🧪 Soil Physical & Chemical Properties
Texture, structure, colour, porosity, density, soil water, consistency, soil colloids, CEC, pH, EC for CUET Agriculture
Soil properties — both physical and chemical — govern how well a soil can support plant growth. Physical properties affect water movement, root penetration, and aeration, while chemical properties determine nutrient availability and soil reaction. Understanding these properties is essential for proper soil management in agriculture.
Physical Properties of Soil
1. Soil Texture
Soil texture refers to the relative proportion of sand, silt, and clay particles in soil. It is one of the most fundamental properties because it influences almost every other soil characteristic — water holding capacity, permeability, aeration, nutrient retention, and workability.
ISSS Classification (International Society of Soil Science):
| Particle | Size (mm) |
|---|---|
| Coarse Sand | 2.0 – 0.2 |
| Fine Sand | 0.2 – 0.02 |
| Silt | 0.02 – 0.002 |
| Clay | < 0.002 |
USDA Classification (United States Department of Agriculture):
The USDA system divides particles into more detailed size classes, which allows for finer distinctions:
| Particle | Size (mm) | Feel |
|---|---|---|
| Very Coarse Sand | 2.0 – 1.0 | Gritty |
| Coarse Sand | 1.0 – 0.5 | Gritty |
| Medium Sand | 0.5 – 0.25 | Gritty |
| Fine Sand | 0.25 – 0.10 | Gritty |
| Very Fine Sand | 0.10 – 0.05 | Slightly gritty |
| Silt | 0.05 – 0.002 | Smooth, floury |
| Clay | < 0.002 | Sticky, plastic |
NOTE
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Soil properties — both physical and chemical — govern how well a soil can support plant growth. Physical properties affect water movement, root penetration, and aeration, while chemical properties determine nutrient availability and soil reaction. Understanding these properties is essential for proper soil management in agriculture.
Physical Properties of Soil
1. Soil Texture
Soil texture refers to the relative proportion of sand, silt, and clay particles in soil. It is one of the most fundamental properties because it influences almost every other soil characteristic — water holding capacity, permeability, aeration, nutrient retention, and workability.
ISSS Classification (International Society of Soil Science):
| Particle | Size (mm) |
|---|---|
| Coarse Sand | 2.0 – 0.2 |
| Fine Sand | 0.2 – 0.02 |
| Silt | 0.02 – 0.002 |
| Clay | < 0.002 |
USDA Classification (United States Department of Agriculture):
The USDA system divides particles into more detailed size classes, which allows for finer distinctions:
| Particle | Size (mm) | Feel |
|---|---|---|
| Very Coarse Sand | 2.0 – 1.0 | Gritty |
| Coarse Sand | 1.0 – 0.5 | Gritty |
| Medium Sand | 0.5 – 0.25 | Gritty |
| Fine Sand | 0.25 – 0.10 | Gritty |
| Very Fine Sand | 0.10 – 0.05 | Slightly gritty |
| Silt | 0.05 – 0.002 | Smooth, floury |
| Clay | < 0.002 | Sticky, plastic |
NOTE
The key difference between ISSS and USDA systems is the silt-sand boundary: ISSS uses 0.02 mm while USDA uses 0.05 mm. CUET may test this distinction.
Properties Comparison:
The three particle types have contrasting physical behaviours. Clay particles are so small they have enormous surface area relative to their volume, which explains their high water-holding capacity:
| Property | Sand | Silt | Clay |
|---|---|---|---|
| Water holding capacity | Low | Medium | High |
| Permeability | High | Medium | Low |
| Porespace % | Low total | Medium | High total |
| Aeration | Good | Medium | Poor |
Textural Classes (USDA system — 12 classes): Sandy, Loamy sand, Sandy loam, Loam, Silt loam, Silt, Sandy clay loam, Clay loam, Silty clay loam, Sandy clay, Silty clay, Clay
- Ideal soil for most crops: Loam (balanced mix of sand, silt, clay) — it combines good drainage with adequate moisture retention
- Texture is determined using the Textural Triangle (graphical method) or Bouyoucos Hydrometer Method (laboratory method based on particle settling rates)
- Soil texture is a basic/fundamental property — it cannot be changed by cultivation or management. This is a key distinction from structure.
- Silica constitutes approximately 31% of soil by weight
IMPORTANT
Remember: Texture cannot be changed; Structure can be changed. This is one of the most frequently tested distinctions in soil science.
