Basics of Soil Science for Competitive exam

Soil Science

Soil Science


  • The term soil is derived from Latin word “Solum” means ‘Floor’ or ‘Ground’.
  • Soil is the interphase between atmosphere and the mantle of rocks making up the earth’s crust called “lithosphere”.
  • Regolith: All loose material above bedrock (the unconsolidated material of weathered rock and soil material).
  • Soil Survey: Systematic examination, description and classification of soils.
  • Pedology: Science dealing with genesis, survey, classification and laws of geographic distribution of soils as a body in nature.
  • Edaphology: It is the study of soils from stand point of higher plants.


  • “Rocks” are defined as the aggregates of one (or) more minerals. Eg: Limestone – aggregation of calcite mineral.
  • Rocks have no definite chemical, morphological composition or definite symmetrical form like minerals.
  • Based on origin, rocks are classified as
  1. Igneous rocks
  2. Sedimentory rocks
  3. Metamorphic rocks.

I. Igneous rocks: These are the most abundant and makeup 95% of all the earth crust. Igneous rocks are “oldest rocks” and are also known as “crystalline (or) massive (or) fire rocks. These rocks mainly consist of primary minerals and more than half of the igneous rock is containing “feldspar” minerals. Igneous rocks based on their mode of formation are divided into two groups.

a. Extrusive (or) volcanic rocks: These are formed at the surface from the volcanic magma. Eg: Basalt, diorite

b. Intrusive (or) plutonic rocks: These are formed by the cooling of the original magma and occur below the earth crust. Eg: Granite. Igneous rocks are also classified based on their chemical composition.

  1. Acid rocks – contain more than 65% silica – Granite.
  2. Neutral rocks – contain 50% – 65% silica – Diorite.
  3. Basic rocks – contain less than 50% silica – Basalt.
  4. Sedimentary rocks: These rocks are formed through the transportation and deposition of weathered sedimens. The sedimetary rocks are  mostly formed through the agency of water, which are called clastic, aqueous or stratified rocks. Based on origin, sedimentary rocks are classfied into 4 groups.
  5. Residual sediments: These are formed from the products of weathering in situe that is at same place. Eg: Laterite, bauxite.
  6. Mechanical sediments: These are formed due to the deposition of pebbles, sand and silt. Eg: Standstone, shale.
  7. Chemical sediments: These are formed due to evaporation of water, precipitation and consequent accumulation of sediments. Eg: Limestone.
  8. . Organic sediments: These are fomred due to partial decompostion of organic remains under anaerobic conditions. Eg: Peat

III. Metamorphic rocks: These are formed from the rocks by the action of heat and pressure on   pre-existing igneous and / or sedimentary rocks.

Eg: Sandstone – Quartzite, Shale – Slate, Limestone – Marble, Granite – Gnesiss, Basalt – Schist, Coal – Graphite.


A mineral is a naturally occuring, homogenous element or inorganic compound that has a definite chemical composition and a characteristic geometric form. The minerals can be identified by many of their physical properties like colour, lustre, streak, hardness etc.

Lustre – general appearance of mineral in reflected light.

Fracture – property of the mineral to break along an irregular surface i.e., surface produced when the mineral breaks in direction other than cleavage plane. • Streak – The colour of the powder of the mineral that is obtained by rubbing the mineral against the unglazed porcelain plate.

Soil forming minerals mainly belong to the group of alumino – silicates. Minerals are classified based on quantity (essential and accessory), origin (primary and secondary), specific gravity      ( light and heavy) and chemical composition (native elements, oxides, sulphates, sulphides, carbonates, halides, silicates).

Primary minerals –They are the original components of the rock, which have not been altered chemically. They are formed from crystallization of magma (molten mass). Eg: Quartz, feldspar etc.

Secondary minerals –are resulted from the decomposition and / or alteration of primary minerals. These are formed due to subsequent changes in the rocks. Eg: All clay minerals like kaolinite, illite except micas. Micas are primary minerals.

