Soil Orders 

In soil taxonomy, there are twelve soil orders and they replace the zonal, iintrazonal and azonal orders of 1938 Genetic System. The characteristics and distribution of 12 soil orders are detailedd below.

1. ENTISOLS (Recent)

Concept

These are very recently developed mineral soils with no diagnostic horizons other than an ochric or anthropic epipedon. There is only slight degree of profile development. In earlier systems these were called the Azonal soils. The parent materials may be recent alluvium, sand dunes or variety of rocks.

Distribution

These are widely distributed inn steep slopes, flood plains, near the oceans and in sand dunes irrespective of climate, vegetation and topography. In India these are common in rocky humid or subhumid mountain regions and in sand bars of Rajasthan, Haryana and Central Punjab.

Major suborders

Aquents, Fluvents, Psamments, Arrents and Orthents

Land Use

Coarse textured with poor water and nutrient holding capacities. Physical conditions are poor especially soil structure and is susceptible to wind erosion. Short duration crops, grains, vegetables, pastures can be grown on conserved moisture. Entisols are the least developed and Oxisols represent the most advanced stage of weathering in evolution of soils.

2. INCEPTISOLS (L. Inceptum, Beginning)

Concept

They represent early stage of soil formation. They have accumulation of clay in their subsurface, but it is insufficient to qualify it as an argillic horizon, which is diagnostic for Alfisols and Ultisols. They have dark coloured surface horizon which is limiting in depth, organic matter and base saturation to qualify it as a mollic epipedon (Mollisols).

These soils are usually not dry and have one or more of the diagnostic horizons (cambic, umbric or mollic with low base saturation). They have developed rather recently owing to the alteration of the parent material but without much leaching and accumulation of materials in sub soil. The soils have too weak profile development to be called zonal yet have horizonation enough not to be termed as Azonal. The order includes many agriculturally productive soils. It also includes others whose productivity is limited because of imperfect drainage and sodicity. The soils develop under varied climatic and vegetation conditions. The woodlands are typical for their formation.

Distribution

These are widely distributed the world. They occur in most climatic regimes except the arid regions such as Thar desert in Western Rajasthan. They are common in inter tropical regions under all topographical positions.

Major suborders

Aquepts, Anthrepts, Cryepts, Ustepts, Xerepts and Udepts.

Land Use

Limiting soil temperature (cryic) and moisture (ustic and xeric regimes permit farmers to have one crop a year. Petrocalcic horizons pose physical limitations and sulphuric acid poses chemical limitations. These are agriculturally productive and provide natural grazing grounds. Pastures, vegetables and grain crops are commonly cultivated.

3. VERTISOLS (L.Verto, Turn or Churn)

Concept

These are uniform, thick (atleast 50 cm) tropical, black and dark coloured, cracking-clay, mineral soils that have high clay content (> 30 %). These soils swell on wetting and shrink on drying. The swell – shrink process induces the development of wide, deep cracks associated with gilgai microrelief or intersecting slickensides (Polished ped faces in the subsoil). They develop on basic parent materials such as basalt, limestone, shale or marls of smectitic composition. They occupy lower topographical position or flat terrain. High clay content and swell and shrink nature of clay is essential for the formation of vertisols.

Distribution

They are found in sub humid to semi arid climates where annual precipitation variesfrom 500 to 1500 mm. These soils  are widely observed in India, NE Africa, Ausralia, S.USA, Sudan, Ethiopia etc., In India they are observed in Madhya Pradesh, Maharashtra, Gujarat, Andhra Pradesh, Tamil Nadu and Rajasthan. In India these are termed as Black cotton soils (Regur).

Major suborders

Aquerts, Cryerts, Xererts, Torrerts, Usterts, Uderts.

Land Use

Because of shrink-swell characters, they pose many tillage problems during cultivation, or when used for building foundations, laying drainage and gas pipelines, installing irrigation networks etc., Tilting of trees, fences, poles and sinking of floors are characteristics of this order. Grasses, cotton, sorghum, pigeonpea, paddy etc., may be cultivated. They are very productive, if managed properly.

