< HKDSE Geography < M6

In tropical rainforests, because of the high temperature and rainfall, infiltration is rapid, and thus chemical weathering of the bedrock occurs rapidly. For example, carbon dioxide, sulphur dioxide and humic acid are dissolved in rainwater to form solutions which wear the rock. Organic acids (from living plants) and humic acids (from dead plants) also wear the rock. As the bedrock is weathered, they turn into regolith and then soil. This process is covered in E1.

The soil characteristics of tropical rainforests are summed up in this table:

This is not from a TRF. It is used for explaining soil terminology only.
AspectCharacteristicReason

With reference to organisms and nutrients...

Level of organic matter (thickness of litter[1])SmallFast decomposition rate
Decomposition rateRapid, creates a thin layer of humusRapid nutrient uptake
Nutrient uptake rateRapidLuxuriant vegetation
Soil fertility (soil nutrient content)Low, soil nutrient concentrated on topsoilRapid nutrient uptake
Leaching[2]Rapid, washes away potassium, sodium, magnesium and calcium, as well as siliconHeavy rainfall

With reference to weathering...

Chemical weathering rateRapidHigh temperature and rainfall
Soil typeReddish, yellowish and brownish oxisolOxidation occurs as the soil is wet and exposed to the air. Iron and aluminium oxides are thus formed by oxidation.
Soil particle sizeFine and clayeyRapid chemical weathering
Soil permeability and soil porosityLow and lowFine and clayey soils
Water retention capabilityHighFine and clayey soils
Soil profile[3]Thick, up to 30 m, supports vegetationRapid chemical weathering
Soil horizons[4]Mature, merging boundariesRapid chemical weathering
Soil pHSlightly acidic, around 6Presence of iron oxides

Notes on Soil Terminology

Soil terminology not yet introduced in previous chapters are explained here:
  1. ↑ Litter refers to dead organic matter on the forest floor, mainly dead leaves and branches.
  2. ↑ The process by which rainfall washes away mineral nutrients in soil.
  3. ↑ The image on the right is a soil profile.
  4. ↑ The soil 'layers' on the right are soil horizons.


Soil Processes

Chemical Weathering

Leaching

Laterisation

 
HKDSE Geography : Core Module 6: Disappearing Green Canopy – Who should pay for the massive deforestation in rainforest regions?
Tropical Rainforests (TRFs)
Climate · Soil · Vegetation · Microclimate · Animals and Micro-Organisms
TRFs as Ecosystems
Trophic Levels · Energy Flow · Nutrient Cycle · Responses to Disturbances
Human Activities
Agriculture · Natural Resource Extraction · Settlement · Tourism · Underlying Causes
Impacts of Deforestation
Local Climate and Hydrosphere · Global Climate · Lithosphere · Biosphere · Economy · Society
Solutions to Deforestation
Measures · Stakeholders
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