What is soil and water degradation?
Degradation means getting worse. Soil is degraded when it loses its topsoil, gets too salty, loses its nutrients or structure, or holds poison. Water is degraded when it carries mud, salt or chemicals.
Soil and water are joined: rain moves through soil into rivers and groundwater. So damage to one almost always passes to the other.
- Slow to make, fast to lose: about 1 cm of topsoil may need hundreds of years to form, but strong erosion can remove it in a few years.
- Soil feeds crops, stores water, filters pollution and keeps carbon. When it is degraded, all four jobs suffer.
Deforestation and erosion
A forest protects soil in three ways: leaves break the force of raindrops, leaf litter soaks up water and roots hold the soil together. Cut the trees and all three go.
Then rain splashes soil loose, runs down the slope and carries it away. This is soil erosion. Effects:
- Thin, poor soil: the crops yield less.
- Muddy rivers: mud settles in dams and canals, and in the river bed, causing floods.
- Gullies cut deep into slopes. Wind can also blow away dry bare soil.
Overgrazing and ploughing up and down a slope do the same job as cutting trees: they leave soil bare.
Farming and irrigation: salinisation
All irrigation water, even from a clean canal, carries a little dissolved salt. Plants take up the water, and sun evaporates some of it. The salt stays behind in the soil.
If the land has no good drainage, extra water collects and the water table rises (waterlogging). Water then climbs up through the soil by capillary action, evaporates at the top and leaves a white crust of salt. This is salinisation.
- Salt makes it harder for roots to take in water, so plants wilt even in wet soil.
- Too much sodium also makes clay soil sticky and hard.
- Farmers abandon such fields, and large irrigated areas in dry regions are affected.
Other farming effects: too much fertiliser, pesticides, and monoculture (the same crop every year) that wears out nutrients.
Industry, pollution and what can be done
Factories, mines, towns and farms can pollute soil and water: heavy metals (lead, mercury, cadmium), oils, acids, plastics, sewage, and extra fertiliser nitrates. Rain washes surface pollution into rivers and lets it seep down to groundwater. Some pollutants, such as heavy metals, do not break down and stay for decades. Excess nitrate and phosphate in rivers feeds algae that use up the oxygen (eutrophication).
Protecting soil and water:
- Roots: plant trees and cover crops, use terraces and contour ploughing, avoid overgrazing.
- Drain: build drainage, use drip irrigation, add only the water the crop needs, grow salt-tolerant crops.
- Clean: treat factory waste and sewage before release, use fertiliser and pesticide only as needed, use compost, keep buffer strips of plants along streams.
Try it: a rain test on a tray
Fill two trays with garden soil and tilt them. Leave one bare. Put grass or small plants on the other. Pour the same amount of water from a watering can above each. Collect the runoff in clear cups. Compare the mud in each cup. Now press the Protect button in the 3D after raising the damage: it shows the same lesson.
Key formulas and definitions
- Soil loss: 1 cm of soil on 1 hectare ≈ 100 m³ ≈ 130 tonnes (density about 1300 kg/m³)
- Years to lose a depth = (tonnes per hectare in that depth) ÷ (loss per year in t/ha/yr)
- Salt added = water volume (litres) × salt in water (g per litre) ÷ 1000 → kg
- 1 hectare = 10 000 m²; 1 mm of water over 1 ha = 10 m³ = 10 000 litres
- Key terms: erosion, salinisation, waterlogging, pollution, eutrophication, runoff
Worked examples
1. A hill farm loses 13 tonnes of soil per hectare each year. How long to lose 1 cm of topsoil? (1 cm over 1 ha is about 130 tonnes.)
Time = 130 ÷ 13 = 10 years. At that speed the 20 cm of topsoil would be gone in about 200 years.
2. If nature builds 1 cm of topsoil in about 200 years, and erosion removes 1 cm in 10 years, how many times faster is the loss than the building?
200 ÷ 10 = 20 times faster. That is why soil is called a non-renewable resource on a human timescale.
3. A field gets 500 mm of irrigation water a year. The water has 0.4 g of salt per litre. How much salt is added to one hectare each year?
500 mm over 1 ha = 500 × 10 m³ = 5000 m³ = 5 000 000 litres. Salt = 5 000 000 × 0.4 g = 2 000 000 g = 2000 kg = 2 tonnes. If drainage does not wash it out, 2 t of salt builds up every year.
4. Why does a white crust appear on a field that is watered often but has no drains?
Extra water cannot drain, so the water table rises. Water moves up through the soil, evaporates at the surface and leaves its dissolved salt behind as a white crust. That is salinisation.
5. A factory drains acid waste into a stream. Name two effects on soil and water downstream.
The water becomes acidic and poisonous for fish and plants, and metals from the waste dissolve more easily. When the stream floods or irrigates land, the soil takes up the pollution. The soil then holds harmful metals and the crops grown in it may be unsafe.
6. A farmer sees brown runoff after every rain from a sloping field. Suggest three fixes.
(1) Plough along the contour, not up and down. (2) Build terraces or grass strips across the slope. (3) Keep the soil covered with a cover crop or mulch and plant trees on the steepest part.
Common mistakes
- Thinking soil is renewable quickly. Topsoil takes hundreds of years to form but can be lost in a few.
- Thinking salinisation comes only from sea water. Even clean irrigation water leaves salt behind.
- Thinking more irrigation always helps. Without drainage it raises the water table and brings salt up.
- Thinking soil and water problems are separate. Erosion muddies rivers, and chemicals in soil reach groundwater.