Introduction to Soil Science PDF

Introduction to Soil Science ICAR e-Course For B.Sc (Agriculture)  Index SSAC 121 - INTRODUCTION TO SOIL SCIENCE SN Chapter Page No 1 Soil...

Introduction to Soil Science ICAR e-Course For B.Sc (Agriculture)  Index SSAC 121 - INTRODUCTION TO SOIL SCIENCE SN Chapter Page No 1 Soil – Pedological and edaphological concepts 1-4 2 Origin of the earth – Earth’s crust – Composition 5-15 3 Rocks and minerals 16-36 4 Weathering 37-47 5 Soil formation factors and processes – Components of soils 48-61 6 Soil profile 62-64 7 Soil physical properties – Soil texture – Textural classes – Particle size analysis 65-72 8 Soil structure – Classification 73-81 9 Soil aggregates – significance – Soil consistency – Soil crusting – Bulk density and 82-89 particle density of soils & porosity - their significance and manipulation 10 Soil compaction – Soil Colour – Soil water 90-98 11 Retention and potentials – Soil moisture constants 99-106 12 Movement of soil water – Infiltration, percolation, permeability – Drainage – 107-113 Methods of determination of soil moisture 13 Thermal properties of soils – Soil temperature – Soil air – Gaseous exchange – 114-122 Influence of soil temperature and air on plant growth 14 Soil colloids – Properties, nature, types and significance 123-129 15 Layer silicate clays – their genesis and sources of charges 130-145 16 Adsorption of ions–Ion exchange–CEC& AEC – Factors influencing ion exchange - 146-155 Significance. 17 Soil organic matter – Composition – Decomposability 156-164 18 Humus – Fractionation of organic matter 165-168 19 Carbon cycle – C: N ratio. Soil biology – Biomass – Soil organisms – Their 169-189 beneficial and harmful roles.  Introduction to Soil Science 01. Soil – Pedological and Edaphological concepts Soil science is the study of soil as a natural resource on the surface of the earth including soil formation, classification and mapping; physical, chemical, biological, and fertility properties of soils; and these properties in relation to the use and management of soils. Sometimes terms which refer to branches of soil science, such as pedology (formation, chemistry, morphology and classification of soil) and edaphology (influence of soil on organisms, especially plants), are used as if synonymous with soil science. The diversity of names associated with this discipline is related to the various associations concerned. Indeed, engineers, agronomists, chemists, geologists, physical geographers, ecologists, biologists, microbiologists, sylviculturists, sanitarians, archaeologists, and specialists in regional planning, all contribute to further knowledge of soils and the advancement of the soil sciences. Soil scientists have raised concerns about how to preserve soil and arable land in a world with a growing population, possible future water crisis, increasing per capita food consumption, and land degradation. Soil occupies the pedosphere, one of Earth's spheres that the geosciences use to organize the Earth conceptually. This is the conceptual perspective of pedology and edaphology, the two main branches of soil science. Pedology is the study of soil in its natural setting. Edaphology is the study of soil in relation to soil-dependent uses. Both branches apply a combination of soil physics, soil chemistry, and soil biology. Due to the numerous interactions between the biosphere, atmosphere andhydrosphere that are hosted within the pedosphere, more integrated, less soil-centric concepts are also valuable. Many concepts essential to understanding soil come from individuals not identifiable strictly as soil scientists. This highlights theinterdisciplinary nature of soil concepts. Soil Science “The science dealing with soil as a natural resource on the surface of the earth, including Pedology (soil genesis, classification and mapping), physical, chemical, biological and fertility properties of soil and these properties in relation to their management for crop production.” Soil Science has six well defined and developed disciplines Soil fertility : Nutrient supplying properties of soil 1 www.AgriMoon.CoM  Introduction to Soil Science Soil chemistry : Chemical constituents, chemical properties and the chemical reactions Soil physics : Involves the study of physical properties Soil microbiology : Deals with micro organisms, its population, classification, its role in transformations Soil conservation : Dealing with protection of soil against physical loss by erosion or against chemical deterioration i.e excessive loss of nutrients either natural or artificial means. Soil Pedology : Dealing with the genesis, survey and classification Views on Soil (Science) The term SOIL was derived from the Latin Word “SOLUM” Means FLOOR  For a Layman soil is dirt or debris  For an Agriculturist soil is a habitat for plant growth (to grow crops)  For a Mining Engineer soil is a debris covering the Rocks  For a Civil Engineer soil is a material on which road bed or house bed is formed  For a Home Owner soil is a mellow or loamy or hard material Definitions Generally soil refers to the loose surface of the earth as identified from the original rocks and minerals from which it is derived through weathering process. Whitney (1892): Soil is a nutrient bin which supplies all the nutrients required for plant growth Hilgard (1892): Soil is more or less a loose and friable material in which plants, by means of their roots, find a foothold for nourishment as well as for other conditions of growth” Dokuchaiev (1900): Russian scientist - Father of soil science - Soil is a natural body composed of mineral and organic constituents, having a definite genesis and a distinct nature of its own. Joffe (1936): “Soil is a natural body of mineral and organic constituents differentiated into horizons - usually unconsolidated - of variable depth which differs among themselves as well as from the underlying parent material in morphology, physical makeup, chemical properties and composition and biological characteristics”. Jenny (1941): Soil is a naturally occurring body that has been formed due to combined influence of climate and living organisms acting on parent material as conditioned by relief over a period of time. Ruffin and Simonson (1968): Soil is a mixture of Earth’s uppermost mantle of weathered rock and organic matter 2 www.AgriMoon.CoM  Introduction to Soil Science Buckman and Brady (1969 ): Soil is a dynamic natural body on the surface of the earth in which plants grow, composed of mineral and organic materials and living forms Soil Science Society of America (1970) (i) Soil is the unconsolidated mineral matter on the surface of the earth that has been subjected to and influenced by genetic and environmental factors of parent material, climate (including moisture and temperature effects), macro and microorganisms and topography, all affecting over a period of time and producing a product, that is “SOIL” that differs from the material from which it is derived in many, physical, chemical, biological and morphological properties and characteristics. (ii) The unconsolidated mineral material on the immediate surface of the earth that serves as a natural medium for the growth of land plants. Dr. W.E.H. Blum Soils not only serve for agriculture and forestry, but also for filtering, buffering and transformation activities between the atmosphere and the ground water, protecting the food chain and drinking water against pollution and biodiversity As soil provides nutrients, water, air and anchorage and supports life on Earth, it can be called as Soul Of Infinite Life (SOIL) List of International Soil Scientists 1. Van Helmont (1577 – 1644) 2. Theoder De Saussure 3. John Woodward 4. Boussingault (1802 – 1882) 5. J.V. Liebig (1803 – 1873) 6. J.B.Laws & J.H. Gilbert (1855) 7. J.T.Way (1856) 8. R.Warrington (1876) 9. E.W. Hilgard (1860) 10. V.V. Dokuchaiev (1846-1903) 11. K.D.Glinga (1914) 12. C.F.Marbut (1927) 13. Hens Jenny (1941) Indian Scientists 1. J.W.Leather (1906) 2. Madam Scholasky (1932) 3 www.AgriMoon.CoM  Introduction to Soil Science 3. Wadia et al. (1935) 4. Viswanath & Ukil (1943) Soil as a three dimensional body Soil is a three dimensional body having length, breadth and depth. They form a continuation over the land surface and differ in properties from place to place. Its upper boundary is air or water and lower boundary is the rock lithosphere. Composition of soil on volume basis (Soil components) Mineral matter : 45% Organic matter : 5% Soil water : 25% Soil air : 25% Soil can be compared to various systems of animals Digestive system : Organic matter decomposition Respiratory system : Air circulation & exchange of gases Circulatory system (blood) : Water movement within the soil Excretory system : Leaching out of excess salts Brain : Soil clay Colour : Soil colour Height : Soil depth Approaches of Soil Study Two Concepts: One treats soil as a natural body, weathered and synthesized product in nature (Pedology) while other treats soil as a medium for plant growth (Edaphology). Pedological Approach: The origin of the soil, its classification and its description are examined in Pedology. (From Greek word pedon, means soil or earth). Pedology is the study of soil as a natural body and does not focus on the soil’s immediate practical use. A pedologist studies, examines and classifies soil as they occur in their natural environment. Edaphological Approach: Edophology (from Greek word edaphos, means soil or ground) is the study of soil from the stand point of higher plants. Edaphologists consider the various properties of soil in