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
Soil maturity is based on:
- The number of horizons present
- The thickness of horizons
- How distinct horizons are from one another
Mature soils have
thick, distinct horizons
Immature soils have
few thin, poorly developed surface horizons
Rapid burial halts:
further pedogenesis.
If the burial is slow, new sediment may be added to existing soil
What types of soils exist?
Soils can be differentiated based on the type of parent material they develop in
- Residual soil
- Transported soil
- Residual Soil
- Weathering in situ: develop in place on underlying bedrock
- Transported Soil
- Form on eroded materials (transported regolith)
- No R horizon
- Named for the erosional process that transported and deposited the parent material
Alluvial soils
rivers
Eolian soils
Wind
Glacial soils
Glaciers
Volcanic soils
Volcanoes
Soil fertility increases with increasing nutrients (N, P, K)
- Volcanic soils tend to be fertile because ash is easily chemically weathered and adds many ions plants use
- Glacial soils also tend to be fertile because they contain rocks from many sources
Loess
Windblown silt (small grains) from glacial deposits and deserts
- Covers 20% of the USA surface, 10% global surface (Chin, Ukraine)
- Very fertile -> loose sediment allows roots to penetrate easily, retains water, releases nutrients as it weathers
- Can be excavated for housing
Soils can also be differentiated based on:
- Texture
2. Colour
Soil texture
Distribution of grain size (relative abundance of sand, silt, and clay)
- Texture determines looseness, drainage, workability, etc.
- Clay-rich soils are generally heavy, harder to work, sticky when wet
- Coarse soils are generally more porous and permeable, allowing water to drain freely through the soil with little retention
Soil drainage
- Rain or irrigation fills pores with water
- If all pores are filled with water, the soil is saturated
- After rain or irrigation has stopped, part of the water present in the larger pores will move downward. This process is called drainage or percolation
- Plants need air and water in the soil
- Extended saturation will harm most plants (except rice)
Soil drainage is affected by:
- Amount and distribution of rainfall (varies seasonally, annually or longer)
- Soil texture, which affect infiltration rate
- Coarse textured (sandy) soils, drainage is completed within a period of a few hours
- Fine textured (clay) soils, drainage may take 2-3 days
- Depth to groundwater: if GW is close to surface (coastal areas) or intersects the surface (low-lying areas like wetlands, swamps), soils will be saturated
Soil Colour
Indicates humus content (OM) and iron content
- Dark brown/black: high humus
- Yellow: iron compounds are hydrated (soil is generally poorly-drained)
- Gray-green: iron compounds are reduced (soil is generally poorly-drained and waterlogged, oxygen is absent)
- Red: iron is oxidized (soil is generally well-drained, pore spaces are filled with oxygen)
Soil Classification
- Environmental condition cause a certain set of soil processes to occur, which produce a distinctive set of horizons at the time we observe in the soil
- These soil horizons are the basis for classifying the soil in the Canadian System of Soil Classification
Which types of soils are problematic?
- Some soils prove problematic when humans try to build infrastructure on them or deforest them for other uses
Expansive Soils
Vertisols (high clay, little OM) contain swelling clays
- Certain clay minerals can absorb water and expand, then dry and shrink
- Foundations can move unevenly over time, causing structural and cosmetic damage
- In Canada, vertisols are not too bad
Heavily Weathered Soils
Oxisols (or laterites) are heavily weathered and leached tropical soils
- Form due to abundant rainfall and warm temperatures
- Even quartz may dissolve, leaving only Fe and AL
- Heavily leached, so few nutrients can be derived from minerals
- New nutrients come from decay of plants, but are quickly consumed
- Deforestation removes nutrient source, making soil infertile
- Could add fertilizer, but exposure to sun dries soil out and it becomes brick-hard “ironstone)
- Within a few years, the soil becomes impenetrable to water and roots
- Water pools in the rainy season, creating swamps
- Repeated efforts to clear rainforests for agriculture over time have destroyed vast tracts of rainforest
Permafrost
Soils and surficial sediments that are frozen for several years at a time
- Found in Alaska, Canada, Siberia
- Up to 1500 thick
- Continuous at high latitudes, but becomes patchier towards middle latitudes where it is warmer
Permafrost table: top of permanently frozen layer
Active layer: above permafrost, seasonal freezing and thawing, always unstable in summer
Human activities can thaw near-surface ice:
- Roads can absorb more heat and thaw the permafrost underneath causing the roads to warp
- Pipelines
- Buildings absorb heat and can sink into the ground, buildings are also heated
Compacted Soils
Cannot hold much water because pore spaces are collapsed
Desert Soils
- Fragile and plant growth is limited by water
- Often only have a thin crust of algae, bacteria, and lichen, and a natural pavement of stones to resist erosion and weathering
- When broken, these soils erode easily and create dust
What is soil erosion?
Soil is continually formed, but its formation takes so long (100s to 1000s of years) that it is ~ a non-renewable resource
- Removes soil due to wind, water, and human activities
- In the US, 3-5x more soil is lost than created
- Required for food production, but disappearing rapidly around the world
Are the high-risk areas for wind erosion the same as those for water erosion?
Lots of overlap, so generally speaking yes
What is controlling the distribution of erosion by wind and water?
Distribution of vegetation
Types of Erosion Processes
- Erosion caused by water
- Erosion caused by wind
- Erosion caused by wind
- Erosion caused by water
- Sheet erosion: water removes thin layers, hard to detect, combat with cover crops
- Rill erosion: streamlets in soil
- Gullying: rills become gullies (ditches with small stream in bottom?)
- Erosion caused by wind
Wind removes soils from bare lands
- Rates are lower than water erosion, except during a drought
- Can create dust storms
- Erosion caused by human activities
Roots helps anchor soil and limit erosion. Any activities that remove vegetation increase rates or erosion.
- Bad agricultural practices (tilled land lies unseeded and without vegetation)
- Deforestation
- Overgrazing
- Recreation/military activities (ORV use on delicate soils)
- Construction
- Mining
How do we mitigate soil erosion?
Although cropland development contributes to soil erosion, we need crops to support the global population. To protect our soils, we have adopted better farming practices:
- Terracing
- Strip-cropping
- Crop rotation
- Conservation-tillage
- No-till/minimum till
- Terracing
Turning a slope into steps to limit erosion and runoff
- Strip-cropping
Groundcover crops (alfalfa) and widely-spaced crops (corn) are sown in alternative strips
- Crop rotation
Yearly alternation of crops so soil does not become depleted in one set of nutrients, the ground is never left bare
- Conservation-tillage
Minimized plowing and retain crop residues (stubble) limits erosion and increases water retention
- No-till/minimum till
Seeds plants through crop residues and weeds controlled with chemicals, requires special equipment and chemicals, so these practices are only minimally used in developing countries
