At HL, soil formation in IB ESS subtopic 5.1 is influenced by climate, organisms, geomorphology, geology (parent material) and time. These interacting factors determine a soil's profile, texture, fertility, drainage and development rate.
| Factor | Influence on soil formation |
|---|---|
| Climate | Temperature controls physical and chemical weathering, while precipitation affects leaching, erosion and water availability. Warm, wet conditions generally accelerate chemical weathering and decomposition; cold or dry conditions slow development. |
| Organisms | Plants add dead organic matter; roots break parent material; and decomposers form humus and recycle nutrients. Animals mix and aerate soil. Farming and deforestation can alter inputs and accelerate erosion. |
| Geomorphology | Slope, elevation and landscape shape affect drainage, erosion and deposition. Steep slopes usually have thinner soils because material is removed; valleys may accumulate sediment and develop deeper soils. |
| Geology | Parent material influences mineral content, particle size, pH and permeability. Limestone-derived soils are often calcium-rich, whereas sandy material commonly produces freely draining soils. |
| Time | Continued weathering, decomposition, leaching and translocation differentiate soil horizons. Young soils usually show less developed horizons than older soils under comparable conditions. |
These factors operate within a soil system: climate supplies precipitation and energy, organisms transform organic matter, and geomorphology controls transfers of water and particles. Soil formation therefore reflects interacting inputs, outputs, storages and flows.
The Amazon rainforest illustrates this interaction: high rainfall and temperature promote rapid weathering and decomposition, but intense leaching can leave soils nutrient-poor despite abundant vegetation. Deforestation removes organic inputs and exposes soil to erosion, affecting ecosystem productivity and local and Indigenous communities.
A common misconception is that climate alone determines soil type. It does not: soil properties result from all five interacting factors.
Exam technique: For an explain question, give a cause-and-effect link for each factor rather than listing it. Use terms such as weathering, leaching, decomposition and erosion, then include a named example.