SAC 101 - Full T.M 5 - Agri Junction PDF

The Indian Agriculture College, Radhapuram, Tirunelveli Dist. Tamil Nadu, Pin : 627111 THEORY NOTES SAC 101 Fundamentals of Soil Science (2+1) Course : Mr. Vinothraj J, M.Sc., (Agriculture) Teacher Asst. Professor (SS & AC) Department of Soil Science and Agricultural Chemistry The Indian Agriculture College, Radhapuram 1|Page Lecture Schedule 1. Soil definition - Soil as a three dimensional natural body, Pedological and edaphological concepts of soil 2. Components of soil – soil a three phase system- Composition of Earth's crust. 3. Soil genesis: soil forming rocks-definition, formation, Classification of rocks- igneous, sedimentary and metamorphic rocks 4. Brief description of important rocks - mineralogical composition 5. Minerals- definition, occurrence, classification of important soil forming primary minerals - silicate and non silicate minerals, ferro and non-ferro magnesium minerals 6. Formation of secondary minerals - clay minerals and amorphous minerals 7. Weathering - Rocks and minerals - Physical, chemical and biological weathering 8. Factors of soil formation- Passive and active soil forming factors soil forming factors 9. Soil forming process- Fundamental - Simenson's four fold soil forming process - eluviation, illuviation, translocation and humification 10.Specific Soil forming processes - podzolization, laterization, salinization, alkalization, calcification, decalcification and pedoturbation 11.Soil Profile – Horizons, Master horizons and subordinate horizons, subdivisions, Lithological discontinuity. 12.Soil physical properties: Soil texture - particle size distribution - textural classes - textural triangular diagram - significance of soil texture 13. Soil structure - classification - genesis - factors influencing structural stability - significance of soil structure 14.Soil bulk density, particle density and porosity - factors influencing – significance. 15.Soil colour - causes and measurement - Munsell colour chart - factors influencing soil colour – Significance of soil colour. 16. Soil consistence - cohesion, adhesion, plasticity, Atterberg's constants - upper and lower plastic limits, plasticity number- significance of soil consistence 17. Mid semester Examination 18. Soil water- forms of water, units of expression and pF scale 19. Soil water potentials - gravitational, matric, osmotic- Soil moisture constants and Soil moisture measurements. 20. Movement of soil water - Saturated and unsaturated flow - infiltration, hydraulic conductivity, percolation, permeability and drainage 21. Soil air, composition, gaseous exchange – Problem and its effect on crop growth. 22. Source, amount and flow of heat in soil, soil temperature and crop growth. and crop growth. 23. Soil reaction (pH) - definition, pH scale, soil acidity and alkalinity, buffering, effect of pH on nutrient availability and factors affecting soil pH 24. Soil Electrical Conductivity - Factors affecting EC – its significance 25. Soil colloids - inorganic and organic 26. Silicate clays: constitution and classification - 1:1, 2:1 expanding and non expanding - 2:2 clay minerals, amorphous minerals and their properties 27. Sources of charge, ion exchange – positive and negative charge – isomorphous substitution, pH dependant charge. 2|Page 28. Ion exchange - Cation and anion exchange capacity and base saturation 29. Soil organic matter: composition, properties and its influence on soil properties 30. Humic substances – fractionation, nature and properties, Theories of humus formation. 31. Soil Biology- Soil organisms: macro and micro organisms, their beneficial and harmful effects, Soil enzymes 32. Soil carbon sequestration and carbon trading 33. Soil pollution - behaviour of pesticides and inorganic contaminants 34. Prevention and mitigation of soil pollution 3|Page LECTURE 1 SOIL DEFINITION - SOIL AS A THREE DIMENSIONAL NATURAL BODY - PEDOLOGICAL AND EDAPHOLOGICAL 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” DISCIPLINES OF SOIL SCIENCE There are 6 well defined and developed disciplines and are: 1. Soil fertility : Nutrient supplying properties of soil 2. Soil chemistry : study of chemical constituents, and their interactions 3. Soil physics : Involves the study of physical properties 4. Soil biology : Deals with the effect of plants, animals, micro organisms on the chemical composition and physical conditions of the soil 5. 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. 6. Soil Pedology : Dealing with the genesis, survey and classification of soils VIEWS ON SOIL The term SOIL was derived from the Latin Word “SOLUM” Means FLOOR ∗ Layman - soil is dirt or debris ∗ Agriculturist - soil is a habitat for plant growth (to grow crops) ∗ Mining Engineer - soil is a debris covering the Rocks ∗ Civil Engineer - soil is a material on which road bed or house bed is formed ∗ 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. According to many researchers the definitions varies and are given below: ∗ 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” 4|Page ∗ Dokuchaev (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 ∗ Buckman and Bardy (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) : 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. ∗ 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) 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. 5|Page 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). 1. 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. 2. 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 6|Page 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. 7|Page LECTURE 2 - Composition of Earth Crust 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: 5to 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 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). Atmosphere The envelop of air that covers both the lithosphere and hydrosphere, is called the atmosphere. It contains water molecules and dust, which may ct as nuclei for the condensation of wate vapour to form clouds or fog. 8|Page 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 solid surface (continents, ocean basins, plains, plateaus and mountains, valleys, sand dunes, lava flows and fault scarps) and the interior of the earth, that is composed of rocks and minerals, and comprises 93.06 per cent of the earth’s mass. It is covered by gaseous and watery envelop, called the atmosphere. Composition of earth crust Most of the hard, naturally-formed substance of the earth is referred to as rock. Rock is composed of elements, which in turn, are made up of atoms. Out of 106 elements known, 8 are sufficiently abundant to constitute 98.6 percent (by weight) of the earth’s crust up to 16 km. Elements present in the Earth’s crust Element Amount Oxygen 46.60 75 % of earth Non-metalic Silicon 27.72 crust Aluminium 8.16 Iron 5.00 25% of earth’s Calcium 3.50 Metals crust Sodium 2.83 Magnesium 2.09 Potassium 2.59 9|Page 10 | P a g e LECTURE 3 - Rocks ♣ Definition, Formation and Classification Igneous, Sedimentary and Metamorphic Rocks Rocks are the materials that form the essential part of the Earth’s solid crust. A rock may be defined as a hard mass of mineral matter comprising two or more rock forming minerals. Petrology (in Greek, petra means rock, logos means science) deals with science of rocks. It consists of a. Petrography which deals with the description of rocks. b. Petrogenesis which is the study of the origin of rocks. Formation of rocks: Cooling and consolidation of molten magma within or on the surface of earth results in the formation of Igneous or Primary rocks Disintegration and decomposition lead to the breaking down of pre-existing rocks. Transportation and cementation of primary rocks results in the formation of Sedimentary or Secondary rocks The primary and the secondary rocks when subjected to earth’s movement and to high temperature and pressure are altered to new rocks called Metamorphic rocks Classification of rocks According to the mode of formation the rocks are divided into the following three main classes. 1. Igneous or Primary rocks 2. Sedimentary or Secondary rocks 3. Metamorphic rocks 1. Igneous rocks (primary or massive rocks) These are first formed in the earth crust due to the solidification of molten magma. They are the source of parent material for other rocks and ultimately for soils. 