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:
With reference to organisms and nutrients...
|Level of organic matter (thickness of litter)||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||Rapid, washes away potassium, sodium, magnesium and calcium, as well as silicon||Heavy rainfall|
With reference to weathering...
|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||Thick, up to 30 m, supports vegetation||Rapid chemical weathering|
|Soil horizons||Mature, merging boundaries||Rapid chemical weathering|
|Soil pH||Slightly acidic, around 6||Presence of iron oxides|
Notes on Soil Terminology
|Soil terminology not yet introduced in previous chapters are explained here: