Key Idea: Most marks in topic 5.1 come from soil texture and productivity, and from soil outputs. At HL, also learn profiles, soil-forming factors, the texture triangle and soil carbon.
1. Texture and productivity (most marks)
- Sand drains fast and holds few nutrients; clay holds water and nutrients but can waterlog.
- A loam, a mix of sand, silt and clay, is the most productive.
- Humus holds water and nutrients and binds the soil together.
2. Soil as a system
- Inputs: dead organic matter, minerals from weathering, rain, fertiliser.
- Outputs: erosion, leaching, uptake by plants, carbon dioxide from decomposition.
- Transfers move matter; transformations change it (decomposition, weathering, salinization).
3. Outputs and HL ideas
- Bare soil is lost to wind and water erosion far faster than new soil forms.
- Leaching washes nutrients down and out of reach of roots.
- At HL: profiles and horizons, the five soil-forming factors, the texture triangle and soil carbon.
How it is examined:
| Question | How to score |
|---|---|
| [1] Identify an input, output or flow from a diagram | Use the exact name: leaching, not 'washing away'. |
| [2] to [4] Outline transfers and transformations | Say which kind each one is, with an example. |
| [2] to [3] Explain how texture affects productivity | Link drainage, water and nutrients to plant growth. |
| [3] to [4] Draw a soil systems diagram | Storages in boxes, labelled arrows in and out. |
Key terms, one line each:
| Term | In one line |
|---|---|
| Humus | dark, decomposed organic matter in soil |
| Leaching | nutrients washed down through the soil by water |
| Loam | a soil with a balanced mix of sand, silt and clay |
| Salinization | salt building up in soil, often from irrigation |
| Horizon | a layer in the soil profile |
| Podzol | an acid, leached soil under coniferous forest |
| Soil texture triangle | a chart that names a soil from its sand, silt and clay percentages |
Last-minute check: answer, then tap to see.
Name the five soil-forming factors. Climate, organisms, relief, parent material, time.
Which horizon holds the most organic matter? The O horizon at the top.
Why do waterlogged soils release methane? No oxygen, so microbes break matter down anaerobically.
What does the texture triangle tell you? The soil's name from its sand, silt and clay percentages.
A podzol forms under coniferous forest on sandy rock in a cool, wet climate. Explain how the soil-forming factors produce a podzol.
Model answer plan
See the mark-by-mark plan — for / against / judgement, with marking guidance — in study mode.
Exam tips
- Name each flow exactly and say if it is an input, output, transfer or transformation.
- Link texture to water, nutrients and air, then to plant growth.
- In a soil diagram, label every box and arrow.
- Learn one soil profile in detail, with its horizons.