MECHANICAL MEASURES The mechanical or engineering methods most widely used include:
(a) excavation of different types of ditches and construction of terraces for the removal of excess of water from the fields.
(b) construction of dams for checking the erosive velocities of the water.
The main objectives are to increase the time of concentration by intercepting the runoff and thereby providing an oppottunity for the infiltration of water; and to divide a long slope into several short ones so as to reduce the velocity of the runoff and thus prevent erosion.
Basin Listing Small interrupted basins are made along the contour with a special implement called a basin lister. It helps to retain rain water as it falls and is specially effective on retentive soils having mild slopes.
Subsoiling It involves in breaking with a subsoiler the hard and impermeable subsoil to conserve more rain water by improving "the physical conditions of a soil. This operation does not involve soil inversion but promotes greater moisture penetration into the soil and reduces both runoff and soil erosion.
Contour Bunding It involves making a comparatively narrow-based embankment at intervals across the slope of the land on a level that is along the contour. It conserves soil and water in arid and semi-arid areas.
Graded Bunding or Channel Terraces This method is used in areas receiving rainfall of more than 80 cm per year irrespective of soil texture. It may be narrow or broad based. Generally in India the terraces are broad-based, that is, wide and low embankments constructed on the lower edge of the channel from which the soil is excavated.
Bench Terracing It consists of a series of platforms having suitable vertical drops along contours or on suitably graded lines across the general slope of the land. The vertical drop may vary from 60 to 180 cm, depending upon the slope and soil conditions.
Thursday, October 29, 2009
SOIL CONSERVATION
SOIL CONSERVATION
For prevention and control of soil erosion, two types of measures are undertaken: agronomic and mechanical.
AGRONOMIC MEASURES These include various methods of crop cultivation to ensure protection of the top soil. The idea is to help intercept raindrops and reduce the splash effect, obtain a better intake of water rate by the soil by improving the content of organic matter and soil structure, reduce the overland runoff through contour cultivation, mulches, dense growing crops, strip cropping and mixed cropping.
Contour Farming When the soil cannot absorb all the rain that falls on it then the excess water flows down the slope under the influence of gravity. If farming is done up and down the slope, the flow of water is accelerated as each furrow serves as a rill. Major part of the rain is drained away and the top fertile soil is washed off. All this results in a scanty and uneven growth of crops.
A simple practice of farming across the slope keeping the same level as far as possible has many beneficial effects. The ridges and rows of plants placed across the slope form a continual series oI-miniature barriers to the water moving over the soil surface. Their effect is great in reducing runoff, soil erosion and loss of plant nutrients.
Mulching Mulches prevent soil from blowing off and being washed away, reduce evaporation, increase infiltration to keep down weeds, improve soil structure and increase crop yields.
Strip Cropping It is another form of controlling the runoff erosion and thereby maintaining the fertility of soil. Strip cropping employs several good farming practices such as crop rotation, contour cultivation, stubble mulching and cover cropping. Strip cropping includes contour strip crop
ping, field strip cropping, wind strip cropping and permanent or temporary buffer cropping.
Contour strip 'cropping involves growing a soil-exposing and erosion-permitting crop in strips of suitable widths across the slopes on contour, z.lternating with strips of soilprotecting and erosion-resisting crops. It shortens the length of the slope, checks the movement of runoff water and helps to desilt it, and increases the absorption of rainwater by the soil.
Wind strip cropping consists of planting tall growing crops and low growing crops in alternately arranged straight and long but relatively narrow parallel strips, laid out light across the direction of the prevailing wind regardless of the contour.
In permanent buffer strip cropping, the strips are established to take care of the steep or highly-eroded slopes in fields under contour strip cropping. The strips are generally legumes, grasses or shrubs.
Mixed Cropping The important objectives of mixed cropping are a better and. continuous cover of the land, good protection against the beating action of the rain and good protection against soil erosion. The line sowing of mixed crops gives rise to the practice of intercropping.
For prevention and control of soil erosion, two types of measures are undertaken: agronomic and mechanical.
AGRONOMIC MEASURES These include various methods of crop cultivation to ensure protection of the top soil. The idea is to help intercept raindrops and reduce the splash effect, obtain a better intake of water rate by the soil by improving the content of organic matter and soil structure, reduce the overland runoff through contour cultivation, mulches, dense growing crops, strip cropping and mixed cropping.
Contour Farming When the soil cannot absorb all the rain that falls on it then the excess water flows down the slope under the influence of gravity. If farming is done up and down the slope, the flow of water is accelerated as each furrow serves as a rill. Major part of the rain is drained away and the top fertile soil is washed off. All this results in a scanty and uneven growth of crops.
A simple practice of farming across the slope keeping the same level as far as possible has many beneficial effects. The ridges and rows of plants placed across the slope form a continual series oI-miniature barriers to the water moving over the soil surface. Their effect is great in reducing runoff, soil erosion and loss of plant nutrients.
Mulching Mulches prevent soil from blowing off and being washed away, reduce evaporation, increase infiltration to keep down weeds, improve soil structure and increase crop yields.
Strip Cropping It is another form of controlling the runoff erosion and thereby maintaining the fertility of soil. Strip cropping employs several good farming practices such as crop rotation, contour cultivation, stubble mulching and cover cropping. Strip cropping includes contour strip crop
ping, field strip cropping, wind strip cropping and permanent or temporary buffer cropping.
Contour strip 'cropping involves growing a soil-exposing and erosion-permitting crop in strips of suitable widths across the slopes on contour, z.lternating with strips of soilprotecting and erosion-resisting crops. It shortens the length of the slope, checks the movement of runoff water and helps to desilt it, and increases the absorption of rainwater by the soil.
Wind strip cropping consists of planting tall growing crops and low growing crops in alternately arranged straight and long but relatively narrow parallel strips, laid out light across the direction of the prevailing wind regardless of the contour.
In permanent buffer strip cropping, the strips are established to take care of the steep or highly-eroded slopes in fields under contour strip cropping. The strips are generally legumes, grasses or shrubs.
Mixed Cropping The important objectives of mixed cropping are a better and. continuous cover of the land, good protection against the beating action of the rain and good protection against soil erosion. The line sowing of mixed crops gives rise to the practice of intercropping.
ZONES OF SOIL EROSION IN INDIA
ZONES OF SOIL EROSION IN INDIA It has been estimated by agricultural experts that yearly loss of soils in India due to water and wind erosions is to the tune of five billion tonnes. On the basis of major causes of soil erosion, the country has been divided into the following zones.
(i) North-eastern region (Assam, West Bengal, etc.) The main causes of soil erosion are heavy rains and floods and widespread bank-cutting.
(ii) Shiwalik ranges of the Himalayas The primary cause is destruction of vegetation. Flood in rivers due to heavy deposition of debris is another important cause. As a result of floods, sediments are deposited on fertile agricultural lands.
(iii) River banks (Yamuna, Chambal, Mahi, Sabarmati, etc.) A sizeable portion of agricultural land in Uttar Pradesh, Rajasthan, Gujarat and Madhya Pradesh has been transformed into ravines due to soil erosion.
(iv) Hills of South India (Nilgiris) The heavy soil erosion in the southern hills may be attributed to steep slopes, heavy rainfall and defective methods of cultivation.
(v) Arid regions of Rajasthan and South Punjab Some parts of Rajasthan and Punjab such as Kota, Bikaner, Bharatpur, Jaipur and Jodhpur are subject to soil erosion by wind action.
(i) North-eastern region (Assam, West Bengal, etc.) The main causes of soil erosion are heavy rains and floods and widespread bank-cutting.
(ii) Shiwalik ranges of the Himalayas The primary cause is destruction of vegetation. Flood in rivers due to heavy deposition of debris is another important cause. As a result of floods, sediments are deposited on fertile agricultural lands.
(iii) River banks (Yamuna, Chambal, Mahi, Sabarmati, etc.) A sizeable portion of agricultural land in Uttar Pradesh, Rajasthan, Gujarat and Madhya Pradesh has been transformed into ravines due to soil erosion.
(iv) Hills of South India (Nilgiris) The heavy soil erosion in the southern hills may be attributed to steep slopes, heavy rainfall and defective methods of cultivation.
(v) Arid regions of Rajasthan and South Punjab Some parts of Rajasthan and Punjab such as Kota, Bikaner, Bharatpur, Jaipur and Jodhpur are subject to soil erosion by wind action.
SOIL EROSION
Soil erosion is the wearing away and redistribution of the earth's soil layer. It is caused by the action of water, wind and ice, and also by improper methods of agriculture. If unchecked, soil erosion results in the formation of deserts. It has been estimated that 20 per cent of the world's cultivated topsoil was lost between 1950 and 1990.
If the rate of erosion exceeds the rate of soil formation (from rock), then the land will decline and eventually become infertile.
The removal of forests or other vegetation often leads to serious soil erosion because plant roots bind soil, and without them the soil is free to wash or blow away, as in the American dust bowl. The effect is worse on hillsides, and there has been devastating loss of soil where forests have been cleared from mountain sides, as in Madagascar. Improved agricultural practices such as contour ploughing are needed to combat soil erosion. Windbreaks, such as hedges or strips planted with coarse grass, are valuable, and organic farming can reduce soil erosion by as much as 75 per cent.
In India, severe soil erosion is a characteristic of areas with heavy rainfall and improper land management. Various types of erosion take place. Normal or geologic erosion is a slow, long process which ensures an equilibrium between soil removal and formation. Accelerated soil erosion is erosion of the soil surface due to animal or human interference at a rate faster than that involved in the soil's building up.
In arid and semi-arid parts with high wind velocity, wind is a major cause of erosion. Finer particles are generally carried farther than coarse particles are. Erosion by water is of three types. Sheet erosion is when rainfall pounds soil grains loose, carrying them downwards. As the huge sheet of water running downslopes gains acceleration, erosion of the soil underneath also takes place. When sheet erosion continues for long, the silt-laden runoff forms many finger-shaped grooves over a large area.
This is rill erosion. Again due to sheet erosion, concentration of water along depressed areas results in heavy erosion in these areas which is termed gully erosion. In hilly parts, heavy rainfall and earthquakes result in limdslides, which bring about slip or landslide erosion. When cobbles, silt and boulders get deposited on the torrent bed, the bed level of the torrent is raised. This reduces the transporting capacity of the torrent, causing erosion (stream-bank erosion).
If the rate of erosion exceeds the rate of soil formation (from rock), then the land will decline and eventually become infertile.
