🪨 Soil Formation & Soil Profile
Soil definition, minerals, weathering, soil formation (pedogenesis), soil profile, horizons, soil types of India for CUET Agriculture
Understanding how soil forms is foundational to agriculture. Soil is not just "dirt" — it is a dynamic, living system made up of minerals, organic matter, water, and air. The journey from rock to soil involves complex processes spanning thousands of years, and the type of soil that forms determines what crops can thrive in a given region.
Soil Minerals
Minerals are naturally occurring inorganic substances with a definite chemical composition and crystalline structure. They are the primary source of plant nutrients in soil. When rocks break down over time, the minerals within them become part of the soil, releasing nutrients that plants absorb through their roots.
Primary Minerals
Primary minerals are those that have not been altered chemically since their formation in igneous or metamorphic rock. They are dominant in the sand and silt fractions of soil (particle size > 0.002 mm). Because they have not undergone chemical transformation, they retain the composition of the original parent rock.
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Understanding how soil forms is foundational to agriculture. Soil is not just "dirt" — it is a dynamic, living system made up of minerals, organic matter, water, and air. The journey from rock to soil involves complex processes spanning thousands of years, and the type of soil that forms determines what crops can thrive in a given region.
Soil Minerals
Minerals are naturally occurring inorganic substances with a definite chemical composition and crystalline structure. They are the primary source of plant nutrients in soil. When rocks break down over time, the minerals within them become part of the soil, releasing nutrients that plants absorb through their roots.
Primary Minerals
Primary minerals are those that have not been altered chemically since their formation in igneous or metamorphic rock. They are dominant in the sand and silt fractions of soil (particle size > 0.002 mm). Because they have not undergone chemical transformation, they retain the composition of the original parent rock.
| Mineral | Chemical Formula | Key Features |
|---|---|---|
| Quartz | SiO₂ | Most resistant to weathering; makes up ~36% of earth's crust; dominant in sandy soils |
| Feldspar | KAlSi₃O₈ (Orthoclase) | Most abundant mineral (~48% of earth's crust); source of K |
| Na-Feldspar (Albite) | NaAlSi₃O₈ | Source of Na |
| Ca-Feldspar (Anorthite) | CaAl₂Si₂O₈ | Source of Ca |
| Mica | — | Makes up ~12% of earth's crust |
| K-mica (Muscovite) | White mica | Source of K |
| Mg-mica (Biotite) | Black mica | Source of K and Mg |
IMPORTANT
Composition of minerals in earth's crust: Feldspar (48%) > Quartz (36%) > Mica (12%). This order is a frequently tested fact — remember it as F-Q-M (descending).
Secondary Minerals
Secondary minerals are formed by the weathering of primary minerals. Unlike primary minerals, these have undergone chemical transformation and are dominant in the clay fraction (particle size < 0.002 mm). Their small size gives them a very high surface area, making them chemically active and crucial for nutrient retention in soil.
| Mineral | Examples | Key Features |
|---|---|---|
| Iron oxides | Hematite (Fe₂O₃) — Red colour; Limonite (Fe₂O₃·3H₂O) — Yellow; Magnetite (Fe₃O₄) | Give red/yellow colour to soils |
| Aluminium oxides | Gibbsite — Al(OH)₃ | Common in laterite soils |
| Silicate clays | Kaolinite, Montmorillonite, Illite, Vermiculite | Most important secondary minerals for soil properties |
| Carbonates | Calcite (CaCO₃) | Found in alkaline/calcareous soils |
| Sulphates | Gypsum (CaSO₄·2H₂O) | Source of Ca and S |
The silicate clays (Kaolinite, Montmorillonite, Illite) are especially significant because they determine a soil's ability to hold water and nutrients. We will study these in detail under Soil Colloids.
