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  2. Soils and Soil Profiles • What is a soil? • Factors influencing the nature of a soil • Weathering and soil development • Soil formation • Soil profile description • Soil classification systems

  3. What is a Soil? • ‘uncemented or weakly cemented accumulation of mineral particles • formed by the weathering of rocks • the void space between the particles containing water and/or air’ (Craig 1998)

  4. Silt-size grains from an estuarine sequence in central Scotland. Shows grains and voids. Photo B.F.Barras

  5. Clay-size grains from an estuarine sequence in central Scotland. Shows structure of clay particles and voids. Photo B.F.Barras

  6. Clay-size grains from an estuarine sequence in central Scotland. Shows structure of clay particles and voids. Photo B.F.Barras

  7. Factors influencing the nature of a soil • Soil is the result of the functioning of the following factors: • climate of the locality • nature of parent material • mass and character of the vegetation • age of the landscape • relief of the locality

  8. Weathering and Soil Development

  9. Weathering • Weathering is the breakdown of existing bedrock to form a covering of secondary materials (soil) • Soils form from the weathering of rocks by means of • Physical weathering • Chemical weathering • Biological weathering


  11. Physical weathering • Physical weathering is a loose term for a group of processes that operate without obvious chemical action. • They mainly involve changes in temperature, perhaps combined with freeze-thaw action if the material contains water. • Includes glacial action, wind and rain.

  12. Physical weathering • Physical processes rarely act in isolation. • A more likely effect is that physical changes will lead to some damage to the rock fabric which permits the ingress of water and thus enables chemical weathering to occur.

  13. Uluru (formerly Ayres Rock) Australia photo (m.a.paul)

  14. Uluru - weathering patterns

  15. Corestones

  16. Physical weathering • Thermal expansion/contraction. (To be effective, this is usually combined with other processes such as freeze-thaw or salt hydration). • This leads to exfoliation in some layered materials. In the case of already frozen soils it can lead to the formation of ice-wedge cracks.

  17. Physical weathering • Freeze-thaw. This is probably the most effective physical process. The 9% volume expansion of water on freezing can generate internal stresses (in excess of 10MPa). This easily exceeds the strength of most rocks.

  18. Physical weathering • Freeze-thaw can also result in the characteristic spheriodal or “onionskin” weathering of some rocks .

  19. Corestones

  20. Physical weathering • Desiccation (drying). When a porous material is subject to extreme drying, high internal suction stresses can be generated. • This leads to shrinkage and consequent cracking. • This process is accelerated if the soil is subject cycles of wetting and drying (slaking).

  21. Physical weathering • The effectiveness of physical processes is dependent on the nature of the rock. • Clearly porous rocks are more susceptible than massive crystalline materials. • There are degrees of susceptibility in porous rock, which depend upon the particle size and extent of pore connectedness. • Similarly, coarser-grained types of crystalline rock are more susceptible (due to internal cracking at grain boundaries) than are fine-grained materials.


  23. Chemical weathering • In general chemical weathering is very much more effective than physical weathering. A number of mechanisms are known.

  24. Chemical weathering • Silica hydrolysis: natural acids attack rock materials through the breakage of the silicon-oxygen bond in silica tetrahedra. • This releases coordinated cations and forms colloidal silica. Secondary reactions reform some silica into sheet minerals such as clays.

  25. Chemical weathering • Particular examples of this process include the formation of kaolinite (china clay) from feldspars and the formation of swelling minerals (smectites) from amphiboles and pyroxenes. • The stability of minerals in soils is proportional to the order in which they crystalise from a silicate melt (i.e. from molten rock) .

  26. Olivine Augite Calcic Plagioclase Hornblende Intermediate Plagioclase Biotite Alcalic Plagioclase Potassium Feldspar Muscovite Quartz Increasing mineral stability during weathering

  27. Chemical weathering • Acid hydrolysis: this involves hydrogen ion attack on the carbonate anion (CO32-) to produce the bicarbonate (HCO3-), which is soluble. • This is the basic mechanism for the solution of limestones.