2. Soil Structure
Soil structure refers to the arrangement of soil particles into aggregates (clumps). While individual particles are sand, silt, and clay, structure describes how these particles group together. Good structure creates pore spaces for air and water movement.
- Ped = Natural soil aggregate (formed by natural processes like wetting-drying cycles)
- Clod = Artificial soil aggregate (formed by tillage operations)
| Type | Shape | Ped Size | Found In |
|---|---|---|---|
| (a) Platy/Laminar | Flat, plate-like; horizontal axis > vertical axis | 1–10 mm | Forest soils; E-horizon; impedes water movement because plates stack horizontally |
| (b) Prismatic | Vertical columns; vertical axis > horizontal axis | 10–100 mm | B-horizon of arid soils |
| — Prismatic | Flat top | — | Subsoil |
| — Columnar | Rounded top (pillar-like) | — | Sodic/alkaline soils (diagnostic feature) |
| (c) Blocky | Cube-shaped; equal axes | 5–50 mm | B-horizon (subsoil) |
| — Angular | Sharp edges/corners | — | Subsoil |
| — Sub-angular | Rounded edges | — | Subsoil |
| (d) Spheroidal | Roughly spherical | 1–10 mm | A-horizon of cultivated soils |
| — Granular | Less porous | 1–10 mm | Surface soil |
| — Crumby | More porous | 1–5 mm | Best for crop production |
| Single grain | No aggregation | — | Sandy soils (particles too large to aggregate) |
| Massive | No structure | — | Heavy clay soils (particles too fine, stick together) |
TIP
For CUET: Crumby structure is best for crops (porous, good aeration). Columnar structure indicates sodic soils. Platy structure impedes water infiltration.
How can soil structure be improved?
Structure can be improved by: (1) adding **organic matter** (FYM, compost) which acts as a binding agent, (2) **liming** acidic soils (Ca²⁺ promotes flocculation), (3) applying **gypsum** to sodic soils (Ca²⁺ replaces Na⁺), (4) **crop rotation** including deep-rooted crops and legumes, and (5) avoiding excessive tillage when soil is too wet or too dry.3. Soil Colour
Soil colour is a quick visual indicator of soil composition and drainage conditions:
- Determined using the Munsell Colour Chart (standardized colour system using Hue, Value, and Chroma)
- Dark/Black → High organic matter content
- Red/Yellow → Presence of iron oxides (hematite = red, limonite = yellow)
- Grey/Blue → Waterlogged conditions (gleying — iron is in reduced Fe²⁺ form)
- White → Calcium carbonate or salt accumulation on the surface
4. Soil Porosity
Porosity is the fraction of soil volume occupied by pore spaces (filled with air and water):
- Porosity = Volume of pore space / Total soil volume × 100
- Sandy soils: large pores (macropores), but low total porosity (~35-40%)
- Clay soils: small pores (micropores), but high total porosity (~50-60%)
- Ideal porosity: ~50% (50% solid, 25% water, 25% air)
NOTE
Macropores (in sandy soils) allow rapid drainage and good aeration but poor water retention. Micropores (in clay soils) hold water tightly but restrict drainage and aeration. The ideal soil has a balance of both — which is why loam is preferred.
5. Bulk Density and Particle Density
These two density measurements help characterize soil compaction and porosity:
| Property | Formula | Typical Values |
|---|---|---|
| Bulk Density (Db) | Mass of dry soil / Total volume (including pores) | 1.1–1.6 g/cm³ |
| Particle Density (Dp) | Mass of dry soil / Volume of solids only | ~2.65 g/cm³ (constant for most mineral soils) |
- Porosity = (1 – Db/Dp) × 100 — this formula connects all three properties
- Low bulk density → Good aeration, easy root growth, healthy soil
- High bulk density → Compacted soil, poor root penetration, restricted water movement
- Organic matter addition decreases bulk density (organic matter is lighter than mineral particles)
6. Soil Water
Soil water exists in different forms depending on how tightly it is held by soil particles. Not all soil water is available to plants:
| Type | Held Between | Availability |
|---|---|---|
| Gravitational water | Saturation to Field Capacity | Not available (drains away quickly under gravity) |
| Capillary water | Field Capacity to PWP | Available to plants — this is the water crops actually use |
