Soil Genesis: Pedogenesis, Soil-Forming Processes and Factors of Soil Formation

• Soil is the uppermost weathered layer of the Earth’s crust, formed over thousands of years by the continuous, interacting action of climate, organisms, relief, parent rock and time on exposed rock material.
• Pedogenesis (soil genesis) is the process by which soil originates from parent material and develops distinguishable layers, or horizons, that together make up a soil profile.
• Pedology (Greek pedon, ground) is the scientific study of soils: their formation, classification and distribution. Its founder, Vasily Dokuchaev, first treated soil as an independent natural body with a history of its own.

Soil and Pedogenesis

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What soil is

• Soil is a relatively thin surface layer of mineral matter that normally contains a considerable amount of organic material and is capable of supporting living plants.
• It occupies that part of the “outer skin” of the Earth extending from the surface down to the maximum depth to which living organisms penetrate, essentially the depth of plant roots.
  • It is marked by its ability to produce and store plant nutrients through the interaction of water, air, sunlight, rock, plants and animals.
• Although thinly spread over the land, soil is the interface where the atmosphere, lithosphere, hydrosphere and biosphere meet.
  • Since its bulk is inorganic, it is conventionally placed in the lithosphere, but it is shaped by, and shapes, the other three spheres.
• Soil formation is a global biospheric process: climate and vegetation act on rock together, and the soil they produce in turn feeds back on vegetation and on human use of the land.
  • As a result, soil develops features absent in the parent rock, such as horizons, humus, structure and a living population, which set it apart from its source material.
• Two ways of seeing soil:
  • Pedology studies soil as a natural body in the landscape, asking how and why it formed.
  • Edaphology studies soil as a medium for plant growth, asking how it serves crops and forests.

Constituents and composition

• Soil has four essential constituents: mineral matter, organic matter, water and air. Their proportions and arrangement decide everything from fertility to load-bearing capacity.
  • The mineral fraction is divided by particle size into sand, silt and clay, which set the soil’s texture and other properties.
  • The organic fraction, chiefly humus, plays an outsized role in fertility although it is present only in small quantities.
• A typical loam is roughly 45% mineral particles, 25% water, 25% air and 5% organic matter by volume.
  • The organic 5% comprises humus, living organisms and fresh plant residues.
  • Water and air share the pore space, so a wet soil is poorly aerated and a dry soil well aerated.
• Soil therefore holds both biotic and abiotic components:
  • biotic: all living and once-living things, such as roots, insects, earthworms, fungi and bacteria;
  • abiotic: minerals, water and air.
• Plant nutrients come from both. Nitrogen, phosphorus and potassium are the major nutrients, with calcium, magnesium and sulphur next in importance.
  • Plants cannot use atmospheric nitrogen gas directly. Soil nitrogen comes from decomposing organic matter and from biological fixation by bacteria, and is taken up as nitrate and ammonium.

From Rock to Soil: Weathering, Regolith and the Developing Profile

Weathering

• Soil development begins with the physical and chemical disintegration of rock exposed to the atmosphere and to water percolating from the surface, a process termed weathering.
  • Its basic result is the weakening and breakdown of solid rock into progressively smaller fragments.
• Physical weathering (frost wedging, thermal expansion, salt crystallisation, unloading) breaks rock without changing its minerals. It dominates in cold and dry climates.
• Chemical weathering alters the minerals themselves. It dominates in warm, humid climates.
  • Hydrolysis turns feldspar into clay minerals such as kaolinite, the main source of soil clay.
  • Oxidation of iron gives the red and yellow colours of tropical soils; carbonation and solution remove calcium carbonate.
• Biological weathering links the two: roots prise rock apart, and lichens and decomposing litter release organic acids that dissolve minerals.

Regolith

• The main product of weathering is a layer of loose, inorganic material called regolith, sometimes described as “blanket rock” because it lies like a blanket over the unbroken bedrock.
• Residual (sedentary) regolith has weathered from the rock beneath it.
  • It shows a crude gradation of particle size, with the largest, least-weathered pieces at the bottom next to the bedrock.
• Transported regolith has been brought from elsewhere by wind, water or ice.
  • Its composition may vary greatly from place to place, independent of the bedrock below.
• Soils formed on the two are called in-situ (residual) and transported soils respectively.

