IB DP Environmental Systems and Societies – Soil, agriculture and food Study Guide

This study guide teaches the soil, agriculture and food unit of IB Diploma Programme Environmental Systems and Societies (ESS). It is aligned to the IB Diploma Programme Subject Brief, Environmental systems and societies, and covers syllabus sections 5.1 (Soil) and 5.2 (Agriculture and food) of Topic 5, Land. Topic 5 is studied at both SL and HL: the brief recommends 8 teaching hours at SL and 15 at HL, and 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.

When you have worked through this page, test yourself with the soil, agriculture and food revision notes and the practice questions. The IB DP ESS course hub and the printable syllabus checklist show where Topic 5 sits. This unit builds directly on the nutrient cycling and energy flow in the Topic 2 Ecology study guide and the perspectives, systems and sustainability ideas in Topic 1 Foundations.

What this unit covers

Syllabus section	What you must be able to do	SL/HL
5.1 Soil	Describe soil as a system of storages, flows, inputs and outputs; explain how texture affects soil properties; explain causes of soil degradation and evaluate conservation methods	SL and HL
5.2 Agriculture and food	Compare farming systems; explain why eating lower in the food chain feeds more people; discuss food security, food waste and the sustainability of food production	SL and HL

The three key concepts — perspectives, systems and sustainability — run through both sections.

5.1 Soil

Soil as a system

Soil is made of mineral particles, organic matter (including humus), water, air and living organisms. Treat it as an open system, just as you treated ecosystems in Topic 2.

Part of the system	Examples
Inputs	Dead organic matter (leaf litter, dead roots, animals), weathered parent rock, precipitation, energy from the Sun, gases from the atmosphere
Storages	Organic matter, nutrients, minerals, water, air, soil organisms
Transfers (move without changing)	Infiltration and percolation of water, leaching of dissolved nutrients downwards, mixing by earthworms, translocation of clay between layers
Transformations (change state or form)	Decomposition, humification, weathering of rock, nitrogen fixation, nutrient release by decomposers
Outputs	Leaching out of the system, uptake by plants, erosion by wind and water, evaporation, gases released by respiration

A soil profile is a vertical section through the soil. Most profiles show layers called horizons:

• O horizon — leaf litter and partly decomposed organic matter at the surface.
• A horizon (topsoil) — dark, humus-rich mineral soil with most roots and organisms.
• B horizon (subsoil) — less organic matter; minerals leached from above often accumulate here.
• C horizon — weathered parent material.
• Beneath this lies bedrock.

Soil forms very slowly. Five factors control soil formation: parent material, climate, organisms, relief (slope) and time. Because soil can take hundreds of years to form a few centimetres, it is effectively non-renewable on a human timescale. That is why soil loss matters so much for sustainability.

Soil texture and its effects

Texture is the proportion of sand, silt and clay particles. Sand particles are the largest and clay particles the smallest. Soil scientists classify texture with a soil texture triangle, which turns three percentages into a named class such as sand, loam or clay.

Property	Sandy soil	Clay soil	Loam
Pore spaces	Large	Very small	Mixed sizes
Drainage and aeration	Fast, well aerated	Slow, often waterlogged	Good
Water-holding capacity	Low	High	Medium to high
Nutrient-holding capacity	Low (nutrients leach)	High (clay particles hold nutrient ions)	Medium to high
Primary productivity	Low without inputs	Can be high, limited by poor drainage	Usually highest

Loam is a mixture of sand, silt and clay. It combines the drainage of sand with the water and nutrient retention of clay, so it usually supports the highest primary productivity and suits most crops.

Worked example 1 — measuring water and organic matter. A fresh soil sample has a mass of 32.0 g. After drying in an oven it has a mass of 25.0 g. After heating strongly to burn off organic matter (loss on ignition), its mass is 23.6 g. Calculate (a) the water content as a percentage of fresh mass and (b) the organic matter content as a percentage of dry mass.

(a) water = 32.0 − 25.0 = 7.0 g
    7.0 / 32.0 × 100 = 21.9 %  (of fresh mass)

(b) organic matter = 25.0 − 23.6 = 1.4 g
    1.4 / 25.0 × 100 = 5.6 %   (of dry mass)

Always state which mass you divided by.

Soil degradation

Soil degradation is the decline in soil quality, and so in its ability to support plant growth. The main forms are:

• Erosion by water and wind. Bare soil is removed by rain splash, surface runoff and wind. Deforestation, overgrazing and ploughing leave soil exposed.
• Salinization. In hot, dry areas, irrigation water evaporates and leaves dissolved salts behind at the surface. Salts build up until crops cannot take up water.
• Nutrient depletion. Harvesting removes nutrients every year. Without fertiliser, manure or fallow periods, fertility falls.
• Compaction. Heavy machinery and trampling livestock squeeze out pore spaces, reducing infiltration and aeration and increasing runoff.
• Contamination and acidification. Pollutants and some fertilisers harm soil organisms or lower pH.
• Desertification. In drylands, a combination of the above with drought can turn productive land into desert-like land.

Worked example 2 — soil loss as depth. A field loses 24.0 tonnes of soil per hectare per year. The soil has a bulk density of 1.2 tonnes per m³. One hectare is 10 000 m². Calculate the depth of soil lost each year.

mass per m² = 24.0 / 10 000 = 0.0024 t per m²
depth = 0.0024 / 1.2 = 0.002 m = 2.0 mm per year
over 25 years: 2.0 × 25 = 50 mm

Compared with formation rates of a few centimetres over centuries, this loss far exceeds replacement.