| Hygroscopic water | Below PWP | Not available (held too tightly as thin films around particles) |
IMPORTANT
Key moisture constants:
- Field Capacity (FC): Water held at –1/3 bar (–33 kPa) — the upper limit of available water
- Permanent Wilting Point (PWP): Water held at –15 bar (–1500 kPa) — the lower limit; plants wilt permanently below this
- Available Water = FC – PWP — this is the usable water range for crops
7. Soil Consistency
Consistency describes the resistance of soil to deformation at various moisture levels. It is important for determining the right time for tillage:
- Wet: Stickiness and plasticity (soil sticks and can be moulded)
- Moist: Friability — ideal condition for tillage because soil crumbles easily without compacting
- Dry: Hardness (soil resists deformation, difficult to plough)
Chemical Properties of Soil
1. Soil pH
Soil pH measures the hydrogen ion concentration in the soil solution and indicates whether soil is acidic, neutral, or alkaline:
- Formula: pH = –log[H⁺]
- Scale: 0 to 14 (7 is neutral)
| pH Range | Classification |
|---|---|
| < 4.5 | Extremely acidic |
| 4.5 – 5.5 | Strongly acidic |
| 5.5 – 6.5 | Moderately acidic |
| 6.5 – 7.5 | Neutral |
| 7.5 – 8.5 | Moderately alkaline |
| 8.5 – 9.5 | Strongly alkaline |
| > 9.5 | Extremely alkaline |
- Ideal pH for most crops: 6.0 – 7.5
- Acidic soil correction: Liming (CaCO₃) — the Ca²⁺ and OH⁻ ions neutralize acidity
- Alkaline soil correction: Gypsum (CaSO₄·2H₂O) to replace Na⁺ with Ca²⁺, or elemental sulphur which bacteria oxidize to H₂SO₄
2. Cation Exchange Capacity (CEC)
CEC is the ability of soil to hold and exchange positively charged ions (cations) such as Ca²⁺, Mg²⁺, K⁺, Na⁺, H⁺, and Al³⁺. It is a measure of a soil's nutrient-holding power — higher CEC means the soil can store more nutrients for plants.
- Expressed in meq/100g or cmol(p⁺)/kg (these units are equivalent)
- Higher CEC → Greater nutrient-holding capacity → More fertile soil
| Soil Type | CEC (meq/100g) |
|---|---|
| Sand | 1–5 |
| Silt | 10–30 |
| Clay (Kaolinite) | 3–15 |
| Clay (Montmorillonite) | 60–100 |
| Humus | 100–300 |
IMPORTANT
Humus has the highest CEC of any soil component — up to 300 meq/100g. This is why adding organic matter dramatically improves a soil's ability to hold nutrients. Among clay minerals, Montmorillonite has the highest CEC.
3. Base Saturation
Base saturation (BS%) indicates what percentage of the CEC is occupied by basic cations (as opposed to acidic H⁺ and Al³⁺):
- BS% = (Sum of base cations / CEC) × 100
- Base cations: Ca²⁺, Mg²⁺, K⁺, Na⁺
- Higher BS% → Higher fertility and higher pH
- Soils with BS% > 80% are generally fertile and near-neutral in pH
4. Electrical Conductivity (EC)
EC measures the concentration of soluble salts in soil and is the primary indicator for identifying saline soils:
- Normal soil: < 4 dS/m
- Saline soil: > 4 dS/m
- Measured using a conductivity meter on a saturated paste extract (standard method)
Soil Organic Matter
Soil organic matter (SOM) is the organic fraction of soil that includes plant and animal residues at various stages of decomposition, cells and tissues of soil organisms, and substances synthesized by soil organisms. Despite typically comprising only 2-5% of soil by weight, it has an outsized influence on soil properties.
Composition of Organic Matter
Different organic compounds decompose at very different rates. Lignin is the most resistant, which is why woody materials take the longest to break down:
| Component | Percentage | Resistance to Decomposition |
|---|---|---|
| Starch, Simple proteins | Low % | Least resistant (decompose first) |
| Crude proteins | — | Low |
| Hemicellulose | — | Medium |
| Cellulose | 40–50% | Medium-high |
| Lignin | 10–30% | Most resistant (decomposed by Actinomycetes and Basidiomycetes only, NOT by bacteria/fungi) |
| Proteins | 1–15% | — |
| Fats & Fatty acids | 1–8% | — |
Humus
Humus is the stable, dark-coloured, amorphous organic matter that remains after extensive decomposition. It is colloidal in nature (very fine particles with enormous surface area), which is why it has such high CEC and water-holding capacity.