Soil: the living top of the regolith

• The upper half-metre or so of the regolith differs from the material below, above all in the intensity of biological and chemical processes taking place in it. This upper portion is soil.
  • It is made largely of finely fragmented mineral particles, the ultimate product of weathering.
  • It also holds an abundance of living and dead plant parts, microscopic plants and animals, and variable amounts of air and water.
• Soil is not the end-product of a process but a stage in a never-ending continuum of physical, chemical and biological change. It is dynamic, not static.

Stages of soil development

The Four Classes of Soil-Forming Processes

• Modern pedology recognises four classes of soil-forming processes: additions (enrichment), losses (removal), translocations and transformations.
  • Each operates simultaneously and continuously within a developing profile.
  • Their net balance explains why two soils formed on identical parent rock look entirely different after a few centuries under different climates.

1. Additions (enrichment)

• In soil enrichment, matter, organic or inorganic, is added to the soil.
  • Surface mineral enrichment: deposition of silt from river floods or wind-blown dust and volcanic ash.
  • Organic enrichment: litter fall, root growth and humus carried by water from the surface litter (O horizon) into the mineral topsoil (A horizon).
  • From the air and water: rain brings dissolved salts; bacteria fix atmospheric nitrogen; groundwater and runoff bring dissolved substances to lower slopes.

2. Losses (removal)

• In removal processes, material is taken out of the soil body altogether.
  • Erosion carries soil particles into streams and rivers.
  • Leaching removes soluble compounds and bases, dissolved in water flowing to lower levels or out of the soil body entirely.
  • Gaseous losses: organic matter decomposes to carbon dioxide, and waterlogged soils lose nitrogen by denitrification.
• Cheluviation is closely related to leaching. It is the downward movement of iron and aluminium bound to organic acids (chelating agents) released by decomposing litter and roots, rather than by water alone.

3. Translocation

• Translocation is the movement of materials upward or downward within the soil without removal from the profile as a whole. It produces the most visible horizon differentiation.

(a) Downward translocation

• Eluviation washes fine particles, particularly clays and colloids, downward. It leaves behind coarser sand and silt as the pale, leached E horizon.
• Illuviation is the complementary process: material brought down from the E horizon, namely clay, humus and sesquioxides of iron and aluminium, accumulates in the B horizon.
  • Mechanical washing of clay alone is called lessivage; it builds the clay-rich Bt horizon.
• The resulting colour sequence: humus moving down from the O horizon gives the A horizon a brownish colour, eluviation leaves a whitened E horizon, and illuviation gives the B horizon orange-red colours from iron sesquioxide.
  • This sequence shows clearly where the topmost layer is a thin deposit of wind-blown silt or dune sand.
• Decalcification: in moist climates, surplus water moves down to the groundwater zone and leaches calcium carbonate from the entire soil body.
  • Such soils are usually acidic and low in bases. Adding lime or pulverised limestone corrects the acidity and restores calcium, an important plant nutrient.
• Calcification: in dry climates, rainfall is too little to flush carbonate out of the soil. It is carried down only as far as the B horizon and deposited as white grains, plates or nodules.
  • Calcification can form a cemented hardpan (calcrete; kankar in Indian usage). It blocks drainage and root penetration and interrupts the exchange of water and nutrients between horizons, lowering fertility.
• Iron pans: in cold, humid climates, illuvial iron and aluminium oxides can form a thin pan. It blocks drainage, keeps the soil saturated and creates chemically reducing (waterlogged, oxygen-poor) conditions.

(b) Upward translocation

• Upward translocation is typical of dry climates, especially in low-lying, poorly drained flats where groundwater lies close to the surface.
• As water at the surface evaporates, groundwater is drawn upward by capillary tension, much as a cotton wick draws oil up in an oil lamp.
• This groundwater is often rich in dissolved salts. When it evaporates, the salts are left behind and accumulate: salinisation.
  • Large amounts of salt are toxic to most plants. When salinisation strikes irrigated land in a dry climate, the soil can be effectively ruined without major reclamation.

