UNIT 1 :ITS COMPOSITION AND FORMATION

Structure 1.1 Introduction Objectives 1.2 Soil : A Natural Medium of Plant Growth 1.3 Composition of Top Soil 1.4 Soil Phases 1.5 Soil Genesis 1.5.1 Weathering of Rocks and M~nerals 1.5.2 Soil Forniation 1.5.3 Factors affecting Soil Formation 1.5.4 Age of Land 1.5.5 Nomenclatures in Soil Fornling Procasa 1.6 Rocks and 1.6.1 Primary Minerals 1.6.2 Role of Primary Minerals 1.6.3 Secondary Minerals 1.6.4 Role of Secondary Mnerals 1.6.5 Igneous Rocks 1.6.6 Sedimentary Rocks 1.6.7 Metamorphic Rocks 1.7 1.7.1 fiaraderisation of Soil Profile 1.7.2 Soil Horizons 1.7.3 Soil Micromorphology 1.8 Sunlmary 1.9 Key Words 1.10 Answers to SAQs

1.1 INTRODUCTION Soil is a complex dynamic system. It is difficult to give a unique definition of soil as its observation is ~ub~jectivein nature. Joffe (1949) defined soil as a natural body of and organic constituents, differentiated into horizons of variable depth that differs in material below in morphology, physical make up, chemical properties, compsition and biological characteristics. However, soil for the purpose of is defined as a dynamic natural body formed due to pedogenic processes during and after weathering of rocks possessing chemical, physical, mineral and biological properties and acting as a medium for plant growth. are formed as a result of weathering of rocks and minerals by physical and chemical processes. Before the processes of soil formation start, the parent material may be transported through the action of air, water, ice and gravity and deposited to produce soil. Coupled with weathering. climate (precipitation, temperature), organism (flora and fauna) and relief (elevation, slope) act slowly over time (age) on parent material to form the soil. The soil forming factors act sufficiently for long periods of time (20-50 years) at lower depths and cause the development of soil morphology representing the succession of soil horizons called soil profile. The soil, thus, in effect is a dynamic system. Objectives After studying this unit, you should be able to define soil, state soil as a medium for plant growth, describe the composition and phases of the soil, list the processes involving the genesis of soil. Physical Properties of Soils explain the activities behind soil formation, explain the role of rocks and minerals in soil, characterise soil profile, and identify rnicromorphologic feature of soil.

1.2 SOIL: A NATURAL MEDIUM OF PLANT GROWTH Although there are various uses of soil, but our paramount interest in soil will be as an agricultural resource - a medium for plant growth. There are six major factors that control growth and development of plant : 1) Light energy, 2) Heat energy, 3) Air, 4) Water, 5) Nutrients, and 6) Mechanical support. Plant enjoys from the soil the following inputs: - soil provides anchorage to roots enabling plants to stand erect, - soil acts as a store house of water and nutrients for plant growth, - soil acts as an abode for flora and fauna. It suitably transform nutrient for uptake by plant roots and detoxify the harmful chemicals present, - soil provides space for air and aeration which create a healthy environment for the biological activity of soil organisms, and - soil provides space to diffuse light and temperature in it.

1.3 COMPOSITION OF TOP SOIL Soil is composed of partially weathered, unweathered and transformed products of rocks, minerals and organic matter. The soil particles are present partly as an individual and partly as aggregates or ped. The organic matter is often firmly combined with mineral particles forming aggregates of various sizes and shapes.

1.4 SOIL PHASES From a physical standpoint, soil may be viewed as a combinatiof solid, liquid and gas. Approximate composition of these in a typical top soil can be represented in Figure 1.1.

