Soils | Biogeography

  • Soil is the thin layer of material covering the earth’s surface and is formed from the weathering of rocks.
  • It is made up mainly of mineral particles, organic materials, air, water and living organisms—all of which interact slowly yet constantly.
  • Most plants get their nutrients from the soil and they are the main source of food for humans, animals and birds. Therefore, most living things on land depend on soil for their existence.
  • Soil is a valuable resource that needs to be carefully managed as it is easily damaged, washed or blown away.
  • If we understand soil and manage it properly, we will avoid destroying one of the essential building blocks of our environment and our food security.

The early phase of soil formation starts by disintegrating the rock under the influence of climate.

    • Rainwater will dissolve rock elements, temperature fluctuations will cause cracks and fissures in the rocks.
    • Freezing and thawing of water captured in the rock will widen existing cracks and cavities.
    • Pioneer vegetation, at first lichens, will settle and their roots will further loosen the rock.
    • Moreover, decaying plant debris will produce organic acids, which further disintegrates the rock.
    • Organic matter will start to accumulate and be mixed with the mineral material provided by the rock.
  • Over time, rock minerals will be dissolved or transformed.
  • Elements released from the rock will precipitate and new minerals may be formed. For example, iron will be oxidized and precipitate as iron oxides or hydroxides, giving the soil reddish or yellowish-brownish colours.
  • Soil fauna will settle and mix (‘homogenize’) the soil. The soil will grow in depth through newly formed soil material at the bottom.
  • The soil matures.
  • Given sufficient time under stable environmental conditions, soils will reach a steady state, whereby soil build-up matches their breakdown.
  • Production of humus from decaying vegetation debris will equal its consumption by soil microbae, fauna and flora.
  • Transformation of rock minerals into soil minerals will keep pace with the removal of earlier formed soil minerals.
  • Slow surface wash of topsoil is matched by new formation of soil material from the bedrock.
  • The soil has aged.

Five soil-forming factors

  • Active factors: Which represent agents that supply energy that act upon the mass for process of soil formation – They are climate and biotic organisms.
  • Passive factors: Which represent the source of the soil-forming mass and conditions affecting it – They are parent material, relief, and time.


  • Temperature and rainfall are the most important factors in soil formation.
  • They determine the effectiveness of weathering of the parent material, the quantity of water seeping through the soil and the type of micro-organisms present therein.
  • 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.
  • 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 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 colour.
  • In Rajasthan and the adjoining arid and semi-arid regions, excess of evaporation makes soils lime accumulating. Hence the soil is pedocal in nature [Pedocal is a subdivision of the zonal soil order. It is a class of soil which forms in semiarid and arid regions. It is rich in calcium carbonate and has low soil organic matter].
  • In cold climates of the Himalayan region, the process of vegetation decay is very slow and the soils are acidic in nature.

Biotic organisms:

  • 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.
  • Vegetation aids control of soil erosion
  • They facilitate percolation and drainage and bring about a greater dissolution of minerals through the action of carbon dioxide and other acidic substances.
  • Fauna: Burrowing animals, rodents, earthworms, ants etc., are highly important in soil formation. Burrowing animals cause constant mixing within the soil profile.
  • The role of microorganisms as soil formers is intimately related to humification and mineralizations.

Parent material:

  • 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.
  • In some cases, the soil formed may or may not have the same physical properties of the parent rock.
  • Climatic factors induce chemical changes which also affect physical properties of the soil.
  • 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.
  • For example, In Indian Conditions, parent material is generally categorized into:
    • Ancient crystalline and metamorphic rocks – red soils_ the red colour of the soils is due to the presence of iron oxides.
    • Cuddapah and Vindhyan rocks- calcareous and argillaceous soils_as the rocks are ancient sedimentary rocks devoid of metalliferous minerals.
    • Gondwana rocks- less mature soils with low fertility_as the rocks are younger sedimentary rocks
    • Deccan basalts- regur or black soils_ as basalts are rich in titanium, magnetite, aluminum and magnesium giving the soil its dark colour


  • The relief is the most important factor for soil formation in places with steep slopes like the hilly regions, edges of plateaus etc.
  • 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.
  • The exceptions in the plateau are river basins where the soil layers are sufficiently deep.


  • The length of time required for a soil to develop horizons depends upon many interrelated factors, such as climate, nature of the parent material, burrowing animals and relief.
  • Certain soils are termed mature or immature which give some idea of the time factor.

