22 Soil

Babita Khosla

23. Introduction

23.1 Definition of Soil

There have been several conceptions held about soil in the historic past. This has led to several definitions of soil by different people at various times. Soil is a natural resource, capable of supporting all life forms on earth. It can be defined as the solid material on the Earth’s surface that results from the interaction of weathering and biological activity on the parent material or underlying hard rock and act as a basic medium for the plant to grow. It is defined as the coarse mineral or organic material on the earth surface providing as a natural medium for plants to grow. It forms at the surface of land so is also called as the “skin of the earth.” Soils differ from one part of the world to another. Inorganic material is derived from mainly weathering of Parents rock materials mainly consisting of sand, silt and clay Fig -23.1

Fig -23.1 Composition of soil

Organic matter (OM) is derived from the decomposition of dead plants and animals. The relative amount of water and soil depends upon the climatic features of the region. However all these components determine the soil properties. Further are some concepts that have affected the definition of soil in the past include-

• Soil as a thin mantle over the land surface
• Soil as a medium for plant growth
• Soil as a basic matter of the universe
• Soil as a mantle of loose and weathered rock
• Soil as an organized natural bodies

Also, the definition of soil varies with the profession and perception of the person defining soil. What a soil is to civil engineers is different from the definition of soil by Geologists, geomorphologists, archeologists and soil scientists.

On basis of particle size the soil is differentiated into three groups i.e clay <silt <sand. Most of the soil texture is being defined by mixture of these three: The texture of clay loam soil exhibit nearly equal parts of sand , silt and clay. Soil texture triangle is shown fig 23.2 reveals the 12 different types of soil texture exhibiting in fix percentages of sand, silt or clay. Each texture corresponds to specific percentages of sand, silt, or clay.

Figure: 23.2- Soil texture triangle Source: https://www.tes.com/lessons/mGns8I_wjDSFBQ/copy-of-soil-triangle

Soil texture is highly correlated with a range of soil chemical and physical properties. Fine textured soils with high clay contents generally have higher nutrient and water holding capacities than do coarse textured soils. However, fine textured soils often do not have drainage characteristics that are ideal for plant growth, especially if the soil does not have good structure. While texture can be determined through quantitative analyses, structure is described in more qualitative terms (blocky, sub angular blocky, prismatic, granular, etc.). Maintaining good soil structure is important for plant growth. Texture really does not change over a period of only a couple of hundred years or so, but structure can be changed rapidly, especially through management practices.

Texture is generally used to reference the proportions of sand, silt, and clay. The particle sizes in each of these three soils separates ranges between specific limits (Fig-23.3). The distinctions among the size groups are more or less arbitrary. They have been arrived at after many trials in developing classes that can be used consistently, conveniently, and best describe the nature of the separates. The scheme we will use is that adopted by the United States Department of Agriculture (USDA).

Sand – 2.0 to 0.05 mm

Silt – 0.05 to 0.002 mm

Clay – less than 0.002 mm

Fig-23.3 Particle sizes of sand, silt and clay.

Source: https://en.wikipedia.org/wiki/Soil_texture The study of soils as naturally occurring phenomena is called pedology (from the Greek word pedon, meaning soil or earth). Pedology takes into account:

Soil characteristics

Distribution of soil types.

23.2 SOIL CLASSIFICATION-

The characterizing of soil has been done number of systems. The classification has been developed specifically to test the suitability of soil for use in particular soil engineering projects. The most common system of classification is as under:

1. Geological Classification:

Soil types may be classified on the basis of their geological origin. The formation of soil depends on its constituents agencies responsible for its present status. During the period of geological cycle soils are originated by weathering of rocks. Over the time due cementation and compaction by heat and pressure these are again formed. Soil classification based on its constituents based as Inorganic soil and Organic soil.

