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Here is a term paper on ‘Soil’. Find paragraphs, long and short term papers on ‘Soil’ especially written for school and college students.
Term Paper on Soil
Term Paper # 1. Introduction to Soil:
Soil has been defined differently by workers in different fields. Thus, for an agriculturist, the soil is simply the upper layer of ground which is useful for supporting life. Geologists define it as a surface layer of rock waste in which the physical and chemical processes of rock weathering cooperate intimately with organic processes.
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In a technical way, however, the term soil may be defined as “the upper layer of the ground made of unconsolidated material produced due to weathering agencies from the rocks and generally modified subsequently by a variety of mechanical, chemical and organic processes all operating constantly in a complex manner.”
The branch of science dealing with soils is called pedology. It embraces the study of formation and development of soils as also soil mapping.
Constitution:
The essential constituents of soil are:
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(i) The solid matter
(ii) Air and water
(i) The Solid Matter:
It consists chiefly of inorganic particles derived from rocks and minerals. These particles are of different size and composition and reflect the type of parent rocks from which the soils are derived. Organic matter is also present in many soils in subordinate proportion. It is derived from roots and remains of life forms that live on or within the soils such as rodents, worms, insects and bacteria.
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(ii) Air and Water:
The solid particles in a lump of soil are seldom closely packed. In fact, soils are generally characterised by a porous texture and the pores between the solid particles may be filled completely or partially by air (various gases) or water or by both. The soil water is in fact a complex chemical solution having many essential nutrients and other components which are very important in the biochemical and organic processes taking place in the soils. Similarly, many engineering properties of soil are due to the presence and proportion of air and water in the soils.
Term Paper # 2. Physical Properties of Soil:
1. Colour:
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It is an important property that has been commonly used by farmers and scientists alike for a broad classification of soils e.g. red soils, grey soils, and black soils and so on. Colour of the soil depends upon its composition, drainage condition and also on its age. Thus, presence of humus (decayed organic material) in good proportion will impart a black or deep brown colour to the soil.
Similarly, finely dispersed iron oxide is responsible for the red colour and iron hydroxide for the yellow colour of the soils. Absence of these components may result in colourless or grey soils. Further, effective drainage may result in dilution of the original colours whereas time may make a soil more mature and concentrate the original colour if there is not much drainage.
2. Texture:
In soils, texture is understood to define their composition and make up in terms of particle size of the solid components. (Fig. 21.1).
Commonly, the following size-limits are followed for describing features of solid fraction:
(i) Gravel fraction (diameter between 6 – 2.00 mm.)
(ii) Sand fraction (diameter between 2.00 – 0.05 mm.)
(iii) Silt fraction (diameter between 0.05 – 0.002 mm.)
(iv) Clay fraction (diameter less than 0.002 mm.)
While determining the texture of a soil sample, the relative proportions of these fractions are found out by sieving and expressed in percentage terms. A coarse texture soil will have the first two fractions as predominating components while in the fine textured soils, clay grade would be predominant.
It is also customary to describe soil texture in terms of composition as determined on the basis of dominant grades. Three principal textural types are – sandy soils, silty soils and clayey soils. A fourth textural type, loam or loamy soils is also recognized when none of the above grades dominates or in other words, sand, silt and clay are almost equally represented.
The textual classification is conveniently represented by a triangular diagram (Fig. 21.2). The textural type of soils is determined by locating its position (composition wise) in this diagram.
Thus, if it happens to fall within or near:
(i) The upper corner, it is a clayey soil,
(ii) The right corner, it is a silty soil,
(iii) The left corner, it is a sandy soil, the middle region, it is a loamy soil.
3. Structure:
It signifies the large-scale arrangement of soil particles and refers to the nature of aggregation or union of these particles in a lump of soil. If the particles are of uniform size and spherical in outline, the resulting structure is of granular type. Similarly, if the particles are flattened or oblate, the soil is said to have a platy structure.
More often the component grains are of irregular shape resulting in blocks of soils having sharp corners and edges; the structure is then called blocky. Other structural types are- prismatic, columnar and laminar. The structure of soil is a property that is influenced greatly by climate, organic activity and temperature variations with special reference to freezing and thawing of soil moisture.