Nutrient Sources from Minerals
Each essential plant nutrient ultimately comes from a mineral source — except nitrogen, which primarily comes from organic matter and atmospheric fixation.
| Nutrient | Mineral Source |
|---|---|
| Nitrogen (N) | Organic matter (not from minerals) |
| Phosphorus (P) | Apatite |
| Potassium (K) | Feldspar, Mica |
| Calcium (Ca) | Calcite, Feldspar, Gypsum |
| Magnesium (Mg) | Biotite, Dolomite |
| Iron (Fe) | Hematite, Magnetite, Limonite |
| Sulphur (S) | Gypsum, Pyrite |
Classification of Minerals
Minerals can be classified on several bases. Understanding these groupings helps in predicting soil behaviour:
| Basis | Types |
|---|---|
| By origin | Primary (from parent rock) and Secondary (from weathering) |
| By crystallinity | Crystalline (quartz, feldspar) and Amorphous/Non-crystalline (allophane) |
| By chemical composition | Silicates (quartz, mica, kaolinite), Oxides (hematite), Sulphates (gypsum), Carbonates (calcite), Native minerals (gold, silver, sulphur) |
Weathering of Rocks
The breakdown of rocks and large particles into smaller particles by physical, chemical, and biological forces is called weathering. It is the first step in soil formation — without weathering, there would be no soil. The type and intensity of weathering depends largely on climate.
1. Physical Weathering (Disintegration)
Physical weathering involves the conversion of large particles into smaller particles by physical forces without changing chemical composition. The rock simply breaks apart into smaller pieces of the same material.
| Agent | Mechanism |
|---|---|
| Temperature | Day: expansion; Night: contraction → cracking (called exfoliation or thermal stress) |
| Water | River erosion, abrasion by running water carrying sediment |
| Ice/Frost | Water freezes in cracks → 9% volume increase → rock shatters (called frost wedging) |
| Glacier | Grinding and scraping of rocks as glaciers move slowly downhill |
| Wind | Abrasion in arid regions where wind-blown sand strikes rock surfaces |
| Lightning | Shattering of exposed rocks through sudden intense heat |
TIP
Frost wedging is particularly important in hilly and temperate regions. The 9% volume expansion when water freezes is a commonly asked value.
2. Chemical Weathering (Decomposition)
Chemical weathering involves the conversion of large particles into smaller particles through chemical reactions. Unlike physical weathering, new substances are formed — the mineral composition changes.
| Reaction | Process | Example |
|---|---|---|
| Hydrolysis | Water molecules split minerals by reacting with them | KAlSi₃O₈ + H₂O → HAlSi₃O₈ + KOH (Orthoclase → new mineral + potash) |
| Hydration | Minerals absorb water molecules into their crystal structure, expanding | CaSO₄ + 2H₂O → CaSO₄·2H₂O (Anhydrite → Gypsum) |
| Oxidation | Minerals react with oxygen, changing their oxidation state | 2Fe₃O₄ + ½O₂ → 3Fe₂O₃ (Magnetite → Hematite — grey to red) |
| Carbonation | CO₂ dissolves in water forming carbonic acid, which attacks minerals | CaCO₃ + H₂CO₃ → Ca(HCO₃)₂ (Calcite → soluble bicarbonate) |
| Solution | Direct dissolution of minerals in water | NaCl → Na⁺ + Cl⁻ |
Hydrolysis is the most important chemical weathering reaction in soils, responsible for the breakdown of most silicate minerals. Carbonation is particularly important in limestone regions.
3. Biological Weathering (Disintegration + Decomposition)
Biological weathering involves both physical and chemical processes driven by living organisms:
| Agent | Mechanism |
|---|---|
| Human and Animals | Digging, burrowing creates pathways for water and air |
| Plant Roots | Physical pressure in cracks; organic acid secretion dissolves minerals |
| Micro-organisms | Produce organic acids that dissolve minerals; decompose organic matter producing CO₂ (which forms carbonic acid) |
| Insects | Burrowing, mixing of soil layers (e.g., termites, ants) |
Factors Affecting Weathering
NOTE
- Tropical regions → Chemical weathering dominates because high temperature speeds up chemical reactions and high rainfall provides abundant water for hydrolysis and carbonation.
- Arid and Temperate regions → Physical weathering dominates because of extreme temperature fluctuations (day/night) and limited moisture for chemical reactions.
Soil Formation (Pedogenesis)
Soil is formed by the weathering of rocks and decomposition of organic matter over thousands of years. The process of soil formation is called pedogenesis (from Greek: pedon = ground + genesis = origin).