The removal of forests or other vegetation often leads to serious soil erosion because plant roots bind soil, and without them the soil is free to wash or blow away, as in the American dust bowl. The effect is worse on hillsides, and there has been devastating loss of soil where forests have been cleared from mountain sides, as in Madagascar. Improved agricultural practices such as contour ploughing are needed to combat soil erosion. Windbreaks, such as hedges or strips planted with coarse grass, are valuable, and organic farming can reduce soil erosion by as much as 75 per cent.
In India, severe soil erosion is a characteristic of areas with heavy rainfall and improper land management. Various types of erosion take place. Normal or geologic erosion is a slow, long process which ensures an equilibrium between soil removal and formation. Accelerated soil erosion is erosion of the soil surface due to animal or human interference at a rate faster than that involved in the soil's building up.
In arid and semi-arid parts with high wind velocity, wind is a major cause of erosion. Finer particles are generally carried farther than coarse particles are. Erosion by water is of three types. Sheet erosion is when rainfall pounds soil grains loose, carrying them downwards. As the huge sheet of water running downslopes gains acceleration, erosion of the soil underneath also takes place. When sheet erosion continues for long, the silt-laden runoff forms many finger-shaped grooves over a large area.
This is rill erosion. Again due to sheet erosion, concentration of water along depressed areas results in heavy erosion in these areas which is termed gully erosion. In hilly parts, heavy rainfall and earthquakes result in limdslides, which bring about slip or landslide erosion. When cobbles, silt and boulders get deposited on the torrent bed, the bed level of the torrent is raised. This reduces the transporting capacity of the torrent, causing erosion (stream-bank erosion).
Desert Soils, Saline and Alkaline Soils, Peaty and Other Organic Soils'
Desert Soils Such soils prevalent between the Indus and the Aravallis (in Punjab and Rajasthan) are covered under a mantle of blown sand. Some of the soils contain a high percentage of soluble salts but are poor in organic matter. They are rich enough in phosphate though poor in nitrogen.
Haryana, Rajasthan and Punjab have land covered with such soils. In all, they cover about 1.42 lakh sq km of land surface.
A number of crops can be cultivated. These soils in many parts of Rajasthan, for instance, have emerged suitable for growth of cotton and cereals. .
Saline and Alkaline Soils These salt-impregnated and infertile soils, also known as reh, usar and kallar, form an important soil group. Salts that make up saline soils include free sodium and those that compose alkali soils include sodium chloride. These soils have undecomposed mineral fragments that on weathering produce magnesium, sodium and calcium salts.
Spread across arid and semi-arid northern India such as parts of Punjab, Uttar Pradesh, Haryana, Rajasthan and also Bihar, the saline and alkali soils cover a land surface of 170 lakh hectares.
Peaty and Other Organic Soils' Peaty soils with a high quantity of soluble salts and organic matter are found in parts of Kerala (Alleppey and Kottayam districts). They, however, lack potash and phosphate. Marshy salts, high in vegetable matter, are found in northern Bihar, coastal parts of O..rissa, Tamil Nadu and West Bengal and parts of Uttar Pradesh.
Haryana, Rajasthan and Punjab have land covered with such soils. In all, they cover about 1.42 lakh sq km of land surface.
A number of crops can be cultivated. These soils in many parts of Rajasthan, for instance, have emerged suitable for growth of cotton and cereals. .
Saline and Alkaline Soils These salt-impregnated and infertile soils, also known as reh, usar and kallar, form an important soil group. Salts that make up saline soils include free sodium and those that compose alkali soils include sodium chloride. These soils have undecomposed mineral fragments that on weathering produce magnesium, sodium and calcium salts.
Spread across arid and semi-arid northern India such as parts of Punjab, Uttar Pradesh, Haryana, Rajasthan and also Bihar, the saline and alkali soils cover a land surface of 170 lakh hectares.
Peaty and Other Organic Soils' Peaty soils with a high quantity of soluble salts and organic matter are found in parts of Kerala (Alleppey and Kottayam districts). They, however, lack potash and phosphate. Marshy salts, high in vegetable matter, are found in northern Bihar, coastal parts of O..rissa, Tamil Nadu and West Bengal and parts of Uttar Pradesh.
Laterite and Lateritic Soils, Forest Soils
Laterite and Lateritic Soils These soils possess a compact to vesicular mass in the sub-soil, composed mainly of hydrated oxides of iron and aluminium. Laterisation is said to be due to loss of silica from the soil profile in humid regions where the process of leaching is widespread. It is the in situ decay and decomposition of basalts and other aluminous rocks tmder warm, humid and monsoonic conditions which is thought to be responsible for forming
laterites of India. Lateritic soils are deficient in nitrogen.
They are chiefly found on the summits of hills of the Deccan, in Karnataka, Kerala, Madhya Pradesh, Orissa, Assam and the Malabars. They cover about 1.26 lakh sq km of land surface. Generally of low fertility, m<1ooring and other activities render them suitable for growing crops such as ragi, rice and sugarcane.
Forest Soils Such soils are mostly found in forests and mountains and they occur along the slopes or in depressions and valleys in forested regions. Their mode of formation and character is controlled by geology, topography, climate, vegetation of the mountain ranges and other factors. Such soils have a high content of organic matter and nitrogen and generally show a great range in their chemical and mechanical composition. Forest soils are deficient in potash, phosphorus and lime. Fertilisation of these soils is a must for good yields.
Such soils are found in the Punjab, Himachal Pradesh, the Nilambur teak forests of Malabar, Jammu and Kashmir, Karnataka, Manipur, etc. Tea, coffee, tropical fruits and spices are obtained from plantations on these soils especially in south India. Wheat, maize, barley are cultivated in some states.
laterites of India. Lateritic soils are deficient in nitrogen.
They are chiefly found on the summits of hills of the Deccan, in Karnataka, Kerala, Madhya Pradesh, Orissa, Assam and the Malabars. They cover about 1.26 lakh sq km of land surface. Generally of low fertility, m<1ooring and other activities render them suitable for growing crops such as ragi, rice and sugarcane.
Forest Soils Such soils are mostly found in forests and mountains and they occur along the slopes or in depressions and valleys in forested regions. Their mode of formation and character is controlled by geology, topography, climate, vegetation of the mountain ranges and other factors. Such soils have a high content of organic matter and nitrogen and generally show a great range in their chemical and mechanical composition. Forest soils are deficient in potash, phosphorus and lime. Fertilisation of these soils is a must for good yields.
Such soils are found in the Punjab, Himachal Pradesh, the Nilambur teak forests of Malabar, Jammu and Kashmir, Karnataka, Manipur, etc. Tea, coffee, tropical fruits and spices are obtained from plantations on these soils especially in south India. Wheat, maize, barley are cultivated in some states.
Alluvial Soils
Alluvial Soils Essentially transformed soils, they form the most important and largest of all the soil groups. These soils, unlike those of the other types, have undergone very little pedogenic (soil formation) evolution since their deposition. The alluvium is of two types-the khadar and the bhanger. The khadar is light in colour, more siliceous in composition and composed of newer deposits while the bhanger or the older alluvium is composed of lime nodules (kankar) and has a clayey composition. It is dark in colour, Vertically, there is no clear differentiation between the alluvium types and the profile often lacks' stratification. Alluvial soils vary in different regions owing to factors like climate, vegetation and surface conditions. Alluvial soils are generally deficient in nitrogen and humus.
Alluvial soils are found throughout the plains of northern India. In the Indian Peninsula, they are confined mainly to the river deltas on the east coast, the lower valleys of the Narmada and the Tapti, northern Gujarat and Chhattisgarh plains, They occupy 7.71akh sq km or about 24 per cent of India's land area. The soils are suitable for the cultivation of cereals, pulses, oilseeds, cotton, sugarcane and vegetables. These favour jute cultivation in the eastern parts -of the Indian plains.
Alluvial soils are found throughout the plains of northern India. In the Indian Peninsula, they are confined mainly to the river deltas on the east coast, the lower valleys of the Narmada and the Tapti, northern Gujarat and Chhattisgarh plains, They occupy 7.71akh sq km or about 24 per cent of India's land area. The soils are suitable for the cultivation of cereals, pulses, oilseeds, cotton, sugarcane and vegetables. These favour jute cultivation in the eastern parts -of the Indian plains.
SOIL GROUPS
SOIL GROUPS
The Indian Council of Agricultural Research (ICAR) has divided Indian soils into eight major types. These are red soils, black soils, the Indo-Gangetic alluvium or alluvial soils, the lateritic and laterite soils, forest and hill soils, alkaline and saline soils, desert soils and peaty and organic soils.
Red Soils They are mainly formed due to decomposition of ancient crystalline rocks like granites and gneisses and from rock types rich in minerals such as iron and magnesium. The term 'red soil' is due to the wide diffusion of iron oxides through the materials of the soil. The red soils are generally poor in nitrogen, hosphorous, humus but rich in potash. They are siliceous or aluminous in character. The clay fraction of the red soils generally consists of kaolinitic minerals.
Morphologically, they are divided into the cloddy structured red loams which have little of concretionary substances and the loose, friable red earths which have much of secondary concretions of sesquioxide clays.
Red soils cover almost the whole of Tamil Nadu, Karnataka, Andhra Pradesh, south-eastern Maharashtra, eastern parts of Madhya .Pradesh, parts of Orissa, Jharkhand and Bundelkhand. They practically encircle the entire black soil region on all sides. They extend northwards in the west along the Konkan coast of Maharashtra.
Though red soils are suitable for cultivating almost all crop types, they are most suitable for growing vegetables, rice, ragi and tobacco. Heavy clays at low-levelled lands are suitable for sugarcane cultivation. Red soils of coarse texture at high levels are good for potato and groundnut crops. Irrigation is a must for these soils.
Black Cotton Soils Also called regur, the soils are derived from basalts of Deccan Traps. They derive their name from their black colour which may be owing to presence of titanium, iron or some other organic compounds. Black soils are fine-grained and highly argillaceous and consist of calcium and magnesium carbonates. One 0f their chief characteristics is their swelling or shrinkage with increase or decrease in the moisture content. Black soils contain high quantities of iron, aluminium, lime and magnesia and generally show poor percentages of phosphorous, nitrogen and organic matter.
Black soils are spread over north-west Deccan Plateau and are made up of lava flows. They cover the plateaus of Maharashtra, Saurashtra, Malwa, Madhya Pradesh, and parts of Chhattisgarh. They extend eastwards in the soutli along Godavari and Krishna valleys. The climatic conditioru and the nature of parent-rock materials are equally impor. tant in the formation of black soils. In. Madhya Pradesh, two distinct kinds of black soils are found: (i) deep heav} black soils covering the Narmada Valley, and (ii) shallo¥. black soils in other areas. The cotton-growing areas arE mainly covered by the deep heavy black soil.