Based on the agencies responsible for their present state, soils may be classified under following types

Residual Soils

Transported Soils

Alluvial or sedimentary soils Aeolian soils

Glacial soils

Lacustrine soils Marine soils

2. Classification by Structure:

On the basis of structure, average grain size and conditions of the soils so formed and deposited have been characterized into following types:

1. Soils of single-grained structure
2. Soils of honey-comb structure
3. Soils of flocculent structure

1. Classification based on grain-size:

The grain-size classification of soils is based on the terms such as gravel, sand, silt. Sand size, silt size names have been given natural soil containing mixtures of all particle size have been named as particle size, silt size. A number of gain-size classifications have been evolved, but the commonly used ones are S. Bureau of Soils and Public Roads Administration (PRA) System of U.S.A. International Classification, proposed at the International Soil Congress at Washington, D.C., in 1927 Massachusetts Institute of Technology (MIT) System of Classification of U.S.A. Indian Standard Classification (IS: 1498-1970).

4.  Unified soil classification system

This system was originally developed by A. Cassagrande and adopted by the U.S. Corps of Engineers in 1942 as ‘Airfield Classification’. It was later revised for universal use and redesignated as the “Unified Soil Classification” in 1957.In this system soils are classified into three broad categories:

1.  Coarse-grained soils with up to 50% passing No. 200 ASTM Sieve

2.  Fine-grained soils with more than 50% pass No. passing No. 200 ASTM Sieve

3.  Organic soils

5. Preliminary Classification by soil types:

Familiarity with common soil types is necessary for an understanding of the fundamentals of soil behavior. In this approach, soils are described by designation such as

Boulders

Gravel

Sand

Silt

Clay

Rock

flour

Peat China Clay

Topsoil

Bentonite

Black Cotton soil

Boulder Clay

Caliche

Hardpan

Laterite

Loam

Loess

Marl

Moorum

Varved Clay

23.3 SOIL CLASSIFICATION IN INDIA

Geologically, Indian soils can broadly be divided into two main types:

1.  Soils of peninsular India

2.  Soils of extra-peninsular India

The soils of Peninsular India are those which have been formed by the decomposition of rocks in situ, i.e. directly from the underlying rocks. They are transported and re deposited to a limited extent and is known as sedentary soils. On the other hand, the soils of the Extra-Peninsula are formed due to the depositional work of rivers and wind. They are mainly found in the river valleys and deltas. They are very deep and constitute some of the most fertile tracts of the country. They are often referred to as transported or azonal soils. The Indian Council of Agricultural Research (ICAR) set up an All India Soil Survey Committee in 1953 which divided the Indian soils into eight major groups. This is a very logical classification of Indian soils and has gained wide acceptance. A brief account of these eight soils is given as under:

1. Alluvial soils: Alluvial soils are important and cover almost quarter of India’s land area i.e. 46.5 % the total land area of the country and covering almost 15 lakh square km. These soils are suitable for crop production and hence considered as agricultural wealth and support our country population. This type of soils is derived from the sediments deposited by either river in the plain and seas in the coastal areas have been formed by the sea waves. Thus the parent material of these soils is all of transported origin consisting of fine particles of sand, silt and clay called alluvium. The products of weathering of rocks from the mountains being carried by the streams and deposit them in the low-lying areas. The alluvial soils are deficient in nitrogen and humus therefore considered immature and having weak profiles. The soil occasional consist of kankar beds whereas pebble, stone or gravel soils are rarely observed in this group. The chemical composition of soils makes this group of soils as one of the most fertile in the world whereas the proportion of nitrogen is generally low, potash, phosphoric acid and alkalis are adequate, while iron oxide and lime vary within a wide range. Hence, the porosity and texture provide good drainage and other conditions favorable for bumper crops. The alluvial soils are mostly found in the Great Indo-Gangetic Plain running from

Punjab in the west to West Bengal and Assam in the east; also occur in deltas of the Mahanadi valley in chattisgarh, the Godavari valley in Andhra Pradesh, the Krishna and the Cauvery valley in Tamil Nadu. Some alluvial soils are found in the Narmada and Tapi valleys in M.P and Northern parts of Gujarat.

