SOIL, CLASS 7

SHARMA CLASSES BHARATPUR

SOIL

Soil is one of the most important natural resources. It is the uppermost layer of earth’s crust which is blackish-brown in colour. Major functions of the soil are:

1.    It supports plant growth by holding the roots firmly and supplying water and nutrients to the plants.

2.    It acts as a natural habitat for many organisms like an earthworm, fungi, bacteria, ants, etc.

3.    It is also essential for agriculture which provides us with food, clothing and shelter for all.

4.    It supplies water and nutrients to plants.

Humus

Humus is the organic component present in the soil formed by the decomposition of plant materials by microorganisms present in the soil. Humus is the rotting dead matter in the soil.

Weathering

Weathering is the breaking down or dissolving of rocks and minerals on Earths surface. Water, ice, acids, salt, plants, animals, and changes in temperature are all agents of weathering.

Weathering is a very slow process as it takes thousands of years for a huge rock to turn into fine particles. The nature of the soil depends upon the rocks from which it has been formed and types of vegetation that grow on it.

Parent Rock - The nature of the soil that is its texture and availability of minerals depends upon the rock from which it is formed. This rock is often called as the Parent Rock.

Soil Profile

Soil is formed by breaking down of rocks by the action of wind, water or climate by the process called weathering. During this process, the rocks are worn away to form small particles by long exposure to the elements of weather.

The soil profile is a vertical section through different layers of the soil.

soil profile consists of different layers of soil which are called horizon. Each horizon differs in texture, colour, depth and chemical composition. Different horizons of the soil are discussed further.

 

Horizon A

1.    This layer is also called the topsoil. It is visible to us.

2.    Humus helps in making the topsoil porous

3.    It contains large amounts of humus and minerals which makes it dark in colour.

4.    The soil is rich in nutrients because of the presence of humus.

5.    The topsoil has a soft texture and can retain water easily. That is why plants roots grow in the topsoil region.

6.    The topsoil is a home to many living organisms as well like insects, worms, beetles, rodents and moles.

Horizon B or the Middle Layer

1.    It is the next layer of the soil which does not contain much humus.

2.    The minerals are found in large quantities in this layer.

3.    This layer has a hard texture, light colour and is more compact than the topsoil.

Horizon C or Third Layer: The third layer of the soil consists of small rocks with cracks in them. These rocks are partly weathered.

Bedrock:

1.    The last layer of the soil is called the Bedrock.

2.    It contains large pieces of rocks that are not weathered or exposed to any winds or water.

3.    Bedrock cannot be dug with the help of a spade. It is very hard in texture.

How is Soil Formed?

We know that soil is formed from weathering of the parent rock and the texture of the soil depends upon the parent rocks only. This process takes time, maybe a hundred years, and then the fine soil is formed.

In the first stage of soil formation, the soil is generally non-porous in nature. Then it slowly turns into soil having air and water in the pores.

We can define soil as a mixture of rock particles and humus. Based on the size of the particles and the textures of the soil it can be divided into various types.

Types of Soil

 

Sandy Soil

·      Sandy soil has big particles that have large spaces between them.

·      The spaces between these particles are filled with air. Hence, sandy soils are called well-aerated soils.

·      Because of large spaces, water can easily penetrate through the particles of sand. Sandy soils, however, cannot hold water.

·      Hence, sandy soils are light aerated and dry in nature.

·      Sandy soils lack much nutrients hence do not support the diverse growth of plants.

Clayey Soil

·      It consists of fine particles which have less space between them.

·      Since there is not much space between the particles clayey soils are not well-aerated like sandy soils.

·      The tiny gaps between the particles although allow absorption of water in the clayey soils easily

·      They are able to hold water hence are suitable for the growth of different kinds of plants.

Loamy Soils

·      Loamy soil contains a similar amount of large and small particles in them.

·      They are combination of sandy, clayey and silty soil.

·      They also contain humus.

·      They can hold water in appropriate amounts and therefore support the growth of plants.

·      They are also called Agricultural Soils because of their fertility and appropriate texture.

·      They contain good amounts of calcium and have a high pH level.

Silt Soil

·      The silt soil particles are smaller than that of sandy soils but larger than clayey soils.

·      Silt soil can hold water to some extent because of its fine quality.

·      They are generally found near the water bodies like river banks and lakes.

·      They are rich in nutrients, highly fertile and hence are suitable for agriculture.

·      They are often mixed with other soils to improve the fertility of the soil. 

