The study of electrochemistry is an integral part of Chemistry Topics, as it explores the relationship between electricity and chemical reactions.
What is Soil profile? and Explain Soil Profile Nature Factors
The uppermost layer of earth’s crust (in which plants grow) is called soil (see picture above). Soil is a dark brown (or black) solid material which is a mixture of‘rock particles of various sizes’ and decayed plant and animal matter called ‘humus’. Soil also contains air, water and countless living organisms.
Some of the living organisms which are present in the soil are bacteria, fungi, insects (such as ants and beetles), worms (like earthworms), rodents, moles and plant roots. Many of the living organisms present in soil are too small to be seen with naked eyes. Thus, soil is the home for many living organisms. Soil is called mitti in Hindi. Soil is an important part of land. We can see soil all around us.
Soil is one of the most important natural resources. In fact, soil is essential for the existence of life on the earth. It is in the soil that the plants and trees grow. Soil supports the growth of plants (and trees) by holding their roots firmly and by supplying them with water and nutrients.
If there were no soil on the earth, there could be no grass, no crop plants, no trees and hence no food for us or other land animals. Thus, soil is essential for agriculture. Agriculture provides us food, clothing and shelter (housing). Some of the important uses of soil are given below :
- Soil is used for growing food (like grains, pulses, fruits and vegetables, etc.).
- Soil is used to grow trees for obtaining wood for building purposes (timber), for burning as fuel (firewood) and for making paper.
- Soil is used to grow cotton plants which give us cotton clothes. Soil is also used to grow mulberry trees for rearing silkworms which provide us silk for making silk clothes.
- Soil is used to make bricks and mortar for building houses.
- Soil is used to make earthenware or pottery (such as matkas, surahis, etc.), crockery (cups and plates), toys and statues, etc.
Before we describe how soil is formed, we should know the meaning of the terms ‘weathering’ and ‘humus’. The process by which huge rocks are broken down into small particles by the action of sun’s heat, wind, rain and flowing river water, etc., is called weathering. It is called weathering because the rocks are worn away to form small particles by long exposure to the elements of weather (such as heat, wind, rain, etc.).
Weathering of rocks is a very, very slow process. It takes thousands of years to weather huge rocks (or break down huge rocks) into fine particles fit to make soil. When plants and animals die, their bodies decompose in nature to form organic matter. The organic matter formed by the decomposition of dead plants and animals by the micro-organisms (like certain bacteria and fungi) is called humus.
Humus contains nutrients (like nitrogen and phosphorus, etc.) which are needed by the plants for their growth. Keeping these points in mind, we can now describe how soil is formed.
How Soil is Formed
Soil is formed from rocks by the process of weathering. In weathering, rocks are broken down very slowly by the action of sun’s heat, wind, rain, flowing river water, etc., to form tiny rock particles. These tiny rock particles then mix up with humus to form fertile soil. The nature of any soil depends on the rocks from which it has been formed and the type of vegetation that grows in it.
If we look carefully on the sides of a trench made by digging soil from the surface of earth downwards, we will find that the soil consists of three different layers. A vertical section (or cutting) through the soil showing the different layers of soil is called ‘soil profile’. In most simple words, the side view of dug up soil is called soil profile. Soil profile consists of three different layers of soil. Each layer of soil is called a horizon. The three layers of soil in the soil profile are :
- A-horizon (or Top-soil)
- B-horizon (or Sub-soil)
- C-horizon (or Sub-stratum)
The soil profile (consisting of three different layers of soil) is shown in Figure. Each layer of soil differs in depth, colour, texture (feel) and chemical composition. We will now describe all the three layers of soil in detail, one by one.
1. A-Horizon (or Top-Soil)
The top layer of soil is called A-horizon (see Figure 1). The top layer of soil is also called top-soil. Top-soil is dark in colour. This is because top-soil is rich in minerals and humus. The plant roots grow in the top-soil. The top-soil contains many living things. For example, top-soil is the home of many living organisms like insects (such as ants and beetles), worms (such as earthworms), rodents and moles, etc.
Bacteria and fungi also occur in top-soil. Top-soil contains a lot of decayed dead plants and animal remains. In other words, top-soil contains a lot of humus. This humus makes the top-soil very fertile. The top-soil is soft and porous, and can hold more of water. The top-soil is rich in minerals which the plants need for growth. In fact, the plants get all the essential nutrients from the top-soil. Top-soil is the most useful part of the soil.