Fractions of Humus:
| Fraction | Solubility | Colour | Properties |
|---|---|---|---|
| Fulvic acid | Soluble in both acid and base | Light yellow | Most susceptible to decomposition; lowest molecular weight |
| Humic acid | Insoluble in acid, soluble in base | Dark brown | Intermediate stability |
| Humin | Insoluble in both acid and base | Dark black | Most resistant to decomposition; highest molecular weight |
TIP
Memory aid: Think of resistance increasing with darkness — Fulvic (light yellow, least stable) → Humic (dark brown, intermediate) → Humin (dark black, most stable). The most resistant fraction is insoluble in everything.
- Humus contains high amounts of C, H, O, N, and S
- Humus is an excellent water absorber — it can hold up to 20 times its weight in water
C:N Ratio
The carbon-to-nitrogen ratio determines how quickly organic material decomposes and whether nitrogen is released to plants (mineralization) or temporarily locked up (immobilization):
| Material | C:N Ratio |
|---|---|
| Normal soil | 8:1 to 15:1 (average 10–12:1) |
| Humus | 10:1 |
| Microbes (general) | 4:1 to 9:1 |
| Bacteria | 5:1 |
| Fungi | 10:1 |
| FYM (well-decomposed) | 20:1 |
| Legume crops | 9:1 |
| Cereal crops (straw) | 80:1 to 100:1 |
IMPORTANT
Key Rules for C:N ratio:
- C:N < 25:1 → Mineralization dominates → net nitrogen release → plants benefit
- C:N > 25:1 → Immobilization dominates → microbes consume available N → temporary N deficiency for crops
This is why incorporating cereal straw (C:N 80-100:1) directly can cause nitrogen deficiency, and why it is recommended to add extra nitrogen when incorporating high C:N residues.
Factors Affecting Decomposition of Organic Matter
- Plant age — Young tissue (low lignin) decomposes faster than mature tissue
- Soil pH — Optimum pH 7.0 (neutral conditions favour microbial activity)
- Soil aeration — High aeration → faster aerobic decomposition
- Soil temperature — Optimum 30–40°C (tropical soils decompose OM faster)
- Soil moisture — Optimum 60–80% moisture content
- Nutrient status — High nutrients → faster microbial growth → faster decomposition
- C:N ratio — Low C:N → faster decomposition
Changes in Soil After Addition of Humus/OM
Adding organic matter triggers a cascade of improvements in soil:
- Soil structure improves → crumby/spheroidal structure forms
- Water holding capacity increases (humus absorbs water)
- Pore space increases (better aggregation)
- Soil aeration improves
- Soil temperature regulation (humus has 2.5× specific heat of mineral soil, buffering temperature swings)
- Erosion decreases (aggregates resist erosion)
- CEC increases (humus has very high CEC)
- Nutrient supply improves
Soil Colloids
The word "Colloid" is derived from Greek: Kolla (gum) + Oids (like). In 1861, Thomas Graham (called the Father of Colloidal Chemistry) divided water-soluble substances into crystalloids (easily diffuse through semipermeable membrane) and colloids (slowly diffuse). Soil colloids are the most chemically active fraction of soil.
Size of Colloids
- Size: 1–500 nm (less than 0.0005 mm or 0.5 μm)
- Because of their extremely small size, colloids have an enormous surface area relative to their mass, which gives them high chemical reactivity
Types of Soil Colloids
| Type | Examples |
|---|---|
| Inorganic (mineral) colloids | Clay minerals — Kaolinite, Montmorillonite, Illite, Vermiculite, Chlorite |
| Organic colloids | Humus |
General Properties of Colloids
- Heterogeneity — composed of different-sized particles
- Negative charge on surface (predominant in most soils)
- Adsorption of cations onto their surface (basis of CEC)
- Tyndall effect (scattering of light beam passing through a colloidal solution)
- Brownian movement (random motion of colloidal particles due to bombardment by water molecules)
- Coagulation/Precipitation when electrolytes are added
Properties of Soil Colloids
- Cohesion and Adhesion — sticking together (cohesion) and to other particles (adhesion)
- Plasticity — ability to be moulded when wet
- Swelling and Shrinkage — absorption of water (swelling) and loss of water (shrinkage)
- Flocculation and Deflocculation — two opposing processes with major agricultural significance:
- Flocculation = Formation of soil aggregates (promoted by Ca²⁺) → improves soil structure
- Deflocculation = Breakdown of aggregates (promoted by Na⁺) → destroys soil structure
- This is why gypsum (Ca²⁺ source) is used to reclaim sodic soils — it replaces Na⁺ with Ca²⁺, promoting flocculation
Clay Mineral Types
The type of clay mineral present dramatically affects soil behaviour. This table is one of the most important for CUET:
| Property | Kaolinite | Illite | Montmorillonite | Vermiculite | Humus |
|---|---|---|---|---|---|
| Type | 1:1 non-expanding | 2:1 low-expanding | 2:1 high-expanding | 2:1 limited-expanding | — |
| Formula | Al₄Si₄O₁₀(OH)₈ | KAl₄Si₈O₂₀(OH)₄ | Al₄Si₈O₂₀(OH)₄ | — | — |
| CEC (cmol/kg) | 3–15 | 20–40 | 60–100 | 100–150 | 150–300 |
| Cohesion/Plasticity | Very low | Medium | High | — | — |
| Swelling/Shrinkage | Very low | Medium | High | Limited | — |
| Permeability | High | Medium | Low | — | — |
| Dominant in | Red soils | Alluvial soils | Black soils | — | Organic soils |
| P fixation | High | — | Low | — | — |
NOTE
1:1 vs 2:1 explained: The ratio refers to layers of silica (tetrahedral) and alumina (octahedral) sheets. Kaolinite has 1 silica + 1 alumina layer (tightly bonded, no space for water to enter → non-expanding). Montmorillonite has 2 silica + 1 alumina (weak bonds between layers → water enters → massive swelling).
