Soil – Composition and Factors Affecting Soil Formation

• The loose material or the upper layer of the mantle rock consisting mainly of very small particles and humus which can support the growth of plants is known as Soil.
• In Simple Words, Soil is the thin top layer on the earth’s surface comprising of rock particles, minerals mixed with organic matter.

• Soil mainly consist of minerals, a certain proportion of decayed organic matter, soil water, soil air, and living organisms which exist in a complex relationship with each other.
• It is usually formed from weathered rock or regolith (a layer of loose, heterogeneous material covering solid rock) changed by chemical, physical and biological process. It is mainly related to the parent rock material, surface relief, climate and natural vegetation.
• Pedogenesis is the natural process of soil formation that includes a variety of processes such as weathering, leaching, calcification etc. Pedology is the study of soils in their natural environment.
• The soil is formed by the breaking down of rocks by the action of wind, water and climate. This process is called weathering.

Composition of Soils

The exact ratio of the components of soils depends on various factors like geographical location and the past treatment of soil – by humans, by climate, by time.

Generally, Soils are mainly composed of following components –

• • Mineral matter – includes all minerals inherited from the parent rocks as well as those formed by combination 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. It is this organic portion that differentiates soil from geological material occurring below the earth’s surface which otherwise may have many of the properties of a soil. (Note: The end product of breakdown of dead organic material is called humus.)
  • Air & Water – Generally, 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 in the soil. Thus the relationship between air and water in soils is continually changing.
  • The ratio of the components by volume is generically indicated as :

How is Soil Formed?

Soil formation is a process taking many thousands of years.

The Pedogenic Processes

The above-explained conversion from rocks to soils happens via four basic processes, these soil forming processes are:

a. Accumulation of materials or Addition of Material

Materials are added to the soil such as organic matter and decomposing materials or new mineral materials deposited by the forces of ice, water or wind and they accumulate over time. 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. Some plants with the help of bacteria fix atmospheric nitrogen and ammonia compounds into the soil as nitrates.

b. Leaching and losses

Through the movement of water, wind, ice or the uptake of the accumulated materials by plants, the new particles including clay, organic matter, silt or other chemical compounds are leached and eroded away or taken up from the soil by plants. As a result, the physical and chemical compositions of the new accumulated materials together with the soil parent material are altered. Losses occur both from the surface and from the deep subsoil. Materials suspended or dissolved in water are the main forms of losses from the subsoil. A prime example is the leaching out of some carbonates, magnesium and other minerals.

c. Podsolization and translocations

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. Podsolization takes place when strong acidic solutions breakdown the clay minerals. Accordingly, aluminium, silica and iron form complex materials together with organic compounds in the soil. They commonly move from the surface horizon to a subsurface horizon. Conversely, in very dry climates salts are moved upwards in solution by capillarity, and in very cold climates solid mineral fragments are moved upwards by frost action.

d. Transformation and illuviation

Here the soil particles held in the suspension after the leaching such as clay are transformed after which they accumulate. Transformation is the chemical weathering of silt, sand, and the formation of clay minerals as well as the change of organic materials into decay resistant organic matter.

After, the clay and other accumulated materials are washed from the upper horizons and deposited in the lower horizons. The plants and animals are also responsible for transformation of the soil by physically and chemically breaking down the materials. The soil begins to take shape on its own through transformation, which improves natural drainage and nutrient composition.

All these processes occur to a greater or lesser extent in all soils. The properties that characterise one soil are the result of a particular balance among all the processes.

The two driving forces for these processes are:

• • • climate (temperature and precipitation), and
    • Organisms (plants and animals).

The Passive factors affecting Soil Formation includes:

• • • Parent material is usually a rather passive Factors affecting Soil Formation because parent materials are inherited from the geologic world.
    • Topography (or relief) is also rather passive Factors affecting Soil Formation, mainly modifying the climatic influences of temperature and precipitation.

Factors Affecting Soil Formation

Factors that Affecting soil formation in Indian Conditions are

• • Parent Material
  • Relief
  • Climate
  • Natural Vegetation

a. Parent Material

• • Parent Material are those rocks from which soil formation takes place.
  • The parent material determines the coloration, mineral composition and texture of the soil.
  • In some cases, the soil formed may or may not have the same physical properties of the parent rock. For Eg. Alluvial fertile soils consist of fine silts and clay. These soils have little relation with the original rocks. On the other hand, the soils of peninsular plateau are generally coarse-grained and are closely related to the parent rocks. The peninsular soils are generally less fertile.
  • The surface rocks are exposed to the process of weathering. In this process, the rocks are converted into fine grains and provide a base for the soil formation.
  • In Indian Conditions, parent material is generally categorized into:
    • • Ancient crystalline and metamorphic rocks
      • Cuddapah and Vindhyan rocks
      • Gondwana rocks
      • Deccan basalts
      • Tertiary and Mesozoic sedimentary rocks of extra peninsular India
      • Recent and Sub-recent rocks

1. Ancient crystalline and metamorphic rocks

• • • They are the Oldest rocks which were formed due to solidification of molten lava (pre-Cambrian era)
    • They form the ‘Basement Complex’ of peninsular India.
    • They are basically granite’s, gniesse’s and schist’s.
    • These rocks are rich in ferromagnetic materials and on weathering give rise to Red Soil.
    • The red color of these soils is due to the presence of iron oxide.

