IB DP Environmental Systems and Societies – Soil, agriculture and food Revision Notes

For full explanations and worked examples, start with the soil, agriculture and food study guide. These notes condense the same unit for the final weeks before the exam.

They cover IB Diploma Programme Environmental Systems and Societies, aligned to the IB Diploma Programme Subject Brief, Environmental systems and societies: syllabus sections 5.1 (Soil) and 5.2 (Agriculture and food) of Topic 5, Land. The topic is studied at SL and HL (8 and 15 recommended teaching hours); HL students study some topics in extra depth. It follows the IB ESS subject brief for first assessment 2026 — the course examined in the May and November 2026, 2027 and 2028 sessions.

The brief gives Topic 5 Land 8 teaching hours at SL and 15 at HL. It does not list the subtopics or learning outcomes for this topic, so the numbered subtopics and outcomes on this page follow the syllabus numbering used in the printable ESS checklist, not the brief itself.

Links: ESS course hub · printable checklist · practice questions · Topic 2 Ecology revision notes · Topic 1 Foundations revision notes · ESS assessment revision notes

Key definitions

• Soil — a mixture of mineral particles, organic matter, water, air and living organisms at the land surface.
• Humus — dark, stable organic matter formed when dead material decomposes; it holds water and nutrients.
• Soil profile — a vertical section showing the horizons (O, A, B, C, then bedrock).
• Soil texture — the proportions of sand, silt and clay.
• Leaching — dissolved nutrients carried down and out of the soil by percolating water.
• Soil degradation — a decline in soil quality that reduces its ability to support plant growth.
• Salinization — build-up of salts at the surface when irrigation water evaporates.
• Desertification — degradation of dryland soils to desert-like conditions.
• Food security — all people, at all times, having access to enough safe and nutritious food.
• Energy ratio (farming) — food energy out ÷ energy in.

Soil as a system

Category	Examples
Inputs	Leaf litter and dead organisms, weathered parent rock, precipitation, solar energy, atmospheric gases
Storages	Organic matter, nutrients, minerals, water, air, organisms
Transfers	Infiltration, percolation, leaching, mixing by earthworms, translocation between horizons
Transformations	Decomposition, humification, weathering, nitrogen fixation
Outputs	Uptake by plants, leaching out, erosion, evaporation, respiration gases

Soil formation factors: parent material, climate, organisms, relief, time. Formation is very slow, so soil is effectively non-renewable on a human timescale.

Texture — must-know table

	Sand	Clay	Loam
Particle size	Largest	Smallest	Mixed
Drainage and aeration	High	Low	Good
Water retention	Low	High	Good
Nutrient retention	Low	High	Good
Primary productivity	Low	Medium (drainage limits)	Highest

Use a soil texture triangle to turn three percentages into a class. The three percentages must add up to 100.

Method in steps

Soil water content (% of fresh mass)

1. Weigh the fresh sample.
2. Dry in an oven until the mass stops changing.
3. Water = fresh mass − dry mass.
4. Divide by fresh mass and × 100. State “of fresh mass”.

Organic matter by loss on ignition (% of dry mass)

1. Weigh the oven-dried sample.
2. Heat strongly to burn off organic matter.
3. Loss = dry mass − mass after ignition.
4. Divide by dry mass and × 100.

Percentage change (erosion rates, yields, salinity) (new − original) ÷ original × 100. A fall gives a negative change; “percentage reduction” is quoted as a positive number.

Trophic-level land comparison

1. Find annual energy need per person (daily need × 365).
2. People fed on crop = crop energy ÷ need.
3. Energy as animal product ≈ 10% of crop energy (rule of thumb).
4. People fed = animal-product energy ÷ need.

Small reminder: a field losing 24.0 t/ha/yr of soil with bulk density 1.2 t/m³ loses 24.0 ÷ 10 000 ÷ 1.2 = 0.002 m, or 2.0 mm, each year.

Degradation and conservation — cause → fix

Problem	Main causes	Matching conservation methods
Water erosion	Bare soil, ploughing downslope, deforestation	Contour ploughing, terracing, cover crops, mulching, no-till
Wind erosion	Dry bare fields, overgrazing, removal of hedges	Windbreaks, cover crops, controlled grazing
Salinization	Irrigation in hot, dry climates with poor drainage	Drip irrigation, drainage channels, flushing salts
Nutrient depletion	Continuous cropping, no fallow	Crop rotation with legumes, manure, compost, fertiliser
Compaction	Heavy machinery, trampling	Fewer passes, controlled traffic, lower stocking rates
Acidification	Some fertilisers, acid deposition	Lime

Every conservation method should be linked to a mechanism: slows water, slows wind, covers the surface, or restores organic matter and nutrients.

Agriculture and food

Classify farms by pairs: subsistence vs commercial; intensive vs extensive; arable vs pastoral vs mixed; conventional vs organic. The pairs are independent — a farm can be subsistence and intensive.

