Soil Formation: Genesis of Soil, Its Five Factors and Soil-Forming Processes

Soil Formation and Pedogenesis: Meaning, Composition and Stages

Pedogenesis: Factors and Processes

Pedogenesis is the genesis of soil, the long chain of changes by which loose rock material becomes a body with layers, structure and life. The word comes from the Greek pedon, meaning soil, and pedology is the branch of soil science that studies how soils form and are characterised. Its partner, edaphology, studies soil as a medium for plant growth.

Two ideas keep the topic straight. A soil-forming factor is a control that decides what kind of soil develops, while a soil-forming process is a change that actually happens inside the soil, such as the washing of clay downwards. Factors act from outside and processes work within.

What Soil Is: Components and Composition

Soil is a dynamic medium in which chemical, physical and biological activities go on constantly. It is a result of decay and also the medium for growth, a changing and developing body whose temperature, moisture and organic matter swing with the seasons. Collectively the Earth's body of soil is called the pedosphere, which meets the lithosphere, hydrosphere, atmosphere and biosphere.

A typical soil is about 50 per cent solids (45 per cent mineral and 5 per cent organic matter) and 50 per cent pore space, which is half filled with water and half with gas. The mineral part is made of sand, silt and clay, and the organic part is mostly humus, the finely divided, dark, stable remains of dead plants and animals.

• Mineral particles: Weathered fragments of rock, from sand to clay, that form the framework of the soil.
• Humus: Dark organic matter that supplies nutrients, helps the soil retain moisture and glues particles into crumbs.
• Water and air: They fill the pores in roughly equal parts, though their shares vary far more than the mineral and organic content.
• Living organisms: Roots, bacteria, fungi and animals that keep the soil working.

Stages of Soil Formation: From Bare Rock to Mature Soil

Soil formation depends first on weathering. The weathering mantle, the depth of weathered material, is the basic input from which soil forms, and life then turns that material into soil. The stages follow one another in a fixed order.

1. Weathering: Rock breaks down in place into a mantle of weathered material, or transported deposits are laid down.
2. Colonisation: Bacteria and simple plants such as mosses and lichens settle on the material, and small organisms shelter in it.
3. Humus accumulation: The dead remains of organisms and plants build up as humus.
4. Spread of plants: Grasses and ferns grow, and later bushes and trees start from seeds brought by birds and wind.
5. Mature soil: Roots push down, burrowing animals bring particles up, and the material turns porous and sponge-like, able to hold water and let air pass, so that a mature soil, a complex mixture of mineral and organic products, forms.

Microbes speed this up from the start. Bacteria, archaea and fungi feed on the simple compounds released by weathering and produce organic acids that weather the minerals further, while their remains add to humus.

Weathering: The Starting Point of Soil Formation

Weathering, Regolith and the Weathering Mantle

Weathering is the mechanical disintegration and chemical decomposition of rocks through the actions of various elements of weather and climate. Very little or no motion of material takes place, so it is an in-situ, or on-site, process. Its three groups, chemical, physical and biological, very rarely operate alone, though one usually dominates.

The result is regolith, a blanket of unconsolidated, loose, heterogeneous deposits covering solid rock. Where rock is chemically weathered deep down, the lower zone is saprolite, which keeps the colour of its ferric compounds. Weathering is usually confined to the top few metres of geologic material, because physical, chemical and biological stresses fade with depth.

Climate decides both the kind of weathering and the depth of the mantle. Chemical decomposition is a function of mineral solubility, and its rate doubles with each 10 °C rise in temperature. Rocks that decompose in a few years in the tropics can remain unaltered for millennia in deserts.

Physical Weathering: Thermal Stress, Frost, Salt and Unloading

Physical, or mechanical, weathering breaks rock without changing its chemistry. The forces that do the work are gravitational (overburden pressure, load and shearing stress), expansion forces (temperature changes, crystal growth or animal activity) and water pressures set by wetting and drying cycles. Most physical weathering is caused by thermal expansion and pressure release, and the damage comes from continued fatigue as rocks contract and expand again and again.