Apart from cotton cultivation, these fertile soils arE suitable for growing cereals, oilseeds, citrus fruits ane vegetables, tobacco and sugarcane. Their moisture-retentiveness makes them suitable for dry farming. As they havE large amounts of water-soluble salts, heavy irrigation 0f these soils must be avoided.
The Indian Council of Agricultural Research (ICAR) has divided Indian soils into eight major types. These are red soils, black soils, the Indo-Gangetic alluvium or alluvial soils, the lateritic and laterite soils, forest and hill soils, alkaline and saline soils, desert soils and peaty and organic soils.
Red Soils They are mainly formed due to decomposition of ancient crystalline rocks like granites and gneisses and from rock types rich in minerals such as iron and magnesium. The term 'red soil' is due to the wide diffusion of iron oxides through the materials of the soil. The red soils are generally poor in nitrogen, hosphorous, humus but rich in potash. They are siliceous or aluminous in character. The clay fraction of the red soils generally consists of kaolinitic minerals.
Morphologically, they are divided into the cloddy structured red loams which have little of concretionary substances and the loose, friable red earths which have much of secondary concretions of sesquioxide clays.
Red soils cover almost the whole of Tamil Nadu, Karnataka, Andhra Pradesh, south-eastern Maharashtra, eastern parts of Madhya .Pradesh, parts of Orissa, Jharkhand and Bundelkhand. They practically encircle the entire black soil region on all sides. They extend northwards in the west along the Konkan coast of Maharashtra.
Though red soils are suitable for cultivating almost all crop types, they are most suitable for growing vegetables, rice, ragi and tobacco. Heavy clays at low-levelled lands are suitable for sugarcane cultivation. Red soils of coarse texture at high levels are good for potato and groundnut crops. Irrigation is a must for these soils.
Black Cotton Soils Also called regur, the soils are derived from basalts of Deccan Traps. They derive their name from their black colour which may be owing to presence of titanium, iron or some other organic compounds. Black soils are fine-grained and highly argillaceous and consist of calcium and magnesium carbonates. One 0f their chief characteristics is their swelling or shrinkage with increase or decrease in the moisture content. Black soils contain high quantities of iron, aluminium, lime and magnesia and generally show poor percentages of phosphorous, nitrogen and organic matter.
Black soils are spread over north-west Deccan Plateau and are made up of lava flows. They cover the plateaus of Maharashtra, Saurashtra, Malwa, Madhya Pradesh, and parts of Chhattisgarh. They extend eastwards in the soutli along Godavari and Krishna valleys. The climatic conditioru and the nature of parent-rock materials are equally impor. tant in the formation of black soils. In. Madhya Pradesh, two distinct kinds of black soils are found: (i) deep heav} black soils covering the Narmada Valley, and (ii) shallo¥. black soils in other areas. The cotton-growing areas arE mainly covered by the deep heavy black soil.
Apart from cotton cultivation, these fertile soils arE suitable for growing cereals, oilseeds, citrus fruits ane vegetables, tobacco and sugarcane. Their moisture-retentiveness makes them suitable for dry farming. As they havE large amounts of water-soluble salts, heavy irrigation 0f these soils must be avoided.
THE SOILS OF INDIA
THE SOILS OF INDIA
Soils can be divided into two broad types: (i) sedentary or residual soils which are formed directly from the rocks under them or in situ and transported; (ii) azonal soils, the soils from in situ that are transported to valleys and deltas by water and wind. To the former tYpe belong black soils red, laterite, podzolic soils fo~d in forests, saline and alkaline soils and peaty soils. The azonal type includes the soils of the forests in the northern mountains, soils constituting the coastal lowland and plains of India and desert soils.
Indian soils can be geologically classified into those of the extra-peninsular region and those that are found in the peninsular region. The young extra-peninsular soils may be sandy, loamy or of the clay-type :hey are either formed from the debris of the Deccan Plateau or the Himalayan region or from the deposited silt of plains and valleys. They include the alluvial, saline, alkaline, terai, desert, chestnut, peaty and marshy soils of the Ganga, Brahmaputra and Sutlej plains. This region includes the Montane region that bears hill and mountain soils, mountain meadow and the acidic podzolic soils. Soils of the Deccan Plateau comprise black soils, red soils, laterites, alluvial soils, saline and alkaline soils, mixed red and black, and yellow and red soils.
Soils can be divided into two broad types: (i) sedentary or residual soils which are formed directly from the rocks under them or in situ and transported; (ii) azonal soils, the soils from in situ that are transported to valleys and deltas by water and wind. To the former tYpe belong black soils red, laterite, podzolic soils fo~d in forests, saline and alkaline soils and peaty soils. The azonal type includes the soils of the forests in the northern mountains, soils constituting the coastal lowland and plains of India and desert soils.
Indian soils can be geologically classified into those of the extra-peninsular region and those that are found in the peninsular region. The young extra-peninsular soils may be sandy, loamy or of the clay-type :hey are either formed from the debris of the Deccan Plateau or the Himalayan region or from the deposited silt of plains and valleys. They include the alluvial, saline, alkaline, terai, desert, chestnut, peaty and marshy soils of the Ganga, Brahmaputra and Sutlej plains. This region includes the Montane region that bears hill and mountain soils, mountain meadow and the acidic podzolic soils. Soils of the Deccan Plateau comprise black soils, red soils, laterites, alluvial soils, saline and alkaline soils, mixed red and black, and yellow and red soils.
CLASSIFICATION OF WORLD SOILS
CLASSIFICATION OF WORLD SOILS
Soil scientists recognise that soils can be subdivided into three orders: the zonal soils formed under conditions of good soil drainage through prolonged action of climate and vegetation; intrazonal soils formed under conditions of very poor drainage (like in bogs or flood-plain meadows) or upon limestones whose influence is dominant; and azonal soils with no well-developed profile characteristics either because they are on steep slopes or for lack of sufficient time to develop. While zonal and intrazonal soils can be classified and have distinctive profile characteristics due to long development, the azonal soils have poorly developed profiles and cannot be classified.
Podzol soils are zonal soils of cool humid climates and are most widely distributed. Podzol soils require cold winter and adequate precipitation throughout the year. Found in the northern Great Lakes states of USA, Asia, and mountain parts of New England, these soils are low in fertility and cannot produce crops to feed large populations. Coniferous forests are associated with podzol soils.
Grey-brown podzolic soils contain important bases but are somewhat acid. Deciduous forests (oak, beech, maple) are associated with this soil. They are found over western Europe, north China and northern Japan and eastern-central USA.
Red-yellow podzolic soils lie in the zone of increasingly warmer climate but abundant precipitation, occupying the southern United States, southern Brazil and southeastern Paraguay; smaller coastal zones in south Africa, Australia and New Zealand. Natural vegetation of this soil type is dominantly rain forest of both tropical and temperate classes.
Latosols are soils of humid tropical and equatorial zones, corresponding closely with wet equatorial climate and the tropical wet-dry climate, favourable for luxuriant growth of broad leaf evergreen rain forest and woodland. The local accumulations of iron and aluminium sesquioxides develop into layers that can be cut out as building brickscalled laterites. Areas of occurrence are India, south-east Asia, Amazon Basin and Congo Basin.
Tundra soils are sometimes classified as intrazonal because of being poorly drained. Tundra climates of the northern continental fringes provide favourable conditions for tundra soils, Le., tundra regions of Siberia and North America.
Chernozem soils or black earth is the most widely distributed of the zonal soils in semi-arid climate. They are associated with humid continental climates, hot summers and cold winters and drought periods. Naturai vegetation of these soils are steppe grasslands and prairies. Rich in calcium, the, outstanding feature of these soils is their productivity for small grain crops-wheat, oats, barley and rye. They occur in Ukraine, USA and in the Deccan in India.
Prairie soils (Brunizem soils) are similar to the chernozems in the general profile and appearance but do not have an excess of calcium carbonate. Prairie soils are extremely productive with somewhat moist climate; most important crop is corn. These are found in the Mississippi valley and the great plain states of the United States.
Chestnut soils are similar to chernozems but contain less humus and so are lighter in colour. These soils are fertile under conditions of adequate rainfall or irrigation. But they lie in the hazardous' belt in which years of drought and adequate rainfall alternate; the semi-arid middle latitude steppe lands, in North America and Asia.
Brown soils replace the chestnut soils in the still more arid regions, with still less humus and thus have a lighter colour. These are tYpical of the middle-latitude steppes and support a light growth of grasses suitable for livestock grazing.
Grey-desert soils and red-desert soils are soils of middle latitude deserts and tropical deserts. Grey desert soils contain little humus due to sparse vegetation growth. Red desert soils are found in more arid, hotter tropical deserts. Humus is minimum. The activity of plants and animals reaches the minimum in red desert soils. The colour is derived from small amounts of oxides of iron.
Hydromorphic soils (intrazonal) are associated with marshes, swamps, bogs, or poorly drained flat uplands. Holomorphic soils (intrazonal) are formed by the pedogenic process of salinisation. The areas where silts and clays make up a large proportion of the soil-body are called holomorphic soil. These are classified into saline soils (containing chlorides, sulphates, carbonates and bicarbonates of sodium, calcium, IJ1<1gnesium and potassium); and alkali soils (predominantly sodium salts, especially sodium carbonate NaC03). The most famous salt area is the salt flats of Great Salt Lake in Utah, on which numerous automobile speed records have been set.
Calcimorphic soils are another class of intrazonal soils whose characteristics are strongly related to the presence of lime-rich parent material. The process of calcification (introduction of calcium) is dominant in the formation of calcimorphic soils, The soils of sub humid tropics are productive agriculturally, yielding cotton, corn and alfalfa.
Soil scientists recognise that soils can be subdivided into three orders: the zonal soils formed under conditions of good soil drainage through prolonged action of climate and vegetation; intrazonal soils formed under conditions of very poor drainage (like in bogs or flood-plain meadows) or upon limestones whose influence is dominant; and azonal soils with no well-developed profile characteristics either because they are on steep slopes or for lack of sufficient time to develop. While zonal and intrazonal soils can be classified and have distinctive profile characteristics due to long development, the azonal soils have poorly developed profiles and cannot be classified.
Podzol soils are zonal soils of cool humid climates and are most widely distributed. Podzol soils require cold winter and adequate precipitation throughout the year. Found in the northern Great Lakes states of USA, Asia, and mountain parts of New England, these soils are low in fertility and cannot produce crops to feed large populations. Coniferous forests are associated with podzol soils.
Grey-brown podzolic soils contain important bases but are somewhat acid. Deciduous forests (oak, beech, maple) are associated with this soil. They are found over western Europe, north China and northern Japan and eastern-central USA.