Farm as a system: inputs (seed, water, fertiliser, labour, fuel, machinery, sunlight) → outputs (food, waste, runoff, greenhouse gases).

Energy ratio: high for labour-based subsistence farms; low for intensive livestock, which eats crops people could eat.

Trophic levels and diet: roughly 90% of energy is lost at each step, so eating plants feeds many more people per hectare than eating animals fed on crops. Grazing land that cannot grow crops is the main exception.

Food waste pattern:

• Lower-income countries — losses mostly early (harvest, storage, transport).
• Higher-income countries — waste mostly late (retail, households).

Impacts of intensive farming: habitat loss and biodiversity decline; eutrophication from fertiliser runoff; pesticide harm to non-target species; methane (cattle, rice paddies), nitrous oxide (fertilised soils) and carbon dioxide (machinery, clearance); water depletion; soil degradation.

Sustainable strategies: cut waste; plant-rich diets; crop rotation and polyculture; integrated pest management; agroforestry; precision farming; soil conservation.

Must-know distinctions

• Transfer vs transformation — leaching and erosion move material (transfers); decomposition and weathering change it (transformations).
• Sand vs clay — sand drains and warms quickly but holds few nutrients; clay holds water and nutrients but drains poorly.
• Intensive vs extensive — about inputs per unit area, not farm size or purpose.
• Food loss vs food waste — loss happens early in the supply chain; waste happens late, at retail and in homes.
• Undernutrition vs overnutrition — both are forms of malnutrition.
• Technocentric vs ecocentric food solutions — technology-led (GM crops, precision farming, alternative proteins) vs change-in-behaviour and low-input approaches (local food, diet change, agroecology).

HL students apply the three HL-only lenses — environmental law, environmental and ecological economics, environmental ethics — to issues such as fertiliser pollution or fair access to food.

Quick self-test

1. Name the five factors that control soil formation.
2. Is leaching a transfer or a transformation? Explain in one line.
3. A soil has 25% clay and 35% silt. What percentage is sand?
4. An oven-dried soil of 40.0 g has a mass of 37.2 g after ignition. Calculate the organic matter percentage.
5. Give two reasons loam supports higher primary productivity than sand.
6. Explain how salinization develops in irrigated land.
7. A farm uses 12 GJ of energy and produces 54 GJ of food energy. Calculate its energy ratio.
8. A crop provides 50 000 MJ of food energy. Assuming 10% transfer, how much energy is available if it is fed to livestock?
9. Soil electrical conductivity (a measure of salinity) rises from 1.6 to 4.0 dS/m over 10 years. Calculate the percentage increase and the mean rate of increase per year.
10. State where most food is lost or wasted in a lower-income country and in a higher-income country.
11. After five years of cover cropping, soil organic matter in a field rises from 2.5% to 3.1%. Calculate the percentage change.
12. Explain one way a windbreak and one way contour ploughing reduce soil erosion.

Answers

1. Parent material, climate, organisms, relief, time.
2. A transfer — nutrients move downwards in solution but do not change form.
3. 100 − 25 − 35 = 40% sand.
4. (40.0 − 37.2) ÷ 40.0 × 100 = 7.0% of dry mass.
5. Better water retention; better nutrient retention (less leaching). Loam still drains and aerates well.
6. Irrigation water evaporates in hot, dry conditions; dissolved salts are left behind and build up near the surface, especially where drainage is poor.
7. 54 ÷ 12 = 4.5.
8. 50 000 × 0.10 = 5000 MJ.
9. (4.0 − 1.6) ÷ 1.6 × 100 = 150%; 2.4 ÷ 10 = 0.24 dS/m per year.
10. Lower-income: early stages (harvest, storage, transport). Higher-income: late stages (retail and households).
11. (3.1 − 2.5) ÷ 2.5 × 100 = +24%.
12. A windbreak lowers wind speed across the field, so less soil is lifted; contour furrows slow runoff moving downslope, so less soil is carried away.

Where marks are usually lost

• Writing “soil erosion is bad for farming” without the mechanism (loss of humus-rich topsoil, nutrients and water-holding capacity).
• Mixing up the properties of sand and clay, especially drainage and nutrient retention.
• Dividing by the wrong mass in water or organic matter calculations, or not stating the basis.
• Giving percentage change with the new value as the denominator.
• Naming a conservation method with no link to the problem it solves or the place it suits (terraces on flat land).
• Explaining salinization as “salty irrigation water” and missing evaporation.
• Treating the 10% rule as exact rather than an approximation.
• Using “intensive” to mean “large” or “commercial”.
• In “evaluate” or “to what extent” answers, listing points with no judgement, or giving only one perspective.
• Quoting data from a table without units or without the comparison the question asks for.

Official syllabus

International Baccalaureate Organization, Diploma Programme Subject Brief, Environmental systems and societies, first assessment 2026. These notes cover syllabus sections 5.1 (Soil) and 5.2 (Agriculture and food) of Topic 5, Land. The brief lists subtopics for Topic 1 only; the subtopic numbering on this page follows the printable ESS checklist.