Soil: Composition, Characteristics & Types of Soils | IBPS PO Prelims & Mains Preparation - Bank Exams PDF Download

COMPOSITION OF SOIL

Soil is the loose material which forms the upper layer of the mantle rock, i.e. the layer of loose fragments which covers most of the earth's land area. It has definite and constant composition. It contains both decayed plants and animal substances.

The four main constituents of soils, present in varying proportion, are:
(i) Silica, present in soil in small crystalline grains form, is the chief constituent of sand. It is derived mainly from the breaking up of rocks, which is a very slow process.

(ii) Clay is a mixture of silicates and contains several minerals such as iron, potassium, calcium, sodium and aluminium. Particles of clay absorb water and swell.

(iii) Chalk (calcium carbonate) provides calcium, the most important element for the growth of plants.

(iv) Humus is not a mineral, it is an organic matter. It is formed by decomposed plant remains, animal manure and dead animals and is the most important element in the fertility of the soil. It helps retain moisture in the soil and helps the plant in absorbing materials from the soil for building its body. A soil looks dark on account of the presence of humus.

TOP SOIL AND SUB SOIL

Soil consists of two layers, namely topsoil and subsoil. Topsoil (the upper layer) is of greater importance. Good topsoil means good crops. It varies considerably in depth and also in character and ability to grow crops. It is only a few metres deep. Millions of bacteria, insects and worms live in it. Top soils develop very slowly. It may take years to form topsoil suitable for plants, but it can be washed away in a few years if proper precautions are not taken. The resources of subsoil are replaceable but when the topsoil itself is gone there is a complete loss.

Topsoil and subsoilThe subsoil consists of the parent material from which soil is formed. It also contains plant food and moisture but it is not as productive as topsoil. It has to be converted into the soil and it may take years to convert subsoil into the soil. Below the subsoil generally, there is solid rock.

Formation of Soil

• The natural soil is formed by weathering, deposition and biochemical processes.
  Weathering is the process of disintegration of rocks into soil. Mechanical weathering involves fragmentation of rocks by frost and temperature changes. Chemical weathering involves crunching of rocks by the chemical action of air, water or both.
• Deposition is the process of progressive laying down of rock particles carried by rivers, ice, marine currents, wind or tides.
• Biochemical processes involve the biological action of the tree roots of burrowing animals which assist the mechanical and chemical weathering of rocks. As vegetation decays, acids are produced which weaken the rock. The roots of the plants and the burrowing of certain animals decrease the resistance of the ground to other agents of weathering.

Soil formation depends upon the following factors:

• Nature of the parent rock which provides the inorganic substances for the soil;
• Climate, i.e. temperature and rainfall conditions: excessive rains tend to leach away the soluble minerals of the soil;
• Natural vegetation, which provides much of the organic substances of the soil;
• Topography or relief; soils are usually thinner and poorer on steep slopes than on gentle slopes;
• The length of time that these factors have been at work.

CHARACTERISTICS OF DIFFERENT SOILS

1. Sandy soil (light soil)

It contains more than 60 % sand and less than 10% clay. Its particles are loosely bonded because there is not enough cementing material. They are easily permeable by air and water. This allows good airing for plant roots but they dry up easily. Sandy soil is easy to cultivate and is favoured for fruits and vegetables. It improves if humus in the form of decayed leaves is added to it.

2. Clayey Soil

It has a high proportion of clay. It becomes sticky when mixed with water. It is not aerated and plant roots find it difficult to dig and plough when dry. It becomes waterlogged when there is too much moisture. The addition of sand and chalk or lime improves it. A soil very rich in clay is called 'heavy'.

3. Loam

It is rich soil and consists of a mixture of sand and clay, together with silt and humus in good balance. It has the qualities of both sand and clay. Loam may be 'sandy loam' depending on whether sand or clay is present in it in a higher proportion. All loamy soils are good for farming and general gardening.

