Soil Science and Soil Fertility
Soil Science and Soil Fertility
This domain examines soil physical and chemical properties, nutrient availability, soil testing and fertiliser management, with emphasis on Rajasthan's sandy, saline and sodic soils.
Core rules and concepts
- Texture, structure, bulk density, porosity and water-holding capacity control root growth, infiltration and drought behaviour.
- Soil reaction is described by pH, electrical conductivity (EC), cation exchange capacity (CEC), base saturation and organic carbon.
- Major nutrients are N, P, K, S, Ca and Mg; micronutrients include Zn, Fe, Mn, Cu, B and Mo. Deficiency symptoms help diagnosis but soil testing is the basis for recommendation.
- Integrated nutrient management combines organic manure, crop residues, biofertilisers and mineral fertiliser.
- Saline soil has EC above about 4 dS/m with pH below 8.5; sodic soil has high exchangeable sodium percentage (often above 15) and pH above 8.5. Gypsum is used for sodic soil; leaching with drainage is used for saline soil.
Worked example
A crop needs 60 kg N, 40 kg P2O5 and 20 kg K2O per hectare. DAP contains 46% P2O5 and 18% N. DAP required = 40 ÷ 0.46 = 87 kg, which supplies 87 × 0.18 = 15.7 kg N. Remaining N = 60 − 15.7 = 44.3 kg; urea required = 44.3 ÷ 0.46 = 96.3 kg. MOP at 60% K2O gives 20 ÷ 0.60 = 33.3 kg. Reason: fertiliser grades are expressed as nutrient oxides, and N supplied by DAP must be credited.
Traps that cost marks
Treating P2O5 as elemental P; forgetting the N in DAP; applying gypsum to saline rather than sodic soil; recommending lime for Rajasthan's alkaline soils; ignoring soil-test critical limits.