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Pedogenic Processes
Soil Morphology, Genesis & Classification (SPS 350)
Introduction
Many processes are responsible for transforming the accumulated parent material into soil horizons.. The exact combinations of these processes and reactions are however not known. a If the combination is dominated by a particular process, it is usually assigned a name. e.g., laterization, podzolization were commonly used as simplifications.
Major Pedogenic Processes
The pedogenic processes can be grouped into four. These are: – Additions or Gains – Losses or Removals – Translocations or Transfers – Transformations Each major pedogenic process is made up of a number of processes acting singly or in combination
Additions or Gains
• • • •
Enrichment
– General term for addition of material to a soil body. e.g., adjoining pedons as in depressions.
Cummulization
– Aeolian and hydrologic additions of mineral particles to the surface of a soil. The effects are more pronounced in depressions.
Littering
– The accumulation of vegetable and associated faunal debris (litter) including humus on the mineral soil surface to a depth of less than 30cm
Melanization
– The darkening of light-colored mineral materials which are initially unconsolidated by admixture of organic matter. Melanization involves some translocation.
Losses or Removals
• • Erosion – surficial removal of material from the surface layer of a soil This is effected by raindrop splash, runoff waters, wind, creep, and other mass wasting processes. Leaching – General term for washing out or eluviating soluble material from the solum.
Translocation or Transfers
• • • • Eluviation – Movement of material out of a portion of a soil profile as in an albic horizon.
Illuviation – Movement of material into a portion of soil profile as in argillic or spodic horizon Lessivage – Washing in suspension of fine clay and lesser amounts of coarse clay and fine silt down cracks and other voids in a soil body; leading to the depletion of clay in the A horizon and enrichment of clay in the B horizon. Pedoturbation – Biologic, physical (freeze-thaw and wet-dry cycles) churning and cycling of soil materials thereby homogenizing the solum in varying degrees
• • • • • • Calcification – Processes including the accumulation of calcium carbonate in a Ck and possibly other horizons of a soil. Deca1cification – Reactions that remove calcium carbonate from one or more soil horizons. Salinization – The accumulation of soluble salts such as sulfates and chlorides of calcium, magnesium, sodium and potassium in salty (salic) 'horizons. Desalinization – The removal of soluble salts from salic horizons. Alkalization (solonization) – The accumulation of sodium ions on the exchange sites in a soil. Dealkalization (solodization) – The leaching of sodium ions and salts from sodium-rich (natric) horizons.
• • • • • Leucinization – The paling of soil horizons by disappearance of dark organic materials either through transformation to light- colored ones or through removal from the horizons. Braunification, Rubification and Ferrugination – Release of iron from primary minerals and the dispersion of particles of iron oxide in increasing amounts; the progressive oxidation or hydration, giving the soil mass brownish, reddish-brown and red colors, respectively. (Also transformation). Gleization – The reduction of iron under anaerobic water-logged soil conditions, with the production of bluish to greenish-gray matrix colors, with or without yellowish brown, brown and black mottles and ferric and manganiferous concretions. (Also transformation: Podzolization (silication) – The chemical migration of aluminum and iron and/or organic matter, resulting in the concentration of silica (i.e. silication) in the layer eluviated. (Also transformation) Laterization (desilication, ferrugination, ferritization, allitization) – The chemical migration of silica out of the soil solum and thus the concentration of sesquioxides in the solum (goethite, gibbsite, etc.)
Transformations
• • • • Decomposition – The breakdown of mineral and organic materials Mineralization – The release of oxide solids during decomposition of organic. It concerns the biochemical breakdown of dead plant tissues by soil micro-organisms to produce simple-structured soluble organic substances, purely mineral compounds like nitrates, and metal cations and gases (mostly carbon dioxide) Humification – Transformation of raw organic material into humus. In this the simple structured soluble organic substances are grouped into lager molecules by polymerization They then become poorly soluble and are stabilized to form a major component of humus Synthesis – The formation of new particles of mineral and organic species.
• • Paludization – The accumulation of deep (> 30cm) layers of organic matter as in mucks and peats. Ripening – This refers to the chemical, biological, and physical changes in organic soil after air penetrates the organic deposits, making it possible for microbial activity to flourish.
The processes in action
• • • • • • Each of the major pedogenic processes may affect many compounds and components which make up the soil. For example, there may be additions, losses, transfers and transformations of organic matter, soluble salts, carbonates, sesquioxides. Organic matter may be added to the soil by littering; it may be transformed by decomposition; it may be translocated by podzolization and it may be lost by erosion. These processes can all be going on singly or in combination with other processes to give rise to the soil profile.
However, not all of the processes will necessarily promote horizon differentiation. Some of the processes may actually retard or offset differentiation e.g. pedoturbation.
The processes in action
Additions Transformations Translocations Translocations Losses
Soil Horizon Development
• • A-Horizon development – Accumulation of organic matter – Clumping of individual soil particles – Distinct from parent material and other layers B and C horizon development – Carbonic and organic acids are carried by water into soil where dissolve various minerals (transformations) – Soluble materials (ions –Ca 2+ , CO3 2 , SO4 2 , etc) are carried by water and precipitate in the soil from upper to lower horizons (translocation) – Weathering of primary minerals into secondary minerals – Wetting and drying cracks soils and makes structures.