Soil: Composition, Characteristics & Types of Soils

Table of Contents
1. Composition of Soil
2. Top Soil and Sub Soil
3. Formation of Soil
4. Characteristics of Different Soils
5. Soil Fertility
6. Types of Soil Erosion
7. Causes and Consequences of Soil Erosion
8. Soil Conservation
9. Different Types of Soils Found in India
10. Reclamation of Acidic and Salt be reclaimed
11. Methods of Soil Conservation
12. Additional Information
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Composition of Soil

Soil is the loose, heterogeneous material that forms the uppermost layer of the Earth's crust. It is made up of mineral particles, organic matter, water and air, and it supports plant life. Soil contains both decomposed plant and animal remains and inorganic mineral particles derived from the parent rock. The proportions of these constituents vary from place to place and determine soil properties and fertility.

The main constituents of soil, present in varying proportions, are:

• Silica - occurs mainly as small crystalline grains and is the chief constituent of sand. It is produced by the physical breakdown of rocks (mechanical weathering).
• Clay - a mixture of silicates containing minerals of iron, potassium, calcium, sodium and aluminium. Clay particles absorb water and swell; they impart plasticity and cohesion to soil.
• Chalk (calcium carbonate) - supplies calcium, an important plant nutrient and a component that influences soil reaction (pH).
• Humus - organic matter formed by the decomposition of plant remains, animal manure and other organic residues. Humus darkens the soil, increases its moisture-holding capacity, improves soil structure and is essential for fertility.

Top Soil and Sub Soil

Topsoil is the uppermost layer of soil and is the most important for plant growth. It is typically rich in organic matter, microorganisms, insects and earthworms. Topsoil depth varies from place to place and develops very slowly; it may take many years to form but can be lost rapidly by erosion. Good topsoil is essential for good crops.

Subsoil lies below the topsoil and consists largely of the parent material from which soil forms. Subsoil contains some plant nutrients and moisture but is generally less fertile and less biologically active than topsoil. Conversion of subsoil into productive topsoil is a slow process that may take many years.

Topsoil and subsoil

Formation of Soil

Soil formation (pedogenesis) is the result of physical, chemical and biological processes acting on parent rock and sediments over time. The principal processes are:

• Weathering - the disintegration and decomposition of rocks. Mechanical weathering breaks rocks into fragments by frost, temperature changes, root action and abrasion. Chemical weathering alters rock minerals by reactions with water, oxygen, carbon dioxide and acids.
• Deposition - accumulation of rock particles transported and laid down by rivers, glaciers, wind, marine currents and tides.
• Biochemical processes - biological activity (plant roots, soil organisms, burrowing animals) accelerates mechanical and chemical weathering; decomposing vegetation produces organic acids which further break down minerals.

Soil formation depends on several interrelated factors:

• Nature of the parent rock (mineral composition supplies inorganic nutrients).
• Climate - temperature and rainfall; heavy rainfall promotes leaching of soluble minerals.
• Vegetation - provides organic matter and protects soil from erosion.
• Topography - slope and relief; soils are usually thinner and less developed on steep slopes.
• Time - length of time the above factors have been active.

Characteristics of Different Soils

1. Sandy Soil (light soil)

Sandy soils contain a high proportion of sand (>60%) and usually less than 10% clay. Particles are loosely bonded, making the soil highly permeable to air and water. This provides good aeration for roots but causes rapid drainage and quick drying. Sandy soils are easy to cultivate and suitable for many fruits and vegetables. Their fertility improves markedly with the addition of humus (decayed organic matter).

2. Clayey Soil

Clayey soils have a high proportion of fine clay particles. They become sticky when wet and hard when dry. Poor aeration and compaction can restrict root growth; they may become waterlogged under excess moisture. Addition of sand and lime or chalk can improve texture and drainage. Very clay-rich soils are often called heavy soils.

3. Loam

Loam is a desirable agricultural soil composed of a balanced mix of sand, silt and clay together with adequate humus. It combines the drainage and aeration of sandy soils with the moisture- and nutrient-holding properties of clay. Loams are generally fertile and ideal for farming and gardening. Variants include sandy loam (higher sand content) and clayey loam (higher clay content).

4. Alluvial Soil

Alluvial soils are formed by the deposition of sediments by rivers and are among the most extensive and agriculturally important soils. In the plains they are deposited by major Himalayan rivers (Sutlej, Ganga and Brahmaputra) and their tributaries, and by peninsular rivers in deltas and coastal plains.

Alluvial soils contain varying proportions of sand, silt and clay and are often very fertile. Geologically they are classified into:

• Khadar - newer, frequently deposited alluvium found close to river channels; generally lighter in colour and texture.
• Bhanger - older alluvium, often more clayey and darker in colour; forms higher ground away from active river channels.