relation to plant production. They are practical and have the production of food and fibre as their ultimate goal. They must determine the reasons for variation in the productivity of soils and find means for improvement. ************** 4 www.AgriMoon.CoM  Introduction to Soil Science Origin of the earth – Earth’s crust – Composition Origin of earth Earth is one of the 9 planets (8 excluding Pluto) orbiting the SUN in the Solar System. The Universe is composed of several Galaxies. Our solar system is part of Milky way galaxy which is disk shaped with about 1, 00,000 million stars of varying sizes. Our solar system consists of 9 planets and 31 satellites, a belt of asteroids. Theories about origin of Earth Nebular hypothesis (Kant and Laplace, 1755) This hypothesis suggests that the solar system formed through the condensation of a nebula which once encircled the Sun. The outer planets formed first, followed by Mars, the Earth, Venus, and Mercury. This hypothesis suggests a sequential origin from outermost planet to innermost. As per this hypothesis, Mars evolved earlier than the Earth. This hypothesis is widely accepted to explain origin of different planets. During the past, the entire solar systems existed as a hot gaseous mass called nebula rotating in space. With time, the gaseous mass (nebula) cooled and contracted. Due to contraction, there developed a bulge at the equatorial region. This bulge subsequently separated into several rings. The ring coalesces in the form of a globe and continues to revolve around the nebula. In similar manner, ten rings were formed of which nine of them gave rise to nine planets. One broke down into smaller fragments to form the group of planetoids, while the remnant of the pre-existing nebula formed the central incandescent mass of the solar system and is known as the Sun. The planets were originally gaseous but were subsequently cooled down into liquid and ultimately to the solid state. 5 www.AgriMoon.CoM  Introduction to Soil Science Planetismal hypothesis (Chamberlain and Moulton, 1905) Planets were formed as a result of mutual interaction between the sun and another star of suitable size. This is the theory of biparental origin of the solar system. This theory postulates that due to the near approach of a larger star, tidal distortions were raised upon the surface of the sun and these in conjunction with the eruptive force prevalent in the sun (known as the solar prominence) brought about a description of the mass of the sun and a number of gaseous bulbs were shot forth, in space, to great distances. These gaseous solar materials thus ejected in space were subjected to immediate chilling, resulting in the formation of a number of minute solid particles known as planetisimals These planetisimals continued to rotate round the sun in highly elliptical orbits. The orbits must have happened to intersect one another and at points of intersection, they must have collided whereby the small planetisimals continued to coalesce gradually giving rise to the planets. During collision and coalescence of the planetisimals, large quantities of heat must have been generated and were dissipated in space before the next collision could occur and accordingly the planets must have been solid all the time during the growth. This theory accepts that the collision must have taken place in quick succession and accumulation of heat might have caused a melting of the masses of the planets. Gaseous tidal hypothesis (Jeans and Jeffreys, 1925) This also accepts the idea of biparental origin like that of planetisimal hypothesis but refuses to consider the disruptive forces in the sun (i.e. the solar prominence) had anything to do with the formation of the planets. According to Jeans and Jeffreys, during the ancient past, an extremely large star, while moving in space chanced to approach the sun. Due to progressive and nearer approach of the star, a tidal pull was raised on the surface of the sun and this increased in size with the 6 www.AgriMoon.CoM  Introduction to Soil Science nearer approach of the passing star. At the stage, where the passing star began to recede, the tidal pull on the surface of the sun, thus formed, was detached from the body of the sun in the shape of a spindle bulging near the centre and tapering at both ends. This very large mass of gas, thus ejected in space, was naturally extremely unstable and was immediately broken down into a number of small fragments. In all, ten such pieces were formed, nine giving rise to the planets and one, which was broken down into pieces, to the group of planetoid. These fragments formed into globular masses revolving round the sun along definite orbits and cooled down gradually from the gaseous to the liquid and ultimately to the solid state. This hypothesis is the most popular one due to do simplicity and capability of explaining the co-planar placement of the planets and the features relating to the distribution of mass and density in them and the density stratification that exists within the earth. Tectonic plate theory Tectonic plates are layers of rock inside the earth. This crust and upper mantle form lithosphere. Under the lithosphere lies a fluid rock layer called asthenosphere. The rocks in Tectonic plates are able to float upon the asthenosphere. There are seven large and several small plates. The largest plates include the pacific, North American, Eurasian, Antarctic and African plates. Continental drift theory The huge land masses of the earth are in constant movement. The continents and the oceans rest upon rock plates. They move relative to each other at rates of few centimeters a year. Until 200 million years ago, the continents were connected into one single land mass called Pangaea. This Pangaea splits and pieces drifted apart creating 2 new continents160 million years ago. The land mass that was to become India separated and 7 www.AgriMoon.CoM  Introduction to Soil Science moved north ward (140 million years ago) and hit another landmass leading to the eruption of the Himalayas. Australia separated from Antarctica (100 million years ago). The two super continents then pulled apart east and west opening the Atlantic oceans. The continents are still moving even today. The continental drift makes the Pacific Ocean smaller, the Atlantic Ocean larger and the Himalayan Mountains taller. The Atlantic oceans become wider, Mediterranean Sea collapses and Australia reaches equator in 60 million years. Age of the earth So far scientists have not found a way to determine the exact age of the Earth directly from Earth rocks because Earth's oldest rocks have been recycled and destroyed. Nevertheless, scientists have been able to determine the probable age of the Solar System and to calculate an age for the Earth by assuming that the Earth and the rest of the solid bodies in the Solar System formed at the same time and are, therefore, of the same age. The ages of Earth and Moon rocks and of meteorites are measured by the decay of long-lived radioactive isotopes of elements that occur naturally in rocks and minerals and that decay with half lives of 700 million to more than 100 billion years to stable isotopes of other elements. These dating techniques, which are firmly grounded in physics and are known collectively as radiometric dating, are used to measure the last time that the rock being dated was either melted or disturbed sufficiently to rehomogenize its radioactive elements. These ancient rocks have been dated by a number of radiometric dating methods and the consistency of the results give scientists confidence that the ages are correct to within a few percent. An interesting feature of these ancient rocks is that they are not from 8 www.AgriMoon.CoM  Introduction to Soil Science any sort of ‘primordial crust’ but are lava flows and sediments deposited in shallow water, an indication that Earth history began well before these rocks were deposited. In Western Australia, single zircon crystals found in younger sedimentary rocks have radiometric ages of as much as 4.3 billion years, making these tiny crystals the oldest materials to be found on Earth so far. The source rocks for these zircon crystals have not yet been found. The ages measured for Earth's oldest rocks and oldest crystals show that the Earth is at least 4.3 billion years in age but do not reveal the exact age of Earth's formation. The best age for the Earth (4.54 Ga) is based on old mineral grains (zircon) with U- Pb ages of 4.4 Ga have recently been reported from sedimentary rocks in west-central Australia. The oldest dated moon rocks, however, have ages between 4.4 and 4.5 billion years and provide a minimum age for the formation of our nearest planetary neighbour. The age of 4.54 billion years found for the Solar System and Earth is consistent with current calculations of 11 to 13 billion years for the age of the Milky Way Galaxy (based on the stage of evolution of globular cluster stars) and the age of 10 to 15 billion years for the age of the Universe. Geological Time Scale of Earth – Development of Life Era Period Age (m.Yr) Organism Archean Archean > 3500 Lifeless Precambrian Precambrian -do- Soft bodied plant & animals Palaeozoic Cambrian 500 Algae and shell bearing molluscus Ordovician 400 Molluscus & sea weeds Silurian 360 Land plants & breathing land animals Devonian 320 True fishes and ampibians Carboniferous 280 Coal forming materials , non flowering plants Permian 246 Reptiles Mesozoic Triassic 235 Marine life & reptiles Jurassic 185 Dinosours Cretaceous 139 Many plants & fishes Cenozoic Tertiary Eocine 20 