Formation & importance of soils, lecture 27 Flashcards
Soil
The outermost layer of the Earth
How is soil formed?
Weathering of rocks
Phrase to describe soil
‘living mantle of the Earth’
Is soil a renewable or non-renewable resource?
non-renewable resource
Rough soil formation time
thousands of years to form
Soils are at the interfere of
4
- Lithosphere
- Atmosphere
- Hydrosphere
- Biosphere
Soils at the interface of
the Lithosphere
Soils formed from rock constituents.
Soils formed from rock constituents.
the atmosphere
Air overlies and intermingles with soil.
Soils are at the interface of
the hydrosphere
Water flows through soil.
Soils are at the interface of
the biosphere
Biota live in and on soil.
Soil science is…
interdisciplinary
Key functions of soil
6
(according to DEFRA)
- Food and Fibre Production
- Environmental Interaction
- Support of Ecological Habitat and Biodiversity
- Protection of Cultural Heritage
- Providing a platform for construction
- Providing Raw Materials
There is unlocked potential in soil resources for…
biological and medical resources
Regulation of water by soil,
Soil misuse or over-exploitation can have significant consequences for…
example?
groundwater, rivers, estuaries and coastal waters
eg: overuse of fertilisers
Link of soil to history
Dominant nations had good soil,
facilitates productive agriculture
Soils quality reflects
3 aspects
all chemical physical biological properties
Main threats to soil function and quality
5
- Erosion
- Compaction and sealing
- Contamination
- loss of SOM (Soil organic matter)
- Salinisation
Threats to soil function and quality
Erosion
(landslides)
loss of productivity and biodiversity
used linked to overuse
Threats to soil function and quality
Compaction and sealing
intensive use of heavy equipment or grazing whilst the soil is saturated
(loss of biodiversity, flooding)
Threats to soil function and quality
contamination
Metals and organic contaminants could degrade soil quality and functions, implications for the wider environment
Threats to soil function and quality
loss of SOM
Loss of SOM, loss of fertility, increased risk of erosion, also reduces water retention capacity, linked to climate change
Threats to soil function and quality
Salinisation
- Excess accumulation of soluble salts due to saline water use in farming or irrigation practices
- generally in hotter countries, Middle East
Percentage of the World’s freshwater stored in soil for plant use
66%
How should soil be viewed?
as a finite resource
Weathering
Physical and chemical breakdown of rocks and other materials
Soils relate to what
the rock they originate from
Factors controlling soil formation
5
- parent material
- climate
- biota
- topography
- time
Factors affecting soil formation
parent material
unconsolidated material or rock in which soil development occurs
2 examples of parent material deposits
organic deposits - peat
glacial ice transport - glacial till
Example of parent material
organic deposits - peat
Organic material deposited when accumulation exceeds decomposition: stratified deposit of peat.
Example of parent material
glacial ice transport - glacial till
Glacial till, material deposited directly by ice, is a heterogeneous (unstratified) mixture of rounded boulders, stones, sand, silt, and clay.
Factors affecting soil formation
climate
Temperature and precipitation (rainfall, snowfall)
Which is the most influential of the four factors acting on parent material?
climate
Which two climatic factors have major effects on physical and chemical weathering and biological processes
precipitation and temp.
What allows carbonates to accumulate at shallow depths?
low rainfall
Why do acidic soils form in humid areas?
intense weathering and leaching of basic cations - Ca, K, Mg
Effect of water induced soil erosion on sloping land?
constant removal of developing layers
When developing soil layers eroded from slopes due to erosion by water, what is the effect on soil downslope?
Deposition of soil materials downslope covers developing soil layers
Weathering, leaching and erosion in warm and humid regions
more intense and occur for a longer period of time
Why does rapid weathering not occur in cold areas such as Alaska?
soil is too cold
Factors affecting soil formation
Biota
Living organisms, plants, organic residue, microbes, soil animals
Soil in humid forest vegetation
soils are leached but have many layers of slowly decomposing organic matter
Grassland soils
generally rich in well decomposed organic matter
Arable soils
generally have low content of organic matter
Soils that harbour many burrowing animals have…
fewer but deeper soil horizons.
