IB Environmental Systems and Societies

1. Topic 5
Soil systems and terrestrial food
production systems and societies
5.3 Soil degradation and
conservation

2. 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

3. 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

5. 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

6. 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

7. 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

8. 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

9. 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

10. 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
Desertification
Combating desertification

11. 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

12. 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

13. 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

14. 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

15. 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

16. 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

17. 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