IB DP Environmental Systems & Societies — Topic 5: Land
Soil Systems & Ecosystems
Subtopic 5.1 — New Syllabus (First Assessment 2026)
Standard Level + Higher Level
14 SL Points
5.1.1 – 5.1.14
7 HL Extensions
5.1.15 – 5.1.21
Overview
What You Need to Know
This subtopic models soil as an open dynamic system — examining its composition, profile horizons, inputs, outputs, transfers, transformations, role in biogeochemical cycles, texture, humus, and function as a global carbon sink.
A. Soil System & Structure
Dynamic open system, composition, horizons (O, A, B, C, R)
Dynamic open system, composition, horizons (O, A, B, C, R)
B. Processes & Texture
Inputs/outputs, transfers/transformations, texture triangle, humus
Inputs/outputs, transfers/transformations, texture triangle, humus
C. Classification & Carbon (HL)
Soil classification (-isols), 5 formation factors, soil analysis, CO₂ vs $\text{CH}_4$ release
Soil classification (-isols), 5 formation factors, soil analysis, CO₂ vs $\text{CH}_4$ release
Guiding Questions
- How do soils play a role in sustaining natural systems?
- How are human activities affecting the stability of soil systems?
Time allocation: minimum 3 hours SL + additional 2 hours HL. First subtopic in Topic 5: Land.
5.1.1 & 5.1.2
Soil as a Dynamic System & Composition
Key UnderstandingSoil is an open dynamic system consisting of inorganic minerals (sand, silt, clay), organic matter (humus, biota), water, and air.
45%
Inorganic Minerals
Sand, silt & clay
5%
Organic Matter
Humus, roots, organisms
25%
Soil Water
Dissolved nutrients
25%
Soil Air
Pore spaces ($\text{O}_2, \text{CO}_2$)
| System Component | Function & Ecological Role |
|---|---|
| Inorganic Minerals | Derived from rock weathering; provide structural support and mineral nutrients ($\text{K}^+, \text{Ca}^{2+}, \text{Mg}^{2+}$) |
| Organic Matter & Humus | Provides plant nutrients ($\text{N, P}$), enhances water retention, feeds decomposers, improves soil crumb structure |
| Soil Water & Air | Dissolves nutrients for root absorption; provides oxygen for root and microbial respiration |
5.1.3
Soil Profile & Horizons
Key UnderstandingSoils develop distinct vertical layers known as horizons (O, A, B, C, R) through long-term physical, chemical, and biological weathering.
| Horizon | Layer Name | Key Characteristics |
|---|---|---|
| O Horizon | Organic Layer | Decomposed leaf litter, plant residues, and dark humus accumulation |
| A Horizon | Topsoil / Mixed Layer | Rich in organic humus, active plant roots, and soil life. Most fertile & most vulnerable layer |
| B Horizon | Subsoil | Accumulates leached minerals (iron, aluminum oxides, clay) washed down from A horizon |
| C Horizon | Parent Material | Partially weathered bedrock and coarse rock fragments with minimal organic content |
| R Horizon | Bedrock | Solid unweathered bedrock layer foundation |
5.1.4 – 5.1.7
System Inputs, Outputs, Transfers & Transformations
Key UnderstandingSoil operates via inputs (litter, precipitation), outputs (erosion, leaching, gases), transfers (infiltration, biological mixing), and transformations (decomposition, weathering).
📥 Inputs & Transfers
- Inputs: Plant litterfall, organic manure, precipitation minerals, nitrogen fixation
- Transfers: Infiltration, downward percolation, earthworm biological mixing, leaching
📤 Outputs & Transformations
- Outputs: Wind & water topsoil erosion, nutrient leaching, plant harvesting, $\text{CO}_2/\text{CH}_4$ gas release
- Transformations: Organic decomposition to humus, rock weathering, nitrification
5.1.8 & 5.1.9
Soil in Biogeochemical Cycles & Plant Medium
Key UnderstandingSoil provides essential mineral nutrients ($\text{N, P, K}$) and water for terrestrial plants. Exception: Carbon is obtained from atmospheric CO₂ via photosynthesis.
| Biogeochemical Cycle | Soil Storage & Flow Role |
|---|---|
| Water Cycle | Infiltration, soil water storage, percolation to aquifers, uptake for plant transpiration |
| Carbon Cycle | Stores organic carbon in humus & peat; releases CO₂ via root and microbial respiration |
| Nitrogen Cycle | $\text{N}_2$-fixing bacteria convert nitrogen to $\text{NH}_4^+$; nitrifying bacteria convert $\text{NH}_4^+$ to $\text{NO}_3^-$ for plant absorption |
Exam distinction: Plants absorb mineral nutrients ($\text{N, P, K, Ca, Mg}$) from soil water via roots, but synthesize carbon compounds using atmospheric CO₂ during photosynthesis.
5.1.10 & 5.1.11
Soil Biodiversity & Ecosystem Functions
Key UnderstandingSoil hosts massive biodiversity (bacteria, fungi, earthworms, nematodes) driving nutrient recycling, soil aeration, and mycorrhizal symbiosis.
| Soil Organism Group | Ecological Role & Ecosystem Service |
|---|---|
| Decomposers & Fungi | Break down complex lignin & cellulose into plant-available nutrients and humus |
| Mycorrhizal Fungi | Symbiotic association with plant roots — greatly expands root surface area for phosphorus & water absorption |
| Earthworms & Burrowers | Biological mixing of O & A horizons, creating macropores for aeration and water infiltration |
| $\text{N}_2$-Fixing Bacteria | Rhizobium in legume root nodules convert atmospheric $\text{N}_2$ gas into bio-available nitrates |
5.1.12 & 5.1.13
Soil Texture Triangle & Productivity
Key UnderstandingSoil texture depends on relative proportions of sand, silt, and clay. Loam offers the optimum balance for primary productivity.
| Particle Type | Particle Size | Water Retention | Drainage & Aeration | Nutrient Holding |
|---|---|---|---|---|
| Sand | 0.05 – 2.0 mm | Low | Excessive / High | Low |
| Silt | 0.002 – 0.05 mm | Moderate | Moderate | Moderate |
| Clay | < 0.002 mm | Very High | Poor / Waterlogged | High ($\text{CEC}$) |
| Loam (Balanced) | Mix + Humus | Optimum | Optimum | High |
5.1.14
Soils as Carbon Sinks, Stores & Sources
Key UnderstandingSoil carbon balance depends on the ratio of organic litter inputs to microbial decomposition rates. Cold or waterlogged soils form major carbon sinks.