Soil

I I I So11d Phase Wquid Phase Gas Phase x Volume (50) (25) (25) I ~nor'ganic ~r~anic Water Salts Soil Air x Volume (40) (10) - RH lOOX X Weight (85) (5) Cations Aniom 2 % caZC- HCO; C% 0.5% I Humus bfgeC SO:- Ar 0.92 Colloids KC C1- - N$ Primary Minerals Secondary Minerals H+ NB HPO; - Quartz Layered Silicates A?+ Feldspar Hydrous Oxide H2PO; Mica Colloids 0.001 - 0.01 M or 100 - 1000 ppm in pores and film on eurfaces

Figure 1.1 :Composition ol Top Soil Soil :Its Con~position and Formation

1.5 SOIL GENESIS Soil genesis has two distinct steps. The first is weathering that involves disintegration and decomposition of rocks and minerals whereas the second is the deposition involving development of soil by pedogenic processes. This can be represented as follows : Soil Genesis A Two Step Process Step 1 Transported - Water

Parent ' Rocks -Weathering -Regolith Wind Material (Unconsolidated) Gravity I Ice Residual (Sedentary! I Step 2

Soil Profile Parent Material -Fnrrning Processes b Soil Profile

1.5.1 Weathering of Rocks and Minerals Weathering is the combined activity of destruction and synthesis of rocks in which rocks are broken down physically into smaller sizes and further to minerals of which they are made up. 111 the process the minerals and rocks are also attacked by chemicals. Weathering and soil development proceed almost simultaneously in case of soft rocks, whereas in case of hard rocks weathering preceeds soil development. Weathering takes place through physical, chemical and biological agents singly or in combination as discussed below : Physicai Weathering Physical weathering retains its original chemical and mineral composition. This is carried out through following processes : - Freezing and thawing - expansion and contraction, - Heating and cooling, - Abrasion, - Rooting and mineral activity, - Gravity - unloading and overburden, and - Columnation - due to chemical weathering. Chemical Weathering This gives rise to drastic alterations in the constituent minerals, partially or wholly with the formation of secondary minerals which differ markedly from the primary minerals. The secondary products inay be formed by alteration in situ or by precipitation from solutions under conditions favourable for it. Principal reactions involved in chemical weathering are oxidation, reduction, hydration, hydrolysis, solution and carbonation. Agents of chemical weathering are water, oxygen and carbon dioxide as given below: Physical Properties oPSoils H20Response Solution AlCl) + H20 (1) + A13' (aq) + 3C1-(aq) Hydration A13' (aq) + 6H20 (I) +AI(H~O)',~ (aq) Hydrolysis AI(H~O): + AI(H~O)~(aq) + H' (aq) + OK (aq) Combination Response SiO, (s) + 2H20 (1) + Si(OH)4 (aq)

(solution and hydrolysis)

CaA12Si20, (s) + 9H20 +C~(HP)T (aq) + 2 OK (aq) + A12Si20s(OH)4) (Solution, l~ydration (Clay mineral) and hydrolysis) C02Response C02 + H20 (1) + H2C03 (a@ +~'(aq) + HCO; (aq) O2 Response 4Fe0 (s) + O2 (g) + 2 H20 (1) +4FeO(OH) (s) (Ferrous) (Ferric)

- Fe in primary minerals, ferromagnesians, is ferrous ,and makes these minerals vulnerable to weathering. - In poorly drained soils (excess H20), ~e~~can be reduced to ~e~+by mottling and gleyzation. Biological Weathering In the strict sense, physical and chemical weathering is brought about by biological agents. Plant roots evolve - Carbon dioxide, - Organic acids, - Chelating agents, - reducing environment, and - widening cracks and crevices. The weathering process could then be summarised as follows : Disintegration 1) Rock Gravel, sand, silt ( i.e., increase in surface area )

H20 Decomposition Product of

Soluble constituents (aq) + Secondary minerals 2, .isintegration + [ g2] i) Clay minerals i) ~a*,~g*, ~a', K' ii) Hydrous oxides of HCOT, SOT -, C1- Fe, A1 and Si ii) Si(OH)4 iii) Carbonates iv) Sulphates

1.5.2 Soil Formation Parent materials formed due to weathering of rocks are transported from the place of their origin and redeposited before they become subject to modification. The parent materials transported are named according to the main force responsible for the transport and redeposition as shown in Table 1.1. Soil :Its Composition and Formation Table 1.1 : Nomenclature of Soil Deposits