Mammals 9 www.AgriMoon.CoM  Introduction to Soil Science Myocine 29 Mammals Pliocine 10 Mammals Quartanary Plistocine 01 Mammals Recent time Mammals Radius, distance from sun and revolution period S.No Planet Radius (km) Distance from sun(m.k) Revolution period (day) 1 Mercury 2439 57.9 88 2 Venus 6052 108.2 245 3 Earth 6378 149.4 365.25 4 Mars 3397 227.9 687 5 Jupiter 71398 778.3 4333 6 Saturn 60000 1427 10743 7 Uranus 23620 2870 30700 8 Neptune 24300 4496.5 60280 9 Pluto 1150 5970 90130 Present condition of the earth Our earth is supposed to have been formed in similar manner. It must have been hot and plastic when it was first formed. At present the outside of the earth is quite cool and solid but the interior is very hot and in a fluid condition. The outer cool layer, which we know as the earth’s crust rests upon a denser molten substratum in which various gases are dissolved at high pressure Interior of Earth The Earth Ball consists of 3 concentric rings namely Crust, Mantle and Core. Crust: 5 to 56 km on the surface of Earth. Density of rocks is 2.6 to 3.0 g cc -1. Distance: 5 to 11 km in oceans and 35 to 56 km in the continents. The crust has been divided into two sub-zones called Sial and Sima. The Sial is a heterogeneous mixture of rocks. The Sima is a homogenous plastic or semi-plastic concentric layer that behaves like a solid. The Sial floats on the Sima, which in turn floats on the lower concentric layer called the Mantle. Sial contains about 65-75 % silica. Aluminium is the next important element in the Sial, represented by the most common rocks like granite, and rhyolite. Silica decreases to about 50-60 percent in the Sima where aluminium has largely been replaced by magnesium 10 www.AgriMoon.CoM  Introduction to Soil Science with minor quantities of iron. Sima is represented by the most common rocks like basalt and gabbro at the upper level and by olivine rich rocks at the lower level. Mantle A massive solid to semi solid layer below the crust; 2900 km in thickness; comprises mixed metals and silicate and basic rocks with density of 3.0 to 4.5 g cc-1. Core Innermost portion of Earth, 3500 km in thickness, contains molten metals like Nickel, iron; average density: 9.0 to 12.0 g cc-1. Exterior of Earth Solid lithosphere, Liquid hydrosphere and gaseous atmosphere. The atmosphere is of 320 km above the lithosphere / hydrosphere. (70% of Earths surface is covered by water (Hydrosphere). 11 www.AgriMoon.CoM  Introduction to Soil Science Composition of atmospheric air By volume (%) By weight (%) N2 78.08 76.5 O2 20.9 23.1 CO2 0.033 0.04 Other gases 0.93 1.36 (H, NH3, H2S, SO2, O3, He, Argon, Neon, Krypton, Xenon) Hydrosphere It is the layer of water surrounding the lithosphere. It is present in the form of seas and oceans. It covers 70% of the earth leaving only about 30% above sea level. The surface of the waters of the various seas is in one level in contrast with the surface of the land. This surface is known as the sea level. The seawater has a higher specific gravity than terrestrial water due to the salts it contains in solution. The average density is 1.026, but it varies slightly from place to place. Sea water contains 3.5% salts (minerals) It is least dense at the places where river enters the sea and very heavy at places where evaporation is high. Lithosphere It is the inner most body within the gaseous and watery envelops. That portion of the lithosphere, which rises above the seawater, is visible to us and is known as land. The land is only about ¼th of the total surface of the earth. Most of this land is situated in the northern hemisphere. The lithosphere consists of two portions, viz., 1. The upper or outer cool solid surface. 2. The inner hot and molten mass. 12 www.AgriMoon.CoM  Introduction to Soil Science It is the heaviest of the three spheres. Its mean density is 5.5 compared to that of water as one. The outer crust has a density of about 2.5 to 3.0, while the inside core, consists of much heavier materials. The outer solid layer, called as the earth’s crust is estimated to be about 10 to 20 miles thick. It consists of the various rocks together with a more or less thin mantle of soil enveloping them. It is on this crust that life, both animal and plant sustains. The inner mass, which forms the interior of the earth, is in molten condition. According to one belief, the whole of the inner core is a molten mass of materials, upto the centre. According to another view, the interior of the earth consists of a molten magma, about 50 to 100 miles thick, surrounding a gaseous centre. A gradation exists from the central gaseous nucleus, through the intermediate molten mass, to the outer solid crust. The interior of the earth is intensely hot and is at the same time under enormous pressure. There is progressive increase in temperature with depth. Though, the increase is not uniform at all depths and all places, there is a rise of 1 oF for every 64 feet on an average. Assuming the temperature to increase at this rate, at a depth of 25 to 50 miles, it should be sufficiently high to melt all substances known to us. This indicates that the interior of the earth is in molten condition. Composition of the earth’s crust The Earth’s crust is principally compassed of mineral matter. This mineral matter is made up of various elements combined together to form compounds. Some elements exist as such without forming compounds. Almost all the elements known to us at present, except the inert gases, are present in the earth’s crust. The elements do not exist in the earth’s crust as such. Each element is in combination with one or more other elements to form definite chemical compounds known as minerals. Many of these minerals in turn combine together to form aggregates, which we know as rocks. Almost all the mineral mater is present in the form of rocks in the earth’s crust. Rock is composed of elements, which in turn are made up of atoms. Out of 106 elements knows, 8 are sufficiently abundant as to constitute about 99 percent by weight of the Earth’s Crust (upto 16 Km). The elements are geochemically distributed into five main groups based on their bonding characters.  Lithophile elements- which ionize readily or form stable oxyanions, viz. O,Si, Ti, Fe, Mn, A1, H, Li, Na, K, Rb, Cs, Be, Mg, Ca, Sr, Ba, B, Ga, Ge, Sn, Sc, Y, F,C1, Br, I, C, HF,Th, P,V, Nb, Ta, Cr,W, U, Zr, (Mo), (Cu), (Zn), (Pb), (T1), (As), (Sb), (Bi), (S), (Se), (Te), (Ni), (Co) and rare earths. 13 www.AgriMoon.CoM  Introduction to Soil Science  Chalcophile element – which tend to form covalent bonds with sulphide, viz, S,Se, Te, (Fe), Ni, Co, Cu,Zn, Pb, Mo, Ag,Sb,(Sn), Cd, In,T1,Pb, As,Bi, Re, (Mn), (Ga) and Ge).  Siderophile elements – which readily form metallic bonds, viz, Fe, Ni, Co,Ru, Rh,Pd, Ir, Os and Au.  Atmosphile elements- which tend to remain in atmospheric gases, viz. N, (O) He, Ne, Ar, Kr, Xe.  Biophile elements- which tend to be associated with living organisms, viz. C,H,O,N, P,S,C1, I, B, Ca, Mg, K, Na, Mn, Fe, Zn, Cu, Ag, Mo, Co, Ni, Au, Be, Cd, Se, T1,Sn, Pb, As and V. Composition of Earth crust (% by weight) Non - metallic Oxygen O2 - 46.60% 74.32% ( ¾th) Silica Si4+ 27.72% 3+ Metallic Aluminium Al 8.13% ¼th of the total Iron Fe2+ 5.00% Calcium Ca2+ 3.63% + Sodium Na 2.83% Potassium K+ 2.59% Magnesium Mg2+ 2.09% Others - 1.41% Eight elements are abundant – 98.6% 14 www.AgriMoon.CoM Introduction to Soil Science ************* 15 www.AgriMoon.CoM  Introduction to Soil Science Rocks and minerals What is Rocks? Rocks are the materials that form the essential part of the Earth’s solid crust. “Rocks are hard mass of mineral matter comprising one or more rock forming minerals”. Rocks are formed from the molten material known as magma. The study of rocks is called Petrology (in Greek, petra means rock, logos means science). Petrology deals with the description of rocks; petrogenesis is the study of the origin of rocks. Formation of rocks 1. Cooling and consolidation of molten magma within or on the surface of earth = Igneous or Primary rocks 2. Transportation and cementation of primary rocks = Sedimentary or Secondary rocks 3. Alteration of the existing primary and secondary rocks = Metamorphic rocks 16 www.AgriMoon.CoM  Introduction to Soil Science Rocks Igneous Sedimentary Metamorphic Intrusive Extrusive Clastic Chemical Foliated Nonfoliated Gabbro Basalt Sandstone Limestone Slate Quartzite Granite Rhyolite Shale Schist Marble 1. Igneous rocks (primary or massive rocks) These are first formed in the earth crust due to the solidification of molten magma. Based on the mode of formation, they are further classified as extrusive and intrusive rocks 17 www.AgriMoon.CoM  Introduction to Soil Science Extrusive rocks or volcanic rocks These rocks are formed due to the consolidation of magma on the surface of the earth. The magma, when it flows on the Earth surface is called LAVA. E.g. Basalt. Intrusive rocks or plutonic rocks These rocks are produced due to solidification of magma below the surface of the earth. Plutonic – intrusive rocks solidifies at greater depth and Hypabassal rocks solidifies at shallow depth from the surface. E.g. Granite, syenite, diorite, Gabbro etc. Rocks formed in vertical cracks are called dykes and in horizontal cracks are called sills. Vesicular rocks: Molten magma cools on the surface. Steam of water is entrapped into rocks and forms vesicles. Based on the silica content, rocks are also classified as 1. Acid rocks : >65% SiO2 (Granite, Rhyolite) 2. Intermediate : 56 to 65% SiO2 (Sub acid rocks 60 to 65% SiO2 (Syenite and Trachyte)) (Sub basic rocks 