11 | P a g e Based on the mode of origin Igneous rocks are classified as  Extrusive rocks (or volcanic rocks) These rocks are formed due to the consolidation of magma on the surface of the earth. The magma, when flows on the Earth surface are called LAVA. eg. Basalt  Intrusive rocks (or plutonic rocks) These rocks are produced due to solidification of magma below the surface of the earth. These intrusive rocks solidifies at greater depths. eg. Granite. Based on chemical composition Igneous rocks may be divided into  Acid rocks : > 65% silica (Granite, Rhyolite)  Sub acid rocks: 60-65% silica (Syenite and Trachyte)  Sub basic rocks: 55-60% silica (Diorite and Andesite)  Basic rocks : 45-55% silica (Gabbro, Basalt) 2. Sedimentary rocks (Clastic or stratified rocks) The sedimentary rocks are formed from sediments, derived from the breaking down of pre-existing rocks. The sediments are transported to new places and deposited in new arrangements and cemented to form secondary rocks. Sediments may contain various size particles cemented together by substances like SiO2, Fe2O3 or lime. These rocks are called as clastic rocks. Stratification is the most common feature of these rocks and are also termed as stratified 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 12 | P a g e 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 3. Metamorphic rocks These are formed from igneous and sedimentary rocks under the influence of heat, pressure, chemically active liquids and gases. Changes may occur in mineral composition of rocks or texture or both. The change due to water is called hydrometamorphism, due to heat is thermometamorphism and due to pressure is called dynamometamorphism. Sand stone : Quartizite Shale : Slate/mica, schist Lime stone : Marble Granite : granite gneiss Dolerite : Hornblende gneiss Relative abundance of rocks in earth’s crust Composition of Earth’s crust as a whole ♦ Igneous Rocks - 95% ♦ Sedimentary Rocks - 5% - Shales - (4.0%) - Sandstones - (0.75%) - Limestones - (0.025%) Composition of the upper 5 km of the Earth’s crust ♦ Sedimentary Rocks Shales - 52% Sandstones - 15% (74%) Limestones and dolomite - 7% ♦ Igneous Rocks 13 | P a g e Granite - 15% (18%) Basalt - 3% ♦ Others - 8% (8%) _____________________________ ♦ Total 100 100 _____________________________ Rock cycle 14 | P a g e LECTURE 4 - Brief Description of Important Rocks ♣ Mineralogical Composition Igneous Rocks S.No Rocks Origin Essential Common Average Remarks minerals minerals specific gravity i. Granite Plutonic Quartz Hornblende, 2.64 Light holocrystalline (20 to 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 Blackish Holocrystalline augite, olivine ilmenite v. Dolerite Hypabasal Labradorite, Hornblende, 3.0 Blackish augite, olivine ilmenite vi. Basalt Volcanic Labradorite, Hornblende, 3.0 Blackish crystalline with augite, olivine ilmenite glassy mass 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 matter Light to dark thinly laminated 3. Limestone Mainly calcite with some dolomite, iron oxides, Light grey to yellow, fine clay, phosphate and organic matter grained and compact 15 | P a g e Metamorphic rocks S.No. Rock Mineral composition Colour and structure 1. Gneiss Formed from granite Alternating light and dark colours, banded and foliated 2. Schist Formed from basalt or shale As original rock, foliated 3. Quartzite Formed from sandstone Light ot brown, compact and uniform texture, foliated structure 4. Slate Formed from shale Grey to black, compact and uniform texture, foliated structure 5. Marble Formed from lime stone Light red, green, black, compact fine to coarse texture, foliated structure 16 | P a g e LECTURE 5 -Minerals ♣ Definition, occurrence ♣ Classification of primary minerals Mineral: A mineral is a naturally occurring inorganic solid with a definite chemical composition and a crystalline structure. More than 90% of all of the minerals in the Earth’s Crust are made up of compounds containing Silicon and Oxygen, the two most abundant elements on Earth. There are over 2000 minerals on Earth, but only 100 are commonly found. 30 minerals make up the majority of the rocks on Earth. A mineral, by definition, must satisfy five conditions: It must be naturally occurring. It must be inorganic. It must be a solid element or compound. It must have a definite composition. It must have a regular internal crystal structure Occurrence: Of the 2000 known minerals, only few occur in abundance in the Earth crust. The relative abundance of important rock forming minerals is given below: Minerals (arranged in the order of their Important constituents Percent 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 17 | P a g e ♦ Muscovite K, Al, OH Non-Ferro Magnesium minerals Tecto Silicates ♦ Feldspars 61.0 Anorthite Ca, Al Albite Na, Al Orthoclase K, Al ♦ Quartz Secondary minerals ♦ Clay minerals Na, K, Ca 11.6 ♦ Others Mg, Fe, Al, OH 6.0 Formation of minerals ♦ When the molten magma solidifies, the different elements present in it freely arrange themselves in accordance with attractive forces and geometric form. ♦ The earth’s crust contains dominant amount of oxygen (46.60%) followed by silicon (27.72%). ♦ In order to achieve neutrality between the negatively – charged oxygen and the positively – charged silicon, there would be a greater tendency for silicon and oxygen to combine to form the basic compound, called the silicon – oxygen tetrahedron (SiO4)4-. ♦ This explains the dominance (>90%) of silicate minerals in the earth’s crust. ♦ Geometrically, it is possible to arrange only 4 oxygen anions around a central silicon cation so that all are touching each other. This is the arrangement of a tetrahedron. ♦ The amount of charge carried by silicon ion is 4+ and by oxygen is 2-. In order to attain neutrality, one silicon (4+) ion would combine with two oxygen ion (2 x 2-) to form SiO2 but geometrically stable structure is formed when 1 silicon combines with 4 oxygen ions to form tetrahedron (SiO4)4- which carries a net negative charge of 4-. The silicate tetrahedron (SiO4)4- is the fundamental building block of all the silicate minerals 18 | P a g e The Si-O bonds are very strong with mixed covalent and ionic character. Classification of minerals i. Based on the mode of origin Primary mineral: A mineral that forms the original component of the rock is known as the primary mineral eg. Feldspar, Hornblende, Mica, Quartz etc; Secondary mineral: A mineral that has been formed, deposited or introduced as a result of subsequent changes in the rock is known as secondary mineral. Eg: Limonite, Gibbsite etc., and clay minerals like Kaolinite, Montmorillonite etc., ii. Based on its importance or amount Essential mineral: Those minerals which form the chief constituents of a rock, and are known as essential minerals. They are present in large quantities varying from 95-98% eg. Calcite and Silicate minerals. 19 | P a g e Accessory minerals: Those minerals which occur only in small quantities and whose presence / absence is of no consequence as far as the character of the rock is concerned is called accessory minerals. Eg. Tourmaline, magnetite, pyrites etc Silicate minerals Ferro magnesian silicate minerals Inosilicates (Pyroxenes and amphiboles) The pyroxenes and amphiboles are two groups of ferromagnesian minerals. The structure consists of long chains of linked silica tetrahedra. The pyroxenes consist of a single chain (2 oxygen shared in each tetrahedron) whereas amphiboles consist of double chains (alternately 2 and 3 oxygen atoms share the successive tetrahedra). These chain silicates are sometimes referred to inosilicates. Pyroxene Amphibole Phyllosilicates The phyllosilicates are an important group of soil – forming minerals and are represented by micas (biotite, muscovite). They have sheet structure of tetrahedra where each silicon ion shares three 20 | P a g e oxygen ions with adjacent silicon ion to form a honey – comb like pattern. The fourth unshared oxygen ion of each tetrahedron stands above the plane of all others. The basic structural unit of phyllosilicates is formed by the condensation of two sheets of silicon-tetrahedra with one sheet of aluminum or magnesium octahedron. Non Ferro magnesian minerals Tectosilicates: The most common minerals of this group are feldspars and quartz. Feldspars  Feldspars are aluminosilicates of K, Na and Ca. The feldspar structure consists of tetrahedral which are attracted by sharing each oxygen atom between neighbouring tetrahedra.  The tetrahedra contain mainly Si ions 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 (KAlSi3O8) include orthoclase and microcline. Orthoclase and microcline are more common in the plutonic and metamorphic rocks. 21 | P a g e  The potassium feldspars occur commonly in the silts and sands of soils and also abundant in clay-size fractions of soil (ii) plagioclase feldspars- a series consisting of solid solution of albite (NaAlSi3O8) high in sodium and anorthite (CaAl2Si3O8) 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, and free from any substitution. It is the most abundant mineral next to feldspars. 