HKDSE Geography/M6/Soil

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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.
Aspect Characteristic Reason

With reference to organisms and nutrients...[edit]

Level of organic matter (thickness of litter[1]) Small Fast decomposition rate
Decomposition rate Rapid, creates a thin layer of humus Rapid nutrient uptake
Nutrient uptake rate Rapid Luxuriant vegetation
Soil fertility (soil nutrient content) Low, soil nutrient concentrated on topsoil Rapid nutrient uptake
Leaching[2] Rapid, washes away potassium, sodium, magnesium and calcium, as well as silicon Heavy rainfall

With reference to weathering...[edit]

Chemical weathering rate Rapid High temperature and rainfall
Soil type Reddish, yellowish and brownish oxisol Oxidation occurs as the soil is wet and exposed to the air. Iron and aluminium oxides are thus formed by oxidation.
Soil particle size Fine and clayey Rapid chemical weathering
Soil permeability and soil porosity Low and low Fine and clayey soils
Water retention capability High Fine and clayey soils
Soil profile[3] Thick, up to 30 m, supports vegetation Rapid chemical weathering
Soil horizons[4] Mature, merging boundaries Rapid chemical weathering
Soil pH Slightly acidic, around 6 Presence of iron oxides

Notes on Soil Terminology[edit]

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[edit]

Chemical Weathering[edit]

Leaching[edit]

Laterisation[edit]