4. Density:
Soil may be dense or light in terms of weight per unit volume. This depends upon its structure and mineralogical composition. Generally, soil density is described either as particle density or as bulk density. The particle density, as the term indicates, is the weight per unit volume of the solid particles alone.
In the bulk density, the solid particles as well as voids etc. are considered together. Obviously, it would be lesser than the particle density. There is generally not much variation in the particle density of the soils of different types. However, considerable variation may be observed in the bulk density of different soils.
Term Paper # 3. Engineering Properties of Soil:
1. Void Ratio and Porosity:
The component grains of soil are seldom thoroughly packed. In most cases there are always present some open-spaces or voids between them. The total volume of voids is of considerable significance in the determination of engineering properties of soils. The Void ratio is defined as the ratio between the volume of voids and volume of soil particles in a given soil mass.
Numerically, void ratio, e, is given by the relationship:
e = Vv/Vs
where Vv = volume of voids; Vs = volume of solids.
The porosity, n, of the soil mass is, however, ratio between the total volume of voids and the volume of the soil sample:
n = (Vv/V) × 100
where V = total volume of the sample
It is expressed in percentage terms.
2. Permeability:
It is defined as the capacity of soil to transmit water through it. Permeability is, in fact, one of the most important physical characteristics of soils. It is of utmost importance in the design of earth dams, dikes and embankments on or passing through the soils. Moreover, permeability has direct bearing on such other properties of soil that depend upon moisture content.
Darcy’s Law is the basis of most studies of permeability of soils. This property is determined in laboratory by instruments known as “permeameters” of which many types are in use. It is generally necessary to know the permeability of soil while preparing the design of any earth structure.
Two facts must always be given due consideration in such cases:
First:
The coefficient of permeability shows great variation for different types of soils and also for similar types of soils under different conditions.
Second:
The coefficient of permeability is considerably influenced by factors like size, shape and arrangement of the component grains, etc.
The approximate range of coefficient of permeability, k, for different types of soils, is as follows:
Sand:
3000 – 5000 (*10-4 cms/sec) for coarse types to 50 – 150 (*10-4 cm/sec) for medium and the fine textured varieties.
Silts:
1 to 20 (*10-4 cm/sec) for coarse types to 0.01 to 0.1 (*10-4 cm/sec) for fine types.
Clays:
Less than 0.01 (*10-4 cm/sec).
These values merely indicate a broad range and should not be used where practical determination is indicated as necessary and important.
3. Shearing Strength:
It is defined as the resistance of soils to shearing forces and is regarded as one of the most important engineering properties of soils.
It is the net result of at least three qualitative characters of the soil, such as:
(a) The frictional resistance existing between the solid components of the soil;
(b) The degree of cohesion and adhesion between the soil particles;
(c) The textural arrangement of the soil particles such as degree of interlocking etc.
Each of the above basic factors is, in turn, subject to a number of variables such as moisture content, stress history, age of the soil and physical and chemical environment surrounding the soil in question. Hence, shearing strength of the soil is always considered as a highly complex problem in engineering and design consideration. Obviously, the problem has been dealt with exhaustively both on theoretical and practical levels and many generalizations have been obtained.
Thus, the shear strength, s, of cohesionless soils (c = 0) is generally expressed by the relationship:
s = σ tan φ … (i)
where σ is directed force and φ is angle of internal friction. In the above relationship, soil is assumed to be dry.
But when saturated, s in the cohesionless soil is given by:
s = (σ – u) tan φ … (ii)
where u is pore-water pressure.
Similarly, the shear strength of cohesive soils is given by the relationship:
s = c + σ tan φ … (iii)
where c = cohesive force or cohesion between the solid particles which is controlled by a number of factors.
The shear strength of soils is determined in the laboratory by direct shear test, unconfined compression tests and triaxial compression tests. A number of methods are also available for determination of shear strength of soils in the field itself. Among them the Vane-shear tester, the penetrometers and the split-spoon sampler are used commonly.
4. Soil Compressibility:
Many natural soils undergo considerable deformation when loaded from above.
This deformation commonly takes the shape of a decrease in volume in vertical direction which may be due to:
(i) Expulsion of air and/or water from within the voids;
(ii) Collapse of soil particles by closure of voids;
(iii) Deformation of solid particles.