Alluvial soil

Alluvial soils are widely used for agriculture and contain adequate potash, phosphoric acid and lime in many areas. Their fertility is due to the diversity of rock material carried from different regions, fine-grained texture and ease of tillage.

Two main problems associated with alluvial soils are:

• Tendency to allow rapid percolation so that soils may not retain moisture for crops that need constant root-zone moisture in regions with infrequent rainfall.
• Relative deficiency in nitrogen and humus in many areas, which requires replenishment through manuring and fertilisation.

Alluvial soils respond well to irrigation and are suitable for crops such as rice, wheat, sugarcane, cotton, jute, maize, oilseeds, tobacco, vegetables and fruits.

5. Black Soils (Regur or Cotton Soils)

• These are dark to black in colour and are best known for cotton cultivation; hence called cotton soils. They cover about 5.46 lakh sq. km and are typical of the Deccan trap (basalt) region.
• They are formed from the weathering of basaltic lava flows and occur over plateaus of Maharashtra, Saurashtra, Malwa and parts of southern Madhya Pradesh, extending eastwards in the Godavari and Krishna valleys and in parts of Karnataka and Tamil Nadu.
• Black colour is attributed to the presence of iron and aluminium compounds, titano-magnetite, accumulated humus and colloidal hydrated silicates.
• These soils are rich in iron, lime and potash but generally deficient in nitrogen and phosphorous and organic matter.
• They are fine-textured, clayey and become sticky when wet; they crack deeply on drying which assists natural aeration and facilitates absorption of atmospheric nitrogen by some processes.
• Highly moisture-retentive, black soils are well-suited to dry farming but can be unsuitable for some types of heavy irrigation due to soluble salt concentration.
• Suitable crops include cotton, linseed, castor, safflower, varieties of cereals, vegetables and some fruits.

Black soil

6. Red Soils

Red soils occur over large parts of the peninsular region and are estimated to cover about 5-18 lakh sq. km (distribution varies among sources). They reach up to the Rajmahal Hills in the east, Jhansi in the north and Kutch in the west. The red soils generally encircle the black soil region and cover areas such as the Chhotanagpur plateau, Odisha, eastern Madhya Pradesh, Telangana, Nilgiris, Tamil Nadu plateau and Karnataka.

Red soil

Red colour is due to iron oxide. These soils are typically light-textured, porous and friable with low quantities of carbonates. They are generally deficient in phosphoric acid, organic matter, lime and nitrogen. Their consistency, colour, depth and fertility vary widely: upland red soils are often thin and gravelly, while lowland red soils in valleys and plains may be deeper and more fertile. Crops vary accordingly - millets and groundnut on poorer uplands, rice and vegetables in richer lowlands.

7. Laterite Soils

Laterite soils form in regions of intense leaching under heavy rainfall (often >200 cm). During prolonged weathering in humid tropics, silica and carbonates are leached, leaving soils rich in iron and aluminium oxides which give a red colour. These soils cap uplands and are common in the Western Ghats and other high-rainfall upland areas.

Laterite soil

Laterite soils are generally poor in nitrogen, phosphoric acid, potash, lime and organic matter. They support scrub and grassland in many places but respond well to manuring; where improved, they can support crops such as rice, ragi, tapioca and cashew.

8. Forest and Mountain Soils

These soils occur over hilly and mountainous regions and are highly variable depending on climate and parent rock. They are often described as soils in the making. Humus content is typically high in forest soils but may be more raw and acidic at higher elevations. Examples include podzolic soils in temperate hill regions.

Forest and mountain soil

Forest soils are generally low in potash, phosphorus and lime and often require manuring for agriculture. Where rainfall is adequate, they can be fertile and support plantation crops (tea, coffee, spices) and tropical fruits; in temperate hill areas they support fruits, maize, wheat and barley.

9. Arid and Desert Soils

Arid and desert soils occur under arid and semi-arid climates in north-western regions. They occupy about 1.42 lakh sq. km, notably in Rajasthan, south Haryana, north Punjab and the Rann of Kutch. The Thar Desert covers approximately 1.06 lakh sq. km.

Arid and desert soil

These soils are sandy (often with wind-blown loess), contain high percentages of soluble salts and have very low organic matter and moisture. They may be rich in phosphates but poor in nitrogen. Where irrigation has been introduced (for example parts of Ganganagar), cereal and cotton production has increased.

10. Saline and Alkaline Soils

Saline and alkaline soils occur in arid and semi-arid tracts and in areas with a high water table where capillary rise concentrates salts near the surface. They are locally called reh, kallar, rakar, usar, karl and chopan. These soils are typically infertile because of excess soluble salts (sodium, calcium, magnesium) in the surface layers.

