Explanation:

Explanation: Alluvial soils are formed mainly due to silt deposited by Indo-Gangetic-Brahmaputra  Rivers. In coastal regions some alluvial deposits are formed due to wave action. Rocks of the  Himalayas form the parent material. Thus the parent material of these soils is of transported origin. They are the largest soil group covering about 15 lakh sq km or about 46 per cent of the total area. They support more than 40% of the India’s population by providing the most productive agricultural  lands.

A. Characteristics of Alluvial Soils:

1. They are immature and have weak profiles due to their recent origin.
2. Most of the soil is Sandy and clayey soils are not uncommon
3. Pebbly and gravelly soils are rare. Kankar (calcareous concretions) beds are present in some  regions along the river terraces.
4. The soil is porous because of its loamy (equal proportion of sand and clay) nature.
5. Porosity and texture provide good drainage and other conditions favorable for agriculture.
6. These soils are constantly replenished by the recurrent floods.
B. Chemical properties of Alluvial Soils:
1. The proportion of nitrogen is generally low.
2. The proportion of Potash, phosphoric acid and alkalies are adequate
3. The proportion of Iron oxide and lime vary within a wide range.
C. Distribution of Alluvial Soils in India:
1. They occur all along the Indo-Gangetic-Brahmaputra plains except in few places where the top  layer is covered by desert sand.
2. They also occur in deltas of the Mahanadi, the Godavari, the Krishna and the Cauvery, where  they are called deltaic alluvium (coastal alluvium)
3. Some alluvial soils are found in the Narmada, Tapi valleys and Northern parts of Gujarat.
D. Crops in Alluvial Soils:
1. They are mostly flat and regular soils and are best suited for agriculture.
2. They are best suited to irrigation and respond well to canal and well/tube-well irrigation.
3. They yield splendid crops of rice, wheat, sugarcane, tobacco, cotton, jute, maize, oilseeds,  vegetables and fruits.
E. Geological divisions of alluvial soils: Geologically, the alluvium of the Great plain of India is divided into newer or younger khadar and older bhangar soils.
Bhabar: The bhabar belt is about 8-16 km wide running along the Shiwalik foothills. It is a porous,  northern most stretch of Indo-Gangetic plain.Rivers descending from the Himalayas deposit their 
load along the foothills in the form of alluvial fans. These alluvial fans (often pebbly soils) have 
merged together to build up the bhabar belt.The porosity of bhabar is the most unique feature. The 
porosity is due to deposition of huge number of pebbles and rock debris across the alluvial fans.The 
streams disappear once they reach the bhabar region because of this porosity. Therefore, the area 
is marked by dry river courses except in the rainy season.The area is not suitable for agriculture and 
only big trees with large roots thrive in this belt.
Terai: Terai is an ill-drained, damp (marshy) and thickly forested narrow tract (15-30 km wide) to 
the south of Bhabar running parallel to it.The underground streams of the Bhabar belt re-emerge in 
this belt. It is a swampy lowland with silty soils.The terai soils are rich in nitrogen and organic matter  but are deficient in phosphate.These soils are generally covered by tall grasses andforests but are  suitable for a number of crops such as wheat, rice, sugarcane, jute etc..This thickly forested region  provides shelter to a variety of wild life.
Bhangar: The Bhangar is the older alluvium along the river beds forming terraces higher than the  flood plain (about 30 metres above the flood level).It is of a more clayey composition and is generally dark colored.A few metres below the terrace of the bhangar are beds of lime nodules known as “Kankar”.
Khadar: The Khadar is composed of newer alluvium and forms the flood plains along the river  banks.The banks are flooded almost every year and a new layer of alluvium is deposited with every  flood. This makes them the most fertile soils of Ganges.They are sandy clays and loams, more dry  and leached, less calcareous and carbonaceous (less kankary). A new layer of alluvium is deposited  by river flood almost every year.