Soil: Introduction to Soil, Functions of Soil, Composition of Soil, Formation of Soil, Physical Properties of Soils (For CBSE, ICSE, IAS, NET, NRA 2022)

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Introduction to Soil

  • It can be defined as a mixture of organic matter, minerals, gases, liquids, and organisms that together support life.
  • It is commonly referred to as Earth or dirt.
  • It is a product of several factors:
    • The influence of climate.
    • Relief (elevation, orientation, and slope of terrain) .
    • Organisms.
    • Soil՚s parent materials (original minerals) interacting over time.
Darkened Topsoil and Reddish Subsoil Layers

Composition of Soil

  • A typical soil is about 50 % solids (45 % mineral and 5 % organic matter) , and 50 % voids (or pores) .
  • The soil texture is determined by the relative proportions of:
    • Individual particles of sand silt, and clay that make up the soil.
  • Composition of a loam soil by percent volume:
    • Water (25 %)
    • Gases (25 %)
    • Sand (18 %)
    • Silt (18 %)
    • Clay (9 %)
    • Organic matter (5 %)

Functions of Soil

It has four major functions.

  • It acts a medium for plant growth.
  • It also acts a means of water storage, supply, and purification.
  • It acts as a habitat for organisms.
  • It acts as a modifier of Earth՚s atmosphere.

Formation of Soil

  • Soil formation is also known as pedogenesis.
  • Pedogenesis is the combined effect of physical, chemical, biological and anthropogenic processes.
  • Soil is said to be formed when:
    • Organic matter has accumulated.
    • Colloids are washed downward.
    • Leaving deposits of clay, humus, iron oxide, carbonate, and gypsum.
    • Producing a distinct layer called the B horizon.
  • When there is a gradual breakdown of rocks soil formation occurs.
  • The rocks get broken down into tiny particles through many processes such as weathering and erosion.
  • Also, soil can be obtained by adding organic matter into the sand.
  • For soil formation to take place, rocks must undergo disintegration under certain environmental conditions.

Five Classic Factors Influencing Soil Formation

Five Classic Factors Influencing Soil Formation

Water Infiltration and Movement in Soil

Water Infiltration and Movement in Soil

Types of Soil

Sandy Soil

  • This soil is light, warm, dry and tend to be acidic and low in nutrients.
  • Due to high proportion of sand with little clay, this soil is also known as light soil.
  • Natural vegetation in sandy soils is usually composed of grasses and forests.
  • They can be profitably used for a variety of cereals and vegetables.
  • These soils are common in drylands.

Clay Soil

  • It is a heavy soil type that benefits from high nutrients.
  • It remains wet and cold in winter and dry out in summer.
  • This soil can hold a high amount of water due to the spaces found in between the clay particles.
  • These soils drain slowly and take longer to warm up.
  • They swell when wetted and shrink when dried.

Silt Soil

  • It is a light and moisture retentive soil type with a high fertility rating.
  • They are well drained and hold moisture well.
  • The silt particles can be bound into more stable clumps with the addition of organic matter.
  • This soil is more prone to water erosion.

Peat Soil

  • It is high in organic matter.
  • It retains a large amount of moisture.
  • This soil is rarely found in a garden.
  • These soils are formed due to the natural accumulation of partially decayed biomass.

Chalk Soil

  • It can be either light or heavy but always highly alkaline. Calcium carbonate or lime is present within its structure.
  • This soil will not support the growth of ericaceous plants.
  • Rhododendron, Camellia, Azalea, Pieris are some of the Ericaceous plants.

Loam Soil

  • It is a mixture of sand, silt, and clay.
  • This soil is fertile, easy to work with and provide good drainage.
  • They can be either sandy or clay loam depending on their predominant composition.

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