The alluvial soils are divided into a) old alluvium (bhangar) (b) new alluvium khadar (c) bhabar

• 2. Black Soils: The black soils are generally called black cotton soil because cotton is the most important crop grown on these soils. These soils are also called regur (from the Telugu word Reguda) and black cotton soils. Several theories have been put forward regarding the origin of this group of soils but most pedologists believe that these soils have been formed due to(d) They are pale in colour and sandy however less kankary too. Khadar soil are mainly found in low lying areas which remain flooded throughout the year on the other hand bhangar soil are found on higher reaches of about 30 meters above the flood level. Bhangar soil is clayey and is generally dark colored and below the surface of the bhangar are beds of lime nodules known as kankar. Khadar soils are more fertile than bhangar soils. Along the Shiwalik foothills  there is zone of 8-16 Km wide known as bhabar. The area is not suitable for agriculture purpose and big trees with large roots thrive in this zone, there are alluvial fans having coarse, often pebbly soils which merged together to form the belt. To the south of the bhabar running parallel to it is a long narrow strip of swampy lowland with silty soils is called tarai. Tarai is ill drained, marshy and thick forested narrow tract best suited for variety of wild life. The soils are rich in nitrogen and organic matter but are deficient in phosphate. These soils are suitable for a number of crops such as wheat, rice, sugarcane, jute and soybean under reclaimed conditions. Due to their softness of the strata and fertility the alluvial soils are best suited to irrigation and respond well to canal and well/tube-well irrigation if alluvial soils are properly maintained can yield splendid crops of rice, wheat, sugarcane, tobacco, cotton, jute, maize, oilseeds, vegetables, and fruits.the solidification of lava spread over large areas during volcanic activity in the Deccan Plateau, thousands of years ago. Most of the black soils are derived from two types of rocks, the Deccan and the Rajmahal trap, and ferruginous gneisses and schists occurring in Tamil Nadu. The former are sufficiently deep while the later are generally shallow. According to Krebs the regur is essentially a mature soil which has been formed climatic and relief factor in spite of particular type of rock. This soil occurs mainly where rainy days range between 30 to 50 days and rainfall is 50 to 80 cm. The occurrence of this soil in the west deccan where the rainfall is about 100 cm and the number of rainy days more than 50, is considered by him to be an exception. The materials deposited by river in some parts of Gujarat and Tamil Nadu brought down from the interior of Peninsula covered with lava. Geographically Black soils are spread 16.6 per cent over total area in country. This is the region of high temperature and low rainfall. It is, therefore, a soil group of the dry and hot regions of the Peninsula. These soils are mainly found in Maharashtra, Madhya Pradesh, parts of Karnataka, Andhra Pradesh, Gujarat and Tamil Nadu. Some scientists attributed black color of soil due to small to the presence of a small proportion of titaniferous magnetite, compound of iron and black, compound of aluminium constituents of the parent rock. The black colour of soil may be due to crystalline schists and basic gneisses. In Tamil Nadu and some parts of Andhra Pradesh. tints of the black color such as deep black, shallow black, medium black or even a mixture of red and black.

The black soil are impermeable and can hold moisture for longer time. During rainy season these soils often swells and become sticky due to which plowing becomes difficult. In hot dry

BLACK SOIL

season, water and moisture evaporates, the soil shrinks develops deep cracks often 10-15 cm wide upto a meter deep good for the soil to have extraordinary fertility. The soil “swallows” itself and retains soil moisture easily and gives the property of self – ploughed. Due to the large clay factor i. e. 62 per cent or more, without gravel or coarse sand the black soil appears to be tightly packed. Chemical composition of this soil is 10 per cent of alumina, 10 per cent of iron oxide and 8 percent of lime and magnesium carbonates. Potash varies (less than 0.5 per cent) and phosphates, nitrogen with low humus. In all regur soils particularly derived from ferromagnesian schists there is a layer rich in kankar nodules formed by segregation of calcium carbonate at lower depths. As a general rule, black soils are formed by volcanic activity. These soils are found rich in Iron, aluminium, magnesium and lime however, poor in organic matter, nitrogen and Phosphorous. Cotton, wheat, jowar, linseed, Virginia tobacco, castor, sunflower and millets are the major crops grown in black soil where proper irrigation facilities are available rice and sugarcane are also grown.

