NCERT Class 7 Science Chapter 9: Soil YouTube Lecture Handouts

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NCERT Class 7 Science Chapter 9: Soil

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BRAIN TEASERS

  • What is porosity, permeability and capillarity?

  • Is clayey soil highly porous and less permeable?

  • Which soil is best topsoil for growing plants?

  • Which soil has highest and least percolation?

  • Why are Sohagpuri earthen pots famous?

  • Which soil is best for scorpio and snails?

LET’S BEGIN!!

  • Soil

  • Provide anchorage

  • Supplies water and nutrients

  • Important for agriculture – which provides food, clothing and shelter

  • Fragrance after rain is refreshing

  • Earthworms in rainy season

  • Polythene and plastic bags pollute it and kill organisms

  • Waste products and chemicals should be treated

  • Using pesticides should be minimized

Soil Profile

  • Has distinct layers

  • The rotting dead matter in the soil is called humus

  • 👉 Particle size – clay < sand < gravel

  • Weathering – breaking down of the rocks by wind, water and climate

  • Nature of soil depends on rocks from which it is formed & type of vegetation that grows

Layers

  • Vertical section – soil profile

  • Layers are called horizons – different color, texture, depth and chemical composition

  • Layers can be seen on side of road or while digging a well

  • Uppermost horizon – dark in color with humus, makes soil fertile, soft, porous and can retain water – called topsoil or A horizon – provides shelter for living organisms; roots are embedded in topsoil

  • B-horizon – less humus, more minerals, harder, compact and middle layer

  • C-Horizon – small lump with cracks and crevices

  • Bedrock – lowermost and hard to dig

Soil Types

  • Soil is mixture of rock particles and humus

  • Bacteria, plant roots and earthworm are important part of soil

  • Sandy soil – has bigger particles; larger spacing; well aerated, light and dry

  • Clayey soil – finer particles; less air; heavy; can hold more water

  • Loamy soil – best topsoil for plant growth – mix of sand, clay and silt; silt occurs as deposits in river beds and size is between sand and clay – it has humus and right water holding capacity

- Properties of soil

- Properties of Soil

- Properties of soil

👉 Water holding capacity is decided by size of soil particles and in between spaces.

Properties of Soil

  • Percolation Rate of Water – water absorbs on kutcha road and not on pucca road

  • Highest percolation in sandy soil

  • Least percolation in clayey soil

Moisture in Soil

  • Amount of water in soil

  • On heating – water evaporates, moves up and condenses cooler inner wall

  • On hot summer day – vapor reflect sunlight and air above soil seems to shimmer

👉 Soils contains water. On heating, the water of the soil gets evaporated and soil becomes loose.

Absorption of Water by Soil

  • Water retained in soil is amount of water in cylinder minus what you start

  • Grams and kilogram are units of mass

  • A mass of 1 gram weighs 1 gram weight, and a mass of 1 kilogram weighs 1 kilogram weight.

Soil and Crops

  • Soil is affected by wind, rainfall, temperature, light and humidity – these affect soil profile and structure

  • Climatic factors and soil components determine vegetation and crops

  • Clayey and loamy soil are good for cereals like wheat and gram – good water retention

  • Paddy – soil rich in clay and organic matter are good

  • Lentils – Loamy soil which drain water easily

  • Cotton – sandy loamy and loamy soil – hold plenty of air

  • Wheat – fine clayey soil – rich in humus and fertile

  • Adding quick lime or slaked lime will make acidic soil neutral

Clayey Soil for Making Pots

  • Burnt horse dung helps open up the pores in the soil. So that water could percolate out of the matkas and surahis, evaporate and cools the water inside. You know Sohagpuri surahis and matkas are famous in far off places like Jabalpur, Nagpur, Allahabad etc.

  • Kalla (earthen frying pans)

Soil Erosion

  • Removal of top soil (due to absence of plants in topsoil) – prevent deforestation and increase green areas

  • North India rivers – bring alluvial soil – fertile and support half f population

  • Water-logged soils – roots are deprived of oxygen and harmful for plants

  • Dense forest has less soil erosion due to huge tree canopy which prevents rainwater from falling on ground

  • Benefits of soil digging

  • For enabling easy root growth;

  • For easier percolation of water;

  • For aerating the soil/enabling air to get into deeper layers of soil;

  • For removing the weeds.

  • Bore well more deep in urban areas than rural areas – due to excessive depletion in urban areas; asphalted roads and vast areas of soil are concreted. As a result, rain water cannot percolate to recharge ground water and the ground water level further decrease.

  • Where animals are found?

  • Lizard on ground surface

  • Crabs on sand and beaches

  • Rodents found in soil burrows

  • Scorpio in deep narrow holes of dry soils

  • Snails and slugs in surface of shaded moist soils

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