summary of topic 5.1
TRANSCRIPT
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Topic 5Soil systems and terrestrial food production
systems and societies
5.1 The Soil System
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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
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The Soil System• Any soil is an open system (there are inputs and outputs of matter and energy)• Inputs
– Organic matter (e.g. decomposing leaf litter)– Inorganic matter (e.g. water)– Energy (sunlight, heat)
• Outputs– Uptake of all inputs by plants– Radiation of heat from soil surfaces– Erosion
• Storages– Organic matter– Nutrients– Organisms– Minerals– Air– Water
• Transformations (Processes)– Weathering of minerals
Soils are ecosystems – biotic and abiotic factors occurring in a self-supporting system
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Soil ProfilesHorizon Characteristics
Organic horizon (O) undecomposed litter partly decomposed litterwell-decomposed humus
Mixed mineral-organic horizon (A)
humusploughedgleyed or waterlogged
Eluvial or leached horizon (E)
strongly leachedweakly leached
Illuvial or deposited horizon (B)
iron depositsclay depositshumus deposits
Bedrock or parent material (C/R)
rockunconsolidated loose deposits
The boundaries between horizons are often blurred due to earthworm activity
soil horizons
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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
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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
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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
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Soil Structure
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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