The net result to this compression is called consolidation of soils which takes place at variable rate with time, i.e. it is a time-related process.
Granular, cohesionless soils consolidate at a fast rate compared to fine structured cohesive soils. However, the total consolidation may be much less in the first type of soils, where it may be completely achieved within a short span compared to in cohesive soils where this process may keep going on for many years.
Consolidation may lead to settlement of the structures built over the soil and if this settlement happens to be beyond the allowable limits, collapse or deformation of built-up structures may follow. As such, the soil engineer is always required to investigate thoroughly the compressibility related characteristics of the soil by practical methods.
5. Bearing Capacity:
Bearing capacity may be defined as the capacity of a soil to withstand building loads without undergoing excessive settlement or shear failure. Hence this forms most important field property that needs firm evaluation before any construction programme is proposed over a soil.
In practice, ultimate bearing capacity is determined by loading the soils to be tested through contact (bearing) plates and observation of settlement. From this allowable bearing capacity is determined for design purpose. Conventionally, for ordinary types of building construction in a planned residential colony local building codes are prepared and followed in a general way with respect to bearing capacity.
For major construction, such as multi-storeyed buildings and industrial buildings, however, elaborate tests are carried out to arrive at safe values of allowable bearing capacity. A number of factors have then to be taken into consideration such as soil compressibility, water table, depth of soil, cover, shape of the foundations to be given on the soil and so on. The subject has been dealt by different workers in different ways.
Term Paper # 4. Soil Formation:
Soil is the end product of processes of decay and decomposition of the rocks on the surface under the influence of certain natural agencies. It has also been established that formation of soils is a continuous and time-dependent process that may require hundreds and thousands of years to reach a stage where the type of soil evolved is in equilibrium with the surrounding conditions.
Soil Development is controlled by a number of factors, which often work in very close cooperation to convert an original material into a soil.
These factors are:
i. Climate,
ii. Parent material,
iii. Vegetation,
iv. Topography and
v. Time.
i. Climate:
It is the single most important soil-forming factor and operates through precipitation (rainfall) and temperature (hot and cold climate).
a. Annual Rainfall is responsible for a variety of rock-decomposing processes. All the biological and chemical activities require some amount of moisture. Hydration and hydrolysis of rocks are important weathering processes, which are actually chemical reactions requiring water. Similarly, all organisms – plant and animal type – require water for their growth and proliferation.
The heavy amount of rainfall itself may result in removal of some components, e.g. silica from the decomposed rocks (leaching and desilication). The shortage of rainfall may result in evaporation of water from the surface and cause precipitation of some chemicals (carbonates and sulphates) among the top layers of soil.
b. Temperature controls soil formation indirectly by exerting positive or negative influence on the growth of organisms. Milder temperatures in moist regions allow profuse growth whereas freezing temperatures in the humid regions may bring such activities to a standstill. In arid dry regions, a rise and fall in temperature may induce considerable expansion and contraction in rocks leading to their disintegration.
Frost action is described as a rock disintegrator in cold-humid climates. In warm humid regions, bacterial growth is highly favoured. Consequently in such regions, all dead vegetable matter is consumed by these bacteria so that resulting soil is free from humus. Compared to this, lot of vegetable matter accumulates in cold humid regions as bacteria are not available. These vegetable accumulations decay and develop organic acids resulting in a different type of infertile soil rich in humus.
ii. Parent Material:
The texture, structure and composition of parent rocks also contribute tremendously towards the rate of development of soils and also the type of soils evolved. The same rock exposed in different climates may result in different types of soils and at different rates. Granites, for example, may result in lot of clays in hot humid climates due to hydration, hydrolysis and carbonation whereas in cold climates, these may simply be fractured and fragmented without much decomposition.
Sandstones, quartzites and other such silica rich rocks may suffer no decomposition even in hot, humid climates resulting in soils rich in quartz formed due to mechanical disintegration. Numerous such examples can be cited where the parent rock has exerted a major influence on the type of soil evolved.
