Chapters :

SOIL – 01

Soil is the thin top layer on the earth’s crust comprising rock particles mixed with organic matter. Pedology is the study of soils in their natural environment. Pedogenesis is the natural process of soil formation that includes a variety of processes such as Weathering, Leaching, Calcificationetc. The Soil formation is mainly related to the parent rock material, surface relief, climate and natural vegetation.


Soils have four main constituents:
  • MINERAL MATTER – It includes all minerals inherited from the parent material as well as those formed by recombination from substances in the soil solution.
  • ORGANIC MATTER – It is derived mostly from decaying plant material broken down and decomposed by the actions of animals and microorganisms living in the soil. (Note: The end product of breakdown of dead organic material is called humus.)
  • AIR
Normally, both air and water fill the voids in soil. Air and water in the soil have a reciprocal relationship since both compete for the same pore spaces. For example, after a rain or if the soil is poorly drained, the pores are filled with water and air is excluded. Conversely, as water moves out of a moist soil, the pore space is filled with air. Thus the relationship between air and water in soils is continually changing. The ratio of the components by volume is generically indicated as:


Knowing a soil’s water, mineral, and organic components and their proportions can help us determine its productivity. Some of the important characteristics are:
  1. COLOR ( A-3)
A soil’s colour is generally related to its physical and chemical characteristics. Black  Soils rich in humus tend to be dark because decomposed organic matter is black or brown. Soils with high humus content are usually very fertile, so dark brown or black soils are often referred to as ‘rich’. [Note – Some dark soils may be dark because of other soil forming factors and may have little or no humus] Red or yellow soils typically indicate the presence of iron.
The soil texture refers to the Coarseness/Finenessof the mineral matter in the soil. It is determined by the proportion of the sand, silt and clay particles: Clay: Particle Size – diameters less than 0.002 millimetres Silt: Particle Size – diameters between 0.002 millimetres to 0.05 millimetres. Sand: Particle Size – diameters between 0.05 and 2 millimetres. [Rocks larger than 2 millimetres are regarded as pebbles, gravel, or rock fragments                 and technically are not soil particles.] The soil texture directly affects:
  • The soil water content
  • Water flow
  • Retention of nutrients
  • Extent of aeration
Note: Generally speaking, Good Soils = Clay + Humus. The clay-humus complex is essential for a fertile soil as it provides it with a high water and nutrient holding capacity. Humus acts as cement binding the soil particles together and thus reducing the risk of erosion.


While the soil texture describes the size of soil particles, soil structure refers to the arrangement of the soil particles. The way in which sand, silt, clay and humus bond together is called soil structure.  


  • PERMEABILITY – The ease with which liquids/gases can pass through rocks or a layer of soil is called permeability. It depends on the size, shape and packing of particles. It is usually greatest in sandy soils and poor in clayey soils.
  • POROSITY – The volume of water which can be held within a soil is called its porosity. It is expressed as a ratio of volume of voids (pores) to the total volume of the material.


An important aspect of soil chemistry is Acidity, Alkalinity (baseness), or Neutrality. Low pHvalues indicate an acidic soil .                 High pH indicates alkaline conditions.
  • In Arid And Semi-Arid regions, soils tend to be alkaline and soils in humid regions tend to be acidic.
  • To correct soil alkalinity and to make the soil more productive, the soil can be flushed with irrigation water.
  • Strongly acidic soils are also detrimental to plant growth, but soil acidity can generally be corrected by adding lime to the soil.


The major factors affecting the formation of soil are Relief, Parent Material, Climate, Vegetation and other life-forms and time. Besides these, human activities also influence it to a large extent. PARENT ROCK The rocks from which soils are formed are called parent materials. In most of the cases, the parent material determines the colouration, mineral composition and texture of the soil. The peninsular soils reflect the parent rock very much.
  • The ancient crystalline and metamorphic rocks which are basically Granite, gneiss and schist form red soils on weathering because they contain iron oxide.
  • Soils derived from Lava Rocks are black coloured.
  • Sandy soils are derived from Sandstone.


  • The role of climate is to vary the inputs of heat and moisture. It affects the rate of weathering of the parent rock. Hot And Humid environments, in general, witness the most rapid weathering of parent materials.
ROLE OF PRECIPITATION: In areas that experience a lot of rainfall water percolating down through soil tends to leach nutrients and organic matter out of the upper layers, unless modified by other soil components like plant roots.
  • g. the soils underlying tropical rain forests tend to be nutrient-poor because of intensive leaching due to heavy rains; most of the nutrients are stored in the lush vegetation itself.
  • Conversely, in arid regions with little annual precipitation, high rates of evaporation encourage the accumulation of salts in the soil.
ROLE OF TEMPERATURE: Temperature increases the rate of reaction,  such as chemical reactions, evapotranspiration and biological processes. Wide fluctuations in temperature, especially in the presence of water cause shrinking and swelling, frost action and general weathering in soils.
  • g. Laterite soils are found in alternate wet and dry climate.
  • In Rajasthan, both granite and sandstone give birth to sandy soil irrespective of parent rock because of high temperature and wind erosion.


Biota, in conjunction with climate, modifies parent material to produce soil.
  • The kind and amount of plants and animals that exist bring organic matter into the soil system as well as nutrient elements. This has a great effect on the kind of soil that will form.
E.g. Soils Formed Under Trees are greatly different from soils formed under grass even though other soil-forming factors are similar.
  • The Roots Of Plants also hold the soils and protect them from wind and water erosion. They shelter the soils from the sun and other environmental conditions, helping the soils to retain the needed moisture for chemical and biological reactions.
TOPOGRAPHY (Relief, Altitude and Slope) Topography is often considered a passive factor modifying the effects of climate.
  • The relief is the most important factor for soil formation in places with steep slopes like the Hilly Regions, Edges Of Plateaus
  • Soil erosion on barren slopes is rampant and it hinders soil formation. Example: Chambal ravines, higher reaches of Himalayas where there is minimal or no forest cover (most on the steep southern slopes) etc.
  • The areas of low relief or gentle slope generally experience deposition and have deep soils. Example: Indo-Gangetic plain.

 Soils can take many years to form. Younger soils have some characteristics from their parent material, but as they age, the addition of organic matter, exposure to moisture and other environmental factors may change its features. With time, they settle and are buried deeper below the surface, taking time to transform. Eventually, they may change from one soil type to another.


Conversion from rocks to soils happens via four basic processes: ADDITIONS: Most additions occur at the surface. The most obvious ones include solar energy, water controlled by climate, and organic material derived principally from the vegetation. LOSSES: Losses occur both from the surface and from the deep subsoil. For instance, water is lost by evapotranspiration and carbon dioxide by diffusion at the surface and, on a more catastrophic level, large masses of soil can be stripped by erosion.


It refers to the physical movement of material within the soil. The material can be in the solid, liquid or gaseous form, the movement can be in any direction from and to any horizon. For instance clay, organic matter and iron and Aluminium Hydrous Oxides are commonly moved from the surface horizon to a subsurface horizon.


Transformations, involve the change of some soil constituent without any physical displacement. Chemical and physical weathering and the decomposition of organic matter are included here.
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