4. Alluvial Soil

This is the most important and widespread group of soils. It covers about 15 lakh sq. km of the land area in Great Plains from Punjab to Assam and also in the valleys of the Narmada and Tapti, Mahanadi, Godavari, Krishna and Cauvery. These soils have been brought down and deposited by three great Himalayan rivers- Sutlej, Ganga and Brahmaputra and their tributaries.

These soils consist of varying proportions of sand, silt and clay. These are predominant in coastal plains and deltas. Geologically, the alluvium is divided into Khadar and bhangar. Khadar is the newer alluvium which is sandy, light coloured and occurs near river beds where deposition takes place regularly and bhangar or older alluvium is of clayey composition, darker in colour. This is because most Deccan rivers flow through the black soil region, whence they carry away large quantities to the delta. Examples are the soils in the valleys of the Narmada, Tapti, Godavari and  Krishna.

Alluvial soilAlluvial soils as a whole are very fertile and therefore the best agricultural soils of the country. Generally, they contain adequate potash, phosphoric acid and lime. The fertility of the soils is due to :

(i) Mixing up of the debris derived from rocks of the Himalaya.
(ii) Presence of great variety of salts in these soils drawn from different rocks.
(iii) Their very-fine grainy texture, porous nature and light weight (due to which they are easily tilled).
 

Two main problems associated with these soils are:

• They allow water to sink into the lower strata and are therefore unsuitable for the growth of those crops which require the retention of great deal of moisture about their roots and thus they cause infertility in regions where showers are not frequent.
• These soils though rich in potash, phosphoric acid, lime and organic matter, are generally deficient in nitrogen and humus; this necessitates heavy fertilisation particularly with nitrogenous fertilisers.
• These soils are suitable for irrigation, particularly well adapted to canal irrigation because of the abundance of sub-soil water and softness of the strata to be penetrated. Under irrigation, these soils are suitable for rice, wheat, sugarcane, cotton, jute, maize, oilseeds, tobacco, vegetables and fruits. The regions of these soils constitute the 'wheat and rice bowls of India.

5. Black Soils

• As the name indicates, these soils are black in colour and since they are ideal for growing cotton, they are also called cotton soils in addition to their local nomenclature of regur soils. These soils, covering an area of 5.46 lakh sq. km, are most typical of the Deccan trap (Basalt) region spread over north-west
• Deccan plateau and are made up of lava flows. They occupy the plateaus of Maharashtra, Saurashtra, Malwa and southern Madhya pradesh and extend eastwards in the south along the Godavari and Krishna valleys. In addition to the parent rock material, climatic conditions are also important in the formation of these soils. Hence they extend much beyond the lava plateau itself.
• In the southern to the parent rock material, climatic conditions are also important in the formation of these soils. Hence they extend much beyond the lava plateau itself. In the southern and eastern parts of the country where rainfall is heavy, black soils often occur in close proximity to red soils, the former occupying valleys and low-lying areas and the latter higher slopes and hill tops.

Black soil

• The black colour of regur is variously attributed to the presence of titaniferous magnetite, compounds of iron and aluminium, accumulated humus and colloidal hydrated double iron and aluminium silicate. 
• These soils are generally rich in iron, lime, potash, aluminium, calcium and magnesium carbonates but are deficient in nitrogen, phosphorus and organic matter. 
• They are clayey, fine-textured and become sticky when wet and hence almost unworkable in rainy season as the plough gets stuck in the mud. They develop deep wide cracks on drying which helps in the process of self aeration and absorption of nitrogen from the atmosphere. 
• These soils are highly retentive of moisture.
• Due to their high degree of moisture retentiveness, fineness and chemical composition, these soils are endowed with inexhaustible fertility. As the content of water-soluble salts is high, they are unsuitable for heavy irrigation. 
• Due to their moisture retentive qualities black soils are ideally suited to dry farming. These soils are suitable for cotton, cereals, like linseeds, castor and safflower, many kinds of vegetables and citrus fruits.

