse (sandy) soils

The soil moisture regime (SMR) is a partial function of climate, soil and landform. It is

defined in terms of the number of days the SMCS remains moist, with moisture tensions between 33 kPa (1/3 - bar) and 1500 kPa (15 - bar). Besides, controlling crop growth, SMR is important not only in understanding pedogenesis and nutrient availability, but also, in the classification of soil at different categoric levels, such as Soil Family, Suborder and occasionally at Order level. The dominant Soil moisture regimes are

1. Saturated

Condition when all soil pores are completely filled with water leading to anaerobic conditions and not conducive to plant growth. These soils are water saturated for at least enough time (several days) so that reduction conditions exist. Low chroma mottles normally are indicative of this condition.

   2.Non leaching

The water into the SMCS for a very short period in a year and is completely withdrawn by high PET demand (potential evaporative).

3.Leaching

Water moves into the soil almost throughout the year (if not frozen). 

Between the leaching and non leaching environment, there exists two partially leaching soil moisture regimes viz., ustic and xeric. The taxonomic classes of soil moisture regimes are aquic, udic, ustic, xeric and aridic.

i. Aquic

The soils remain saturated with wate for at least a few weeks so the reducing conditions exist within 25cm of the soil surface. Low chroma mottles are indicative of these conditions.

ii. Aridic

SMCS is moist for <60days and dry throughout for more than 180 days and may not support maturity for even short duration crops

iii. Udic moisture regime

In most years, these soils are not dry for as long as 90 cumulative days. Common in soils of humid climate with well distributed rainfall.

iv. Ustic moisture regime

SMCS is moist for 90-150 days and may support a single crop of short or medium duration.

v. Xeric moisture regime

These soils are associated with Mediterranean climate and are dry for 45 or more consecutive days within 4 months only in the temperature (non-iso) areas and have dry summers and moist winters. These soils are usually dry > 45 consecutive days with 4 months following summer solistice.