Soil and productivity
Big idea: Soil quality directly affects net primary productivity (NPP). Healthy soils support more plant growth, which supports more consumers and decomposers throughout the ecosystem.
How soil properties affect productivity
- Nutrient availability — N, P, K essential for plant growth; limiting factors
- Water-holding capacity — plants need consistent water supply
- Aeration — roots need oxygen for respiration
- Soil depth — deeper soils allow more root growth
- pH — affects nutrient availability and microbial activity
- Organic matter — improves all of the above
Soil organisms and productivity
Soil is teeming with life! One teaspoon contains billions of bacteria, fungi, and other organisms.
- Decomposers (bacteria, fungi) — break down dead matter, release nutrients
- Nitrogen-fixing bacteria — convert N2 to usable forms (in legume root nodules)
- Mycorrhizae — fungi that help plant roots absorb nutrients and water
- Earthworms — aerate soil, mix organic matter, improve drainage
PAST PAPER PATTERN (4-7 marks): "Explain how soil properties influence productivity." You need multiple properties, each with a CAUSE-EFFECT link to productivity. Example: "High organic matter increases CEC, allowing soil to hold more nutrients, which plants absorb for growth, increasing NPP."
IB-style question — what makes a soil productive
Outline four characteristics of a soil that make it highly productive for agriculture. [4]
How the marks are earned
- Feeds the plant
• high nutrient content (N, P, K) and organic matter / humus
• retains moisture without waterlogging - Lets roots thrive
• loamy texture — good drainage and aeration
• near-neutral pH (~6–7) with active soil organisms
Final answer
Any four distinct characteristics score 1 mark each — briefly link each one to plant growth to secure the mark.
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