Minerals which form the chief constituent of rock and regraded as the characteristic components of the rock are known as “Essential Minerals”. Eg: Quartz and feldspars.

Accessory minerals : The minerals which occur in small quantity in rocks They are not concerned for naming or the nomenclature of the rocks.  Such minerals are called as “accessory minerals”. Eg: Apatite, pyrite, magnetite etc. These are not required for the fomration of any rock.

Primary silicate minerals: 

  • Quartz – SiO2.
  • Potash feldspar (Orthoclase) – KAlSi3O8, Soda feldspars (Albite) – NaAlSi3O8
  • Lime feldspar (Anorthite) – CaAl2Si2O8; Albite and anorthite combine to form plagioclase or soda lime feldspars.
  • Feldspars are easily attacked by “water containing H2CO3”. The weathering process is called as carbonation.
  • Plagioclase weathers more rapidly than orthoclase. Orthoclas is commonly occurring feldspar mineral in acid igneous rocks.
  • Micas are the double silicates of K and Al with or without iron. These are plate like structures.
  1. Muscovite (white mica) – KAl3Si3O10(OH)2
  2. Biotite (black mica) – occurs both in acidic and basic rocks.
  3. Phlogopite – Occurs as a primary mineral in igneous rocks. Biotite is easily weatherable than muscovite.
  • Pyroxenes and amphiboles: These are the double silicates of Fe, Mg, Al and Ca.
  • Pyroxene – Augite (dark green)
  • Amphibole – Hornblende (green – black)
  • Olivines are the “thin silicates of Fe and Mg”. Eg: Fayalite, forsterite.
  • Sedimentary rocks have more of secondary minerals.
  • Muscovite alters to “hydrous mica”.
  • The insoluble residual material left behind during weathering is called as “saprolite”.
  • The phenomenon of weathering of surface layer of rocks due to differential co-efficient of expansion and contraction leading to ultimate disintegration is called “exfoliation”.
  • The material deposited due to melting of ice or glacier in warm regions forms a structureless mass and is termed “moraine or till”.
  • Chemical weathering of feldspar produces clay mineral.
  • Basalt decomposes more easily than granite.
  • Ease of weathering of minerals Quartz > Feldspar > Micas > Olivines > Hornblende.
  • Weathering is a denstructive process whereas soil formation is a constructive porcess in nature.
  • Relief: It is defined as the elevations and inequalities of a land surface considered collectively. “Topograpy” is similar to relief to be used on contour maps.
  • The time devoted by nature to the formation of soil is known as “pedogenic time”.
  • The process leading to the development of “soil profile” is called “pedogenic process”.
  • “Humification” is the process of decomposition of raw O.M into humus. This process usually takes place in surface or O horizon.
  • ‘Eluviation” is the process of removal of constituents by percolation from upper layer to lower layer (wash out).
  • “Illuviation” is the deposition of dissolved material in the lower layers (wash in).
  • “Podzole” means ash like under. Podzolisation is humid temperate type of soil forming process. It is opposite to “calcification”.
  • Laterization is the process of soil formation in tropics and sub-tropics. Laterization is the process of removal of “silica” instead of “sesquioxides” from the upper layers.
  • Laterization and podzolisation form soils belonging to the group of “pedalfer”.
  • “Calcification” occurs in areas where there is insufficient rainfall.
  • The soils which are having high saturation of ‘Ca’ are called as “pedocals”.
  • “Decalcification” is the removal of ‘Ca’ ions (or) CaCO3 by leaching.
  • “Cation exchange capacity” is expressed as me/100 g of soil or cmol (p) kg-1 soil.
  • Soil cations are sometimes called as “swarm ions” because they resemble swarm of bees around a beehive. • The area in which the ions are moving around root (or) clay particle in soils is called “oscillation zone”. • CEC of kaolinite increases as the pH of soil increases.

