Soil Science and Land Management
What you must understand
Soil is a mixture of mineral particles, organic matter, air, water and living organisms. Know the texture classes - sand, silt, clay and loam - and their properties: sand drains fast and holds little water or nutrients; clay holds water and nutrients but drains poorly and compacts; loam balances both. Understand soil structure, the meaning of pH and its effect on nutrient availability, cation exchange capacity in simple terms, and how lime raises pH while sulphur or organic acids lower it. Learn the causes and control of erosion (water and wind), the value of contour ploughing, terracing, cover crops, windbreaks and mulching, and the importance of organic matter in water retention and structure. Know the difference between soil fertility and soil productivity.
Traps in questions
- Confusing soil texture (particle size) with structure (how particles clump).
- Thinking lime lowers pH; lime raises pH (makes soil less acid).
- Assuming clay is the best soil for all crops; drainage and aeration limit it.
- Forgetting that most nutrients are most available near pH 6–7.
- Erosion questions: splash erosion, sheet, rill and gully erosion occur in that order of severity.
Worked example
A soil test shows pH 5.2 and the crop prefers pH 6.5. The gap is 1.3 units. Lime requirement is estimated from the soil's buffer capacity, so the recommendation is read from a lime table rather than guessed. Rule of thumb: correct acidity before applying fertiliser, because nutrients are wasted at very low pH.