Soils of India – Part 1 – Indian Geography
Soil is an important layer of earth’s outermost layer known as crust. It is the mixture of organic matter, minerals, gases, water and air and other organisms. It is the anchor for vegetation which is the key to a food cycle on Earth.
How is Soil Formed?
Soil originally comes from rocks, also called the ‘parent material’. Soil formation starts when rock is exposed to the atmosphere. Properties of a soil will depend upon the type of parent material and how the soil is formed. Atmospheric conditions play a vital role in both the process and rate of physical and chemical decomposition of the parent material.
Types of Soil
The soils of India has been classified depending on the how they are formed, their color, composition etc.
- Alluvial soils
- Black soils
- Red soils
- Laterite and Lateritic soils
- Forest and Mountain soils
- Arid and Desert soils
- Saline and Alkaline soils and
- Peaty and Marshy soils
- 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 6 per cent of the total area.
- They support more than 40% of the India’s population by providing the most productive agricultural lands.
Characteristics of Alluvial Soils
- They are immature and have weak profiles due to their recent origin.
- Most of the soil is Sandy and clayey soils are common.
- Pebbly and gravelly soils are rare. Kankar (calcareous concretions) beds are present in some regions along the river terraces.
- The soil is porous because of its loamy (equal proportion of sand and clay) nature.
- Porosity and texture provide good drainage and other conditions favorable for agriculture.
Distribution of Alluvial Soils in India
- They occur all along the Indo-Gangetic-Brahmaputra plains except in few places where the top layer is covered by desert sand.
- They also occur in deltas of the Mahanadi, the Godavari, the Krishna and the Cauvery, where they are called deltaic alluvium (coastal alluvium)
- Some alluvial soils are found in the Narmada, Tapi valleys and Northern parts of Gujarat.
Crops in Alluvial Soils
- They are mostly flat and regular soils and are best suited for agriculture.
- They respond well to canal and well/tube-well irrigation.
- They yield splendid crops of rice, wheat, sugarcane, tobacco, cotton, jute, maize, oilseeds, vegetables and fruits.
- The parent material for most of the black soil are the volcanic rocks that were formed in the Deccan Plateau (Deccan and the Rajmahal trap).
- In Tamil Nadu, gneisses and schists form the parent material. The former are sufficiently deep while the later are generally shallow.
- These are the region of high temperature and low rainfall. It is, therefore, a soil group typical to the dry and hot regions of the Peninsula.
Colour of Black Soils
- The black colour is due to the presence of a small proportion of titaniferous magnetite or iron and black constituents of the parent rock.
- In Tamil Nadu and parts of Andhra Pradesh, the black colour is derived from crystalline schists and basic gneisses.
- Various tints of the black colour such as deep black, medium black, shallow black , a mixture of red and black may be found in this group of soils.
Chemical Composition of Black Soils
- 10 per cent of alumina,
- 8-10 per cent of iron oxide,
- 6-8 per cent of lime and magnesium carbonates,
- Potash is variable (less than 0.5 per cent) and
- phosphates, nitrogen and humus are low.
Distribution of Black Soils
- Spread over 46 lakh sq km (16.6 per cent of the total area) across Maharashtra, Madhya Pradesh, parts of Karnataka, Telangana, Andhra Pradesh, Gujarat and Tamil Nadu.
Crops in Black Soils
- These soils are best suited for cotton crop. Hence these soils are called as regular and black cotton soils.
- Other major crops grown on the black soils include wheat, jowar, linseed, virginia tobacco, castor, sunflower and millets.
- Rice and sugarcane are equally important where irrigation facilities are available.
- Large varieties of vegetables and fruits are also successfully grown on the black soils.
- Red soils along with its minor groups form the largest soil group of India.
- The main parent rocks are crystalline and metamorphic rocks like acid granites, gneisses and quartzites.
Characteristics of Red Soils
- The texture of these soils can vary from sand to clay, the majority being loams.
- On the uplands, the red soils are poor, gravelly, and porous. But in the lower areas they are rich, deep dark and fertile.
Chemical Composition of Red Soils
- They are acidic mainly due to the nature of the parent rocks. The alkali content is fair.
- They are poor in lime, magnesia, phosphates, nitrogen and humus.
- They are fairly rich in potash and potassium.
Color of Red Soils
- The red colour is due to the presence of iron oxide.
- When limestone, granites, gneisses and quartzites are eroded the clay enclosed within the rocks remains intact with other forms of non-soluble materials.
- In oxidizing conditions, rust or iron oxide develops in the clay, when the soil is present above the water table giving the soil a characteristic red colour.
- The colour is more due to the wide diffusion rather than high percentage of iron oxide content.
Distribution of Red Soils
- These soils mostly occur in the regions of low rainfall.
- They occupy about 3.5 lakh sq km (10.6 per cent) of the total area of the country.
- These soils are spread on almost the whole of Tamil Nadu.
- Other regions with red soil include parts of Karnataka, south-east of Maharashtra, Telangana, Andhra Pradesh, Madhya Pradesh, Chhattisgarh, Odisha, Chota Nagpur plateau; parts of south Bihar, West Bengal, Uttar Pradesh; Aravalis and the eastern half of Rajasthan (Mewar or Marwar Plateau), parts of North-Eastern states.
Crops in Red Soils
- The red soils are mostly loamy and hence cannot retain water like the black soils.
- The red soils, with the proper use of fertilizers and irrigation techniques, give good yield of cotton, wheat, rice, pulses, millets, tobacco, oil seeds, potatoes and fruits.
Laterite – Lateritic Soils
- Laterite soils are mostly the end products of weathering.
- They are formed under conditions of high temperature and heavy rainfall with alternate wet and dry periods.
- Heavy rainfall promotes leaching (nutrients gets washed away by water) of soil whereby lime and silica are leached away and a soil rich in oxides of iron and aluminium compounds is left behind.
- ‘Laterite’ means brick in Latin. They harden greatly on loosing moisture.
- Laterite soils are red in colour due to little clay and more gravel of red sand-stones.
Chemical composition of Laterite – Lateritic Soils
- Laterite soils are rich in bauxite or ferric oxides.
- They are very poor in lime, magnesia, potash and nitrogen.
- Sometimes, the phosphate content may be high in the form of iron phosphate.
- In wetter places, there may be higher content of humus.
Distribution of Laterite – Lateritic Soils
- Laterite soils cover an area of 2.48 lakh sq km.
- Continuous stretch of laterite soil is found on the summits of Western Ghats at 1000 to 1500 m above mean sea level, Eastern Ghats, the Rajmahal Hills, Vindhyan, Satpuras and Malwa Plateau.
- They also occur at lower levels and in valleys in several other parts of the country.
- They are well developed in south Maharashtra, parts of Karnataka etc. and are widely scattered in other regions.
Crops in Laterite – Lateritic Soils
- Laterite soils lack fertility due to intensive leaching.
- When manured and irrigated, some laterites are suitable for growing plantation crops like tea, coffee, rubber, cinchona, coconut, etc.
- In some areas, these soils support grazing grounds and scrub forests.