Soil Geography: Formation, Pedogenic Processes, Profile, Classification and Conservation

These soil geography notes for OPSC OAS cover the five factors of soil formation and the difference between active and passive formers, the key pedogenic processes, the soil profile and its horizons, the zonal–intrazonal–azonal and USDA classifications, the major soil types with their climates and vegetation, soil degradation and conservation, and the soils of Odisha. Each section is written for both prelims facts and mains-level analysis, and the page closes with previous year practice questions.

Soil Formation: The Five Factors

Soil is the thin, weathered mantle over the Earth's surface produced by two sets of processes: rock weathering, which breaks down parent rock physically and chemically, and pedogenesis, the biological reorganisation of that weathered material into soil. Five factors control the outcome.

Climate and biological activity are active agents because they supply the energy and the organic input that transform regolith into soil. Parent material, relief and time are passive: they condition the process but do not drive it. Relief works through slope: steep slopes such as the Chambal ravines shed water and soil and prevent deep profile development, while gentle slopes and basins accumulate thick fertile alluvium as in the North Indian plain. Parent material fixes the initial texture and mineralogy — a granite parent gives sandy, silica-rich material, a basalt parent gives clay-rich, iron-rich material.

UPPSC 2018 asked which term is concerned with soil — edaphic. Edaphic factors are the soil conditions of texture, structure, pH, moisture and nutrients that affect plants.

Pedogenic Processes: How Soil Differentiates

Pedogenic processes are the physical, chemical and biological processes that create horizons and distinct soil types. They are environment-specific: the same parent material can become a podzol in a cool coniferous climate and a laterite in a hot humid one.

Eluviation is the removal of material, chiefly mechanical, from the upper horizon; leaching is the removal of material in chemical solution. Illuviation is the deposition of that material in the B-horizon. The paired terms describe a single vertical transfer: the E-horizon loses, the B-horizon gains.

NDA 2009 and OPSC-style questions repeatedly test the laterisation–podzolisation–salinisation triad. Laterisation removes silica and leaves iron and aluminium; podzolisation removes iron and aluminium as chelates; salinisation accumulates salts at the surface.

Soil Profile and Horizons

A soil profile is the vertical section from the surface down to the parent rock, showing the sequence of horizons. A horizon is a layer approximately parallel to the surface that differs in colour, texture, structure and composition from the layers above and below.

The solum is the true soil, comprising the A, E and B horizons. The regolith extends from the surface through the C-horizon to the top of the bedrock, including the loose weathered material. Horizonation, the differentiation of soil with depth, is produced by eluviation, illuviation and humification.

NDA 2012 asked what a soil profile is — the vertical arrangement of horizons up to the parent rock. CDS 2014 asked why clay layers are poor aquifers — their inter-particle spaces are the smallest.

Classification of Soils: Zonal, Intrazonal and Azonal

The zonal system groups soils by the dominant soil-forming factor. Zonal soils are mature soils whose profile reflects climate and vegetation, such as podzols, laterites, chernozems and sierozems. Intrazonal soils are shaped by local factors such as waterlogging or salinity, overriding climate, and include hydromorphic, halomorphic and calcimorphic soils. Azonal soils are young soils with little horizon development because time or transport has prevented profile formation; lithosols, regosols and alluvial soils are examples.

Zonal soils are further divided into pedalfers and pedocals. Pedalfers are leached soils rich in aluminium and iron, characteristic of humid regions with a water surplus, and include podzols, laterites and tundra soils. Pedocals are calcium-rich soils of regions with a water deficit where evaporation exceeds rainfall, and include chernozems, chestnut and brown steppe soils and desert soils.

The USDA soil taxonomy, by contrast, uses twelve orders defined by measurable soil properties. Entisols are young soils with minimal horizons, vertisols are clay-rich and cracking, spodosols correspond to podzols, oxisols to laterites and latosols, histosols are organic soils such as peats, and mollisols are the dark, base-rich grassland soils including chernozems.

SSC CGL 2018 asked which is an azonal soil — lithosol. CDS 2019 matched entisols with alluvial soils, vertisols with grumusols, spodosols with podzols and oxisols with latisols.