2. B-Horizon (or Sub-Soil)
The layer of soil which is just below the top-soil is called B-horizon (see Figure 1). It is also known as sub¬soil. The sub-soil is made up of slightly bigger rock particles than that of top-soil. It is somewhat harder and more compact than the top-soil. Sub-soil is also lighter in colour than the top-soil. The sub-soil contains very little living organisms. The roots of some of the trees are, however, able to reach sub-soil. The sub-soil has very little humus (decayed organic matter). Due to this, sub-soil is much less fertile as compared to the top¬soil. Sub-soil is, however, rich in soluble minerals.
3. C-Horizon (or Sub-Stratum)
The layer of soil which is just below the sub-soil is called C-horizon (see Figure 1). It is also called sub-stratum. Sub-stratum is made up of small lumps of broken rocks (or stones) formed by the partial weathering of bed-rock (or parent rock). In other words we can say that sub-stratum consists of partially weathered rocks.
In the sub-stratum part of soil, the rock pieces are still breaking down to form smaller and smaller particles. Below the C-horizon we have unweathered solid rock called bed-rock (or parent rock) (see Figure 1). It is this bed-rock (or parent rock) which has produced the soil over a long period of time.
We usually see the top surface of the soil, not the layers below it. We can, however, see the soil profile at a place (showing the inner layers of soil) in the following situations:
- Soil profile can be seen by looking at the sides of a recently dug trench (or ditch).
- Soil profile can be seen while digging a well or foundation of a building.
- Soil profile can be seen at the sides of a road on the hill or a steep river bank.
All these situations give us a side view of soil which tells us that soil consists of three different layers. Wind, temperature, rainfall, light and humidity are some important climatic factors which affect the soil profile and bring changes in the soil structure.
The most important part of soil for us is the top-soil. So, we will discuss the top soil in detail. Please note that in the following discussion when we talk of soil, it will actually mean top-soil (which is the most fertile soil). We will now discuss the composition of soil.
Composition of Soil
The soil is mainly made up of different sized rock particles and humus. In addition to their other functions, the rock particles present in soil also provide mineral salts needed for plant growth. Air, water and living organisms are also essential components of a fertile soil.
Thus, the soil is made up of six components : Rock particles (of different sizes), Minerals, Humus (Organic matter), Air, Water and Living organisms. The rock particles present in soil are of different sizes and chemical compositions. On the basis of their sizes, the rock particles present in soil can be divided mainly into four groups : Clay, Silt, Sand and Gravel. These are described below.
(i) The smallest rock particles present in soil form clay. Thus, clay has the smallest sized rock particles in it. Since the size of clay particles is very small, we cannot see single clay particles. Because of its very small particles, clay feels smooth to touch. Clay is known as ‘chikni mitti’ in Hindi.
(ii) The rock particles in soil which are a little larger than clay particles form silt. Thus, silt is made up of rock particles somewhat bigger than that of clay. Due to its slightly bigger sized particles, silt is not so smooth. Actually, the size of silt particles is in-between that of clay and sand. The size of silt particles is bigger than that of clay particles but smaller than those of sand particles. Silt occurs as a deposit in river beds. Floods in rivers deposit the silt from rivers in the fields. Silt is known as ‘gaad’ in Hindi.
(iii) The rock particles in soil which are larger than silt particles form sand. Thus, sand is made up of particles larger than that of silt. Being quite large, sand particles can be easily seen by us. And because of its large sized particles, sand is coarse to touch. Sand is called ‘reit’ or balu’ in Hindi.
(iv) The largest sized rock particles present in soil are called gravel. Gravel is a kind of tiny stones. The amount of gravel present in a good top-soil is very small. Gravel is known as ‘kankad’ or ‘bajri’ in Hindi.
We will now describe an activity to show that soil consists mainly of rock particles of different sizes and humus.
To Show that Soil Consists Mainly of Rock Particles of Different Sizes and Humus
We take a tall gas jar and fill it three-fourths with water. Add about 200 grams of soil into the water in the gas jar. Stir the contents of the gas jar with a glass rod to mix the soil and water completely. Allow the gas jar to stand undisturbed until the soil has settled into different layers (see Figure).
If we look carefully at Figure, we will find that the rock particles present in the sample of soil settle at the bottom of the gas jar. The rock particles of different sizes form different layers in the gas jar on the basis of their density (or heaviness). The largest particles present in the soil called ‘gravel’ being heaviest, form a layer at the bottom of the gas jar (see Figure).
The sand particles being lighter than gravel form a layer above the gravel. The silt particles being still lighter form a layer above the sand particles. And clay particles being the lightest rock particles, form a layer above the silt (see Figure). The humus present in soil is lighter than water, so humus floats on the surface of water in the gas jar (see Figure).
From this activity we conclude that the soil is made up mainly of the rock particles (like clay, silt, sand and gravel), and humus. In other words, the major components of soil are rock particles and humus.