1 Agents Deposited in or by Name of Deposit Stream (flood plain, terraces) Alluvium Lacustrine Ocean Marine

Ice Till, Moraine [Ice IMeltwater 1 (Silt). } I Dunes (Sand) I I I 1 Gravity I Gravity Colluvium 1 1.5.3 Factors affecting Soil Formation Soil is the result of combined activity and reciporcal influence of parent material, plant and chemical organisms, climate, age of land and topography as outlined below : 1) Climate Precipitation and temperature 2) Organisms Forest, grassland and microbes The above factors vary on a large scale geographically (i.e., regional) 3) Parent material Mineral composition and texture 4) Topography Erosion and drainage 5) Time The above three factors vary on a small scale geographically (i.e., locally). Climate The precipitation and temperature are two most important climatic factor that affects physical, chemical and biological weathering. The rise in temperature also increases rate of biochemical reactions. The rainfall in substantial quantities create favourable environment for plant growth. The vegetation varies from areas of high rainfall to low rainfall and hence varying organic matters in soils. Organisms Activity of soil organisms plays a significant role in soil formation by accumulating organic matter and cycling of nutrients. In addition the living organism, bacteria (azotobacter) fix atmospheric nitrogen into the compounds which can be easily used by plants. Parent Material is greatly influenced by parent material which in turn affects movement of water and nutrients. Similarly, mineralogical and chemical composition of parent material have direct bearing on weathering. For example, soil acidity in humid condition can be delayed in case of soils made up of limestone. Topography Topography is generally described in terms of slope, elevation, etc. The steep slope is vulnerable to erosion whereas flat land weaken the process of erosion. Time The time determines the effect of weathering. The alluvial material generally does not get sufficient time to develop in comparison to upland soils. In fact two or more of the factors described above act simultaneously and affect the soil formation. Hence, their interaction and interdependence make it difficult to assess how a given soil was formed. 1.5.4 Age of Land The span of period from inception to the present stage is termed as age. Soil formation is very slow process; therefore, the age may vary from few years to several thousand years. Approximate age of soil can be arrived at by the following methods: Physical Properties of Soils - Radioactive dating - Pollen analysis - Carbonate carbon in layer Mohr and Van Baren recognised five stages of development of tropical soils as follows : 1) Initial stage Unweathered parent material 2) Juvenile stage - Weathering just started 3) Virile stage Easily weatherable minerals which have been decomposed for Ule greater part. The clay content has increased and certain mellniness is discernible. The content of soil components less susceptible to weathering is still appreciable. 4) Senile stage Decomposition arrives as a final stage, and only the most resistant minerals have survived. 5) Final stage Soil development has completed and the parent material is fully weathered. In , soils are described as young or mature. Young Soil This means the soil where the fdctors of soil formation and pedogenic process are still operative and changing the properties of soil in the profile and the processes have not made a distinct impression on soil profile clay content which also decreases with depth. Mature Soil Clay is more or less in eyuillibrium with primary minerals. Clay content also increases with depth of profile, accumulating at an intermediate depth. If the soil is removed by erosion or deposited over by a fresh transported parent material, a new cycle of soil formation starts. 1.5.5 Nomenclature in Soil Forming Process The fundamental processes involved in the formation of soil are - addition of organic and mineral matters of soil (humification) - inaterial from the surface to a horizon (eluviation) - translocation of material from one profile and deposition at another (illuviation) - Transformatioil of the mineral and organic matter in the soil and formation of definite profile (horizonation) The specific processes involved in soil formation are Calcification :Formation & accumulation of CaC03. Gypsification :Formation & accumulation of gypsum. Decalcification :Removal of CaC03 from . Podzolization : Eluviation of oxides of Fe and Al, and humus under acidic condition. Laterization : Migration of silica out of soil solum. Salinization :Accumulation of soluble salts such as SO:- and C1 of Na, K, Ca and Mg . Desalinization : Leaching thus removalof soluble salts. Alkalisation : Increase of soil pH, i.e., accumulation of sodium ion. Dealkalisation : Decrease of soil pH. Gleization : ReducTion of iron due to anaerobic (i.e., water logged) condition. Pedoturbation : Intermixing of soil horizon of iron in water logged soils with function of mottles and concrets of Fe and Mn. Argillation : Leaching of dispersed clay particle from upper horizon to lower horizon. Soil : Its Composition and Formation