6. Red Soils

These soils occupy about 5-18 lakh sq. km over the peninsula reaching upto Rajmahal Hills in the east, Jhansi in the north and Kutch in the West. In fact the north-western half of peninsula is covered by the black soils and the remanining south-eastern half is covered by red soils. They practically encircle the entire black soil region on all sides and cover the eastern part of peninsula comprising Chhotanagpur plateau, Orissa, east Madhya Pradesh, Telangana, the Nilgiris, Tamilnadu plateau and Karnataka.

Red soilThese soils have a reddish colour due to iron compounds and are characterised by light texture, porous and fiable structure, absence of kankar and free carbonates, and presence of soluble salts in small quantity. They are deficient in phosphoric acid, organic matter, lime and nitrogen. They differ greatly in consistency, colour, depth and fertility. In uplands, they are thin, light coloured, poor and gravelly (coarse) which are suitable for bajra, Groundnut and potatoes, But on the lower plains and valleys they are rich, deep coloured, fertile loamy which are suitable for rice, ragi, tobacco and vegetables.

7. Laterite Soils

Laterite soil

These soils are generally poor in nitrogen, phosphoric acid, potash, lime and organic matter and support only pastures and scrub forests. While they are poor in fertility, they respond well to manuring and are suitable for rice, ragi, tapioca and cashew nuts.

8. Forest and Mountain Soils

These soils occupy about 2.85 lakh sq.km in the hilly regions of the country. These soils are highly variable depending upon the differences in the climate and geology. They are described as soils in the making. Humus predominates in all forest soils and it is more raw at higher level leading to acidic conditions. These soils are found in the Himalayas and the other ranges in the north and high hill summits in the Sahyadris, Eastern Ghats and the Peninsula.

Forest and mountain soilThe forest soils are poor in potash, phosphorus and lime and require manuring for cultivation. In areas of good rainfall, these are rich in humus and suitable for cultivation of plantation crops of tea, coffee, spices and tropical fruits, as in Karnataka, Tamilnadu, Kerala and Manipur. Temperate fruits, maize, wheat and barley are grown in Jammu and Kashmir and Himachal Pradesh where soils are mostly podzols which are acidic in reaction.

9. Arid and Desert Soils

These soils occur under arid and semi-arid conditions in the northwestern parts of the country and occupy about 1.42 lakh sq. km area in Rajasthan, south Haryana, north Punjab and Rann of Kutch. The Thar Desert alone occupies an area of 1.06 lakh sq. km. The soil mainly consists of sands including wind-borne loess as well. These soils contain high percentage of soluble salts and a low to very low organic matter; they are also friable and low in moisture content.

Arid and desert soil

They are quite rich in phosphate but poor in nitrogen. Crops grown in these soils include coarse millets, jowar and bajra. The Ganganagar district of Rajasthan, where canal irrigation has been introduced recently, has become a leading producer of cereals and cotton.

10. Saline and Alkaline Soils

These soils occupy about 170 lakh sq. km of arid and semi-arid areas of Rajasthan, Punjab, Haryana, Uttar Pradesh, Bihar and whole of Maharashtra. These soils have saline and alkaline effervescence mainly of sodium, calcium and magnesium which have rendered the soils infertile and hence uncultivable. The injurious salts are confined to the top layers of the soil as a result of the capillary transference of solutions from lower layers in places under canal irrigation and in areas of high subsoil water table as in the coastal areas of Maharashtra and Tamilnadu.

Saline and alkaline soil These soils, variously called reh, kallar, rakar, usar, karl and chopain, are infertile. Texturally they are sandy to loamy sand. Saline soils contain free sodium and other salts whereas alkaline soils contain large quantities of sodium chloride. These soils can be reclaimed by methods based on irrigation, application of lime or gypsum where necessary and cultivation of salt resistant crops like rice and sugarcane. Crops grown on these soils include rice, wheat, cotton, sugarcane and tobacco.

