Soil Science and Soil Management
Soil Science and Soil Management
This domain examines soil texture and structure, bulk density and porosity, soil reaction, cation exchange, nutrient availability and erosion control. Numerical questions on bulk density and porosity are common.
Core rules
- Texture refers to the relative proportions of sand (2–0.05 mm), silt (0.05–0.002 mm) and clay (<0.002 mm).
- Bulk density = oven-dry soil mass ÷ total soil volume. Particle density is about 2.65 g/cm³ for most mineral soils.
- Porosity = 1 − (bulk density ÷ particle density), expressed as a percentage.
- Most crops grow best near pH 6.5–7.5. Acid soils are corrected with lime; sodic soils with high exchangeable sodium are corrected with gypsum.
- Cation exchange capacity measures the soil's capacity to hold exchangeable cations; base saturation is the proportion of CEC occupied by basic cations.
- Control erosion with contour bunding, terracing and vegetative cover; sheet, rill and gully erosion are water-erosion forms.
Worked example
A 100 cm³ core of oven-dry soil weighs 130 g. Bulk density = 130 ÷ 100 = 1.3 g/cm³. Porosity = 1 − (1.3 ÷ 2.65) = 1 − 0.491 = 0.509, or about 51%. The reasoning is to use oven-dry mass, not field-moist mass, and to divide by the core's total volume.
Mark-losing traps
- Confusing field capacity with permanent wilting point; the difference is available water.
- Treating a saline soil as sodic, or recommending gypsum without checking exchangeable sodium percentage.
- Using fresh soil weight in the bulk density formula.