3. Red Soils: This comprehensive term along with its major group form the largest soil group of India. The main parent rocks are acid granites and gneisses, quartzitic and felspathic. Most of the red soils have come into existence due to weathering of ancient crystalline and metamorphic rocks. Due to the presence of iron oxide The color of these soils is generally red, often ranges into brown, chocolate, yellow, grey or even black. The red color is due to the wide diffusion rather than to high percentage of iron content. The red soils extend a upto area of about 3.5 lakh sq km i.e 10.6 per cent of the total geographical area of the country. These soils are found in Tamil Nadu, parts of Karnataka, south-east of Maharashtra, eastern parts of Andhra Pradesh and Madhya Pradesh, Chhattisgarh, Orissa and Chota Nagpur in Jharkhand. In the north large parts of south Bihar, Bankura and Birbhum in West Bengal; Banda and Hamirpur districts of Uttar Pradesh, Mirzapur, Aravallis, Jhansi and the eastern half of Rajasthan, parts of Assam, Manipur, Tripura, Nagaland, , Mizoram, and Meghalaya districts covered by red soil. By and large, these soils are deficient in lime, magnesia, phosphates, nitrogen and humus, fairly rich in potash & potassium. In their chemical composition they are mainly siliceous and aluminous; with free quartz as sand the alkali content is fair, some parts being quite rich in potassium. The texture of these soils exhibit texture which varies from sand to clay whereas the majority being loams. The nature of uplands, red soils are thin, poor and gravelly, sandy or stoney and porous, but in the lower areas they are rich, deep dark and fertile with proper use of fertilizers and irrigation techniques nthe productive capacity can be increased and can give excellent yields of cotton, wheat, rice, pulses, millets, tobacco, oil seeds, potatoes and fruits.

4.   Laterite Soils: Laterite has been derived from latin word ‘later’ which means bricks and it was first applied by Buchanan in 1807 for clayey rock in Malabar where he observed hardening on exposure, to high temperature . But many authors agree with Fermor’s restriction of this term to soils formed as to 90-100 per cent of iron, aluminium, titanium and manganese oxides. According to majority opinion, the hopan soil is formed under conditions of high temperature and heavy rainfall with alternate wet and dry periods. According to Polynov, hopan soils

LATERITE SOIL

may be “the end products of weathering given sufficiently long time”. In the opinion of George Kuriyan, “It is probably the end product of decomposition found in regions of heavy rainfall, more than 200 cm” In such soil lime and silica are leached away due to intense tropical rain and soil rich in iron and aluminium oxides, the compounds left behind. We have numerous varieties of hopan which have bauxite at one end and an indefinite mixture of ferric oxides at the other. Almostall hopan soils are very poor in lime and magnesia and deficient in nitrogen. Sometimes in these soils the phosphate content may be high, probably present in the form of iron phosphate but potash is deficient. At some places, there may be higher content of humus. Laterite and

lateritic soils are widely spread in India and cover an area of 2.48 lakh sq km. They are mainly found on the summits of Western Ghats at 1000 to 1500 m above mean sea level, Eastern Ghats, the Rajamahal Hills, Vindhyas, Satpuras and Malwa Plateau.

They also occur at lower levels and in valleys in several other parts of the country. They are well developed in south Maharashtra, parts of Karnataka, Orissa, West Bengal, Jharkhand, Assam and Meghalaya. In Tamilnadu, A.P and Kerala these soils are known as red laterite soil which are found very suitable for crops like cashew nut. Due to intensive leaching and low base exchange

MOUNTAIN SOIL

capacity, typical hopan soils generally lack fertility. Hence laterites soil are not considered suitable for cultivation purpose whereas by applying manure and fertilizer can be used for cultivation of plantation crops like tea, coffee, rubber, cinchona, coconut, arecanut and in low lying areas paddy is also grown For eg. soils in Kerala, Karnataka, Chota Nagpur region of Jharkhand, Orissa and Assam can with the application of fertilizers like nitrogen, phosphorus and potassium give good results. In some areas, these soils support grazing grounds and scrub forests. Laterite and lateritic soils have a unique distinction of providing valuable building material. These soils can be easily cut with a spade but hardens like iron when exposed to air. Because it is the end- product of weathering, it cannot be weathered much further and is indefinitely durable and hence more often used as bricks for use in house construction.

