Key Idea: Most marks in topic 5.1 come from soil texture and productivity, and from soil outputs. Learn the soil as a system of inputs, outputs, transfers and transformations.
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. Losing soil
- 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.
- Draw the soil system as boxes (storages) and arrows (flows).
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 |
Last-minute check: answer, then tap to see.
Which soil is best for crops? Loam: it drains well but holds water and nutrients.
Why can clay soils waterlog? Tiny particles leave small pores, so water drains slowly.
Is decomposition a transfer or a transformation? A transformation: dead matter becomes humus and nutrients.
Name two outputs from soil. Any two: erosion, leaching, plant uptake, carbon dioxide.
IB-style questionExplain[4 marks]
A farmer has a field of sandy soil and a field of loam. Explain why crops grow better on the loam.
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 example of soil erosion, such as the Dust Bowl.