11. Peaty and  Marshy Soils

These soils cover an area of about 150 sq. km in the Kottayam and Allepey districts of Kerala. Peaty soils have formed under humid condition as a result of an accumulation of large amounts of organic matter. They contain considerable amount of soluble salts but are deficient in phosphate and potash. They are suitable for paddy cultivation.

Peaty and marshy soilMarshy soils are found in coastal areas of Orissa, West Bengal and Tamilnadu, in central and northern Bihar and in Almora district of Uttar Pradesh. The soils are formed as a result of water logging and anaerobic conditions of soils and the presence of iron and a high quantity of vegetable matter. These soils are not suitable for cultivation but certain buttressed root plants grow in these soils.

SOIL FERTILITY

The factors responsible for deficiency of Indian soils are:

1. Loss of Soil Nutrients Through Crop Removal:

   • Harvested crops lead to the removal of nutrients from the soil. This is a common occurrence in agriculture, where crops absorb essential elements from the soil during their growth cycle. To maintain soil fertility, it is crucial to replenish these nutrients.

2. Leaching and Its Impact on Soil Nutrients:

   • Leaching is the process by which nutrients are washed away from the soil due to heavy monsoonal rains. Sandy soils are more susceptible to leaching than heavier soils. Additionally, bare soils are more prone to nutrient loss compared to soils covered by plants. This underlines the importance of soil cover and soil structure in nutrient retention.

3. Soil Erosion and Nutrient Loss:

   • Soil erosion, the removal of surface soil, is another factor contributing to the loss of soil nutrients. Preventing soil erosion is crucial for maintaining the fertility of the land.

4. Optimum Manuring of Crops:

   • The passage emphasizes that, after irrigation, the most important factor for increasing crop yield is the optimum manuring of crops. This involves supplying essential nutrients such as nitrogen, phosphorus, and potash to the soil.

5. Nutrient Deficiency in Indian Soils:

   • Indian soils are primarily deficient in nitrogen, phosphorus, and potash. These deficiencies can be addressed through the application of organic manures and fertilizers.

6. Organic Manures:

   • Various organic manures, such as cow dung, compost, farmyard manure, bone meal, and animal refuse, can be used to enrich the soil with essential nutrients.

7. Fertilizers:

   • Different types of fertilizers are mentioned, including phosphate fertilizers (phosphoric acid), nitrogenous fertilizers (nitre, saltpetre, ammonium sulphate), and potassic fertilizers (potassium sulphate, potassium nitrate, wood ashes). These fertilizers provide specific nutrients needed for plant growth.

8. Methods for Restoring Fertility:

   • Strategies for restoring soil fertility include crop rotation, fallowing (allowing the land to remain uncultivated for a season), and mixed farming (cultivating two or more crops in the same field). These practices help maintain soil health and prevent nutrient depletion.

Types of Soils Erosion

Usually, there are two types: 

1. Water Erosion

• The important types of erosion are sheet, rill and gully. In sheet erosion thin layer of soil is removed by the water during heavy rains from the hill slopes. Bare fallow land or cultivated land whose plant cover has been thinned. If the erosion continues unchecked, numerous finger-shaped grooves may develop all over the area as a result of the silt-laden run-off. This is called rill erosion. 
• The rills may deepen and enlarge into gullies if the erosion continues further. U-shaped gullies are formed where the sub-soil is resistant, while V-shaped gullies are formed where the underlying soil is softer and easily eroded than the upper one.
  
