Why do plains have fertile soil?
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Answer: Rivers deposit sediment
Flat land drains well — Wrong. Drainage doesn't create fertility. Plains are fertile because rivers deposit nutrient-rich sediment during floods over millennia.
Rivers deposit sediment ✓ — Correct! Many fertile plains (Ganges, Nile, Mississippi) formed from river floodplains. Rivers erode upstream rock, carrying nutrient-rich sediment. During floods, water spreads over plains, depositing minerals and organic matter. Centuries of deposition create deep, fertile soil perfect for agriculture. Alluvial plains = breadbaskets!
Volcanic ash enriches soil — Wrong. Some plains (Java) have volcanic soil, but most fertile plains formed from river sediment deposition over time.