Similarly, the texture of the soil is often related to the texture of the parent material, at least in the initial stages. Thus, fine-textured soils develop from fine-grained rocks like basalts, limestones and shales whereas relatively coarse soils result from granites, sandstones and cherty limestones.
iii. Vegetation:
Vegetation comprising higher types (macroflora- trees, shrubs and herbs) and lower types (microflora- bacteria, algae and fungi) influences the formation of soils, especially their differentiation into organic and inorganic-types. Plants act as conduits of dispersion of certain bases such as calcium, sodium and potassium from lower layers to the upper layers of the soil.
Similarly, accumulation of dead parts of plants forms the source of humus in the soils. Bacteria act in a number of ways to change the composition and texture of soils, as for example by consuming humus, by fixing nitrogen from the atmosphere and so on.
iv. Topography:
This effects the soil development through slope factor. Steeper slopes do not allow accumulation of loose particles for greater thickness; hence, though soil-thickness on the slopes may be less, the rate of decomposition may be more as fresh surface is repeatedly exposed.
Similarly, drainage is more effective and fast on the slopes; the slope soils may be particularly poor in some constituents that might get leached out soon after their formation. Poorly drained soils are often rich in original residues and in soluble mineral components.
v. Time:
The factor of time operates in a purely logical manner. Since every soil forming process requires definite time for its completion, it is evident that the type of soil in an area would be defined by the time that has been allowed to it to develop into the present form. On slope, for instance, a layer of soil may not be allowed to rest for any considerable time.
As such soils on slopes may be young in age, meaning, they are of only recent formation. Similarly, in the older soils a number of well-defined zones or horizons may be found indicative of operation of soil forming processes. The age of soil may vary from place to place.
In certain areas soils may be as young as a hundred or couple of hundred years; in other areas they may be mature enough and have been in existence for thousands of years in more or less the same general form. There are regions in the world where soils developed during the glaciation periods (thousands of years ago) are still intact.
Term Paper # 5. Soil Profile:
During the development of soil from a parent material, the actual transformation proceeds through certain well defined stages. In mature soils, these stages appear as a series of horizons, or layers with contrasting properties. Such horizons, when arranged in descending order, are collectively said to form a Soil Profile for that particular area.
A typical Soil Profile, beginning from surface and proceeding downwards, is generally made up of four main horizons (there may be many sub-horizons in each main horizon):
(i) The A-Horizon:
It is characterized generally by finely divided particles and extends from a few centimeters to as much as a meter or more. It may contain, in humid regions, loose leaves, incompletely decomposed organic matter and good amount of humus. In the basal zone of A-horizon, leaching effects may be seen. (Fig. 21.3)
(ii) The B-Horizon:
This zone lies immediately below the A-horizon and is often free from the staining of particles by humus as conspicuous in the overlying zone. Moreover, in arid regions, it may contain nodules of calcium carbonate or gypsum. Colloid accumulation is maximum in this zone. The lower region of B-horizon becomes more pebbly and coarse indicating transition to the C-horizon.
The A and B horizon together form the true soil, called Solum.
(iii) The C-Horizon:
It is more a zone of weathered rock than a true soil. In texture, it is often coarse grained and pebbly; in composition, it retains all the evidence of its parent rock although operations of processes of soil formation having begun in it are also indicated unmistakably.
(iv) The D-Horizon:
In a true soil profile, a sample from this horizon is the parent rock itself – unaltered as yet. In some cases, it may not be the parent rock, but the solid rock mass on which the other zones are resting.
Term Paper # 6. Classification of Soils:
Soils have been classified in a number of ways. At present many classification systems are available, each system serving to a particular objective. The agricultural classification of soils is useful for crop scientists and is basically different from engineering classification used by a civil engineer or the one used by a geologist. Only geological classification of soils will be dealt with here, for obvious reasons.
Geological Classification:
It is perhaps the simplest of soil classifications and divides all the soils into two main groups:
A. Residual soils and
B. Transported soils.
This is based primarily on the evidence whether soils are found over and above the rocks from which these are derived by decomposition, or, these have been accumulated in the region of their present occurrence after transportation for considerable distances.
A. Residual Soils:
Those soils that have suffered very little or no transport during or after their formation and are found more or less covering their parent rocks are classified broadly as residual soils.
Their most important characters are summarized as below:
(i) Thickness:
Thickness of such soils varies from place to place depending upon factors like climate, lithology, topography and the extent of time for which the soil forming processes have been operative in that region. Deep residual soils may be either due to a rapid rate of soil formation or to prolonged soil forming activities without any obstruction.
