Soils Flashcards
What are soils?
Mixtures of organic matter (OM), minerals, gases, and liquids that support life at the surface of the Earth
- OM mostly from dead plants
- Minerals are products of weathering
Soils can be classified based on their composition:
- Organic soils
- >20% OM
- OM accumulates in anoxic wetlands (bogs, swamps…) and doesn’t break down
- High amounts of leaf litter accumulates - Mineral soils
- <20% OM
- Dominated by minerals and rock fragments that form from weathered rocks
Soils form from the weathering of rock/regolith (fragmented rock material at or just below Earth’s surface) + accumulation of OM
- Weathering: physical and chemical breakdown of rock due to exposure to air, water, ice, and organisms
- Regolith: sediment resulting from weathering. Covers most of the Earth’s surface.
- Soil: regolith that supports plant growth
Soils form through
pedogenesis: a series of chemical, physical, and biological weathering processes
Soil Formation Steps
- Soil begins to form
- Simple organisms
- Horizons begin to form
- Well-developed soil
- Soil begins to form
- First, rocks break down into regolith
- Water percolates into cracks causing physical and chemical weathering (frost wedging, water causes dissolution/hydration/hydrolysis of mineral, dissolved ions and clays transported downwards with water)
- During dry periods, cracks fill with air and minerals oxidize. Pore waters may evaporate, concentrating dissolved ions and causing minerals to precipitate at lower levels.
- Simple Organisms
- Nutrients released by weathering becomes food for bacteria
- Bacteria produce CO2, which combines with water to produce carbonic acid (=weathering)
- They die and leave behind organic residues
- Residues support growth of simple photosynthetic algae, lichens, or mosses
- Horizons Begin to Form
- Bacteria also fix nitrogen (convert inert N2 from atmosphere into a usable form), which support growth of more complex plants
- Root growth causes additional physical weathering
- Thickness of soil is starting to increase
- Well-developed soil
- pedogenesis gradually increases thickness of soil
- Destroys original structures (e.g sedimentary structure, foliation, igneous textures)
- Produces layers called horizons
- A soil profile comprises all horizons of a given soil
Soil horizons are named according to the following scheme:
From top down:
O, A, E, B, C, R
O - Humus
- Dominated by OM (humus) of variable thickness
- Only present in organic soils
- Provides CO2 and organic compounds that make percolating water slightly acidic
- In the rock record, the O horizon forms coal layers
A - Zone of leaching of soluble salts and minerals (topsoil)
- Consists of minerals and OM
- Minerals dissolved and removed by percolating, slightly acidic water
E - “bleach horizon”
- May or may not be present below A horizon
- Zone of “eluviation” or maximum leaching of clay/Fe/Al
B - Zone of accumulation of dissolved elements (subsoil)
- Zone of illuviation (addition of minerals from downward percolating water)
- If the soil is saturated, the ions will generally stay in solution and may be flushed from this zone
- Multiple B horizons may developed numbered B1, B2..
- May also add suffixes to describe composition/character (e.g Bt = clay accumulation, Bf = frozen, Bk = carbonate accumulation)
Bk layers contain accumulations of caliche (CaCO3)
- Forms in deserts/dry environments
- Rainwater leaches CaCO3 from above and redeposits it at deeper levels
- Often forms nodules
- Nodules can coalesce to form thick, hard layers that are very difficult to dig through
C - Weathered Parent Material (Bedrock)
- Consists of pedogenically unaltered regolith
- May transition into unweathered parent rock
R horizon
Unweathered parent rock
- not always visible
How do horizons form?
- Over time, weathering can make surface horizons thicker and cause the C horizon to move downward
- Weathering produces more regolith, increasing thickness of sediment that may be pedogenically altered
- Clays and element percolated downward with rainwater, increasing thickness of elluviated horizons (O, A, E)
- Clays and elements redeposit deeper in the profile, increasing thickness of illuviated horizons (B)
What controls rates of chemical weathering?
- Climate
- Relief of the land
- Surface area
- Composition of parent rock
- time
- Climate
- Rates of chemical weathering are strongly controlled by water availability and temperature
- Fast: hot, humid
- Slow: hot, dry
- Almost none: cold, dry
- Relief of the land
- Water will tend to flow downhill: regional control on chemical weathering
- Typically more infiltration on flat/shallowly dipping surfaces compared to steep surfaces
- Surface area
- In general, greater SA = increased chance of chemical weathering
- Fractures, cracks, and exposure surfaces (natural and anthropogenic) will increase susceptibility to weathering
- Weathering rounds out rocks = more SA
- Composition of parent rock
- Some minerals (and therefore rocks) are more susceptible to chemical weathering
- Most susceptible to least (high crystallization temp to low): olivine/plagioclase (Ca rich), pyroxene, hornblende, biotite/plagioclase (Na rich), feldspar, muscovite, quartz
- Time
- The longer a rock is exposed, the more it will be weathered
- Rates of chemical weathering increase over time as SA increases
How do we determine how old a soil is?
Difficult to determine the age of a soil:
- Soils are built on pre-existing sediment
- Soils are younger than the sediment, but difficult to date the deposition of sediment
- Could carbon date OM, but new OM constantly being added = unreliable
- An old soil may be poorly developed if the rates of weathering are particularly slow (arctic)
- A very young soil may be well-developed if the rates of weathering are particularly fast (tropics)
Therefore, we describe soil maturity