Soil conservation

Conservation methods work in one of three ways: they slow water or wind, keep the surface covered, or maintain organic matter and nutrients.

Method	How it works
Contour ploughing	Furrows follow the contours, so each ridge slows runoff down the slope
Terracing	Steps cut into steep slopes reduce slope length and gradient
Windbreaks (shelter belts)	Rows of trees or hedges reduce wind speed across fields
Cover crops and mulching	Keep the soil covered between harvests, protecting it from rain splash
Reduced tillage or no-till	Less disturbance keeps soil structure and organic matter intact
Crop rotation	Different crops use and return different nutrients; legumes add nitrogen
Adding organic matter	Compost and manure improve structure, water retention and nutrient supply
Lime	Raises the pH of acidic soils
Controlled grazing	Stocking rates kept low enough to keep vegetation cover
Drip irrigation and drainage	Less water applied and salts flushed away, reducing salinization

To evaluate a method, weigh its benefit against cost, labour, yield effects and local suitability. Terraces suit steep slopes but take great effort to build; no-till reduces erosion but may rely on herbicides.

5.2 Agriculture and food

Farming systems

A farm is a system. Inputs include seed, water, fertiliser, labour, machinery, fuel and energy from the Sun. Outputs include crops, livestock products, waste, runoff and greenhouse gases. Learn to classify farms using pairs of terms:

Pair	Meaning
Subsistence vs commercial	Food grown mainly for the farmer’s household vs grown to sell for profit
Intensive vs extensive	High inputs of labour or capital per unit area and high yields vs low inputs spread over large areas
Arable vs pastoral vs mixed	Crops vs livestock vs both
Conventional vs organic	Synthetic fertilisers and pesticides permitted vs natural inputs only

Farming systems differ in their energy efficiency. A simple measure is the energy ratio: food energy out divided by energy in (fuel, fertiliser, machinery, feed). Labour-intensive subsistence farming often has a high ratio. Intensive livestock farming usually has a low one, because animals are fed crops that people could have eaten.

Why diet choices change land use

In Topic 2 you learned that only a small fraction of energy passes from one trophic level to the next. A common rule of thumb is about 10%. Feeding crops to animals, and then eating the animals, adds a trophic level and loses most of the energy.

Worked example 3 — people fed per hectare. A hectare of cereal gives 40 150 MJ of food energy per year. One person needs 11 MJ per day. Assume 10% of the crop’s energy passes on if it is fed to livestock. Compare the number of people fed.

need per person per year = 11 × 365 = 4015 MJ
eat the crop directly: 40 150 / 4015 = 10 people
feed to livestock:     40 150 × 0.10 = 4015 MJ
                       4015 / 4015 = 1 person

Eating at the lower trophic level feeds ten times as many people from the same land, so plant-rich diets need less land. But some grazing land cannot grow crops, and livestock can turn grass on steep or dry land into food.

Food security

Food security exists when all people, at all times, have access to enough safe and nutritious food for an active, healthy life. Food insecurity can come from poverty, conflict, low production or waste. Malnutrition includes both undernutrition (too little food or too few nutrients) and overnutrition (too much energy, leading to obesity).

Food waste happens at every stage. In lower-income countries more is lost early — in the field, in storage and in transport — because of pests, poor storage and weak cold chains. In higher-income countries more is wasted late, in shops and homes, because of cosmetic standards, large portions and over-buying.

Environmental impacts and sustainable food production

Intensive agriculture raised yields, especially during the Green Revolution, through high-yielding varieties, fertilisers, pesticides and irrigation. The costs include:

• habitat loss and deforestation as farmland expands, reducing biodiversity
• eutrophication of rivers and lakes from fertiliser runoff
• pesticide effects on non-target species, including pollinators
• greenhouse gases: methane from cattle and flooded rice fields, nitrous oxide from fertilised soils, carbon dioxide from machinery and land clearance
• heavy water use and falling water tables
• soil degradation (section 5.1)

Ways to make food production more sustainable include reducing waste, shifting towards plant-rich diets, crop rotation and polyculture, integrated pest management, agroforestry, precision application of fertiliser and water, and protecting soil with the conservation methods above.

These choices reflect environmental value systems. A technocentric view favours new technology such as genetically modified crops, precision farming and alternative proteins. An ecocentric view favours small-scale, low-input farming, local food and changes in consumption. Strong answers present both and reach a justified judgement. HL students also apply the HL lenses (environmental law, environmental and ecological economics, environmental ethics) to issues such as fertiliser pollution or fair access to land and food.

Common errors

• Listing soil components without classifying them as inputs, outputs, storages, transfers or transformations.
• Calling leaching a transformation. It is a transfer: nutrients move but do not change.
• Saying clay “drains well” or sand “holds nutrients”. It is the other way round.
• Dividing by the wrong mass in soil moisture and organic matter calculations.
• Describing salinization as “salt in the water” without explaining evaporation leaving salts behind.
• Naming a conservation method without saying how it reduces erosion or loss of fertility.
• Using the 10% rule as an exact law. Say “approximately” or “assume”.
• In evaluation questions, giving only advantages, or ending without a judgement.

Where to go next

Next, use the revision notes and the practice questions. For how Paper 1 case studies and Paper 2 essays are set up, read the ESS exam preparation guide. The syllabus guide sets out the structure of the whole course.

Official syllabus

International Baccalaureate Organization, Diploma Programme Subject Brief, Environmental systems and societies, first assessment 2026. This guide covers 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.