IB Environmental Systems and Societies

1. Topic 3.4
The Soil System

2. Soils
• The interface between:
– Atmosphere
– Lithosphere
– Biosphere
– Hydrosphere
• Comprised of:
– Regolith (weathered bedrock)
– Organic matter (living and non-living)
– Air
– Water
• Exist in all 3 states
– Solid (organic and inorganic matter)
– Liquid (water from precipitation, seepage and groundwater)
– Gas (volatiles in atmosphere and within pores)
• Soils take so long to develop that they are generally considered to be
a non-renewable resource
http://www.youtube.com/https://www.youtube.com/watch?v=iGfco7kNzJA?v=a1_WPMu0ZiI

3. Soil Profiles
Horizon Characteristics
Organic horizon (O) undecomposed litter
partly decomposed litter
well-decomposed humus
Mixed mineral-
organic horizon (A)
humus
ploughed
gleyed or waterlogged
Eluvial or leached
horizon (E)
strongly leached
weakly leached
Illuvial or deposited
horizon (B)
iron deposits
clay deposits
humus deposits
Bedrock or parent
material (C/R)
rock
unconsolidated loose deposits
The boundaries between horizons are often blurred due to earthworm activity
soil horizons

4. How Are Soils Formed?
• They are considered to be open systems in steady-state
equilibrium
• The main processes of formation are:
– Weathering
– Translocation (movement of substances) *
– Organic changes (largely near the surface)
– Gleying (waterlogging)
• At the surface, humus is created (humification) and
eventually decomposed completely (mineralisation) – they
always occur together
• Human activity is having severe effects on soil formation
* Translocation usually occurs downwards due to the movement of water and dissolved
substances. However in arid environments movement is upwards due to evaporation
soil formation animation

5. Using Soils
• The main human use of soils is for
cultivation (also peat extraction to a
lesser extent)
• For cultivation, the ideal soil has a
good balance between water-
retention and drainage (porosity)
and aeration
• These properties are based on soil
texture
• Texture depends on the proportions
of different sized particles
(sand/silt/clay)
• The ideal balance of particle size is
achieved in loamy soils
http://www.youtube.com/watch?v=7V5qUusgYLw

6. Porosity vs Surface Area
• Pore size determines the rate of drainage of water and
how easily it is aerated
• Particle size/ surface area determines how easily water
and dissolved nutrients are retained (against gravity)
– Light soils (> 80% sand) – coarse texture, easily drained;
low primary productivity
– Heavy soils (> 25% clay) – fine texture, small pores (<
0.001mm), water and nutrient retentive, chemically active,
not easily worked (ploughed)*; low primary productivity
– Medium soils – somewhere in between (loam); high
primary productivity
Ploughing a clay-rich soil

7. Soil Structure

9. Porosity vs Surface Area
• A variety of pore sizes is required to allow root
growth, water drainage, aeration and water
storage
– Pores > 0.1 mm are needed for root growth
– Pores < 0.05 mm are needed for good water
storage
• Overall soil structure depends on:
– Soil texture (see the triangle)
– Amount of dead organic matter
– Earthworm activity

10. Soil Degradation
• Reduction in quantity and quality of soil
• It may be caused by:
– Erosion (by wind and water)
– Biological degradation (loss of humus and living
material)
– Physical degradation (loss of structure and
accompanying changes in porosity)
– Chemical degradation (acidification, loss of
nutrients, changes in pH, changes in salinity
(salinisation)

11. The Universal Soil Loss Equation (USLE)
• A = predicted soil loss (tonnes acre-1 year-1)
• R = erosivity factor
• K = soil erodibility factor
• L = slope length
• S = slope gradient
• C = cover and management factor
• P = erosion control practice factor
A = R K L S C P
Usually combined as the LS factor

12. The Universal Soil Loss Equation (USLE)
Factor Description
R Total rainfall, intensity and seasonal distribution. Maximum for regular,
high intensity storms. Greatest if rain occurs on newly ploughed soil.
Lower for gentle rain, if crop is established or soil is frozen
K Depends on infiltration capacity, structural stability and ability to
withstand rain splash
L and S Affect the movement and speed of water flow and its ability to transport
particles. Linked to erodibility (K)
C Crops, grass and forest cover provide protection against erosion. This is
greatest for plants with extensive root systems and greatest foliage.
Fallow land or exposed cropland provide little protection
E Soil conservation measures such as terracing can reduce erosion and slow
runoff

13. Soil Degradation
• Water erosion - surface, gully, rill, tunnel
• Wind erosion
• Acidification - increases the availability of toxins such as
heavy metals
• Eutrophication - nutrient enrichment of pore water and
groundwater
• Salinisation - capillary action brings salts to the surface
• Atmospheric deposition - entry of heavy metals and
persistent organic pollutants to soil from the air
• Desertification – a cause and a result of soil degradation
• Climate change
• Human activity
http://www.youtube.com/watch?v=403sT9CGRl0

14. Soil Degradation
• Climate change may increase soil
degradation by:
– Increasing decomposition rates of organic
matter (reducing nutrient and water storage)
– Increased precipitation and flooding may lead
to higher rates of soil erosion
– Increased drought may lead to higher rates of
wind erosion
– Increasing the need for agricultural land to
compensate for the loss of degraded land
– Increased crop yield due to the CO2 fertilisation
effect
These
effects may
cancel each
other out

15. Human Activity
• Removal of woodland or pasture
– Bare soil may be exposed, loss of root systems which bind soil
together increase erosion rates
• Cultivation
– Bare soil is exposed after harvesting and before planting. Cultivation of
slopes causes rills and gullies. Irrigation in arid areas leads to
salinisation
• Grazing
– Reduces vegetation cover leading to increased erosion, animals also
trample existing vegetation and waste causes eutrophication
• Road building
– Reduced infiltration causes rills and gullies
• Mining
– Exposure of bare soil
The effects are often most severe in LEDCs which are highly dependent on agriculture
15% of the world’s soil is thought to be degraded

17. Soil Conservation
• Better management by farmers
– Aforestation
– Maintaining crop cover for longer
– Keeping root systems in place after harvesting
– Planting a grass crop
– Use of fencing, treelines or hedgerows to reduce wind erosion
– Multicropping (crop rotation)
• Prevention of erosion
– Physical barriers (wind and water breaks)
– Improved infiltration
– Contour ploughing (at right angles to the slope)
– Terracing
– Dams and dykes to prevent flooding

18. Soil Conservation
• Prevention of salinisation
– Regular flushing soil to wash away salts
– Application of chemicals to remove sodium salts
– Reduction of evaporation losses to prevent
capillary action

19. Questions
1. Create a flow diagram of the soil system. Try
to show links with the lithosphere,
atmosphere, hydrosphere and biosphere
2. Compare and contrast the structure and
primary productivity of sandy, clayey and
loamy soils
3. Outline the processes and effects of soil
degradation
4. Evaluate a number of soil conservation
measures