1.6 ROCKS AND MINERALS Rock is the solid material contained in tlle shell or outer crust of the earth, a layer about 16 to 40 krn thick. It is defined as an aggregate of one or more minerals; most commonly, two or more. All rocks are classified into three types based on their mode of origin. 1) Igneous : Formed by the cooling and consequent solidification of magma, a hot fluid mass or rock melt. 2) Sedimentary : Forn~edfrom material derived from the weathering of pre-existing rock as a result of erosion, transportation, deposition, and lithification, principally by action of water; much less frequently, involving ice (glacial) or wind action. 3) Metamorphic : Fomned by chemical and/or physical transformation (metamorphism)of igneous or sedin~entaryrcxk by means of heat and/or pressure. Water and other chemical agents often aid in this process. A mineral is a naturally cxcurring chemical element or compound formed as a product of inorganic processes. Soil scientists, however, often include within the term soil mineral the inorganic, amorphous (non-crystalline) components fornled in soil. Plant scientists also use mineral to connote those essential pl'ant nutrients derived from the soil. The intended meaning is dependent on the context in which it is used. 1.6.1 Primary Minerals Based on origin, minerals are classified into two groups, primary and secondary. Primary minerals are formed at temperatures andlor pressure higher than Uiat normally encountered at the earth's surface (one atmosphere, and < 100'C) ,and found in igneous ,and metamorphic rocks. Primary minerals are those minerals. which have gone through little change since they were formed, e.g., quartz, mica, feldspar etc. You have already gone through the composition and physical properties of various minerals in Unit 1 of Block 2 of Earth and its Environment. Now, we shall discuss their role in soils. 1.6.2 Role of Primary Minerals Quartz Quartz is the most common and abundant mineral contained in the sand silt in most soils. Because of its abundance in sand, quartz and s,md have often been used synonymously. However, if mineralogical determinations are made, sand in soil is found to contain many other minerals. Quartz is very insoluble, both in water or acids. Because of no cleavage and high hardness, it is not readily reduced in particle size. A low thermal coefficient of exp'ansion results in insensitivity to rapid temperature change ,and the consequent reduction in particle size. As a result of this it has high resistance to chemical and physical weathering. Feldspar Gmup In soil, feldspar is found in the sand 'and silt and, only in small amounts, in the clay. The amount of feldspar in a soil depends on the coniposition of the rock from which it has formed and the amount of weathering that has taken place. In soil, feldspar may be a source of the plant nutrients, K and Ca. Weathering of it releases small quantities of Potassium and Calcium which is too slow to meet immediate plant needs with in a growing season, however, it may meet long term supply of K and Ca. Weathering of feldspar by reaction with water results in the release of K+, ca2+,~1'+ in Physical Properties ofsoils some form, and silicon as silicic acid, Si(OH)4. Layered Silicate Group The layered silicates are found in the sand and silt particles in soil. The kind of layered silicate depends on the typ of rock from which the soil is formed. Muscovite is most common because of its greater resistance to weathering than either biotite or chlorite. Upon weathering these layered silicates may be altered to vermiculite or montmorillonite, which are layer type silicates. Other Ferromagnesian Silicates Their presence in soil depends on the rock type being weathered and the weathering intensity. On weathering these minerals form hydrous iron oxide along with some clay mineral or gibbsite. These are long-term sources of Ca, Mg, and Fe for plant nutrition. Apatite Apatite is the original source of soil phosphate. The amount found depends on the apatite content of the parent rock and the intensity and duration of weathering that has occurred. In acid soil, it. is slowly transformed to an iron or aluminum phosphate, or adsorbed by clay, to form an insoluble form in the clay. Since phosphate anions readily react with various soil constituents - clay minerals, iron and aluminium hydrous oxides. It is very immobile in soil. 1.6.3 Secondary Minerals Secondary minerals are formed under conditions of temperature and pressure found at the earth's surface by the weathering of preexisting minerals. In this process, with water being an almost ubiquitous participant, elements are released into solution. Part of these elements are leached out of the soil material and end up in ground water, streams and eventually, ocean. Some of the released elements are adsorbed and retained on the surface of the fine soil particle and serve as a readily available source of plant nutrients. Other released elements recombine, along with water, to form secondary minerals. Minerals such as silicate clays and iron oxide, which are formed by breakdown and weathering of less resistant materials, are the examples of secondary minerals. 1.6.4 Role of Secondary Minerals The secondary minerals most commonly formed are the clay minerals and hydrous oxide. Other secondary minerals found in soil are forms of silica, calcite, gypsum and apatite. Silica The sand fraction of soil derived from consists in part of grains of chert (Si02). Soils derived from some limestones often contain large amounts of golf ball to fist-sized pieces of chert. Being resistant to weathering, they persist as coarse fragments in soil often in sufficient quantity to result in a cherty fraction of . They are called phytoliths literally plant rocks. Silica oxide released into solution by weathering is absorbed by plant (grass are notable for this) and deposited along the cell walls. Upon the death of the plant and subsequent decomposition of the organic remains the opal (Si02.nHzO)is left in the surface soil. Clay Minerals Layer silicated of various types and an amorphous silicon aluminum water containig material are common in soils. Besides 02-, OH, AI?+ ; they can contain Mg, Fe, K in large amounts. The clay minerals occur characteristically as particles less than 2 pm in size. The colour of pure mineral (consisting of only one mineral) depends on chemical composition. One is white and is used as a coating on paper to produce high-quality, glossy finish. Other types vary from gray to light yellow to dark-brown. In soil the clay sized partcles are invariably a mixture of various clay minerals and are coated with hydrous iron oxides and/or organic material (humus) resulting in various shades of yellow to brown, red, orange and even black. A characteristic of clay minerals is to swell on wetting with water. It can absorb water within the crystal particles and swell to a much greater extent than the others. It is often referred to as the expanding lattice type, clay minerai. Lattice in this context means crystal. Mixtures of clay minerals are present in almost all soil and are the dominant constituents of the clay fraction. The kinds of clay minerals found in a soil is dependent, primarily on the type of the rock from which the soil formed and the weathering intensity to which it has been subjected. Because of their small particle size and consequent high surface area, they are the source of chemical property of soil (cation adsorption and release or cation Soil :Its Composition exchange) which is important in . and Formation Oxides, Hydroxides or Hydrous Oxide Gmup The red, orange and/or yellow to brown colouration in soils is caused by the presence of geothite and hematite which commonly occurs as coating on the surface of soil particles of . all sizes but in particular the clay particles. Gibbsite is found in highly weathered soil in the tropics or semi-tropics. In some of these soils gibbsite and hydrous iron oxides are the most abundant consituents. Weathering is sufficiently intense to remove, by leaching, practically all elements except ferric iron and aluminum, which form highly-insoluble oxides or hydrous oxides and are, therefore, very immobile. In the past, these soils were called ' laterites but are known today as oxidols. Besides being a possible source of plant nutrient, these iron minerals provide an indicator of leaching and of the drainage status of a soil. In soils called spodosols (formerly ) a white, bleached layer occurs near the surface from which all the hydrous iron oxides have been removed. Drainage status of a soil may also be indicated by the hydrous iron oxides. Well-drained soils have sufficient oxygen present to maintain iron in a ferric state, which results in bright-red, brown and orange colours. If water fills the pore space in soil, reducing the oxygen supply for extended periods of time during the year, some ferric iron is reduced to ferrous iron and gray colour becomes mixed which is called motting. Degree of development of motting can be used as a measure of the drainage status. Carbonate group Calcium carbonate remains in a soil in arid or sem-iarid areas. Lesser the rainfall, carbonates are found closer to the surface. Calcite is hard, cemented layer in a soil, resulting from the accumulation of carbonates. When rainfall exceeds about 640 mm, per year, as in humid areas, carbonates are completely leached out of a soil. Limestone-derived soils form insoluble residue (mostly silicates) gets accumulated as the carbonates are leached away.

Sulphate I Gypsum is found only in some arid area soils. It is by far the most water-soluble and therefore, the mostreadily leached. In soil, it is found below calcite which is less soluble and hence less mobile. Small quantities are used as sources of the plant nutrients, e.g., calcium and sulphur. It can be used to amends the highly alkaline soils in arid areas that contain excessive sodium (sodic soils) and as a result, are unsuitable for agricultural purposes. Large application of gypsum result in replacement of sodium by calcium, reducing alkalinity. The physical condition of the soil is also greatly improved. Apatite - Rock Phosphate Small amounts of apatite are found in the silt and sand even in highly weathered soil. The insolubility of apatite, under most soil conditions, prevents it from supplying adequate amounts of Phosphate for plant growth. Ground rock phosphate if used as a fertilizer, must be very finely ground and be applied at much higher rates than manufactured phosphate fertilizers. 1.6.5 Igneous Rocks Approximately 95% of the 10 mile thick crust at the surface is igneous in origin. This rock forms by the cooling of a molten magma, which results in formation of a hard mass of interlocking crystals. Thus igneous rocks are often referred to as crystalline or hard rock. Occurrence of fossils is precluded because of the high temperatures involved. Igneous rocks are classified on the basis of two properties, texture and mineral composition. Texture, which refers to size of mineral grains, IS determined by the rate of cooling of the magma. Rapid cooling such as would occur at the earth's surfact; with lava (extruded magma) results in fine textures with grains too small to be seen with the unaided eye. Slow cooling as happens when magma is enclosed within the crust, forms crystals sufficiently coarse to be seen and identified with the naked eye. Minera!~in igneous rock are divided into essential minerals which are present in large amounts and determine the rock type and varietal and accessory minerals which nay be present and occur in smaller anlounts. Essential nunerals are a combinatin of one or more of the primary minerals. Quartz, feldspar and ferromagnesians, amphibole and pyroxene. Two igneous rocks, granite and basalt are the most abundant types within the crust and our attention will therefore be devoted to these. Physical Properties of Soils Granite This is a coarse-grained rock which is granular or crystalline in appearance. Orthoclase and quartz comprise about 90% of the rock, giving it a white to pink colour. The black speckeled appearance is due to the occurrence of biotite and or amphibole which make up most of the remaining 10%. It is major rock type underlying the continents. Basalt Basalt is a fine-grained dark-coloured rock which underlines the ocean basin. Most lava eruptions result in formation of basalt. Pyroxene and calcium-rich plagioclase in about equal proportions, comprising approximately 95% of rock. A minor amount of olivine or amphibole may be contained. A coarse-grained equivalent of basalt is called gabbro. Diabase is a smaller rock intermediate in grain size between basalt and gabbro. SAQ 3 Which rock type is known as hard rock '' Nnrllc two varelies.

