Soil Class 7 Notes Science

Soil is not just dirt and mud, it is full of life and vital nutrients for plants. All living organisms depend on soil directly or indirectly. 

Let's learn more about soil.

What is Soil?

The uppermost layer of the Earth's surface is called soil. It is composed of sand, mud, rock particles, minerals, and humus. Humus is a layer of dead and decayed plant and animal remains.

Soil Formation

Soil formation is a slow and stepwise process. It takes thousands of years to form a thin layer of soil. It is a result of continuous weathering of rocks. Weathering is the disintegration of rocks on the Earth's surface caused by natural forces such as wind, water, frost, roots of plants, etc.
Weathering is of two types: physical and chemical.

Physical Weathering
It is a mechanical process and does not involve any change in the characteristics of the original rock. Rocks are broken down into smaller pieces. It may be caused by temperature differences, frost, growing roots of plants, movement of animals, etc.

Chemical Weathering
It is a process in which the existing minerals are broken down into new mineral components. The chemical nature of the rock changes. Water is one of the main agents of chemical weathering.

Things found in Soil
Radha took a handful of soil and placed it under a lit table lamp. Six different things that are found in soil:

• Stones or pebbles
• Sand or grit
• Silt or clay
• Water
• Air
• Organic matter

Factors Affecting Soil Formation

Soil formation is a complex process that involves a number of factors. These factors include climate, characteristics of the parent rock, and slope of land. In this chapter, we will discuss each of these factors in detail.

• Climate
  Climate plays an important role in soil formation. Climatic factors such as temperature, rain, and wind affect the process of weathering. Soil formation is fastest in hot, moist climates and slowest in cold, dry climates. In the warm and wet tropics, bacterial activity is high, resulting in soil with a rich content of humus. Desert regions have very little vegetation and animal life, and so limited decaying material is available for the soil. Lack of rainfall leads to the formation of coarse-textured soil in dry and arid regions. In areas with severe winter, bacterial activity is very slow causing dead plant and animal matter to build up, leading to slow soil formation.
• Characteristics of the Parent Rock
  The composition of the parent rock also determines the kind of soil formed. If the parent rock is rich in calcium, the soil formed by its weathering would be rich in calcium too. Other elements found in the parent rock such as potassium, iron, and magnesium also contribute to soil formation.
• Slope of Land
  The physical features of a place play an important role in the formation of soil. Hilly areas generally have a thin layer of soil because running water and wind tend to carry the soil down the slope. On flat plains, soil remains relatively undisturbed leading to the formation of a thick layer. The angle of slope also affects soil formation. Steep slopes tend to have shallow, poorly developed soils while gentle slopes have deeper, more developed soils.

Soil Profile

Soil profile is a vertical section of soil from the ground surface to the parent rock. Soil profile shows the different layers of soil or the horizons. Soil profile is formed due to the physical, chemical, and biological activities that take place during soil formation.

• Horizons of Soil Profile
  Typically, soil profile has four distinct layers or horizons. These horizons are named as O, A, B, and C, with R being the parent rock layer.
• O Horizon or Topsoil
  O horizon is the uppermost layer of soil. It consists of fine particles and is usually rich in humus. O horizon is soft, porous, and can hold water. It is dark in colour due to the presence of humus. O horizon lies just above the topsoil and is rich in organic matter.
• A Horizon or Topsoil
  A horizon is also known as topsoil. It lies just below the O horizon. A horizon is also rich in humus and is soft and porous. It is darker in colour than the B and C horizons.
• B Horizon or Subsoil
  B horizon is rich in minerals that seep down along with water. It also contains compactly packed fine particles of soil. B horizon is located below the A horizon. It is lighter in colour than the A horizon.
• C Horizon
  C horizon contains partly weathered pieces of rock. It is usually characterized by cracks and crevices. C horizon is very difficult to dig beyond.
• R Horizon or Bedrock
  R horizon is the lowermost layer of soil. It mainly consists of the parent rock. The properties of the parent rock greatly determine the composition of the soil.

Composition of Soil

Soil is a mixture of different substances that include water, sand, clay, silt, pebbles, and humus. The proportion of these substances determines the type of soil. This chapter aims to explain the constituents of soil and their properties.

The Constituents of Soil
The main constituents of soil are water, sand, clay, silt, pebbles, and humus. The differences in the proportion of these constituents lead to the formation of different kinds of soils.

"Activity
To separate the various constituents of a soil sample into different layers.
Materials needed: Soil sample, beaker, glass jar, and water.
Method: Take two-three handfuls of soil from a nearby garden. Mix it with a glass of water in a beaker. Stir the mixture well and pour it into a glass jar. Allow it to stand undisturbed for one or two hours. Observe the jar.
Observation: Formation of different layers of soil can be observed.
Conclusion: Soil is made up of various constituents that occur in layers."

Air and Water in Soil
Soil particles have empty spaces between them called pore spaces, which are filled with air or water. The amount of pores that a soil has is called porosity. Porosity depends on the size and number of pores in the soil. Larger the size of particles, larger will be the spaces between soil particles.