5. Forest And Mountain Soils: These soils are found mainly in areas covered by forest and heavy rainfall hill slopes and forests covering 8.67 per cent of the total land area of India. The organic matter derived from forest growth is responsible for the formation of these soils. Parent rocks, ground-configuration and climate give these soils heterogeneous character in nature. Even though they occur in close proximity to one another covered  with  soil  cover  in  southern  slopes.  The

forest soils also occur on Western and Eastern Ghats and in some parts of the Peninsular plateau. These soils are however infertile for field crops and requires good deal of fertilizers for high yield. The forest soils are deficient in lime, phosphorus and potash whereas rich in humus. They are only useful for plantations of spices, coffee, tea and tropical fruits in Karnataka, Tamil Nadu and Kerala and barley, maize, wheat and temperate fruits in Uttaranchal Jammu and Kashmir and Himachal Pradesh.

6. Arid and Desert Soils: In Northwest arid and semiarid regions of India where annual rainfall is less than 50 cm. this soil is found. These soils are extensively found in Rajasthan, parts of Haryana and Punjab and extend upto Rann of Kutch in Gujrat. These deserted soil covering an area of 1.42 lakh sq km or 4.32% of total land area in India. The soil growth is inhibit in these areas due to cap of sand over the soil. This sand has originated from either mechanical breakdown of the ground rocks or may be blown from prevailing south-west monsoon winds. These barren sandy soils without clay factor are also commonly observed in coastal regions of Kerala, Tamil Nadu and Orissa.

Soluble salts are present in these soils at high percentages varying degree of calcium carbonate also makes these soils alkaline in some parts. The subsoil has ten times more calcium as compared to top soil and it keeps on increasing as we go down. In normal alluvial soils the phosphate content of these soils is as high. Nitrogen deficiency may be improved by making nitrogen available in the form of nitrates. Thus, the presence of phosphates and nitrates make them fertile soils wherever moisture is available. If proper irrigation facilities are available these soils can be reclaimed and one of the living example of such is the cropping pattern in the Indira Gandhi Canal Command Area. In these kinds of deserted soils, only the drought resistant and salt tolerant crops support the vegetation such as barley, rape, cotton, wheat, millets, maize and pulses. And hence, this soil supports less density of population.

7. Saline and Alkaline Soils: These soils are found in Andhra Pradesh and Karnataka and also in the drier parts of Bihar, Uttar Pradesh, Haryana, Punjab, Rajasthan and Maharashtra, there are salt-impregnated or alkaline soils occupying 68,000 sq km of area. These soils are known by different names such as reh, usar, thur, rakar, karl and kalar. Rocks and mineral fragments on weathering liberate sodium, magnesium and calcium salts and sulphurous acid.
8. 

The salts are transported in solution by the rivers, which percolates in the sub-soils of the plains. In canal irrigated areas and process is common in canal irrigated areas. In areas of high sub-soil water table due to evaporation in dry season theinjurious   salts   are   transferred   from   below    to the top soil by the capillary action. The accumulation of these salts makes the agricultural soil infertile and unfit for crop production. It has been estimated that usar has affected 1.25 million hectares of land in Uttar Pradesh about and in Punjab 1.21 million hectares. In Gujarat, the area round the Gulf of Khambhat is affected by the sea tides carrying salt-laden deposits. Vast areas comprising the estuaries of the Narmada, the Tapi, Mahi and the Sabarmati have thus become infertile.

8. Peaty and Marshy Soils: Peat soils originate in humid low lying areas as a result of deposition of huge amounts of organic matter in the soils undr water logged condition contain considerable amount of soluble salts and organic matter is present in these soils. Soils belonging to this group is called kari found in districts Kottayam and Alappuzha of Kerala. Marshy soils with a high proportion of vegetable matter also occur in the coastal areas of Orissa and Tamil Nadu, Sunderbans of West Bengal, in Bihar and Almora district of Uttaranchal.

These soils are black, heavy and highly acidic and deficient in potash and phosphate. Most of the peaty soils are under water during the rainy season but as soon the monsoon passes they are used for paddy cultivation.

Summary:

Soil acts as a basic medium for the plant to grow. Understanding as well as recognizing the various soil properties and their interactions with one another is important for making sound decisions regarding use and management of soil. A brief account of eight different soil types has also been studied.