The Biosphere- soils Flashcards
What is soil?
Soil is a major component of the Earth’s ecosystems at the interface between the atmosphere, lithosphere,
biosphere and hydrosphere.
Performs a wide range of essential functions…
– Medium for plant growth (also animals)
– Water reservoir
– Recycles plant and animal material
Components of soil
Mineral 45%
Air 25%
Water 25%
Organic 5%
Components of soil
- Mineral particles
– Weathering of parent material
– Primary/secondary minerals
Components of soil
- Soil organic matter
– Litter, humus and biomass – Breakdown of nutrients by microorganisms – mineralization – Peat forms where decomposition is slowed – Important – binding, water holding capacity, porosity, nutrients
Components of soil
- Soil water
– Ecological function
– Different types of soil water – gravitational water, capillary water,
hygroscopic water, plant available water
– Different soil states – saturation, field capacity, wilting point
– Amount of water held in each state depends on soil texture,
structure and organic matter
Components of soil
- Soil air
– Occupies pores not filled with water – Aeration = ventilation of soil air – High CO2 content – Composition varies from place to place – Composition varies diurnally and seasonally
Soil profile
Vertical section through soil from ground surface – parent material
Soil horizons
Distinct horizontal layers in the profile
Some profiles have distinct boundaries whereas others
are gradual
Translocation of materials by movement of water through
the soil
– eluviation
– illuviation
Horizon classification
Lettered according to genesis and relative position in the profile
• Solum = A, E, and B horizons
Pedogenesis
Process of soil formation – takes hundreds and thousands of years
Pedogenesis
Main inputs
Parent material weathering, surface accumulation, solutes
and particles from precipitation and wind, energy from the sun and gases from the atmosphere
Pedogenesis
Main losses
Through wind and water erosion, and leaching
Pedogenesis
Mixing
Soil animals, microbes and plant roots, freeze–thaw cycles, shrinking–swelling, ploughing, chemical processes
Translocations and transformations
Luvisols/brown soils
Where rainfall > evapotranspiration
– Clay eluviated and redeposited in argillic horizons
Podzolization
– Where there is intense leaching and translocation (acidic)
– Organic acids compound with iron and aluminium
– Freely drained sites under forests and heath plants
Gleying
– Waterlogging leads to reduction, mobilization and removal or
redeposition of iron compounds
– Alternate phases can result in mottling
Laterization (ferralitization)
– Tropical and subtropical oxisols
– Heavy leaching – depletion of base cations
Factors affecting soil formation
• Climate (cl) – most influential
– Determines moisture and temperature regime under the soil
• Organisms (o) – plants, animals, microorganisms and
humans
• Relief (r) – altitude, slope and aspect of the landscape
– Can hasten or delay the influences of climatic factors
– Soil relationship with slop is a catena
• Parent material (p) – influence of the weathered material
on the soil
– Some minerals are more susceptible to weathering than others
• Time (t) – soil is continually forming
– Soil rarely reaches steady state because of changes in
environmental factors, e.g. Quaternary glacial cycles
Physical properties of soil
• Soil colour
– Use to determine the nature of soil properties and horizons (e.g. OM content, oxidation–reduction)
– Munsell colour chart – hue, value and chroma
Physical properties of soil
• Soil texture
– Proportion of sand, silt and clay-sized fractions
– As particles become smaller they have different properties (e.g.
influence of surface areas on water-holding capacity, cation
exchange capacity and rate of weathering)
Physical properties of soil
• Soil structure
– Shape (or type), size and distinctiveness of soil aggregates (peds)
– Blocky, spheroidal, platy and prismatic
– The finer textured the stronger the structure
– Determines soil porosity – affects water and air movement
pH
A logarithmic method of expressing the concentration of hydrogen cations (H+) i.e. acidity
Most soils – pH 3.5 – 9
