GENESIS OF SOILS (PEDOGENESIS OR SOIL FORMATION) | Biogeography Optional for UPSC

• Soil genesis or pedogenesis is the process of soil formation as a result of various factors including the region, environment, and history.
• The evolution of soils and their properties is called soil genesis or soil formation. 

The formation of the soils is the result of 

• Soil forming factors: There are five fundamental soil formation factors. These five “state factors” are parent material, topography, climate, organisms, and time.
• Soil forming processes: There are five fundamental soil formation processes. Lateralization, Calcification, Salinization, Podzolization, and Gleization.

Thinkers’ Perspective

• Soil develops through a series of changes. The starting point is weathering of freshly accumulated parent material. Its culmination is seen in the development of complete soil profile with well stratified horizons.” – Hans Jenny (1994) in his “Factors of soil formation: a system of quantitative pedology”
• “Alterations within the soils due to biogeochemical processes lead to the development of soil horizons. These horizons are distinguished by differences in color, structure, texture, and chemistry.” – Stanley and Southard in “Soil genesis and classification” (2011). 

Factors Affecting Soil Formation 

• Hans Jenny (1941) in his “Factors of soil formation: a system of quantitative pedology” views that “Soil is the function of climate, organisms, reliefs, parent materials and time”.
  S = f (cl, o, r, p, t, …)
• Dots in the equation are meant for the inclusion of other factors of local importance. e.g. as seen in intrazonal soils. 

Hans Jenny divided these factors into two groups:  

• Active factors: These represent the agents that supply energy for the process of soil formation. These are climate and biotic organisms. Their effects can be immediately observed.
• Passive factors: These represent the source of the soil-forming and conditions affecting it. Time, topography, and parent material are the passive factors because their effects are not immediately observed. These factors can, however, control the role of climate and organisms.

(1) Climate 

• The climatic factors in paedology include precipitation, temperature, and storm patterns etc.

Precipitation 

• It controls the downward movement of nutrients in soils by translocation. 
• If precipitation is high, water will wash nutrients deeper into the soil. It leads to increase in eluviation, decrease in pH level, and therefore increase in the acidification of the soils.
• If precipitation is low, salts will build up in the soil and restrict fertility.
• E.g. Laterite soils are found in the alternate wet and dry climate.
• Desert soil is developed through the calcification process under high temperatures and very low precipitation.
• Peaty soil is developed through the Gleization process under waterlogging areas.
• Saline soils are developed through the calcification process (when evaporation is greater than precipitation) under capillary action.

Temperature 

• Soil horizons are better developed in a warm and moist climate due to higher rate of weathering.
• Below 10°C, biological activities are slowed. Thus, decomposition is slow in cold climates, and so organic matter accumulates to form a thick O horizon. This material becomes humus, which is carried downward to enrich the A horizon.
• In contrast, bacteria rapidly decompose plant material in the warm and moist climates of low latitudes. O horizons are generally lacking.
• Wide fluctuations in temperature, especially in the presence of water cause shrinking and swelling, frost action, and general weathering in soils.
• Solar energy, usually expressed as temperature, controls the form of water falling onto the soil surface as well as in the soil. 
• In the Podozolization process, iron and aluminium move downward to form acidic soils.
• In Rajasthan, both granite and sandstone give birth to sandy soil irrespective of parent rock because of high temperature and wind erosion.

Climosequence 

• It is a sequence of soil profiles that differ in climatic conditions.
• Example of climosequence along the Cascade and Sierra Nevada ranges: The vegetation varies from shrubs in the dry south to needle-leaved trees in the humid north, with extensive grasslands in between.

(2) Parent Materials 

• The weathering of parent rocks is a prerequisite condition for the formation of soils.
• Parent material include the structure of rocks, weathering, and nature of transportation and deposition.

Structure 

• Parent rock may be igneous, metamorphic, or sedimentary. It determines the color, texture, chemical properties, and minerals of the soil. 
• Parent material consist of consolidated rocks. It can also include unconsolidated deposits such as river alluvium, lake or marine sediments, glacial tills, loess, volcanic ash, and organic matter.
• The structure of the parent rocks influences the properties of the soils. e.g., degree of consolidation, nature of cementation, bedding planes, joints, chemical composition etc.
• Soil gets sand, silt, and clay from parent rock.

