1. Soil -
ü Soil is one of the most important natural resources.
ü It is the uppermost layer of earth’s crust which is blackish-brown in colour.
ü Major functions of the soil are-
n It supports plant growth by holding the roots firmly and supplying water and nutrients to the plants.
n It acts as a natural habitat for many organisms like an earthworm, fungi, bacteria, ants, etc.
n It is also essential for agriculture which provides us with food, clothing and shelter for all.
n It supplies water and nutrients to plants.
2. Types Of Soil -
ü The weathering of rocks produces small particles of various materials, these include sand and clay.
ü The relative amount of sand and clay depends upon the rock from which the particles were formed.
ü The rock particles present in soil are of different sizes and chemical compositions.
ü On the basis of sizes, the rock particles present in soil can be divided mainly into four major groups-
(i) Clay, These are formed by the smallest rock particles. These are so small that we cannot see a single clay particle and it feels smooth.
(ii) Silt, These are made up of somewhat bigger rock particle than clay. It is not so smooth as clay. Silts are found deposited at river beds. Floods deposit the silt from rivers in the field.
(iii) Sand, These are made up of larger particles enough than that of silt and can be easily seen. These are coarse to touch due to their larger size.
(iv) Gravel, These are the largest sized rock particles that are present in the soil. These are the tiny stones that are present in topsoil in very small quantity.
ü The soil in different geographical areas contains different proportions of sand, clay and silt in them. It also contains a small amount of hummus in them.
ü Depending upon its composition, soil can be classified as sandy soil, clayey soil and loamy soil. The size of the particles in the soil influences its properties.
Figure 01- Types of soil
(i) Sandy soil, These contain mainly sand. These are quite large particles. There are large spaces between them which are filled with air. Water can drain quickly through these spaces. The sandy soil tends to be light, well aerated and rather dry. Since it is aerated, can be ploughed easily and cannot retain water, so it is not good for plants. Since it is light, it can be easily blown away if left bare. It does not possess humus, thus it is less fertile. It can be made fertile by adding manure in them which increases its water-holding capacity. This soil is not sticky and therefore, these cannot be used to make pots, bricks, toys and statues.
(ii) Clayey soil, Clay particles are smaller and packed tightly so that it leaves a little space for air. Water drains very slowly through clayey soil. These also contain very little humus. It is heavier than sandy soil. Clayey soil is more fertile than sandy soil.
The fertility of clayey soil can be improved by adding some sand and humus to it. Clayey soil is very sticky and can be used to make pots, bricks, toys, statues, etc.
(iii) Loamy soil, It is a mixture of sand, clay and silt with a small amount of humus in it. Therefore, it is very fertile and the best topsoil for growing plants. It has water holding capacity and excess water can also be drained out through it easily. It is a smooth, partially gritty and sticky soil.
3. Soil Profile -
ü Soil profile consists of different layers of soil which are called horizon.
ü Each horizon differs in texture, colour, depth and chemical composition. Different horizons of the soil are discussed further.
A-Horizon or Topsoil
It is the uppermost layer of soil and is usually dark in colour because of the presence of minerals and humus in it. It provides the nutrients in plants since the roots of most plants are confined to this layer. Humus is the dark brown coloured layer which consists of dead, rotting remains of plants and animals.
Soil Class 7 Notes Science Chapter 9
Humus helps in making the topsoil porous. This layer is generally soft and retains more water in it. This layer provides shelter for many living organisms like earthworm, rodents, moles and beetles.
B-Horizon or Subsoil
This layer of soil lies just below the topsoil. It is made up of slightly bigger rock particles than that of the topsoil. It does not have much humus and is lighter in colour than topsoil. It is somewhat harder and more compact than the topsoil. This layer is less fertile than topsoil and contains few living organisms. The subsoil is rich in minerals and iron oxides.
C-Horizon or Substratum
The layer of soil which lies just below the subsoil is called C-horizon. It consists of small pieces of broken rocks with cracks and crevices, formed by the weathering of bedrock or parent rock.
D- Bedrock or Parent Rock
Below the C-horizon un-weathered solid rock is found which is called as bedrock. It is non-porous and can produce the soil over a long period of time. Rainwater gets collected over it to form the water table.
This layer is hard enough and is difficult to dig with a spade.
Figure 02- Horizons of soil
4. Important Soil Dynamics -
(a) Percolation Rate of Water in Soil- Soil is porous, i.e. it has tiny pores in it. When water is poured over it, then some water gets absorbed in the soil and rest passes down the soil. The process of passing down water slowly through the soil is called percolation of water. Percolation rate is the amount of water (in mL) that is percolated through the soil in unit time, i.e. in minutes. The percolation rate differs in different soil types.
ü The rate of percolation can be calculated by using the following formula-
Figure 03- Measuring rate of percolation
ü For example, if water in bottle ‘A’ percolates in 20 min, in ‘B ’, it percolates in f 15 min in bottle. While in bottle ‘C’, it percolates in 25 min, then the percolation rate (mL/min) will be calculated as follows-
n For bottle ‘A’, Rate of percolation = 200mL20min =10 mL/min
n For bottle ‘B Rate of percolation = 200mL15min = 13 mL/min
n For bottle ‘C’, Rate of percolation = 200mL25min = 8 mL/min
ü 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.
ü 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.
(b) Moisture in Soil - The soil contains some water in it which is called soil moisture. Usually, moisture is present as a thin film around the soil particles. This moisture is absorbed by the roots of plants. Thus, the moisture content of the soil is very important for the growth of crops.
Figure 04- Removing moisture from soil
(c) Absorption of Water by Soil- Soil contains moisture in it but it can still absorb or soak a lot of water. But soil has the limit to absorb water in it. 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 = Amount of water absorbed(mL)/Amount of Soil(g) × 100
ü 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.
Figure 05- Abortion of moisture
(d) Water Retention - The ability of soil to hold water is called water retention. The space between soil particles is called pores provide the passage for gases and moisture within the soil. The ability of soil to retain water is strongly related to the particle size. Water molecules hold more lightly to the fine particles of the clayey soil than the coarser particle of sandy soil.
ü Sandy soil will absorb less water and allows more water to percolate.
ü A clayey soil will absorb more water but allow less water to percolate.
ü Sandy soil will absorb less water than clayey soil because of the large spaces between the soil particles.
ü The area where, there is a lot of clay in the soil, stagnant water collects above the soil whenever it rains.
5. Soil And Crops-
ü Different types of soil are found in different parts of India.
ü Soil is mainly affected by wind, rainfall, temperature, light and humidity.
ü Some climatic factors also affect the soil profile and bring changes in the soil structure.
ü The plants that grow on the surface of the earth are called vegetation.
ü It includes green grass, herbs, shrubs, bushes, crop plants and trees.
Types of Soil | Crop Grown |
Clayey Soil | Wheat, Gram and Paddy |
Loamy Soil | Lentil, Tomatoes, Corn and Oats |
Sandy-loam Soil | Potatoes, Cotton and Cereal Rye |
Table 01- Soil and crop grown
ü Clayey and loamy soils are both suitable for growing cereals like wheat and gram. Such soils are good at . retaining water.
ü For paddy, soils rich in clay and organic matter and having a good capacity to retain water are ideal.
ü For lentils (masoor) and other pulses, loamy soils which drain water easily, are required.
ü For cotton, sandy-loam or loam, which drain water easily and can hold plenty of air, are more suitable.
ü Crops such as wheat are grown in the fine clayey soils because they are rich in humus and are very fertile.
6. Soil Erosion-
ü The removal of land surface by water wind or ice is known as erosion.
ü The topsoil is very fertile and in the absence of it, the plants cannot grow.
ü In the absence of plants, the soil becomes loose. As the plant roots bind to the soil.
ü Soil erosion is mainly caused by the large scale cutting of forest trees and plants.
ü This process of cutting down of trees is called deforestation.
ü Erosion of soil is more severe in the areas of little or no surface vegetation like the desert or barren land.
ü Therefore, cutting of trees and deforestation must be prevented.
ü The effects of soil erosion are, famines, flood desertification and damage or spoilage of environment.
Figure 06- Soil erosion
NCERT PRACTICE QUESTIONS
1. In addition to the rock particles, the soil contains
(i) air and water (ii) water and plants (iii) minerals, organic matter, air and water (iv) water, air and plants
Ans.- (iii) minerals, organic matter, air and water.
2. The water holding capacity is the highest in
(i) sandy soil (ii) clayey soil (iii) loamy soil (iv) mixture of sand and loam
Ans.- (ii) clayey soil
3. Match the items in Column I with those in Column II-
Ans.-
Column I | Column II |
(i) A home for living organisms | (b) All kinds of soil |
(ii) Upper layer of the soil | (c) Dark in colour |
(iii) Sandy soil | (a) Large particles |
(iv) Middle layer of the soil | (e) Lesser amount of humus |
(v) Clayey soil | (d) Small particles and packed tight |
4. Explain how soil is formed.
Ans.- Soil is formed due to the weathering of rocks. Weathering is a process in which physical breakdown and chemical decomposition of minerals takes place primarily by wind, water and climatic changes. In the weathering process, rocks are converted to small pieces, which eventually turn to soil particles to form a layer of soil.
5. How is clayey soil useful for crops?
Ans.- It has excellent water holding capacity. Clayey soil is rich in organic matter. Clayey and loamy soils are suitable for growing cereals like wheat. Water holding-capacity of clayey soil supports the growth of crops like paddy which require more water to grow
6. List the differences between clayey soil and sandy soil.
Ans.-
Clayey Soil | Sandy Soil |
1. Particles are finer | Particles are larger |
2 Particles are tightly packed | Particles are loosely packed |
3. Holds a good amount of water | Water holding capacity is low |
4. It is heavy in weight | It is light in weight |
5. Rich in humus and organic nutrients | Not rich in humus and organic nutrients |
6. Very little air is trapped between the particles | More air is trapped between the particles |
7. Sketch the cross-section of soil and label the various layers.
Ans.-
8. Razia conducted an experiment in the field related to the rate of percolation. She observed that it took 40 min for 200 mL of water to percolate through the soil sample. Calculate the rate of percolation.
Solution:
Amount of water = 200 ml
Percolation time = 40 minutes
Percolation rate = 5ml/min
9. Explain how soil pollution and soil erosion could be prevented
Ans.- (a) Soil pollution can be controlled by the following measures:
· By reducing the use of plastics, we can reduce soil pollution.
· By controlling the use of chemical fertilisers and pesticides, we can control soil pollution.
(b) Soil erosion can be controlled by taking the following steps
· Planting more and more trees will result in the reduction of soil erosion.
· By stopping deforestation and avoiding overgrazing of animals.
10. Solve the following crossword puzzle with the clues given-
2. Plantation prevents it. 5. Use should be banned to avoid soil pollution. 6. Type of soil used for making pottery. 7. Living organism in the soil. 1. In desert soil erosion occurs through. 3. Clay and loam are suitable for cereals like. 4. This type of soil can hold very little water. 5. Collective name for layers of soil.
Ans.- Across - 2. Erosion 5. Polythene 6. Clay 7. Earthworm
Down - 1. Wind 3. Wheat 4. Sandy 5. Profile
Junior Science Topic - Soil. Chapter notes, video lectures, power point presentation and NCERT exercise solution. (Class 7, CBSE syllabus, chapter 09)
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