Soil Profile

  • As soils develop over time, layers (or horizons) form a soil profile.
  • Most soil profiles cover the earth as 2 main layers—topsoil and subsoil.
  • Soil horizons are the layers in the soil as you move down the soil profile. A soil profile may have soil horizons that are easy or difficult to distinguish.
  • Most soils exhibit 3 main horizons:
    • A horizon—humus-rich topsoil where nutrient, organic matter and biological activity are highest (i.e. most plant roots, earthworms, insects and micro-organisms are active). The A horizon is usually darker than other horizons because of the organic materials.
    • B horizon—clay-rich subsoil. This horizon is often less fertile than the topsoil but holds more moisture. It generally has a lighter colour and less biological activity than the A horizon. Texture may be heavier than the A horizon too.
    • C horizon—underlying weathered rock (from which the A and B horizons form).
  • Some soils also have an O horizonmainly consisting of plant litter which has accumulated on the soil surface.
  • The properties of horizons are used to distinguish between soils and determine land-use potential.

Soil Classification concerns the grouping of soils with a similar range of properties (chemical, physical and biological) into units that can be geo-referenced and mapped. Soils are a very complex natural resource, much more so than air and water.

Soils contain all naturally occurring chemical elements and combine simultaneously solid, liquid and gaseous states. Moreover, the number of physical, chemical and biological characteristics and their combinations are nearly endless. Many different approaches have been proposed to come to a sensible grouping of different soils.

Soil can be categorised into sand, clay, silt, peat, chalk and loam types of soil based on the dominating size of the particles within a soil.

Sandy Soil

Classification of Soils

  • Sandy Soil is light, warm, dry and tend to be acidic and low in nutrients.
  • Sandy soils are often known as light soils due to their high proportion of sand and little clay (clay weighs more than sand).
  • It is most extensively used construction material.
  • It consists of particles of rock and hard minerals, such as silicon dioxide.
  • They are the largest type of soil particles, where each particle is visible to naked eye.
  • The large, relatively stable sand-particle size increases soil aeration, improves drainage in tight soils and creates plant-growth supporting qualities, or tilt.
  • The particle size of course sand ranges from 2 – 4.75mm, Medium sand ranges from 0.425 – 2 mm and fine sand ranges from 0.075 – 0.425 mm.
  • The bigger particle size of the sand gives wet or dry sandy soil a grainy texture when you rub it between your fingers, and it makes the soil light and crumbly even when you try to stick it together in your hand.
  • The particle shape is angular, sub angular, rounded, flat or elongated.
  • The texture is rough, smooth, or polished.
  • These soils have quick water drainage and are easy to work with.
  • The addition of organic matter can help give plants an additional boost of nutrients by improving the nutrient and water holding capacity of the soil.

Clay Soil

  • Clay Soil is a heavy soil type that benefits from high nutrients.
  • Clay soils remain wet and cold in winter and dry out in summer.
  • Clay particles are the finest of all the soil particles, measuring fewer than 0.002 mm in size.
  • It consists of microscopic and sub-microscopic particles derived from the chemical decomposition of rocks.
  • Clay is a fine grained cohesive soil.
  • They stick together readily and form a sticky or gluey texture when they are wet or dry.
  • These soils are made of over 25 percent clay, and because of the spaces found between clay particles, clay soils hold a high amount of water.
  • Clay expand when in contact with water and shrink when getting dry.
  • Compared to sand particles, which are generally round, clay particles are thin, flat and covered with tiny plates.
  • Organic clay is highly compressible and its strength is very high when dry, which is why it is used in construction as mud mortar.

Silt Soil

Silt Soil

  • Silt Soil is a light and moisture retentive soil type with a high fertility rating.
  • As silt soils compromise of medium sized particles they are well drained and hold moisture well.
  • As the particles are fine, they can be easily compacted and are prone to washing away with rain.
  • Silt is a sediment material with an intermediate size between sand and clay.
  • Carried by water during flood it forms a fertile deposit on valleys floor.
  • The particle size of silt ranges from 0.002 and 0.06 mm.
  • Due to its fineness, when wet it becomes a smooth mud that you can form easily into balls or other shapes in your hand and when silt soil is very wet, it blends seamlessly with water to form fine, runny puddles of mud.
  • By adding organic matter, the silt particles can be bound into more stable clumps.

Peat Soil

Peat Soil

  • Peat soil is high in organic matter and retains a large amount of moisture.
  • This type of soil is very rarely found in a garden and often imported into a garden to provide an optimum soil base for planting.

Chalk Soil

Chalk Soil

  • Chalk soil can be either light or heavy but always highly alkaline due to the calcium carbonate (lime) within its structure.
  • As these soils are alkaline they will not support the growth of ericaceous plants that require acidic soils to grow.
  • If a chalky soil shows signs of visible white lumps then they can’t be acidified and gardeners should be resigned to only choose plants that prefer an alkaline soil.

Loam Soil

Loam Soil

  • Loam soil is a mixture of sand, silt and clay that are combined to avoid the negative effects of each type.
  • These soils are fertile, easy to work with and provide good drainage. Depending on their predominant composition they can be either sandy or clay loam.
  • As the soils are a perfect balance of soil particles, they are considered to be a gardeners best friend, but still benefit from topping up with additional organic matter.

Classification of soils in India

India has varied relief features, landforms, climatic realms and vegetation types. These have contributed in the development of various types of soils.