Red-yellow podzolic soils lie in the zone of increasingly warmer climate but abundant precipitation, occupying the southern United States, southern Brazil and southeastern Paraguay; smaller coastal zones in south Africa, Australia and New Zealand. Natural vegetation of this soil type is dominantly rain forest of both tropical and temperate classes.
Latosols are soils of humid tropical and equatorial zones, corresponding closely with wet equatorial climate and the tropical wet-dry climate, favourable for luxuriant growth of broad leaf evergreen rain forest and woodland. The local accumulations of iron and aluminium sesquioxides develop into layers that can be cut out as building brickscalled laterites. Areas of occurrence are India, south-east Asia, Amazon Basin and Congo Basin.
Tundra soils are sometimes classified as intrazonal because of being poorly drained. Tundra climates of the northern continental fringes provide favourable conditions for tundra soils, Le., tundra regions of Siberia and North America.
Chernozem soils or black earth is the most widely distributed of the zonal soils in semi-arid climate. They are associated with humid continental climates, hot summers and cold winters and drought periods. Naturai vegetation of these soils are steppe grasslands and prairies. Rich in calcium, the, outstanding feature of these soils is their productivity for small grain crops-wheat, oats, barley and rye. They occur in Ukraine, USA and in the Deccan in India.
Prairie soils (Brunizem soils) are similar to the chernozems in the general profile and appearance but do not have an excess of calcium carbonate. Prairie soils are extremely productive with somewhat moist climate; most important crop is corn. These are found in the Mississippi valley and the great plain states of the United States.
Chestnut soils are similar to chernozems but contain less humus and so are lighter in colour. These soils are fertile under conditions of adequate rainfall or irrigation. But they lie in the hazardous' belt in which years of drought and adequate rainfall alternate; the semi-arid middle latitude steppe lands, in North America and Asia.
Brown soils replace the chestnut soils in the still more arid regions, with still less humus and thus have a lighter colour. These are tYpical of the middle-latitude steppes and support a light growth of grasses suitable for livestock grazing.
Grey-desert soils and red-desert soils are soils of middle latitude deserts and tropical deserts. Grey desert soils contain little humus due to sparse vegetation growth. Red desert soils are found in more arid, hotter tropical deserts. Humus is minimum. The activity of plants and animals reaches the minimum in red desert soils. The colour is derived from small amounts of oxides of iron.
Hydromorphic soils (intrazonal) are associated with marshes, swamps, bogs, or poorly drained flat uplands. Holomorphic soils (intrazonal) are formed by the pedogenic process of salinisation. The areas where silts and clays make up a large proportion of the soil-body are called holomorphic soil. These are classified into saline soils (containing chlorides, sulphates, carbonates and bicarbonates of sodium, calcium, IJ1<1gnesium and potassium); and alkali soils (predominantly sodium salts, especially sodium carbonate NaC03). The most famous salt area is the salt flats of Great Salt Lake in Utah, on which numerous automobile speed records have been set.
Calcimorphic soils are another class of intrazonal soils whose characteristics are strongly related to the presence of lime-rich parent material. The process of calcification (introduction of calcium) is dominant in the formation of calcimorphic soils, The soils of sub humid tropics are productive agriculturally, yielding cotton, corn and alfalfa.
SOIL FORMING FACTORS
SOIL FORMING FACTORS
Many types of processes and influences, together known as soil formers, act to develop a soil. Some of these are passive conditions; others are active agents. Many years ago, Russian pedologist Dukuchaiev firmly established five main soil formers: (i) parent material; (H) landform; (Hi) time; (iv) climate; and (v) biological activity. The main ways in which the parent material is liable to have a lasting effect on soils are through texture and fertility. Thus, sandstone grit stones give rise to fr~e-draining coarse-textured soils and shales produce finer soils. Of the relief features, slope and situation determine erosion and depositional factors; topography also affects drainage conditions and controls exposure to the sun. Time is a factor in creating mature soils. Young soils are thin, having evolved, for example, from recently deposited river alluvium of glacial till. However, it is difficult to be precise about the role of time factor in soil formation.
Of the active soil formers, climate is perhaps the most important. (a) Moisture conditions affect the soil; an excess of precipitation over evaporation tends to leach away the soil, while in dry climates when evaporation exceeds precipitation, there is a soil-water deficit, and the soil is dry. (b) Temperature influences chemical activity (which is generally increased by higher temperature and decreased by lower temperature), and the bacterial activity, which is increased by warmer soil temperatures, and consequently, humus is reduced as in the humid tropics. (c) Winds may increase the evaporation from soil surfaces and may remove surface soil in acid regions. Wind-blown dust may accumulate and thus provide the parent material of soil. Both plants and animals, from microscopic bacteria to large mammals including man, influence soil development. Plants. help to che~k erosion and maintain soil fertility.
Influences of animals in the soil is largely mechanical, sometimes also chemical. Earthworms are important agents in humid areas. They rework the soil by burrowing and also change the texture and chemical composition of the soil as it passes through their digestive systems. Ants and termites bring large quantities of soil from lower layers to the surface. Soil characteristics closely determine the type of animals present in the soil. Soils and the ecosystem share a close relationship.
Many types of processes and influences, together known as soil formers, act to develop a soil. Some of these are passive conditions; others are active agents. Many years ago, Russian pedologist Dukuchaiev firmly established five main soil formers: (i) parent material; (H) landform; (Hi) time; (iv) climate; and (v) biological activity. The main ways in which the parent material is liable to have a lasting effect on soils are through texture and fertility. Thus, sandstone grit stones give rise to fr~e-draining coarse-textured soils and shales produce finer soils. Of the relief features, slope and situation determine erosion and depositional factors; topography also affects drainage conditions and controls exposure to the sun. Time is a factor in creating mature soils. Young soils are thin, having evolved, for example, from recently deposited river alluvium of glacial till. However, it is difficult to be precise about the role of time factor in soil formation.
Of the active soil formers, climate is perhaps the most important. (a) Moisture conditions affect the soil; an excess of precipitation over evaporation tends to leach away the soil, while in dry climates when evaporation exceeds precipitation, there is a soil-water deficit, and the soil is dry. (b) Temperature influences chemical activity (which is generally increased by higher temperature and decreased by lower temperature), and the bacterial activity, which is increased by warmer soil temperatures, and consequently, humus is reduced as in the humid tropics. (c) Winds may increase the evaporation from soil surfaces and may remove surface soil in acid regions. Wind-blown dust may accumulate and thus provide the parent material of soil. Both plants and animals, from microscopic bacteria to large mammals including man, influence soil development. Plants. help to che~k erosion and maintain soil fertility.
Influences of animals in the soil is largely mechanical, sometimes also chemical. Earthworms are important agents in humid areas. They rework the soil by burrowing and also change the texture and chemical composition of the soil as it passes through their digestive systems. Ants and termites bring large quantities of soil from lower layers to the surface. Soil characteristics closely determine the type of animals present in the soil. Soils and the ecosystem share a close relationship.
DESILICATION and LATERISATION
DESILICATION It is the process by which silica, together with many bases, is removed from a soil profile by intense weathering and leaching. It is characteristic of humid, tropical areas and leads to development of ferralsol soils. Such soils often have low organic content because of rapid decomposition by micro-organisms.
LATERISATION This is a weathering process by which iron and aluminium oxides become concentrated in the upper layer of the soil. It is common in tropical and subtropical areas where wet and dry seasons alternate.
LATERISATION This is a weathering process by which iron and aluminium oxides become concentrated in the upper layer of the soil. It is common in tropical and subtropical areas where wet and dry seasons alternate.
PODZOLISATION
PODZOLISATION (CHELUVIATION) It is a process widespread in acidic soils. In these cases, because of the different solubility of the various minerals, a situation develops in which the upper layers of the soil become rich in silica, tending towards pure quartz, and take on a cha:-1.cteristic ash-grey appearance. The lower alluvial horizon is rich in sesquioxides of iron. The basic cause of these'
translocations lies in the leaching action of certain organic compounds known as chelating agents. Podzol profiles are usually associated with coniferous and heathland vegetation. Podzol soils extend in a broad belt across Russia and North America.
GLEYING The process occurs in wet or waterlogged soils. The anaerobic (oxygen-deficient) conditions lead to the process of red.uction in which ferric oxide is reduced to ferrous oxide. This gives the soil a blue-grey colour. This horizon is called gley, a compact layer of sticky structureless day. The gley horizon usually occurs within the zone of permanent groundwater situation. Where the soil periodically dries out, the ferrous solutes may oxidise back into the ferric state. Since this process is not uniform, it gives the soil a mottled or blotchy look, typified by patchy red colours.
translocations lies in the leaching action of certain organic compounds known as chelating agents. Podzol profiles are usually associated with coniferous and heathland vegetation. Podzol soils extend in a broad belt across Russia and North America.
GLEYING The process occurs in wet or waterlogged soils. The anaerobic (oxygen-deficient) conditions lead to the process of red.uction in which ferric oxide is reduced to ferrous oxide. This gives the soil a blue-grey colour. This horizon is called gley, a compact layer of sticky structureless day. The gley horizon usually occurs within the zone of permanent groundwater situation. Where the soil periodically dries out, the ferrous solutes may oxidise back into the ferric state. Since this process is not uniform, it gives the soil a mottled or blotchy look, typified by patchy red colours.
PODZOLISATION
PODZOLISATION (CHELUVIATION) It is a process widespread in acidic soils. In these cases, because of the different solubility of the various minerals, a situation develops in which the upper layers of the soil become rich in silica, tending towards pure quartz, and take on a cha:-1.cteristic ash-grey appearance. The lower alluvial horizon is rich in sesquioxides of iron. The basic cause of these'
translocations lies in the leaching action of certain organic compounds known as chelating agents. Podzol profiles are usually associated with coniferous and heathland vegetation. Podzol soils extend in a broad belt across Russia and North America.
GLEYING The process occurs in wet or waterlogged soils. The anaerobic (oxygen-deficient) conditions lead to the process of red.uction in which ferric oxide is reduced to ferrous oxide. This gives the soil a blue-grey colour. This horizon is called gley, a compact layer of sticky structureless day. The gley horizon usually occurs within the zone of permanent groundwater situation. Where the soil periodically dries out, the ferrous solutes may oxidise back into the ferric state. Since this process is not uniform, it gives the soil a mottled or blotchy look, typified by patchy red colours.
translocations lies in the leaching action of certain organic compounds known as chelating agents. Podzol profiles are usually associated with coniferous and heathland vegetation. Podzol soils extend in a broad belt across Russia and North America.