Source of Negative Charge on Clay
Clay particles carry negative charges on their surface, which attract and hold positively charged cations. These charges arise from two sources:
- Open crystal edge — Unsatisfied valence of oxygen atoms at broken edges; this charge is pH-dependent:
- Low pH: SiO⁻ + H⁺ → SiOH (charge decreases as H⁺ neutralizes it)
- High pH: SiOH → SiO⁻ + H⁺ (charge increases)
- Isomorphous substitution — Replacement of one cation by another of similar size but different charge (e.g., Al³⁺ replacing Si⁴⁺ creates a net negative charge); this is pH-independent and creates a permanent charge
Importance of Soil Colloids
Soil colloids are the "powerhouse" of soil chemistry:
- Nutrient storage increases (cations held on surfaces)
- Soil fertility increases
- CEC increases
- Water holding capacity increases (especially humus colloids)
- Buffering capacity improves (resists sudden pH changes)
Ion Exchange
Cation Exchange Capacity (CEC): The total capacity of soil to hold exchangeable cations. It is a direct measure of the negative charge on soil colloids.
- CEC is mainly pH-dependent (increases with pH because more edge charges develop)
- Expressed in cmol(p⁺)/kg or meq/100g
Anion Exchange Capacity (AEC): The capacity of soil to hold exchangeable anions (negatively charged ions). Generally much lower than CEC because soil surfaces are predominantly negatively charged.
- AEC is highest at low pH (when positive charges develop on edges)
Rules of Ion Exchange:
- Higher valence ions replace lower valence ions: Ca²⁺ replaces K⁺
- At same valence, larger hydrated radius ions are held less tightly (easier to replace)
- Lyotropic series (replacing power): Al³⁺ > Ca²⁺ > Mg²⁺ > K⁺ = NH₄⁺ > Na⁺
TIP
The lyotropic series explains why Na⁺ is the easiest cation to displace — it has the lowest replacing power. This is the principle behind reclaiming sodic soils with gypsum: Ca²⁺ from gypsum replaces Na⁺ on clay surfaces.