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 22 | P a g e 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. 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. 23 | P a g e LECTURE 6 -SECONDARY MINERALS ♣ Clay minerals ♣ Amorphous minerals Formation of secondary mineral The secondary minerals are formed at the earth’s surface by weathering of the pre- existing primary minerals under variable conditions of temperature and pressure. These clay minerals are of size muscovite > K feldspar > Na and Ca feldspar > biotite, hornblende, augite > olivine > dolomite and calcite > gypsum (least resistant) In conclusion, during chemical weathering igneous and metamorphic rocks are involved in the destruction of primary minerals and the production of secondary minerals. In sedimentary rocks, which are made up of primary and secondary minerals, weathering acts initially to destroy relatively weak bonding agents and the particles are freed. These particles are then individually subjected to weathering. III. 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 by the prevailing environment. 32 | P a g e 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 make holes in them and thus help in 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. Higher Plants and Roots The roots of trees and other plants penetrate into the joints and crevices of the rocks. As they grew, they exert a great disruptive force and the hard rock may break apart. (e.g.) peepal tree growing on walls/ rocks. The grass roots form a sponge like mass, prevent erosion and conserve moisture and thus allow 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 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. 33 | P a g e They 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 are closely associated with the decay of plant and animal remains and thus liberate nutrients for the next generation plants and also produce CO2 and organic compounds which aid in mineral decomposition. Relative stabilities of common minerals under weathering 34 | P a g e LECTURE 8 -SOIL FORMING FACTORS ♣ Active and Passive soil forming factors 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 formation of true soil from regolith takes place by the combined action of soil forming factors and processes. 1. The first step is accomplished by weathering (disintegration & decomposition) 2. The second step is associated with the action of Soil Forming Factors Factors of soil formation Dokuchaiev (1889) established that the soils develop as a result of the action of soil forming factors viz., 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 act simultaneously at any point on the surface of the earth to produce soil Active soil forming factors: i) Climate, ii) organism (vegetation or biosphere) These factors supply energy and act upon other soil forming factors Passive soil forming factors i) Parent material ii) Relief iii) Time 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. 35 | P a g e Parent Material: It is that mass (consolidated material) from which the soil has formed. i) Sedentary: Formed in original place. It is the residual parent material. The parent material differ as widely as the rocks ii) Transported: the parent materials are 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 c) by glacial ice - Moraine d) by lake water - Lacustrine e) by wind- Aeolian (sandy texture) - Loess (silty texture) 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: Soils wherein the composition of parent material subdues the effects of climate and vegetation. These soils develop under the influence of parent material. 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 36 | P a g e 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 clay and humus colloids.  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 Slope 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 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. 37 | P a g e 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). 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 formation is a very slow process requiring thousands of years to develop a mature pedon. The period taken by a given soil from the stage of weathered rock (i.e. regolith) up to the stage of maturity is considered as time. The matured soils mean the soils with fully developed horizons (A, B, C). It takes hundreds of years to develop an inch of soil. The time that nature devotes to the formation of soils is termed as Pedologic Time. It has been observed that rocks and minerals disintegrate and/or decompose at different rates; the coarse particles of limestone are more resistant to disintegration than those of sandstone. However, in general, limestone decomposes more readily than sandstone (by chemical weathering). Weathering stages in soil formation Stages Characteristic 1. Initial Un weathered parent material 2. Juvenile Weathering started but much of the original material still un weathered 3. Virile Easily weatherable minerals fairly decomposed; clay content increased, slowly weatherable minerals still appreciable 4. Senile Decomposition reaches at a final stage; only most resistant minerals survive 5. Final Soil development completed under prevailing environments 38 | P a g e  The soil properties also change with time, for instance nitrogen and organic matter contents increase with time provided the soil temperature is not high.  CaCO3 content may decrease or even lost with time provided the climatic conditions are not arid  In humid regions, the H+ concentration increases with time because of chemical weathering. B. Active Soil Forming Factors The active soil forming factors are those which supply energy that acts on the mass for the purpose of soil formation. These factors are climate and vegetation (biosphere). 1. Climate Climate is the most significant factor controlling the type and rate of soil formation. The dominant climates recognized are:  Arid climate: The precipitation here is very less than the water-need. Hence the soils remain dry for most of the time in a year.  Humid climate: The precipitation here is much more than the water need. The excess water results in leaching of salt and bases followed by translocation of clay colloids.  Oceanic climate: Moderate seasonal variation of rainfall and temperature.  Mediterranean climate: Moderate precipitation in winters and summers are dry and hot.  Continental climate: Warm summers and extremely cool or cold winters.  Temperate climate: Cold humid conditions with warm summers.  Tropical and subtropical climate: Warm to hot humid with isothermal conditions in the tropical zone. Climate affects the soil formation directly and indirectly. ♦ Directly, climate affects the soil formation by supplying water and heat to react with parent material. ♦ Indirectly, it determines the fauna and flora activities which serve a source of energy in the form of organic matter. This energy acts on the rocks and minerals in the form of acids and salts are released. ♦ Precipitation and temperature are the two major climatic elements which contribute most to soil formation. Precipitation: 39 | P a g e ♦ Precipitation is the most important among the climatic factors. The rain water percolates and moves from one part of the parent material to another. ♦ It carries with it substances in solution and suspension. ♦ The substances so carried are redeposited in another part or completely removed from the material. ♦ The soluble substances move with it and are translocated to the upper layer. ♦ Thus rainfall brings about a redistribution of soluble substances in the soil body. Temperature  Temperature is another climatic agent influencing the process of soil formation.  High temperature hinders the process of leaching and causes an upward movement of soluble salts.  High temperature favors rapid decomposition of organic matter and increase microbial activities in soil while low temperatures favour the accumulation of organic matter by slowing down the process of decomposition.  Temperature thus controls the rate of chemical and biological reactions taking place in the parent material. Jenny (1941) computed that in the tropical regions the rate of weathering proceeds three times faster than in temperate regions and nine times faster than in arctic. 2. Organism & Vegetation Organism  The active components of soil ecosystem are plants, animals, microorganisms and man.  