56 to 60 % SiO2 (Diorite and Andesite)) 3. Basic rocks : 40 to 55% (Gabbro, basalt) Igneous rocks S.No Rocks Origin Essential Common Average Remarks minerals minerals specific gravity i. Granite Plutonic Quartz (20 to Hornblende, 2.64 Light holocrystalline 30%) magnetite, coloured mica white or reddish ii. Syenite Plutonic Quartz, Hornblende, 2.80 Light Holocrystalline orthoclase magnetite, coloured biotite white or reddish iii. Diorite Plutonic Quartz Hornblende, 2.85 Darker Holocrystalline magnetite, biotite iv Gabbro Plutonic Labradorite, Hornblende, 3.0 Blakish Holocrystalline augite, ilmenite olivine 18 www.AgriMoon.CoM  Introduction to Soil Science v. Dolerite Hypabasal Labradorite, Hornblende, 3.0 Blakish augite, ilmenite olivine vi. Basalt Volcanic Labradorite, Hornblende, 3.0 crystalline with augite, ilmenite glassy mass olivine 2. Sedimentary rocks These rocks are formed from the consolidation of sediments accumulated through wind or water action at the surface of the earth. Many are deposited in layer or formed through chemical reactions as precipitates from aqueous solutions. Sediments may contain various size particles cemented together by substances like SiO 2, Fe2O3 or lime. These rocks are also called as clastic rocks. Based on the origin, the sedimentary rocks are classified as 1. Residual : Laterite 2. Transported a. Deposited as solids in suspension : Sandstone, shale b. Deposited by chemical precipitation : Limestone, ironstone c. Deposited through agency of organic matter: Peat, Phosphatic deposits Based on the grain size, sedimentary rocks are classified as 1. Rocks with boulder pebbles sized minerals (Rudaceous) : Conglomerate 2. Rocks with sand size particles (Arenaceous) : Sandstone 3. Rocks with silt size particles (silt rocks) : Siltstone 4. Rocks with clay size particles (Argillaceous) : Shale Sedimentary rocks S.No Rock Mineral composition Colour and structure 1. Sandstone Mainly quartz with some CaCO3, iron oxides Light to red, granular and clay 2. Shale Clay minerals, quartz and some organic Light to dark thinly matter laminated 3. Limestone Mainly calcite with some dolomite, iron Light grey to yellow, fine oxides, clay, phosphate and organic matter grained and compact 19 www.AgriMoon.CoM  Introduction to Soil Science 3. Metamorphic rocks These are formed from igneous and sedimentary rocks under the influence of heat, pressure, chemically active liquids and gases. Change may occur in mineral composition or texture or both. The changes due to water is called hydro metamorphosis and due to pressure is called dynamo metamorphosis Sand stone : Quartizite Shale : Slate/mica, schist Lime stone : Marble Granite : granite gneiss Dolerite : Hornblende gneiss Metamorphic rocks S.No. Rock Mineral Colour and structure composition 1. Gneiss Formed from granite Alternating light and dark colours, banded and foliated 2. Schist Formed from basalt or As original rock, foliated shale 3. Quartzite Formed from Light or brown, compact and uniform texture, 20 www.AgriMoon.CoM  Introduction to Soil Science sandstone foliated structure 4. Slate Formed from shale Grey to black, compact and uniform texture, foliated structure 5. Marble Formed from lime Light red, green, black, compact fine to coarse stone texture, foliated structure Brief description of important rocks-Mineralogical composition Sedimentary rocks Formed through the agency of water. Also called as aqueous rocks. Formed from sediments brought by water. The sediment may contain various types of substances and sizes of particles. The particles are cemented by silica, iron oxide or lime to give a consolidated form. The rocks are mostly deposited in layers or strata – so called as stratified rocks. Sometimes they are formed by cooling, evaporation or by direct chemical precipitation. Anyway they are of secondary origin. Sedimentary rocks divided into six groups as follows 1. Arenaceous: Formed of the deposits of coarse grained particles. They are composed of siliceous material derived from the disintegration of older rocks. The fragmental material so derived is deposited in beds of varying thickness through the agency of water. Depending upon the nature of cementing material present, some arenaceous rocks are hard and refractory, but most are loose and fall away very easily. E.g. Sandstone, grit, conglomerate and breccia. 2. Argillaceous rocks: Consist of small sized particles known as clay. They are composed of hydrated silica of alumina in admixture with sand, various other silicates and calcareous matter. When clay is deposited mainly of silicate of alumina, it is known as kaolin or China clay. E.g. clay, mudstone, shale and fuller’s earth. 21 www.AgriMoon.CoM  Introduction to Soil Science 3. Calcareous rocks: Consists of carbonate of lime or lime and magnesia. They may be of sedimentary origin or formed by chemical precipitation or by organic agency. When, they are of organic agency, they are composed mainly of debris from plant and animal life. They are formed either by growth and decay of organisms in situ or by the transport and subsequent accumulation of their remains. The rocks so formed are found in layers, which vary considerably in depth of thickness. When formed by chemical precipitation, the calcareous material is deposited in the form of layers/sheets from waters containing calcium carbonate in solution. The precipitate when first formed is usually soft and chalky, but soon acquires a hard, compact structure and crystalline texture. The important calcareous rocks of aqueous origin are limestone, chalk, magnesian, ferruginous limestones, dolomite, marks of various varieties and coral. 4. Carbonaceous rocks: Formed from decomposing vegetation under anaerobic conditions. When plants undergo decomposition under restricted air supply, is greater portion of the carbonaceous matter is retained and the material is slowly converted into coal. E.g. peat, lignite, coal, anthracite. 5. Siliceous rocks: Siliceous rocks of organic origin formed from parts of minute plants and animals like diatoms, radiolaria etc, some are soft and friable and crumble to powder very easily. Others like flint and chert are hard and compact. 6. Precipitated salts: Consist mainly of deposits formed as rock masses either by cooling, evaporation or by chemical precipitation. Water charged with acid or alkaline material, acting under pressure as it does under subterranean regions, dissolves various mineral substances from rocks with which it comes in contact. The salts thus formed deposit as rocks and such rocks vary in composition. They are i. Oxides: e.g. hematite, limonite, bauxite and quartz. ii. Carbonates: e.g. stalactite, stalagmite, magnetite and limestone. iii. Sulphates: e.g. gypsum and anhydrite iv. Phosphates: e.g. phosphorite v. Chlorides: e.g. rock salt. Metamorphic rocks The igneous and sedimentary rocks after they were first formed sometimes undergo a change. When the change is considerable, the rock is said to have undergone metamorphosis and the new rock is known as a metamorphic rock. The metamorphism is brought about by the action of water, heat or pressure or by the combined action of any one of these or all. The change brought about by water is hydro-metamorphism. The change brought about by heat is thermo-metamorphism. The change brought about by pressure is 22 www.AgriMoon.CoM  Introduction to Soil Science dynamo- metamorphism. The changes that are brought about are both physical and chemical in character. In some cases the metamorphism is so pronounced that the new rock looks quite different from the original. The action of water tends to remove some material or introduce new materials. By the introduction of a cementing material like silica, lime or iron oxide, loose sand may be turned into sandstone or sandstone into a quartzite. By the removal of certain constituents by percolating waters, basalt or granite may be converted into a laterite. The action of heat hardens the rock and develops new crystals in it. Crystalline marble is produced this way from amorphous limestone by the action of heat and pressure. Due to pressure, the crystals of the original rock get pressed or flattened and the new rock is foliated. When foliation is slight, the layers are inseparable and it is called as gneiss. It foliation is complete with distinct and separable layers it is called as schist. Mineralogical composition of important rocks SI. Rocks Grain size Essential minerals Most Averag Remarks No common e accessory specific minerals gravity (i) Igneous rocks 1. Granite Plutonic Predominant quartz Hornblende 2.64 Light Holocrystalli 20-35 orthoclase mica, coloured, ne magnetite White or reddish 2. Syenite - do - Predominance quartz Hornblende, 2.08 - do - 100% plus of biotite, orthoclase, nepheline magnetite and albite 3. Granodi - do - Intermediate quartz - do - 2.07 Medium orite plagioclase exceeds coloured orthoclase reddish 4. Diorite - do - Intermediate quartz - do - 2.85 Darker absent plagioclase 5. Gabbro - do - Labradorite augite + Hornblende, 3.00 Blackish olivine ilmenite 6. Dolerite Hypabyssal - do - - do - 3.00 - do – ophitic - texture 23 www.AgriMoon.CoM  Introduction to Soil Science 7. Basalt Volcanic - do - - do - 3.00 - do - micro crystalline with glassy mass SI.N Name of the Mineral composition Colour and structure o type (ii) Sedimentary rocks 1. Sandstone Mainly quartz with same Light to red, usually granular in contents, such as structure calcium carbonate, iron oxides and clays 2. Shale Clay minerals, quartz Light to dark thinly laminated and some organic structure matter 3. Limestone