• Soil texture is largely determined by the size of mineral grains within the parent material. E.g. 
  
  • The coarse texture of granitic rocks leads to a coarse and loamy soil texture. 
  • The fine texture of basaltic rocks yields soils with a loam or clay-loam texture. 
• Development of E horizons: Water percolates easily to greater depths through soils with coarse texture. Hence, a clearly defined E horizons may develop.

Soil inherits many properties from the structure of parent material. Some examples are as follows:

• The peninsular soils reflect the parent rock very much.
• Black soil is made up of weathered materials of basaltic volcanic rocks.
• The soils of the northern plains are transported in nature. Hence, these have little relation to in-situ parent material.
• Sandy soils are derived from sandstone.
• Red soil is made up of igneous rock.

Soils of different clay minerals groups 

• There are six main types of clay minerals. The types are: 1. Kaolinite 2. Smectite Group 3. Illite Group 4. Vermiculite 5. Chlorite 6. Allophanes.

• The dark coloured ferromagnesian rocks (e.g. ancient crystalline and metamorphic rocks) produce soils with a high content of iron compounds. These have clay minerals in the kaolin or smectite groups. For example, Red soils.
• Light-coloured siliceous rocks produce soils that are low in iron compounds. These contain clay minerals in the illite or vermiculite groups. 

Transportation 

• Most soils derive from transported materials that have been moved many miles by wind, water, ice and gravity.
• Aeolian processes (movement by wind) form loess soils. Loess contains 60–90 % silt. Clay is seldom moved by wind as it forms stable aggregates.
• Water-transported materials are classed as either alluvial, lacustrine, or marine. 
• Alluvial materials are deposited by flowing water. 
• Sedimentary deposits settled in lakes are called lacustrine. 
• Marine deposits, such as soils along the Atlantic and Gulf Coasts, are the beds of ancient seas that are revealed due to land upliftment.
• Ice moves parent material and makes deposits in the form of terminal and lateral moraines.
• Parent material moved by gravity is deposits at the base of steep slopes as talus cones and is called colluvial material.
• Cumulose parent material is not moved but originates from deposited organic material. This includes peat and muck soils and results from preservation of plant residues by the low oxygen content of a high water table.

Weathering 

• The weathering of parent material includes physical weathering, chemical weathering and biological weathering.

Lithosequence 

• It is a sequence of soils where the changing character of the soil is related to the changing lithology of rocks.

(3) Topography or Relief 

Topography determines the nature of drainage and erosion. Topography includes the following: 

• Aspect (the compass orientation of a landform), 
• Slope configuration (i.e., either convex or concave), and 
• Relative position on a slope (that is, from the toe to the summit). 

Slope 

• The slope gradient, slope length and slope aspect affect the properties of the soils.
• Soil horizons are thick on gentle slopes and thin on steep slopes. 
• Steep slope encourages rapid soil loss by erosion. It allows rainfall to run off before enter into the soil and hence there is little leaching.

Slope Aspect 

• slope aspect is the direction of slope in relation to the sun.

• Slopes facing away from the Sun are sheltered from direct insolation, and so tend to have cooler, moister soils. 
• Slopes facing toward the Sun are exposed to direct solar rays, raising soil temperatures and increasing evapotranspiration.

Slope configuration 

• Slope configuration is either convex or concave. 

Relative position on a slope 

• It is the position from the toe to the summit. 

Drainage 

• The flat upland surface is characterized by ideal drainage and least erosion.
• Topography influences the way the hydrologic cycle affects due to runoff processes and evapotranspiration. 
• Precipitation may run off the land surface which cause soil erosion. It may percolate into soil profiles and become part of subsurface runoff.
• Runoff from uplands creates saline soils at the lowlands.