The role of microorganisms in soil formation is related to the humification and mineralization of vegetation  The action of animals especially burrowing animals is to dig and mix-up the soil mass  Man influences the soil formation through his manipulation of natural vegetation, agricultural practices etc.  Compaction by traffic of man and animals decrease the rate of water infiltration into the soil and thereby increase the rate of runoff and erosion. Vegetation  The roots of the plants penetrate into the parent material and act both mechanically and chemically. 40 | P a g e  They facilitate percolation and drainage and bring about greater dissolution of minerals through the action of CO2 and acidic substances secreted by them.  The decomposition and humification of the materials further adds to the solubilization of minerals.  Forests – reduces temperature, increases humidity, reduce evaporation and increases precipitation.  Grasses reduce runoff and result greater penetration of water in to the parent material. 41 | P a g e LECTURE 9 -FUNDAMENTAL SOIL FORMING PROCESSES ♣ Eluviation, Illuviation ♣ Podzolization, Laterization The basic soil forming processes involved in soil formation (Simonson, 1959) includes the following.  Gains or Additions of water, mostly as rainfall, organic and mineral matter to the soil.  Losses of the above materials from the soil.  Transformation of mineral and organic substances within the soil.  Translocation of soil materials from one point to another within the soil. It is usually divided into two aspects. 1) movement in solution (leaching) and  Movement in suspension (eluviation) of clay, organic matter and hydrous oxides A. Fundamental Soil Forming Processes Eluviation:  It is the mobilization and translocation of certain constituents viz., Clay, Fe2O3, Al2O3, SiO2, humus, CaCO3, other salts etc. from one point of soil body to another.  Eluviation means washing out. It is the process of removal of constituents in suspension or solution by the percolating water from the upper to lower layers.  The eluviation process involves mobilization and translocation of soil constituents resulting in textural differences.  The horizon formed by the process of eluviation is termed as eluvial horizon (A2 or E horizon). Translocation depends upon relative mobility of elements and depth of percolation. Illuviation: The process of deposition of soil materials (removed from the eluvial horizon) in the lower layer is termed as Illuviation. o The horizon formed by this process is termed as illuvial horizon (B-horizon, especially Bt). 42 | P a g e o This is the horizon of gains having the property of stabilizing translocated clay materials. Podzolization: It is a process of soil formation resulting in the formation of Podzols and Podzolic soils. The process operates under favorable combination of the following environments. i) Climate: A cold and humid climate is most favorable for podzolization ii) Parent material: Siliceous (Sandy) material, having poor reserves of weatherable minerals, favor the operation of podzolization as it helps in easy percolation of water. iii) Vegetation: Acid producing vegetation such as coniferous pines is essential iv) Leaching and Translocation of Sesquioxides: In the process of decomposition of organic matter various organic acids are produced. The organic acids thus formed act with Sesquioxide and the remaining clay minerals, forming organic- sesquioxide and organic clay complexes, which are soluble and move with the percolating water to the lower horizons. As iron and aluminium are removed, the A horizon gives a bleached grey or ashy appearance. The Russians used the term Podzols (pod means under, the zola means ash like i.e. ash-like horizon appearing beneath the surface horizon) for such soils. To conclude, the Podzolization is a soil forming process which prevails in a cold and humid climate where coniferous and acid forming vegetations dominate. The humus and Sesquioxide become mobile and leached out from the upper horizon s and deposited in the lower horizon Laterization:  The term laterite is derived from the word later meaning brick or tile and was originally applied to a group of high clay Indian soils found in Malabar hills of Kerala, Tamil Nadu, Karnataka and Maharashtra.  It refers specifically to a particular cemented horizon in certain soils which when dried, become very hard, like a brick.  Such soils (in tropics) when massively mixed with sesquioxides (iron and aluminium oxides) to an extent of 70 to 80 per cent of the total mass, are called laterites or latosols (Oxisols). 43 | P a g e  The soil forming process is called Laterization or Latozation.  Laterization is the process that removes silica, instead of sesquioxides from the upper layers and thereby leaving sesquioxides to concentrate in the solum. The process operates under the following conditions. i) Climate: Unlike podzolization, the process of laterization operates most favorable in warm and humid (tropical) climate with 2000 to 2500 mm rainfall and continuous high temperature (25°C) throughout the year. ii) Natural vegetation: The rain forests of tropical areas are favorable for the process. iii) Parent Material: Basic parent materials having sufficient iron bearing ferromagnesian minerals (Pyroxene, amphiboles, biotite and chlorite), which on weathering release iron that are congenial for the development of laterites. Horizonation: It is the process of differentiation of soil in different horizons along the depth of the soil body. The differentiation is due to the fundamental processes of eluviation and illuviation. 44 | P a g e LECTURE 10- SPECIFIC SOIL FORMING PROCESSES ♣ Salinization and Alkalinization ♣ Calcification and Decalcification ♣ Carbonation and Gleization ♣ Pedoturbation and humification The basic pedologic processes provide a framework for later operation of more specific processes Salinization ♦ It is the process of accumulation of salts, such as sulphates and chlorides of calcium, magnesium, sodium and potassium in soils in the form of a salty (salic) horizon. ♦ It is quite common in arid and semi arid regions. ♦ It may also take place through capillary rise of saline ground water and by inundation with seawater. ♦ Salt accumulation may also result from irrigation or seepage in areas of impeded drainage. Desalinization ♦ It is the process of removal of excess soluble salts from horizons that contained enough soluble salts to impair the plant growth. ♦ Leaching is done by ponding water and improving the drainage conditions by installing artificial drainage network. Solonization or Alkalization ♦ The process involves the accumulation of sodium ions on the exchange complex of the clay resulting in the formation of sodic soils (Solonetz). ♦ All cations in solution are engaged in a reversible reaction with the exchange sites on the clay and organic matter particles. ♦ The reaction can be represented as: Ca.Mg.2NaX → Ca++ + Mg++ +2Na+ + X (3CO3 2- → Na2CO3 + MgCO3 + CaCO3) (Where X represents clay or organic matter exchange sites) Solodization or Dealkalization 45 | P a g e + ♦ The process refers to the removal of Na from the exchange sites. This process involves dispersion of clay. Dispersion occurs when Na+ ions become hydrated. ♦ Much of the dispersion can be eliminated if Ca++ and or Mg++ ions are concentrated in the water. These Ca and Mg ions can replace the Na on exchange complex, and the salts of sodium are leached out as: 2NaX + CaSO4 ------------ Na2SO4 + CaX (leachable) Calcification ♦ It is the process of precipitation and accumulation of calcium carbonate (CaCO3) in some part of the profile. The accumulation of CaCO3 may result in the development of a calcic horizon. ♦ Calcium is readily soluble in acidic soil water and/or when CO2 concentration is high in root zone as:  CO2 + H2O → H2CO3  H2CO3 + Ca → Ca (HCO3)2 (soluble)  Ca (HCO3)2 → CaCO3 + H2O + CO2 (precipitates) ♦ The process of precipitation after mobilization under these conditions is called calcification. Decalcification It is the reverse of calcification that is the process of removal of CaCO3 or calcium ions from the soil by leaching CaCO3 + CO2 + H2O → Ca(HCO3)2 (insoluble) (soluble) Carbonation  It occurs when carbon dioxide interacts chemically with minerals. When carbon dioxide is dissolved in water, it forms weak carbonic acid.  When carbonic acid comes in contact with the surface of the earth it dissolves large masses of limestone, creating caves and caverns.  Other common terms associated with carbonation are sink holes, karst topography, stalactites and stalagmites. Gleization:  The term glei is of Russian origin means blue, grey or green clay.  