• Thermal stress: Daily heating and cooling expand and contract the rock, and it matters most in deserts, where the day-night range is large.
• Frost: When water freezes its volume increases by 9.2 per cent, and the ice widens cracks.
• Salt crystal growth: Salt solutions seep into cracks and evaporate, leaving crystals that pry the rock apart; it is commonest in arid climates and along coasts.
• Unloading: When erosion removes overlying rock, the rock expands and cracks into curved sheets.

Exfoliation, the flaking of curved sheets from bedrock, is a result of these processes and not a process itself. Exfoliation domes form by unloading, and tors by thermal expansion.

Chemical Weathering: Solution, Carbonation, Hydration, Oxidation and Reduction

Chemical weathering decomposes, dissolves or reduces rock to a fine clastic state through chemical reactions. Water and air (oxygen and carbon dioxide) along with heat must be present to speed up all chemical reactions, and decaying plants and animals add carbon dioxide underground. The reactions are very like those in a laboratory.

Of these, hydrolysis and carbonation are the most effective in regions of high rainfall and temperature. Rain is a weathering agent because it carries dissolved carbon dioxide, which drives carbonation, and atmospheric oxygen, which drives oxidation.

Biological Weathering and the Preparation of Parent Material

Biological weathering is the removal or addition of minerals and ions in the weathering environment, together with physical changes caused by the growth or movement of organisms. Earthworms, termites and rodents burrow and wedge, which exposes new surfaces to chemical attack and lets moisture and air in. Plant roots exert a tremendous pressure that mechanically breaks rock apart, and lichens pluck grains from the surface.

Decaying plant and animal matter produces humic, carbonic and other acids that enhance decay and the solubility of some elements. People disturb vegetation, plough and cultivate soils, and so mix soil and create new contacts between air, water and minerals.

Weathering is an important process in the formation of soils, because it breaks rock into smaller fragments and prepares the way for regolith and soil. Basalt weathers more easily than granite, because it forms at higher temperatures and has fine grains and volcanic glass, and in tropical settings it rapidly weathers to clay minerals. That fact matters for the black soils of the Deccan.

The Five Factors of Soil Formation

Soil-Forming Factors: Dokuchaev, Jenny and the CLORPT Model

Five basic factors control the formation of soils, namely parent material, topography, climate, biological activity and time. They act in union and affect the action of one another, so no single factor explains a soil.

• Dokuchaev, 1883: The Russian geologist Vasily Dokuchaev, commonly regarded as the father of soil science, concluded from field studies of Russian soils that soil forms over time under the influence of climate, vegetation, topography and parent material. His best-known work is Russian Chernozem (1883).
• Jenny, 1941: The American soil scientist Hans Jenny wrote soil formation as a state equation, S = f(cl, o, r, p, t, ...), for climate, organisms, relief, parent material and time (CLORPT), leaving the end open for more factors.

The factors split into two groups by how they act. Parent material, topography and time are passive control factors that set the stage. Climate and biological activity are the active factors that do the work.

Parent Material: The Passive Base of Soil

Every soil starts from a parent material, which is any in-situ weathered rock debris (giving residual soils) or any transported deposit (giving transported soils). Soil formation depends on the texture (sizes of debris) and structure (arrangement of grains) as well as the mineral and chemical composition of the debris. Rock, whether igneous, sedimentary or metamorphic, is the source of all soil minerals and of every plant nutrient except nitrogen, hydrogen and carbon.

Transported material is grouped by the agent that laid it down, such as alluvial (flowing water), lacustrine (lakes), marine, glacial (moraines), aeolian (wind, which makes loess) and colluvial (gravity, at the foot of slopes). Cumulose material is organic matter that grows and piles up in place, as peat does.

The link with the bedrock is not a fixed rule, since similar bedrock can carry different soils, and different bedrocks can carry similar soils. Very young soils show strong links with the type of parent rock, and so do soils in some limestone areas, where solution weathering dominates.

India gives clear cases. Black soil is typical of the Deccan trap, the basalt region made up of lava flows, and climate along with the parent rock is the main influence on its formation. It covers the plateaus of Maharashtra, Saurashtra, Malwa, Madhya Pradesh and Chhattisgarh and extends south-east along the Godavari and Krishna valleys. Red soil develops on crystalline igneous rocks in the eastern and southern parts of the Deccan plateau, and the alluvial soil of the northern plains was deposited by the Indus, the Ganga and the Brahmaputra.

