Definitions and Concepts
Full Form of SOIL; S – Stratum / Soul, O – Of, I – Infinite, L – Life
Concepts of Soil Science
- Pedology
- Pedon (soil) + Logos (study) = Study of soil genesis, classification, and description.
- Focus: Natural body in its undisturbed condition
- Concerned with soil profile, useful in construction, roadways, and geology.
- Edaphology
- Edaphos (ground) + Logos = Study of soil in relation to plants
- Focus: Soil fertility, crop growth, and plant nutrition
Soil Composition (Ideal Soil)
|
Component |
Proportion (%) |
|
Mineral Matter |
45% |
|
Organic Matter |
5% |
|
Water (Pore Space) |
25% (20–30%) |
|
Air (Pore Space) |
25% (20–30%) |
Regolith
- Unconsolidated weathered rock above bedrock
- Can be residual or transported
- Regosol: Azonal soil without defined horizons formed from loose material like sand or glacial drift
Soil Types by Organic Matter
|
Soil Type |
OM Content |
|
Organic Soil |
≥20% (in low clay soils), ≥30% (in high clay soils) |
|
Mineral Soil |
<20% OM |
Soil Profile and Horizons
A soil profile is a vertical section through the soil showing distinct horizons. All horizons above parent material (C) form the Solum.
Master Horizons
|
Horizon |
Description |
|
O |
Organic matter-rich, found in forest soils |
|
A |
Topmost mineral horizon, rich in humified OM, dark in color |
|
E |
Zone of maximum eluviation (leaching) of clay, Fe, Al |
|
B |
Zone of illuviation (accumulation of Fe, clay, OM) |
|
C |
Unconsolidated parent material, least weathered |
AB/EB horizons are transition zones between layers.
Physical Properties of Soil
The physical properties of soil influence its fertility, water dynamics, and aeration.
Soil Texture
- Refers to the relative proportions of sand, silt, and clay
- A permanent property, unaffected by normal cultivation
- Determined using hydrometer method
Particle Size Classification (International System)
|
Soil Component |
Diameter (mm) |
|
Stone |
> 250 |
|
Gravel |
2 – 75 |
|
Coarse Sand |
0.2 – 2.0 |
|
Fine Sand |
0.02 – 0.2 |
|
Silt |
0.002 – 0.02 |
|
Clay |
< 0.002 |
Textural Classes of Soil
|
Soil Type |
Sand (%) |
Silt (%) |
Clay (%) |
Remarks |
|
Sandy |
>85 |
— |
<15 |
Light soil |
|
Silty |
— |
>80 |
— |
Smooth, low cohesion |
|
Clayey |
— |
— |
>40 |
Heavy, sticky |
|
Loamy |
Balanced |
Balanced |
Balanced |
Best for agriculture |
- Loam is a mix of sand, silt, and clay in nearly equal proportions.
- Light soils = less resistance to tillage (sandy), Heavy soils = more resistance (clay), despite lighter weight.
Crop Suitability Based on Texture
|
Soil Type |
Suitable Crops |
|
Heavy soils (clay, silty clay) |
Rice, Cotton, Sorghum, Coriander |
|
Medium soils (loam, silt loam) |
Wheat, Maize, Pulses, Most crops |
|
Light soils (sandy, loamy sand) |
Groundnut, Tobacco, Pearl millet, Potatoes, Leguminous fodder |
Key Concepts for Competitive Exams
Law of Minimum – Von Liebig (1840)
“The crop yield is limited by the most deficient essential nutrient, even if all others are adequate.”
Relationship of Texture with Other Soil Properties
|
Property |
Influenced By |
|
Water holding capacity |
↑ in clay, ↓ in sand |
|
Nutrient retention |
↑ in clay and loam |
|
Aeration |
Better in sandy soils |
|
Plasticity |
Highest in clay |
|
Ease of tillage |
Higher in sandy (light) soils |
|
Surface area |
↑ in clay, ↓ in sand |
Summary Table
|
Soil Component |
Physical Trait |
Impact on Soil |
|
Sand |
Large particle size |
Good drainage, low fertility |
|
Silt |
Smooth texture |
Holds water, moderate fertility |
|
Clay |
Smallest particle |
High nutrient retention, poor drainage |
Competitive Exam Pointers
- Loamy soils = Best for agriculture
- Clay = Highest plasticity and surface area
- Sand = Light texture, low nutrient retention
- O horizon = Found in forests, not in cultivated soils
- Soil Profile = O → A → E → B → C → R
- Gravel size = 2–75 mm
- Hydrometer method = For texture analysis
- Von Liebig’s Law = Most limiting nutrient determines yield
