{"id":537,"date":"2019-03-11T04:38:20","date_gmt":"2019-03-11T04:38:20","guid":{"rendered":"http:\/\/esp16.epgpbooks.inflibnet.ac.in\/?post_type=chapter&p=537"},"modified":"2019-04-25T07:37:41","modified_gmt":"2019-04-25T07:37:41","slug":"soil-i-formation-structure-and-chemical-composition","status":"publish","type":"chapter","link":"https:\/\/ebooks.inflibnet.ac.in\/esp16\/chapter\/soil-i-formation-structure-and-chemical-composition\/","title":{"rendered":"Soil – I (Formation, Structure and Chemical Composition)"},"content":{"raw":"
\"epgp<\/a>\r\n<\/span><\/div>\r\n \r\n\r\nContents:<\/strong>\r\n
    \r\n \t
  1. Soil Definition<\/li>\r\n \t
  2. Formation of Soil<\/li>\r\n \t
  3. Weathering<\/li>\r\n \t
  4. Soil Forming factors<\/li>\r\n \t
  5. Soil Composition<\/li>\r\n \t
  6. Soil Structure: Formation and Description<\/li>\r\n \t
  7. Soil Grade<\/li>\r\n \t
  8. Soil Profile<\/li>\r\n \t
  9. Inorganic Components in soil<\/li>\r\n \t
  10. Soil Humus<\/li>\r\n \t
  11. References<\/li>\r\n<\/ol>\r\n \r\n\r\nSoil Definition<\/strong>\r\n

    The word soil is derived from word\u2018solum\u2019<\/em>(Latin word) which means earthly material in which plants grow. The study of soils as naturally occurring phenomena is called pedology (from the Greek word pedon<\/em>, meaning soil or earth). Soil possess complexity and heterogeneity. It documentation is as early as 5000 BC. Vedas and Upanishads regarded soil as \u2018Mother of all<\/em>\u2019 that supports and nourishes all life on Earth. By definition, Soil is a dynamic natural body developed as a result of pedogenic processes during and after weathering of rocks, consisting of mineral and organic constituents, possessing definite chemical, physical, mineralogical and biological properties, having a variable depth over the surface of the earth and providing a medium for plant growth for land plants.<\/p>\r\n \r\n\r\nFormation of Soil<\/strong>\r\n

    Soil is formed as a result of long term process of complex interactions disintegration and decomposition of rocks due to weathering leading to the production of mineral matrix in close association with interstitial organic matter. Two types of rocks that are recognized are (1) Soft rock that is formed fromdecomposition of rocks and soil development proceed simultaneously and (2) in hard rockdecomposition of rocks and rock minerals occurs prior to development of soil.<\/p>\r\n \r\n\r\nWeathering<\/strong>\r\n

    The disintegration and decomposition of rocks and rock minerals by physical, chemical and biological agents in single or in combination is termed as weathering. Two types of weatheringare recognized:<\/p>\r\n \r\n

    (1) Geochemical weathering occurs before soil is developed and continues beneath soil cover. It is also termed as \u201cgeogenesis\u201d and (2) Pedochemical weathering that occurs as result of disintegration and transformation of minerals that take place within the soil. It is also termed as \u201cpedogenesis\u201d. Weathering on the basis of agents caused can be classified into following three types. The Physical\/ Mechanical weathering takes place when rock is split or broken into smaller pieces of the same material without changing its composition. Agents include temperature, water, ice, gravity, wind, mutual rubbing of rock fragments and air blast of sand grains. It is more predominant in the desert and arctic regions. Biological weathering occurs due to weakening and subsequent disintegration of rock by plants, animal and microbes. Their agents include micro-organisms such as bacteria, fungi, protozoa, lichens and mosses and also burrowing animals like earthworms. These micro- organisms decompose a part of the organic matter and add inorganic nutrients to soil. Chemical weathering includes processes whereby rock material are decomposed by alteration of present material. Their agents include atmospheric gases, especially oxygen, water and acids. It takes place through six different chemical reactions:<\/p>\r\n \r\n

    (a) Solution- Weathering of water soluble minerals (limestone, gypsum) by solvent action. It increases in presence of carbon dioxide and organic acids formed as a result of decay of organic plant and animal remains.<\/p>\r\n

    (b) Oxidation- Oxides dissolve in water weakening rock and causing weathering. It occurs best in well aerated and well drained soils.<\/p>\r\n\"\"\r\n

    (c) Hydrolysis- Here, weathering by the chemical reaction occurs between hydrogen ions and hydroxyl ions of water and a mineral\u2019s ions.<\/p>\r\n\"\"\r\n\r\n(d) Reduction- It occurs in poorly aerated deep zones of earth\u2019s crust\r\n\r\nThe red ferric oxide reduces to grey ferrous oxide\r\n\r\n\"\"\r\n

    (e) Carbonation- Combination of carbon dioxide and water to form carbonic acid. Carbonic acid combines with hydroxides of Ca, Mg to form carbonates and bicarbonates.<\/p>\r\n

    (f) Hydration- Attack of water molecule to the rock material resulting in increment in volume and softening of hydrated mineral with its subsequent weathering.<\/p>\r\n\"\"\r\n\r\n \r\n\r\nSoil Forming Factors<\/strong>\r\n