1.6.6 Sedimentary Rocks Although comprising only 5% of the 16 km crust, sedimentary rock tornls about 75% of the surface exposed rock and as sucli are very important as a source of soil parent material. Sedimentary rock, formed from the consolidation (lithification) of loose accumulations of usually having been transported by and deposited in some body of water. They may be subdivided, depending on the manner of deposition into: (1) mechanical type, which form from particle settling out of suspension and (2) chemical types, which form by precipitation of ions from solution, either inorganically or by a biological process. When settled from water, sedimentary rocks characteristically have stratified or layered appearance. Many of these rocks contan tossils of organisms now extincl thal lived in the remote geologic past. Machanical a) is a coarse-grained secondary rock, which consists of sand grains cemented together by finer material. The sand composed principally of quartz, chert and rock fragmerils along with smaller an~ountsof potassium, feldspar and muscovite mica. The cement consists of hydrous oxides, clay minerals, carbonate and or silica. The colour may be white, gray, yellow, brown or red. Conglomerate, which consists of consolidated gravel, pebbles or boulders is much less abundant than sandstone. b) Shale is a fine-grained rock consisting of silt and clay sized particles with a finely layered or laminated structure. Although clay minerals are dominant in shale quartz, mica feldspar, hydrous oxides and calcite also occur. It is most abundant seconda~yrock type. Chemical Limestone is a fine grained chemical precipitate which often contains numereous fossil remains of organisms. Calcite and dolomite are the principal minerals. Various amounts of quartz, chert, feldspar clay minerals, and hydrous oxide may be found in limestone. Dolomitic limestone forms from alteration of a previous calcitic limestone by magnesium replacing part of the calcium. The dolomitic limestone is somewhat hard and more resistant to weathering than the calcite limestone. The physical appearance of the two rocks is very similar but they may be distinguished by the rapid effervescence of the calcitic type when treated with a dilute acid. Dolomite must be powdered to produce a notable reaction. Of the sedimentary rocks on the earth's surface about 80% are shale, 15% sandstone (including conglomerate) and 5% limestone. SAQ 4 Hnw do sedimentary rocks orig~nate'! Nal~lcd~cir vanclics. Which IS 111enlost abundant sedimentary rock on earth's hut tackh ' 1.6.7 Metamorphic Rocks Soil :Its Composition and Formation When igneous or sedimentary rocks are subjected to high temperatures and pressures, as a result of earth crust movements, for example, recrystallization of elements and alignment of crystals frequently occur causing a bonding, foliation or finer lamination, the process is called metamorphism. Minerals present in metamorphic rock are primary. The . metamorphosed equivalents of the igneous and sedimentary rocks described are as follows :