How do micro-organisms help soil formation?
by decomposing organic matter and forming weak acids
Role of worms in soil
factors affecting soil formation - biota
important for mixing of soil
Factors affecting soil formation
Topography
The Earth’s surface contour.
Slope, aspect, elevation and landscape position
Horizons of soils on steep hillsides
thin
Landlocked depressions
runoff, productivity, decomposition rates
receive more runoff, greater production of vegetation, but slower decomposition.
Waterlogged, and high productivity of vegetation and low rate of decomposition results in…
organic (peat and muck) soils
Slopes facing sun are…
warmer and drier than those not; water limitation may affect vegetation cover and warmer temperature can affect the breakdown of organic matter.
Slopes tend to
- soil depth, drainage, erosion
decreased soil depth,
better drainage,
increased wind and water erosion
What is relief vs topography?
Relief = height difference between a high and low point (measured by contours on a map not elevation) Topography = What the land looks like based on elevation
Factors affecting soil formation
Time
Tine since parent materials were exposed
What soil maturity based on?
development stage rather than age
Is younger or older soil generally more fertile?
why?
younger soil is, as it had more minerals
Which factors determine the length pf time for soil formation?
4
- climate
- the nature of the parent material
- the organisms
- topography
What type of climate is soil formation faster in?
warm, humid climates
Time periods of glacial till and sediments
short, <200 years
Weathering of rock time period
varies, but can take thousands/millions of years
Processes during soil formation (genesis)
5
- additions
- removals
- mixing
- translocation
- transformation
Soil Horizons
Different layers of soil
Soil profile
A section through soil horizons
How many soil horizons are there?
six
What are the six soil horizons?
in order
O - horizon A - horizon E - horizon B - horizon C - horizon R - horizon
O - horizon of soil
A thin layer which is at the top of the soil and is composed of leaf litter and organic matter
A - horizon of soil
The soil layer below the top layer (o-horizon) which is composed of the most humus so is the darkest generally.
E - horizon of soil
The layer below A - horizon with a high clay content and insoluble minerals such as quartz, soluble materials will have leached from this layer.
B - horizon
The layer below the E - horizon where there is little organic matter content, yet soluble minerals and iron oxides build up here.
C - horizon
Bedrock which is mixed with clay and is broken, decayed and slightly altered.
R - horizon
The bottom soil horizon which made up of unaltered bedrock.
Processions during soil formation (genesis)
Additions
9
Water (from the surface, and by ground water discharge)
Suspended and dissolved materials carried by water
Solids transported by wind
Gases from the air
Energy from the sun
Organic carbon by plants in the form of roots and root-derived material
Organic carbon by photoautotrophic bacteria
Organic nitrogen by nitrogen-fixing bacteria
Plant and animal remains on and in the soil
Processes during soil formation (genesis)
Removals
7
Material removed by wind erosion
Material removed by water erosion
Dissolved and suspended material leached out
Uptake of nutrients from the soil by plants
Carbon dioxide gas produced by plant root, microbial and faunal respiration
Other gases such as nitric oxide, nitrous oxide and nitrogen produced by denitrifying bacteria
Other gases such as methane which are produced under anaerobic conditions
Processes during soil formation (genesis)
Mixing
4
Soil animals
Tree fall and gravity
Shrinking and swelling due to water content changes
Freezing and thawing
Processes during soil fomation (genesis)
Translocation
3
Gradients in water potential and chemical concentrations withinthe soil pores
Soluble minerals, colloidal material, organic compounds, andiron may move up or down the profile, between horizons, with water movement.
Biological activity may cause gradients in the chemical composition of the water and air-filled pores of the soil.
In the formation of soils, translocation means…
vertical movement of materials
Processes during soil transformation (genesis)
Transformation
3
Soil components are transformed by chemical and biological reactions.
Organic compounds decay, some minerals dissolve, other minerals precipitate.
These transformations result in the development of soil structure, and in changes in colour, relative to the parent material.
