Soil Science
Domain 2 tests soil as a growing medium: texture and structure, pH and salinity, organic carbon, cation exchange, and the diagnosis of problem soils that decides which crop and amendment a farmer should use.
Physical properties
- Textural classes from sand, silt and clay; loam is ideal for most vegetables.
- Structure, bulk density and porosity control aeration and root penetration.
- Field capacity, permanent wilting point and available water determine irrigation depth.
Chemical properties
- pH governs nutrient availability; most horticultural crops prefer 6.0-7.0.
- Cation exchange capacity rises with clay and organic matter and buffers fertiliser additions.
- Organic carbon below 0.5% is low and calls for organic manures.
- Electrical conductivity measures soluble salts; above about 1 dS/m in the saturation extract, sensitive crops begin to suffer.
Problem soils
- Saline: high EC, normal ESP; leach with good-quality water and choose tolerant crops.
- Sodic: high pH (above 8.5) and ESP above 15; apply gypsum, then leach.
- Acidic: pH below 6; apply agricultural lime.
Worked example
A soil test reports pH 8.9, EC 0.35 dS/m and ESP 22. Reasoning: high pH with high ESP but low EC is sodic, not saline. Treatment is gypsum to replace exchangeable sodium, followed by leaching and organic matter; lime would raise pH further. Choose a tolerant crop and avoid sodium-based fertilizers such as sodium nitrate.
Marks-losing traps
- Treating sodic and saline soils as one problem.
- Applying lime to sodic soil.
- Reading pH as a linear scale; it is logarithmic.
- Quoting EC without stating that it is measured in the saturation extract.