Toposequence 

• It is a sequence of soils that are characteristic of a topography.
• Adjacent soils that show differing profile characteristics reflecting the influence of local topography are called toposequences.
• As a general rule, soil profiles on the convex upper slopes in a toposequence are shallower.
• In a normal toposequence, soil at the hilltop is red. As we move down, it gradually changes into brown and then yellow.
• It is related to hydrology and drainage. 

(4) Organic Factor 

The organic factors include vegetation, animals and micro-organisms. 

Flora 

• The nature of tree canopy affects the soil moisture and soil temperature.
• The tree roots bind the soil, hence controls soil erosion. 
• On the other hand, the roots may promote soil erosion.

Animals 

• Animals play a major role in influencing the major soil forming processes e.g. transformations and translocations. 
• The macro and meso-fauna affect and modify the soil properties through burrowing, transporting, and mixing of organic and inorganic materials. 
• The macrofauna like rabbits, moles, prairie dogs, squirrels, beavers, rats, etc. are amongst the largest modifiers of the soils. 
• Among the mesofauna which modify the soils, important are earthworms, mites, springtails, termites etc. 
• Micro-organisms decompose the organic matter.

Human 

• Human is also part of the biological influx that influences soil formation. 
• Severe influence: Eg. wholesale removal of an entire soil profile due to mining, burial of an entire soil profile by urbanization.
• Subtle influence: E.g. gradual modification of organic matter by agriculture, gradual modification of soil structure by irrigation. 
• Agriculture: The chemical and physical properties of soils often are affected significantly by cultural practices. The problems created for agriculture by cultural practices are loss of arable land, erosion, the buildup of salinity, and the depletion of organic matter.

Biosequence 

• The development of soils can be significantly affected by vegetation, animal inhabitants, and human populations. 
• Any array of contiguous soils influenced by local flora and fauna is termed a biosequence.
• A sequence of soils that contain distinctly different soil horizons because of the influence that vegetation had on the soils during their development. 
• Example: prairie soils in a dry environment, oak-savannah soils as a transition zone, and forested soils in a wetter environment. 

(5) Time

• The soil-forming factors of parent material and topography are site-related (attributes of the terrain), whereas those of climate and organisms are flux-related (inputs from the surroundings). 
• As a soil-forming factor, time is independent from the other four factors. Time is neither a property of the terrain nor a source of external stimulus. 
• Time has a critical role in the development of the thickness of the soil profile. 
• Soil-forming processes are generally very slow. Soil develops relatively quickly from the sediments, and at a relatively slower from the bedrock.
• The conceptual independence of time from its four companion factors means simply that “soil evolution can occur while site attributes and external inputs remain essentially unchanged”.

Topography and time 

• Topography is susceptible to great changes over time. 
• Soil erosion by water or wind removes A horizons and exposes B horizons to weathering. 
• Major portions of entire soil profiles can move downslope.
• Catastrophic events, such as volcanic eruptions and earthquakes, can have obvious consequences for the instability of geomorphologic patterns.

Paleosol 

• Paleosols are ancient soils that formed in the past.
• A paleosol is a former soil preserved by burial under either sediments (alluvium or loess) or volcanic deposits (volcanic ash). 
• Exhumed soils: Paleosol may be buried beneath a covermass of igneous or sedimentary origin for a long duration of time. Over time, the surrounding softer rock erodes away, and it leads to formation of exhumed soils.
• The term paleosol is used to designate the "fossil soils" buried within sedimentary and volcanic deposits.
• Paleosols are always exceedingly infertile soils, containing lower phosphorus levels. This has forced highly specialised evolution amongst Australian flora which can sustain on minimal nutrient supplies.
• Paleosols often contain the most exceptional biodiversity due to the absence of competition.

Chronosequence

• Chronosequences are developed over the adjacent areas over different periods of time with relatively small differences in other soil-forming factors. 
• Such groups of sites are used to assess the influence of time as a factor in pedogenesis.

Processes of Soil Formation

Soils develop from parent material by various weathering processes. 

The zone of humification and weathering where pedogenic processes are dominant and where biota play an important role is termed the solum.

