Essay topics:(Due by noon on Friday 21 May) • 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 • Parent material – lithosequence • Climate – climosequence • Biota – biosequence • Topography – toposequence • Time - chronosequence
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 • 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 • 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.