Short essay on Alluvial Soils

The alluvial soils occupy about 15 lakh sq. km of area stretching from the river Satluj in the west to the Brahmaputra valley in the east.

They also occur in the valleys of Narmada and Tapi in Madhya Pradesh and Gujarat; Mahanadi in Chhattisgarh and Orissa; Godavari in Andhra Pradesh; Krishna in Karanataka and Andhra Pradesh and Kaveri in Tamilnadu. Along the coast of Kerala these are called coastal alluvium and in the deltas of the Mahanadi, Godavari, Krishna and Kaveri as deltaic alluvium.

These soils are mainly derived from the debris brought down from the Himalayas or from the silt left out by the retreating sea. Their colour varies from light grey to ash grey and the texture is sandy to silty loam.

These soils are both well drained and poorly drained. At some places these have high textured sub soil while at other places the subsoil is low textured. These soils have an immature profile in undulating land while the flat lands have a more mature profile. The coastal allu­vial soils are of tidal origin.

The two sub-types-(i) Alluvial loam, and (ii) Calcareous-clavey alluvial are identical with the low-lying younger, floodplain alluvium locally known as Khadar and the older alluvium seldom liable to inundation known as Bhangar respectively. "The Khadar occupies the rive rain flood plains of the rivers and is enriched by fresh silt deposits every year. Its alluvium is light coloured and poor in calcareous matter consisting of deposits of sand, silt, mud and clay" (Tiwari, 1984, p. 3).

A little of Khadar can be assigned to the Upper Pleistocene and most to the recent age (Krishnan, 1960, p. 574). The Bhangar lands are level plains above the normal flood limit. Here the alluvium is of dark colour (pale reddish brown) generally rich in concretions, and nodules impure calcium carbonate or Kankar (Mukerji, 1964, p. 100). In drier areas it also exhibits stretches of saline and alkaline efflorescences known as reh. The main constituent of Bhangar is clay while at places it gives way to loam and sandy loam (Shafi, 1968, p. 31). It also occupies the Pleistocene terrace dating back to middle and upper Pleistocene periods.

The depressions in Bhangar containing clay loam to clay (grey to dark grey in colour) are better suited to rice crops, while the rest is used for other crops. Bhangar soils require recurrent manuring to main­tain their fertility. The Khadar soils, although rich in humus, yield only one crop during rabi season after the retreat of the floods.

Alluvial soils are rich in potash, phosphoric acid, lime and organic matter but deficient in nitro­gen and humus contents. These soils are capable of fixing nitrogen very rapidly through leguminous crops. The percentage of phosphoric acid in the oven dry soil varies from 0.005 to 0.02; humus from 0.8 to 0.9; potash from 0.1 to 0.35; and lime less than 7.0.

These soils are well suited to irrigation which helps in raising a number of crops like rice, jute, sugarcane, wheat, cotton, maize, oil seeds, fruits and vegeta­bles. These constitute the rice and wheat bowls of India and support large clusters of population.