Unit 1 Nature and Formation of Soil
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UNIT 1 NATURE AND FORMATION OF SOIL Structure 1.1 Introduction Objectives 1.2 Soil and its Importance 1.3 Soil Morphology Characteristics of Soil Profile Soil Horizons 1.4 Soil Genesis: Origin and Formation of Soil Minerals and Rocks Weathering and Soil Formation Factors Affecting Soil Formation 1.5 Soil Classification Soil Types of India 1.6 Summary 1.7 Terminal Questions 1.8 Answers 1.1 INTRODUCTION Evolution of Earth took place five to eight billion years ago when it was a red hot magma (molten rock). It took millions of years for the magma to cool down and during this process it got transformed in many ways. In the initial stages there was no water on Earth’s surface, it rained for some million years and filled up valley like places to make oceans and seas. In these early stages of evolution, Earth conditions were quite drastic in which formation of many silicates, like orthoclase, took place. These in contact with water slowly hydrolysed to give open structured silicates called clay minerals. This transformation took another million years. Sometimes in between, life came into existence in water and evolved. This evolution was accompanied by the evolution of soil organisms which helped in the formation of soil. The process of soil formation, factors affecting the soil formation, soil morphology, and the common soil types are discussed in this unit. The next unit deals with different characteristic parameters of soil which determine the nature and quality of the soil. To begin with, it is important to understand what is soil and how is it important for all living beings. We will discuss this in the next section. Objectives After studying this unit you should be able to: • define soil and explain its significance, • describe the soil profile and soil horizons, • identify different types of soil horizons, • define rocks and minerals, • explain the process of weathering, • describe the factors which affect soil formation, and • describe soil taxonomy in general and the different types of soils of India. 1.2 SOIL AND ITS IMPORTANCE Everyone knows about soil as a resource which fulfils the basic requirement of human The noun soil is derived from the latin solum , which kind by supporting variety of plants and other vegetation. It is difficult to give a means floor or ground. unique definition for it, as its observation is quite subjective in nature. In a very broad As a transitive verb the manner, soil may be visualised as a “thin layer” of Earth’s crust which serves as a word 'soil' means "to make natural medium for the growth of plants. However, it has different meanings to dirty" as in the case of different people. To a farmer, soil is that portion of Earth’s surface which he can soiled dishes or clothing. 7 Soil plough and grow crops to fulfill needs of family and animals. For a civil engineer, soil is the foundation for all construction activity like roads, buildings, embankment of canals and drains etc. For an oil technologist, soil clays and clay minerals are the sources of petroleum cracking agents. Thus, soil is a vital natural resource which needs proper understanding and management to maintain and improve its productivity, which in turn, helps to maintain a healthy and green environment. The definition of soil has changed a great deal from a thin “outer layer of Earth’s crust” to “a collection of natural bodies on the surface of the Earth ”. The currently accepted definition of soil given by Soil Survey Staff of United States Department of Agriculture (USDA) is an elaborate one and is beyond the scope of this course. The operational part of the definition visualises the soil as “the collection of natural bodies in the Earth's surface, in places modified or even made by man of Earthy materials, containing living matter and supporting or capable of supporting plants out-of-doors”. To be soil, a natural body: • must contain living matter, • should be capable of supporting plants. Having defined soil, let us look into why does one need to study about soil and what is Why study soils? its importance? You would be aware that, from the dawn of agriculture, cultivators - People and society were attracted to fertile soils of river valleys. Early Greek and Roman writers have depend on soil - Soil is a vital natural described farming systems that involved leguminous plants, use of ashes and sulphur resource as soil supplements. German chemist Justus von Liebig (1840) found that crop yields - Soils are the major were increased by adding minerals to soils. He proposed that the mineral elements in source of food, fiber the soil added as manures and fertilizers are essential for plant growth. According to and renewable or reusable resources him certa in factors are essential for plant growth and if any one of these factors is (biomass etc.) limiting (controlling) plant production would be reduced. Soils are studied today to ascertain which of these factors is below desired level and how its limitation to plant growth can be removed. Pedology focuses on soil formation (from nature In the last two centuries of scientific study, two concepts or approaches to the study of source), its classification soil have evolved. One called pedology treats soil as a natural entity, a biochemically and composition. weathered and synthesised product of nature and deals with its classification and description as it occurs. The other named edaphology, treats soil as a natural habitat Edaphology deals with for plants and studies it in terms of its productivity and means of conserving and soil productivity, its conservation and, improving it, the production of food and fibre being their ultimate goal. We however management (for are going to follow a combined approach. Having understood about soil and its efficient production of importance let us study about the general features of soil. Before that try to answer the food and fibre). following SAQ. SAQ 1 How is pedology different from edaphology? ………………………………………………………………………………………… ………………………………………………………………………………………… ………………………………………………………………………………………… 1.3 SOIL MORPHOLOGY Parent material: The Soil morphology is the description of the soil body and its general characteristics. The primary material from morphology of the soil is expressed by number, kinds and arrangements of the which the soil is formed. different layers constituting it and their observable and measurable characteristics. The development of soil from the parent material is the result of soil forming 8 processes occurring under the influence of soil forming factors (discussed in next section). These processes take a long time- may be a few thousand years to create a Nature and Formation soil. As the time passes, the soil matures and generally becomes deeper and develops of Soil distinct layers called horizons. A soil horizon may be defined as “a horizontal layer of regolith, approximately parallel to the soil surface, and possessing relatively Regolith: the loose Earth homogeneous physical, chemical, and biological properties produced by soil forming material above solid rock. processes”. In a vertical section of the soil (as can be seen in a freshly exposed pit) several characteristic horizons can be identified. Such a section is called as a profile or soil profile. It is commonly conceived as a plane at right angles to the surface. The upper part of a soil profile above the parent material in which processes of soil formation occur and within which most plant roots and soil animals are found is called solum. A study of the soil profile refers to the identification and description of all the horizons as completely as possible. Different horizons are differentiated on the basis of the characteristics observable in the field, though sometimes laboratory data is also needed for complete description. In practice, however, a description of a soil profile includes soil properties that can be deter mined only by inspecting volumes of soil. The characteristics of soil do not change vertically downwards only, there are lateral Pedon: Greek word variations also as soil is a three dimensional body. Therefore, a three dimensional meaning soil or Earth. sample within a soil, called a pedon , is chosen as a unit to represent the nature and The area of a pedon arrangement of horizons and variability in the properties of soil. A group of similar 2 pedons that are surrounded on all sides by “non-soil” or by pedons of unlike character ranges from 1 to 10 m . is called a polypedon. A schematic sketch of a soil pedon in relation to the soil, polypedon and soil profile is shown in Fig. 1.1. In the following subsection you will study about the characteristics of profile of a given soil. It is important for the classification of the soil as well as in putting the soil to an appropriate use. Polypedon Soil pedon O A B Soil profile Soil horizons C Fig. 1.1: A pedon in relation to the soil, polypedon and soil profile. 1.3.1 Characteristics of Soil Profile Colours are caused by iron oxides and The profile characteristics studied in the field consist of locating the soil hor izons organic matter that (based on its colour description that includes the colour name, the Munsell notation), coat the surface of soil the water state, and the physical state. The physical state is recorded as broken, particles. Clays tend rubbed, crushed, while the water state of a sample is given either as "moist" or "dry". to be orange - red due The Munsell notation is a soil colour system devised originally in the USA and now to redox reactions, whereas darker colours widely accepted. It is based on the three variables of colour viz., hue, value and are caused from chroma.