Soil Science
Soil science questions test whether you understand soil as a three-phase system (minerals, organic matter, pore space) and can identify the major soil groups of Maharashtra and their management problems.
Core rules
- Soil forms by weathering of parent rock plus organic matter addition, and is described by a profile: O (litter), A (topsoil, humus-rich), B (subsoil, zone of illuviation), C (weathered parent material), R (bedrock).
- Texture is the proportion of sand (0.05–2 mm), silt (0.002–0.05 mm) and clay (below 0.002 mm). Loam is a balanced mixture and is agriculturally best. Structure is how particles clump - granular is ideal, platy and columnar indicate problems.
- Bulk density = oven-dry mass ÷ bulk volume; particle density of most soils is about 2.65 g/cm³.
- Black cotton soil (regur) from Deccan basalt: high clay, swelling and shrinking, high water holding, poor drainage, difficult tillage when wet, pH 7.5–8.5.
- Red soils from granite and gneiss: well drained, low organic matter and low fertility. Laterite soils are leached and acidic. Alluvial soils are transported and generally fertile.
- Most nutrients are most available at pH 6.5–7.5.
Worked example
A soil sample has a bulk density of 1.33 g/cm³. Find its porosity. Reasoning: porosity = (1 − bulk density ÷ particle density) × 100 = (1 − 1.33 ÷ 2.65) × 100 = (1 − 0.502) × 100 ≈ 50 per cent. This means half the soil volume is pore space holding air and water.
Traps that cost marks
- Confusing texture (particle size) with structure (arrangement).
- Assuming black soil is best for every crop; its drainage and workability are poor.
- Mixing up bulk density with particle density.