Pre-existing Rock Metamorphic Equivalent I Granite Gneiss I I Basalt Sandstone Quartzite I Shale , Slate I I Limestone 1 Marble I

Gneiss Gneiss is a crystalline rock with a banded appearance. Light-coloured minerals, feldspar, quartz, and mica roughly alternate with bands of dark, fenomagnesian minerals. Schist Schist is a finely-foliated or laminated rock composed principally of micas and or chlorite, together with some quartz and ferromagnesian minerals. Slate This is a very finely textured rock with minerals tocr small to be seen. It characteristically splits into relatively thin smooth sheets. Mica, chlorite and quartz are the principal mineral in slate. Quartzite It is non-foliated rock, composed almost wholely of quartz grains cemented together by silica cement. This resultsin a very tough hard rock. It is distinguished from sandstone by fracture surface passing through, rather than around quartz grains. Marble Marble is a non-foliated crystalline rock, composed of calcite or dolomite. Individual grains may range in size from very fine to grains sufficiently coarse to manifest the typical rhombohedra1 cleavage of carbonates.

1.7 SOIL MORPHOLOGY Soil morphology is the description of the soil body, its appearance, features and general characteristics which are expressed in the profile of a soil. This is also expressed by number, kids and arrangements of the different horizons and their observable and measurable characteristics. A soil horizon is a layer of soil approximately parallel to the soil surface with characteristics produced by soil fonning processes. An individual soil body may be literally bounded by other soil bodies or by non-soil materials. This is a natural unit in the landscape characterised by particle size, slope, profile and other features. A pedon in the smallest volume that can be recognised as a soil individual and has the smallest area for which we should describe and sample the soil to represent the nature and arrangement of horizons and variability in other properties of the sample. The area of pedon varies from 1-10 m2 and the depth unit is somewhat vague extending upto depths of plant root or genetic soil horizons. A schematic sketch of a soil pedon is given in Figure 1.2. A group of similar pedons that are bounded on all sides by "non-soil" or by pedons of unlike character is called a plypedon. Physical Properties of Soils ------

'\ \ \ t

Solum Po!ypedon (ind~v~dualroil)

Figure 1.2 :Schematic of a Soil Pedon

1.7.1 Characterisation of Soil Profile Characteristics studied in the field include leaching the soil horizons, based on colour difference. Where this is not possible, horizons are differentiated on the basis of variations in other soil characteristics. Each of the differentiated soil horizons are described in tenns of the following characters: Genetic characters Thickness PH Colour (munsell notations) Ec Texture CO; - Structure HCO;