Types of Soil

The difference in particle size of the various soil constituents gives rise to a property called texture. The easiest way to identify the type of soil is to take some soil in hand, wet it, and roll it between your fingers. Sandy soil is very gritty and pale brown. Clayey soil makes a shiny, smooth coating on your fingers and is greyish-brown. Loamy soil is flaky and dark. Based on the proportion of sand, silt, clay, and humus present, soil can be sandy, clayey, or loamy. The differences between them will be explained in more detail in the following sections.

Properties of Soil

Soil is an essential component of the earth's ecosystem. It is a natural resource that supports plant growth and is a source of food for all living organisms. Soil has unique properties that determine its types. In this chapter, we will discuss the properties of soil that are important for plant growth and their significance.

• Absorption of Water in Soil
  Plants require water to grow, and the soil should be able to hold water for plants to grow properly. The water absorption tendency of soil determines the amount of water that a particular type of soil can absorb. Soil that can absorb more water is better for plant growth.
• Moisture in the Soil
  Soil contains moisture in the form of water droplets that condense near the mouth of the test tube when heated. The moisture content in the soil can be estimated by the look and feel of it. Soil moisture is essential to determine when to irrigate and how much water is required for proper plant growth.
• Percolation Rate of Water in Soil
  Percolation rate is the rate at which water exits the soil. Different soils have different percolation rates. The percolation rate of water in soil is significant for proper plant growth. Well-drained soil allows water to pass through it rapidly, whereas soil with poor percolation holds too much water for a long time, which can be harmful to plants.

"Activities
An activity can be conducted to determine the water absorption tendency of different types of soil. Students can measure the amount of water that different types of soil can absorb and compare their results.
Another activity can be conducted to check the percolation rate of water in soil. Students can observe the time taken for water to pass through different types of soil and compare their results.
Conclusion
The properties of soil are significant for plant growth and determine the type of soil. Water absorption tendency, soil moisture, and percolation rate are the essential properties of soil that should be considered for proper plant growth."

Soil Suitability for Crops

Soil is not uniform everywhere, and different crops grow better in different types of soil. Soil fertility is essential for crops to grow well, and it depends on various factors like nutrients, light, moisture, and temperature.

Factors Affecting Soil Fertility

• Nutrients: The soil should have adequate amounts of nutrients and minerals to support plant growth.
• Light: The amount of sunlight the soil receives affects the photosynthesis process, which is vital for plant growth.
• Moisture: The soil should have the right amount of water to support the plants.
• Temperature: The soil temperature should be appropriate for the crops to grow.

Adjustments to Soil Properties

• Alteration of pH: pH affects the ability of plant roots to absorb minerals from the soil. A neutral pH is ideal for most crops. Lime is added to acidic soils to raise the pH, while peat is added to alkaline soil to lower the pH.
• Alteration of air content: Soil with low air content is heavy and challenging to work with. Drainage and ploughing can help improve soil aeration.
• Addition of fertilizers: Fertilizers provide essential nutrients like nitrate, phosphate, potassium, and magnesium to the soil, making it more fertile.

Crop Suitability in Different States
Punjab, Uttar Pradesh, and Haryana are major wheat-producing states. Punjab, Andhra Pradesh, and Uttar Pradesh are major rice-producing states.

Soil Erosion

Soil erosion is the removal of topsoil by wind and water, which leads to a decrease in soil productivity.

Causes of Soil Erosion:

• Deforestation: Removal of trees on a large scale leaves soil particles loose and prone to be carried away by wind and water.
• Overgrazing: Repeated grazing on the same patch of land without sufficient recovery periods leads to the removal of grass, making it easy for wind and water to erode soil.
• Floods and heavy rainfall: Wash off soil, especially when there are no trees and the land is bare.
• Improper farming: Improper farming on hills makes it easy for wind and water to erode soil.

Prevention of Soil Erosion:
Soil conservation is the reduction or prevention of soil erosion. Planting of trees in large numbers on deforested land (reforestation) or in areas where there was no forest previously (afforestation). Grasses and herbs can be used to cover large patches of loose soil. Overgrazing by animals should be avoided. Animals should be moved to a different area for grazing after some time. Floods can be controlled to a large extent by building dams. Embankments or mud walls should be constructed around hill slopes or fields to stop the flow of water. Terrace farming should be adopted in hilly areas. This method involves growing suitable crops on a slope, which is cut into large steps called terraces. This reduces the speed with which water flows down, thereby reducing soil erosion.

Soil Pollution

• Soil is a valuable natural resource on Earth
• Pollution of soil is caused by human activities
• Measures must be taken to control soil pollution

Sources of Soil Pollution:

• Acid rain and acid water from factories, mines, and industries
• Improper dumping of garbage and sewage wastes in soil
• Excessive use of pesticides and fertilizers
• Waste materials such as plastic and metals
• Spilling or leakage of chemicals

Effects of Soil Pollution

• Soil pollution can lead to decreased crop production
• Polluted soil can also affect the quality of food grown in it
• It can lead to the contamination of underground water sources
• Soil pollution can also harm animals and other living organisms that depend on the soil

Control of Soil Pollution
Solid wastes should not be dumped on land. Proper measures for disposal of sewage should be adopted. Animal and domestic wastes should be used to produce biogas. Organic pesticides should be used instead of excessive chemical pesticides and fertilizers. Liquid wastes from factories and mines should be properly treated before release into water bodies.

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