Properties of Soils

1. Percolation of water through the Soil

Percolation can be defined as the property of the soil by which it allows the flow of water through it. The rate at which water percolates or moves through soils may vary in different kind of soils. Some soils absorb water while others allow it to flow through them. The rate of percolation can be calculated by:

Percolation rate is highest in sandy soil because it is very loose. On the other hand, clayey soil is very compact and therefore has the lowest rate of percolation. The rainwater moves or reaches to well faster and in a greater amount from sandy soil.

Since clayey soil can retain water in them. These are the best soil to grow paddy because paddy requires standing water in fields. The kutcha (unpaved) road due to percolation of water becomes dry after rain while pakka road does not.

2. Moisture

Moisture is the amount of water that is present in the soil. Even a dry soil has some amount of moisture in the air. However, the clayey soil has the highest content of moisture.

3. Absorption

Every soil has a water absorption capacity which depends upon how porous the soil is. Clayey and loamy soils are most porous hence can retain water in large quantities. That is why crops can grow over these soils. Sandy soils, on the other hand, do not absorb water and hence do not support much vegetation.

The ability or capacity of the soil to absorb a limit of water is called absorption percentage. It can be calculated as follows:

Percentage of water absorbed = (Amountofwaterabsorbed(inmL)/AmountofSoil(g)) × 100

 

Different types of soil can absorb water to a different extent, i.e. some absorb more water while other absorbs less water. When we talk about the percentage of water absorbed by the soil, it means the mass of water absorbed by 100 g of soil.

Percentage of water absorbed by soil =

[ Initial weight or initial volume of water absorbed – Final weight or final volume of water absorbed/weight of soil ]×100
Weight of water absorbed by the soil = (u – v) g
Volume of water absorbed by the soil = (u – v) mL
where u is the initial volume of water/initial weight of water v is the final volume of water/final weight of water absorbed by the soil.

4. Texture

The texture is the size of particles of the soil. Different kinds of soils have a different texture.

5. Colour

Different soils have different colours as well. This is because of the minerals and nutrients present in the soil. For instance, some soils are black in colour because of the presence of humus and minerals while some soils are red in colour because they have iron in large quantities in them.

7. Air Content

Since soil is made up of particles of different sizes these particles can be loosely bound or tightly bound. The air often occupies the space in between these particles. This allows life to sustain in the soil such as microorganisms.

 

When we perform this activity with different soil samples, we will see that.

·         Sandy soil will absorb less water and allows more water to percolate.

·         A clayey soil will absorb more water but allow less water to percolate.

·         Sandy soil will absorb less water than clayey soil because of the large spaces between the soil particles. The area where, there is a lot of clay in the soil, stagnant water collects above the soil whenever it rains.

 

Soil and Crops

The type of crops that will grow in the soil depends upon these factors as well as the properties of a soil.

The component of soil along with various climatic factors determine the type of vegetation in a particular region.

·         Clayey and loamy soils are both suitable for growing cereals like wheat and gram. Such soils are good at . retaining water.

·         For paddy, soils rich in clay and organic matter and having a good capacity to retain water are ideal.

·         For lentils (masoor) and other pulses, loamy soils which drain water easily, are required.

·         For cotton, sandy-loam or loam, which drain water easily and can hold plenty of air, are more suitable.

·         Crops such as wheat are grown in the fine clayey soils because they are rich in humus and are very fertile.

 

Type of Soil	Crops Grown
Sandy	Potato, Lettuce, Corn, Peppers
Clayey	Sprouts, Broccoli, Kale, Beans, Cabbage Wheat, Gram and Paddy
Loamy	Apples, Carrots, Tomatoes, Cucumber, Lentil, Tomatoes, Corn and Oats
Sandy-loam Soil	Potatoes, Cotton and Cereal Rye

What is soil erosion?

·      When the top layer of soil gets removed it is called soil erosion.

·      The soil erosion mainly occurs when the soil is left loose without vegetation or when deforestation occurs.

·      In such a situation, strong winds and flowing water or rainwater takes away the topsoil and therefore decrease its quality.

·      Also, this kills the organisms living inside the soil.

·      The roots of the plants and trees keep the soil together and allow several microorganisms to grow and survive there. Therefore, it is always advised to plant more trees and avoid deforestation.

 

Soil Pollution
Sometimes people throw polythene bags and plastic articles in the soil. They pollute the soil and also hill the useful organisms living in the soil. Some other substances like chemicals and pesticides also pollute the soil.

To prevent the soil pollution, the use of polythene bags must be banned, and the waste products and chemicals must be treated before they are released into the soil. The use of pesticides in the agricultural field must also be minimised in order to prevent soil pollution.