Factors of Soil Formation (Jenny's Equation)
Hans Jenny (1941) expressed soil formation as a function of five factors:
S = f(Cl, O, R, P, T)
| Factor | Symbol | Description |
|---|---|---|
| Climate | Cl | Temperature, rainfall — most important factor overall |
| Organisms | O | Plants, animals, microorganisms — contribute organic matter |
| Relief/Topography | R | Slope, elevation, aspect — controls drainage and erosion |
| Parent Material | P | Rock from which soil is derived — determines initial mineral composition |
| Time | T | Duration of soil formation — older soils are more developed |
IMPORTANT
V.V. Dokuchaev (the Father of Soil Science) originally proposed 3 factors of soil formation: Climate, Parent material, and Biosphere/Organism. Hans Jenny (1941) expanded this to 5 factors by adding Relief and Time.
Pedology vs Edaphology — What's the difference?
**Pedology** (Greek: *Pedon* = soil + *logos* = study) is the study of soil **origin, classification, and properties** — it treats soil as a natural body. **Edaphology** (Greek: *Edaphon* = soil) studies soil **in relation to plant growth and crop production** — it treats soil as a medium for plants. For agricultural purposes, edaphology is more directly relevant, but pedology provides the scientific foundation.Basic Soil Forming Processes
These three fundamental processes operate in all soils:
- Humification — Decomposition of organic matter and formation of humus; this process builds the O horizon (organic layer) at the surface
- Eluviation — Removal (washing out) of materials (clay, humus, iron, aluminium oxides) from the upper horizon by percolating water. Think of it as the "exit" of materials — the E in E-horizon stands for eluviation
- Illuviation — Deposition (washing in) of leached materials in the lower horizon. The materials removed by eluviation accumulate here, enriching the B-horizon
Specific Soil Forming Processes
These processes operate under specific environmental conditions and produce distinctive soil types:
| Process | Description | Region |
|---|---|---|
| Laterization | Accumulation of Fe and Al oxides (sesquioxides); silica is leached out. Produces hard, brick-like soil | Tropical, high rainfall |
| Podzolization | Leaching of Fe and Al from A-horizon; silica accumulates (opposite of laterization) | Cool, humid (coniferous forests) |
| Calcification | Accumulation of CaCO₃ in subsoil due to upward movement of calcium-rich water | Arid/semi-arid regions |
| Decalcification | Removal of CaCO₃ from soil by leaching (reverse of calcification) | Humid regions |
| Salinization | Accumulation of soluble salts (NaCl, Na₂SO₄) at soil surface due to evaporation | Arid, poor drainage |
| Desalinization | Removal of soluble salts by leaching with water (reclamation process) | Reclamation of saline soils |
| Gleization | Reduction of Fe under waterlogged conditions → grey/blue colour (indicates anaerobic environment) | Wetlands, paddy fields |
| Pedoturbation | Mixing of developed soil horizons, disrupting profile development | Various |
TIP
Memory aid for Laterization vs Podzolization: In Laterization (tropical), Fe and Al stay while silica leaves. In Podzolization (cold), silica stays while Fe and Al leave. They are opposite processes.
Types of Pedoturbation:
- Floral Pedoturbation — by plants (root growth pushing soil layers)
- Faunal Pedoturbation — by insects, ants, earthworms, rodents (burrowing and mixing)
- Argilli Pedoturbation — mixing due to swelling and shrinkage of clay → found in Black soils (Vertisols). The high montmorillonite clay content causes deep cracks in summer and swelling in the rainy season, churning the soil.
Soil Profile
A vertical cross-section of soil from the surface to the parent rock is called a soil profile. It consists of distinct layers called horizons, each with different physical and chemical properties.
Soil Horizons
| Horizon | Name | Description |
|---|---|---|
| O | Organic horizon | Surface layer of organic matter (litter, humus); found mainly in forests |
| A | Topsoil | Zone of eluviation; dark colour due to humus; most fertile layer |
| E | Eluvial horizon | Zone of maximum leaching; lighter in colour due to loss of clays and oxides |
| B | Subsoil | Zone of illuviation; accumulation of clay, iron, aluminium washed down from above |
| C | Parent material | Partially weathered rock; little biological activity |
| R | Bedrock | Unweathered, solid rock |
IMPORTANT
Key definitions to remember:
- Solum = A + B horizons (or A + E + B) — the true soil where biological and chemical processes are active
- Regolith = A + B + C — all unconsolidated material above bedrock
- Master Horizons = 5 (O, A, E, B, C)
- Furrow Slice = top 15 cm of soil (plough layer); Weight of 1 ha furrow slice = 2.25 x 10⁶ kg/ha; 1 cm soil = 150 tonnes/ha
TIP
The A-horizon is the most important for agriculture as it contains maximum organic matter and biological activity. When we talk about "topsoil erosion," we are talking about loss of this critical A-horizon.
