Lecture 26 : Soil Formation, Classification and Distribution

Topics Covered in this Lecture

  1. Soil :- Composition and Structure
  2. Properties of Soil
  3. Soil Formation :- Factor and Processes
  4. Classification of Soil
  5. Global Distribution of Soil

Introduction :-

Soil is the thin top layer on the earth’s crust comprising rock particles mixed with organic matter. Penology is the study of soils in their natural environment. Pedogenesis is the natural process of soil formation that includes a variety of processes such as weathering, leaching, clarification, etc.

The soil formation is mainly related to the parent rock material, surface relief, climate, and natural vegetation. The soil is formed by the breaking down of rocks by the action of wind, water, and climate. This process is called weathering.

Soil types sandy, loamy, and clayey:

The soil is classified on the basis of the proportion of particles of various sizes.

  • If the soil contains a greater proportion of big particles it is called sandy soil.

  • If the soil proportion of fine particles is relatively higher, then it is called clayey soil.

  • If the amount of large and fine particles is about the same, then the soil is called loamy.

  • water can drain quickly through the spaces between the sand particles so, sandy soils, tend to be light, well-aerated, and dry.

  • clay particles, being much smaller, pack tightly together, leaving little space for air. Unlike sandy soil, water can be held in the tiny gaps between the particles of clay.

  • So, clay soils have little air. But they heavy as they hold more water than the sandy, clay and another type of soil particles know as silt. Silt occurs as a deposit in river beds. The size of the silt particles in between those if sand and clay the loamy soil also has humans in it. It has a theme right water holding capacity for the grout.

Suitability of soils

  • Clayey and loamy soils : cereals like wheat, and gram.

  • Loamy soils : lentils (masoor) and other pulses, which drain water easily.

  • Sandy loam or loan : cotton.


  • A Vertical section through different layers of the soil is called the soil profile. Each layer differs in feel (texture) , colour, depth and chemical composition.

  • These layers are referred to as horizons. A soil horizon is a layer generally parallel to the soil surface, whose physical characteristics differ from the layers above and beneath.

  • Horizons are defined in most cases by obvious physical features, chiefly colour and texture. The uppermost horizon is generally dark in color as it is rich in humus and minerals.

  • The humus makes the soil fertile and provides nutrients to growing plants. This layer is generally soft porous and can retain more water. It is called the topsoil or the A-horizon.

  • The next layer has a lesser amount of humus but more of minerals. This layer is generally harder and more compact and is called the B-horizon or the middle layer. The third layer is the C-horizon, which is made up of small lumps of rocks with cracks.

Factors influencing soil formation :

Parent rock : determines colour, texture, chemical properties mineral, continent, permeability.

Relief :- altitude and slope, determine accumulation of soil.

Flora, fauna and micro- organism :- affect the rate of humans formation.

Climate :- temperature, rainfall influence rate of weathering and humus.

Time :- determines thickness of soil profile.  

Explanation :-

1. Parent material:

This refers to the mineral material or organic material from which the soil is formed will carry the characteristics of its parent material such as color, texture, structure, mineral composition, and so on. For example, if soils are formed from an area with large rocks aren’t rocks (parent rocks) of red sandstone, the soils will also be red in color and have the same feel as it’s parent material.

2. Time:

Soils can take many years to form. younger soils have some characteristics from their parent material, but as they age, the addition of organic matter, exposure to moisture, and other environmental factors may change its features. With time, they settle and are buried deeper below the surface, taking time to transform. Eventually, they may change from one soil type to another.

3. Climate:

This is probably the most important factor that can shape the formation of soils. Two important climatic components, temperature, and precipitation are key. They determine how quickly weathering will be, and what kind of organic materials may be available on and inside of the soils. Moisture determines the chemical and biological reactions that will occur as the soils are formed. A warmer climate with more rainfall means more vegetative cover and more animal action. It also means more runoff, more percolation, and more water erosion. They all help to determine the kind of soils in an area.

4. Relief

This refers to the landscape position and the slopes it has. Steep, long slopes mean water will run down faster and potentially erode the surfaces of slopes. The effect will be poor soils on the slopee and richer deposits at the foot of the slopes. Also, slopes may be exposed to more direct sunlight, which may dry out soil moisture and render less fertile.

5. Organisms:

The source and richness of organic matter are down to the living things (plants and animals) that live on and in the soils. Plants, in particular, provide lots of vegetative residues that are added to soils. Their roots also hold the soils and protect them from wind and water erosion. They shelter the soils from the sun and other environmental conditions, helping the soils to retain the needed moisture for chemical and biological reactions. Fungi, bacteria, insects, earthworms, and burrowing animals help with soil aeration. Worms help break down organic matter and aid decomposition. Animal droppings, dead insects, and animals result in additional decaying organic matter. Microorganisms also help with mineral and nutrient cycling and chemical reactions.

Major Soil Types of India:

Geologically, Indian soils can broadly be divided into soils of peninsular India and soils of extra-peninsular India. The soils of Peninsular India are formed by the decomposition of rocks in situ, ie. directly from the underlying rocks. Soils of Peninsular India are transported and re-deposited to a limited extent and are known as sedentary soils. The soils of the Extra-Peninsula are formed due to the depositional work of rivers and wind. They are very deep. They are often referred to as transported or azonal soils.

Major soil types:

1. Alluvial soils

2. Black soils

3. Red soils

4. Laterite soils,

5. Forest and Mountain soils

6. Arid and Desert soils

7. Saline and alkaline soils

8. Peaty and Marshy soils

Alluvial Soils :-

Alluvial soils are formed mainly due to silt deposited by Indo-Gangetic-Brahmaputra rivers and in coastal regions due to wave action. They are the largest soil group covering about 15 lakh sq km or about 46 percent of the total area. they support more than 40% of 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.

  • 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.

  • These soils are constantly replenished by the recurrent floods in the khadar region.

Chemical properties of Alluvial Soils

  • The proportion of nitrogen is generally low.

  • The proportion of Potash, phosphoric acid, and alkalies are adequate.

  • the proportion of iron oxide and lime vary within a wide range.

Distribution of Alluvial Soils in India –

  • They occur all along the Indo-Gangetic-Brahmaputra plains except in a few places where the top layer 1s covered by desert sand.

  • They also occur in deltas of the Mahanadi, the godavar?, 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 Aluvial Soils –

  • They are mostly flat and regular soils and are best suited for agriculture.

  • these are best suited to irrigation and respond well to the canal and well/tube-well irrigation.

  • yield splendid crops of rice, wheat, sugarcane, tobacco, cotton, Jute, maize, oilseeds, vegetables, and fruits.

Black Soils –

  • 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).

Characteristics of Black Soils

  • A typical black soil is highly argillaceous [Geology (of rocks or sediment) consisting of or containing clay] with a large clay factor, 60% or more.

  • In general, black soils of uplands are of low fertility while those in the valleys are very fertile.

  • The black soil is highly retentive of moisture. It swells greatly on accumulating moisture. strenuous effort is required to work on such soil in the rainy season as it gets very sticky.

  • In summer, the moisture evaporates the soil shrinks and is seamed with broad and deep cracks. the lower layers can still retain moisture. the cracks permit oxygenation of the soil to sufficient depths and the soil to sufficient depths and the soil has extraordinary fertility.

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.

Chemical composition of Black Soils

  • 10 percent of alumina,
  • 9-10 percent of iron oxide,
  • .6-8 percent of lime and magnesium carbonates.
  • Potash 18 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 crops. Hence these soils are called regur 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.

  • This soil has been used for growing a variety of crops for centuries without adding fertilizers and manures, with little or no evidence of exhaustion.

Red Soils

Red soils along with its minor groups form one of the largest soil groups of India. The main parent rocks are crystalline and metamorphic rocks like acid granites, gneisses, and quartzite.

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.

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.

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 Soils –

Laterite soils are mostly the end products or 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) or sol whereby lime and silica are leached away and a soil rich in oxides of iron and aluminium compound 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.

characteristics of Laterite Soils:-

Laterite and lateritic soils provide valuable building material. These soils can be easily cut into cakes but hardens like iron when exposed to ar. As it is the end-product of weathering, it cannot be weathered much further and is durable.