4. MOLLISOLS (L.Mollis, Soft)

Concept

These are soils of grassland vegetation under semi arid to humid environment. They have a dark coloured, base-rich, well-structured (granular or crumb) surface horizon, that is rich in organic matter called the mollic horizon. The subsurface horizons, rich in illuviated clay (argillic), calcareous (calcic) or gypsiferous (gyspic) materials may be present. Roots of grassy vegetation decompose resulting in dark stable compounds associated with soil burrowing activity of diverse populations of soil invertebrates, especially earthworms. They develop under a wide range of temperatures from the equator to the poles and in lowlands to high mountain meadows. They are formed in parent materials rich in bases such as limestone, basalt, marl or alluvium. They have high base saturation and abundant calcium.

Distribution

This order is observed in the central USA, Russia and in the central South America. In India, such soils are observed in the ‘Tarai’ region of Uttar Pradesh and Uttaranchal , in Himalayas of Himachal Pradesh, northern Bihar and in Maharashtra and Madhya Pradesh in association with vertisols.

Major suborders

Albolls, Aquolls, Rendolls, Cryolls, Xerolls, Ustolls and Udolls.

Land Use

The mollisols are inherently the best agricultural soils without real constraints. In India, these soils produce optimum yields both under irrigated and unirrigated conditions with minimum inputs. Maize, sorghum, field and vegetable crops are cultivated in this soil.

5. ARIDISOLS (L.Aridus, Dry)

Concept

These are mineral soils of arid and semi-arid regions and of areas of high groundwater table. The soils remain dry for most part of the year and salts accumulate at the surface and / or in the solum resulting in the development of a salic, gypsic or calcin horizon.

Distribution

They occur widely in the arid climatic environments of the world like the Sahara, Gobiand Thar deserts and occupy over a biollion hectares of land area. They are dominantly observed in S.Asia, N and SE Africa, Australia, South America, SW and N.USA, S.Africa and Russia. In India they occur in Rajasthan, Gujarat, Haryana and in Leh and Ladak regions. The important soil forming processes responsible for the evolution of such soils are salinization and calcification. If the groundwater is brackish, salt accumulation takes place in the profile by capillary fringe. The parent material is alluvium, loess or till.

Major suborders

Cryids, Salids, Durids, Gypsids, Argids, Calcids and Cambids

Land Use

The vegetation in this order is sparse and xerophytic. They have limitations of aridic climatic conditions and brackish groundwater close to the surface. The sub surface horizons hamper root growth and water movement. If the gypsiferous landscapes are irrigated, they develop sink holes due to dissolution of gypsum and the land sinks behind leaving uneven surfaces for cultivation. These soils require irrigation for crop growth. They are best suited for sparse native grazing.

6. ALFISOLS (Pedalfer of Marbut)

Concept

Alfisols are base – rich, timbered, mineral soils of sub humid and humid regions. They have light coloured ochric epipedon, over a subsurface argillic horizon that is rich in exchangeable cations with base saturation of > 35%. Alfisols are more strongly weathered thatn the inceptisols, but less than the Ultisols. The Alfisols develop under semi-arid to humid climates of temperate and tropical climates and where the vegetative cover is from grassland to forest. Though the soils are formed in varying topographic conditions, steep slopes are not conducive because of surface run off. Decalcification (when rainfall exceeds potential evaporation, eluviation and illuviation results in the formation of argillic horizon. In sodic soils, high sodium is responsible for the dispersion and mobilization of clay.

Distribution

These soils are widely distributed in the USA, South America, Northern Europe, Africa, Brazil and SE Asia. In India, these soils are found in Andhra Pradesh, Assam, Bihar, Arunachal Pradesh, Himachal Pradesh, Maharashtra, Karnataka, Tamil Nadu, Orissa and Madhya Pradesh. They are found in localized depression in soils having high sodium on the exchange complex.

Major suborders

Aqualfs, Cryalfs, Ustalfs, Xeralfs, Udalfs

Land Use

Alfisols are naturally fertile and productive. They are used for forestry, grazing and for growing fruits, vegetables and grain crops. The sodium-rich Alfisols need ameliorative measures such as gypsum application before bringing under cultivation.