GLEYING The process occurs in wet or waterlogged soils. The anaerobic (oxygen-deficient) conditions lead to the process of red.uction in which ferric oxide is reduced to ferrous oxide. This gives the soil a blue-grey colour. This horizon is called gley, a compact layer of sticky structureless day. The gley horizon usually occurs within the zone of permanent groundwater situation. Where the soil periodically dries out, the ferrous solutes may oxidise back into the ferric state. Since this process is not uniform, it gives the soil a mottled or blotchy look, typified by patchy red colours.
SOIL FORMING PROCESS
The soil forming processes include gains and losses of material to the soil profile, movement of matter from one part of the profile to the other and chemical transformation within individual horizons. The major soil processes include weathering, translocation, organic changes, gleying, podzolisation and desilication.
TRANSLOCATION This term includes several kinds of movement of material within the soil body, mainly by the agent of water. Leaching is the downward movement of material in solution or colloid suspension. Eluviation is the physical downward washing of clay or other fine particles. Leaching and eluviation may move material right out of the soil system. But commonly the solutes and particles are redeposited at the base of the soil profile, forming an alluvial or enriched horizon.
In calcification, calcium carbonate accumulates within soils to form a oncentration. This takes place in arid or semi-arid environments where potential vapotranspiration exceeds precipitation. The movement of soil solution is likely to be upward, drawn by capillary attraction towards the drying surface. The calcification is enhanced in grasslands. Grass uses calcium, drawing it up from the lower layers of soil, and returning it to the soil when it dies. Salinisation or aikalisation is the process by which soils are enriched with salt. In cases where evaporation is very intense, calcium or sodium salts may form a whitish layer on the soil surface, harmful to plant growth. Such accumulation is the result of capillary rise of water from a water table that is saline and close to the surface. Or it is induced by man by irrigation praCtices where evaporation is intense and produces salinisation unless counteracted by regular flushing, deep ploughing or chemical treatment.
ORGANIC CHANGES Organic accumulation in the soil profile takes place mainly at the ground surface with the decay of plant material. Its forms are: degradation or the action '.If fungi, algae, small insects and worms, reducing the surface litter to its skeletal material; humification or formation of humus of the dead organic content of the soil, mainly through bacteriological activity; mineralisation or the process of decomposition of humus which releases nitrogen compounds into the soil. All these processes always accompany each other.
TRANSLOCATION This term includes several kinds of movement of material within the soil body, mainly by the agent of water. Leaching is the downward movement of material in solution or colloid suspension. Eluviation is the physical downward washing of clay or other fine particles. Leaching and eluviation may move material right out of the soil system. But commonly the solutes and particles are redeposited at the base of the soil profile, forming an alluvial or enriched horizon.
In calcification, calcium carbonate accumulates within soils to form a oncentration. This takes place in arid or semi-arid environments where potential vapotranspiration exceeds precipitation. The movement of soil solution is likely to be upward, drawn by capillary attraction towards the drying surface. The calcification is enhanced in grasslands. Grass uses calcium, drawing it up from the lower layers of soil, and returning it to the soil when it dies. Salinisation or aikalisation is the process by which soils are enriched with salt. In cases where evaporation is very intense, calcium or sodium salts may form a whitish layer on the soil surface, harmful to plant growth. Such accumulation is the result of capillary rise of water from a water table that is saline and close to the surface. Or it is induced by man by irrigation praCtices where evaporation is intense and produces salinisation unless counteracted by regular flushing, deep ploughing or chemical treatment.
ORGANIC CHANGES Organic accumulation in the soil profile takes place mainly at the ground surface with the decay of plant material. Its forms are: degradation or the action '.If fungi, algae, small insects and worms, reducing the surface litter to its skeletal material; humification or formation of humus of the dead organic content of the soil, mainly through bacteriological activity; mineralisation or the process of decomposition of humus which releases nitrogen compounds into the soil. All these processes always accompany each other.
CHEMICAL ASPECT OF SOIL
CHEMICAL ASPECT OF SOIL Tiny particles with unusual chemical properties called soil colloids are included in the clay fraction of the soil; these are in the form of thin flakes, which remain suspended indefinitely in water. Colloids may be organic, made up of a very finely divided humus or mineral, in which case they are known as day minerals. Together, the two types make up a clay-humus complex. Most soils have more clay minerals than organic colloids. Clay minerals are of great importance because they are in a state of continuous chemical change, which is fundamental to soil formation.
Unusual chemical properties of colloids result from their vast surface area for a given weight. Colloids have a property of being electrically charged and can therefore attract and hold ions. Ions of calcium, magnesium and potassium are known in soil science as bases. These bases may be given by the colloids to plants, which require them for growth, by a process known. as base exchange. Some bases are more readily given up than others. Particularly the metallic ions, like potassium and sodium, tend to be replaced by hydrogen.
The hydrogen in the soil solution makes for an acid condition. The concentration of hydrogen ions in soil solution is known as the pH of the soil, and is the measure of soil acidity or alkalinity. Soil acidity is thus the property related to the proportion of exchangeable hydrogen in soil with relation to other elements. Over a period of time, this base exchange makes the soil more acid, unless bases ate replenished. The decomposition of plants and animals helps in recycling of bases to the soil, under natural conditions. Artificial supply of the bases in the form
of fertilisers is required where the vegetation is removed by man by cutting or cropping.
Unusual chemical properties of colloids result from their vast surface area for a given weight. Colloids have a property of being electrically charged and can therefore attract and hold ions. Ions of calcium, magnesium and potassium are known in soil science as bases. These bases may be given by the colloids to plants, which require them for growth, by a process known. as base exchange. Some bases are more readily given up than others. Particularly the metallic ions, like potassium and sodium, tend to be replaced by hydrogen.
The hydrogen in the soil solution makes for an acid condition. The concentration of hydrogen ions in soil solution is known as the pH of the soil, and is the measure of soil acidity or alkalinity. Soil acidity is thus the property related to the proportion of exchangeable hydrogen in soil with relation to other elements. Over a period of time, this base exchange makes the soil more acid, unless bases ate replenished. The decomposition of plants and animals helps in recycling of bases to the soil, under natural conditions. Artificial supply of the bases in the form
of fertilisers is required where the vegetation is removed by man by cutting or cropping.
SOIL PROFILE
Soil profile is a vertical section of the soil through all its horizons and extends up to the parent materials. The soil profile consists of the weathered material derived from the rock. But the parent material itself does not form a part of it. Nor has it any horizontal layers termed as horizons. A soil profile generally has four main horizons in it-true soil at the top (Horizon A), subsoil (Horizon B), weathered rock (Horizon C) and bedrock (Horizon D). Each horizon is quite distinct from the other with its own physical and chemical composition.
Horizon' A; may consist of sub-horizons richer in organic matter intimately mixed with mineral matter. There is loss of clay, iron or aluminium due to high concentration of quartz. In Horizon 'B', there is a dominant concentration of clay, iron, aluminium, of humus alone or in combination. The 'C' Horizon excludes the bedrock from which' A' and 'B' Horizons are presumed to have been formed.
The soil profile reveals the surface and the sub-surface characteristics and qualities- depth, texture, structure, drain age conditions and soil-moisture relationships-of the soil which directly affect plant growth. Hence, its study is important from the viewpoint of crop husbandry. The soil profile is taken as a unit of study which helps 'the investigators both to classify the soils and to understand soilmoisture-plant relationships. The study of soil profile thus - furnishes a base which has to be supplemented by physical, chemical and biological properties of the soils.
Horizon' A; may consist of sub-horizons richer in organic matter intimately mixed with mineral matter. There is loss of clay, iron or aluminium due to high concentration of quartz. In Horizon 'B', there is a dominant concentration of clay, iron, aluminium, of humus alone or in combination. The 'C' Horizon excludes the bedrock from which' A' and 'B' Horizons are presumed to have been formed.
The soil profile reveals the surface and the sub-surface characteristics and qualities- depth, texture, structure, drain age conditions and soil-moisture relationships-of the soil which directly affect plant growth. Hence, its study is important from the viewpoint of crop husbandry. The soil profile is taken as a unit of study which helps 'the investigators both to classify the soils and to understand soilmoisture-plant relationships. The study of soil profile thus - furnishes a base which has to be supplemented by physical, chemical and biological properties of the soils.
SOIL COLOUR
The physical property of 'colour' of the soil, though a minor factor, can tell much about its formation and constituents. Soil horizons are distinguishable by col~ur differences. One sequence of colours ranges from white, through brown to black, as a result of an increasing quantity of humus. In the middle latitudes soils range from black or dark brown in the cool, humid areas to light brown or grey in the semi-arid steppe lands and deserts. Desert soils have little humus.
Red and yellow are common soil colours and are the results of small quantities of iron compounds. Red colour is particularly associated with iron oxide (Fep3)' Red colour indicates that the soil is well-drained, but locally the colour may be derived from a red source rock, like red shale or sandstone. Yellow colour may indicate the presence of the same iron compound combined with water (hydrated iron oxide). Greyish and bluish colour in soils of humid climates often means the presence of reduced iron compounds in the soil and indicates poor drainage or bog conditions. Greyish soils in dry climates mean a meagre amount of humus; a white colour may be the result of salt depositions in soil. Although some recently formed soils retain the colour of the parent material, the colour of a fully developed soil is independent of what lies beneath it.
Red and yellow are common soil colours and are the results of small quantities of iron compounds. Red colour is particularly associated with iron oxide (Fep3)' Red colour indicates that the soil is well-drained, but locally the colour may be derived from a red source rock, like red shale or sandstone. Yellow colour may indicate the presence of the same iron compound combined with water (hydrated iron oxide). Greyish and bluish colour in soils of humid climates often means the presence of reduced iron compounds in the soil and indicates poor drainage or bog conditions. Greyish soils in dry climates mean a meagre amount of humus; a white colour may be the result of salt depositions in soil. Although some recently formed soils retain the colour of the parent material, the colour of a fully developed soil is independent of what lies beneath it.
SOIL TEXTURE AND STRUCTURE
SOIL TEXTURE AND STRUCTURE
Soil texture refers to particle sizes composing the soil. Particles are classified as various grades of gravel, sand (20.2 mm), silt (0.2-0.0002 mm) and clay (less than 0.002 mm), in decreasing order of size. Most soils contain some of each category. A well-balanced mixture (i.e., a loam soil) gives the optimum conditions of water holding, temperature and supply of plant nutrients. It is often possible to identify soil texture by its 'feel'; e.g., sand feels gritty to touch. Texture results principally from the mineral composition of the parent material.
