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
