Soil Science
Soil Science - This domain tests soil physical properties, texture, structure, density, pH, salinity and erosion, and the link between soil condition and crop growth.
Core rules
- Texture is the proportion of sand, silt and clay; the textural triangle converts percentages to a class such as loam or clay loam.
- Structure is the arrangement of particles into aggregates (granular, blocky, platy); good structure improves aeration and infiltration.
- Bulk density (BD) = oven-dry mass ÷ total volume; particle density (PD) is about 2.65 g/cm3 for mineral soil. Porosity = 1 − BD/PD.
- Soil pH: acidic below 6.5, neutral 6.5–7.5, alkaline above 7.5. EC measures soluble salts.
- Saline soil: EC above 4 dS/m, pH usually below 8.5; sodic soil: pH above 8.5 with high exchangeable sodium.
- Erosion types: sheet, rill, gully and wind; contour bunding and mulching reduce loss.
Worked example
A core sample has oven-dry mass 150 g and volume 100 cm3. BD = 150 ÷ 100 = 1.5 g/cm3. Porosity = 1 − (1.5 ÷ 2.65) = 1 − 0.566 = 0.434, i.e. 43.4%. Reasoning: always use oven-dry mass, not field-moist mass, and keep units consistent.
Traps
- Do not confuse bulk density with particle density.
- A pH of 8.9 with high ESP points to sodic, not saline, soil.
- Texture is not the same as structure.
- Read the textural triangle from the correct axis; clay percentage is read from the right side.
- Organic matter improves structure and water-holding capacity but is not a texture class.