Class 7 Science Soil Chapter Notes

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Soil
Soil is very important natural resource and a gift from mother earth. It is capable of supporting life on earth. Soil forms a very thin layer on the surface of the earth, ranging from few mm to 3 to 4 meters. It is in this layer of the soil that the nutrients are produced and held along with air and water. These nutrients nourish the plants and support their growth. Plants, as we now provide food for all land animals directly or indirectly through different food chains. Soil holds the roots of plants firmly. Soil is also the home of many living organisms.
Thus, there would be no life on earth without soil. But surprisingly, there are places on the earth which have no soil at all. Large areas of Antarctica and the Arctic are covered with snow and have no soil for plants to grow on.

SOIL FORMATION

Soil is formed when large rocks on the surface of the earth break into smaller pieces by the action of natural agents such as rain, wind, the roots of plants, etc. Soil has not always existed on earth. In very early times, millions of years ago, the ground was mostly solid rock. It partially changed into soil by the process called weathering. Thus, the process of breakdown of large rocks into smaller pieces by action of natural force, such as sun, water, wind, glaciers, roots of plants, etc., is called weathering. Weathering is a very slow process and occurs continuously.
Some of the agents which bring about weathering of rocks are:
1. Physical components like rain, wind and temperature.
2. Biological components like plants, animals and microbes.

Physical Components

Rain: Rain water is one of the causes of the weathering of rocks. It enters the crevices of rocks. In winter, this water freezes and form ice. Since water expands on freezing, the ice in the crevices of the rocks which further break down to form even smaller pieces.

Wind: The broken pieces roll down by the force of flowing water or strong winds. These pieces further break down on hitting other rocks. The river currents break these into still smaller pieces which mix with humus to form soil.

Temperature: The rocks get heated during the day and cooled at night at places which show large variation in the day and night temperatures. Alternate heating and cooling cracks the rocks and causes weathering. The rocks expand during the day and contract at night. This continuous expansion and contraction weakens the rocks, and they crack and crumble.
Minerals present in the rocks react with the atmospheric oxygen during hot and humid climates. Oxidation produces acid and gases which further break the rocks to form soil.

Biological Components
Plants: Roots of plants can penetrate to great depth for the absorption of water and nutrients. They push their way across rocks in the earth’s crust and exert lot of pressure on the rocks. Thus, cracks are formed in the rocks and cause weathering. The acid produced by the roots of some plants also play a role in breaking down rocks.

Animals and Microbes: The organic acids formed by organisms bring about the biological weathering of the rocks. The weathered mineral matter and decomposed organic matter get mixed to from soil. Atmospheric weathering is brought about by the influence of temperature changes and frost. Chemical
weathering involves the action of atmospheric gases and acids dissolved in water. Weathering of rocks takes a very-very, long time. It takes hundred of years to produce a few centimeters of soil. Small plants such as mosses, lichens and ferns grows in small cracks and fissures in rocks. When they die, the residue mixes with the soil to become part of the humus. Humus is the nutrient rich part of soil that makes its fertile.

AGENTS OF GRADATION
Any natural force which is capable of transporting the weathered rock material is called an agent of gradation. Running water, ground water, glaciers, wind, sea waves are the chief agents of gradation. These perform the work of erosion, transportation and deposition of rock materials. They build different soil types and land forms.

Soil Profile
Soil profile is the side view of the vertical section cut through the soil to the underlying layer of solid rock (Fig. 1). The process of soil formation directly leads to the development of the soil profile. The colour of the soil changes as we dig deeper. Similarly, the size of the rock pieces also changes. Soil profile shows up different layers called horizons, which are called the A, B and C layers. These three horizons correspond to the three main layers of
soil, viz, top soil, subsoil and parent rock followed by the impervious or nonporous layer of bed rock.

Top Soil or A-Horizon
This is the upper most layer which is dark in colour containing organic matter, the humus. It is soft and porous consisting of fine particles of soil. Humus makes the soil fertile. Minerals like phosphorus, potassium, magnesium and calcium are found in this layer. These minerals are very useful nutrients for healthy growth of plants. Roots of small plants are buried in this layer. This layer also provides shelter to organism like worms, beetles and moles.

Subsoil or B-Horizon
Below the top soil is sub-soil which contains weathered rock materials silt and clay. It is generally coarse and is rich in minerals that have been washed down from the top layer. Sub-soil is poor in humus, so it is lighter in colour. It provides minerals to plants with deep roots.

Weathered Rock or C-Horizon
Below the sub-soil is the zone of infertile, partly weathered rock. It does not contain any humus and is not fertile. This layer is first stage in the soil formation process eventually forms the upper two layers.

Bed Rock or Parent Rock or D-Horizon
The fourth layer consists of unweathered, non-porous and impervious parent rock. This is known as bed rock. This layer provides as a base to other three layers. The soil forming materials are derived from weathering of this layer.

Soil Composition

About one-fifth of the area of the Earth’s surface is covered with soil. It is the topmost layer of the Earth’s crust. But not all top soil is the same. The composition varies for different types of soil and depends upon the type of rock from which it is formed. The colour and size of the soil particles also vary with the minerals present in the rocks. Some kinds of rocks break into very small particles, whereas others break into larger ones.

Some basic components present in all types of soils are:

1. Rock Particles: These are of different sizes and can be a part of clay, sand and gravel.

2. Minerals: Clay and sand contain minerals like silver, iron, magnesium, potassium, calcium necessary for healthy plant growth.