  28. Chemical weathering • Oxidation-reduction: there are a number of reactions involving the change of oxidation state (valency) of the cation (usually iron) in compounds such as oxides, hydroxides and sulphides. • These reactions are responsible for the colour changes in weathered materials and the release of iron oxides from many minerals. They are particularly important in pedogenesis (soil formation).

  29. Chemical weathering • Salt hydration: many minerals absorb water to form hydrated compounds. These may involve an increase in volume which is capable of exerting considerable pressure in a confined space. • A common example is the formation of gypsum (calcium sulphate), which can exert pressures up to 100 Mpa.


  31. Biological weathering • Due to the action of animals and plants (e.g. burrowing organisms and root growth) which exploit existing fractures in the rock and and increase weathering rates. • The boring of limestone rocks by molluscs and bivalves facilitate the ingress of water into the rock fabric.


  33. Soil formation • The formation of soil from weathered inorganic substrate involves physical, chemical and biotic processes and imparts a well-defined structure to the material. • Many of the processes are identical to those of chemical weathering, although they have evolved their own terminology.

  34. Soil formation • Five general descriptive terms are often applied: • Humification: the addition of organic material (humus) to the inorganic soil particles. The humus is provided by the decomposition of plant material and is mixed largely by the action of burrowers such as earthworms.

  35. Soil formation • Translocation: a general term for the downward movement of chemical compounds by leaching. • The most prominent of these are iron compounds which provide the characteristic red-brown colours of most soils. • Translocation involves solution in the more acid upper zone of the soil profile and deposition as hydroxides in the more alkaline lower zone.

  36. Soil formation • Lessivation: the downward movement of clay particles under the action of percolating water. • The movement of these particles typically causes a fining-downwards profile, possible with the formation of distinctive porous horizons near surface and dense, impermeable bands lower in the profile.

  37. Soil formation • Structuration: a general term for the acquisition of soil micro-structure, including the formation of peds and fissures. • Processes include wetting and drying, ingestion/excretion by soil organisms and cation exchange, particularly the uptake of calcium and loss of sodium/potassium.

  38. Soil formation • Gleying: a visible mottling, usually attributed to alternating changes in oxidation state in iron compounds due to a fluctuating water table.


  40. Soil profile description • The operation of the above processes, combined with the downward leaching of their products through the developing soil, usually lead to the formation of a number of distinctive horizons in the soil profile. • There is a standard nomenclature for these horizons, although not all will be present in any one soil.

  41. H or O Horizon A Horizon E Horizon B Horizon C Horizon R Horizon

  42. Soil profile description • H-horizon (or O): Formed by the accumulation of organic material deposited on the surface. • H- horizon: saturated with water for prolonged periods. Contains 20% or more organic matter. • O-horizon: Saturated with water for no more than a few days of a year. Contains 35% or more of organic matter.

  43. Soil profile description • A-horizon: the uppermost organic rich mineral layer in the soil, characteristic of forest soils or others with a high input of humified organic detritus. May be disturbed by ploughing. • E-horizon: a distinctive pale coloured horizon in the upper part of the profile. Characterised by a loss of iron compounds and clays, leading to a silty, pale layer. Well developed in those soils where acidic conditions are developed in the upper layers. Also termed the eluvial (hence E) horizon.

  44. Eluviation and illuviation under humid, semiarid and arid conditions. (

  45. Soil profile description • B-horizon: the main soil horizon, usually subdivided according to the degree of iron translocation. Characteristically richer in iron compounds and fines due to the downward percolation of these materials. Sometimes termed an illuvial horizon for this reason. Shows variable stucturation. • C-horizon: the insitu substrate, may be weathered but not affected by soil formation. • R-horizon: Very hard bedrock.