The Gleization is a process of soil formation resulting in the development of a gley horizon in the lower part of the soil profile above the parent material due to poor drainage 46 | P a g e condition and where waterlogged conditions prevail. Such soils are called hydromorphic soils.  The process is not particularly dependent on climate (high rainfall as in humid regions) but often on drainage conditions.  Under such conditions, iron compounds are reduced to soluble ferrous forms.  The solubility of Ca, Mg, Fe, and Mn is increased and most of the iron exists as Fe++ organo - complexes in solution or as mixed precipitate of ferric and ferrous hydroxides.  This is responsible for the production of typical bluish to greyish horizon with mottling of yellow and or reddish brown colors. Pedoturbation: Another process that may be operative in soils is pedoturbation. It is the process of mixing of the soil. Mixing to a certain extent takes place in all soils. The most common types of pedoturbation are:  Faunal pedoturbation: It is the mixing of soil by animals such as ants, earthworms, moles, rodents, and man himself  Floral pedoturbation : It is the mixing of soil by plants as in tree tipping that forms pits and mounds  Argillic pedoturbation: It is the mixing of materials in the solum by the churning process caused by swell-shrink clays as observed in deep Black Cotton Soils. Humification:  Humification is the process of transformation of raw organic matter into humus. It is extremely a complex process involving various organisms.  First, simple compounds such as sugars and starches are attacked followed by proteins and cellulose and finally very resistant compounds such as tannins, are decomposed and the dark coloured substance, known as humus, is formed. 47 | P a g e LECTURE 11 -SOIL PROFILE ♣ Profile description, Master horizons, Pedon and polypedon Definition of soil profile: The vertical section of the soil showing the various layers from the surface to the unaffected parent material is known as a soil profile. The various layers are known as horizons. Soil Horizons (layers): Soil is made up of distinct horizontal layers; these layers are called horizons. They range from rich, organic upper layers (humus and topsoil) to underlying rocky layers (subsoil, regolith and bedrock). The master horizons are O, A, B and C horizons. Most soils have a distinct profile or sequence of horizontal layers. Generally, these horizons result from the processes of chemical weathering, eluviation, illuviation, and organic decomposition. Up to five layers can be present in a typical soil: O, A, B, C, and R horizons. Figure: Typical layers found in a soil profile. The O horizon is the topmost layer of most soils. It is composed mainly of plant litter at various levels of decomposition and humus. A horizon is found below the O layer. This layer is composed primarily of mineral particles which has two characteristics: it is the layer in which humus and other organic materials are mixed with mineral particles, and it is a zone of translocation from which eluviation has removed finer particles and soluble substances, both of which may be deposited at a lower layer. Thus the A horizon is dark in color and usually light in texture and porous. The A horizon is commonly 48 | P a g e differentiated into a darker upper horizon or organic accumulation, and a lower horizon showing loss of material by eluviation. The B horizon is a mineral soil layer which is strongly influenced by illuviation. Consequently, this layer receives material eluviated from the A horizon. The B horizon also has a higher bulk density than the A horizon due to its enrichment of clay particles. The B horizon may be colored by oxides of iron and aluminum or by calcium carbonate illuviated from the A horizon. The C horizon is composed of weathered parent material. The texture of this material can be quite variable with particles ranging in size from clay to boulders. The C horizon has also not been significantly influenced by the pedogenic processes, translocation, and/or organic modification. The final layer in a typical soil profile is called the R horizon. This soil layer simply consists of unweathered bedrock. Master horizons and sub horizons O horizon - It is called as organic horizon. It is formed in the upper part of the mineral soil, dominated by fresh or partly decomposed organic materials. This horizon contains more than 30% organic matter if mineral fraction has more than 50 % clay (or) more than 20 % organic matter if mineral fraction has less clay. The organic horizons are commonly seen in forest areas and generally absent in grassland, cultivated soils. O1 - Organic horizon in which the original forms of the plant and animal residues can be recognized through naked eye. O2 - Organic horizon in which the original plant or animal matter cannot be recognized through naked eye. A horizon - Horizon of organic matter accumulation adjacent to surface and that has lost clay, iron and aluminium. A1 - Top most mineral horizon formed adjacent to the surface. There will be accumulation of humified organic matter associated with mineral fraction and darker in Colour than that of lower horizons due to organic matter. A2 - Horizon of maximum eluviation of clay, iron and aluminium oxides and organic matter. Loss of these constituents generally results in accumulation of quartz and other sand and silt size resistant minerals. Generally lighter in Colour than horizons above and below. A3 - A transitional layer between A and B horizons with more dominated properties of A1 or A2 above than the underlying B horizon. This horizon is sometimes absent. Solum. B horizon - Horizon in which the dominant features are accumulation of clay, iron, aluminium or humus alone or in combination. Coating of sesquioxides will impart darker, stronger of red Colour than overlying or underlying horizons. 49 | P a g e B1 - A transitional layer between A and B. More like A than B. B2 - Zone of maximum accumulation of clay, iron and aluminium oxide that may have moved down from upper horizons or may have formed in situ. The organic matter content is generally higher and Colour darker than that of A2 horizon above. B3 - Transitional horizon between B and C and with properties more similar to that of overlying B2 than underlying C. C horizon - It is the horizon below the solum (A + B), relatively less affected by soil forming processes. It is outside the zone of major biological activity. It may contain accumulation of carbonates or sulphates, calcium and magnesium. R - Underlying consolidated bed rock and it may or may not be like the parent rock from which the solum is formed Special Features Pedon: A pedon is the smallest volume that can be recognized as a soil individual. It has three dimensions and its area ranges from 1 to 10 square meters, depending on the variability in the horizons. The shape of the pedon is roughly hexagonal. A soil volume that consists of more than one pedon is termed a polypedon Polypedon: A polypedon is defined as a continuous similar pedons bounded on all sides by "not- soil”or by pedons of unlike characters. It is a real physical soil body which has a minimum area of more than 1 sq. km and an unspecified maximum area. Regolith: The unconsolidated materials above the parent rock including A, B and C horizons are called the weathered soil material including O, A and B horizons are called solum. Subordinate Distinctions within master horizons New Old Short description A - Highly decomposed organic material B B Buried genetic horizon C Cn Concretions or nodules D D Physical root restriction E - Organic material of intermediate decomposition F F Frozen soil G G Strong gleying H H Illuvial accumulation of organic matter I I Slightly decomposed organic material K Ca Accumulation of carbonates 50 | P a g e M M Cementation or induration N N Accumulation of sodium O O Residual accumulation of sesquioxides P P Tillage or other disturbance by cultivation Q Si Accumulation of silica R R Weathered or soft bed rock S Ir Illuvial accumulation of sesquioxides and organic matter Ss Ss Presence of silikensides T T Accumulation of silicate clay V - Plinthite W - Development of colour or structure X - Fragipan Y Cs Accumulation of gypsum Z Sa Accumulation of salts Designations of master horizons Horizon Designation Short description New O Organic horizon A Mineral horizon E Mineral horizon C Horizons or layers excluding hard bed rock R Hard bed rock Lithological discontinuity When two or more genetically unrelated materials are present in a profile as in the case of alluvial or colluvial soils, then the phenomenon is known as Lithological discontinuity. This is indicated by the use of Roman letters as prefixes to the master horizons. 