Soil as a system

Soil forms slowly from weathered rock plus decayed organic matter, and behaves like any system:

• Inputs — weathered parent rock (minerals), organic matter (leaf litter, dead organisms), water and air.
• Storages — mineral particles, humus, soil water, soil air and the soil biota.
• Outputs — leaching of dissolved nutrients, uptake by plants, erosion, and losses of carbon by decomposition.

A fertile soil holds a balance of these flows; degradation occurs when outputs (erosion, harvest) exceed inputs.

Soil horizons and the soil profile

A vertical soil profile shows distinct layers, or horizons:

• O horizon — surface litter of fresh and decaying organic matter.
• A horizon (topsoil) — dark, humus-rich, biologically active; where most roots and nutrients are.
• B horizon (subsoil) — minerals leached down from above accumulate here.
• C horizon — weathered parent rock, the raw material of the soil.

Which horizon feeds the crop?

Soil texture and the texture triangle

Soil texture is the proportion of sand, silt and clay particles; the three always add up to 100%. Texture controls drainage, aeration and nutrient retention:

• Sandy soils — large particles, drain fast, well aerated, but low water and nutrient retention.
• Clay soils — tiny particles, hold water and nutrients well, but drain poorly and can waterlog.
• Loam — a balanced mix of all three; the ideal agricultural soil, combining good drainage with good retention.

Your turn — soil texture

Reading the triangle

How soils are degraded

• Soil erosion — wind and water strip topsoil, worsened by ploughing, overgrazing and removing vegetation.
• Salinisation — in dry climates, irrigation water evaporates and leaves salts behind, poisoning the soil.
• Desertification — degradation of drylands into desert through overgrazing, deforestation and drought.
• Nutrient depletion and compaction — from monoculture and heavy machinery.

Soil forms at roughly 1 cm per century, so on human timescales it is effectively non-renewable — degradation is very slow to reverse.

Naming the damage

Comparing food production systems

Food systems vary in their inputs, scale and impacts:

• Subsistence (grown to feed the family) vs commercial (grown for sale).
• Intensive — high inputs (fertiliser, machinery, labour) per unit area for high yields (e.g. feedlots, greenhouse crops).
• Extensive — low inputs spread over a large area (e.g. nomadic herding, ranching).

Higher yields often come at higher environmental cost — fossil-fuel energy, water use, pollution and biodiversity loss. Sustainability means maximising food while minimising these.

Sort the farming features

Tap a feature, then tap the category it belongs to.

Trophic efficiency and food choice

Because only about 10% of energy passes between trophic levels, eating lower down the food chain feeds far more people from the same land and energy.

Producing meat means growing crops, feeding them to animals (losing ~90% of the energy), then eating the animal. A plant-based diet cuts out that step, so it needs less land, water and energy per Calorie of food.

This is why livestock, especially cattle, have a large ecological footprint — though grazing marginal land unsuitable for crops can still be an efficient use of it.

Your turn — feeding cattle

Feeding more people

Your turn — farm energy efficiency

Conserving soil and sustainable food

Soil-conservation methods keep production sustainable by cutting erosion and rebuilding fertility:

• Contour ploughing and terracing — slow run-off on slopes.
• Crop rotation and cover crops — restore nutrients (especially nitrogen from legumes) and protect bare soil.
• Agroforestry and windbreaks — tree roots bind soil and reduce wind erosion.
• Reduced tillage and organic matter (mulch, manure) — build humus and soil structure.

Protecting the soil

Match the soil and food terms

Tap a statement on the left, then its matching answer on the right.

The big ideas to know

You can now explain how soil forms, degrades and feeds us sustainably. Press Finish to see your score.