Soil Science
What the exam asks
Soil Science tests soil formation, texture, structure, profile, pore space, bulk density, pH, cation exchange capacity (CEC) and organic matter, with examples from Uttar Pradesh soils. Items often ask you to interpret a simple soil test or distinguish related terms.
Core rules
- Soil has mineral matter, organic matter, water and air. Texture is the proportion of sand, silt and clay; structure is the arrangement of particles into aggregates.
- Bulk density = oven-dry soil mass ÷ bulk volume. Porosity = 1 − (bulk density ÷ particle density); particle density is commonly about 2.65 g cm−3.
- CEC is the soil's capacity to hold exchangeable cations, expressed in cmol(+) kg−1 or meq 100 g−1; clay and organic matter raise it.
- Soil pH governs nutrient availability. Most field crops prefer pH 6.0–7.5; strongly acidic and sodic soils need amendment.
- Uttar Pradesh has largely alluvial soils, with red and black soils in parts of Bundelkhand and varied textures along rivers.
Worked example
An oven-dry soil sample has mass 100 g and bulk volume 60 cm3. Bulk density = 100 ÷ 60 = 1.67 g cm−3. Porosity = 1 − (1.67 ÷ 2.65) = 1 − 0.63 = 0.37, or 37%. Reasoning: use oven-dry mass, not field-moist mass; bulk volume includes pores. A porosity of 37% is low for a surface soil and suggests compaction.
Traps
Texture is not structure; bulk density is not particle density; CEC is not total exchangeable bases alone. Do not use field-moist mass for bulk density. pH 7 is neutral, not automatically best for every crop. Field capacity and permanent wilting point are water-status terms, not soil types.