'Soil structure refers to the way in which soil grains are grouped together into larger pieces held together by colloids. Soil structure influences the rate at which water is absorbed by the soil, the susceptibility of soil to erosion and the ease of soil cultivation. Soil structure may be: (a) blocky or nutlike, where the lumps or pods of soil have irregular pieces with sharp comers and edges; (b) crumb or granular structure, with more or less spherical pieces; (c) columnar or prismatic structure made up of- vertical columns or prisms; (d) platy structure, consisting of plates of flat pieces, in a horizontal position.
Soil texture refers to particle sizes composing the soil. Particles are classified as various grades of gravel, sand (20.2 mm), silt (0.2-0.0002 mm) and clay (less than 0.002 mm), in decreasing order of size. Most soils contain some of each category. A well-balanced mixture (i.e., a loam soil) gives the optimum conditions of water holding, temperature and supply of plant nutrients. It is often possible to identify soil texture by its 'feel'; e.g., sand feels gritty to touch. Texture results principally from the mineral composition of the parent material.
'Soil structure refers to the way in which soil grains are grouped together into larger pieces held together by colloids. Soil structure influences the rate at which water is absorbed by the soil, the susceptibility of soil to erosion and the ease of soil cultivation. Soil structure may be: (a) blocky or nutlike, where the lumps or pods of soil have irregular pieces with sharp comers and edges; (b) crumb or granular structure, with more or less spherical pieces; (c) columnar or prismatic structure made up of- vertical columns or prisms; (d) platy structure, consisting of plates of flat pieces, in a horizontal position.
PHYSICAL AND, CHEMICAL CHARACTERISTICS OF SOIL
PHYSICAL AND, CHEMICAL CHARACTERISTICS OF SOIL
Soils contain matter in three states.
(i) The solid portion is of two types. (a) The organic
part consists of living and decayed plant and animal materials like roots, leaves and worms. Humus is the endproduct of decay. It is black, amorphous organic matter, which is responsible for the fertility of soil. (b) The inorganic part (mineral part) of soil is made up of particles derived from the par~nt material, i.e., the rocks, which weather to form the soil.
(ii) The Liquid portion or soil water is the water temporarily held in the soil, derived from direct precipitation and from run-off, seepage and groundwater. Soil water is a dilute but complex chemical solution of substances like bicarbonates, sulphates, chlorides, nitrates, phosphates and silicates of calcium, potassium, magnesium, sodium and iron.
(Hi) The gaseous portion or soil air or soil atmosphere occupies the pore spaces of the soil when it is not saturated with water. Soil air has been analysed and found to contain an excess of carbon dioxide, but a deficiency of oxygen and nitrogen. Soil air and soil water are present in inverse proportion to each other.
Soils contain matter in three states.
(i) The solid portion is of two types. (a) The organic
part consists of living and decayed plant and animal materials like roots, leaves and worms. Humus is the endproduct of decay. It is black, amorphous organic matter, which is responsible for the fertility of soil. (b) The inorganic part (mineral part) of soil is made up of particles derived from the par~nt material, i.e., the rocks, which weather to form the soil.
(ii) The Liquid portion or soil water is the water temporarily held in the soil, derived from direct precipitation and from run-off, seepage and groundwater. Soil water is a dilute but complex chemical solution of substances like bicarbonates, sulphates, chlorides, nitrates, phosphates and silicates of calcium, potassium, magnesium, sodium and iron.
(Hi) The gaseous portion or soil air or soil atmosphere occupies the pore spaces of the soil when it is not saturated with water. Soil air has been analysed and found to contain an excess of carbon dioxide, but a deficiency of oxygen and nitrogen. Soil air and soil water are present in inverse proportion to each other.
Soil
Soil scientists restrict the word 'soil' to the surface material which, over a long period of time, has come to have distinct layers or horizons. A soil has certain distinctive physical, chemical and biological qualities, which permit it to support plant growth and which differentiate it from the infertile substratum lying below. While soil is composed of poth mineral and organic particles, the underlying layers may usually be composed of mineral material.
Soil is a dynamic layer-a changing and developing body-as many complex chemical, physical and biological activities continue constantly in it Soils become adjusted to conditions of climate, landform and vegetation and change internally when the controlling conditions change.
The scientific study of the characteristics, development and distribution of soils is called pedology; the process of soil formation is known as pedogenesis.
Soil is a dynamic layer-a changing and developing body-as many complex chemical, physical and biological activities continue constantly in it Soils become adjusted to conditions of climate, landform and vegetation and change internally when the controlling conditions change.
The scientific study of the characteristics, development and distribution of soils is called pedology; the process of soil formation is known as pedogenesis.
HYDROLOGICAL CYCLE AND GLOBAL WATER BALANCE
HYDROLOGICAL CYCLE AND GLOBAL WATER BALANCE
The cyclic movement of water between the atmosphere, the land and the sea is called the water cycle. It involves the movement of water in a great series of continuous interchanges 'of both geographical position and physical change. Water is released into the atmosphere as water vapour through evaporation from the oceans, rivers, and lakes, and through evapo-transpiration from plants and the ground surface. Oceans are the biggest reservoirs of free water, from which the evaporation totals about 455,000 cu km per year. Evaporation from soil, plants and water surfaces of the continents totals 62,000 cu km per year.
Within the atmosphere water vapour condenses to form clouds and is returned'to the land and to its water bodies as precipitation. This quantity of precipitation must be equal to the quantity of evaporation that takes place. Precipitation is unevenly distributed between lan~ and oceans. The amount of precipitation gained by the land is more than the evaporation that takes place from the land surface.
This excess quantity flows over or under the ground surface to reach the sea; collectively called runoff, this forms a continuous cycle. However, great inequalities exist in the global amounts of water stored in gaseous, liquid and solid states.
Water flowing exposed or ponded upon land is surface water; water occupying openings in the soil, overburden or bedrock is subsurface water.
The cyclic movement of water between the atmosphere, the land and the sea is called the water cycle. It involves the movement of water in a great series of continuous interchanges 'of both geographical position and physical change. Water is released into the atmosphere as water vapour through evaporation from the oceans, rivers, and lakes, and through evapo-transpiration from plants and the ground surface. Oceans are the biggest reservoirs of free water, from which the evaporation totals about 455,000 cu km per year. Evaporation from soil, plants and water surfaces of the continents totals 62,000 cu km per year.
Within the atmosphere water vapour condenses to form clouds and is returned'to the land and to its water bodies as precipitation. This quantity of precipitation must be equal to the quantity of evaporation that takes place. Precipitation is unevenly distributed between lan~ and oceans. The amount of precipitation gained by the land is more than the evaporation that takes place from the land surface.
This excess quantity flows over or under the ground surface to reach the sea; collectively called runoff, this forms a continuous cycle. However, great inequalities exist in the global amounts of water stored in gaseous, liquid and solid states.
Water flowing exposed or ponded upon land is surface water; water occupying openings in the soil, overburden or bedrock is subsurface water.
WORLD PATTERNS OF PRECIPITATION
WORLD PATTERNS OF PRECIPITATION The pattern of rainfall in the world is very complex and only a very broad zonal pattern can be detected. Equatodal areas have the most precipitation due to high temperatures and consequent large moisture holding capacities of air, and also the presence of large oceanic water surfaces to supply the moisture. Most of the rainfall is convectional. Conversely, polar areas receive scanty rainfall because of low air temperatures.
Middle latitudes, between these two extremes, have a complicated distribution where high rainfall patterns relate particularly to the westerlies in both the hemispheres and also to their cyclone tracks. Regions of the lowest rains coincide with regions of subsiding air because of high pressure conditions. This occurs mainly in the sub tropics and on the eastern sides of the oceans (or western margins of continents), e.g., the Sahara desert. Rainfall pattern is greatly affected by the mountain ranges; for example, the Rockies and the southern Andes, where the windward sides record high rainfall, while the leeward sides are marked by rain-shadow areas. Altitude also plays an important role on the local scale. There is a general increase in precipitation with height up to about 2 km, while beyond this, precipitation diminishes due to the coolness of air.
All places having the same rainfall are represented on a map by lines called isohyets.
Middle latitudes, between these two extremes, have a complicated distribution where high rainfall patterns relate particularly to the westerlies in both the hemispheres and also to their cyclone tracks. Regions of the lowest rains coincide with regions of subsiding air because of high pressure conditions. This occurs mainly in the sub tropics and on the eastern sides of the oceans (or western margins of continents), e.g., the Sahara desert. Rainfall pattern is greatly affected by the mountain ranges; for example, the Rockies and the southern Andes, where the windward sides record high rainfall, while the leeward sides are marked by rain-shadow areas. Altitude also plays an important role on the local scale. There is a general increase in precipitation with height up to about 2 km, while beyond this, precipitation diminishes due to the coolness of air.
All places having the same rainfall are represented on a map by lines called isohyets.
MEASURING PRECIPITATION
MEASURING PRECIPITATION Rainfall is usually measured by an instrument called the rain guage, which can be operated on a simple level merely by setting out a straightsided,' flat-bottomed fan and measuring the depth to which water accumulates in a particular period. Precipitation is generally stated in units of inches or centimetres that fall per unit of time. Snowfall is measured by melting a sample column of snow and reducing it to an equivalent in water.
CONDITIONS FOR PRECIPITATION
CONDITIONS FOR PRECIPITATION There are three possible ways by which precipitation is produced.
(i) Convectional Precipitation is caused by heating of moist air in the lower layers of atmosphere which rises, expands, and is cooled adiabatically to its dew point. Towering cumulonimbus clouds may form. Convection rain is often accompanied by lightning and thunder. In tropical latitudes this type of rain is usually torrential. It occurs in regions near the equator in the afternoon as a result of the constant high temperature and high humidity, It is most common in equatorial regions and regions having a tropical monsoon climate.
(ii) Orographic means 'related to mountains'. The precipitation is caused by moisture-laden air being forced to rise over a relief barrier (mountain ranges). As the air rises on the windward side, it is cooled at the adiabatic rate. If sufficiently cooled, precipitation results; when the air descends on the leeward side, it gets warmed and dry, having no source from which to draw up moisture. A belt of dry climate, often called a rainshadow, may exist on the leeward side.
Several of the important dry deserts of earth are of this type. Orographic rainfall is most common where no-shore winds rise up over hilly mountain regions lying parallel to the coast, e.g., British Columbia (Canada) and Scotland. In USA, prevailing westerlies bring moist air from the Pacific Ocean over the coast ranges of central and northern California and the great Sierra Nevada Range, Heavy rainfall is experienced on the windward side, while on the leeward side, air descends and becomes dry down the eastern face of sierras. This produces part pf America's great desert zone. Much of orographic rainfall is actually of convectional type, in that it takes the form of heavy convectional showers and thunderstorms. In monsoon areas, this type greatly augments the normal monsoon rainfall e.g., at Cheerapunji, on the windward slopes of the Khasi hills.
