Lecture 4 Flashcards
What are the 4 soil forming processes?
Transformations
Transfers
Additions
Losses
How does soil organic matter change with depth?
Only a small quantity of SOM present as determined by weight (1-8% by weight in mineral topsoil)
Soil development on residual material?
Highlighting the distinction between soil developed on residual and transported material
Soil developed on residual basalt bedrock
Soil development on transport material?
Transport by:
Wind
Water
Soil developed on basal till (ice transported material)
Define soil
Soil is the upper and biochemically whether portion of the regolith
Define sediment
Sediment is material of varying size both mineral and organic that is being or has been moved from its state of origin by the action of wind water gravity or ice and comes to rest elsewhere on the surface
List some transformations (internal change)
Physical and chemical weathering of minerals
Decomposition of organic matter
Waterlogging and gleying
Soil ripening
List some transfers (internal rearrangement)
Pedoturbation
Infiltration and percolation
Eluviation / illuviation
The development of soil profiles
Soils develop through a combination of additions, transformations, transfers and losses
Physical weathering is the brittle fracture of rocks by the physical processes of…
A) unloading (removal reducing pressure on underlying rocks causing fractures to form eg granite or sandstone)
B) growth and expansion of ice (termed gelifraction) or salts (termed haloclasty) in rock pores and fissures
When does chemical weathering occur?
When parent rock materials (primary minerals) react with acidic or oxidising substances, usually in the presence of water, to produce more stable secondary minerals
Why are chemical weathering rates particularly high adjacent to plant roots?
Because of biological activity and CO2 production from respiration
When are secondary minerals formed?
In weathering
They play critical roles in soils and ecosystem processes
Define dissolution
The breakdown of minerals into component ions under the action of water
Define hydration
Combination of a compound with water whereby the water is absorbed within the mineral lattice
Define hydrolysis
The replacement of metal cations in minerals by a hydrogen ion from acidic soil water
Waterlogging and gleying
Causes of saturation?
Rainfall (intensity and duration)
Rising water table
Overbank flooding
What happens with waterlogging and gleying
Saturation slows down rate of organic matter decomposition
Gleyed soils are produced when waterlogging is intermittent or seasonal
When does soil ripening occur
When a soil that is waterlogged or flooded dries out
Transfers
Processes which can move material either physically or chemically within the soil, both upwards and downwards
Pedoturbation - physical mixing of soil profile eg soil creep
Bioturbarion - biological mixing of the soil by plants and animals
Define cryoturbation
Physical mixing of the soil by frost
Define the transfer of infiltration
Process by which water enters the soil and transports chemicals into the soil profile
Transfers
Define eluviation
General term for the transfer of soil material in solution from one horizon, comprising 2 processes:
Leaching: movement of soluble organic and inorganic soil components in percolating water
Cheluviation: soluble organic complexes move metal cations through soil profile (especially iron and aluminium)
Transfers
Define lessivage
Movement old clays in colloidal suspension without change in chemical composition
Produces soils such as acid brown earths
Transfers
Define illuviation
Accumulation of materials in a lower B horizon after eluviation from A horizon
Transfers: illuviation
How does material become deposited with a soil horizon?
- Mechanical sieving effect
Depends on pore size of transported material relative to pore space in the lower part of the profile - Chemical effect
Reduced solubility and subsequent precipitation of colloidal and chelated material in the lower part of the profile
What is pozolisation and example of?
Leaching, eluviation and illuviation
Where can additions come from?
Outside or inside the ecosystem
Produced by outside:
Inflow of material from upslope
Aeolian input eg Iceland, tephra from volcanic ash
Produced by organisms from inside adding organic matter and nitrogen
Losses
Wind erosion
Rain erosion
What is the effect of climate on soil depth?
Increased temperatures and moisture availability together enhance weathering
Increased weathering leads to an increase in the depth of soil
Soil catena and topography
Predictable series of soils arranges down a slope due to the differences in moisture, erosion and sediment transport along with pedogenic processes
Effects of vegetation type on soil development?
A) ultisol development: forests
Soil with low base status and clay enriched subsoil
Red colour due to iron oxides
B) mollisol
Development: grasslands
Very fertile soil with organic matter
Organic soils: HISTOSOLS
Consist of organic debris which accumulated at the surface under wet or dry conditions