Mainly calcite or calcite Usually light grey to yellow usually and dolomite with some fine grained iron oxides, clay, and compact phosphate and organic matter (iii) Metamorphic rocks 1. Gneiss Formed from granite, Alternating light and dark colours. mineral composition like Banded and that of granite foliated structure 2. Sehist Formed from basalt or Much as original rock, foliated shales. Mineral structure composition much as that of original rock 3. Quartzite Formed from sandstone Light to brown. Compact and uniform and of same texture, composition foliated structure 4. Slate Formed from shale and Grey to black, compact, and uniform of same composition texture 24 www.AgriMoon.CoM  Introduction to Soil Science foliated structure 5. Marble Formed from limestone Light red, green, black, compact, fine to coarse texture, non-foliated structure Minerals Minerals are naturally occurring solids with a definite chemical composition and crystal structure. “Solid substances composed of atoms having an orderly and regular arrangement” When molten magma solidifies, different elements present in them freely arrange in accordance with the attractive forces and geometric form. Silica tetrahedron is the fundamental building blocks for the formation of different minerals. (SiO2). Different silicate minerals are ortho silicates, ino-silicates, phyllosilicates and tectosilicates. There are non- silicate minerals also. These are different oxides, carbonates, sulphates, phosphates etc. Minerals that are original components of rocks are called primary minerals. (feldspar, mica, etc.). Minerals that are formed from changes in primary minerals and rocks are called secondary minerals (clay minerals). Those minerals that are chief constituents of rocks are called as essential minerals (Feldspars, pyroxenes micas etc) and those which are present in small quantities, whose presence or absence will not alter the properties of rocks are called accessory minerals (tourmaline, magnetite etc). 25 www.AgriMoon.CoM  Introduction to Soil Science Of the >2000 known minerals, only few occur in abundance in the Earth crust. Minerals (arranged in the order Important constituents Percent of their crystallization) distribution Primary minerals Ferro magnesium minerals Ortho-ino silicates 16.8 Olivine Fe, Mg Pyroxenes Ca, Na, Fe, Mg Amphiboles Ca, Na, Fe, Mg, Al, OH Phyllo Silicates 3.6 Biotite K, Fe, Mg, Al, OH Muscovite K, Al, OH Non-Ferro Magnesium Tecto Silicates Feldspars 61.0 Anorthite Ca, Al Albite Na, Al Orthoclase K, Al Quartz Secondary clay minerals Minerals Na, K, Ca 11.6 Others Mg, Fe, Al, OH 6.0 Ferro magnesium minerals Pyroxenes and amphiboles: The pyroxenes and amphiboles are two groups of ferromagnesian minerals (heavy group) the structure of which consists of long chains of linked silica tetrahedral. The pyroxenes consist of a single chain (2 oxygen shared in each tetrahedron) whereas amphiboles consist of a double chains (alternately 2 and 3 oxygen atoms shared successive tetrahedral). These chain silicates are sometimes referred to inosilicates. The pyroxene group of minerals comprised of different minerals namely enstatite, hypersthene, diopside and augite, of which augite is the most important minerals in soils and it is found in basic rocks. The amphibole group of minerals are common in acidic rocks and it can be represented by the isomorphous series between tremolite actinolite olivine and hornblende. Hornblende weathers fairly rapidly. Olivine (olive-green) minerals from an isomorphous series between foresterite (Mg2 SiO4) and fayalite (Fe2SiO4). 26 www.AgriMoon.CoM  Introduction to Soil Science Pyroxenes are more basic in character and therefore it weathers more rapidly than amphiboles. Micas: Micas occur extensively in soils. They are primarily originated from the parent rock from which the soil is derived. Generally soils are inherited from well-ordered and imperfectly ordered micas. Well-ordered micas are derived from sedimentary rocks. The most common well ordered micas are muscovite, paragonite, biotite and phlogopite (trioctahedral). The imperfectly ordered micas contain less potassium and more water as compared to well-ordered micas and this type of micas are most abundant in the clay fraction of soils. Among the ordered micas, biotite weathers more rapidly than muscovite. In imperfectly ordered micas, many of the illite-type specimens as well as the disordered micas of soils exhibits some mixed-layering with phases of vermiculite, smectite group of minerals, chlorite and intergrades of several of these species. Non-ferromagnesium minerals Feldspars: Feldspars are anhydrous aluminosilicates of K, Na and Ca and occasionally of other large cations such as Ba. The feldspar structure consists of tetrahedral which are attracted by sharing each oxygen atom between neighbouring tetrahedran. The tetrahedral contain mainly Silicons with sufficient Al substitution. It belongs to the group of minerals that are light in weight. There are two groups of feldspars: (i) potassium feldspars (KA1Si3O8) include orthoclase, microcline, adularia and sanidine. Orthoclase and microcline are more common in the plutonic and metamorphic rocks. The potassium feldspars occur commonly in the silts and sands of soils and also abundant in clay-size, (ii) plagioclase feldspars- a series consisting of a solid solution of albite (NaA1Si 3O8) high in sodium and anorthite (CaA12Si2O8) high in calcium. Plagioclase weathers more rapidly than orthoclase. Quartz: It is very densely packed and occurs in a high degree of purity. It is strongly resistant to weathering as the structure is densely packed, electrically neutral and free from any substitution. It is the most abundant mineral next to feldspars. Serpentine, a hydrous magnesium silicate occurs more commonly as a secondary product. Garnets are characteristic of metamorphic rocks and are very hard and most resistant to weathering. Silicate minerals Ortho/ Neosilicates The minerals in this group are composed of single tetrahedral linked together by Mg or Fe. To effect a break down, it is considered sufficient to sever the weaker Mg-O or Fe-O bonds. Non-withstanding the bond energy considerations susceptibility of the minerals in this group to breakdown by weathering appears to vary considerably from one mineral to another, 27 www.AgriMoon.CoM  Introduction to Soil Science e.g., zircon makes the mineral comparatively hard. On the other hand, the looser packing of oxygens in olivine makes the mineral weather faster. Inosilicates The inosilicate group has in its structure single-chain (pyroxenes) and double chain (amphiboles) silica tetrahedral linked together by Ca, Mg, or Fe. Because of the presence of many weak spots provided by the Ca-O, Mg-O, or Fe-O bonds, these minerals tend to weather rapidly Phyllosilicates Linkages of silica tetrahedral and Alumina octahedral sheets by mutually shared oxygen atoms from the basis for the structure of this group. Some of the minerals, e.g., biotite and muscovite, are relatively susceptible to weathering, whereas others, like clay minerals, are resistant weathering products and further breakdown of clays is difficult. Disruption of interlayer ions, or through cleavage of A1-O bonds in tetrahedral and octahedral positions. Tectosilicates The minerals are considered solid solution minerals with a framework of silica tetrahedral, in which the cavities are occupied by Na, Ca, and so on. The minerals in this group may also vary considerably in their resistance to weathering, e.g., leucite and plagioclase versus potash fertilizers. The relative degree of close packing of atoms in their structural frame work may be the reason for such variability in weathering. Increased substitution of A1 and Si in tetrahedral of plagioclase mineral is also considered a factor that makes these minerals weaker than potash feldspars. Non-silicate minerals Oxides: Hematite (Fe2O3) Limonite (Fe2O3, 3H2O) Goethite (FeO (OH) H2O) Gibbsite (Al2O3H2O) The red, yellow or brown colours in soils are due to the presence of goethite and hematite, which occur as coatings on the surface of soil particles. Carbonates: Calcite (CaCO3) Dolomite (CaMgCO3) Sulphates: Gypsum (CaSO4.2H2O) Phosphates: Apatite (Rock phosphate Ca3 (PO4)2 - primary source of phosphorus 28 www.AgriMoon.CoM  Introduction to Soil Science Physical properties of minerals 1. Color 2. Streak 3. Fracture/ cleavage 4. Hardness 5. Luster 6. Crystal form 7. Taste 8. Specific gravity 9. Magnetism 10. Effervescence (fizz) 11. Birefringence 12. Fluorescence Additional reading Color  Denotes the natural colour of the mineral  The most obvious, but least reliable.  Calcite has more colours  Sulfur and Pyrite have same colour Streak  Refers to the colour of the powder form of the mineral When an unknown mineral is rubbed against a piece of unglazed porcelain (streak plate) it produces a colored line.  