51 | P a g e LECTURE 12-SOIL PHYSICAL PROPERTIES ♣ Soil texture ♣ Particle size distribution ♣ Textural classes and textural triangular diagram ♣ Significance of soil texture The physical properties of a soil are the result of soil parent materials being acted upon by climatic factors (such as rainfall and temperature), and being affected by relief (slope and direction or aspect), and by vegetation, with time. A change in any one of these soil-forming factors usually results in a difference in the physical properties of the resulting soil. Physical properties (mechanical behaviour) of a soil greatly influence its use and behaviour towards plant growth. The plant support, root penetration, drainage, aeration, retention of moisture, and plant nutrients are linked with the physical condition of the soil. Physical properties also influence the chemical and biological behaviour of soil. The physical properties of a soil depend on the amount, size, shape, arrangement and mineral composition of its particles. These properties also depend on organic matter content and pore spaces. Important physical properties of soils 1. Soil texture 2. Soil structure 3. Surface area 4. Soil density 5. Soil porosity 6. Soil colour 7. Soil consistence Soil Texture Definition: Soil texture refers to the relative proportion of the three soil separates viz., sand, silt and clay or simply refers to the size of soil particles. 52 | P a g e The proportion of each size group in a given soil (the texture) cannot be easily altered and it is considered as a basic property of a soil. The soil separates are defined in terms of diameter (in millimeters) of the particles. Soil particles >2 mm in diameter are excluded from soil textural determinations. Particles less than 2 mm are called fine earth, normally considered in chemical and mechanical analysis. The size limits of these fractions have been established by various organizations. There are a number of systems of naming soil separates.  The American system developed by United States Department of Agriculture(USDA)  The English system or British system developed by British Standard Institute (BSI )  The International system developed by International Society of Soil Science (ISSS)  European system i) USDA Soil Separates Diameter (mm) Clay < 0.002 Silt 0.002 – 0.05 Very Fine Sand 0.05 – 0.10 Fine Sand 0.10 – 0.25 Medium Sand 0.25 - 0.50 Coarse Sand 0.50 - 1.00 Very Coarse Sand 1.00 – 2.00 ii)BSI Soil separates Diameter (mm) Clay < 0.002 Fine Silt 0.002 – 0.01 Medium Silt 0.01 – 0.04 Coarse Silt 0.04 – 0.06 53 | P a g e Fine Sand 0.06 - 0.20 Medium Sand 0.20 - 1.00 Coarse Sand 1.00 – 2.00 iii) ISSS Soil separates Diameter (mm) Clay < 0.002 Silt 0.002 – 0.02 Fine sand 0.02 – 0.2 Coarse sand 0.2 – 2.0 (v) European System Soil separates Diameter (mm) Fine clay < 0.0002 Medium clay 0.0002 – 0.0006 Coarse clay 0.0006 – 0.002 Fine silt 0.002 - 0.006 Medium silt 0.006 - 0.02 Coarse silt 0.02 - 0.06 Fine sand 0.06 - 0.20 Medium sand 0.20 - 0.60 Coarse sand 0. 60 - 2.00 Sand:  Usually consists of quartz but may also contain fragments of feldspar, mica and occasionally heavy minerals viz., zircon, Tourmaline and hornblende.  Has uniform dimensions  Can be represented as spherical  Not necessarily smooth and has jagged surface Silt:  Particle size intermediate between sand and clay  Since the size is smaller, the surface area is more  Coated with clay  Has the physico- chemical properties as that of clay to a limited extent 54 | P a g e  Sand and Silt forms the SKELETON Clay:  Particle size less than 0.002 mm  Plate like or needle like in shape  Belong to alumino silicate group of minerals  Some times considerable concentration of fine particles which does not belong to alumino silicates. (eg). iron oxide and CaCO3  These are secondary minerals derived from primary minerals in the rock  FLESH of the soil  Knowledge on Texture is important  It is a guide to the value of the land  Land use capability and methods of soil management depends on Texture Particle size distribution/ determination The determination of relative distribution of the ultimate or individual soil particles below 2 mm diameter is called as Particle size analysis or Mechanical analysis. Two steps are involved i) Separation of all the particles from each other ie., Complete dispersion into ultimate particles ii) Measuring the amount of each group Separation S.No Aggregating agents Dispersion method 1 Lime and Oxides of Fe & Al Dissolving in HCl 2 Organic matter Oxidizes with H2O2 3 High concentration of Precipitate and decant or filter with suction electrolytes (soluble salts) 4 Surface tension Elimination of air by stirring with water or boiling After removing the cementing agents, disperse by adding NaOH Measurement Once the soil particles are dispersed into ultimate particles, measurement can be done i) Coarser fractions – sieving – sieves used in the mechanical analysis corresponds to the desired particle size separation 55 | P a g e  For 2 mm, 1 mm and 0.5 mm – sieves with circular holes  For smaller sizes, wire mesh screens are used ( screening) ii) Finer fractions – by settling in a medium The settling or the velocity of the fall of particles is influenced by  Viscosity of the medium  Difference in density between the medium and falling particles  Size and shape of object Stokes' Law: Particle size analysis is based on a simple principle i.e. "when soil particles are suspended in water they tend to sink. Because there is little variation in the density of most soil particles, their velocity (V) of settling is proportional to the square of the radius 'r' of each particles. Thus V = kr2, where k is a constant. This equation is referred to as Stokes' law. Stokes (1851) was the first to suggest the relationship between the radius of the particles and its rate of fall in a liquid. He stated that "the velocity of a falling particle is proportional to the square of the radius and not to its surface. The relation between the diameter of a particle and its settling velocity is governed by Stokes' Law: 2 2 (ds - dw) V= gr 9 η where V - velocity of settling particle (cm/sec.) g - acceleration due to gravity cm/ sec2 (981 ) ds - density of soil particle (2.65) dw - density of water (1 ) η - coefficient of viscosity of water (0.0015 at 4°C) r - radius of spherical particles (cm). Assumptions and Limitations of Stokes' Law  Particles are rigid and spherical / smooth. This requirement is very difficult to fulfill, because the particles are not completely smooth over the surface and spherical. It is established that the particles are not spherical and irregularly shaped such as plate and other shapes.  The particles are large in comparison with the molecules of the liquid so that in comparison with the particle the medium can be considered as homogenous. Ie the particles must be big enough to avoid Brownian movement. The particles less than 0.0002 56 | P a g e mm exhibit this movement so that the rate of falling is varied.  The fall of the particles is not hindered or affected by the proximity (very near) of the wall of the vessel or of the adjacent particles. Many fast falling particles may drag finer particles down along with them.  The density of the particles and water and as well as the viscosity of the medium remain constant. But this is usually not so because of their different chemical and mineralogical composition.  The suspension must be still. Any movement in the suspension will alter the velocity of fall and such movement is brought by the sedimentation of larger particles (> 0.08 mm). They settle so fast and create turbulence in the medium.  