Major Soil Types and Their Environments

Laterite soils

Laterite takes its name from the Latin later, meaning brick, because the moist soil can be cut into blocks that harden on exposure. It forms under high temperature and heavy tropical rainfall through intense leaching: lime and silica are washed away, leaving a residue rich in iron oxide and aluminium compounds. Laterites are therefore poor in organic matter, nitrogen, phosphate and calcium but rich in iron oxide and potash, and they are porous and often vesicular.

In India, laterites occur in Kerala, Karnataka, Tamil Nadu, Maharashtra, Chhattisgarh, and the hilly areas of Odisha and Assam, chiefly on hill summits and uplands. They support tree crops such as cashew, rubber, tea and coffee rather than cereals, and they are widely cut as building bricks.

NDA and OPSC-style questions ask why laterites are porous — the leaching of soluble minerals and silica leaves a porous iron-rich residue.

Podzols and spodosols

Podzols form under cool, humid coniferous forest, where acidic litter produces organic chelates that carry iron and aluminium down to a dark, indurated B-horizon. The profile is sharply differentiated, with a bleached E-horizon over the illuvial B. Podzols are acidic, base-poor and infertile for grain crops, and they require liming and fertiliser. In USDA taxonomy they are spodosols, and their formation process is cheluviation.

Chernozems and temperate grassland soils

Chernozems are deep, dark, humus-rich soils of the temperate grasslands such as the Ukrainian and Russian steppes. They form where evaporation and precipitation are nearly balanced, so calcium carbonate accumulates and organic matter is not fully leached away. They are among the most fertile soils in the world and are the classic pedocal.

Vertisols and black cotton soils

Vertisols, also called grumusols, are clay-rich soils of tropical and subtropical grasslands with a marked dry season. Their swelling clays crack deeply in summer and swell on wetting, producing a self-mulching surface. They are rich in clay and bases but poor in humus and nitrogen, which is why they need fertiliser despite their reputation for fertility.

Oxisols and ferralsols

Oxisols are the deeply weathered, highly leached soils of the humid tropics, rich in iron and aluminium oxides. They correspond to laterites and latosols, and their FAO equivalent is ferralsols. They are old, strongly structured soils with low nutrient reserves.

Azonal and intrazonal soils

Lithosols are thin soils over bare rock, regosols are soils of loose coarse rock fragments, and alluvial soils are transported and deposited by rivers — khadar being the new alluvium of the floodplain and bangar the older alluvium of the terrace. Hydromorphic soils develop under waterlogging and show gleying; halomorphic soils are salt-affected; calcimorphic soils are calcium-rich.

CDS 2017 asked where vertisols develop — tropical grasslands. CDS 2018 asked which statement about vertisols is not correct — that they are rich in humus, which they are not.

Soil Degradation and Conservation for Mains

Soil degradation is the decline in soil quality and fertility through erosion, salinisation, waterlogging, nutrient depletion and loss of organic matter. In India, water erosion is the dominant process, followed by wind erosion, salinity and alkalinity. Erosion rates are projected to rise from about 35% to 40% of the land area under degradation, with the Chambal ravines, the Shiwaliks, the Deccan black soil belt and the coastal sands among the worst-affected regions.

Causes include deforestation, overgrazing, shifting cultivation, improper irrigation, excessive tillage and encroachment on marginal lands. Consequences include declining yields, reservoir siltation, desertification and reduced groundwater recharge.

For mains answers, link soil degradation to food security, rural livelihoods and SDG 15.3 on land degradation neutrality, and cite specific Indian programmes such as the National Mission for Sustainable Agriculture, soil health cards and watershed development.

Odisha Angle: Eight Soil Groups

Odisha's soils are highly heterogeneous because of the interplay of climate, topography and parent material, and this heterogeneity is one reason average crop productivity in the state is low. The state's soils are conventionally divided into eight broad groups.

Laterite soils of Odisha are naturally poor in nitrogen and phosphorus and need heavy fertilisation, while the deltaic alluvial soils are the state's most fertile. The coastal saline belt needs drainage and gypsum for reclamation, and the red and laterite uplands are prone to erosion and respond well to contour bunding and afforestation.

Practice Questions