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Difficulty: Very hardSoil Types, Distribution, and Soil Conservation in Nigeria

In the humid tropical zone of southeastern Nigeria, intense seasonal rainfall causes deep leaching of sedimentary rocks, producing acidic, iron-rich ferrallitic soils (red earths) that suffer severe gully erosion upon vegetation clearance. Which of the following best explains why biological conservation measures (such as reforestation and cover cropping) are essential for restoring long-term stability to these specific soils compared to physical terracing alone?

  1. Biological measures rebuild organic humus to aggregate porous soil particles and bind loose subsoils against deep hydraulic piping, whereas physical terracing does not improve soil structural cohesion.Answer
  2. B
    Physical terracing only prevents in-situ chemical weathering of parent bedrock, whereas afforestation primary stops down-slope mass wasting of fully formed soil layers.
  3. C
    Ferrallitic soils are formed by marine sedimentary deposition of metamorphic slates, which causes terraced slopes to chemically dissolve during rainy seasons.
  4. D
    Ferrallitic red soils are restricted to the semi-arid Sudan and Sahel belts where wind erosion renders physical terraces ineffective compared to moisture-retaining cover crops.

Answer

Biological measures rebuild organic humus to aggregate porous soil particles and bind loose subsoils against deep hydraulic piping, whereas physical terracing does not improve soil structural cohesion.
Biological conservation practices like cover cropping and afforestation supply vital organic litter to heavily leached ferrallitic soils. Humus acts as a binding agent to aggregate soil grains, while plant root networks structurally anchor loose subsoils, preventing subsurface piping and gully incision that physical terraces cannot stop on highly permeable sands.

Step-by-Step Solution

1
Identify the pedological characteristics of ferrallitic soils in southern Nigeria.
Ferrallitic soils are heavily leached, kaolinitic red soils derived from sedimentary rocks in high-rainfall zones, possessing poor structural stability and low organic content when stripped of canopy.
Understanding the physical and chemical vulnerability of the soil profile is necessary to evaluate effective conservation methods.
2
Analyze the mechanics of gully erosion in southeastern Nigerian soil formations.
Gully erosion in places like Anambra and Imo is driven by surface runoff and underground water seepage (piping) through porous, un-aggregated sandy subsoil layers beneath steep slopes.
Physical terracing alters slope gradient but does not bind loose subsoil particles or restore lost soil organic matter.
3
Evaluate the specific function of biological soil conservation (reforestation and cover cropping).
Plant roots bind subsoil layers and reduce subterranean piping, while decaying organic litter adds humus that improves soil aggregation, water retention, and structural resistance to erosion.
Biological methods target the root cause of ferrallitic soil degradation by restoring soil structure and vegetative protection.

Key Concept

Ferrallitic Soil Characteristics, Distribution, and Conservation Mechanisms in Nigeria
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Topics

Soil Types, Distribution, and Soil Conservation in Nigeria

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