UPSC Prelims 2023 Q63: Tropical Rainforest Soils — Explained
Consider the following statements:
Statement-I:
The soil in tropical rain forests is rich in nutrients.
Statement-II:
The high temperature and moisture of tropical rain forests cause dead organic matter in the soil to decompose quickly.
Which one of the following is correct in respect of the above statements?
- Both Statement-I and Statement-II are correct and Statement-II is the correct explanation for Statement-I
- Both Statement-I and Statement-II are correct and Statement-II is not the correct explanation for Statement-I
- Statement-I is correct but Statement-II is incorrect
- Statement-I is incorrect but Statement-II is correct
Answer: (d) Statement-I is incorrect but Statement-II is correct
Statement-wise explanation
- Statement-I: incorrect — Most tropical rainforest soils are poor in nutrients, even though the forest above them is lush. Nearly all of the ecosystem's nutrients sit in the living plants and in the thin layer of litter on the forest floor, not in the soil. Heavy rain all year washes soluble bases such as calcium, magnesium and potassium down out of the root zone, a process called leaching. What remains is mostly iron and aluminium oxides, as in oxisols and laterite soils. This is why cleared rainforest land loses its fertility after a few harvests.
- Statement-II: correct — Year-round heat and high humidity keep termites, fungi and bacteria active, so fallen leaves and other dead matter break down within weeks or months. This is much faster than in temperate or boreal forests, so little humus builds up in the soil. Statement-II does not explain Statement-I, because Statement-I is false. Fast decomposition is actually part of why the soil stays poor: the released nutrients are quickly taken up by shallow roots or washed away by rain, and very little stays in the soil.
How to eliminate
In a Statement-I/II question, first judge each statement on its own and only then ask whether one explains the other. Statement-II is basic biology: warm, wet places speed up decay. That rules out the option where Statement-II is incorrect. Statement-I is the trap. It sounds right because the forest looks so rich. The fact to remember is the paradox: a lush rainforest stands on poor soil, because its nutrients are held in the vegetation and leached out of the ground. Two more clues help. The 'explains' options would require fast decay to make the soil rich, but fast decay means less humus stays in the soil. And slash-and-burn farmers in rainforests move on after two or three years because the soil wears out, which a nutrient-rich soil would not do. This question can only be solved if you know the paradox.
Know more
Rich forest, poor soil: the rainforest paradox
In a tropical rainforest, nutrients cycle quickly through a closed loop:
- Leaves, branches and dead animals fall onto the forest floor.
- Decomposers such as termites, fungi and bacteria break them down fast in the heat and humidity.
- Dense, shallow roots, often helped by mycorrhizal fungi, take the freed nutrients back up almost at once.
- Anything the roots miss is leached downward by more than 2,000 mm of rain a year.
So the stock of nutrients sits in the biomass, not the soil. Cut and burn the forest, and the ash gives one or two good harvests. After that, rain strips the soil and yields collapse. This is why jhum, the shifting cultivation of north-east India, needs long fallow periods.
| Feature | Tropical rainforest | Temperate deciduous forest |
|---|---|---|
| Decomposition | Very fast | Slow (cold winters) |
| Humus in soil | Low | High |
| Where nutrients are stored | Living vegetation | Soil |
| Leaching | Intense, all year | Moderate |
| Typical soil | Oxisol / laterite (red, acidic, Fe-Al rich) | Brown forest soil (fertile) |
The link to India. Laterite soils form in the same way. High temperature and heavy rainfall leach out silica and bases, leaving iron and aluminium oxides behind. They are found in the Western Ghats, Kerala, Karnataka, Odisha, Assam and Meghalaya. NCERT notes that laterite soils are poor in organic matter, nitrogen, phosphate and calcium, because bacteria that thrive in high temperatures remove humus quickly. Laterite soil hardens when exposed and is cut into bricks. With manure and fertiliser, it supports plantation crops such as tea, coffee, rubber and cashew.
Remember: high temperature + heavy rain → fast decay + leaching → nutrient-poor soil under a nutrient-rich forest.
NCERT Concepts to Revise
- Leaching: Year-round heavy rain in rainforests drives leaching, which decides how many nutrients a rainforest soil can hold.
- Laterite soil: Laterite is the classic soil of hot, wet climates and India's closest example of a rainforest-type soil.
- Amazon rainforest: The Amazon is the standard example used to test ideas about rainforest soils and nutrient cycling.