Fundamentals of Soil Science (1+1)

Horizon Designation

Horizon designation

• Once horizons are recognized and demarcated, it becomes necessary to name each horizon.
• Capital letters, lower case letters and arabic numerals are used to designate horizons.
• The capital letters are used to designate master horizons e.g. A, B, C.
• The lower case letters are used as suffixes to indicate specific characteristics of the master horizon.
• Example : A_p, B_t etc.,
• p - tillage or other disturbance by cultivation
• t - Accumulation of silicate clay.

• Arabic numerals are used as suffixes to indicate additional vertical subdivisions within master horizon,
• Example : B_t1, B_t2, B_t3 etc.
• Lastly Arabic numerals are also used as prefixes to indicate discontinuities in a profile
• Example : 2C, 2BC etc.,
• The names of the orders can best be recalled by remembering a coined compound word ‘AVAAMIHOUSEG’. Names of the orders along with formative elements and salient characteristics are presented in Table-3.

Table 2. Major features of diagnostic horizons

Diagnostic Horizon	Major features
Surface Horizons (Epipedons)- 9
Mollic Thick, dark coloured, high base saturation, strong structure Umbric Same as Mollic except low base saturation Ochric Light coloured, low organic content, may be hard and massive when dry Histic Very high in organic content, wet during some part of the year. Anthropic Man modified Mollic like horizon, high in available P. Plaggen Man made sod like horizoncreated by years of manuring Folistic   Organic soil materials that remain saturated for less than one month Melanic A thick black horizon (>4.0 % OC) at or near, but within 30 cm of the soil surface. Grossarenic A sandy horizon , 100 cm or more thick over an Argillic horizon
Subsurface Horizons( Endopedons)- 19
Argillic Silicate clay accumulation Natric Argillic, high in sodium, columnar or prismatic structure Spodic Organic matter, Fe and Al oxide accumulation Cambic Changed or altered by physical movement or by chemical reactions Agric Organic and clay accumulation just below plough layer due to cultivation Oxic Highly weathered, primarly mixture of Fe, Al oxides, and 1:1 type minerals Duripan Hard pan, strongly cemented by silica Fragipan Brittle pan, usually loamy textured, weakly cemented Albic Light coloured, clay and Fe, Al oxides mostaly removed Calic Accumulation of CaCO3 or CaCO3.MgCO3 Gypsic Accumulation of gypsum Salic Accumulation of salts Sombric Draining horizon, formed due to illuviation of humus and not of aluminium or sodium Kandic Low activity clayswith or without clay skins, it has CEC of < 16 cmol(p+) kg-1soil Sulphuric A mineral or organic soil horizon that has a pH <3.5. Glossic Albic horizon characteristics gradually intruding in to an argillic, a kandic or nitric horizon. Petrocalcic An indurated calcic horizon that has hardness of 3 or more(Mho’s scale) Petrogypsic Astrongly cemented gypsic horizon whose dry fragments do not slake in water. Placic A thin , slowly permeable, dark reddish brown to black coloured Fe or Mn pan Of these Argillic, Natric,Cambic, Kandic, Oxic, Calcic and Gypsic are commonly observed in India

Table 3: Soil orders with soil characteristics

S.No.	Soil Order	Derivation	Pronunciation	Soil Characteristics
1	Entisol	Nonsense symbol	Recent	Soils with little horizon development or beginning of soil formation
2	Inceptisol	L.inceptum, begining	Inception	Soils with altered horizons, but no illuvial horizon containing clay or Fe, Al oxides, may have umbric, ochric or cambic horizon
3	Gelisols	Gk, gel,ice	Frost churning	Soils that have permafrost within 100cm or gelic material with permanent frost within 2m of the surface
4	Histosols	Gk. Histos, tissue	Histology	Soils containing > 30 % organic matter to a depth of 40 cm
5	Spodosols	Gk. Spodos, ash	Podzol; odd	Soils with spodic horizon with in 2 m.
6	Andisols	Jap. And	Ando	Soils that have andic soil properties in 60% or more of the thickness between the soil surface and 60 cm or lithic or paralithic contact
7	Oxisols	Fr. Oxide, oxide	oxide	Highly weathered soils of tropical and subtropical regions with oxic horizon
8	Vertisols	L. verto, turn	Invert	Soils with more than 30% clay in all horizons and crack when dry
9	Aridisols	L. aridus	Arid	Dry soils, ochric epipedon, some have argillic, nitric or salic horizon
10	Ultisols	L.ultimus,last	ultimate	Soils in warm humid regions with argillic horizon and low base saturation(<35% at 2 m depth below the surface
11	Mollisols	L. mollis, soft	mollify	Dark colored, base rich soils of grass land areas, mollic epipedon, many with argillic, nitric, or calcic horizon
12	Alfisols	Nonsense symbol	Pedalfer	High base status (>35%) soils of the humid and sub humid regions with an ochric epipedon and argillic (or nitric) horizon.