Saline and alkaline soil 

Texturally they range from sandy to loamy sand. Saline soils contain free soluble salts, whereas alkaline soils have high proportions of sodium salts that make the soil strongly alkaline. Reclamation methods include proper irrigation and drainage, application of amendments such as lime or gypsum (where appropriate) and cultivation of salt-tolerant crops. With reclamation, crops like rice, wheat, cotton, sugarcane and tobacco can be grown.

11. Peaty and Marshy Soils

Peaty soils are formed under waterlogged, humid conditions with accumulation of large quantities of organic matter (peat). In India, peaty soils occur in small areas such as Kottayam and Alleppey districts of Kerala. They may contain soluble salts but are often deficient in phosphate and potash. Peaty soils are suitable for paddy cultivation after proper management.

Peaty and marshy soil

Marshy soils develop in coastal and low-lying regions subject to waterlogging and anaerobic conditions; they are found in coastal Odisha, West Bengal, Tamil Nadu, parts of Bihar and in some hill valleys. These soils are often unsuitable for conventional cultivation but support special wetland vegetation.

Soil Fertility

Factors responsible for deficiency of Indian soils

1. Loss of soil nutrients through crop removal
   Harvest removes nutrients absorbed during plant growth. Continuous cropping without replenishment depletes soil nutrients.
2. Leaching
   Heavy monsoon rains can wash soluble nutrients downward out of the root zone. Sandy soils and bare soils are particularly vulnerable to leaching.
3. Soil erosion
   Erosion by water and wind removes the fertile topsoil, reducing productive capacity.
4. Insufficient manuring and fertilisation
   Without optimal application of nutrients (nitrogen, phosphorus, potassium), yields decline.
5. Inherent nutrient deficiencies
   Many Indian soils are deficient in nitrogen, phosphorus and sometimes potash; organic matter (humus) is often low.

Measures to maintain and restore fertility:

• Use of organic manures - farmyard manure, compost, green manure, crop residues and animal wastes to improve humus and soil structure.
• Use of fertilisers as required - nitrogenous (e.g., ammonium sulphate), phosphatic (e.g., phosphoric fertilizers) and potassic (e.g., potassium sulphate) - based on soil testing.
• Crop rotation, fallowing and mixed farming to replenish nutrients and break pest and disease cycles.
• Cover cropping and mulching to reduce erosion and leaching and to add organic matter.

Types of Soil Erosion

1. Water erosion

• Sheet erosion - removal of a thin uniform layer of topsoil by rainwater; common on bare or fallow land.
• Rill erosion - formation of numerous small channels (rills) by concentrated runoff; these appear as finger-like grooves across slopes.
• Gully erosion - rills enlarge to form deeper gullies that cannot be removed by normal tillage; gullies may be U-shaped where subsoil is resistant or V-shaped where softer material is exposed.
• River floods and tidal action along coasts also cause significant erosion of soils and agricultural land.

Soil erosion by water

Sheet erosion is common in Himalayan foothills, north-eastern peninsular regions, Assam, the Sahyadris and the Eastern Ghats. Rill erosion occurs in Bihar, Uttar Pradesh, Madhya Pradesh and semi-arid peninsular regions. Gully erosion is severe in parts of northern Haryana, Punjab and the badlands of Madhya Pradesh, Rajasthan and Uttar Pradesh.

2. Wind erosion

Wind erosion occurs mainly in arid and semi-arid regions with sparse vegetation. Strong winds lift and transport fine soil particles (dust storms), leading to loss of fertile topsoil. Rajasthan and adjoining areas of Haryana, Uttar Pradesh and Gujarat are typical regions affected by wind erosion.

Soil erosion by wind

Causes and Consequences of Soil Erosion

Causes include:

• Deforestation and removal of natural vegetation.
• Overgrazing of pastures by livestock.
• Shifting (slash-and-burn) cultivation.
• Poor cultivation practices and removal of crop residues.
• Road ruts, improperly constructed terrace outlets and uncontrolled surface runoff.

Consequences of soil erosion:

• Loss of fertile topsoil and reduction in agricultural productivity.
• Increased runoff and higher risk of floods.
• Lowering of groundwater levels due to reduced infiltration.
• Siltation of rivers, reservoirs and irrigation channels.
• Long-term degradation of landscapes and loss of livelihoods.

Soil Conservation

Soil conservation comprises measures to prevent or reduce soil erosion and to maintain or increase soil fertility. Complete prevention of erosion is seldom possible, but proper measures can substantially reduce its rate and effects.

Natural vegetation prevents erosion in three primary ways:

• Roots bind soil particles together.
• Vegetation reduces wind speed and shields soil from wind erosion.
• Plant canopies reduce the impact energy of raindrops thus lessening splash erosion.