Climate: The Active Factor in Soil Formation

Climate acts through moisture and temperature. Moisture means the intensity, frequency and duration of precipitation against evaporation and humidity, and temperature means seasonal and daily variation. Precipitation gives soil the moisture that makes chemical and biological activity possible. The effective figure is precipitation minus evapotranspiration, and the depth to which water penetrates sets the depth of weathering and soil development.

Excess water carries soil components down the profile (eluviation) and deposits them below (illuviation). In wet equatorial areas of high rainfall, not only calcium, sodium, magnesium and potassium but also a major part of silica is removed, which is called desilication. In dry climates evaporation exceeds precipitation, so water moves up through the soil instead of down and leaves its salts near the surface. In tropical areas of intermediate precipitation, calcium carbonate nodules called kankar form.

Temperature acts in two ways. Chemical activity increases with temperature, falls in cooler conditions (carbonation is the exception) and stops in freezing conditions. That is why tropical soils with higher temperatures show deeper profiles, while the soils of frozen tundra are largely mechanically broken material.

In India the climate difference is easy to see. Laterite soil develops under tropical and subtropical climate with alternate wet and dry seasons and is the result of intense leaching by heavy rain. Arid soils form under a dry climate with high temperature, where evaporation is faster and the soil lacks humus and moisture.

Topography: Slope, Drainage and Aspect

Topography, or relief, is a passive control factor. Its influence is felt through the exposure of the surface to sunlight and the amount of surface and sub-surface drainage over and through the parent material.

• Steep slopes: Soils are thin, because erosion is quick and little water enters the soil.
• Gentle slopes: Soil formation is most favourable, because erosion is slow and percolation of water is good.
• Flat upland areas: Soils are thick and may develop a thick layer of clay with much organic matter, which gives a dark colour.
• Depressions: Runoff collects and the regolith is more deeply weathered, but in the lowest places water saturates the soil and restricts drainage and aeration, so wetland soils such as saline marshes or peat bogs can develop.
• Aspect: Slopes facing the sun's path (the south aspect) are drier than slopes that do not.

Recurring patterns of slope position make a soil catena, in which soils change down a hillside because water, solutes and fine particles move downslope.

Indian mountain soils show the rule on one hillside. Forest soils are loamy and silty on valley sides and coarse grained on the upper slopes. In the snow-covered Himalaya they suffer denudation and are acidic with low humus, while the soils of the lower valleys, on river terraces and alluvial fans, are fertile. Near the break of slope, where rivers leave the hills, the soils are coarse, as in the piedmont plains of the Duars, Chos and Terai.

Biological Activity: Humus, Nitrogen Fixation and Soil Organisms

Biological activity is the second active factor. Plants and organisms, from the first colonisers onwards, add organic matter, retain moisture and add nitrogen.

• Humus: Dead plants supply the litter that becomes humus.
• Cold and warm climates: In cold climates bacterial growth is slow, so humus accumulates and layers of peat develop in sub-arctic and tundra climates. In humid tropical and equatorial climates bacterial action is intense, dead vegetation is rapidly oxidised, and humus stays low.
• Nitrogen fixation: Bacteria and other organisms take gaseous nitrogen from the air and turn it into a form plants can use. Rhizobium lives in the root nodules of leguminous plants and fixes nitrogen for its host.
• Soil animals: Ants, termites, earthworms and rodents rework the soil up and down, and soil passing through an earthworm changes in texture and chemistry.

Animal mixing is called bioturbation. It tends to undo the layering that other processes create, so a soil heavily worked by animals shows fainter horizons.

Plants also protect the soil. They shade it, slow evaporation and hold it against erosion. The laterite soils of India show the effect, because where they support deciduous and evergreen forests they are rich in humus, but under sparse vegetation and in semi-arid conditions they are generally poor in humus.

Time: Soil Maturity, Chronosequences and Paleosols

Time decides how far the other factors can go. The length of time the soil-forming processes operate determines the maturity of a soil and its profile development. A soil becomes mature when all soil-forming processes have acted for long enough to develop a profile. Soils on recently deposited alluvium or glacial till are young and show no horizons, or only poorly developed ones. No specific length of time can be fixed in absolute terms for soils to develop and mature, because it depends on climate, parent material, topography and biological activity.

