Soil properties
Big idea: Soil properties determine how well plants grow. Key properties include texture, structure, pH, organic matter content, and water-holding capacity.
Soil texture
Texture refers to the size of mineral particles. The three main types are:
Sandy soil
- Large particles (0.05-2mm)
- Good drainage and aeration
- Low water and nutrient retention
- Warms up quickly in spring
Clay soil
- Tiny particles (<0.002mm)
- Poor drainage, easily waterlogged
- High water and nutrient retention
- Slow to warm, hard when dry
Loam is the ideal agricultural soil — a balanced mix of sand, silt, and clay with good drainage AND nutrient retention.
Other key properties
- Soil pH — affects nutrient availability (most crops prefer pH 6-7)
- Organic matter / humus — improves structure, holds nutrients and water
- Porosity — pore spaces for air and water
- Water-holding capacity — ability to retain moisture for plants
- Cation exchange capacity (CEC) — ability to hold and release nutrients
PAST PAPER PATTERN: Questions ask you to link soil properties to productivity. Example: "High humus content increases water-holding capacity AND nutrient availability, leading to higher NPP." Always make the cause-effect chain clear!
IB-style question — how texture changes a soil
A gardener compares a sandy soil with a clay soil for growing vegetables. Explain how their textures affect water drainage and nutrient retention. [4]
How the marks are earned
- Sandy soil — large particles, large pores
• water drains quickly → nutrients easily leached away
• low water and nutrient retention - Clay soil — tiny particles, tiny pores
• holds water tightly (risk of waterlogging)
• retains nutrients well
Final answer
Link particle size → pore size → drainage/nutrient effect for BOTH soils to get all 4 marks.