Total exchangeable bases (m.e/100 g.soil) % of base saturation = X 100 CEC

  • Arid region soils have high B.S than soils of humid region.
  •  Soils which have higher B.S one dominated by 2:1 clay minerals like montmorillonite, vermiculite, chlorite, micas.
  • Anion exchange is more in soils high in 1:1 clay.
  • Acid soils are poor in available Ca and Mg.
  • Availability of ‘S’ is not affected by soil reaction as the sulphur compounds are soluble in whole pH range.
  • When pH is low, solubility of Fe, Mn, Al increases.
  • Availability of B, Cu, Zn is reduced when the pH is increased.
  • Availability of Mo is reduced in acid soils.
  • “Buffering” refers to resistance to slight change in pH.
  • The power to resist slight change in pH is called “buffer action”.
  • Horizons in a soil profile are broadly divided into 4 groups and are called A, B, C, D.
  • AB horizons are collectively called as ‘solum’. The solum together with parent material is called “soil profile”.
  • “Horizon” – A layer of soil approximately parallel to the land surface.
  • The diagnostic surface horizons are called “epipedons”.
  • When larger mineral particles dominate, soil is said to be “gravelly” (or) sandy. When the mineral colloids dominate it is “clayey”.
  • Compact soils and sandy soils have high bulk density.
  • B.D. is more in lower layers of the profile because of less O.M.
  • Addition of organic matter lowers the B. D and increases the porespace.
  • Due to leaching of Fe compounds due to high rainfall, “grey (or) grey brown” soils are formed.
  • Hue – refers to the dominant spectral colours.
  • Value – refers to the relative lightness (or) darkness of colour.
  • Chroma – relative purity of a colour.
  • “Soil consistence” is a dynamic property of soils which is expressed by the degree and kind of “cohesion and adhesion”.
  • Non – exchangeable cations in montmorillonite – Mg, illite – K.
  • The organic matter on decomposition gets modified and acquires the properties of “Colloids”. • “Soil survey” is the study and mapping of soils in their natural environment.
  • “Remote sensing” is the science and art of acquiring the information about objects from distance without physically going in contact with the object.
  • Pedalfers – Accumulation of iron – aluminium in soils under high rainfall. Pedocals – Accumulation of ‘Ca’ – in areas of low rainfall

1.Alfisols – They are characterized by clay enriched Bt horizon.

  1. Vertisols – These soils are black soils – Inversion of soil occurs in the profile.
  2. Aridisols – These are the soils of dry regions.
  3. Mollisols – These are developed under grassland vegetation.
  4. Histols – These are the organic soils developed under water saturated environment.
  5. Oxisols – These are very strongly weathered mineral soil.
  6. Ultisols – These are the soils of low base status.
  7. Spodosols – These are the mineral soils with accumulation of humus and sesquioxides.
  8. Entisols – These are recently developed mineral soils horizonisation.


  • In soils, bauxite is the dominant oxide of aluminium.
  • Bluish and greenish colour of soil indicate ill drained condition.
  • The porosity and permeability of 1:1 clay mineral is high.
  • Ca & Mg have specified role of flocculation.
  • Total pore space is more in “clayey soils”.
  • Many fungi are soil inhabitants living as saprophytes on dead organic matter.
  • A larger proportion of plant nutrient present in the soil are in organic form.
  • The CEC of inorganic colloids is less than organic colloids.
  • Saline woils are dominated by chlorides and sulphates.
  • The hydrogen ion concentration of soil solution is called – Active acidity.
  • Limonite – 2Fe2O3. 3H2O.
  • Climate and biosphere are the active factors involved in the soil formation.
  • Humus theory was proposed by “von Liebig”.
  • The number of textural classes in the textural triangle is 12.
  • The steps involved in the development of soil structure are flocculation and aggregation.
  • In the arid regions with imperfect drainage, the most preferred cation for adsorption on soil colloid is Na.
  • The soil/parent material is said to be colluvial if it is formed due to gravity.

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