Hematite - red  Magnetite - Black  Talc - white 29 www.AgriMoon.CoM  Introduction to Soil Science Fracture and Cleavage These terms describe the way a mineral breaks Fracture is the nature of the surface produced as a result of its breakage Conchoidal - curved surface Uneven - Uneven surface Hackly - Jagged surface Earthy - Like chalk Even - Smooth Cleavage Some minerals break along certain well defined planes called cleavage planes. Gypsum - 1 set Calcite - 2 sets Flourite - 3 sets Hardness This is how resistant a mineral is to being scratched. We use the Mohs scale to classify a given minerals hardness. Try to scratch the unknown mineral with various items, such as a fingernail (hardness of about 2.5), a coin (3), a steel nail (5.5) and a steel file (7) MOHS SCALE OF HARDNESS Mineral Hardness Mineral Hardness Talc 1 Feldspar 6 Gypsum 2 Quartz 7 Calcite 3 Topaz 8 Fluorite 4 Corundum 9 Apatite 5 Diamond 10 30 www.AgriMoon.CoM  Introduction to Soil Science Luster The way a mineral reflects light Metallic (Magnetite); sub-metallic, Vitreous (Opal), Resinous (Pyrite), Pearly, Adamentine (Diamond), silky (Asbestos) and greasy. Crystal form Crystal structure is the result of regular grouping of atoms that are homogeneous. A crystal is a polyhedral form, which means it is a geometric solid. It has a specific set of faces, corners and edges, which is consistent with the geometric packing of the atoms There are 6 basic crystal forms 1. Isometric 2. Tetragonal 3. Hexagonal 4. Orthorhombic 5. Monoclinic 6. Triclinic 31 www.AgriMoon.CoM  Introduction to Soil Science Taste This property is used to identify the mineral halite (salt) Specific Gravity This characteristic relates to the minerals density. If the mineral is heavy for its size, then it has a high specific gravity Magnetism Is the mineral magnetic (try using a compass), or is it attracted by a magnet? This property is characteristic of Magnetite. Effervescence When some minerals are exposed to acids, they begin to fizz (calcite). Birefringence This is also known as double refraction. Birefringent minerals split the light into two different rays which gives the illusion of double vision in this Iceland Spar Calcite Fluorescence Some minerals display the phenomenon of photoluminescence. 32 www.AgriMoon.CoM  Introduction to Soil Science They "glow" when exposed to UV light. Opal and Fluorite. Formation of secondary minerals, Clay minerals & Amorphous minerals The secondary minerals are formed at the Earth’s surface by weathering of the preexisting primary minerals under variable conditions of temperature and pressure. During weathering, water accompanied by CO2, from the atmosphere plays an important role in processes, such as hydrolysis, hydration and solution. As a result the primary minerals are altered or decomposed. Feldspar + water — clay mineral + cations + anions + soluble silica Because of weathering, many elements are released into solution; a part of which may be used as a source of plant nutrients, a part may be leased out into the groundwater; still another part together with other constituents of the environment (like CO 2, H2O) may recombine to form secondary minerals. The most commonly formed secondary minerals are clay minerals (e.g. illite, montmorillonite, kaolinite, etc.) and iron and aluminium oxides. Other secondary minerals observed in soils, especially in arid and semi-arid (dry) regions are gypsum, calcite, attapugite and apatite. SILICATES - Clay minerals: hydrous aluminosilicates, with layer structure similar to micas, e.g. illite, montomorillonite, kaolinite, etc. NON-SILICATES --Oxides, hydroxides or oxyhydrates of Si, A1 and Fe Haematite Fe2O3 Goethite; Limonite FeO(OH)n H2O Gibbsite A1 (OH) 3, Clay Minerals Clay minerals in soils are formed from primary minerals due to weathering processes. These clay minerals are of size 7) is alkaline while a soil with low pH (7) is acidic in nature.  pH is expressed in terms of negative log to the base 10 of H + ion concentration. Distribution  While primary minerals are observed in all rocks and in sand and silt fractions of soils, the secondary minerals dominantly occur in the clay fractions of almost all soils and in sedimentary rocks, especially shales. The kind and proportion of mineral(s) observed in a soil depend on the kind of parent material and weathering intensity (to which it has been exposede.) The most common clay mineral observed is illite. Apart from illite, smectite predominates in the cracking-clay soils (of Australia, northern Iraq and central India north- east Africa), kaolinite in the highly-weathered soils of the inter tropical zones (of southern India, South America, S.E. Asia) and southern Iraq, western India). In view of their high 34 www.AgriMoon.CoM  Introduction to Soil Science surface area and negative charge on them, they are considered as a source of cation adsorption and cation release which are so important in acidic soil fertility. Non-Silicates Oxides, Hydroxides or Hydrous-oxides group We have already seen that oxygen is present in great abundance (46.7%) in the Earth’s crust. The oxide minerals are found by the direct combination of elements (present in the Earth’s crust) with oxygen. The oxides are usually harder than any other mineral, except the silicates. The most important soils-forming oxide minerals are: Haematite : Fe2O3 Limonite : Fe2O3 3H2O Goethite : FeO (OH).nH2O Gibbsite : A12O3.H2O Haematite, Fe2O3 It varies in colour from red to blackish and has reddish streak. It has a metallic luster and hardness (H) of about 5. Its presence in rocks is indicative of quick chemical change. Haematite alters to limonite, magnetite, pyrite and siderite. It occurs as coatings on sand grains and acts as a cementing agent. It swells on absorbing water to form hydrated iron oxide, i.e. limonite, 2Fe2O3 3H2O and goethite, FeO (OH).nH2O. Limonite or Bog Iron, 2Fe2O3,3H2O It is hydrated ferric oxide, yellow to brown in colour and is of wide occurrence. It is the final product of most iron minerals and hence is resistant any further change, except for absorption of water. It is an important colouring and cementing agent in soils, iron. Limonite is a common alteration product of pyrite, magnetite, hornblende and pyroxene. It may be present in the form of iron concentration. Goethite, FeO(OH)nH2O Most materials, called limonite, are goethite with some adsorbed water. It is usually white but may pink or grey in colour. Its hardness is 5.3 Gibbsite (Hydragillite), A12O3H2O It is the most common aluminium compound in soils. Its natural colour is white. It is abundantly observed in highly-weathered soils of the tropical environment, supporting Laterites (Oxisols). It’s present in soils suggests extreme degree of weathering and leaching under well drained conditions. The red, yellow or brown colour in soils is due to the presence of goethite and hematite which occur as coating on the surfaces of soil particles, especially clay. 35 www.AgriMoon.CoM  Introduction to Soil Science Carbonate Group The basic compounds, like Mg (OH) 2, and Ca(OH) combine with CO2 or carbonic acid to form carbonates as under: Calcite, CaCO3 A white mineral, with hardness of 3, is widely distributed in sedimentary rocks, like limestone and decomposes easily to calcium bicarbonate as: CaCO3 + CO2 + H2O --- Ca (HCO3)2 (soluble in water) Dolomite, Ca Mg (CO3)2 Dolomite is less-readily decomposed than calcite; it is the chief source of Mg in soils. Siderite, FeCO3 It is an alteration product of other iron-bearing minerals, having hardness of 4 and may itself alter to hematite or limonite. It is an important mineral in waterlogged soils. Sulphate Group Sulphate is a complex group formed by the combination of 1 sulphur and 4 oxygen ions, which further reacts with Ca to form calcium sulphate (anhydrite, CaSO4) On hydration it forms gypsum (CaSO42H2O) Gypsum, CaSO4 2H2O It is a common mineral in desert soils and in sedimentary rocks having a hardness of 2. It is slightly soluble in water and gets most-easily leached. It precipitates as very fine, powdery mycelium from ground waters rich in Ca and SO4 ions (as observed in the Mesopotamian Plain of Iraq where hyper aridic prevail). In India, it is used as an amendment to reclaim sodic soils and also acts as a source of Ca and S for plants. Under the hot, aridic climatic environments of Iraq, the presence of gypsum in high amounts is a problem, as it causes civil structures to collapse and makes sink-holes in soils, resulting in loss of irrigation water. Phosphate Group Apatite, Rock Phosphate It is a primary source of phosphorus in soils. Its hardness is 5 in mho’s scale. It decomposes readily under the influence of carbonic acid. It becomes immobile in calcareous soils as it readily combines with clays, with clays, Fe-A1 hydrous oxides, calcium carbonate to form rock phosphate. It also precipitates under acidic environment, as Fe and/or A1- phosphate. ****************** 36 www.AgriMoon.CoM  Introduction to Soil Science Weathering – Soil formation factors and processes – Components of soils Weathering A process of disintegration and decomposition of rocks and minerals which are brought about by physical agents and chemical processes, leading to the formation of Regolith (unconsolidated residues of the weathering rock on the earth’s surface or above the solid rocks). (OR) The process by which the earth’s crust or lithosphere is broken down by the activities of the atmosphere, with the aid of the hydrosphere and biosphere. (OR) The process of transformation of solid rocks into parent material or Regolith. Parent material It is the regolith or at least it’s upper portion. May be defined as the unconsolidated and more or less chemically weathered mineral material from which soil are developed. Weathering Two basic processes Physical /mechanical Chemical (disintegration) (decomposition) In addition, another process: Biological and all these processes are work hand in hand. Depending up on the agents taking part in weathering processes, it is classified into three types. 