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 a higher content of humus.

Distribution of Laterite – Lateritic Soils –

A continuous stretch of laterite soil is found on the summits of Western Ghats at 1000 to 1500 m and northeast India. They are also found in the regions of the heavy rain of the northeast.

Crops in Laterite – Lateritic Soils –

Laterite soils are poor 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, arecanut, etc.

Forest and Mountain Soils –

These soils occupy about 2.85 lakh sq km or 8.679% of the total land area of India. They are mainly heterogeneous soils found on the hill slopes covered by  forests.

Chemical properties of Forest – Mountain Soils –

  • The forest soils are very rich in humus.
  • They are deficient in potash, phosphorus, and lime.
  • They require good deal of fertilizer for high yields.

Crops in Forest – Mountain Soils –

They are suitable for plantations of tea, coffee, spices, and tropical fruits in the peninsular forest region. Wheat, maize, barley, and temperate fruits are grown in the Himalayan forest region.

Arid -Desert Soils –

The desert soils consist of Aeolian sand (90 to 95 percent) and clay (6 to 10 percent). They cover a total area of 1.42 lakh sq km (4.32o). The presence of sand inhibits soil growth. Desertification of neighboring soils is common due to intrusion of desert sand under the influence of wind [Aeolian sand].

Distribution of Arid – Desert Soils

Occur in arid and semi-arid regions of Rajasthan, Punjab, and Haryana. The sand here is blown from the Indus basin and the coast by the prevailing south-west monsoon winds. Sandy soils without clay factor are also common ln coastal regions of Odisha, Tamil Nadu, and Kerala.

Chemical properties of Arid- Desert Soils –

  • They are usually poor n organic matter.

  • Some desert soils are alkaline with varying degree of soluble salts like calcium.

  • Calcium content increases downwards and the subsoil has ten times more calcium.

  • The phosphate content of these soils is as high as in normal alluvial soil.

  • Nitrogen is originally low but some of it is available in the form of nitrates.

Crops of Arid – Desert Soils –

Phosphates and nitrates make these soils fertile wherever moisture is available. There is a possibility of reclaiming these soils if proper irrigation facilities are available. In large areas, only the drought-resistant and salt-tolerant crops such as barley, cotton, millets, maize, and pulses are grown.

Saline and Alkaline Soils –

This is an outcome of excessive irrigation which promotes Capillary action in saline and Alkaline Soils, the top soil is impregnated (soak or saturate with a substance) with saline and alkaline efflorescence (become covered with salt particles and forms hard pan).. In regions with high sub-soil water table, injurious salts are transferred from below by the capillary action as a result of evaporation in dry season.

Distribution of Saline – Alkaline Soils –

  • Saline and Alkaline Soils occupy 68,000 sq km of area.

  • These soils are found in canal irrigated areas and in areas of the high sub-soil water table.

  • Parts of Andhra Pradesh, Telangana, Karnataka, Bihar, Uttar Pradesh, Haryana, Punjab side effects of improper or excess irrigation), Rajasthan and Maharashtra have this kind of soils.

  • The accumulation of these salts makes the soil infertile and renders it unfit for agriculture.

  • In Gujarat, the areas around the Gulf of Khambhat are affected by the sea tides carrying Salt-laden deposits. Vast areas comprising the estuaries of the Narmada, the Tapi, the Mahi, and the Sabarmati have thus become infertile.

  • Along the coastline, saline sea waters infiltrate into coastal regions during storm surges(when cyclones make landfall) and make the soil unfit for cultivation. The low-lying regions of coastal Andhra Pradesh and Tamil Nadu face this kind of soil degradation.

  • Farmers are advised to add GYPSUM to the soil. Gypsum being acidic reacts and removes the alkaline salts. Gypsum also increases the percolation of water into the ground.

Peaty-Marshy Soils

These are soils With large amount of organie matter.

The most humid regions have this type of soil. They are black, heavy and highly acidic.

