Topic 5Soil systems and terrestrial food production

systems and societies

5.1 The Soil System

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

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

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

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

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

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

Soil Structure

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