(ii) Stratification:
Residual soils generally show no stratification. These would show, however, well-distinguished horizons of soil profile developing through ages by operation of soil forming processes.
(iii) Chemical Composition:
Their chemical composition is defined principally by the nature of parent rock and the trends of chemical activity as determined by the climate of the area. The residual soils in cold climates are often rich in humus.
(iv) Leaching:
The deep residual soils of the humid areas are often highly leached resulting in the removal of soluble plant foods and concentration of salts harmful for fertility. Hence, these are often infertile. The Red and Black soils of India are examples of residual group.
B. Transported Soils:
This group includes all those soils that have been deposited at places far from their parent rocks. During transportation, soils from one place are mixed up with many types from other places and hence result in complex, heterogeneous nature of the resulting accumulations, thereby making it difficult to make an immediate idea about their source rocks.
Transported soils are further classified on the basis of main agent involved in their transport and deposition as follows:
1. Colluvial Soils:
These include the soils that have moved down largely under the influence of gravity and only for short distance. Such soils commonly occur at or near the base of steep slopes and are never well stratified.
2. Alluvial Soils:
It is a major group of soils and includes such soils that have been spread out by streams along their banks on the flood plains during repeated floods. These are made up of fine material and are clearly stratified. In view of their heterogeneous composition, fine size and negligible leaching, these soils are often very fertile. The Indo-Gangetic alluvial soils are the best example of this group.
3. Glacial Soils:
During the Pleistocene Ice age, great glaciers covered vast areas of N. America, Europe and Asia. When the ice melted, all the debris it carried was deposited in the form of heterogeneous soil which has undergone series of changes subsequently. These soils are aptly classed as glacial soils.
4. Eolian Soils:
Wind is a very active agent for transport of dust, silt and sand grade particles. Wind deposited soils composed chiefly of silt and clay fractions form the loess spread over thousands of square kilometers in China and America. The sandy soils of similar origin are also common and form the desert and the semi desert regions of many countries.
A smaller group of soils, formed from accumulation of debris in lakes and other bodies of standing water, is called lacustarine soil. It is rich in organic material and often classed among the organic soils. The Karewas of Kashmir – so famous for supporting ‘saffron’ crops are partly lacustarine in origin.
Term Paper # 7. Soil Stabilization:
Objects:
The main purpose of different processes of soil stabilization is to improve the bearing capacity of soils. This is commonly achieved by the use of admixtures like those of cement, bituminous substance and certain industrial waste products. Soil stabilization becomes an essential process for improving the strength and stability of base and sub-base course in major highway projects especially where these have to pass over weak, compressible type of soils. Another object of soil stabilization is to induce and increase the imperviousness of top layers so that as little as possible water could seep in.
Methods:
Different soils might need separate type of treatment for stabilization. The exact treatment would also depend on the fact whether the object is to simply increase the imperviousness of the top soil or to increase the bearing capacity of the sub-course or to achieve both the ends.
Cement Stabilization:
Portland cement admixtures, when mixed with many types of soils in a proper manner have been found to give good degree of stabilization. All types of soils except the clayey soils are amenable to stabilization by using cement. The clayey soils require too large quantities of cement and costly and prolonged compaction to produce desired results.
For obtaining soil-cement stabilization ten to fifteen percent by weight of cement is added to the soil and the same is compacted to optimum moisture content under humid conditions from 7-28 days. The resulting soil is sufficiently hard to water and frost action. The strength of cement-stabilized soils generally ranges between 30-40 kg/cm2 at 7 days.
Bituminous Stabilization:
Bituminous materials like asphalt, either singularly or in combination with water and oils etc. has been extensively used for increasing the soils impermeability. Bitumen is in liquid form and when mixed with some sand provides a mixture which could be as effective as soil-cement. However, in this case a dry condition is an important factor. When the soil has enough moisture, the bitumen takes lot of time to reach each grain and coat it to make it impervious. Sand mixed with bitumen helps in increasing the strength of the soil.