4. Transformation

• Transformation is the change of material within the soil body itself, without net addition, removal or movement between horizons.
  • Mineral transformation: primary minerals change to secondary ones, as when feldspar weathers into clay minerals.
  • Humification: microorganisms decompose organic matter into humus.
  • Mineralisation: humus itself breaks down, releasing nutrients, carbon dioxide and water.
• In warm, moist climates, organic matter can be turned almost completely into carbon dioxide and water.
  • This is why tropical rainforest soils, despite the luxuriant vegetation above, are so often nutrient-poor: most nutrients are held in the living vegetation, not in the soil.

Pedogenic regimes: the major soil-forming processes

• The four process classes combine differently under each climate and vegetation, producing a few characteristic pedogenic regimes. Each gives a family of soils, and together they explain the broad world pattern of zonal soils, pedalfers and pedocals.

Laterisation

• Laterisation (ferrallitisation) operates in hot, humid tropical climates with heavy rainfall, usually with a dry season.
  • Intense hydrolysis and leaching remove silica (desilication) and bases (calcium, magnesium, potassium).
  • The less mobile sesquioxides of iron and aluminium remain and concentrate, giving deep red and yellow soils rich in kaolinite and gibbsite but poor in nutrients.
  • Organic matter decomposes so fast that little humus survives.
• Where the iron-rich layer (plinthite) is exposed to alternate wetting and drying, it hardens irreversibly into a brick-like crust.
  • The word laterite (Latin later, brick) was coined in 1807 by Francis Buchanan-Hamilton at Angadipuram, Kerala, where local people cut it into building blocks. The Geological Survey of India declared the site a National Geological Monument in 1979.

Podzolisation

• Podzolisation operates under cool, humid climates with coniferous forest or heath, as in the taiga.
  • Needle litter forms an acid mor humus that is poor in bases.
  • Fulvic acids from it chelate iron and aluminium and carry them down (cheluviation).
  • The result is a bleached, ash-grey E horizon over a dark, rusty B horizon of illuvial humus and sesquioxides, sometimes cemented into an iron pan.
  • The name podzol is Russian for “under ash”, after the bleached layer.

Calcification

• Calcification dominates in semi-arid to subhumid continental interiors under grassland, where evaporation exceeds precipitation.
  • Leaching is too weak to remove calcium and magnesium carbonates. They collect at the depth reached by the wetting front, forming a Bk horizon of nodules or a calcrete pan.
  • Dense grass roots constantly recycle bases to the surface, so the soils stay base-rich and neutral to alkaline: the chernozems of the Ukrainian and Russian steppe and the North American prairies.

Gleisation

• Gleisation (gleying) occurs wherever soil is waterlogged for long periods: in poorly drained hollows, floodplains, paddy fields, marshes and the tundra above permafrost.
  • With no oxygen, bacteria reduce red ferric iron to soluble grey ferrous iron, giving bluish-grey or greenish colours.
  • Where air enters along root channels, iron is re-oxidised into rusty mottles.
  • Slow decomposition lets organic matter pile up; extreme cases form peat (paludisation), as in the acid kari soils of Kuttanad.

Salinisation and alkalisation

• Salinisation concentrates soluble salts (chlorides and sulphates of sodium, calcium and magnesium) near the surface in arid and semi-arid areas with a high water table or careless irrigation, often as a white crust.
• Alkalisation (sodification or solonisation) follows when sodium dominates the exchange sites of the clay.
  • The soil turns strongly alkaline (pH above about 8.5), the clay disperses, and the soil becomes sticky when wet, hard when dry and nearly impermeable.
  • Later leaching of the sodium (solodisation) leaves a bleached, acid topsoil.
• India has about 6.73 million hectares of salt-affected soils, roughly half saline and half sodic. They are known as reh, kallar and usar in the north-western plains, and include the coastal and Rann of Kachchh salt flats.

