UNIT 1 SOIL :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 Minerals 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 Soil Morphology 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 mineral 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 agriculture 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. Soils 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 Sand Silt Clay 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 Loess (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 Soil texture 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.