  Soil erosion by water
• Sheet erosion is common in Himalayan foothills, over north-eastern parts of Peninsula, in Assam, Shayadris and the Eastern Ghats. Rill erosion is found in Bihar, Uttar Pradesh, Madhya Pradesh and in semiarid parts of Peninsula in Maharashtra, Karnataka and Andhra Pradesh and Tamilnadu. Gully erosion has resulted into the chaos of northern Haryana and Punjab and badlands of Madhya Pradesh, Rajasthan and Uttar Pradesh.
• The rivers in floods and the tidal waters of the sea also cause soil erosion or considerable damage to the soil along the coast. Steep slopes, heavy rainfall and bare land surface quicken the rate of erosion.

2. Wind Erosion: Wind erosion is mainly confined to arid and semi-arid areas devoid of vegetation. Wind, particularly during sandstorms, lifts and carries away fertile soil. Rajasthan and the adjoining areas of Haryana, Uttar Pradesh and Gujarat show this type of soil erosion.

Soil erosion by wind

Causes of Soil Erosion

Human and animal interference in a variety of ways lead to soil erosion. Deforestation, overgrazing of pastures, shifting cultivation, faulty method of cultivation, ruts in roads, ditches, improperly constructed terrace outlets (along which running water is concentrated) etc., are responsible for soil erosion.

Erosion due to deforestation have resulted in chaos of Punjab and Haryana, and ravines of M.P., Rajasthan and UP. Erosion due to overgrazing is common in J&K., H.P., hilly areas of Madhya Pradesh, Rajasthan and low rainfall areas of Maharashtra, Karnataka and Andhra Pradesh. Shifting cultivation is responsible for soil erosion in Assam, Meghalaya, Mizoram, Tripura, Nagaland, Kerala, Orissa, Andhra Pradesh and parts of Madhya Pradesh.

Consequences of Soil Erosion

Soil erosion leads to loss of soil and badly affects the runoff. It causes:

(i) Heavy floods in rivers;

(ii) Lowering down of subsoil water level;

(iii) Reduction of soil fertility;

(iv) Silting of streams and water courses;

(v) Disappearance and downfall of civilisation.

SOIL CONSERVATION

Soil conservation is an effort made by man to prevent soil erosion in order to retain the fertility of soil. It may not be possible to stop soil erosion entirely.
Any erosion such as gullies already formed should be tackled by construction of dams or obstructions. Ploughing and tilling of land should be done along contour levels so that the furrows run across the slope of land. Bunds should be constructed according to contours. Trees reduce the force of straight winds and obstruct blowing away of dust particles. Plants, grass and shrubs reduce the speed of flowing water. Therefore, such vegetable cover should not be removed indiscriminately, where it does not exist, steps should be taken to plant it.
Natural vegetation cover prevents soil erosion in three ways:
(i) The roots of plants bind together the particles of soil;
(ii) Plants check the force of wind so that it cannot blow away the soil particles, and
(iii) Plants lessen the force of rain as it reaches the ground.
 

Measures for Soil Conservation

(i) Planting cover crops such as grasses, on uncultivated land. Trees should be planted along hill slopes.

(ii) Adoption of correct farming techniques such as contour ploughing and strip cropping. Strip cropping is the practice of planting alternate rows of close-growing plants such as beans and peas, with open-growing crops such as corn. This practice prevents wind erosion.

(iii) Terracing, the practice of cutting steps in hillside, to create level land for cultivation.

(iv) Construction of check dams on steep slopes which prevent gully erosion and spread of gullies.

(v) Creation of windbreaks by planting lines of trees, hedges, or fences which obstruct the path of wind, thereby reducing its speed and hence reducing soil erosion.

(vi) Controlling grazing of pastures.

(vii) Suspending cultivation for one season and more so as to help the soil recover its fertility.

(viii) Realising the deteriorating situation of soil erosion and its impact on the country's economy, a Central Conservation Board was created in 1953 to coordinate the soil conservation include contour bunding, bench terracing, nalla plugging, land levelling, and other engineering and biological measures such as afforestation, grassland development etc.

(ix) Eight regional research-cum-demonstration centres have been established for the study of problems of soil and water conservation. In addition, the Desert Afforestation and Research Station has been set up at Jodhpur for studying the desert problem.