Humification, melanisation and pedoturbation

• Humification builds humus from litter; melanisation is the darkening of the mineral topsoil as that humus is mixed downward by roots and earthworms.
  • It gives the thick, dark, crumbly topsoil of grassland soils, where fibrous roots die back and are renewed each year.
• Pedoturbation is the mixing of the soil that blurs horizons:
  • faunal mixing by earthworms, termites and ants;
  • shrink–swell mixing in clay soils, as in the deep-cracking black soils (regur) of the Deccan, which “plough themselves”;
  • freeze–thaw mixing (cryoturbation) in permafrost soils.

Natural processes of soil enrichment

• Natural soil enrichment is the build-up of nutrients, organic matter and fine material without human inputs. It works through all four process classes:
• Weathering of parent rock releases nutrients: potassium from feldspar and mica, phosphorus from apatite, calcium and magnesium from basalt.
  • Basalt-derived regur is naturally rich in calcium, magnesium and potassium.
• Litter fall and humification return nutrients to the topsoil and build humus, which holds nutrients and water.
• Biological nitrogen fixation: Rhizobium in the root nodules of legumes, free-living bacteria, and cyanobacteria in paddy fields (including the Azolla–Anabaena partnership) turn atmospheric nitrogen into plant-usable forms.
• Mycorrhizal fungi extend root systems and unlock phosphorus; earthworms and termites mix organic matter into the mineral soil.
• Flood silt: rivers renew floodplain soils with fresh silt each year.
  • The khadar of the Ganga–Brahmaputra plain is replenished by floods; the Nile fed Egyptian farming this way until the Aswan High Dam (1970) trapped the silt.
• Wind-blown dust and loess: loess built the fertile soils of the Chinese Loess Plateau, Ukraine and the American Midwest.
  • Satellite (CALIPSO) data for 2007–13 showed that about 27.7 million tonnes of Saharan dust reach the Amazon basin each year, bringing about 22,000 tonnes of phosphorus, roughly what the rainforest loses to rain and floods.
• Volcanic ash weathers quickly into fertile Andosols, which support dense farming populations in Java, Japan and the Andes.
• Impact on food production:
  • The great breadbaskets sit on naturally enriched soils: the Indo-Gangetic alluvium, the Nile valley, the chernozem belt of Ukraine and Russia, the loess lands of northern China, and the rice terraces on Java’s volcanic soils.
  • Nitrogen fixation and flood silt let traditional rice and pulse systems yield for centuries with few inputs.
  • Natural enrichment is slow. Where harvests remove nutrients faster than it can replace them, soils are mined; then manures, legumes and fertilisers must make up the difference.

Factors of Soil Formation

• Soil is best understood as an ever-evolving material. It acts like a sponge, taking in inputs and being acted upon by the local environment, and changes over time as those inputs change.
• Six factors are conventionally recognised: climate, organisms, relief, parent material and time (the classical five), plus human activity, now an increasingly emphasised sixth.
  • Vasily Dokuchaev identified the natural factors from his studies of the Russian chernozems (1883).
  • Hans Jenny (1941) formalised them as the state-factor (CLORPT) equation: S = f(cl, o, r, p, t, …), where the dots leave room for further factors such as human activity.
• Active and passive factors:
  • Active factors, climate and organisms (the biosphere), supply the energy and agents that drive soil formation.
  • Passive factors, parent material, relief and time, provide the base and the setting on which the active factors work.
• The factors interact. Climate decides the vegetation, relief redistributes rainfall, and time lets the climate’s signal overtake that of the parent rock.

