Soils Flashcards

1
Q

What are soils?

A

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
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2
Q

Soils can be classified based on their composition:

A
  1. Organic soils
    - >20% OM
    - OM accumulates in anoxic wetlands (bogs, swamps…) and doesn’t break down
    - High amounts of leaf litter accumulates
  2. Mineral soils
    - <20% OM
    - Dominated by minerals and rock fragments that form from weathered rocks
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3
Q

Soils form from the weathering of rock/regolith (fragmented rock material at or just below Earth’s surface) + accumulation of OM

A
  • 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
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4
Q

Soils form through

A

pedogenesis: a series of chemical, physical, and biological weathering processes

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5
Q

Soil Formation Steps

A
  1. Soil begins to form
  2. Simple organisms
  3. Horizons begin to form
  4. Well-developed soil
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6
Q
  1. Soil begins to form
A
  • 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.
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7
Q
  1. Simple Organisms
A
  • 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
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8
Q
  1. Horizons Begin to Form
A
  • 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
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9
Q
  1. Well-developed soil
A
  • 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
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10
Q

Soil horizons are named according to the following scheme:

A

From top down:

O, A, E, B, C, R

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11
Q

O - Humus

A
  • 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
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12
Q

A - Zone of leaching of soluble salts and minerals (topsoil)

A
  • Consists of minerals and OM

- Minerals dissolved and removed by percolating, slightly acidic water

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13
Q

E - “bleach horizon”

A
  • May or may not be present below A horizon

- Zone of “eluviation” or maximum leaching of clay/Fe/Al

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14
Q

B - Zone of accumulation of dissolved elements (subsoil)

A
  • 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)
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15
Q

Bk layers contain accumulations of caliche (CaCO3)

A
  • 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
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16
Q

C - Weathered Parent Material (Bedrock)

A
  • Consists of pedogenically unaltered regolith

- May transition into unweathered parent rock

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17
Q

R horizon

A

Unweathered parent rock

- not always visible

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18
Q

How do horizons form?

A
  • 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)
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19
Q

What controls rates of chemical weathering?

A
  1. Climate
  2. Relief of the land
  3. Surface area
  4. Composition of parent rock
  5. time
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20
Q
  1. Climate
A
  • Rates of chemical weathering are strongly controlled by water availability and temperature
  • Fast: hot, humid
  • Slow: hot, dry
  • Almost none: cold, dry
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21
Q
  1. Relief of the land
A
  • Water will tend to flow downhill: regional control on chemical weathering
  • Typically more infiltration on flat/shallowly dipping surfaces compared to steep surfaces
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22
Q
  1. Surface area
A
  • 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
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23
Q
  1. Composition of parent rock
A
  • 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
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24
Q
  1. Time
A
  • The longer a rock is exposed, the more it will be weathered
  • Rates of chemical weathering increase over time as SA increases
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25
Q

How do we determine how old a soil is?

A

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

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26
Q

Soil maturity is based on:

A
  • The number of horizons present
  • The thickness of horizons
  • How distinct horizons are from one another
27
Q

Mature soils have

A

thick, distinct horizons

28
Q

Immature soils have

A

few thin, poorly developed surface horizons

29
Q

Rapid burial halts:

A

further pedogenesis.

If the burial is slow, new sediment may be added to existing soil

30
Q

What types of soils exist?

A

Soils can be differentiated based on the type of parent material they develop in

  1. Residual soil
  2. Transported soil
31
Q
  1. Residual Soil
A
  • Weathering in situ: develop in place on underlying bedrock
32
Q
  1. Transported Soil
A
  • Form on eroded materials (transported regolith)
  • No R horizon
  • Named for the erosional process that transported and deposited the parent material
33
Q

Alluvial soils

A

rivers

34
Q

Eolian soils

A

Wind

35
Q

Glacial soils

A

Glaciers

36
Q

Volcanic soils

A

Volcanoes

37
Q

Soil fertility increases with increasing nutrients (N, P, K)

A
  • 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
38
Q

Loess

A

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
39
Q

Soils can also be differentiated based on:

A
  1. Texture

2. Colour

40
Q

Soil texture

A

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
41
Q

Soil drainage

A
  • 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)
42
Q

Soil drainage is affected by:

A
  • 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
43
Q

Soil Colour

A

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)
44
Q

Soil Classification

A
  • 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
45
Q

Which types of soils are problematic?

A
  • Some soils prove problematic when humans try to build infrastructure on them or deforest them for other uses
46
Q

Expansive Soils

A

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
47
Q

Heavily Weathered Soils

A

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
48
Q

Permafrost

A

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

49
Q

Human activities can thaw near-surface ice:

A
  • 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
50
Q

Compacted Soils

A

Cannot hold much water because pore spaces are collapsed

51
Q

Desert Soils

A
  • 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
52
Q

What is soil erosion?

A

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
53
Q

Are the high-risk areas for wind erosion the same as those for water erosion?

A

Lots of overlap, so generally speaking yes

54
Q

What is controlling the distribution of erosion by wind and water?

A

Distribution of vegetation

55
Q

Types of Erosion Processes

A
  1. Erosion caused by water
  2. Erosion caused by wind
  3. Erosion caused by wind
56
Q
  1. Erosion caused by water
A
  • 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?)
57
Q
  1. Erosion caused by wind
A

Wind removes soils from bare lands

  • Rates are lower than water erosion, except during a drought
  • Can create dust storms
58
Q
  1. Erosion caused by human activities
A

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
59
Q

How do we mitigate soil erosion?

A

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:

  1. Terracing
  2. Strip-cropping
  3. Crop rotation
  4. Conservation-tillage
  5. No-till/minimum till
60
Q
  1. Terracing
A

Turning a slope into steps to limit erosion and runoff

61
Q
  1. Strip-cropping
A

Groundcover crops (alfalfa) and widely-spaced crops (corn) are sown in alternative strips

62
Q
  1. Crop rotation
A

Yearly alternation of crops so soil does not become depleted in one set of nutrients, the ground is never left bare

63
Q
  1. Conservation-tillage
A

Minimized plowing and retain crop residues (stubble) limits erosion and increases water retention

64
Q
  1. No-till/minimum till
A

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