    Factors that contribute to soil formation can be classified into three types: Active factors include rainfall, temperature, wind, humidity, evaporation. Passive factors includes parent material, topography (drainage, runoff, soil erosion) while micro-organisms eg.worms, centipedes etc. come under biosphere factors.<\/p>\r\n \r\n\r\nSoil Composition<\/strong>\r\n\r\n\"\"Figure 1: Constituents of soil (by percentage) in a typical top soil<\/strong>\r\n\r\n \r\n\r\n(a) Mineral matter- geochemically classified on basis of their bonding characteristics into five maingroups:\r\n\r\nTable 1: Components of Mineral matter of soil<\/strong>\r\n\r\n \r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n
    Element<\/strong><\/td>\r\nCharacteristics<\/strong><\/td>\r\nExamples<\/strong><\/td>\r\n<\/tr>\r\n
    Lithophile<\/td>\r\nReadily ionized forming oxyanions<\/td>\r\nLi, Na, K, Ca, Mg, Fe, Si, B, Bi, Ni, Co, S, Cu,Zn, halogens, rare earths.<\/td>\r\n<\/tr>\r\n
    Chalcophile<\/td>\r\nForms covalent\u00a0\u00a0bonds\u00a0with\u00a0sulphides.<\/td>\r\nFe, Ni, Co, Cu, Zn, Pb, Ag, Sb, As, Bi, S, Se,\u00a0Te, As.<\/td>\r\n<\/tr>\r\n
    Siderophile<\/td>\r\nCapable of forming metallic bonds<\/td>\r\nFe, Co, Pd, Os, Au.<\/td>\r\n<\/tr>\r\n
    Atmosphile<\/td>\r\nTend to remain in atmospheric gases.<\/td>\r\nHe, Ne, Ar, N, O<\/td>\r\n<\/tr>\r\n
    Biophile<\/td>\r\nRemain\u00a0associated with living\u00a0organisms.<\/td>\r\nC, H, O, N, P, S, Ca, Mg<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n \r\n

    (b) Organic matter -It varies from 3.5 \u2013 5% by weight in top soil. It contains partly decayed plant and animal residues that are microbially synthesized contributing in maintenance and improvement of soil properties. It acts as storehouse of plant nutrients. Five major classes of organic matter exist in soil namely humus, saccharides, fats, resins and waxes, nitrogen organics and phosphorus containing organics:<\/p>\r\n

    (i) Humus- The formation by microbial degradation of plants and animals is called humification<\/em>. The composition of humic substances follows order: C: 45-55%,O :30-45%, H :3-6%, N :1-5%, S :1%. It possesses aromatic character and are instrumental in decomposing soil minerals<\/p>\r\n

    (ii) Saccharides- It assist in stabilization of soil aggregates from major food sources.e.g.sugar, cellulose, starches, gums.<\/p>\r\n

    (iii) Fats, resins and waxes: They are lipid extractable. The enzyme lipase degrades fats into glycerol and fatty acids which further breaks down into carbon dioxide and water.Being phototoxic affects soil properties by repelling water.<\/p>\r\n

    (iv) Nitrogen organics: In this, nitrogen is attached to humus, amino acids and amino sugars<\/p>\r\n

    (v) Phosphorus organics- It is the source of plant phosphate and occurs as phosphate esters, phospholipids<\/p>\r\nSoil Water<\/strong>\r\n

    It provides medium to plants for obtaining necessary nutrients. Itaids in maintenance of texture, arrangement and compactness of soil particles. The total amount of water present in the soil is called holard<\/em>. Water that can be absorbed by plants ischesard <\/em>while unabsorbed is called echard<\/em>.<\/p>\r\n \r\n\r\nSoil Air<\/strong>\r\n

    Its composition nearly similar to atmospheric air except CO2 level being high. Water clogging between air spaces leads to anaerobic condition<\/em> in which gases are unable to diffuse inwards. It is essential for respiration of soil microorganisms and plants and enhances permeability<\/em> of roots to soil.<\/p>\r\n \r\n\r\nSoil Structure<\/strong>\r\n

    The texture and structure of soil influences the amount of water held and hence behavior of any soil. Soil bulk density with soil texture influences water infiltration rate.Clay soil has slow water drainage than sand and volcanic soils. A loam soil with humus containing 34% of air and 66% of water is<\/em> considered best for majority of crops. <\/em>Secondary particles called aggregates or peds formed by the arrangement or grouping of primary particles (sand, clay, slit and organic matter) is defined as soil structure.<\/p>\r\n \r\n\r\nFormation of soil structure<\/strong>\r\n

    The process of soil formation occurs through physio-chemical process characterized by aggregate formation, flocculation and swelling, shrinking of clay masses and biological process where alleviation of aggregates by burrowing animals, binding action of plant roots and production of organic glues by micro-organisms occurs.<\/p>\r\n\"\"\r\n\r\nSoil is known to contain air spaces and generally possess a loose texture<\/em>\r\n\r\n \r\n\r\nFigure 2: Fine structure of soil showing solid, water, air phases<\/strong>\r\n\r\n \r\n\r\nDescribing Soil Structure<\/strong>\r\n

    Soil possess varying shapes or types. Soils having no true structure are termed as structure less.<\/em> Sandy soil possess negligible attraction between sand grains. Deposits of sand in a sand dune are called single<\/p>\r\n

    grain. Clay soil have large cohesive masses on textural basis and are thus massive in structure. Structural units are called Peds<\/em><\/strong>. Peds can take several definite and repeatable shapes such as granular, blocky, prism-like and platy.<\/p>\r\n \r\n