Soils in Rainforests and Savannas

This section explores the characteristics of soils found in tropical rainforests and savannas, focusing on oxisols/latosols and tropical red and brown earths. We will examine their formation, profile characteristics, and implications for vegetation and agriculture.

1. Oxisols / Latosols

Formation

Oxisols (primarily found in rainforests) and Latosols (found in older rainforest areas) are formed under high temperatures and rainfall. The intense weathering processes lead to the leaching of minerals, particularly silica, clay, and soluble elements.

The high temperatures accelerate chemical reactions, breaking down rocks and organic matter. This process is known as intense weathering.

The leaching process removes most of the easily soluble materials, leaving behind insoluble oxides of iron and aluminium. These oxides give the soil its characteristic reddish or yellowish colour.

Soil Profile Characteristics

Oxisols and Latosols exhibit distinct soil horizons:

• O Horizon: A thin layer of organic matter, often decomposed leaf litter.
• A Horizon: Light-coloured, leached layer with reduced organic matter. Often contains iron and aluminium oxides.
• E Horizon: Further leaching occurs, resulting in a pale, sandy layer with even lower concentrations of iron and aluminium oxides.
• B Horizon: The accumulation zone of leached materials – iron and aluminium oxides (parent material), clay, and other minerals. This layer is often hard and dense.
• C Horizon: Parent material, often weathered rock.

Properties and Implications

Colour: Reddish or yellowish due to iron and aluminium oxides.

Texture: Often sandy or clayey, depending on the degree of leaching.

Nutrient Availability: Low in essential plant nutrients due to leaching. However, some nutrients may be present as oxides.

Water Retention: Can vary, but often drains well due to the sandy component.

Agriculture: Generally unsuitable for agriculture without significant soil amendment. Requires extensive use of fertilizers.

2. Tropical Red and Brown Earths

Formation

Tropical red and brown earths are formed in savannas and areas with distinct wet and dry seasons. They are formed through a combination of weathering and the accumulation of iron and aluminium oxides.

The alternating wet and dry seasons promote the oxidation of iron, leading to the formation of red soils. The presence of organic matter contributes to the brown colour in some areas.

Soil Profile Characteristics

The soil profile is less distinct than that of oxisols/latosols, but typically includes:

• O Horizon: A layer of organic matter, often mixed with leaf litter.
• A Horizon: Darker, more fertile layer with a higher proportion of organic matter.
• B Horizon: Accumulation zone of iron and aluminium oxides. May be reddish or brownish in colour.
• C Horizon: Weathered parent material.

Properties and Implications

Colour: Reddish or brownish, depending on the amount of iron oxides present.

Texture: Generally loamy, with a good balance of sand, silt, and clay.

Nutrient Availability: More fertile than oxisols/latosols, with a higher concentration of plant nutrients.

Water Retention: Good water retention capacity due to the loamy texture.

Agriculture: Suitable for agriculture, particularly for growing crops that can tolerate periods of drought. Often supports natural grasslands and savannas.

3. Comparison Table

Feature	Oxisols / Latosols	Tropical Red and Brown Earths
Formation	Intense weathering, leaching, high temperatures and rainfall.	Weathering, alternating wet and dry seasons, iron and aluminium oxide accumulation.
Colour	Reddish/Yellowish	Reddish/Brownish
Texture	Sandy/Clayey	Loamy
Nutrient Availability	Low	Moderate
Water Retention	Variable, often drains well	Good
Agriculture	Generally unsuitable without amendment	Suitable, supports grasslands and savannas

Figure: Suggested diagram: A cross-section showing the soil profiles of Oxisols/Latosols and Tropical Red and Brown Earths, clearly labeling the horizons.