Soil Fertility and Conservation
Here the exam separates soil fertility (the soil's inherent capacity to supply nutrients) from soil productivity (output, which also depends on water, climate, variety and management), and then tests erosion control measures by slope and soil type.
Core rules
- Fertility is maintained by organic matter, legume rotation, balanced fertilizer and periodic soil testing for pH, EC, organic carbon and available N, P, K.
- Water erosion appears as sheet erosion (thin uniform removal), rill erosion (small channels that can be crossed by tillage) and gully erosion (deep channels that cannot). Wind erosion dominates in dry, bare, sandy tracts.
- Agronomic measures: contour farming, strip cropping, mulching, cover crops, crop rotation, green manuring, alley cropping.
- Mechanical measures: contour bunds, graded bunds, bench terraces, contour trenches, gully plugging, check dams, nala bunds, farm ponds.
- Selection guide: 2–6 per cent slope with permeable soil → contour bunds; same slope with heavy, slowly permeable soil → graded bunds to drain surplus water safely; above about 8–10 per cent slope → bench terracing; ravine land → gully plugging with check dams.
Worked example
A 1.2 hectare plot has a 5 per cent slope and heavy black soil that sheds water slowly. Which structure would you recommend and why? Reasoning: the slope falls in the 2–6 per cent band, but the impermeable clay means contour bunds would pond water and risk waterlogging and bund breaching. A graded bund with a safe grassed outlet is chosen, supported by mulching and contour cultivation. On a permeable red soil of the same slope, contour bunds would have been correct.
Traps that cost marks
- Treating fertility and productivity as synonyms.
- Calling a rill a gully, or a gully a rill.
- Recommending bench terraces on nearly level land, or contour bunds on steep land.
- Ignoring wind erosion in sandy, dry districts.