Soil types and profile characteristics: oxisols/latosols, tropical red and brown earths

Soils in Tropical Rainforests and Savannas (Cambridge AS & A Level Geography 9696 – Topic 7)

1. Main Tropical Soil Families (WRB 2022 classification)

• Oxisols (Latosols) – highly weathered, deep, well‑drained soils of humid tropical rainforests.
• Tropical Red Earths (Alfisols/Entisols) – iron‑rich soils of leeward rainforest margins and savanna regions.
• Tropical Brown Earths (Inceptisols/Alfisols) – less leached, more fertile soils of mixed forest‑savanna mosaics and older alluvial deposits.

2. Detailed Soil Profiles

2.1 Oxisols / Latosols

• Typical climate: Humid tropical, annual rainfall > 2000 mm, mean temperature ≈ 25‑27 °C.
• Dominant minerals: Kaolinite, Fe‑Al oxides (laterite), minor quartz.
• Colour & texture: Deep red‑brown to orange; fine‑grained, often a hard‑pan Bt horizon.
• pH & base saturation: pH 4.0‑5.0 (acidic); base saturation ≤ 30 %.
• Typical profile sequence: O‑A‑E‑Bt‑C‑R
  • O – thin litter layer.
  • A – low‑activity humus.
  • E – eluviation of silicates, light‑coloured.
  • Bt – thick, dark, Fe‑Al‑oxide‑rich illuvial horizon.
  • C – weathered parent material.
  • R – unweathered bedrock.
• Nutrient‑cycling characteristics:
  • High Fe‑Al oxides bind cations → very low CEC.
  • Phosphorus strongly adsorbed to iron oxides → limited availability.
  • Scarce organic matter → low microbial activity.
• Agricultural potential: Requires intensive fertiliser (e.g., > 200 kg ha⁻¹ P₂O₅, 2023 FAO data) and liming (≈ 2 t ha⁻¹ CaCO₃). Suitable for plantation crops (oil palm, rubber, cocoa) when inputs are supplied.
• Geographical examples: Brazilian Amazon (Manaus region), Congo Basin, West‑central Java (Indonesia).
• Evaluation prompt: *Assess the long‑term sustainability of intensive oil‑palm on Oxisols, considering nutrient depletion, soil structure and social‑economic factors.*

2.2 Tropical Red Earths (Alfisols/Entisols)

• Typical climate: Humid to sub‑humid, 1500‑2500 mm rainfall.
• Dominant minerals: Hematite, goethite, some kaolinite; moderate silica retention.
• Colour & texture: Bright red due to iron oxides; loamy to sandy‑loam.
• pH & base saturation: pH 4.5‑5.5; base saturation 30‑50 %.
• Typical profile sequence: O‑A‑E‑Bt‑C
  • Bt – distinct red horizon rich in iron oxides, moderate clay accumulation.
• Nutrient‑cycling characteristics:
  • Iron oxides retain phosphorus less strongly than in Oxisols → slightly higher P availability.
  • Moderate CEC; exchangeable bases (Ca, Mg, K) higher than in Oxisols.
• Agricultural potential: Supports shifting cultivation, extensive grazing and low‑input crops (millet, sorghum). Fertiliser improves yields but inputs are lower than for Oxisols.
• Geographical examples: Kenyan savanna (Kajiado County), northern Tanzania, Brazilian Cerrado.
• Evaluation prompt: *Discuss the trade‑offs between extensive grazing and soil conservation on Red Earths.*

2.3 Tropical Brown Earths (Inceptisols/Alfisols)

• Typical climate: Seasonally dry, 800‑1500 mm rainfall.
• Dominant minerals: Mixed clay minerals (kaolinite, illite), moderate Fe‑oxides.
• Colour & texture: Brown to yellow‑brown; well‑structured loam.
• pH & base saturation: pH 5.5‑6.5; base saturation ≥ 50 %.
• Typical profile sequence: O‑A‑Bt‑C
  • Bt – well‑developed clay‑enriched horizon from illuviation.
• Nutrient‑cycling characteristics:
  • Higher organic matter → greater microbial activity and CEC.
  • Phosphorus less strongly fixed; more readily available.
• Agricultural potential: Most versatile; supports subsistence (maize, beans) and cash crops (cocoa, coffee) with modest fertiliser inputs.
• Geographical examples: Ghanaian forest‑savanna zone, southern Nigeria, Western Ghats of India.
• Evaluation prompt: *Evaluate why Brown Earths are preferred for mixed farming systems in terms of productivity, resilience and equity.*

3. Comparative Table of Soil Families

4. Nutrient‑Cycling & Fertility in Tropical Soils (AO2 – Skills)

Use the diagram below (adapted from Gersmehl’s nutrient‑cycling model) to illustrate the main fluxes for each soil family.

