Biosphere, Soils and Ecosystems
What this domain tests
This section tests soil formation and profiles, biomes and ecosystem functioning, including energy flow and nutrient cycling.
Core rules
- Soil-forming factors: parent material, climate, relief, organisms and time.
- Typical horizons: O (organic litter), A (topsoil, humus), E (eluviation, in some soils), B (illuviation), C (weathered parent material), R (bedrock).
- Ecosystems have producers, consumers and decomposers. Energy flows one way; nutrients cycle. About 10% of energy transfers between trophic levels.
- Arid soils are thin, sandy, low in organic matter and prone to salinity where evaporation exceeds rainfall.
Worked example
Why are soils in western Rajasthan sandy and often saline? Rainfall is low, vegetation is sparse, and organic matter input is small. High evaporation draws water upward by capillary action, leaving salts near the surface. Low rainfall also means little leaching of salts downward.
Traps
Humus is concentrated in the A horizon, not the B horizon. The B horizon is a zone of accumulation (illuviation), not the parent rock. Alluvial soils are not confined to deserts. Decomposers are not producers.