The temperature should be kept constant so that convection currents are not set up. Methods of Textural determination Numerous methods for lab and field use have been developed i) Elutriation method – Water & Air ii) Pipette method iii) Decantation/ beaker method iv) Test tube shaking method v) Feel method – Applicable to the field – quick method – by feeling the soil between thumb and fingers Feel Method Evaluated by attempting to squeeze the moistened soil into a thin ribbon as it is pressed with rolling motion between thumb and pre finger or alternately to roll the soil into a thin wire. Four aspects to be seen i) Feel by fingers ii) Ball formation iii) Stickiness iv) Ribbon formation Soil Textural Classes To convey an idea of the textural make up of soils and to give an indication of their physical properties, soil textural class names are used. These are grouped into three main fractions viz., Sand, Silt and Clay. According to the proportion of these three fractions a soil is given a name to indicate its textural composition. Such a name gives an idea not only of the textural composition of a soil but also of its various properties in general. On this basis soils are classified into various textural classes like sands, clays, silts, loams 57 | P a g e etc. Sands: The sand group includes all soils in which the sand separates make up at least 70% and the clay separate 15% or less of the material by weight. The properties of such soils are therefore characteristically those of sand in contrast to the stickier nature of clays. Two specific textural classes are recognized in this group sandy and loamy sand. Silt: The silt group includes soils with at least 80% silt and 12% or less clay. Naturally the properties of this group are dominated by those of silt. Only one textural class - Silt is included in this group. Clays: To be designated clay a soil must contain at least 35% of the clay separate and in most cases not less than 40%. In such soils the characteristics of the clay separates are distinctly dominant, and the class names are clay, sandy clay and silty clay. Sandy clays may contain more sand than clay. Likewise, the silt content of silty clays usually exceeds clay fraction Loams: The loam group, which contains many subdivisions, is a more complicated soil textural class. An ideal loam may be defined as a mixture of sand, silt and clay particles that exhibits the properties of those separates in about equal proportions. Loam soils do not exhibit dominant physical properties of sand, silt or clay. Loam does not contain equal percentage of sand, silt and clay. However, exhibit approximately equal properties of sand, silt and clay. Determination of Textural Class: In the American system as developed by the United State Department of Agriculture twelve textural classes are proposed. The textural triangle: It is used to determine the soil textural name after the percentages of sand, silt, and clay are determined from a laboratory analysis. Since the soil's textural classification includes only mineral particles and those of less than 2mm diameter, the sand plus silt plus clay percentages equal 100 percent. (note that organic matter is not included.) 58 | P a g e Using the Soil Texture Triangle to Determine Soil Texture: Procedure: First, you will need to know the percentages of sand, silt, and clay in your soil, as determined by laboratory particle size analysis. 1. Locate the percentage of clay on the left side of the triangle and move inward horizontally, parallel to the base of the triangle. 2. Follow the same procedure for sand, moving along the base of the triangle to locate your sand percentage 3. Then, move up and to the left until you intersect the line corresponding to your clay percentage value. 4. At this point, read the textural class written within the bold boundary on the triangle. For example: a soil with 40% sand, 30% silt, and 30% clay will be a clay loam. With a moderate amount of practice, soil textural class can also be reliably determined in the field Importance of Soil Texture  Presence of each type of soil particles makes its contribution to the nature and properties of soil as a whole  Texture has good effect on management and productivity of soil. Sandy soils are of open 59 | P a g e character usually loose and friable.  Such type of texture is easy to handle in tillage operations.  Sand facilitates drainage and aeration. It allows rapid evaporation and percolation.  Sandy soils have very little water holding capacity. Such soils can not stand drought and unsuitable for dry farming.  Sandy soils are poor store house of plant nutrients  Contain low organic matter  Leaching of applied nutrients is very high.  In sandy soil, few crops can be grown such as potato, groundnut and cucumbers.  Clay particles play a very important role in soil fertility.  Clayey soils are difficult to till and require much skill in handling. When moist clayey soils are exceedingly sticky and when dry, become very hard and difficult to break.  They have fine pores, and are poor in drainage and aeration.  They have a high water holding capacity and poor percolation, which usually results in water logging.  They are generally very fertile soils, in respect of plant nutrient content. Rice, jute, sugarcane can be grown very successfully in these soils.  Loam and Silt loam soils are highly desirable for cultivation  Generally, the best agriculture soils are those that contain 10 – 20 per cent clay, 5 – 10 per cent organic matter and the rest equally shared by silt and sand 60 | P a g e LECTURE 13 -SOIL STRUCTURE ♣ Definition ♣ Genesis and Classification ♣ Factors influencing soil structural stability. Definition The arrangement and organization of primary and secondary particles in a soil mass is known as soil structure. (or) The grouping and arrangement of individual soil particles is known as soil structure. Genesis of soil structure Soil particles are present as single individual grains and as aggregates i.e. group of particles bound together into granules or compound particles. These granules or compound particles are known as secondary particles. Particles in sandy and silty soils are present as single individual grains while in clayey soil they are present in granulated condition. The individual particles are usually solid, while the aggregates are not solid but they possess a porous or spongy character. Most soils are mixture of single grain and compound particle. Soils, which predominate with single grains are said to be structure-less, while those possess majority of secondary particles are said to be aggregate, granulated or crumb structure. Mechanism of Aggregate Formation The bonding of the soil particles into structural unit is called as the genesis of soil structure. The bonding between individual particles in the structural units is generally considered to be stronger than the structural units themselves. In aggregate formation, a number of primary particles such as sand, silt and clay are brought together by the cementing or binding effect of soil colloids. The cementing materials taking part in aggregate formation are colloidal clay, iron and aluminium hydroxides and decomposing organic matter. Whatever may be the cementing material, it is ultimately the dehydration of colloidal matter accompanied with pressure that completes the process of aggregation. Classification The primary particles - sand, silt and clay - usually occur grouped together in the form of aggregates. 61 | P a g e Natural aggregates are called peds whereas clod is an artificially formed soil mass. Structure is studied in the field under natural conditions and it is described under three categories. 1) Type - Shape or form and arrangement pattern of peds 2) Class - Size of Peds 3) Grade - Degree of distinctness of peds Types of Structure: There are four principal forms of soil structure 1) Plate-like (Platy) In this type, the aggregates are arranged in relatively thin horizontal plates or leaflets. The horizontal axis or dimensions are larger than the vertical axis.  When the units/ layers are thick they are called platy  When they are thin then it is laminar. Platy structure is most noticeable in the surface layers of virgin soils but may be present in the subsoil. This type is inherited from the parent material, especially by the action of water or ice. 2) Prism-like: The vertical axis is more developed than horizontal, giving a pillar like shape.  Vary in length from 1- 10 cm.  Commonly occur in sub soil horizons of Arid and Semi arid regions.  When the tops are rounded, the structure is termed as columnar when the tops are flat / plane, level and clear cut - prismatic. 3) Block like:  All three dimensions are about the same size.  The aggregates have been reduced to blocks.  Irregularly six faced with their three dimensions more or less equal.  When the faces are flat and distinct and the edges are sharp angular, the structure is named as angular blocky.  When the faces and edges are mainly rounded it is called sub angular blocky.  These types usually are confined to the sub soil and characteristics have much to do with soil drainage, aeration and root penetration. 62 | P a g e 4) Spheroidal (Sphere like):  All rounded aggregates (peds) may be placed in this category.  Not exceeding an inch in diameter.  These rounded complexes usually loosely arranged and readily separated.  When wetted, the intervening spaces generally are not closed so readily by swelling as may be the case with a blocky structural condition.  Therefore in sphere-like structure, infiltration, percolation and aeration are not affected by wetting of soil.  The aggregates of this group are usually termed as granular which are relatively less porous.  