In practice bitumen might be used as cut-back bitumen and as wet-sand mix. Bitumen is called cut-back bitumen when a flux or solvent such as petroleum oil or paraffin has been added to it to increase its curing power. Bituminous emulsions are made with water as the emulsifying agent and are specially useful in wet soils that are pretreated with two percent of hydrated lime.
Electro-osmosis has been used in few cases for soil stabilization at local levels. In this method, electrodes are embedded in the soil and connected with the source of electricity. Water in the soil pores flows towards one of the electrodes (generally cathode), thereby reducing its quantity around the other electrode which gets dried. Saturated fine grained soils are easily treatable by electro- osmosis. It is, however, an energy consuming process.
Term Paper # 8. Soil Groups of India:
A number of classifications have been suggested for the soils of Indian sub-continent.
Broadly speaking, following five major soil groups are recognized:
1. The Red Soils,
2. The Black Soils,
3. The Indogangetic Alluvium,
4. The Laterite Soils and
5. The miscellaneous soils.
1. The Red Soils:
These are mainly residual type of soils that have been formed due to decomposition of ancient crystalline rocks like granites and gneisses and also other rocks rich in iron and magnesium bearing minerals. The name (red soil) is due to prevalence of iron oxide which gives them all a red colouration, not due to higher iron content but due to its fair dispersion. At places, the so-called red soils may not show typical red colouration at all.
Chemical Characters:
(i) The red soils are generally poor in nitrogen, phosphorus and humus but rich in potash. They are mainly siliceous or aluminous in character.
(ii) The clay faction is made up mostly of Kaolinitic minerals.
(iii) Red soils are often deficient in soluble exchangeable bases and are rich at places in calcareous concretions of secondary origin.
Distribution:
Red soils cover practically the whole of Chennai, Mysore, south east Mumbai, east of Hyderabad, Madhya Pradesh and Orissa. Isolated patches occur in Bihar, Bengal and Uttar Pradesh.
2. Black Soils (Regur):
These are the typical soils of Mumbai and Gujarat, and adjoining areas. These are also of residual type and derived from basic igneous rocks, the Basalts of Deccan Traps. Black colour is dominating and hence the name. The exact source of black colour is not yet perfectly known; presence of titanium iron or some organic compounds in fine dispersion is thought by some to be responsible for it.
Black soils are fine grained, highly argillaceous soils also containing high proportions of calcium and magnesium. Their behaviour towards moisture is most noteworthy – they become exceedingly sticky when wet and undergo considerable contraction on drying. Owing to this property (of swelling with gain and shrinking with loss of moisture), black soils have been found quite troublesome from engineering point of view.
Black soils also contain quantities of iron, aluminum, lime and magnesia and generally show poor percentages of phosphorous, nitrogen and organic matter. Layers of calcareous nodules – the kankar – are frequently encountered in black soils.
The Base Exchange capacity of black soils is generally fairly high.
Distribution:
Black soils have wide development in Mumbai, western part of Madhya Pradesh, part of Gujarat. They also occur in some parts of Chennai. In the valleys of Tapti, the Narmada, the Krishna and the Godavari rivers, deep layers of black soil – often upto 8 meters – occur widely.
Origin:
It is believed that the black soils have been derived by decomposition of two types of rocks:
(i) Basalts of the Deccan Traps – The soil groups prevalent in Maharashtra and Gujarat are derived from these parent rocks.
(ii) Ferruginous Schists and gneisses – The black soils of part of Tamil Nadu belong to this category.
Black soils are generally distinguished into shallow and deep types, the latter being very fertile with respect to cotton crops.
3. The Indo-Gangetic Alluvium:
This group of soils is the most important and largest soil groups of India. It is spread in the most thickly populated areas of the country. These are essentially transported soils having been deposited by river systems of the Ganges, the Indus, and the Brahmaputra in the sub-Himalayan regions. Weathering products from the fountains were (and are) carried down and deposited in the flood plains and deltaic regions of these river systems.
The Indo-Gangetic-Brahmaputra soils are for a greater part, young soils and have undergone little pedogenic (soil formation) evolution since their deposition. They are yet immature in the strict sense.
The alluvium has been divided into two groups on the basis of the Kankar (calcium nodules) concentration:
(i) The Khadar which is light in colour, having relatively less concentration of kankar and greater silica content.