Alluvial Soils

  • This is the most widely spread and important soil. In fact, the entire northern plains are made of alluvial soil.
  • These have been deposited by three important Himalayan river systems – the Indus, the Ganga and the Brahmaputra.
  • These soils also extend in Rajasthan and Gujarat through a narrow corridor.
  • Alluvial soil is also found in the eastern coastal plains particularly in the deltas of the Mahanadi, the Godavari, the Krishna and the Kaveri rivers.
  • The alluvial soil consists of various proportions of sand, silt and clay.
  • As we move inlands towards the river valleys, soil particles appear some what bigger in size.
  • In the upper reaches of the river valley i.e. near the place of the break of slope, the soils are coarse.
  • Such soils are more common in piedmont plains such as Duars, Chos and Terai.
  • Apart from the size of their grains or components, soils are also described on the basis of their age.
  • According to their age alluvial soils can be classified as old alluvial (Bangar) and new alluvial (Khadar).
  • The bangar soil has higher concentration of kanker nodules than the Khadar. It has more fine particles and is more fertile than the bangar.
  • Alluvial soils as a whole are very fertile.
  • Mostly these soils contain adequate proportion of potash, phosphoric acid and lime which are ideal for the growth of sugarcane, paddy, wheat and other cereal and pulse crops.
  • Due to its high fertility, regions of alluvial soils are intensively cultivated and densely populated.
  • Soils in the drier areas are more alkaline and can be productive after proper treatment and irrigation.

Black Soil

  • These soils are black in colour and are also known as regur soils.
  • Black soil is ideal for growing cotton and is also known as black cotton soil.
  • It is believed that climatic condition along with the parent rock material are the important factors for the formation of black soil.
  • This type of soil is typical of the Deccan trap (Basalt) region spread over northwest Deccan plateau and is made up of lava flows.
  • They cover the plateaus of Maharashtra, Saurashtra, Malwa, Madhya Pradesh and Chhattisgarh and extend in the south east direction along the Godavari and the Krishna valleys.
  • The black soils are made up of extremely fine i.e. clayey material.
  • They are well-known for their capacity to hold moisture.
  • In addition, they are rich in soil nutrients, such as calcium carbonate, magnesium, potash and lime.
  • These soils are generally poor in phosphoric contents.
  • They develop deep cracks during hot weather, which helps in the proper aeration of the soil.
  • These soils are sticky when wet and difficult to work on unless tilled immediately after the first shower or during the pre-monsoon period.

Red and Yellow Soils

  • Red soil develops on crystalline igneous rocks in areas of low rainfall in the eastern and southern parts of the Deccan plateau.
  • Yellow and red soils are also found in parts of Odisha, Chhattisgarh, southern parts of the middle
  • Ganga plain and along the piedmont zone of the Western Ghats.
  • These soils develop a reddish colour due to diffusion of iron in crystalline and metamorphic rocks.
  • It looks yellow when it occurs in a hydrated form.

Laterite Soil

  • Laterite has been derived from the Latin word ‘later’ which means brick.
  • The laterite soil develops under tropical and subtropical climate with alternate wet and dry season.
  • This soil is the result of intense leaching due to heavy rain. Lateritic soils are mostly deep to very deep, acidic (pH <6.0), generally deficient in plant nutrients and occur mostly in southern states, Western Ghats region of Maharashtra, Odisha, some parts of West Bengal and North-east regions.
  • Where these soils support deciduous and evergreen forests, it is humus rich, but under sparse vegetation and in semi-arid environment, it is generally humus poor.
  • They are prone to erosion and degradation due to their position on the landscape.
  • After adopting appropriate soil conservation techniques particularly in the hilly areas of Karnataka, Kerala and Tamil Nadu, this soil is very useful for growing tea and coffee.
  • Red laterite soils in Tamil Nadu, Andhra Pradesh and Kerala are more suitable for crops like cashew nut.

Arid Soils

  • Arid soils range from red to brown in colour.
  • They are generally sandy in texture and saline in nature.
  • In some areas the salt content is very high and common salt is obtained by evaporating the water.
  • Due to the dry climate, high temperature, evaporation is faster and the soil lacks humus and moisture.
  • The lower horizons of the soil are occupied by Kankar because of the increasing calcium content downwards.
  • The Kankar layer formations in the bottom horizons restrict the infiltration of water.
  • After proper irrigation these soils become cultivable as has been in the case of western Rajasthan.

Forest Soils

  • These soils are found in the hilly and mountainous areas where sufficient rain forests are available.
  • The soils texture varies according to the mountain environment where they are formed.
  • They are loamy and silty in valley sides and coarse grained in the upper slopes.
  • In the snow covered areas of Himalayas, these soils experience denudation and are acidic with low humus content.
  • The soils found in the lower parts of the valleys particularly on the river terraces and alluvial fans are fertile.