When the granules are very porous, it is termed as crumb.  This is specific to surface soil particularly high in organic matter/ grass land soils. 63 | P a g e Classes of Structure Each primary structural type of soil is differentiated into 5 size classes depending upon the size of the individual peds. The terms commonly used for the size classes are: 1. Very fine or very thin 2. Fine or thin 64 | P a g e 3. Medium 4. Coarse or thick 5. Very Coarse or very thick The terms thin and thick are used for platy types, while the terms fine and coarse are used for other structural types. Grades of Structure Grades indicate the degree of distinctness of the individual peds. It is determined by the stability of the aggregates. Grade of structure is influenced by the moisture content of the soil. Grade also depends on organic matter, texture etc. Four terms commonly used to describe the grade of soil structure are: 1. Structure-less: There is no noticeable aggregation, such as conditions exhibited by loose sand. 2. Weak Structure: Poorly formed, indistinct formation of peds, which are not durable and much un-aggregated material. 3. Moderate structure: Moderately well developed peds, which are fairly durable and distinct. 4. Strong structure: Very well formed peds, which are quite durable and distinct. Structure naming For naming a soil structure the sequence followed is grade, class and type; for example strong coarse angular blocky, moderate thin platy, weak fine prismatic. Factors Affecting Soil Structure The development of structure in arable soil depends on the following factors: 1. Climate: Climate has considerable influence on the degree of aggregation as well as on the type of structure. In arid regions there is very little aggregation of primary particles. In semi arid regions, the degree of aggregation is greater. 2. Organic matter: Organic matter improves the structure of a sandy soil as well as of a clay soil. In case of a sandy soil, the sticky and slimy material produced by the decomposing organic matter and the 65 | P a g e associated microorganism cement the sand particles together to form aggregates. 3. Tillage: Cultivation implements break down the large clods into smaller fragments and aggregates. For obtaining good granular and crumby structure, optimum moisture content in the soil is necessary. If the moisture content is too high, it will form large clods on drying. If it is too low, some of the existing aggregates will be broken down. 4. Plants, Roots and Residues  Excretion of gelatinous organic compounds and exudates from roots serve as a link  Root hairs make soil particles to cling together. Grass and cereal roots Vs other roots  Pressure exerted by the roots also held the particles together  Dehydration of soil - strains the soil due to shrinkage result in cracks lead to aggregation  Plant tops and residues – shade the soil – prevent it from extreme and sudden temperature and moisture changes and also from rain drop impedance. Plant residues – serve as a food to microbes which are the prime aggregate builders. 5. Animals: Among the soil fauna small animals like earthworms, moles and insects etc., that burrow in the soil are the chief agents that take part in the aggregation of finer particles. 6. Microbes: Algae, fungi and actinomycetes keep the soil particles together by the products of decomposition. 7. Fertilizers: Fertilizer like Sodium Nitrate destroys granulation by reducing the stability of aggregates. Few fertilizers for example Calcium Ammonium Nitrate (CAN) help in development of good structures. 8. Wetting and drying: When a dry soil is wetted, the soil colloids swell on absorbing water. On drying, shrinkage produces strains in the soil mass gives rise to cracks, which break it up into clods and granules of various sizes. 9. Exchangeable cations: Ca, Mg → Flocculating; Good structure H, Na → Deflocculating; Poor structure 66 | P a g e 10. Inorganic cements: CaCO3 and Sesquioxides 11. Clay and Water Importance of Soil Structure Soil structure influences rather indirectly by the formation of an array of pores of various shapes and sizes. These pores are controlling factors governing water, air and temperature in soil. The role of soil structure in relation to plant growth  Soil structure influences the amount and nature of porosity.  Structure controls the amount of water and air present in the soil. Not only the amount of water and air dependent on soil structure, but their movement and circulation are also controlled by soil structure.  Structure can be modified by cultivation and tillage operations while texture is an inherent property of soil and cannot be modified within short period of time.It affects tillage practices.  Structure controls runoff and erosion.  Platy structure normally hinders free drainage whereas sphere like structure (granular and crumby) helps in drainage.  Crumby and granular structure provides optimum infiltration, water holding capacity, aeration and drainage. It also provides good habitat for microorganisms and supply of nutrients. 67 | P a g e LECTURE 14-DENSITIES OF SOIL Density is the weight per unit volume of a substance. It is expressed in gram per cubic centimeter or pound per cubic foot. Two density measurements particle density and bulk density are common for soils. Mass (M) Density (D) = ――――――――― gm / cc Volume (V) Bulk Density (Db) Soil bulk density, like all density measurements, is an expression of the mass tovolume relationship for a given material. Soil bulk density measures total soil volume. ♦ Thus, bulk density takes into account solid space as well as pore space. ♦ Soils that are loose, porous, or well-aggregated will have lower bulk densities than soils that are compacted or nonaggregated. ♦ This is because pore space weighs less than solid space (soil particles). Sandy soils have less total pore than clayey soils, so generally they have higher bulk densities. ♦ Bulk densities of sandy soils vary between 1.2 to 1.8 Mg m-3. Fine-textured soil, such as Clays, silty clays, or clay loams, have bulk densities between 1.0 and 1.6 Mg m-3. Factors Affecting Bulk Density Bulk density is an indirect measure of pore space and is affected primarily by texture and structure. As aggregation and clay content increase, bulk density decreases. Tillage operations do not affect texture, but they do alter structure (soil particle aggregation). Primary tillage operations, such as plowing, generally decrease bulk density and increase pore space, which is beneficial. Secondary tillage (cultivation) generally increases bulk density and decreases pore space. 68 | P a g e The compaction resulting from cultivation can be detrimental to plant growth. Cropped soils generally have higher bulk densities than uncropped soils. The movement of machinery over the field forces solid particles into spaces once occupied by water or air, resulting in less pore space and increased bulk density. Clayey Loamy Sandy Well-aggregated Moderately aggregated Nonaggregated High organic Moderate organic Low organic matter Matter content matter content content >-----------BULK DENSITY INCREASING--------> Figure. Relationship of soil bulk density to texture, organic matter content, and aggregation The density of water is 1.0 Mg m-3 and mineral soils are usually heavier than water. However, organic soils generally have a bulk density less than water. As the organic matter content of mineral soils increases, the bulk density decreases. Manure additions in large amounts tend to lower the surface bulk density of mineral soils because of the addition of low bulk density material and the consequent promotion of soil aggregation. Soil bulk density increases with soil depth primarily because of less organic matter and decreased aggregation. Particle Density (Dp) Soil particle density is a measure of the mass per unit volume of the soil solids only.  Texture and structure do not affect particle density.  However, organic matter, which is a soil solid, readily influences particle density. Organic matter weighs much less per unit volume than soil minerals. 69 | P a g e  Soils high in organic matter have lower particle densities than soils similar in texture that are low in organic matter.  Soil particle density generally increases with soil depth because of the concurrent decrease in organic matter  Particle density varies with the type of soil minerals present as well as the amount of organic matter.  The particle density of most mineral soils is in the range of 2.60 to 2.75 Mg m-3. Particle density is used in the calculation of pore space and bulk density on a coarse fragment free basis.  When unknown, particle

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