(ii) The Bhangar or the older alluvium. It is dark in colour, rich in clay and contains greater proportion of kankar nodules.
4. Lateritic Soils:
Definition:
Laterite is a kind of vesicular rock composed essentially of mixture of hydrated oxides of aluminum and iron with small percentage of other oxides such as manganese and titanium. There is no fixed relation between the principal oxides (iron and aluminium), and this variation is responsible for occurrence of laterite in a number of varieties.
Origin:
Laterite is formed as a result of decay of many types of rocks. As regards the exact mode of origin of laterites, however, there is much controversy among soil scientists. A widely prevailing belief is that laterization is due to loss of silica from soil profile developed in humid regions where the process of leaching is much widespread.
It is the in-situ decay and decomposition of basalts and other aluminous rocks under warm, humid and monsoonic conditions which is thought to be responsible for the origin of most of laterites in India. Under the above-cited conditions of subaerial weathering, the parent rock is first decomposed into kaolin which may further be decomposed into hydrated oxide of aluminium or bauxite.
Distribution:
Laterite soils are well developed in India and are found on the summits of hills of Mysore, Kerala, and at many places in Madhya Pradesh, Chhattisgarh, Orissa and Assam. It has been observed that all laterite soils developed at the above mentioned places are deficient in nitrogen and very poor in lime, magnesia and oxides of potassium. In places the soils show a high content of humus and oxides of phosphorous.
Laterite soils are also developed all along the coastal regions in Chennai, in Mumbai and Ratnagiri in Maharashtra and some parts of Bengal. These sons occur less regularly and often as capping hills and plateaus where these may acquire considerable thickness.
5. Miscellaneous Soil Groups:
Besides the above described major groups of India, certain other well recognized types of soils of this country are:
(i) The forest and the hill soils;
(ii) The desert soil;
(iii) The saline and alkaline soils and
(iv) The peaty and marshy soils.
(i) Forest and Hill Soils:
These are the typical soils of forests and mountains and occur along the slopes or in depressions and valleys in forested regions. The exact mode of formation and character of such soils is always controlled by factors like geology, topography, climate and type of vegetation of the mountain ranges. Forest and hill soils show (e.g. in Assam) high content of organic matter and nitrogen. They generally exhibit a great variation in their chemical and mechanical composition.
In the Terai area of Uttaranchal, the extreme unhealthy conditions are attributed to the excessive soil moisture and prolific growth of vegetation.
Forest and hill soils are developed in the Punjab, Himachal Pradesh, Nilambur teak forests of Chennai and Jammu and Kashmir.
(ii) Desert Soils:
A large part of the country laying between Indus and Aravali (i.e. in Punjab and Rajasthan) is covered under blown sand. Some of the soils of these desert regions contain high percentage of soluble salts and are poor in organic matter and hence unfit for crops. The Rajputana desert is spread over an area of 65,000 square kilometers. Because of scarcity of water, it is completely barren.
(iii) Saline and Alkaline Soils:
These are the salt impregnated soils and form a very important soil group in India developed in many parts of the Punjab, Uttar Pradesh, Rajasthan, Bihar and Maharashtra. The chief feature of such soils is that they contain many undecomposed mineral fragments which on weathering liberate sodium, magnesium and calcium salts.
These injurious salts (known as Reh or Kallar) are confined to the top layers and their concentration in upper parts of the soil is due to capillary action, under the influence of which the saline layers are transferred from the lower layers. The reh salts always damage the fertility of the soil and are highly problematic for the agriculturist.
It has been estimated that lakhs of acres of land in the Punjab and Uttar Pradesh have been affected by these salts. The reclamation of alkaline and saline soils can be achieved by methods involving addition of salts of calcium and growth of salt resistant crops – e.g. rice, sugarcane and barsaem.
(iv) Peaty and Marsh Soils:
The peaty soils developed in some parts of Kerala are highly organic soils rich in soluble salts. They are acidic in character and black in colour. The marshy soils are developed in depressions formed by dried river basins and lakes in coastal areas. These soils are generally rich in ferrous salts and hence may sometimes be bluish in colour. Marshy soils have been recorded along coastal tracts of Orissa, in Sunderbans of Bengal and in Almora district of Uttaranchal.
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