(i) Climate

• Climate, measured mainly through precipitation and temperature, is the most important single factor on a world scale. It is why the broad soil belts follow the climatic regions and biomes.
• Role of precipitation:
  • It controls the downward movement of water, nutrients and compounds. High rainfall washes nutrients deeper and eventually out of the soil, restricting fertility.
  • Soils under tropical rainforest are nutrient-poor because of intensive leaching; most of their nutrients are stored in the vegetation itself.
  • In arid regions, high evaporation draws water upward and salts accumulate (salinisation).
  • The balance of precipitation and evaporation thus separates leached, acidic soils of humid lands (pedalfers) from carbonate-rich soils of dry lands (pedocals).
• Role of temperature:
  • Solar energy controls the form of water in and on the soil and speeds up chemical reactions, evapotranspiration and biological activity. Reaction rates roughly double with every 10 °C rise.
  • Below about 10 °C biological activity slows; at or below freezing it effectively stops and mineral weathering becomes inactive.
  • In cold climates, decomposition is slow, so organic matter builds a thick O horizon, whose humus then enriches the A horizon.
  • In warm, moist low latitudes, bacteria decompose plant material rapidly; O horizons are usually lacking and the whole profile holds little organic matter.
  • Wide temperature swings, especially with water present, cause shrinking and swelling, frost action and general weathering.
• Seasonality matters: laterite forms characteristically where wet and dry seasons alternate.
• Climate can override rock: in Rajasthan, both granite and sandstone give rise to sandy soils, because heat, aridity and wind make mechanical weathering and wind sorting dominate whatever the parent rock.
• Depth of weathering follows climate. Nikolai Strakhov (1967) showed the weathered mantle to be thin in the tundra and in deserts and tens of metres deep under humid tropical forest, with clay type changing from little-altered minerals in the north to kaolinite and iron–aluminium oxides near the equator.

(ii) Organisms

• Living plants and animals, and their non-living organic products, have a major effect on soil.
  • Plant roots mix and disturb the soil, open channels, weather rock and supply organic matter directly to the upper horizons.
  • Soil organisms range from bacteria and fungi to burrowing mammals.
• Earthworms continually rework the soil, by burrowing and by passing soil through their guts. Charles Darwin devoted his last book to them (1881).
  • Their mixing improves soil structure, increases fertility, lessens accelerated erosion and deepens the profile.
  • Many well-nourished earthworms are almost always a sign of productive, or potentially productive, soil.
• Moles, gophers, rabbits, badgers, prairie dogs and other burrowing animals make larger, tube-like openings; termites and ants do the same work in tropical and dry lands.
• Microorganisms decompose litter, form humus, fix nitrogen and drive the redox changes of gleying.

Natural vegetation and the formation of in-situ soils

• In-situ soils form on their own weathered bedrock, so the vegetation growing on them, more than any outside input, shapes what they become.
• Vegetation starts the soil. Lichens and mosses colonise bare rock, release organic acids and trap dust, making the first thin soil (stage II).
• Litter type sets the humus and acidity:
  • Coniferous forest sheds acid, base-poor needles, forms mor humus and drives podzolisation.
  • Deciduous forest returns base-rich leaves, forms mull humus and favours brown forest soils.
  • Grassland adds fibrous roots throughout the topsoil each year and drives melanisation and calcification: the deep, dark chernozems.
  • Tropical rainforest keeps nutrients in a tight, closed cycle; litter decomposes in weeks and the soil beneath is leached towards laterisation.
• Roots deepen the profile and pump nutrients: deep roots draw bases from the weathering zone and return them to the surface in litter.
• Cover protects the profile: canopy, litter and roots shield the soil from rain splash and erosion, so an in-situ profile can thicken undisturbed. Remove the cover and the A horizon is lost first.
• Vegetation moderates soil climate: shade and litter keep soils cooler and moister, slowing organic losses.
• Nitrogen-fixing plants, such as legumes and alder, enrich the soil they grow in.

(iii) Relief

• Relief, the configuration of the ground surface, influences soil formation in four ways:
• Slope and thickness: soils are thick on gentle slopes and thin on steep slopes, because erosion removes soil faster on steep gradients. Valley floors receive the eroded material and have deep soils.
• Aspect: slopes facing away from the sun are sheltered from direct insolation and have cooler, moister soils; sun-facing slopes are warmer and drier, with more evapotranspiration.
  • In the Himalaya, the north-facing slopes generally carry denser forest and deeper, more humus-rich soils than the south-facing ones.
• Drainage: runoff from uplands makes lowlands wetter, sometimes producing saline sloughs, gleyed soils or organic soils.
• Altitude: in mountains, falling temperature and changing rainfall give a vertical zonation of soils, much like the latitudinal one.
• As a redistributor of climatic inputs, topography thus affects soil processes, soil distribution and vegetation. A regular sequence of soils from hilltop to valley floor on the same parent material is a catena, a term introduced by Geoffrey Milne (1935) in East Africa.