Soil Air Composition
Soil air differs from atmospheric air mainly in its CO₂ content, which is much higher because plant roots and microorganisms constantly respire, releasing CO₂:
| Gas | Atmosphere | Soil Air |
|---|---|---|
| N₂ | 79.0% | 79.2% (slightly more) |
| O₂ | 20.95% | 20.60% |
| CO₂ | 0.03% | 0.25–0.30% (8–10 times more than atmosphere) |
| Ar | 0.94% | 0.90% |
NOTE
With increase in depth, CO₂ concentration increases and O₂ concentration decreases. This is because deeper soil has less air exchange with the atmosphere and more root respiration. Waterlogged soils have almost no O₂, which is why drainage is essential.
Soil Types in India
India has remarkably diverse soil types due to its varied climate, vegetation, and parent material. ICAR classifies Indian soils into 8 major groups:
| Soil Type | Area (%) | Colour | Key Features | Distribution |
|---|---|---|---|---|
| Alluvial | ~43% | Light grey to ash | Most fertile; Khadar (new) & Bangar (old) | Indo-Gangetic plain, river deltas |
| Black (Regur) | ~15% | Black/dark grey | High clay (montmorillonite), self-ploughing, high moisture retention | Deccan Plateau (Maharashtra, MP, Gujarat) |
| Red | ~18% | Red/yellow | Rich in iron oxide, poor in N, P, humus | Eastern Deccan, Odisha, Chhattisgarh |
| Laterite | ~3.7% | Red/brown | Leached, acidic, rich in Fe & Al oxides | Western Ghats, NE India, high rainfall areas |
| Desert/Arid | ~6.4% | Sandy/brown | Sandy, poor water retention, high CaCO₃ | Rajasthan, parts of Gujarat, Haryana |
| Forest/Mountain | ~7.9% | Dark brown | Rich in humus, acidic | Himalayan region, Western Ghats |
| Saline & Alkaline | ~1.5% | White crusts | High pH (Usar/Reh), poor structure | Parts of UP, Punjab, Haryana, Gujarat |
| Peaty/Marshy | ~0.4% | Black/dark | Waterlogged, high organic matter, acidic | Kerala, Sundarbans, coastal areas |
Alluvial soils cover the largest area and are the backbone of Indian agriculture, supporting the food production of the Indo-Gangetic plains. Black soils are ideal for cotton cultivation due to their high moisture retention.
Special Terms
- Khadar: New alluvium deposited by recent floods; more fertile, found near river channels
- Bangar: Old alluvium deposited away from flood plain; less fertile, found on higher terraces
- Bhangar: Calcareous concretions (kankar) found in old alluvium
- Regur: Black cotton soil (from Telugu "Reguda" meaning black)
- Usar/Reh: Alkaline soil with white salt efflorescence on the surface
Why are Black soils called 'self-ploughing'?
Black soils are rich in **montmorillonite clay**, which swells enormously when wet and shrinks when dry, forming deep wide cracks. This repeated swelling-shrinking action naturally turns and mixes the soil — hence the term "self-ploughing." In the USDA Soil Taxonomy, these are classified as **Vertisols** (from Latin *vertere* = to turn).Soil Classification Systems
1. USDA Soil Taxonomy (Modern System)
The USDA Soil Taxonomy is the internationally recognized modern classification. It groups soils into orders based on their properties and formation processes:
| Order | Key Feature | Indian Equivalent |
|---|---|---|
| Entisols | No profile development (very young) | Young alluvial soils |
| Inceptisols | Beginning development (incipient) | Most Indian alluvial soils |
| Vertisols | High shrink-swell clay | Black soils |
| Aridisols | Dry conditions, salt accumulation | Desert soils |
| Alfisols | Moderate leaching, clay-rich B horizon | Red & yellow soils |
| Ultisols | Strongly leached, acidic | Laterite soils |
| Mollisols | Dark, organic-rich surface layer | Northern mountain soils |
| Histosols | Organic soils (peat/muck) | Peaty/marshy soils |
TIP
For CUET, focus on matching Indian soil types with their USDA orders: Vertisols = Black, Aridisols = Desert, Alfisols = Red, Ultisols = Laterite.