Important Points for CUET
Quick Revision Checklist
- Soil texture **cannot** be easily changed; soil structure **can** be improved by adding organic matter - **Montmorillonite** clay has **2:1 lattice** structure (high CEC, shrink-swell, dominant in black soils) - **Kaolinite** clay has **1:1 lattice** structure (low CEC, non-expanding, dominant in red soils) - India's soil survey is conducted by **National Bureau of Soil Survey and Land Use Planning (NBSS&LUP)**, Nagpur - **Soil Health Card Scheme** launched in 2015 to test soil and recommend nutrients to every farmer - **Soil Testing instruments:** pH meter for pH, hydrometer for texture, flame photometer for K, spectrophotometer for P - Humus has the highest CEC (150-300 meq/100g) - Available water = Field Capacity – Permanent Wilting Point - Crumby structure is ideal for crops - Particle density of most soils ≈ 2.65 g/cm³Summary Cheat Sheet
| Concept / Topic | Key Details / Explanation |
|---|---|
| Soil Texture | Relative proportion of sand, silt, clay; cannot be changed by cultivation |
| ISSS silt-sand boundary | 0.02 mm; USDA boundary: 0.05 mm |
| Clay particle size | < 0.002 mm (both ISSS & USDA) |
| Sand properties | Low water holding, high permeability, good aeration |
| Clay properties | High water holding, low permeability, poor aeration |
| USDA textural classes | 12 classes; ideal soil = Loam (balanced mix) |
| Texture determination | Textural Triangle (graphical) or Bouyoucos Hydrometer (lab) |
| Texture vs Structure | Texture cannot be changed; Structure can be improved (by adding OM) |
| Soil Structure | Arrangement of particles into aggregates; Ped = natural; Clod = artificial |
| Crumby structure | Best for crop production (porous, good aeration); found in A-horizon |
| Columnar structure | Diagnostic of sodic/alkaline soils; rounded top pillars |
| Platy structure | Impedes water movement; found in forest soils, E-horizon |
| Soil Colour | Determined by Munsell Colour Chart; Dark = OM; Red = Fe oxides; Grey = waterlogged; White = CaCO₃/salts |
| Ideal porosity | ~50% (50% solid, 25% water, 25% air) |
| Macropores vs Micropores | Sandy soils = macropores (rapid drainage); Clay = micropores (water retention) |
| Bulk Density (Db) | Mass dry soil / Total volume; typical 1.1-1.6 g/cm³; OM addition decreases Db |
| Particle Density (Dp) | Mass dry soil / Volume solids only; ~2.65 g/cm³ (constant for mineral soils) |
| Porosity formula | (1 - Db/Dp) × 100 |
| Gravitational water | Saturation → Field Capacity; not available (drains quickly) |
| Capillary water | FC → PWP; available to plants |
| Hygroscopic water | Below PWP; not available (held too tightly) |
| Field Capacity (FC) | Water held at -1/3 bar (-33 kPa) — upper limit of available water |
| Permanent Wilting Point | Water held at -15 bar (-1500 kPa) — lower limit |
| Available Water | FC - PWP |
| Soil Consistency | Wet = sticky/plastic; Moist = friable (ideal for tillage); Dry = hard |
| Soil pH | pH = -log[H⁺]; ideal for most crops: 6.0-7.5 |
| Acidic soil correction | Liming (CaCO₃) — neutralizes acidity |
| Alkaline soil correction | Gypsum (CaSO₄·2H₂O) — Ca²⁺ replaces Na⁺ |
| CEC definition | Ability to hold/exchange cations (Ca²⁺, Mg²⁺, K⁺, Na⁺, H⁺, Al³⁺); measured in meq/100g |
| Humus CEC | 100-300 meq/100g — highest of any soil component |
| Montmorillonite CEC | 60-100 meq/100g — highest among clay minerals |
| Base Saturation | BS% = (Sum of base cations / CEC) × 100; higher BS% = higher fertility |
| EC (Electrical Conductivity) | Measures soluble salts; Normal < 4 dS/m; Saline > 4 dS/m |
| Lignin | Most resistant to decomposition; decomposed by Actinomycetes & Basidiomycetes only |
| Humus fractions | Fulvic acid (light yellow, least stable, soluble in both) → Humic acid (dark brown, soluble in base) → Humin (dark black, most resistant, insoluble in both) |
| C:N ratio — Normal soil | 10-12:1; Cereal straw = 80-100:1 |
| C:N < 25:1 | Mineralization dominates → N released to plants |
| C:N > 25:1 | Immobilization dominates → temporary N deficiency |
| Humus specific heat | 2.5× mineral soil — buffers temperature swings |
| Soil Colloids | Size: 1-500 nm; named by Thomas Graham (1861) — Father of Colloidal Chemistry |
| Flocculation | Aggregate formation; promoted by Ca²⁺ → improves structure |
| Deflocculation | Aggregate breakdown; promoted by Na⁺ → destroys structure |
| Kaolinite | 1:1 non-expanding; low CEC (3-15); dominant in Red soils; high P fixation |
| Montmorillonite | 2:1 high-expanding; CEC 60-100; high shrink-swell; dominant in Black soils |
| Illite | 2:1 low-expanding; CEC 20-40; dominant in Alluvial soils |
| Isomorphous substitution | Creates permanent, pH-independent negative charge on clay |
| Lyotropic series | Al³⁺ > Ca²⁺ > Mg²⁺ > K⁺ = NH₄⁺ > Na⁺ (replacing power); Na⁺ easiest to displace |
| NBSS&LUP | National Bureau of Soil Survey, Nagpur — conducts India's soil survey |
| Soil Health Card Scheme | Launched 2015 — soil testing + nutrient recommendations for every farmer |
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