Soil Science and Soil Conservation Principles
What is tested
Soil formation and profile, texture and structure, pH and electrical conductivity, organic carbon, soil fertility versus productivity, types of erosion, the Universal Soil Loss Equation, land capability classification, and the choice of conservation measures.
Core concepts
- Profile: O (organic litter), A (topsoil, humus-rich), B (subsoil, illuviation), C (weathered parent material), R (bedrock).
- Texture is the proportion of sand, silt and clay; structure is how particles aggregate into crumbs, blocks or plates. Loam with granular structure is best for most crops.
- Erosion types: splash, sheet, rill, gully, and wind erosion; mass movement is landslip or creep. Sheet erosion is the most insidious because it removes fertile topsoil uniformly and is barely visible.
- USLE: A = R × K × LS × C × P - rainfall erosivity, soil erodibility, slope length-steepness, cover management and support practice.
- Measures: contour and graded bunding, bench terracing, contour trenching, check dams and gully plugs, nala bunding, vegetative barriers (vetiver), strip cropping, mulching, crop rotation, farm ponds and percolation tanks.
Worked example
With R = 300, K = 0.25, LS = 2.0, C = 0.4 and P = 0.5, find soil loss, then the loss if bunding cuts P to 0.2. A = 300 × 0.25 × 2.0 × 0.4 × 0.5 = 30 t/ha/yr. With P = 0.2: 300 × 0.25 × 2.0 × 0.4 × 0.2 = 12 t/ha/yr. The reasoning: only P changed, so the loss falls in the same ratio (0.2/0.5), a 60% reduction - the point of a support practice is to cut P, not R or K.
Marks-losing traps
- Confusing soil fertility (ability to supply nutrients) with productivity (yield, which also depends on water, climate and management).
- Calling a 30 cm deep channel a rill; rills are shallow and obliterated by tillage, gullies are not.
- Choosing terracing for gentle slopes - bench terracing is for steep land; gentle slopes take contour or graded bunds.