❄️ Cold Permafrost & Peatlands
Decomposition is extremely slow due to freezing or anaerobic conditions $\rightarrow$ massive long-term carbon sink (e.g. Arctic tundra, peat bogs).
🌴 Tropical Rainforest Soil
Warmth and moisture accelerate decomposition $\rightarrow$ organic carbon rapidly recycled into biomass $\rightarrow$ thin soil carbon store.
🔥 Degraded Agricultural Soil
Deforestation, heavy tillage, and wetland drainage accelerate oxidation $\rightarrow$ turns soil from carbon sink to net CO₂ source.
HL — 5.1.15 & 5.1.18
Soil Classification & 5 Formation FactorsHL
Key UnderstandingSoils are classified by profile appearance (-isols). Soil genesis is shaped by 5 factors: Climate, Organisms, Relief (geomorphology), Parent Material (geology), and Time.
| Soil Order (-isol) | Associated Biome | Profile Features |
|---|---|---|
| Oxisol | Tropical Rainforest | Deep, highly weathered, red/yellow iron oxide rich, nutrient-depleted topsoil |
| Mollisol | Temperate Grassland (Prairie) | Deep, dark, humus-rich A horizon; exceptionally high fertility |
| Spodosol | Boreal Forest (Taiga) | Acidic, leached ash-gray A horizon with mineral accumulation in B horizon |
| Aridisol | Desert | Arid, alkaline, minimal organic matter with salt/calcium carbonate crusts |
HL — 5.1.16 & 5.1.17
A Horizon Value & VulnerabilityHL
Key UnderstandingThe A horizon (topsoil) contains mixed organic humus and mineral nutrients. It is the primary engine of terrestrial plant growth but the most susceptible to erosion.
Topsoil Vulnerability & Degradation Cascade
- Plowing and vegetation removal expose loose A horizon particles to wind and water erosion.
- Erosion removes dark humus $\rightarrow$ loss of Cation Exchange Capacity ($\text{CEC}$) and water-holding ability.
- Subsoil (B horizon) exposure results in poor seedling germination and agricultural desertification.
HL — 5.1.19 – 5.1.21
Soil Analysis & Greenhouse Gas ReleaseHL
Key UnderstandingSoil health is measured via bulk density, organic %, infiltration, and pH. Aerobic soils release CO₂; anaerobic waterlogged soils release $\text{CH}_4$.
| Soil Analysis Parameter | Measurement Method | Ecological Meaning |
|---|---|---|
| Bulk Density | Core dry mass / core volume | High bulk density indicates severe soil compaction, restricting root growth |
| Organic Content (%) | Loss-on-ignition (dry burning at 500°C) | Higher % indicates greater humus content, fertility, and carbon storage |
| Gas Emission Mode | Aerobic vs Anaerobic respiration | Aerobic soils release CO₂; flooded rice paddies/peatlands release $\text{CH}_4$ (28× GHG potency) |
Key Terms Glossary
Essential Vocabulary
| Term | Definition |
|---|---|
| Soil Profile | Vertical cross-section showing distinctive soil horizons (O, A, B, C, R) |
| Humus | Dark, crumbly organic material formed by fully decomposed plant and animal matter |
| A Horizon (Topsoil) | Fertile mixed organic-mineral layer essential for root growth; highly vulnerable to erosion |
| Leaching | Downward removal of dissolved mineral nutrients out of root reach by percolating water |
| Loam | Optimal agricultural soil texture combining balanced sand, silt, clay, and organic humus |
| Mycorrhizae | Symbiotic fungal-plant root association enhancing nutrient (phosphorus) and water uptake |
| Oxisol / Mollisol | Soil orders characteristic of tropical rainforests (Oxisol) and temperate grasslands (Mollisol) |
| Bulk Density | Dry soil mass per unit volume; indicator of soil compaction and aeration pore space |
| Permafrost Carbon Sink | Frozen Arctic soils storing massive amounts of organic carbon under low decomposition rates |
Exam Preparation
Key Takeaways & Exam Tips
Must-Know Concepts
- Soil System Model: Inputs (litter, rain) $\rightarrow$ Transfers (leaching, percolation) $\rightarrow$ Transformations (decomposition, weathering) $\rightarrow$ Outputs (erosion, gas release).
- Plant Nutrients Exception: Plants absorb mineral nutrients ($\text{N, P, K}$) from soil water, but carbon is taken from atmospheric CO₂.
- Texture Triangle: Practice reading Sand %, Silt %, Clay % coordinates to identify Loam vs Clay soil.
- HL Formation Factors: Remember CLORPT (Climate, Organisms, Relief, Parent Material, Time).
Exam tip: In essay questions comparing agricultural sustainability, contrast high-tillage monoculture (A horizon loss, compaction) with conservation tillage & cover cropping.
You've covered all 21 syllabus points ✅
5.1.1 – 5.1.14 (SL) + 5.1.15 – 5.1.21 (HL)