Consistency C a++ Clay lense Mg++ Roots ~a+ Crotorians K+ Pores Total soluble salts Water holding capacity Cation exchange capacity Mottlings Anion exchange capacity Concretions Organic carbon Artefacts Primary and jecondary minerals External characters Form Drainage condition Linearity Ground water level Sloped Erosion You will study these characters in details in subsequent units. 1.7.2 Soil Horizons Soil horizons tell much about the history of soil formation; of the several horizons, the master horizons are the results of the fundamental soil-forming processes viz. accumulation of humus (humification), leaching of organic matter and salts (eluviation) and deposition of Soil :~ts Composition clays (illuviation). These are designated by capital letters 0, A, E, B, and C. 0 represents and Formation organic horizon whereas A, E, B, and C represent mineral horizons. In case of sedimentary soils these horizons overlie the bedrock whereas the alluvium it has above the parent meterial. The master horizons may not be uniform through the depth in the characteristics by which they have been designated and may be subdivided to indicate the deviations. The sub-divisions are indicated by placing arabic numbers after capital letter such as 01, 02, A1, B1,B2 etc. A secondary arabic number is used to indicate further sub-divisions viz. O11, 012, All, B11, etc. In addition to the above designations the following symbols are also used to indicate accumulation of other specific features of the horizons. The symbols used are Cs = accuinulation of calcium sulphate. b = bumed soil horizon. Ca = accunulation of calcium carbonate. Cu = accumulation of concretion or hard non-concreting nodules enriched in sesquioxides with or without phosphorus. f = frozen soil. g = gleying. h = illuvial humus. ir = illuvial iron. m = strong cementation, induration. p = ploughing or other disturbance. Su = accumulation of soluble salts more soluble than calcium sulphate. Se = accumulation by siliceous material soluble in alkali. t = illuvial clay. x = fragipan character (high ). 1.7.3 Soil Micromorphology Examination with optical aid reveals more detailed features which are helpful in understanding . This is known as soil nlicromorphology. This can be done by hand lens to same extent but rigorous examination needs preparation of a thin section of a selected ped. The ped is impregnated with resins which on drying becomes as hard as rock and can be cut into thin sections for examination under a petrographic microscope. A good deal of information, viz., origin of parent material, exogenic processes, soil forming processes and management etc. are deciphered. SAQ 5 Explaln OIP,Alh, B,, .

1.8 SUMMARY In this unit you studied process by which soil is formed and the morphology it bears. Soil is a natural medium of plant growth. It is formed from the disintegration and decomposition of rocks and minerals. Such material when tran~portedand deposited, form the morphology of soil. Physical Properties of Soils Soil is composed of disintegrated and decomposed rocks and minerals. Thus, the mineral nutrients present in them are also found in the soil. Soil are formed involving specific processes viz. calcification, gypsification, decalicification, silication, pddzolification, laterisation, salinisation desalinisation, alkalisation, dealkalisation, gleyation, pedoturbation and argillation. During the process of formation soil may be at initial stage, juvenile stage, virile stage, senile stage or final stage. Pedologically these may be called young or mature soils. Soil formed on its maturity develop soil horizons based on colour differentiation. Various physio-chemical and biological features of these soils are studied to describe the soil profile. Miaomorphologic features of soil are studied to examine origin of parent meterial, exogenic processes and land use management.

1.9 KEY WORDS

Peds : Soil particles or aggregate. Alluvium Deposited by stream. Lacustrine Deposited in lake. Marine Deposited by ocean. I~es Silt transported through winds. Dunes Sand transported through winds. Colluvium Parent material transported through gravity. Young soil No distinct impression on profile. Mature soil Clays accumulated at an intermediate depth. Humification Accumulation and transformation of raw organic material into humus. Eluviation Leaching of organic matter and salts. Illuviation Deposition of clays.

1.10 ANSWERS TO SAQs Please refer preceding text for the answers of all SAQs.