Soil scientists study time by comparing soils of different ages with other factors held nearly equal, a method called a chronosequence. The alluvial plains of India are a natural one, with old alluvial soil called bangar and new alluvial soil called khadar. Bangar has a higher concentration of kankar nodules, while khadar has more fine particles and is more fertile.

Soil is a slow product. The current Class 10 NCERT textbook puts the time to form a few centimetres of soil at millions of years, while the FAO figure is over 1,000 years to make 1 cm; both agree that soil is non-renewable on a human timescale.

Old soils can outlive their climate. Paleosols are ancient soils that formed in the past, and three kinds are recognised, namely buried soils covered by younger sediment or rock, exhumed paleosols uncovered again by erosion, and relict soils that sit at the surface but formed under a very different climate or biome.

Human Activity as a Soil-Forming Factor

Soil scientists now place organisms, including humans, among the main factors, because people change soil formation by removing vegetation, tilling, irrigating and adding fertiliser. Tillage mixes the layers and restarts soil formation by mixing less weathered material into the upper layers. Clearing vegetation can lead to erosion, waterlogging, laterisation or podzolisation, depending on climate and topography.

The FAO soil classification has a group for soils formed or profoundly modified by human activity, such as adding organic matter or household waste, irrigation or cultivation. These are anthrosols, and paddy soils are among them. Irragric anthrosols occur in the irrigation areas of dry regions, including parts of India.

Soil-Forming Processes

Humification and Mineralisation: How Humus Forms

Humification is the formation of humus, the stable form of soil organic matter derived from the decomposition of plant and animal substances. Saprotrophic fungi, bacteria, earthworms and other soil animals break down litter, dead organisms and their excreta, and what remains is a mixture of large, complex molecules formed in part from lignin and in part from microbial products. Humus is black or dark brown because organic carbon has accumulated.

Microorganisms also break down part of the organic matter into inorganic minerals that roots can absorb as nutrients, a step called mineralisation. During humification microbes secrete sticky mucilages that glue particles into crumbs and give soil its crumb structure. Humus holds moisture and binds toxic metals so that they do not leach away.

Climate sets the balance between the two steps, which is why humus builds up in cold climates and stays low in the humid tropics.

Eluviation, Illuviation and Leaching: Moving Material Down the Profile

In eluviation, percolating rain water moves soil material from the upper layers to lower levels, and the accumulation of that material below is illuviation. It happens where precipitation exceeds evaporation. The horizon that loses material is the eluvial horizon, usually a light-coloured zone (the E horizon), and the horizon that gains is the illuvial B horizon.

Illuvium includes organic matter, silicate clay and hydrous oxides of iron and aluminium. Soluble constituents move by leaching, which is the transport of soil solutes, and are deposited where soil chemistry changes, especially pH and redox potential. Water that percolates in this way stimulates weathering and helps to differentiate soil horizons.

Leaching does not have one outcome. In the wet tropics it removes silica and bases and leaves iron and aluminium behind, and under conifers it carries iron and aluminium down and bleaches the top.

Laterisation: Leaching in the Hot and Wet Tropics

Laterite soils develop by intensive and prolonged weathering of the parent rock, usually at high temperature and heavy rainfall alternating with dry periods, in a process called laterisation. Leaching removes the more soluble ions and leaves the insoluble ones, mainly iron and aluminium, so laterite is rusty red from iron oxide. A repeated wet and dry season is essential, because rain leaches the rock in the wet season, and capillary action brings dissolved ions to the surface in the dry season.

The name comes from the Latin later, brick, because the compacted, cemented soil can be cut into brick-shaped blocks. Francis Buchanan-Hamilton described a laterite formation in southern India in 1807.

In India lateritic soils are mostly deep to very deep, acidic (pH below 6.0) and generally deficient in plant nutrients. They occur mostly in the southern states, the Western Ghats region of Maharashtra, Odisha, parts of West Bengal and the North-East. They are prone to erosion and degradation, but with soil conservation they grow tea and coffee in the hilly areas of Karnataka, Kerala and Tamil Nadu, and the red laterites of Tamil Nadu, Andhra Pradesh and Kerala suit cashew nut. Laterite also suits tapioca (cassava), which tolerates drought and poor soil.