Distribution of Peaty – Marshy Soils

Kottayam and Alappuzha districts of Kerala where it is called kari. Also occur in the coastal areas of Odisha and Tamil Nadu, Sunderbans of West Bengal, in Bihar and Almora district of Uttrakhand.

Chemical Properties of Peaty – Marshy Soils

  • They are deficient in potash and phosphate.

Crops of Peaty – Marshy Soils –

Most of the peaty soils are under water during the rainy season but as soon the rains cease they are put under paddy cultivation.

Problems of Indian Soils:

  • The soils of various parts of the country suffer various issues such as Soil erosion (Himalayan region, Chambal Ravines, etc.), deficiency infertility (Red, lateritic and other soils), desertification around.

  • Thar desert, rain-shadow regions like parts of Karnataka, Telangana, etc.), water logging (Punjab-Haryana plain) salinity and alkalinity (excessively irrigated regions of Punjab, Haryana, Karnataka etc.).

  • wasteland, over exploitation of soils due to increase in population and rise in living standards and encroachment of agricultural land due to urban and transport development.

  • Soil degradation in India is estimated to be occurring on 147 million hectares (Mha) of land, including 94 Mha from water erosion, 16 Mha from acidification, 14 Mha from flooding.

  • 9 MHA from wind erosion, 6 Mha from salinity, and 7 Mha from a combination of factors. This is extremely serious because India supports 18% of the world’s human population and 15% of the world’s livestock population, but has only 2.4% of the world’s land area.

  • Despite its low proportional land area, India ranks second worldwide in farm output. Agriculture, forestry, and fisheries account for 17% of the gross domestic product and employ about 50% of the total workforce of the country.

Causes of soil degradation are both natural and human-induced. Natural causes include earthquakes, tsunamis, droughts, avalanches, landslides, volcanic eruptions, floods, tornadoes, and wildfires.

Human-induced soil degradation results from land clearing and deforestation, inappropriate agricultural practices, improper management of industrial effluents and wastes, over-grazing. careless management of forests, surface mining, urban sprawl, and commercial/industrial development. Inappropriate agricultural practices include excessive tillage and use of heavy machinery, excessive and unbalanced use of inorganic fertilizers, poor irrigation and water management techniques, pesticide overuse, inadequate crop residue and/or organic carbon inputs, and poor crop cycle planning. Some underlying social causes of soil degradation in India are land shortage, decline in per capita land availability, economic pressure on land, land tenancy, poverty, and population increase.

Soil conservation –

  • It is the name given to a handful of techniques aimed at preserving the soil. Soil loss and loss of soil fertility Can be traced back to a number of causes including over-use, erosion Salinization, and chemical contamination.

  • Unsustainable subsistence farming and the slash and burn clearing methods used in some less developed regions can often cause deforestation, loss of soil nutrients, erosion on a massive scale, and sometimes even complete desertification.

  • Soil erosion removes the topsoil that is necessary for organic matter, nutrients, micro-organisms that are required for plants to grow and shine. Soil conservation is one such step that protects the soil from being washed away.

  • The soil then ends up in aquatic resources bringing in pesticides and fertilizers used on agricultural land. Healthy soil is important for plants to grow and flourish. There are several ways to conserve  Soil that Can be done through agricultural practices or measures.

Methods and Techniques of Soil Conservation

Many different techniques have been invented throughout the years with the aim of preserving the nutrients level of the soil and preventing erosion.

  • Contour Ploughing-  contour plowing involves plowing grooves into the desired Farmland, then planting the crop furrows in the grooves, and following the contours. It a very effective way for farmland on slopes to prevent run oft improve crop yields.

  • Terrace farming – terracing is a method of carving multiple, flat leveled areas into hills. Steps are formed by the terrace which is surrounded by a mud wall to prevent runoff and hold the soil nutrients in the beds. Very popular in Asia for planting rice.

  • Runoff control – this is the practice of planting trees, shrubs, and ground cover around the perimeter of your farmland which impedes surface flows and keeps nutrients in the farmed soil using the grass way is a specialized way of handling perimeter runoff that uses surface friction to channel and dissipate runoff.