(iv) Parent material

• Soil chemistry is strongly influenced by the parent material.
  • Iron-rich rocks produce soils rich in iron oxides; limestone forms calcium-rich soils; some secondary minerals give soils unique properties.
  • Acid rocks such as granite give base-poor, often acidic soils; basic rocks such as basalt give base-rich, clayey soils.
• Soil texture largely follows the size of mineral grains in the parent material: sandstone gives sandy soils, shale clayey soils.
• Soil thus inherits much of its mineral composition, colour, particle size and chemistry from its parent material.
• Types of parent material:
  • residual, weathered in place from bedrock;
  • transported: alluvial (rivers), aeolian (loess and dune sand), glacial (till), colluvial (slope wash), lacustrine, marine and volcanic.
• The parent-material signal is strongest in young soils and dry climates. Over long periods under a humid climate it fades: granite and basalt in the humid tropics both end as deeply weathered red soils.

Indian case evidence: parent material and regional soils

• Peninsular soils closely reflect their parent rock. Ancient crystalline and metamorphic rocks (granite, gneiss and schist) weather to red soils, coloured by iron oxide.
• Soils derived from lava (the Deccan Trap basalt) are black: the genesis basis of India’s regur (black cotton) soils.
• Sandy soils derive from sandstone, as in parts of Rajasthan and central India.
• The soils of the northern plains (Indo-Gangetic alluvium) were transported from the Himalaya and the Peninsula, so they bear little relation to any rock beneath them: the key distinction between residual and transported soils.
• Laterite in the Western Ghats shows the climate-driven process at work: alternating wet and dry seasons drive leaching of silica and accumulation of iron and aluminium sesquioxides.
• These links are why India’s soil types are often described as reflections of both structure (rock) and process (climate).

(v) Time

• The characteristics of soils take time to develop.
  • A fresh deposit, such as the clean, sorted sand of a dune, may need hundreds to thousands of years to acquire the structure and properties of a mature sandy soil.
  • Many centuries may be needed for even a thin layer of soil on a newly exposed surface. Warm, moist environments develop soil faster.
• Parent material often matters more than elapsed time: soil develops quickly from loose sediments but slowly from hard bedrock.
• Rates: a widely cited rule of thumb holds that about 500 years are needed to form 2.5 cm (1 inch) of topsoil. The FAO puts it more starkly: it can take around a thousand years to create just a few centimetres of topsoil.
  • For practical human purposes, soil lost to erosion is a non-renewable resource.
• Stages of maturity:
  • Young (immature) soils have an A horizon directly over the parent material (A–C) and resemble it closely, as on the fresh khadar alluvium or Himalayan screes.
  • Mature soils have a full A–B–C profile in balance with climate and vegetation.
  • Old soils on ancient, stable surfaces are deeply weathered and leached, like the Ferralsols of the African, Brazilian and Australian shields and the laterites of peninsular India.
• Soils record history. Soils of northern Europe and North America have formed only since the Pleistocene ice sheets melted and are young. Polygenetic soils and buried palaeosols carry features of earlier climates; many laterite caps on Indian plateaus are thought to be relics of wetter past climates.

(vi) Human activity

• Human activity alters both the physical and the chemical nature of soil.
  • Clearing native vegetation for crops can trigger erosion that strips the upper layers rich in organic matter.
  • Centuries of ploughing and planting have greatly changed the structure and composition of large areas of farm soil.
  • Irrigation without drainage causes waterlogging and salinisation; terracing, manuring and puddling for rice build new horizons.
• Such altered soils are recognised as distinct soil classes, as important and as worthy of classification as natural soils.
  • The international World Reference Base (4th edition, 2022) groups them as Anthrosols (long cultivated) and Technosols (urban and industrial soils full of artefacts).
  • The Amazonian dark earths (terra preta), enriched with charcoal and refuse by pre-Columbian peoples, remain fertile today: human activity can enrich as well as degrade.
• The scale today: satellite mapping by ISRO’s Space Applications Centre found about 97.85 million hectares, nearly 30% of India’s area, under land degradation in 2018–19, much of it through human-driven soil erosion and degradation.

Soil genesis in India and the world