CBSE – 7th Standard Science

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Chapter 9 – Soil

CHAPTER 9

SOIL

  • Soil is one of the most important natural resources.
  • It supports the growth of plants by holding the roots firmly and supplying water and nutrients. It is the home for many organisms.
  • Soil is essential for agriculture.
  • Agriculture provides food, clothing and shelter for all.
  • Soil is thus an inseparable part of our life.
  • The earthy fragrance of soil after the first rain is always refreshing.
  • Polythene bags and plastics pollute the soil.
  • They also kill the organisms living in the soil.
  • That is why there is a demand to ban the polythene bags and plastics.
  • Other substances which pollute the soil are a number of waste products, chemicals and pesticides.
  • Waste products and chemicals should be treated before they are released into the soil. The use of pesticides should be minimised.
  • The rotting dead matter in the soil is called humus.

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  • We probably know that the soil is formed by the breaking down of rocks by the action of wind, water and climate. This process is called weathering.

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  • The nature of any soil depends upon the rocks from which it has been formed and the type of vegetation that grows in it.
  • A vertical section through different layers of the soil is called the soil profile.

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  • Each layer differs in feel (texture) colour, depth and chemical composition. These layers are referred to as
  • Soil fertile and provides nutrients to growing plants.
  • This layer is generally soft, porous and can retain more water. It is called the topsoil or the A-horizon.

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  • If we look at the sides of a recently dug ditch, we can see the inner layers of the soil, too.
  • Such a view enables us to observe the soil profile at that place.
  • Soil profile can also be seen while digging a well or laying the foundation of a building.
  • It can also be seen at the sides of a road on a hill or at a steep river bank.
  • The uppermost horizon is generally dark in colour as it is rich in humus and minerals. The humus makes the
  • This provides shelter for many living organisms such as worms, rodents, moles and beetles.
  • The roots of small plants are embedded entirely in the topsoil.
  • The next layer has a lesser amount of humus but more of minerals.
  • This layer is generally harder and more compact and is called the B-horizon or the middle layer.

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  • The third layer is the C-horizon, which is made up of small lumps of rocks with cracks and crevices.

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  • Layer is the bedrock, which is hard and difficult to dig with a spade.

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  • Weathering of rocks produces small particles of various materials.
  • These include sand and clay. The relative amount of sand and clay depends upon the rock from which the particles were formed
  • The mixture of rock particles and humus is called the soil.
  • Living organisms, such as bacteria, plant roots and earthworm are also important parts of any soil.

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  • The soil is classified on the basis of the proportion of particles of various sizes. If soil contains greater proportion of big particles it is called sandy soil.
  • If the proportion of fine particles is relatively higher, then it is called clayey soil.
  • If the amount of large and fine particles is about the same, then the soil is called loamy. Thus, the soil can be classified as sandy, clayey and loamy.
  • The sizes of the particles in a soil have a very important influence on its properties.
  • Sand particles are quite large.
  • They cannot fit closely together, so there are large spaces between them.
  • These spaces are filled with air.
  • We say that the sand is well aerated.
  • Water can drain quickly through the spaces between the sand particles. So, sandy soils tend to be light, well aerated and rather dry.
  • Clay particles, being much smaller, pack tightly together, leaving little space for air.
  • Unlike sandy soil water can be held in the tiny gaps between the particles of clay. So clay soils have little air. But they are heavy as they hold more water than the sandy soils. The best topsoil for growing plants is loam.
  • Loamy soil is a mixture of sand, clay and another type of soil particle known as silt.
  • Silt occurs as a deposit in river beds.
  • The size of the silt particles is between those of sand and clay.
  • The loamy soil also has humus in it. It has the right water holding capacity for the growth of plants.
  • Climatic factors, as well as the components of soil, determine the various types of vegetation and crops that might grow in any 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, sandyloam or loam, which drain water easily.
  • Crops such as wheat are grown in the fine clayey soils, because they are rich in humus and are very fertile.
  • The removal of land surface by water, wind or ice is known as erosion.
  • Plant roots firmly bind the soil. In the absence of plants, soil becomes loose.

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  • So it can be moved by wind and flowing water.
  • Erosion of soil is more severe in areas of little or no surface vegetation, such as desert or bare lands.

Percolation rate of water in soil: The phenomenon of absorption of water by soil is termed as percolation. The rate of absorption is different for different types of soils. The rate of absorption of a soil depends on its composition. A soil with more percolation rate can hold water for longer time. On the other hand a soil with poor percolation rate will hold water for longer time. Percolation rate helps in selection of suitable soil for crop growth.
Percolation rate =Amount of water (ml)/Percolation time (min)

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