Different types of soils found in India

There is a great variation in the distribution of soil in India due to differences in the bedrock and climate. 

Major soil groups are distributed in India as follows:-
1. Alluvial soils in the great plains, river valleys and deltas.
2. Black cotton soils over the Deccan lavas in Maharashtra, Gujarat, Madhya Pradesh, Karnataka, Tamil Nadu, Uttar Pradesh and Rajasthan.
3. Laterites in Madhya Pradesh, West Bengal, Eastern Ghats, Sahyadris, Maharashtra, Karnataka, Kerala, Assam, Rajmahal hills.
4. Red soils in parts of Tamil Nadu, Karnataka, Maharashtra, Andhra, Goa, Madhya Pradesh, Orissa.
5. Saline Alkaline in Maharashtra, Gujarat, Karnataka, Andhra and Tamil Nadu (arid and semi-arid areas).
6. Arid desert mainly in Rajasthan.
7. Peaty in Kerala and marshy areas of Orissa, Western Bengal, Tamil Nadu.

Measures for soil conservation include contour ploughing and terracing, bunding afforestation, control of grazing, etc. to prevent water erosion, vegetal covers and windbreaks against wind erosion and dumping rocks, construction of jetties, etc, against sea erosion.

How can alkaline and acid soil be reclaimed?

The acid and salt affected soils need special nutrient management for reclamation and subsequent crop production. The Central Soil Salinity Research Institute at Karnal has done commendable work in reclamation of these soils. The liming of acid soils according to their requirement corrects the nutrient deficiencies and toxicity. The use of partially water soluble phosphate fertilisers is recommended. The judicious recycling of available organic sources of manure should be dovetailed to increase soil productivity and partially supplement the nutrient needs of crops.

Methods of soil conservation

1. Agronomic Measures

These include various methods of crop cultivation to ensure protection of the top soil.
(i) Contour Farming
(ii) Mulching
iii) Strip Cropping
(iv) Mixed Cropping

2. Mechanical Measures of Erosion Control

These includes excavation of different types of ditches and construction of terraces for the removal of excess of water from the field. Construction of dams for checking the erosive velocities of the water.
(i) Basin Listing
(ii) Sub-soiling
(iii) Contour Bunding
(iv) Graded Bunding or Channel Terraces
(v) Bench Terracing.

Soil Conservation programmes in India
(i) Soil conservation in the catchments of river valley project.
(ii) Integrated Watershed Management in the catchments of flood-prone rivers.
(iii) The scheme for reclamation and development of ravenous area.
(iv) The scheme for control of shifting cultivation.

Q. Where are Point Calimere, Gulf of Mannar, Itanagar located?
Ans.
(a) Point Calimere is in Tanjavur district on Tamil Nadu coast.
(b) Gulf of Mannar separates the Indian main-land from the island of Sri Lanka.
(c) Itanagar is the capital of Arunachal Pradesh.

Soils of Peninsular India
They are mostly diluvial soils formed by decomposition of rocks such as black cotton, red laterite, saline, alkaline, alluvial and mixed red, yellow and black soils.

National parks and game sanctuaries
Game sanctuaries are meant to preserve particular animals and birds where as national parks protect the entire eco-system including flora and fauna of all species.

DIFFERENT TYPES OF PLATEAUS
Plateaus are classified as:
(a) Inter-mountane (between mountains).
(b) Piedmont (between mountains and sea).
(c) Continental (extensive table land rising abruptly from sea or low lands like the Deccan plateau).

Different types of Plateau

Rift valley 
Rift valleys are steep-sided depressions on the earth’s surface resulting from geological faults due to fracturing of the surface. Narmada valley is a rift valley.

Flood plains and coastal plains 
Flood plains are formed by deposition of materials by rivers e.g., Ganga plains. Coastal plain occur by upliftment of continental shelf e.g., eastern and western coastal strips.           `