As per Michael Pidwirny (Soil pedogenesis, 2006), the key soil-forming processes important to macro-scale soil formation are as follows: 

• Laterization        
• Podsolization    
• Calcification    
• Salinization        
• Gleization        

Four groups of pedogenic processes 

The pedogenic processes or soil forming processes are classified into four groups.

• Soil Enrichment        
• Soil Depletion.         
• Translocation        
• Transformation        

(1) Soil Enrichment मृदा संवर्धन

• Addition of inorganic materials by exogenetic processes viz. running water, wind etc.
• The organic materials derived from the litter accumulate in the ‘O’ horizons.
• Addition of materials due to lateral movement of water.
• Addition of materials due to upward movement (capillary action).

(2) Soil Depletion: 

• Loss of surface through lateral movement. e.g., removal of materials by soil erosion caused by surface runoff. 
• Loss of water from plant leaves through transpiration.
• Loss of minerals through the processes of leaching or eluviation.

(3) Translocation of Materials 

It involves two types of vertical movements: 

• Upward movement (capillary action)
• Downward movement (leaching or eluviation).

(4) Transformation 

• The decomposition of primary minerals into secondary minerals
• Formation of new minerals
• Humification (transformation of plant tissues into humus).

Pedoturbation 

• It involves mixing between soil horizons. 
• It includes churning clays, cryoturbation, and bioturbation. 
• Bioturbation includes faunal pedoturbation (animal burrowing), and floral pedoturbation (root growth, tree-uprootings). The zone of active bioturbation is termed the soil biomantle.
• Pedoturbation transforms soils through destratification, mixing, and sorting. It also creates preferential flow paths for soil gas and infiltrating water. 
• Destratification is the process of vertical mixing to eliminate separate layers of temperature, plant life, or animal life.

Eluviation and Illuviation 

• The process of removal of materials from geological or soil horizons is called eluviation or leaching. 
• Eluviation is the transport of soil material from upper layers of soil to lower levels by downward percolation of water.
• Accumulation of this material (illuvial deposit) in lower levels is called illuviation. The materials accumulated in B horizon include clay particles, organic matter or sesqui-oxides of iron and aluminium.
• Eluvium or eluvial deposits are those geological deposits and soils that are derived by in situ weathering or weathering plus gravitational movement.
• The interplay of removal (eluviation) and deposition (illuviation) is also called pedo-translocation, which results in contrasting soil horizons. – McKeague and Arnaud (1969)
• Due to transfer of fine particles downward, coarse particles are left in the upper soil horizon. These mineral grains include quartz and silica. The increase in the proportion of silica in the A horizon is called silication. 

Laterization

• It generally takes place in the regions of high temperature and heavy rainfall, like in tropical and equatorial climates.
• In such climatic conditions, the bacterization process is increased. It leads to lower humus content in the upper part of the soil.
• Heavy rainfall leads to the leaching of minerals from the A horizon to the B horizon. Hence, B horizon is rich in minerals. Hence, laterite soil is suitable for bigger trees with deep roots.

Podsolization 

• It takes place in a cool and humid climate. E.g. Taiga Forests, Coniferous soil, Boreal forests.
• In this region, the bacterial activity is low. The thick dark organic surface having organic compounds can be translocated by heavy rainfall.
• Dissolved organic matter and ions of iron and aluminium form organo-mineral complexes (chelates or Humus Colloids). 

Calcification

• The calcification process takes place in the Savana types of climates (350C temperature and 75 cm precipitation).
• Nutrients are translocated upwards with evaporating water from a lower profile and make soil suitable for grass vegetation and unsuitable for trees.

Salinization or alkalization 

• It generally takes place in hot deserts where precipitation is very low, and the temperature is very high.
• Extreme evaporation translocates the salts due to upward movement (capillary action) to the surface. 
• It takes place in irrigated areas of low rainfall areas. For example, in some areas of Punjab and Rajasthan. 

Gleization

• Gleying takes place in waterlogged and anaerobic (low oxygen) conditions. E.g. Peaty and Marshy Soil.
• It causes iron and manganese to reduce, which makes the soil more acidic and grayer.
• Low oxygen conditions lead to flourishing of some specialized bacterias that use organic matter.