Soil Science and Soil Management
Soil formation and profile
Soil is a three-phase system of mineral matter, organic matter, water and air. It forms by physical, chemical and biological weathering of parent rock. A typical profile has O (organic), A (topsoil), B (subsoil accumulation), C (weathered parent material) and R (bedrock) horizons. The exam tests soil properties, problem soils of Tamil Nadu and management practices.
Physical and chemical properties
- Texture is the proportion of sand, silt and clay. Loam is best for most crops; clay holds water but drains poorly; sandy soil drains quickly but holds few nutrients.
- Structure, porosity, bulk density and colour indicate how a soil behaves.
- Soil pH measures acidity or alkalinity; electrical conductivity (EC) measures soluble salts; organic carbon indicates fertility; cation exchange capacity (CEC) measures nutrient-holding ability.
- Tamil Nadu has red loams, laterites, black cotton soils (vertisols), alluvial delta soils, coastal saline soils and calcareous or sodic soils.
Management
Add FYM, compost, green manure (daincha, sunhemp) and crop residues to build organic matter. Control erosion with contour bunding, terracing, strip cropping, mulching and check dams. Reclaim sodic soils with gypsum, saline soils with drainage and leaching, and acid soils with lime.
Worked example
A sample shows pH 8.4, EC 0.3 dS/m and ESP 22. EC below 4 dS/m rules out a saline soil, while pH above 8.2 with ESP above 15 identifies a sodic soil. Gypsum supplies calcium that replaces exchangeable sodium, and leaching removes the sodium salts. If EC were above 4 dS/m with ESP above 15, the soil would be saline-sodic.
Traps that cost marks
Confusing pH with EC; calling every black soil fertile; applying lime to sodic soil; and sampling only the surface when a profile is required.