Essay topics:(Due by noon on Friday 21 May)

1. Describe how soils affect hydrological processes, and how hydrological processes affect soils.

2. Discuss soil formation in the Grahamstown area with reference to the soil forming factors of parent material, biota, topography, climate and time.

3. 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.

4. 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.

5. 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

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):• 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 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!!!

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

– 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:

• 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 Relates to configuration of land surface

• Elevation• Slope• Landscape position

Can speed up or slow down the work of climatic forces.

Interaction with vegetation. Slope aspect. Salt buildup. Parent material interactions. Catena concept.