Podzolisation: Leaching Under Coniferous Forest

Podzols are the typical soils of coniferous, or boreal, forests. The name means "under-ash", from the Russian pod and zola, and Dokuchaev gave it in 1875 after the ash‑like under-layer that peasants ploughed up.

Podzolisation is a complex process in which dissolved organic matter and ions of iron and aluminium released by weathering form organo-mineral complexes, called chelates, and are moved from the upper part of the profile to deeper parts. The eluvial horizon becomes bleached and ash‑grey, and the illuviated horizon is brown, red or black as the complexes accumulate there. It occurs under forest or heath vegetation in cool and humid climates, where decomposition is inhibited and acidic organic layers build up.

Podzols are generally poor for agriculture, since they are sandy, low in moisture and nutrients, acid, and short of phosphate. In laterite iron stays near the surface as oxide, while in a podzol iron is carried down and the top is bleached.

Calcification and Kankar Formation: Lime in Dry Lands

Calcification is the secondary accumulation of lime in a soil. Calcisols are soils with a substantial secondary accumulation of lime, common on calcareous parent material and widespread in arid and semi-arid environments. In low-rainfall areas lime accumulates as caliche, also called calcrete, which is calcium carbonate that precipitates at depth and binds gravel, sand, clay and silt.

The Indian form is kankar, a term derived from Hindi, for nodular calcium carbonate formed in the soils of semi-arid regions. In the arid soils of India the lower horizons are occupied by kankar because the calcium content increases downwards, and the kankar layers in the bottom horizons restrict infiltration of water. After proper irrigation such soils become cultivable, as in western Rajasthan.

Dryness, stoniness and sometimes a shallow petrocalcic horizon limit the use of calcisols for agriculture. If irrigated and drained, however, they can be highly productive.

Salinisation and Alkalisation: Salts in Dry Lands and Irrigated Fields

Salinisation is the process of increasing the salt content of soil. Soil minerals release salts as they weather. Where rainfall is sufficient the salts are flushed out by drainage water, but in dry areas they rise by capillary action and accumulate near the surface under intense evaporation, where they can harden into a crust called a hardpan.

Salinisation is also caused by people. Almost all water, even rainfall, carries dissolved salts, and when plants use the water the salts are left behind in the root zone. Disrupted drainage, which stops the leaching that would otherwise carry salts away, makes it worse, and in dry lands a shallow salty water table sends salts up to the surface by capillary action. When sodium predominates, soils become sodic and have very poor structure, which limits or prevents water infiltration and drainage.

India's arid soils are generally sandy and saline in nature, so much so that in some areas common salt is obtained by evaporating water. In Punjab, Haryana and western Uttar Pradesh, over-irrigation has caused waterlogging that raises the salinity and alkalinity of the soil. Soil salinity can be reduced by leaching soluble salts out with excess irrigation water of good quality, together with water-table control and drainage.

Gleying and Peat Formation: Soil Formation in Waterlogged Ground

Gleying happens where the soil is saturated with ground water for long enough to develop a characteristic grey-blue colour pattern. When water holds the pore spaces, the oxygen supply in the profile is restricted and anaerobic conditions reduce the iron, which gives a greenish-blue-grey colour. Where air returns for a time, the iron oxidises and reddish, yellow or orange mottles appear. Such soils are gleysols, found in depressions and low landscape positions with shallow ground water. In the tropics and subtropics they are widely planted with rice.

Peat forms where waterlogging or cold slows decomposition so much that plant remains pile up, partially decayed. Soils consisting mainly of peat are called histosols.

Both are intrazonal processes, because local conditions of drainage override the effect of the regional climate.

Soil Profile and Horizons

The O, A, E, B, C and R Horizons

A pit dug into mature soil shows layers called horizons, and their arrangement is the soil profile, the vertical section of soil from the ground surface to the parent rock. A horizon is a layer parallel to the surface whose physical, chemical and biological characteristics differ from the layers above and below it. A layer in which soil-forming processes have not acted is just a layer, and it becomes a horizon only when at least one property results from those processes.

