Soil Survey, Classification and Land Use
The exam checks survey types and scales, soil profile terminology, Soil Taxonomy orders and the land capability classification used to decide what a parcel can safely grow.
Core rules
- Survey intensity: reconnaissance at 1:250,000 to 1:50,000, semi-detailed at about 1:50,000 to 1:25,000, detailed at 1:15,000 to 1:5,000 for farm planning.
- A pedon is the three-dimensional sampling unit; a polypedon is the mapping unit; horizons are designated O, A, E, B, C and R.
- Twelve Soil Taxonomy orders: Gelisols, Histosols, Spodosols, Andisols, Oxisols, Vertisols, Aridisols, Ultisols, Mollisols, Alfisols, Inceptisols and Entisols. In India, Inceptisols and Entisols dominate, followed by Alfisols, Vertisols and Aridisols.
- Land capability classes I to VIII run from no limitation to no cultivation; subclasses use e (erosion), w (water), s (soil/rooting) and c (climate). Class IV is the practical limit for regular cultivation.
- Rajasthan shows Aridisols and Entisols in the arid west and Vertisols in the south-east; remote sensing, GPS and GIS now support mapping (NBSS&LUP).
Worked example
A parcel has 10% slope, moderate erosion and 40 cm depth to a hard layer. A slope above 8% already places it in class IV, so it is IVe, not IIIe, even though the erosion hazard alone might be tolerated at class III. Recording the dominant limitation as the subclass and refusing to upgrade the class is what earns the mark.
Traps that cost marks
- Assigning class III to land steeper than 8%.
- Confusing a pedon (the individual soil body) with a polypedon (the mapping unit).
- Naming the soil order from colour or crop alone instead of diagnostic horizons and moisture regime.