Topic 5Soil systems and terrestrial food production

systems and societies

5.3 Soil degradation and conservation

Case Study: The Dust Bowl• In the 1930s huge dust storms moved across the

mid-west of the U.S. picking up soil and destroying farmland

• It was caused by a mixture of poor farming methods, drought and extreme winds and temperatures

• Intensive farming had removed vegetation, especially grass which bound the topsoil together

• It lead to famine and lung diseases caused by breathing in dust

• It also led to mass migrations of people looking for work

• The same thing could happen in the future although there is now a better understanding of soil conservation The dust bowl

Case Study: The Aral Sea• Human’s may have an indirect effect on soil quality through

their use of water• In the Soviet era, the Aral Sea in Kazakhstan/Uzbekistan was

lost due to use of water for irrigation. The Soviet government tried to grow cotton on a huge scale in the surrounding soils

• As the sea dried, the soil that was left has such a high salt content that vegetation couldn’t grow, leaving a saline desert

• Part of the sea has been recovered by damming rivers, however it is still only about 10% of its original size

• As with the dust bowl disaster there have been huge social problems as a result: mass unemployment and economic migration

The Aral Sea

Fertile soil• Since it takes so long to form (2000 years to make 10 cm of

topsoil), soil is considered a non-renewable resource• Good soils (loam) have a suitable texture for plant growth, a

healthy soil community (to recycle nutrients), a good balance of nutrients (NPK) and mineral ions, and a suitable pH (generally 5.5 – 7.5)

• High acidity may release toxic metal ions which otherwise would be bound in the soil

• High alkalinity releases calcium carbonate which reduces infiltration and percolation

• High and low pH kills off the soil community and further reduces fertility

Fertile soil• Succession and the climax community depends

on the natural pH of the soil (together with other abiotic factors such as rainfall and temperature)

• The carbon cycle makes organic matter available and the nitrogen cycle puts the major nutrient (N) back into the soil. The water cycle ensures water is available for further plant growth

• Nutrient levels may be further enhanced by the use of natural of synthetic fertilisers

Soil degradation• About one third of the world’s soil is considered to be degraded• This is due to processes of:

– Erosion (by wind and water) [this is the major cause]– Chemical degradation (pollution, salinisation, acidification, nutrient depletion)– Physical degradation (e.g. soil compaction)

• Erosion results in partial or complete loss of fertile topsoil. Remaining soil has reduced water retention capability. Lost sediment may pollute or block up nearby watercourses (or cause eutrophication)

• Erosion is generally a result of the loss of vegetation which bind soils together with root systems• As large areas are deforested, windspeeds may increase causing increased levels of erosion

(positive feedback)• Acidification may be caused by bacteria releasing high concentrations of H+ ions due to the

overuse of fertilisers• Nutrient depletion is caused by continually harvesting and not allowing the nutrients removed in

the crop to be replaced• Pollution may be caused by the overuse of pesticides which allows toxic compounds to

accumulate in the soil• Soil compaction is caused by heavy machinery, animals, building of infrastructure and results in

loss of porosity of the soil

Soil degradation

Soil degradation• There are 4 basic human activities which cause soil loss and degradation:

– Urbanisation – Overgrazing– Deforestation– Mismanagement of farmland

• Urbanisation causes soil to be concreted over or moved from place to place. If it is uncovered it is often compacted due to footfall or vehicle movement, or polluted (e.g. from disposal of waste or atmospheric pollution)

• Overgrazing reduces vegetation cover and removes protective root systems

• Deforestation directly removes nutrients from an ecosystem, takes away protective root systems and protection from wind erosion. Water erosion may transfer the remaining nutrients and pollute nearby water systems

Soil degradation• Mismanagement of farmland includes practices such as:

– Increased loss of nutrient content without replacement (e.g. multiple harvests)

– Monoculture which quickly removes key nutrients from soil – Loss of vegetative cover leaving land vulnerable to erosion– Excessive irrigation which may lead to erosion or nutrient loss by

percolation– Pollution (e.g. by pesticides) leading to loss of the soil community– Cultivation of steep slopes, encouraging erosion– Use of marginal land with poor soil characteristics. This may lead

to increased erosion, overuse of fertilisers and pesticides and excessive ploughing

Desertification• Extreme soil degradation may result in

desertification (as occurred with the Aral Sea)• This is the result of human activity which

renders soil infertile• Soil exhaustion is already starting to affect

global food production• It can be reversed by long-term programmes to

return nutrients to soil and prevent erosion

DesertificationCombating desertification

Soil conservation

• Soil degradation may be prevented by:– Reducing wind and water erosion– Reducing salinisation– Managing nutrient levels – Preventing overgrazing– Limiting soil compaction

In fact, it is a matter of trying to limit all of the factors we said were causing soil degradation

Reducing water erosion• Water may be captured by terracing of steep hillsides• Furrowing prevents movement of water across land• Contour tillage along natural contours• Planting crops along natural contours (strip cropping)• Buffer strips – permanent vegetation which prevents runoff across

large areas of land• Planting crops without ploughing• Increased infiltration and percolation through the soil

– adding organic matter (e.g. manure) to improve texture– mulching (adding dry organic material to the surface to reduce evaporation)– avoiding soil compaction– conservation tillage (leaving part of the previous crop on the surface to

decompose and return nutrients) No-till agriculture

Reducing wind erosion• Wind breaks to reduce windspeeds and capture blown

soil particles (simply by planting banks of trees or shrubs)• These also provide habitat for other species and provide

corridors for them to move around• Techniques used to prevent water erosion may also

reduce wind erosion:– Conservation tillage – Vegetation cover– Buffer strips

Reducing salinisation

• Avoiding over-irrigation• Not watering at specific times of the day• Incorporating better drainage (e.g. by avoiding

soil compaction)• Flushing water through the soil periodically to

remove build-up of salts

Managing nutrient levels• Avoiding erosion also allows the soil to retain nutrients• Organic matter may be added. This improves soil texture and

replaces lost nutrients• Sowing of leguminous plants replaces lost nitrogen• Use of synthetic fertilisers replaces N:P:K• Liming (addition of calcium carbonate or calcium hydroxide) raises

the pH and helps keep the soil community healthy• Crop rotation helps to ensure a range of nutrients are used up over

a longer time• Similar results may be achieved by

using polyculture rather than monoculture • (e.g. traditional Mexican milpas)• Land may be left fallow for a time to allow

nutrients to return

Preventing overgrazing

• This is combated by reducing herd sizes and allowing animals to move frequently to new areas of land

• Areas should never be completely stripped of vegetation as this will increase regrowth time

• Fertilisers can be used judiciously to allow vegetation growth

• Erosion should be further reduced (e.g. by the use of windbreaks) to encourage regrowth

Limiting soil compaction

• This is combated simply by reducing herd-sizes on small areas of land

• Urbanisation and road building could be reduced but at significant social cost