Alluvial Soils (Entisols, Inceptisols and Alfisols)
The alluvial soils are the most important soils from the agricultural point of view.
Characteristics – These soils are derived from the deposition laid by the numerous tributaries of the Indus, the Ganges and the Brahmaputra systems. The products of weathering of rocks in the Himalayas are brought down and materials transported by water, ice, gravity and wind. The alluvial soils include the deltaic alluvium, calcareous alluvial soils, coastal alluvium and coastal sands. This is the largest and most important soil group of India. It contributed the largest share to India’s agricultural wealth. Broadly this soil is divided into two types:
Newer alluvium: Sandy, generally light coloured and less kankary.
Older alluvium: More clayey in composition, generally dark and full of Kankar.
Formation of hard pans (impervious layer) is often observed in Indo-gangetic alluvial soils of Uttar Pradesh and West Bengal. In Assam, old alluvium at hills is more acidic than the new alluvial soils along the riverbanks, which are often neutral or alkaline. In general alluvial soils are low in N except in Brahmaputra valley where they are moderate. Alluvial soils are found in Indo-gangetic plains of Uttar Pradesh, West Bengal, Bihar and Brahmaputra valley of Assam. Alluvial soils are fertile and suitable for most of the agricultural crops like lowland rice, pulses, cotton, banana etc.
Black Soils ( Entisols, Inceptisols, Vertisols )
Characteristics – Black Soils are dark grey in colour, which is due to the presence of clay–humus complex.
Black soils are:
• mainly formed from Deccan basalt trap parent material
• occur in monsoon climate, mostly of semi-arid and sub humid type
• alternate dry and wet periods and calcification favours black soil formation
• Cracks are formed on the surface soil (from 0.5-l cm up to 6 cm wide) during summer
• mixing of soil along the entire solum
• highly clayey (35-60% clay); Calcareous with high CEC (30-50 C mol/kg of soil)
• high swelling and shrinkage, plasticity and stickiness
• impeded drainage and low permeability
• high content of exchangeable calcium and management
• poor in organic matter, N and available P2O5.
Suitable crops – Cotton, Sugarcane, Groundnut, Millets, Maize, Pulses, Safflower
Red Soils ( Alfisols, Inceptisols, Ultisols )
Characteristics – The red colour of soils is due to the coating of ferric oxides on soil particles.
Red soils are:
• formed from granites, gneiss and other metamorphic rocks either in-situ or from decomposed rock materials
• with Argillic subsurface horizon
• Occur in semi-arid tropics
• Light textured, friable, absence of lime and CaCO3 and low contents of soluble salts
• Kaolinite with an admixture of illite clay minerals
• Well drained with moderate permeability
• Excess gravelliness, surface crusting, susceptibility to erosion.
Suitable crops : Maize, Wheat, Millets, Groundnut and Pigeon pea.
Laterites and Lateritic Soils ( Ultisols, Oxisols, Alfisols )
• Eluviations of silica and enrichment with oxides of Fe and Al. (Laterization process)
• Occurrence of plinthite or a pallid zone above water table.
High level laterite: not useful for agriculture (thin and gravelly).
Low level laterite: clays and loams in coastal regions.
• Laterization is intensified with increase in rainfall but with low intensity
• Low Silica/Sesquioxide ratio ( SiO2 : R2O3)
• Rich in nutrients and contain 10-20% organic matter ( Low pH)
• Low in Ca and Mg but well drained and porous
• Kaolinite and traces of illite clays ( CEC 2-7 C.mol/kg)
At lower elevations: Rice
At higher elevations: Tea, Coffee, Cinchona, and Rubber
Desert Soils ( Aridisols, Entisols )
• Sand dunes and undulating sandy plains
• Presence or accumulation of alkaline earth carbonates
• Clay content is very low ( <8%)
• Presence of sodic clay (dispersion and less permeable) with pH 8.0-8.8.
• Presence of phosphate and nitrate makes desert soils fertile and productive under water supply
• Dominantly illitic with smaller amount of kaolinite, chlorite, vermiculite
Tarai Soils ( Mollisols )
Tarai soils are derived from the materials washed down by the erosion of mountains (alluvial origin).
• Hard clay, coarse sand and gravel (parent material)
• Relatively high moisture content for the greater part of the year results in luxuriant vegetation
• Organic matter content is high
• Sandy loam to silty loam in texture
Suitable cops: Tall grasses.
Saline and Sodic Soils ( Aridisols, Inceptisols, Alfisols, Entisols, Vertisols )
These soils contain excess amounts of neutral soluble salts dominated by chlorides and sulphates of Na, Ca and Mg, which affect plant growth. White encrustation of salts occurs on the surface of the saline soils hence called as “White alkali”. These soils are characterized by EC: 4dSm-1 at 25°C; ESP : < 15; pH: < 8.5. These soils need leaching and drainage before cropping. The crops grown in these soils are grouped as:
(i) High salt tolerant: Sesbania, rice, sugarcane, oats, berseem, lucerne, indian clover and barley.
(ii) Medium salt tolerant: Castor, cotton, sorghum, cumbu, maize, mustard and wheat.
(iii) Low salt tolerant: Pulses, peas, sunnhemp, gram, linseed, sesamum.
These soils contain high content of CO3 and HCO3 of Na. Hence, they are with high exchangeable sodium percentage (ESP). Generally, they are non-saline and with dark encrustation hence called as “black alkali”. These soils are rich in NaHCO3 and characterized by pH: > 8.5; EC: < 4d Sm-1 ; ESP : > 15. Use gypsum (CaSO4 2H2O) as amendment for reclamation of sodic/alkali soils. Iron pyrites, (FeS2) bulky organic manures (especially green manures) and crop residues which produce weak organic acids are also used for reclamation. Crops having tolerance are grown in the soils.
(i) Tolerant crops: Karnal/rhodes/para/bermuda grass, rice, sugar beet.
(ii) Semi – Tolerant: Wheat, barley, oats, berseem, and sugarcane.
(iii) Sensitive: Cowpea, gram, groundnut, lentil, peas, and maize.
• Low pH with high amounts of exchangeable H+ and Al3+.
• Occur in regions with high rainfall.
• Laterization, Podzolization in areas with sub temperate to temperate climate.
• Significant amount of partly decomposed organic matter.
• Kaolinitic and Illitic.
• Low CEC and high base saturation.
• Liming and judicious use of fertilizers are the management measures suggested.
Suitable crops: Acedophytes (like potato).
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