37 www.AgriMoon.CoM  Introduction to Soil Science Weathering of Rocks Different agents of weathering Physical/ Mechanical Chemical Biological (disintegration) (decomposition) (disint + decomp) 1.Physical condition of rock 1.Hydration 1.Man & animals 2.Change in temperature 2.Hydrolysis 2. higher plants & their roots 3.Action of H O 3.Solution 3.Micro organisms 2 -fragment&transport 4.Carbonation - action of freezing 5.Oxidation - alter. Wet & drying 6.Reduction - action of glaciers 4.Action of wind 5.Atmosp.electric pheno 38 www.AgriMoon.CoM  Introduction to Soil Science Physical weathering The rocks are disintegrated and are broken down to comparatively smaller pieces, without producing any new substances 1. Physical condition of rocks The permeability of rocks is the most important single factor. Coarse textured (porous) sand stone weather more readily than a fine textured (almost solid) basalt. Unconsolidated volcanic ash weather quickly as compared to unconsolidated coarse deposits such as gravels. 2. Action of Temperature The variations in temperature exert great influence on the disintegration of rocks.  During day time, the rocks get heated up by the sun and expand. At night, the temperature falls and the rocks get cooled and contract.  This alternate expansion and contraction weakens the surface of the rock and crumbles it because the rocks do not conduct heat easily.  The minerals within the rock also vary in their rate of expansion and contraction  The cubical expansion of quartz is twice as feldspar  Dark coloured rocks are subjected to fast changes in temperature as compared to light coloured rocks  The differential expansion of minerals in a rock surface generates stress between the heated surface and cooled un expanded parts resulting in fragmentation of rocks.  This process causes the surface layer to peel off from the parent mass and the rock ultimately disintegrates. This process is called Exfoliation 3. Action of Water Water acts as a disintegrating, transporting and depositing agent. i) Fragmentation and transport Water beats over the surface of the rock when the rain occurs and starts flowing towards the ocean  Moving water has the great cutting and carrying force.  It forms gullies and ravines and carries with the suspended soil material of variable sizes.  Transporting power of water varies. It is estimated that the transporting power of stream varies as the sixth power of its velocity i.e the greater the speed of water, more is the transporting power and carrying capacity. 39 www.AgriMoon.CoM  Introduction to Soil Science Speed/Sec Carrying capacity 15 cm Fine sand 30 cm Gravel 1.2 m Stones (1kg) 9.0 m Boulders (several tons) The disintegration is greater near the source of river than its mouth ii) Action of freezing Frost is much more effective than heat in producing physical weathering  In cold regions, the water in the cracks and crevices freezes into ice and the volume increases to one tenth  As the freezing starts from the top there is no possibility of its upward expansion. Hence, the increase in volume creates enormous out ward pressure which breaks apart the rocks iii) Alternate wetting and Drying Some natural substances increase considerably in volume on wetting and shrink on drying. (e.g.) smectite, montmorilonite  During dry summer/ dry weather – these clays shrink considerably forming deep cracks or wide cracks.  On subsequent wetting, it swells.  This alternate swelling and shrinking/ wetting or drying of clay enriched rocks make them loose and eventually breaks iv). Action of glaciers  In cold regions, when snow falls, it accumulates and change into a ice sheet.  These big glaciers start moving owing to the change in temperature and/or gradient.  On moving, these exert tremendous pressure over the rock on which they pass and carry the loose materials  These materials get deposited on reaching the warmer regions, where its movement stops with the melting of ice 4. Action of wind  Wind has an erosive and transporting effect. Often when the wind is laden with fine material viz., fine sand, silt or clay particles, it has a serious abrasive effect and the sand laden winds itch the rocks and ultimately breaks down under its force 40 www.AgriMoon.CoM  Introduction to Soil Science  The dust storm may transport tons of material from one place to another. The shifting of soil causes serious wind erosion problem and may render cultivated land as degraded (e.g) Rajasthan deserts 5. Atmospheric electrical phenomenon It is an important factor causing break down during rainy season and lightning breaks up rocks and or widens cracks Chemical Weathering Decomposition of rocks and minerals by various chemical processes is called chemical weathering. It is the most important process for soil formation. Chemical weathering takes place mainly at the surface of rocks and minerals with disappearance of certain minerals and the formation of secondary products (new materials). This is called chemical transformation. Feldspar + water clay mineral + soluble cations and anions Chemical weathering becomes more effective as the surface area of the rock increases. Since the chemical reactions occur largely on the surface of the rocks, therefore the smaller the fragments, the greater the surface area per unit volume available for reaction. The effectiveness of chemical weathering is closely related to the mineral composition of rocks. (e.g) quartz responds far slowly to the chemical attack than olivine or pyroxene. Average mineralogical composition (%) Composition Granite Basalt Shale S. Stone L.Stone Feldspar 52.4 46.2 30.0 11.5 - Quartz 31.3 - 2.3 66.8 - Pyrox-amphi - 44.5 - - - 41 www.AgriMoon.CoM  Introduction to Soil Science FeO mineral 2.0 9.3 10.5 2.0 - Clay mineral 14.3 - 25.0 6.6 24.0 Carbonates - - 5.7 11.1 76.0 Chemical Processes of weathering: 1. Hydration Chemical combination of water molecules with a particular substance or mineral leading to a change in structure. Soil forming minerals in rocks do not contain any water and they under go hydration when exposed to humid conditions. Up on hydration there is swelling and increase in volume of minerals. The minerals loose their luster and become soft. It is one of the most common processes in nature and works with secondary minerals, such as aluminium oxide and iron oxide minerals and gypsum. Example: a) 2Fe O + 3HOH 2Fe O.3H O 2 3 2 3 2 (Haematite) (red) (Limonite) (yellow) b) Al O + 3HOH Al O.3H O 2 3 2 3 2 (Bauxite) (Hyd. aluminium Oxide) c) CaSO + 2H O CaSO.2H O 4 2 4 2 (Anhydrite) (Gypsum) 42 www.AgriMoon.CoM  Introduction to Soil Science d) 3(MgO.FeO.SiO ) + 2H O 3MgO.2SiO.2H O + SiO + 3H O 2 2 2 2 2 2 (Olivine) (Serpentine) 2. Hydrolysis Most important process in chemical weathering. It is due to the dissociation of H O into 2 + - H and OH ions which chemically combine with minerals and bring about changes, such as exchange, decomposition of crystalline structure and formation of new compounds. Water acts as a weak acid on silicate minerals. KAlSi O + H2O HAlSi O + KOH 3 8 3 8 (Orthoclase) (Acid silt clay) HAlSi O + 8 HOH Al O.3H O + 6 H SiO 3 8 2 3 2 2 3 (recombination) (Hyd. Alum.oxide) (Silicic acid) This reaction is important because of two reasons  clay, bases and silicic acid - the substances formed in these reactions - are available to plants  water often containing CO (absorbed from atmosphere), reacts with the minerals directly 2 ++ ++ + + to produce insoluble clay minerals, positively charged metal ions (Ca , Mg , Na , K ) - - and negatively charged ions (OH , HCO ) and some soluble silica – all these ions are 3 made available for plant growth. 3. Solution Some substances present in the rocks are directly soluble in water. The soluble substances are removed by the continuous action of water and the rock no longer remains solid and form holes, rills or rough surface and ultimately falls into pieces or decomposes. The action is considerably increased when the water is acidified by the dissolution of organic and inorganic acids. (e.g) halites, NaCl +, - NaCl + H2O Na Cl , H O (dissolved ions with water) 2 4. Carbonation: Carbon di oxide when dissolved in water it forms carbonic acid. 2H O + CO2 H CO 2 2 3 43 www.AgriMoon.CoM  Introduction to Soil Science This carbonic acid attacks many rocks and minerals and brings them into solution. The carbonated water has an etching effect up on some rocks, especially lime stone. The removal of cement that holds sand particles together leads to their disintegration. 44 www.AgriMoon.CoM  Introduction to Soil Science CaCO + H2CO3 Ca (HCO ) 3 3 2 (Calcite) (Ca bi carbonate) slightly soluble readily soluble 5. Oxidation The process of addition and combination of oxygen to minerals. The absorption is usually from O dissolved in soil water and that present in atmosphere. The oxidation is more active in 2 the presence of moisture and results in hydrated oxides. (e.g) minerals containing Fe and Mg. 4FeO (Ferrous oxide) + O 2Fe O (Ferric oxide) 2 2 3 4Fe O (Magnetite) + O 6Fe O (Haematite) 3 4 2 2 3 2Fe O ( Haematite) + 3H O 2Fe O.3H O(Limonite) 2 3 2 2 3 2 6. Reduction The process of removal of oxygen and is the reverse of oxidation and is equally important in changing soil colour to grey, blue or green as ferric iron is converted to ferrous iron compounds. Under the conditions of excess water or water logged condition (less or no oxygen), reduction takes place. 2Fe O (Haematite) - O 4FeO (Ferrous oxide) - reduced form 2 3 2 In conclusion, during chemical weathering igneous and metamorphic rocks can be regarded as involving destruction of primary minerals and the production of secondary minerals. In sedimentary rocks, which is made up of primary and secondary minerals, weathering acts initially to destroy any relatively weak bonding agents (FeO) and the particles are freed and can be individually subjected to weathering. Biological Weathering Unlike physical and chemical weathering, the biological or living agents are responsible for both decomposition and disintegration of rocks and minerals. The biological life is mainly controlled largely by the prevailing environment. 