Transcript LECTURE 2 - Rhodes University

Essay topics:
(Due by noon on Friday 21 May)
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Describe how soils affect hydrological processes, and how
hydrological processes affect soils.
Discuss soil formation in the Grahamstown area with reference to
the soil forming factors of parent material, biota, topography, climate
and time.
Discuss the concept of soil classification and explain the South
African soil classification system. Do you think that soil classification
is a necessary concept? Substantiate your answer.
Discuss the roles and importance of soil water and describe the
ways in which it affects soil properties, as well as how it is affected
by these properties.
Explain what is meant by the term “soil pollution” and discuss this
problem with particular reference to South Africa.
LECTURE 2
Factors Influencing Soil
Formation –
Parent Material, Climate and Topography
Definitions…

Regolith
• (B.Halley)

Parent material
• (K.T. Hobbs)

Biosequence
• (K. Mapham)
The 5 factors influencing
soil formation
Parent materials
 Climate
 Biota
 Topography
 Time

Sequences that develop when one of
these factors is dominant in
determining differences
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Parent material – lithosequence
Climate – climosequence
Biota – biosequence
Topography – toposequence
Time - chronosequence
Parent Material
Parent Material

Why is it important?
– Largely influences soil characteristics, e.g. texture,
structure etc.
– In general, dark coloured rocks contain iron and
magnesium and are more easily weathered than
lighter coloured rocks.

There are many different types of parent
materials
– These have been brought to the earth’s surface by
a variety of geological processes and then
weathered and transported in a variety of ways.
How are parent materials
classified?

Often according to the way in which they
were placed in their current locations.
 One suggested classification (Brady & Weil, 1999):
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Residual
Colluvial
Alluvial
Marine
Lacustrine
Glacial
Eolian
Organic
- Formed in situ from rock
- Transported
- Accumulated
Residual Parent Material

Develops in place from weathering of
underlying rock
– Stable vs. active landscapes
– Warm & humid vs. cooler and drier
climates

Widely distributed on all continents
– Highly variable
Colluvial Debris

Also known as colluvium
 Consist of rock fragments detached from
heights above and carried downslope
– Gravity, frost action

Physical rather than chemical weathering has
been dominant
– Material coarse and stony

Easily drained
 Tend to be unstable
Alluvial Stream Deposits
Floodplains
 Alluvial fans
 Delta Deposits

Marine Sediments

Streams deposit their sediment loads in
oceans, estuaries and gulfs.
 Coarser fragments settle out near the shore
and finer particles at a distance.
• Texture highly variable

Sediment builds up, later changes in sea
level can result in these deposits being
exposed.
• Marine sediments have generally been subjected to soil
forming processes for shorter periods of time than upland
sediments.
Parent Materials Transported by
Glacial Ice and Meltwaters

In the past, much of the land surface (about 20%)
was covered by thick ice sheets, some more than
1km thick.

As climate changed and glaciers began to shift, large
quantities of regolith were taken up by the glaciers
and pushed forward as the glacier moved.

When the glaciers melted, a “mantle” of glacial debris
was left behind as a new parent material for soil
formation
• Glacial till
• Glacial outwash and lacustrine sediments
Parent Materials Transported by Wind
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Can be vast quantities transported over vast
distances
Dune sand
• Medium and fine sand grains.
• Consists mostly of quartz (devoid of plant nutrients and
resistant to weathering).

Loess
• Silt, very fine sand and clay, tend to form high fertility soils.
• May be blown for hundreds of km.
• In China, loess deposits can be 30 to 100m in depth.

Aerosolic dust
• Very fine particles carried high in the air; can travel thousands
of km before being deposited before being deposited, usually
with rainfall.
• Sahara
Amazon!!!
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Volcanic Ash
• Soils formed are uniquely light and porous and accumulate
organic matter more rapidly than other soils in the area.
Organic Deposits as Parent
Materials

Accumulate in wet places where plant growth
exceeds rate of residue decomposition.
 Decomposition limited by a lack of oxygen
• Occurs mostly in wetland areas, often in cool climates.
• Results in the formation of peats
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Moss peat (tends to be acid)
Herbaceous peat
Woody peat (can make productive agricultural soils)
Sedimentary peat (generally poor agricultural soils)
NB Wetland preservation!
Climate
Climate
Determines rate and intensity of
weathering over large geographic
areas.
 Indirectly influences soil formation
through its effect on living organisms.
 Most important variables:

• Effective precipitation
• Temperature
Effective Precipitation

To fully promote soil development, water must
enter regolith zone AND percolate through it.
 The amount of water percolating through the
soil profile, and its effectiveness in soil
formation is determined by:
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Total annual precipitation
Seasonal distribution
Temperature and evaporation
Topography
Permeability
Temperature
10°C rise in temp. = more than 2x
increase in the rate of biochemical
reactions.
 Temperature and moisture influence
organic matter content of soil through
their effects on the balance between
plant growth and microbial
decomposition.

Topography
Topography
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Relates to configuration of land surface
• Elevation
• Slope
• Landscape position
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Can speed up or slow down the work of
climatic forces.
Interaction with vegetation.
Slope aspect.
Salt buildup.
Parent material interactions.
Catena concept.