2. ICAR Classification
ICAR classifies Indian soils into 8 major groups (as shown in the soil types table above). This is the most commonly used classification in Indian agricultural context and CUET examinations.
Summary Cheat Sheet
| Concept / Topic | Key Details / Explanation |
|---|---|
| Soil definition | Dynamic living system of minerals, organic matter, water, and air |
| Primary minerals | Not chemically altered; dominant in sand & silt fraction; Feldspar, Quartz, Mica |
| Earth's crust composition | Feldspar (48%) > Quartz (36%) > Mica (12%) — remember F-Q-M |
| Secondary minerals | Formed by weathering of primary minerals; dominant in clay fraction (<0.002 mm); Kaolinite, Montmorillonite, Illite |
| Nutrient sources | N → organic matter (not minerals); P → Apatite; K → Feldspar, Mica; Ca → Calcite, Gypsum; S → Gypsum, Pyrite |
| Physical weathering | Disintegration without chemical change; by temperature, water, ice (9% volume increase in frost wedging), glacier, wind, lightning |
| Chemical weathering | Decomposition — new substances formed; Hydrolysis (most important), Hydration, Oxidation, Carbonation, Solution |
| Biological weathering | By humans, animals, plant roots, microorganisms, insects — both physical + chemical |
| Tropical regions | Chemical weathering dominates (high temp + rainfall) |
| Arid/Temperate regions | Physical weathering dominates (temperature extremes) |
| Pedogenesis | Process of soil formation (Greek: pedon=ground, genesis=origin) |
| Father of Soil Science | V.V. Dokuchaev — proposed 3 factors (Climate, Parent material, Organism) |
| Jenny's Equation (1941) | S = f(Cl, O, R, P, T) — 5 factors: Climate, Organisms, Relief, Parent material, Time |
| Most important factor | Climate (temperature + rainfall) |
| Pedology vs Edaphology | Pedology = soil origin/classification; Edaphology = soil in relation to plant growth |
| Humification | Decomposition of organic matter → humus formation (builds O horizon) |
| Eluviation | Removal (washing out) of materials from upper horizon → E horizon |
| Illuviation | Deposition (washing in) of materials in lower horizon → B horizon |
| Laterization | Fe & Al oxides accumulate, silica leached; tropical, high rainfall; hard brick-like soil |
| Podzolization | Silica accumulates, Fe & Al leached; cool, humid regions; opposite of laterization |
| Calcification | CaCO₃ accumulates in subsoil; arid/semi-arid regions |
| Salinization | Soluble salt accumulation at surface (NaCl, Na₂SO₄); arid, poor drainage |
| Gleization | Fe reduction under waterlogged conditions → grey/blue colour |
| Argilli Pedoturbation | Swelling-shrinkage mixing in Black soils (Vertisols) due to montmorillonite clay |
| Soil Profile | Vertical cross-section: surface to bedrock |
| Soil Horizons | O (organic) → A (topsoil, most fertile, eluviation) → E (max leaching) → B (subsoil, illuviation) → C (parent material) → R (bedrock) |
| Solum | A + B horizons — the true soil |
| Regolith | A + B + C — all unconsolidated material above bedrock |
| Furrow Slice | Top 15 cm; weight = 2.25 × 10⁶ kg/ha; 1 cm soil = 150 tonnes/ha |
| Soil air CO₂ | 0.25-0.30% (8-10× more than atmosphere's 0.03%) |
| ICAR soil groups | 8 major groups of Indian soils |
| Alluvial soil | ~43% area (largest); Khadar (new, fertile) & Bangar (old); Indo-Gangetic plain |
| Black soil (Regur) | ~15%; high clay (montmorillonite); self-ploughing; high moisture; Deccan Plateau; ideal for cotton |
| Red soil | ~18%; rich in iron oxide; poor in N, P, humus; Eastern Deccan |
| Laterite soil | ~3.7%; leached, acidic, rich in Fe & Al oxides; Western Ghats, NE India |
| Desert/Arid soil | ~6.4%; sandy, poor water retention, high CaCO₃; Rajasthan |
| Saline & Alkaline soil | ~1.5%; high pH; Usar/Reh (white salt crusts) |
| Peaty/Marshy soil | ~0.4%; waterlogged, high OM, acidic; Kerala, Sundarbans |
| USDA orders — key matches | Vertisols = Black; Aridisols = Desert; Alfisols = Red; Ultisols = Laterite |
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