Soil Fundamentals and Soil Formation
The Soil Fundamentals and Soil Formation domain tests your grasp of what soil is, how it develops from parent material, and how physical and chemical properties are measured and interpreted - the base on which every later conservation decision rests. Questions are usually definitional or numerical: horizons, texture classes, density, porosity and Indian soil groups.
Composition and profile
- A representative mineral soil is about 45% mineral matter, 5% organic matter, 25% water and 25% air by volume, giving roughly 50% pore space.
- Master horizons: O (litter), A (topsoil with humus), E (eluviation/leaching), B (illuviation/accumulation), C (weathered parent material), R (hard bedrock).
- Five soil-forming factors: parent material, climate, relief, organisms and time.
Properties that matter
- Texture is the sand–silt–clay proportion; structure is how particles are packed into aggregates (granular, blocky, prismatic, platy).
- Bulk density (BD) counts pores; particle density (PD) is about 2.65 g/cm³ for mineral soils.
- Porosity (%) = (1 − BD/PD) × 100.
- Cation exchange capacity rises with clay and organic matter; most nutrients are most available at pH 6.0–7.5.
Worked example
A sample gives BD 1.40 g/cm³ and PD 2.65 g/cm³. Porosity = (1 − 1.40/2.65) × 100 = (1 − 0.528) × 100 = 47.2%. The reasoning: BD/PD is the solid fraction of the volume, so subtracting it from 1 leaves the pore fraction.
Traps
Mixing up texture and structure; quoting bulk density when particle density is asked; forgetting time among the formation factors; labelling black cotton soil as laterite; reporting porosity above 60% for a mineral soil.