(iii) Cyclonic Precipitation (depression or frontal) occurs when large masses of air of different temperatures meet. The warm moist air of one air mass moves over the cold heavier air of another. Or, it is caused by air rising through horizontal convergence in an area of low pressure. Cyclonic rain is common throughout the doldrums where the trade winds meet. It is the precipitation along the frontal surfaces of a depression in mid and high latitudes. In tropical cyclones, the rainfall is often very heavy, but lasts only for a few hours. In temperate cyclones, it is much lighter but lasts for many hours, even days.
Thunderstorm It is an intense local storm accompanied by lightning and thunder that develops in large cumulonimbus clouds, which result from rapid ascent of air under very unstable conditions. Such conditions occur either at the cold front of a depression or when the ground is intensely heated, and there must always be sufficient moisture in the air for cloud formation. The equatorial areas produce the highest frequency of thunderstorms a& the air is usually very humid and the hot sun produces the necessary up currents of air. Very heavy rainfall or hail occurs, accompanied by the discharge of electrical energy, producing lightning and thunder.
Based on the cause of initial lift of air column, resulting in a storm, a classification can be made:
(i) Thermal or air mass thunderstorm set off by thermal convection caused by solar heating of the ground and lower layers of air; time or occurrence is typically in late afternoon when air temperatures near the ground are the highest;
(ii) Orographic thunderstorms occur when the air is forced to rise
over a mountain range; the torrential monsoon rains of the Asiatic and East Indian mountain ranges are largely of this type; (iii) Frontal thunderstorm is caused when a layer of warm air is forced to rise over a layer of cold air.
(i) Convectional Precipitation is caused by heating of moist air in the lower layers of atmosphere which rises, expands, and is cooled adiabatically to its dew point. Towering cumulonimbus clouds may form. Convection rain is often accompanied by lightning and thunder. In tropical latitudes this type of rain is usually torrential. It occurs in regions near the equator in the afternoon as a result of the constant high temperature and high humidity, It is most common in equatorial regions and regions having a tropical monsoon climate.
(ii) Orographic means 'related to mountains'. The precipitation is caused by moisture-laden air being forced to rise over a relief barrier (mountain ranges). As the air rises on the windward side, it is cooled at the adiabatic rate. If sufficiently cooled, precipitation results; when the air descends on the leeward side, it gets warmed and dry, having no source from which to draw up moisture. A belt of dry climate, often called a rainshadow, may exist on the leeward side.
Several of the important dry deserts of earth are of this type. Orographic rainfall is most common where no-shore winds rise up over hilly mountain regions lying parallel to the coast, e.g., British Columbia (Canada) and Scotland. In USA, prevailing westerlies bring moist air from the Pacific Ocean over the coast ranges of central and northern California and the great Sierra Nevada Range, Heavy rainfall is experienced on the windward side, while on the leeward side, air descends and becomes dry down the eastern face of sierras. This produces part pf America's great desert zone. Much of orographic rainfall is actually of convectional type, in that it takes the form of heavy convectional showers and thunderstorms. In monsoon areas, this type greatly augments the normal monsoon rainfall e.g., at Cheerapunji, on the windward slopes of the Khasi hills.
(iii) Cyclonic Precipitation (depression or frontal) occurs when large masses of air of different temperatures meet. The warm moist air of one air mass moves over the cold heavier air of another. Or, it is caused by air rising through horizontal convergence in an area of low pressure. Cyclonic rain is common throughout the doldrums where the trade winds meet. It is the precipitation along the frontal surfaces of a depression in mid and high latitudes. In tropical cyclones, the rainfall is often very heavy, but lasts only for a few hours. In temperate cyclones, it is much lighter but lasts for many hours, even days.
Thunderstorm It is an intense local storm accompanied by lightning and thunder that develops in large cumulonimbus clouds, which result from rapid ascent of air under very unstable conditions. Such conditions occur either at the cold front of a depression or when the ground is intensely heated, and there must always be sufficient moisture in the air for cloud formation. The equatorial areas produce the highest frequency of thunderstorms a& the air is usually very humid and the hot sun produces the necessary up currents of air. Very heavy rainfall or hail occurs, accompanied by the discharge of electrical energy, producing lightning and thunder.
Based on the cause of initial lift of air column, resulting in a storm, a classification can be made:
(i) Thermal or air mass thunderstorm set off by thermal convection caused by solar heating of the ground and lower layers of air; time or occurrence is typically in late afternoon when air temperatures near the ground are the highest;
(ii) Orographic thunderstorms occur when the air is forced to rise
over a mountain range; the torrential monsoon rains of the Asiatic and East Indian mountain ranges are largely of this type; (iii) Frontal thunderstorm is caused when a layer of warm air is forced to rise over a layer of cold air.
PRECIPITATION
PRECIPITATION
Formation of water particles or ice within the cloud that fall towards earth's surface is precipitation. It occurs when condensation takes place rapidly within the cloud. Main types are rain, drizzle, sleet, snow and hail. Any of these may evaporate before reaching the ground surface, appearing as streamers below cloud base-a phenomenon known as virgo.
(i) RAIN Form of precipitation consisting of the drops of water formed by the coalescence of minute condensation droplets within the clouds. In strict terms, these drops have a diameter ranging from about 0.5 mm to 5.0 mm, although smaller drops can be called rain if they are widely scattered.
(ii) DRIZZLE Form of precipitation in which the water droplets are very fine (less than 0.5 mm) and are close together. Normally drizzle is produced by stratus and stratocumulus clouds.
(iii) SLEET Generally it implies. a form of precipitation consisting of either partly-melted snowflakes or rain and snow falling together. In USA, it implies a form of precipitation consisting of frozen raindrops that have subsequently eartially melted.
(iv) SNOW A form of precipitation consisting of crystals of ice. It is produced when condensation takes place at a temperature below freezing point, so that the minute
crystals (spicules) of ice form directly from the water vapour. These may fall as they are but more commonly they combine together to form snowflakes, which display an infinite variety of patterns.
(v) HAIL Precipitation in the form of ice-pellets (hail stones) that develop in and fall from cumulonimbus clouds, either at a cold front or where intense heating of surface causes rapidly-ascending convection currents. The hail stone develops in the updraught of air as water vapour freezes onto the surface of a nucleus embryo of ice in the cloud. When it has grown sufficiently, its weight overcomes the force of the updraught and it falls. Hail consists of rounded lumps of ice, having an internal structure of concentric layers much like an onion.
Formation of water particles or ice within the cloud that fall towards earth's surface is precipitation. It occurs when condensation takes place rapidly within the cloud. Main types are rain, drizzle, sleet, snow and hail. Any of these may evaporate before reaching the ground surface, appearing as streamers below cloud base-a phenomenon known as virgo.
(i) RAIN Form of precipitation consisting of the drops of water formed by the coalescence of minute condensation droplets within the clouds. In strict terms, these drops have a diameter ranging from about 0.5 mm to 5.0 mm, although smaller drops can be called rain if they are widely scattered.
(ii) DRIZZLE Form of precipitation in which the water droplets are very fine (less than 0.5 mm) and are close together. Normally drizzle is produced by stratus and stratocumulus clouds.
(iii) SLEET Generally it implies. a form of precipitation consisting of either partly-melted snowflakes or rain and snow falling together. In USA, it implies a form of precipitation consisting of frozen raindrops that have subsequently eartially melted.
(iv) SNOW A form of precipitation consisting of crystals of ice. It is produced when condensation takes place at a temperature below freezing point, so that the minute
crystals (spicules) of ice form directly from the water vapour. These may fall as they are but more commonly they combine together to form snowflakes, which display an infinite variety of patterns.
(v) HAIL Precipitation in the form of ice-pellets (hail stones) that develop in and fall from cumulonimbus clouds, either at a cold front or where intense heating of surface causes rapidly-ascending convection currents. The hail stone develops in the updraught of air as water vapour freezes onto the surface of a nucleus embryo of ice in the cloud. When it has grown sufficiently, its weight overcomes the force of the updraught and it falls. Hail consists of rounded lumps of ice, having an internal structure of concentric layers much like an onion.
CUMULUS CLOUDS
CUMULUS CLOUDS These are clouds of great vertical extent from 1500 to 9000 metres. They have a low base level (500-2000 m), the base being flat and cloud masses isolated, either rounded or towering, and with a clear outline, resembling the head of a cauliflower. When these clouds are sunlit, they are brilliantly white and are called 'wool clouds'. They occur mainly in summer and are caused by convection. Small cumulus clouds are associated with fair weather.
Cumulonimbus Under different weather conditions, a cumulus cloud may develop into cumulonimbus, the thunderstorm cloud mass of enormous size which brings heavy rainfall, thunder and lightning and gusty winds. It may extend from a height of 300-600 m at the base up to 9000 to 12000 m. When seen from a distance, the top of cumulonimbus may appear white but to observers below, the sky may be darkened to almost night-time blackness.
Cumulonimbus Under different weather conditions, a cumulus cloud may develop into cumulonimbus, the thunderstorm cloud mass of enormous size which brings heavy rainfall, thunder and lightning and gusty winds. It may extend from a height of 300-600 m at the base up to 9000 to 12000 m. When seen from a distance, the top of cumulonimbus may appear white but to observers below, the sky may be darkened to almost night-time blackness.
TYPES OF CLOUDS
Clouds are classified on the basis of appearance, form and altitude. On the basis of form, there are two major groups: (i) stratiform or layered types, and (ii) cumuliform or massive, globular !)'pes.
1. STRATIFORM CLOUDS are blanket-like, often covering vast areas but are fairly thin when compared to their horizontal dimensions. They are sub-divided on the basis of elevation.
(a) High Clouds: 6000 to 12000 metres above sea-level, e.g., (i) Cirrus Cloud, a wispy, fibrous looking cloud, uften indicating fair weather; (ii) Cirrocumulus, a thin cloud often globular and rippled (this is the mackerel sky of popular
description);
(iii) Cirrostratus looks lilse a thin white sheet producing a halo around the sun or moon.
(b) Medium Clouds: 2100 to 6000 metres above sea level.
(i) Altocumulus Clouds: A layer of individual cloud masses
fitted closely together in geometric pattern; the masses appear white or somewhat grey on shaded sides, and the blue sky can be seen between individual patches of rows
of these bumpy-looking clouds with a flattened base. They indicate fine weather. (ii) Altostratus: A banked layer, often smoothly distributed over the entire sky; greyish in appearance, they usually have a smooth under-side and often
show the sun as a bright spot in the clouds. These are associated with development of bad weather.
