Soil Science
Soil Science
This domain examines soil as a three-phase system and tests physical and chemical properties that affect crop growth. For Odisha, expect laterite and red soils, coastal alluvial soils, acidic pH, and salinity or sodicity in some coastal pockets.
Core rules and concepts
- Soil texture is the proportion of sand, silt and clay; soil structure is the arrangement of particles into aggregates. Texture cannot be changed easily, but structure can be improved with organic matter.
- Bulk density (BD) = oven-dry soil mass ÷ total soil volume. Particle density is usually about 2.65 g/cm³ for mineral soils. Porosity (%) = (1 − BD ÷ particle density) × 100.
- Soil pH controls nutrient availability. Most field crops grow best near pH 6.0–7.5; laterite soils are often acidic and may need liming, while coastal soils may be saline or sodic.
- Cation exchange capacity (CEC) is the soil's capacity to hold exchangeable cations; clay and organic matter increase CEC.
Worked example
A core of diameter 7 cm and height 10 cm has radius 3.5 cm. Volume = π × 3.5² × 10 = 384.8 cm³. The oven-dry mass is 500 g. BD = 500 ÷ 384.8 = 1.30 g/cm³. With particle density 2.65 g/cm³, porosity = (1 − 1.30 ÷ 2.65) × 100 = 50.9%. The reasoning is to use dry mass, not fresh mass, and to divide by total core volume.
Traps that cost marks
Do not use moist field mass for bulk density. Do not confuse texture with structure. A pH of 6.5 is acidic to slightly acidic, not 65. Saline soils have excess soluble salts; sodic soils have excess exchangeable sodium, so the management differs.