7. SPODOSOLS (Gk. Spodos – wood ash)

Concept

These are mineral soils with accumulation of sesquioxides and humus in the sub surface horizons. The diagnostic features of these soils is an illuvial horizon, enriched with free sesquioxides and humus underlying a belached, wood-ash-coloured luvial E-horizon. In the past, these soils were classified as podzols. Podzolisation is responsible for the development of spodosols. There is limited polysaccharide production due to reduced bacterial activity. Organic acids result in the formation of organic-sesquioxide-clay complex, which are soluble thus forming a spodic horizon. Spodosols also develop due to degradation of Alfisols.

Distribution

These soils are widely distributed NW Europe, NE USA, Canada etc., In India, true Spodosols are seldom reported to occur because of the lack of siliceous parent material and environmental conditions (cool humid, supporting acid producing vegetation) needed for the formation.

Major suborders

Aquods, Cryods, Humods, Orthods

Land Use

Spodosols are naturally unfertile and are used for forestry, soft-wooded timber production and pasture land. When fertilized they may support one crop in a year because of the limiting climatic condition.

8. ULTISOLS (L.Ultimum – Last)

Concept

The Ultisols are comparable with Alfisols, but have low base saturation (< 35 %) due to advanced stage of weathering. These are base poor, timbered mineral soils of humid regions developed under high rainfall and forest vegetation. They have argillic or kandic subsurface horizon. These soils are completely decalcified by the leaching of bases during long period (thousands of years) of their pedogenic evolution. Base poor clay minerals such as kaolinite , predominates whilst gibbsite is occasionally observed.

Distribution

 The soils are most extensive in the warm, humid and (subtropical) regions of the world. These are most predominant in SE USA, central Africa and SE Asia. In India, they are observed in Kerala, Tamil Nadu, Karnataka, Orissa, Assam and north-eastern states.

Major suborders

Aqults, Humults, Udults, Xerults

Land Use

Because of the low fertility and low base status, these soils pose limitations for agricultural use. Generally, these soils are used for forestry, but may produce good agricultural, vegetable and plantations crops, when they are adequately limed and fertilized. In the tropical regions, such soils are cultivated for pineapple, sugarcane, coffee, cocoa, coconut, rubber etc.,

9. OXISOLS (Oxide - Iron)

Concept

These are extremely and deeply weathered mineral soils of the humid tropics that are very poor in soil fertility. These are characterized by a uniform profile having limited or negligible amount of weatherable minerals, and are dominated by kaolinitic and sesquioxide – rich deep subsurface diagnostic horizon (oxic). Deeply weathered, mottled horizon (plinthite) may also develop. Most oxisols have brick-red colour, but some are also yellow or grey in colour. They develop from basic parent materials with sufficient iron-bearing ferro – magnesian minerals (pyroxene, amphibole, biotite). Silica is removed leaving behind a high proportion of the oxides of iron and aluminium. The hydrated form of sesquioxides, being unstable, get dehydrated irreversibly to Al2O3 / Fe2O3 due to high temperature. Such a material is cut into bricks and used in construction of buildings. These soils are rich in clay content, but unlike Vertisols, the clay in Oxisols is non-sticky and predominates in kaolinite and gibbsite minerals, that have very low exchange capacity.

Distribution

Oxisols are observed predominantly in the tropical world, on gently sloping very old landscapes in Central Africa, South America and South-east Asia. In India, such soils are found in Kerala, Tamil Nadu, Karnataka and Orissa. Their occurrence in other areas is indicative of climatic change in the recent past.

Major suborders

Aquox, Torrox, Ustox, Perox, Udox

Land Use

Oxisols are chemically degraded soils and need careful management and fertilization for agricultural use. They pose the problem of phosphate fixation as Fe and Al. These are mainly used for grazing and forestry. Some are used for growing crops such as coffee, rubber, cocoa, sugarcane and tropical fruit crops (pine apple, coconut, jackfruit etc.,). Shifting agriculture is a common practice adopted in such areas. The cycle is reduced to 35 years, resulting in serious problems of soil erosion by water and nutrient depletion.