2. Cuddapah and Vindhyan rocks

• • • They are ancient sedimentary rocks (4000 m thick).
    • On weathering they give calcareous (containing calcium carbonate) and argillaceous (consisting of or containing clay) soils.
    • The soil is mostly devoid of metalliferous minerals.

3. Gondwana rocks

• • • These rocks are also sedimentary in nature and they are much younger.
    • On weathering they give rise to comparatively less mature soils of more or less uniform character but of low fertility.

4. Deccan basalts

• • • Volcanic outburst over a vast area of the Peninsular India many hundred million years ago gave rise to Deccan Traps. They are covered with Basaltic lava.
    • Basalts are rich in titanium, magnetite, aluminium and magnesium.
    • The weathering of these rocks has given rise to soils of darker color.
    • The Soil is fertile with high moisture holding capacity and is popularly known as ‘regur’ or black cotton soil.

5. Tertiary and Mesozoic sedimentary rocks

• • • Rocks of extra peninsular (plains and Himalayas) India have given rise to soils with high porosity.
    • These soils are generally immature recent and sub recent rocks, result in alluvial soils on weathering.

b. Topography (Relief, Altitude and Slope)

• • Topography is often considered a passive factor modifying the effects of climate.
  • The relief is the most important Factors affecting Soil Formation in places with steep slopes like the hilly regions, edges of plateaus etc.
  • The Degree of Slope also largely determines the fertility of Soil.
  • Topography redistributes the water reaching the soil surface. Runoff from uplands creates wetter conditions on the lowlands, in some cases saline sloughs or organic soils. Thus, as a redistributor of the climate features, topography affects soil processes, soil distribution and the type of vegetation at the site.
  • 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: North Indian Plains.
  • The exceptions in the plateau are river basins where the soil layers are sufficiently deep.

c. Climate

• • 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.
  • Temperature and rainfall are the most important factors in soil formation.
  • 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.
  • Role of Temperature : temperature controls the form of water falling onto the soil surface as well as in the soil. Also, it increases the rate of reactions, 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.
  • Two different parent materials may develop the same soil in the same type of climate. Similarly, the same parent material may produce two different types of soils in two different types of climates. For Eg. –
    • • The crystalline granites produce laterite soil in relatively moist parts of the monsoonal region and non-laterite soil in drier areas.
      • Hot summer and low rainfall develops black soil as is found in some parts of Tamil Nadu irrespective of the parent rock.
      • In Rajasthan, both granite and sandstone give birth to sandy soil under arid climate.
  • In cold climates of the Himalayan region, the process of vegetation decay is very slow and the soils are acidic in nature.

In arid and semi-arid regions, soils are of Light color –

• • In arid and semi-arid regions, evaporation always exceeds precipitation. There is little vegetation and the soils badly lack humus content. Hence the soils are invariably of light color.
  • In Rajasthan and the adjoining arid and semi-arid regions, excess of evaporation makes soils lime accumulating. Hence the soil is pedocal in nature.

In areas of heavy rainfall and high temperature, the soils are red or lateritic –

• • Torrential rainfall during the rainy season washes the upper soil and leaches the materials into deeper horizon.
  • During the dry summer season evaporation exceeds precipitation and through capillary action iron and aluminium oxides are transported to the surface making the soil red.
  • In areas of alternate wet and dry climate, the leached material which goes deep down in the horizon is brought up and the blazing sun bakes the top soil so hard that it resembles a brick. Therefore, this soil is called lateritic which literally means brick.

d. Natural Vegetation

• • Natural vegetation reflects the combined effects of relief and climate.
  • The formation and development of soil is very much influenced by the growth of vegetation.
  • The decayed leaf material adds much needed humus to soil thereby increasing its fertility.
  • The densely forested areas contain some of the best soils in India. There is a close relationship between the vegetation types and soil types in India.

e. Biota (Flora, Fauna and Microorganisms)

• • Biota, in combination 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. For Eg. 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.

f. Time

• • Time is an important Factors affecting Soil Formation as 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.

That’s it for this introductory post on Soil and Composition and Factors affecting Soil Formation.

In the next post (Click here), we would study in detail Soil Profile, Characteristics and Types of Soil in India.