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Lesson 1. Nature and Origin of Soil
1.1 INTRODUCTION
Soil is defined as a dynamic natural body on the surface of the earth in which plants grow, composed of mineral and organic materials and living forms.
The soil is made-up of broken down rock material of varying degree of fineness and changed in varying degrees from the parent rocks by the action of different agencies such that the growth of vegetation is made possible.
1.2 ORIGIN OF THE EARTH
The earth is a minute part of the universe. Moving around the sun are nine spheroidal bodies called planets. Our earth is one of these nine planets. There are several hypotheses regarding the origin of these planets from the sun. According to the early nebular hypothesis, the solar system once existed as an incandescent mass of gases. During the process of cooling, it threw out rings of nebulae. These rings on disruption and further cooling condensed themselves into planets. The 'planetisimal' hypothesis of Chamberlain and Moulton was proposed in 1900 and is the most generally accepted. Millions of stars rush at high velocities through space. One of these stars, probably bigger than our sun, happened to pass near enough to exert a gravitational pull on the sun and tear out a glowing mass of incandescent vaporous material into the limitless emptiness of space. The incandescent mass continues spinning in an elliptical path round the sun. These masses gradually cooled off into solid planetary nuclei or planetisimals. In the course of thousands of millions of years, most of these planetisimals combined by collusion and accretion and formed our solar system.
1.3 THE EARTH'S CRUST AND ITS COMPOSITION
Classically there are three divisions of earth's sphere corresponding to the solid, liquid and gas, which constitute the Earth. The solid zone is the Lithosphere. The outermost 10 mile strata of the lithosphere are called the 'earth crust'. The incomplete covering of water forming seas and oceans is the Hydrosphere and the gaseous envelope over the earth's surface is the Atmosphere.
The earth's crust is principally composed of mineral matter. Altogether, 109 elements known to us are found in earth crust in various combinations to form compounds. Oxygen is most abundant element but it usually occurs in combination with other elements. The composition of earth crust is given below (Table 1).
Table 1.1 : Composition of earth crust
Element |
O |
Si |
Al |
Fe |
Ca |
Mg |
Na |
K |
Others |
Per cent |
46 |
27 |
8 |
5 |
4 |
2 |
3 |
3 |
2 |
Out of 109 elements known, 8 are sufficiently abundant as to constitute 98.6 per cent (by weight) of the earth's crust (up to 16 km). The two elements occurring in greatest abundance are non metallic (oxygen and silicon) and comprise nearly three-fourth (75%) of the total composition of the crust. Most of the hard, naturally formed substance of the earth is referred to as rock. Rock is composed of elements. The materials of the earth's crust fall into two principal categories, namely minerals and rocks.
In the process of cooling, the elements began to combine into compounds. The compounds so formed aggregated into large masses giving rise to a variety of minerals. These mineral masses got cemented or pressed into the rocks, as they are now present in the earth's crust. These rocks were then exposed to the action of weathering agencies over thousands of years. The disintegrated rocks gradually evolved into the soils. Therefore, knowledge of the important soil forming rocks and minerals and the processes of weathering is required.
1.4 COMPONENT OF SOILS (VOLUME BASIS)
The soil consists of four major components (Fig.1.1) i.e. mineral matter (45%), organic matter (5%), soil air (20-30%) and soil water (20-30%).
Fig.1.1 : Four major components of soils (volume basis)
1. Mineral matter
The minerals are extremely variable in size. Some are as large as the smaller rock fragments, others, such as colloids clay particles, are so small that they cannot be seen without the aid of an electron microscope.
2. Organic matter
Soil organic matter represents partially decayed and partially synthesized plant and animal residues. Such material is continually being broken down by the action of soil microorganisms. Consequently, organic matter is a transitory soil constituent and renewed constantly by the addition of plant residues.
3. Soil water
Soil water is the major component of the soil in relation to the Plant growth. The water is held within the soil pores. If the moisture content of a soil is optimum for plant growth, plants can readily absorb water. Soil water dissolves salts and makes up the soil solution, which is important as a medium for supplying nutrients to growing plants. There is an exchange of nutrients between the soil solids and the soil solution and then between the soil solution and plants roots.
4. Soil air
A part of the soil volume that is not occupied by soil particles, known as pore space, is filled partly with soil water and partly with soil air. As the pore space is occupied by both water and air, volume of air varies inversely with water. As the moisture content of the soil increases, the air content decreases and vice-versa.