Lecture 10

• soil covers virtually all of the Earth's surface
• all terrestrial food depends on soil
• the sustainable use of soil is essential if problems of exhaustion, erosion and pollution are to be avoided
• the sustainable use of soils is essential if problems of exhaustion, erosion and pollution are to be avoided
• inappropriate use of soils not only damages the soil but can lead to problems of drainage, flooding and mass movements
• soils are a significant carbon reservoir (2.5 gigatons)
• soils take thousands of years to develop
• soil is a mixture of organic matter, minerals, gases, liquids, and organisms that together support life
• the pedosphere has four major functions:

  • as a medium for plant growth
  • as a means of water storage supply and purification
  • as a modifier of Earth's atmosphere
  • as a habitat for organisms
• regolith


  unconsolidated material found overlaying bedrock. may have been formed in situ or been transported by water, wind or ice. soil is part of the regolith and is usually the top part which contains a high concentration of organic material and is affected by weathering
• mechanisms of chemical weathering:

  • dissolution by acids (low pH) or bases (high pH)
  • redox reactions - the adding or taking away of CO2
  • adding water to mineral structure (hydration)
  • interaction with organic compounds
• dissolution


  two types of dissolution reaction:

  • congruent - mineral dissolved entirely into solution
  • incongruent - produces solutes and forms new minerals (usually clays or oxides)
• distinct horizons form in soil profiles, the whole profile must be considered when studying a soil
• soil formation is controlled by several factors

  pedological processes

  • climate
  • biological activity (organisms)
  • relief
  • parent material (e.g. rock)
  • time
• soil formation processes are linked to soil classification
• Scottish parent material:

  • in situ (residual) shattered and weathered rock
  • glacial till
  • fluvioglacial meltwater deposits of sand and gravel and in highland areas morainic deposits
  • mountain-top detritus; frost shattered debris with common rock and scree
  • recent deposits, including aeolian sand, alluvium raised beach deposits, peat
• arenosols (arena = sand)


  quartz-rich rocks -> sandy soils, easily eroded and leach nutrients
• andosols (volcanic soils)


  parent material of volcanic ash, tuff, pumice etc found all over the world
• Britain:

  • sandstones - cambisols
  • acid rock (e.g. granite) - podzols
  • limestones - cambisols
  • glacial till - cambisols to gley soils
• precipitation

  • high and low rainfall impact on types of weathering
  • lower rainfall - salt crusts/lime layers
  • higher rainfall - leaching of soluble salts, more clays, organic matter, cation exchange capacity and nutrients increase
• temperature:

  • high temperatures lead to rapid breakdown of organic material
  • small increase in clay minerals with temperature
  • evaporation (impact depends on rainfall)
• humid tropics:

  • high temperatures and precipitation
  • highly weathered, nutrients supplied by vegetation
  • e.g. ferralsols, alisols, acrisols (acidic), lixisols (washed out)
• arid and semi-arid

  • high temperatures, low precipitation
  • evaporation and deposition of salts
  • solonetz (salty), gypsisols (gypsum), durisols (hard)
• humid temperatures:

  • season variations in temperatures and precipitation
  • rich organic material near the surface and horizons (very generalised)
  • podzols, liuvisols, umbrisols
• permafrost regions:

  • cryosols - soils showing influence of frost action and permafrost
• histosols:

  • peat, much soils, organic soils - waterlogged organic soils with limited decomposition, low pH
  • found in high latitude, temperate and tropical environments
  • can be productive, but peats contain large carbon reservoirs and need to be conserves
  • have been extensively drained and lost in many areas
  • major stores of carbon - important part of climate change mitigation strategies
• altitude:

  • in UK, colder and wetter conditions lead to accumulation of organic material - e.g. peat
• aspect:

  • warmth of soil
• slope:

  • mass movement, drainage, overland flow, throughflow, creep, water accumulation hollows, base of slopes
• catena 


  a sequence of soils down a slope created by the balance of processes such as precipitation, infiltration and runoff