45 www.AgriMoon.CoM  Introduction to Soil Science 1. Man and Animals  The action of man in disintegration of rocks is well known as he cuts rocks to build dams, channels and construct roads and buildings. All these activities result in increasing the surface area of the rocks for attack of chemical agents and accelerate the process of rock decomposition.  A large number of animals, birds, insects and worms, by their activities they make holes in them and thus aid for weathering.  In tropical and sub tropical regions, ants and termites build galleries and passages and carry materials from lower to upper surface and excrete acids. The oxygen and water with many dissolved substances, reach every part of the rock through the cracks, holes and galleries, and thus brings about speedy disintegration.  Rabbits, by burrowing in to the ground, destroy soft rocks. Moles, ants and bodies of the dead animals, provides substances which react with minerals and aid in decaying process.  The earthworms pass the soil through the alimentary canal and thus brings about physical and chemical changes in soil material. 2. Higher Plants and Roots The roots of trees and other plants penetrates into the joints and crevices of the rocks. As they grew, they exert a great disruptive force and the hard rock may broken apart. (e.g) pipal tree growing on walls/ rocks. The grass roots form a sponge like mass, prevents erosion and conserve moisture and thus allowing moisture and air to enter in to the rock for further action. Some roots penetrate deep into the soil and may open some sort of drainage channel. The roots running in crevices in lime stone and marble produces acids. These acids have a solvent action on carbonates. The dead roots and plant residues decompose and produce carbon dioxide which is of great importance in weathering. 3. Micro- organisms In early stages of mineral decomposition and soil formation, the lower forms of plants and animals like, mosses, bacteria and fungi and actinomycetes play an important role. They 46 www.AgriMoon.CoM  Introduction to Soil Science extract nutrients from the rock and N from air and live with a small quantity of water. In due course of time, the soil develops under the cluster of these micro-organisms. These organisms closely associated with the decay of plant and animal remains and thus liberate nutrients for the use of next generation plants and also produces CO and organic 2 compounds which aid in mineral decomposition. ****** 47 www.AgriMoon.CoM  Introduction to Soil Science Soil formation factors and processes The soil formation is the process of two consecutive stages. 1. The weathering of rock (R) into Regolith 2. The formation of true soil from Regolith The evolution of true soil from regolith takes place by the combined action of soil forming factors and processes.  The first step is accomplished by weathering (disintegration & decomposition)  The second step is associated with the action of Soil Forming Factors Weathering Rock Regolith True soil 2 1 Factors Dokuchaiev (1889) established that the soils develop as a result of the action of soil forming factors S = f (P, Cl, O) Further, Jenny (1941) formulated the following equation S = f (Cl, O, R, P, T, …) Where, Cl – environmental climate O – Organisms and vegetation (biosphere) R – Relief or topography P – Parent material T- Time … - additional unspecified factors The five soil forming factors, acting simultaneously at any point on the surface of the earth, to produce soil. 48 www.AgriMoon.CoM  Introduction to Soil Science Two groups Passive : i) Parent material, ii) Relief, iii) Time Active : iv) Climate, v) Vegetation & organism Passive Soil forming factors The passive soil forming factors are those which represent the source of soil forming mass and conditions affecting it. These provide a base on which the active soil forming factors work or act for the development of soil. Parent Material It is that mass (consolidated material) from which the soil has formed. Two groups of parent material  Sedentary Formed in original place. It is the residual parent material. The parent material differ as widely as the rocks  Transported The parent material transported from their place of origin. They are named according to the main force responsible for the transport and redeposition. a) by gravity - Colluvial b) by water - Alluvial , Marine , Locustrine c) by ice - Glacial d) by wind - Eolian Colluvium It is the poorly sorted materials near the base of strong slopes transported by the action of gravity. Alluvium The material transported and deposited by water is, found along major stream courses at the bottom of slopes of mountains and along small streams flowing out of drainage basins. Lacustrine Consists of materials that have settled out of the quiet water of lakes. Moraine 49 www.AgriMoon.CoM  Introduction to Soil Science Consists of all the materials picked up, mixed, disintegrated, transported and deposited through the action of glacial ice or of water resulting primarily from melting of glaciers. Loess or Aeolian These are the wind blown materials. When the texture is silty - loss; when it is sand. Eolian The soils developed on such transported parent materials bear the name of the parent material; viz. Alluvial soils from alluvium, colluvial soils from colluvium etc. In the initial stages, however, the soil properties are mainly determined by the kind of parent material. Endodynamomorphic soils With advanced development and excessive leaching, the influence of parent material on soil characteristics gradually diminishes. There are soils wherein the composition of parent material subdues the effects of climate and vegetation. These soils are temporary and persist only until the chemical decomposition becomes active under the influence of climate and vegetation. Ectodynamomorphic soils Development of normal profile under the influence of climate and vegetation. Soil properties as influenced by parent material: Different parent materials affect profile development and produce different soils, especially in the initial stages.  Acid igneous rocks (like granite, rhyolite) produce light-textured soils (Alfisols).  Basic igneous rocks (basalt), alluvium or colluvium derived from limestone or basalt, produce fine-textured cracking-clay soils (Vertisols).  Basic alluvium or aeolian materials produce fine to coarse-textured soils (Entisols or Inceptisols).  The nature of the elements released during the decaying of rocks has a specific role in soil formation. (e.g.) Si and Al forms the skeleton for the production of secondary clay minerals.  Iron and manganese are important for imparting red colour to soils and for oxidation and reduction phenomena.  Sodium and potassium are important dispersing agents for day and humus colloids. 50 www.AgriMoon.CoM  Introduction to Soil Science  Calcium and magnesium have a flocculating effect and result in favorable and stable soil structure for plant growth. 2. Relief or Topography The relief and topography sometimes are used as synonymous terms. They denote the configuration of the land surface. The topography refers to the differences in elevation of the land surface on a broad scale. The prominent types of topography designations, as given in FAO Guidelines (1990) are: Land surface with slopes of 1 Flat to Almost flat 0–2% 2 Gently undulating 2-5% 3 Undulating 5 – 10 % 4 Rolling 10 – 15 % 5 Hilly 15 –3 0 % 6 Steeply dissect > 30 % with moderate range of elevation ( 30% with great range of elevation (>300 m) Soil formation on flat to almost flat position On level topographic positions, almost the entire water received through rain percolates through the soil. Under such conditions, the soils formed may be considered as representative of the regional climate. They have normal solum with distinct horizons. But vast and monotonous level land with little gradient often has impaired drainage conditions. Soil formation on undulating topography The soils on steep slopes are generally shallow, stony and have weakly- developed profiles with less distinct horizonation. It is due to accelerated erosion, which removes surface material before it has the time to develop. Reduced percolation of water through soil is because of surface runoff, and lack of water for the growth of plants, which are responsible for checking of erosion and promote soil formation. Soil formation in depression The depression areas in semi-arid and sub humid regions reflect more moist conditions than actually observed on level topographic positions due to the additional water received as runoff. Such conditions (as in the Tarai region of the Uttar Pradesh) favour more vegetative growth and slower rate of decay of organic remains. This results in the formation of comparatively dark- coloured soils rich in organic matter (Mollisols). 51 www.AgriMoon.CoM  Introduction to Soil Science Soil formation and Exposure/ Aspect Topography affects soil formation by affecting temperature and vegetative growth through slope exposures (aspect}. The southern exposures (facing the sun) are warmer and subject to marked fluctuations in temperature and moisture. The northern exposures, on the other hand are cooler and more humid. The eastern and western exposures occupy intermediate position in this respect. 3. Time Soil