Measures for Soil Conservation

• Planting cover crops such as grasses on uncultivated land and planting trees along slopes.
• Contour ploughing - ploughing along contour lines so furrows run across slopes and reduce runoff velocity.
• Strip cropping - alternating strips of close-growing crops (e.g., legumes) with open crops (e.g., cereals) to reduce wind erosion and runoff.
• Terracing - cutting level steps on hillsides to reduce slope length and runoff speed.
• Check dams and nalla plugging - small dams and barriers in gullies and channels to trap sediment and reduce gully growth.
• Windbreaks - lines of trees or hedges to reduce wind speed and protect fields from aeolian erosion.
• Controlled grazing to prevent overuse of grasslands.
• Suspending cultivation (fallowing) where necessary to allow soil to recover.
• Land-leveling, bench terracing, graded bunding and other engineering measures to control water flow and erosion on agricultural lands.

Institutional and programme measures in India include the establishment of the Central Conservation Board (1953) and regional research-cum-demonstration centres for soil and water conservation. The Desert Afforestation and Research Station at Jodhpur focuses on afforestation and reclamation of desert lands.

Different Types of Soils Found in India

Major soil groups and their common locations:

• Alluvial soils - great plains, river valleys and deltas (Ganga, Brahmaputra, etc.).
• Black cotton soils - Deccan lava region: parts of Maharashtra, Gujarat, Madhya Pradesh, Karnataka, Tamil Nadu and adjoining areas.
• Laterite soils - Western Ghats, parts of eastern plateau regions, Kerala, Karnataka, Maharashtra and Assam in patches.
• Red soils - parts of Tamil Nadu, Karnataka, Andhra Pradesh, Maharashtra, Odisha and central India.
• Saline and alkaline soils - arid and semi-arid tracts in Maharashtra, Gujarat, Punjab, Haryana, Uttar Pradesh and coastal regions where water table and evaporation concentrate salts.
• Arid desert soils - mainly Rajasthan.
• Peaty and marshy soils - Kerala (Kottayam, Alleppey) and coastal marshes of Odisha, West Bengal and Tamil Nadu.

Common conservation measures include contour ploughing, terracing, bunding, afforestation, regulating grazing and coastal protection works (jetties, groynes) to defend against sea erosion.

Reclamation of Acidic and Salt be reclaimed

Acid and salt-affected soils require special nutrient and water management for reclamation and sustainable crop production. Recommendations and practices include:

• Liming of acid soils to correct soil pH and reduce toxicities; liming rate should be based on soil testing.
• Application of gypsum or other amendments to displace sodium in alkali soils and improve soil structure.
• Use of partially water-soluble phosphate fertilisers where phosphorus availability is limited.
• Recycling organic manures and crop residues to restore humus and microbial activity.
• Appropriate drainage and irrigation management to leach excess salts below the root zone where feasible.

Institutions like the Central Soil Salinity Research Institute (Karnal) conduct research and develop technologies for reclamation of salt-affected soils.

Methods of Soil Conservation

Agronomic measures

These include various methods of crop cultivation to ensure protection of the top soil.

• Contour farming
• Mulching
• Strip cropping
• Mixed cropping

Mechanical measures

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.

• Basin listing
• Sub-soiling
• Contour bunding
• Graded bunding or channel terraces
• Bench terracing
• Construction of small dams and diversion channels to reduce erosive velocities

National and watershed programmes

• Soil conservation in the catchments of river valley projects
• Integrated watershed management for flood-prone catchments
• Reclamation and development of ravine areas
• Control of shifting cultivation through alternative livelihood and land-use measures

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.

Additional Information

Soils of Peninsular India
Peninsular soils are largely derived in situ (diluvial) from local parent rocks and include black cotton soils, red soils, laterites, saline and alkaline pockets, and mixed soils.

National parks and game sanctuaries
Game sanctuaries are designated to preserve specific species of animals and birds, whereas national parks protect whole ecosystems - flora and fauna across species.

Types of plateaus
Plateaus can be classified as:

• Inter-montane - plateaus between mountain ranges.
• Piedmont - plateaus lying between mountains and sea.
• Continental or extensive plateaus - large tablelands rising abruptly from surrounding lowlands (e.g., the Deccan Plateau).

Rift valley
Rift valleys are steep-sided depressions formed by faulting and crustal extension; the Narmada valley is an example of a rift valley.

Flood plains and coastal plains
Flood plains form by river deposition (e.g., Ganga plains). Coastal plains are formed by the uplift of continental shelves and by sedimentation along the coast (e.g., eastern and western coastal strips of peninsular India).

Different types of Plateau