• Oxisols: Very low CEC → limited exchange of Ca²⁺, Mg²⁺, K⁺; P strongly adsorbed to Fe‑oxides; organic‑matter turnover is slow.
• Red Earths: Moderate CEC; iron oxides retain some P but less than Oxisols; higher base‑cation levels allow better plant uptake.
• Brown Earths: Higher organic matter → higher CEC; P is more mobile; microbial mineralisation replenishes N.

When answering exam questions, students should be able to:

1. Identify the limiting nutrient(s) for each soil family.
2. Explain how climate (rainfall intensity) and vegetation influence nutrient loss (leaching, erosion).
3. Suggest appropriate management (type and rate of fertiliser, liming, erosion control) to improve fertility.

5. Human‑Impact Case Studies (required by the syllabus)

5.1 Amazonian Oxisol Conversion to Oil‑Palm Plantation

• Location: Pará state, Brazil.
• Process: Large‑scale clearing of rainforest → exposure of deep Oxisols.
• Soil challenges: Acidic pH, low CEC, strong P fixation.
• Management (2023 FAO data): > 200 kg ha⁻¹ P₂O₅, 2 t ha⁻¹ CaCO₃ (lime), contour planting and mulching to reduce erosion.
• Outcomes: Initial yield increase, but long‑term sustainability threatened by nutrient depletion, soil compaction and loss of smallholder livelihoods.
• Evaluation prompt: *Assess the economic benefits versus the environmental and social costs of this conversion.*

5.2 Over‑grazing of Kenyan Savanna Red Earths

• Location: Kajiado County, Kenya.
• Process: Expansion of cattle herding on Red‑Earth soils.
• Soil challenges: Removal of vegetation → increased runoff, loss of the thin A horizon, erosion of the Bt horizon, decline in base saturation.
• Consequences: Reduced pasture productivity, heightened desertification risk, disproportionate impact on pastoralist families (often women‑headed households).
• Mitigation measures: Rotational grazing, reseeding with native grasses, construction of stone bunds, community‑managed grazing committees.
• Evaluation prompt: *Critically evaluate whether the proposed mitigation measures are likely to restore soil health and improve equity for local communities.*

6. Implications for Land‑Use Planning (AO2 – Skills & AO3 – Evaluation)

Students should be able to interpret soil‑profile diagrams, extract data from tables, and evaluate land‑use options.

1. Diagram interpretation task: Given a cross‑section showing O‑A‑E‑Bt‑C, identify the soil family and justify your choice using colour, texture and horizon characteristics.
2. Data‑driven comparison: Using the comparative table, list three properties that make the identified soil suitable or unsuitable for a chosen crop (e.g., maize).
3. Management plan (AO3): Propose a plan that addresses:
   • pH amendment (type and rate of lime).
   • Limiting nutrient(s) and appropriate fertiliser (e.g., P‑based for Oxisols, N‑based for Brown Earths).
   • Erosion control measures (contour ridges, cover crops, agroforestry).
   • Social considerations – who will benefit, gender implications, access to inputs.
4. Evaluation question: *To what extent can sustainable intensification be achieved on each soil type without compromising long‑term productivity and equity?*

7. Suggested Revision Diagram

8. References (latest international standards)

• Food and Agriculture Organization (FAO) & International Union of Soil Sciences (IUSS). World Reference Base for Soil Resources 2022. Rome: FAO, 2022.
• FAO. “Fertiliser use on Amazonian Oxisols 2020‑2023.” FAO Statistical Yearbook, 2024.
• Schwartz, F. & S. McCarty. “Nutrient dynamics in highly weathered tropical soils.” Geoderma 401 (2023): 115‑129.
• UNESCO. Soil Atlas of the World, 2022 – chapters on tropical soils.