3. Humus: Humus is made up of dead and decaying organic plant and animal remains. Humus is formed by the action of microbes like bacteria and fungi on these dead organisms. Humus adds to the fertility of the soil by providing with organic nutrients. It improves the soil texture and water holding capacity. It also prevents minerals from being washed away by water.

4. Moisture: Rain water seeps through the soil and collects above the bedrock. The amount of water above bedrock determines the natural level of ground water, which is called ‘The Water Table’. All three types of soil, viz., sandy, clayey and loamy contain some water in the spaces between the soil particles.
This water is taken up to the plant leaves by the root hairs through the xylem tissue of plants. The amount of water contained in a particular type of soil is called its water holding capacity and depends on the particle size of the soil. 

5. Air: Air is present in the spaces between the soil particles. Plants need air to breathe or respire. Other living organisms present in the soil also need air to survive and perform the vital function of converting dead and decaying organic matter to humus.

Texture of Soil

Rock break into smaller pieces of different sizes. This is why the size of soil particles also vary. The texture of the soil thus depends on the relative proportions of the various types of soil particles.

Depending on particle size, soil is of three types:

Finally, depending upon the size of soil particles, soil can be classified into
Gravel 5.0 mm (diameter)
Fine gravel 2.0 mm (diameter)
Coarse sand 0.20 mm (diameter)
Fine sand 0.02 mm (diameter)
Silt 0.002 mm (diameter)
Clay Less than 0.002 mm (diameter)
Soil Types
Different geographical regions have different types of soils with their own characteristic colour, texture and contents. Six types of soils are commonly found in India, as described below:

1. Red Soil or Red Latosol: This soil is red in colour due to the presence of iron oxides. It contains quartz and clay particles but is deficient in nitrogen and humus. This soil can be enriched with nutrients by the addition of manures and fertilizers. This type of soil is found in the interior regions of Kerala and Tamil Nadu, Southern Karnataka, Andhra Pradesh, Orissa and Eastern Madhya Pradesh.

2. Black Soil or Regur: This rich black loamy soil is rich in minerals, humus and is porous in nature. It contains large proportions of iron and magnesium which are derived from basaltic rocks - the rocks formed from the lava of volcanic eruptions. This soil is ideally suited for growing sugarcane and cotton.
In India, regur soil is found in Maharashtra, Andhra Pradesh , Madhya Pradesh and Gujarat.

3. Alluvial Soil or Khadar: It is the most important and widespread of all the soil types. It covers approximately 40% of the land area . It is formed by the deposition of silt which has been brought down by the great Himalayan rivers - Sutlej, Ganga, Brahmaputra and their tributaries. In India it is found in the plains of Haryana, Punjab, Uttar Pradesh, Bihar, West Bengal, Coastal Orissa and Anthra Pradesh.

4. Desert Soil: This soil contains large proportions of sand. It is greyish brown in colour, porous in texture and is rich in soluble salts. It does not have good water holding capacity but it can become fertile when watered. Sandy soil is mainly found in the Rann of Kutchh in Gujrat and in Rajasthan.

5. Mountain Soil: This is highly fertile soil and has the highest humus content among all the soils in India.
The proportion of its contents vary from place to place. It is found in the Himalayan region and North Eastern parts of India and some parts of the Western Ghats and other hilly ranges such as the Malabar range in Kerala..

6. Laterite Soil: It is red in colour and is found in regions of heavy rainfall with highly humid tropical climates. It is clayey in texture and good for growing plantation crops like tea, coffee and coconut. In India, it is mainly found in the Westsern Ghats, parts of Tamil Nadu , Andhra Pradesh, Orissa and Assam.
Water Holding Capacity of Soil The amount of water absorbed and retained by the soil is called its water holding capacity. It depends upon the amount of humus present in the soil and the texture of the soil. Since the texture of soil varies from place to place, its water holding capacity also varies.

Sandy Soil: Sandy soil has very poor water holding capacity because of the large spaces between the soil particles.

Clayey Soil: Clayey soil has very good water holding capacity, so in areas that have a lot of clayey soil, waterlogging is common. Waterlogging mean the collection of stagnant water above the soil whenever there is rain. The water logged soil cannot hold air for plant roots to breathe, hence plants cannot grow well in it.
Loamy Soil: It has good capacity to hold and absorb water. This soil is ideal for agriculture. Soil Quality and Suitability

Quality of soil depends upon:
1. Fertility of Soil: The capacity of soil to sustain plant life is a fertility of soil. Soil fertility can be enhanced artificially by adding to the soil.
a) Chemical fertilizers
b) Biomanures like cow dung compost and wormicast to the soil.
2. Soil Particles: On the basis of the particle size, soil can be classified as gravel, fine gravel, coarse sand, fine sand, silt and clay.
3. Soil Texture: Relative proportion of different size of soil particles forms the texture of the soil. On the basis of texture, soil can be classified as sandy, clayey and loamy.
4. Water Content: Plants are propagated mainly in soil and get their water supply from soil. Chief source of water for soil is rainfall. As we have seen, the water holding capacity of different types of soils varies.

There are different ways in which water finds its way into the soil, and these are known as follows:

a) Gravitational Water: The type of water that moves down through the layers of soil due to the effect of gravity.

b) Capillary water: Water held between the soil particle. This is the water that the plants take from soil.

4. SoilAir: Air is required for respiration in roots and for release of energy.

5. Soil Colour: Colour of soil may be white, yellow, red, grey or black. Red soils are rich in iron oxides. Dark coloured soils contain more organic matter. Red and yellow colour of soil increases as we proceed towards the equator from cold or polar regions.

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