  • Windbreaks – rows of tall trees are used in dense patterns around the farmland and prevents wind erosion. Evergreen trees can provide year-round protection but deciduous trees can be adequate as long as foliage is apparent during the season when the soil is bare.

  • Cover crops/crop rotation – cover crops such as turnips and radishes are rotated with cash crops in order to blanket the soil all year – round and produce green manure the replenishes nitrogen and other critical nutrients. Using cover crops can also suppress weeds.

  • Soil conservation farming – a mixture of farming methods intending the mimic the biology of virgin land. These practices can be used to prevent erosion and even restore damaged soil and encourage plant growth. Eliminating the use of nitrogen fertilizer and fungicides can increase yield and protect crops from drought and flooding.

  • No-till farming-This is the method of growing crops year-round without changing the topography of the soil by tilling or contouring. This technique increases the amount of water that penetrates the soil ad can increase organic matter of the soil which leads to Larger yields.
  • Green Manures- Green manures are a few different crops that can be grown, not for produce or food usage, but grown in order to fertilize the farmland on which it grows. This method can improve the soil structure and suppresses the growth of weeds.

  • Salinity Management – When water evaporates from the soil, it leaves behind its salt. This can lead to damage to soil and nutrient loss. Using humic acids can prevent this or growing crops like saltbush can rejuvenate the soils and replace lost nutrients. High levels of salt in the soil can often be caused by changes made to the water table by damming and other causes.

  • Mulching- It is the practice of covering fields with (organic matter Mulch is a protective covering (as of sawdust, compost, or paper) spread or left on the ground to reduce evaporation, maintain even soil temperature, prevent erosion, control weeds and enrich the soil.

  • Vermicomposting – it is the cultivation of annelid worms (such as earthworms or bloodworms) especially for use as bait or in composting. Earthworms provide great benefits of farmland due to the way they burrow under the ground and provide more are for water to rest after it has infiltrated the soil. When these worms excrete ejecta, this sits in the soils and gives the crops many nutrients which are absorbed via the roots of the plants. Earthworm casts contain a vast amount more nutrients than any natural soil in the world, and for that reason should be invited into the soils of farmland to help prevent erosion and will lead to larger crop yields.

  • Mineralization-To help crops reach higher yields or full potential, sometimes crushed rock or chemical supplements are added to the farmland, this helps combat mineral depletion. Normally used after flooding, it brings substantial amounts of sediment which can damage the nutrient level of the soil.

  • Natural Farming – This method takes advantage of natural and indigenous Microorganisms to produce fertile soils that yield high output and gets rid of the need to use herbicides or pesticides. An improvement in soil health and output is what keeps this method used in the respective areas.

  • Dry Farming- In areas with a very low amount of rainfall, crops which require very little water should be grown, this will lead to the preservation of the natural levels of moisture and nutrients in the soil.

  • Rain Gardens – A rain garden is a shallow depression in the land which holds and collects running water from impervious surfaces and prevents eros1on while saving the nutrients that inevitably get washed away. This also gives you a good bed to grow wetland plants.

  • Re-establish Forest Cover – A dense number of trees in a forest leads to a vast network of deep roots that offer a long-term solution to soil erosion, another benefit is the windbreak that these trees can provide.

  • Maintaining PH levels of soil- Contamination of soils due to acid rains and other pollutants can lead to loss of soil fertility. Use a Ph indicator monthly to check the levels of acids in the soil and treat the soils with eco-friendly chemicals to prevent a loss of crops and low yields.

  • Indigenous Crops-The growth of indigenous crops is a good way to conserve soil, as the plants have a natural need for the nutrients in the soil in your area, they help to prevent soil erosion. If you grow non-indigenous crops a recommended tip is to plant indigenous crops around them, in order to prevent soil erosion.

  • Prevent Overgrazing-Try not to let overgrazing happen by moving herds around Often. If overgrazing Occurs, plant hardier and more nutritious species of forage in order to rebuild the soil. You can also harvest these crops and feed them to the grazers during the winter season.

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