(c) Low Clouds: Below 2100 metres (6900 feet) (i) Stratus: A dense, fog-like, low-lying dark-grey layer; brings 'dull weather, usually accompanied with a drizzle. (ii) Nimbostratus: If rain or snow is falling from a stratus cloud, it is Okta is a unit used in meteorology to measure cloud cover. One okta is equal to a cloud cover of one-eighth of the whole sky, Le., one okta represents one-eighth of cloud cover. Therefore, eight oktas are equivalent to a total cloud cover. called nimbostratus.
The prefix 'nimbo' means that precipitation is coming from the cloud. (Hi) Stratocumulus: A lowlying cloud layer consisting of distinct greyish masses of cloud between which blue sky is' visible; oriented at right angles to the direction of wind and cloud motion, they are generally indicative of fair or clearing weather.
1. STRATIFORM CLOUDS are blanket-like, often covering vast areas but are fairly thin when compared to their horizontal dimensions. They are sub-divided on the basis of elevation.
(a) High Clouds: 6000 to 12000 metres above sea-level, e.g., (i) Cirrus Cloud, a wispy, fibrous looking cloud, uften indicating fair weather; (ii) Cirrocumulus, a thin cloud often globular and rippled (this is the mackerel sky of popular
description);
(iii) Cirrostratus looks lilse a thin white sheet producing a halo around the sun or moon.
(b) Medium Clouds: 2100 to 6000 metres above sea level.
(i) Altocumulus Clouds: A layer of individual cloud masses
fitted closely together in geometric pattern; the masses appear white or somewhat grey on shaded sides, and the blue sky can be seen between individual patches of rows
of these bumpy-looking clouds with a flattened base. They indicate fine weather. (ii) Altostratus: A banked layer, often smoothly distributed over the entire sky; greyish in appearance, they usually have a smooth under-side and often
show the sun as a bright spot in the clouds. These are associated with development of bad weather.
(c) Low Clouds: Below 2100 metres (6900 feet) (i) Stratus: A dense, fog-like, low-lying dark-grey layer; brings 'dull weather, usually accompanied with a drizzle. (ii) Nimbostratus: If rain or snow is falling from a stratus cloud, it is Okta is a unit used in meteorology to measure cloud cover. One okta is equal to a cloud cover of one-eighth of the whole sky, Le., one okta represents one-eighth of cloud cover. Therefore, eight oktas are equivalent to a total cloud cover. called nimbostratus.
The prefix 'nimbo' means that precipitation is coming from the cloud. (Hi) Stratocumulus: A lowlying cloud layer consisting of distinct greyish masses of cloud between which blue sky is' visible; oriented at right angles to the direction of wind and cloud motion, they are generally indicative of fair or clearing weather.
CONDENSATION
CONDENSATION
The physical process of transformation from the vapour to the liquid state is condensation. It occurs only when large masses of air experience a steady drop in temperature below the dew point, or when there is enough water vapour within the air mass for it to reach saturation point.
The rising of air mass to high elevations is necessary for this to happen. Condensation occurs when temperature reaches the dew point and the water vapour in the form of tiny droplets becomes visible in the form of clouds at a height of about one kilometre or more above the sea level.
DEW POINT is the temperature at whlch the air is fully saturated and below which condensation normally occurs; water vapour starts to condense to form water droplets.
DEW is the deposition of water droplets on the ground and objects, such as plants, near the ground. It occurs when the temperature of the ground surface falls and the air in contact with it is cooled below its dew point. Water vapour from the air or diffused from the soil then condenses and is deposited as droplets. The favourable conditions are moist air, light winds and clear night skies to ensure
maximum cooling by radiation.
FROST is a weather condition that occurs when the air temperature is at or below ODe. Moisture on the ground surface and objects freezes to form an icy deposit. Con
ditions favourable for its formation are similar to those in the case of dew formation.
FOG is made of the droplets of water suspended in the lower layers of the atmospher~, resulting from the condensation of water vapour around nuclei of floating dust or smoke particles. A visibility of less ~han 1 kilometre is the internationally-recognised definition of fog.
SMOG, also called smoke fog, is a form of fog that occurs in areas where the air contains a large amount of smoke. Smoke particles provide a high concentration of nuclei around which condensation occurs. Condensation can occur around these nuclei even when the air is not saturated and therefore it forms. earlier, becomes denser and lasts longer than fog that develops in unpolluted air. The smoke, because of its chemical contents, gives an acid taste to fog.
MIST is the term for a reduction of visibility between 1-2 km caused by condensation producing water droplets
within the lower layers of atmosphere. It is intermediate between fog and haze.
HAZE is normally formed by water particles that have condensed around nuclei in the atmosphere, but may also be a result of particles of smoke, dust or salt in the air. In meteorology, it is an obscurity of the lower atmosphere that limits visibility to under 2 km but over 1 km. This term is also used for other phenomena that limit visibility.
CLOUDS are masses of minute water droplets and! or ice crystals formed by the condensation of water vapour and held in suspension in the atmosphere. Condensation, which results from cooling, usually takes place around nuclei such as dust, smoke particles and salt. The cooling may be caused by convection, uplift over mountains or
ascent in depressions. Clouds may be present at heights ranging from ground level up to over 13,000 metres.
The physical process of transformation from the vapour to the liquid state is condensation. It occurs only when large masses of air experience a steady drop in temperature below the dew point, or when there is enough water vapour within the air mass for it to reach saturation point.
The rising of air mass to high elevations is necessary for this to happen. Condensation occurs when temperature reaches the dew point and the water vapour in the form of tiny droplets becomes visible in the form of clouds at a height of about one kilometre or more above the sea level.
DEW POINT is the temperature at whlch the air is fully saturated and below which condensation normally occurs; water vapour starts to condense to form water droplets.
DEW is the deposition of water droplets on the ground and objects, such as plants, near the ground. It occurs when the temperature of the ground surface falls and the air in contact with it is cooled below its dew point. Water vapour from the air or diffused from the soil then condenses and is deposited as droplets. The favourable conditions are moist air, light winds and clear night skies to ensure
maximum cooling by radiation.
FROST is a weather condition that occurs when the air temperature is at or below ODe. Moisture on the ground surface and objects freezes to form an icy deposit. Con
ditions favourable for its formation are similar to those in the case of dew formation.
FOG is made of the droplets of water suspended in the lower layers of the atmospher~, resulting from the condensation of water vapour around nuclei of floating dust or smoke particles. A visibility of less ~han 1 kilometre is the internationally-recognised definition of fog.
SMOG, also called smoke fog, is a form of fog that occurs in areas where the air contains a large amount of smoke. Smoke particles provide a high concentration of nuclei around which condensation occurs. Condensation can occur around these nuclei even when the air is not saturated and therefore it forms. earlier, becomes denser and lasts longer than fog that develops in unpolluted air. The smoke, because of its chemical contents, gives an acid taste to fog.
MIST is the term for a reduction of visibility between 1-2 km caused by condensation producing water droplets
within the lower layers of atmosphere. It is intermediate between fog and haze.
HAZE is normally formed by water particles that have condensed around nuclei in the atmosphere, but may also be a result of particles of smoke, dust or salt in the air. In meteorology, it is an obscurity of the lower atmosphere that limits visibility to under 2 km but over 1 km. This term is also used for other phenomena that limit visibility.
CLOUDS are masses of minute water droplets and! or ice crystals formed by the condensation of water vapour and held in suspension in the atmosphere. Condensation, which results from cooling, usually takes place around nuclei such as dust, smoke particles and salt. The cooling may be caused by convection, uplift over mountains or
ascent in depressions. Clouds may be present at heights ranging from ground level up to over 13,000 metres.
HUMIDITY
Water vapour is amongst the most important atmo
spheric gases. The atmosphere gathers moisture by the process of evaporation, while it loses moisture through
condensation and precipitation. Humidity, which refers to the condition of the air with regard to water vapour, plays a decisive role in evaporation, condensation and precipitation.
HUMIDITY
It is the degree of water vapour present in the air. For any specified temperature, there is a definite limit to the maximum quantity of moisture that can be held by the air. This limit is known as the saturation point. Humidity can be measured by- a hygrometer or sling psychrometer.
RELATIVE HUMIDITY is the ratio between the amount
of water vapour actually present in an air mass and the maximum amount that the air mass can hold at that temperature. It is expressed as a percentage. At the
saturation point, relative humidity is 100 per cent. It varies inversely with temperature, given a fixed amount of water vapour. Secondly, if an exposed water surface is present the relative humidity can be increased by evaporation.
ABSOLUTE HUMIDITY is the actual amount of moisture present in air. The amount of water vapour per unit volume of air is usually expressed in grams per cubic metre. It is a measure of the quantity of water that can be extracted from the atmosphere as precipitation. As the absolute humidity cannot remain a constant figure for the same body of air, modem meteorology makes use of another measure of moisture content-specific humidity.
SPECIFIC HUMIDITY is the ratio of the weight of water vapour to the weight of moist air. Expressed in units of grams of water vapour per kilogram of moist air, specific
humidity is often used to describe the moisture characteristic of a large mass of air.
spheric gases. The atmosphere gathers moisture by the process of evaporation, while it loses moisture through
condensation and precipitation. Humidity, which refers to the condition of the air with regard to water vapour, plays a decisive role in evaporation, condensation and precipitation.
HUMIDITY
It is the degree of water vapour present in the air. For any specified temperature, there is a definite limit to the maximum quantity of moisture that can be held by the air. This limit is known as the saturation point. Humidity can be measured by- a hygrometer or sling psychrometer.
RELATIVE HUMIDITY is the ratio between the amount
of water vapour actually present in an air mass and the maximum amount that the air mass can hold at that temperature. It is expressed as a percentage. At the
saturation point, relative humidity is 100 per cent. It varies inversely with temperature, given a fixed amount of water vapour. Secondly, if an exposed water surface is present the relative humidity can be increased by evaporation.
ABSOLUTE HUMIDITY is the actual amount of moisture present in air. The amount of water vapour per unit volume of air is usually expressed in grams per cubic metre. It is a measure of the quantity of water that can be extracted from the atmosphere as precipitation. As the absolute humidity cannot remain a constant figure for the same body of air, modem meteorology makes use of another measure of moisture content-specific humidity.
SPECIFIC HUMIDITY is the ratio of the weight of water vapour to the weight of moist air. Expressed in units of grams of water vapour per kilogram of moist air, specific
humidity is often used to describe the moisture characteristic of a large mass of air.
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