Soil Science: Formation, Types, Properties and Fertility Management
What this domain covers
The exam tests how soils form, the soil profile, Assam's soil types, physical and chemical properties, and how fertility is corrected through manure, fertilizer, lime and cropping practices.
Core rules and concepts
- Weathering is physical (temperature, frost, water, wind), chemical (hydrolysis, oxidation, solution, carbonation) and biological (roots, microbes, burrowing animals).
- Soil-forming factors: parent material, climate, relief, organisms and time.
- Profile horizons: O (litter), A (topsoil, humus-rich), B (subsoil, illuviation), C (weathered rock), R (bedrock).
- Assam soils: alluvial soils of the Brahmaputra and Barak valleys (fertile but flood-prone), laterite and red soils of the hills (acidic, leached), peat and marshy soils, and hill forest soils. Most are acidic, roughly pH 4.5-6.0, and low in available nitrogen and phosphorus.
- Properties: texture, structure, porosity, bulk density, water-holding capacity, cation exchange capacity, base saturation and organic matter.
- Fertility management: soil-test-based NPK, FYM or compost, green manuring with dhaincha, legume rotation, lime for acidity and biofertilizers.
Worked example
A field tests pH 5.0 with low available phosphorus. At low pH, phosphorus is fixed by aluminium and iron and is poorly available, so the first step is lime applied according to the lime requirement, incorporated well before sowing. FYM is added to raise organic matter and cation exchange capacity, and dhaincha is grown and buried as green manure. Lime is not mixed with ammonium or phosphatic fertilizer in the same operation, because the reaction reduces nutrient availability and can cause ammonia loss.
Traps that cost marks
- Confusing laterite hill soils with fertile alluvial valley soils.
- Believing lime makes soil more acidic; it neutralizes acidity.
- Mixing lime and fertilizer at the same time.
- Collecting a soil sample immediately after fertilizer application.