Soils and Vegetation
Soils and Vegetation
Learn how soil forms from parent rock through weathering, influenced by climate, organisms, relief and time. Understand soil profile horizons: O (organic litter), A (topsoil, humus-rich), B (subsoil, minerals leached down), C (weathered parent material) and R (bedrock). Know soil texture (sand, silt, clay) and its effect on drainage, aeration and fertility; learn leaching, podsolisation, laterisation, humus, pH and cation exchange in simple terms.
Study soil types: brown earths (temperate, fertile), podsols (cool, wet, acidic, leached), laterites (tropical, iron-rich, nutrient-poor), and alluvial soils (river-deposited, fertile). Link soil to vegetation: climate determines natural vegetation belts, and vegetation returns organic matter to soil.
Traps: confusing leaching (downward removal of nutrients) with erosion (physical removal of soil); assuming clay soils drain well (they do not); saying laterite soils are fertile (they are often poor for farming); and muddling topsoil with subsoil.
Rule of thumb: wet climates leach nutrients downward; dry climates accumulate salts near the surface. Match soil type to climate.
Worked example: In a hot, wet tropical climate, heavy rain leaches soluble nutrients from the A horizon, leaving iron and aluminium oxides; the resulting laterite soil is reddish, hard when exposed and low in fertility.