10. HISTOSOL (Gk. Histos - Tissue)

Concept

 These are organic rich soils with high, peaty horizon (histic epipedon) that develop in lower topographic positions in a permanent water saturated environment. As a thumb rule, a soil without permafrost is classified as a Histosol if half or more the upper 80 cm is organic. The main process in their formation is accumulation of peat. The breakdown of organic matter is related to waterlogging as the most effective microbial decomposers require oxygen. Sediment or rock (R,C) is little altered by weathering. There may be some leaching or formation of gley minerals such as pyrite or siderite, but most of the weatherable minerals and structures of the parent materials are retained. Histosols support bog, swamp and marsh. The organic matter content ranges from 20 to 30 percent. In the past, these soils were known as peat, muck and bog soils.

Distribution

Histosols occur in selected parts of Europe and Asia. In India, Histosols have not been reported, if they exist, these may be of rare occurrence. They are found to occur in Kerala and Andaman and Nicobar islands.

Major suborders

Hemists, Folists, Fibrists, Saprists.

Land Use

They are used for growing vegetables. For management, it is necessary to drain these soils for plant growth and to facilitate the operation of farm machinery. When drained, they oxidize and subside making the drainage of such soil difficult. In Kerala, they are used for growing paddy, soon after draining the flood / run off water received during the monsoon period, but keeping the subsurface soils submerged in water to avoid oxidation of pyrite to form sulphuric acid. In Vietnam, these soils are used for growing pineapple. Under natural conditions, they support mangrove vegetation that helps in maintaining ecobalance in coastal regions of Orissa, Tamil Nadu, Goa and Sunderban areas of West Bengal.

11. ANDISOL (Japan, Ando – Black soil)

Concept

Andosols were first recognized in Japan and were named as Volcanic ash material. They are dark coloured, low bulk density soil that have developed on volcanic ash parent material. They donot have an albic horizon but have andic properties : low bulk density (< 0.9 Mg m-3) or 60 percent or more vitric volcanic ash within 60 cm of the mineral soil surface. It has high allophane which gives them a very low bulk density (easy to cultivate) and fluffiness, especially in the B horizon. They are generally very fertile.

Distribution

Andisols are observed in all topographic positions and under all soil moisture and temperature regimes. They are found in the (sub)tropical and Meditterranean climates, but not in cold, temperate conditions where the soils have humus rich surface horizon. Since they are formed from volcanic ash or materials derived from it as volcanic alluvium, volcanic loess, theyNare dominantly observed in the ‘Ring of Fix’ in the Pacific, including Japan, New Zealand, Indonesia and Phillipines. They are also observed in the Rift Valley of Africa, especially Kenya,Nwestern coast of USA and South America. In India, such soils have not been reported to occurNso far, although volcanic activity is locally observed in Andaman and Nicobar islands, where  there is possibility of localized occurrence.

Sub orders

Aquands, Cryands, Torrands, Xerands, Vitrands, Ustands and Udands

Land Use

Andisols that occur on steep slopes, have the limitation of mechanized farming. The major limitation is their high phosphate fixation capacity. Andisols are stable and resist water erosion because of high infiltration and permeability rates. When dry, they are susceptible to wind erosion. They demand careful management for engineering use because of low bulk density, poor compactability and large changes in cohesion on drying. It is advantageous to grow crops that show little response to the application of phosphorus. E.g. Sweet potatoes.

12. GELISOL (Gelic - Churning)

Concept

These are the soils with gelic materials viz., mineral or organic soil materials that show evidence of cryoturbation(frost churning) underlain by permafrost within 2 m of the soil surface. Diagnostic horizons may or may not be present in Gelisols as thawing and freezing play an important role in evolution. Permafrost acts as a barrier to the downward movement of the soil solution. Cryoturbation (frost churning or mixing) is an important process in the development of Gelisols.

Distribution

Gelisols are observed in areas of permafrost, that include extreme northern USA,Canada, Greenland, Russia, Mongolia and other areas in the extreme north of the northern  hemisphere. In India, such soils have not been observed and reported. They may be observed in permafrost conditions (ice caps in the extreme North, represented by Leh and Ladakh and Sikkim regions of the Higher Himalayas)

Sub orders

Histels, Tubels and Orthels.

Land Use

Gelisols are not cultivated because of the limitation of severe cold climate. They are left to support natural vegetation. Orthels of anhydrous condition, support little vegetation.