The same idea is often shown in simpler words, with topsoil, subsoil of weathered rock, sand, silt and clay, and a substratum of weathered parent rock above unweathered parent bedrock. The development of a distinct B horizon is the sign that pedogenesis has really taken place.

Soil Texture and Soil Structure: How Particles Are Arranged

Texture is the proportion of sand, silt and clay in a soil. Clay particles are less than 0.002 mm across and are plate-shaped, which gives them a large surface area. Fine-textured soils generally hold more water and nutrients, and sandy soils hold less. Texture is read from a texture triangle.

Structure is the arrangement of the solid parts of a soil and the pore spaces between them. It forms as grains clump, bind and aggregate into units called peds, and it has a major influence on water and air movement, root growth and seedling emergence.

Humus, root and microbial products and animal excreta bridge the mineral grains into crumbs, while the swelling and shrinking of clay minerals opens cracks. India's black soils show the extreme case, since they are made of extremely fine clayey material, hold moisture well, develop deep cracks in hot weather that aerate the soil, and are sticky when wet.

Mature and Immature Soils: Zonal, Intrazonal and Azonal Soils

Mature and Immature Soils and the Genetic Classification

A mature soil has a fully developed profile with all its horizons. An immature soil shows no horizons or only poorly developed ones, because it is too young or because the material keeps moving or arriving. Soil formation and soil erosion go on at the same time, and generally there is a balance between the two. Deforestation, over-grazing, construction and mining can disturb that balance.

Time also decides which factor dominates. A mature soil reflects the climate and vegetation of its zone, while a very young soil still reflects its parent rock.

The older genetic classification of 1938 sorted soils into three groups by the factor that dominates their formation.

Intrazonal soils have well-defined profiles shaped by a local factor such as drainage or limestone. Calcimorphic soils form on limestone, and rendzinas are thin with limited water-holding capacity, while terra rossa soils are deep red and go with higher rainfall. Hydromorphic soils form in wetland conditions, as gley and peat soils. Halomorphic soils form through salinisation.

Azonal soils form where fine grains made by weathering keep sliding down a mountain slope, so the time needed for soil formation never becomes available. On flood plains new alluvium arrives each year, so the soils stay immature.

Soil Formation in India: The Factors at Work

Parent Rock, Climate and Relief: How India's Main Soils Form

Indian soils are classified on the basis of the factors responsible for their formation, together with colour, thickness, texture, age and chemical and physical properties. India has varied relief features, landforms, climatic realms and vegetation types, and these have contributed to the development of different soils.

Soils that differ this much from place to place need testing farm by farm. The Soil Health Card Scheme, introduced on 19 February 2015 at Suratgarh, Rajasthan, aims to issue soil health cards to all farmers in the country, and each card reports the soil's status on 12 parameters, namely nitrogen, phosphorus, potassium and sulphur; zinc, iron, copper, manganese and boron; and pH, electrical conductivity and organic carbon.

Sources

• NCERT: Class 10 Geography: Contemporary India II, Chapter 1
• NCERT: Class 11 India: Physical Environment, Chapter 6
• NCERT: Class 11 Fundamentals of Physical Geography, Chapter 5
• USGS: Soil formation (Chapter 6)
• USGS: Soils and paleosols
• FAO: Soil formation and soil-forming factors
• FAO: Cherishing the ground we walk on
• PIB: Celebrating a Decade of Soil Health Cards
• Wikipedia: Calcisol
• Wikipedia: Calcrete
• Wikipedia: Cassava
• Wikipedia: Deccan Traps
• Wikipedia: Eluviation
• Wikipedia: Gleysol
• Wikipedia: Humus
• Wikipedia: Illuvium
• Wikipedia: Kankar
• Wikipedia: Laterite
• Wikipedia: Mushroom
• Wikipedia: Peat
• Wikipedia: Pedogenesis
• Wikipedia: Pedology
• Wikipedia: Podzol
• Wikipedia: Regolith
• Wikipedia: Saprolite
• Wikipedia: Soil
• Wikipedia: Soil horizon
• Wikipedia: Soil salinity
• Wikipedia: Soil science
• Wikipedia: Soil structure
• Wikipedia: Soil texture
• Wikipedia: Vasily Dokuchaev
• Wikipedia: Weathering
• Wikipedia: Zonal soil
• UPSC: Civil Services Examination question papers